Cut Emissions Transportation Shift to Alternatives

Enhance Public Transit

Highly Recommended
Train with city in the distance

We define the Enhance Public Transit solution as increasing the use of any form of passenger transportation that uses publicly available vehicles (e.g., buses, streetcars, subways, commuter trains, and ferries) operating along fixed routes. It does not include increasing the use of publicly available forms of transportation without fixed routes, such as taxis, except when these transport options supplement a larger public transit system (for example, to help passengers with disabilities). It also does not include increasing the use of vehicles traveling over long distances, such as intercity trains, intercity buses, or aircraft. The cost per climate unit is the cost to the transit provider, not the passenger.

Last updated June 30, 2025

Solution Basics

million passenger kilometers (million pkm)

tCO2-eq/unit/yr
58.27
units/yr
Current 1.67×10⁷2.2×10⁷4.19×10⁷
Achievable (Low to High)

Climate Impact

GtCO2-eq/yr
Current 0.97 1.282.44
US$ per tCO2-eq
-3,300
Gradual

CO₂, CH₄, N₂O

Additional Benefits

183,187,188
    183
    • 184
    • 185
    • 186
    • 187
    • 188
189,194

Overview

Public transit vehicles are far more fuel-efficient – and thus less GHG-intensive – on a per-pkm basis than fossil fuel–powered cars. Diesel-powered buses emit fewer GHGs/pkm than cars because of their much higher occupancy. Electric buses further reduce GHG emissions (Bloomberg New Energy Finance, 2018), as do forms of public transit that already run on electricity. Finally, a fleet of large, centralized public transit vehicles operating along fixed routes is usually easier to electrify than a fleet of fossil fuel–powered cars. 

Enhancing public transit to reduce emissions from transportation relies on two processes. First is increasing the modal share of existing public transit networks by encouraging people to travel by public transit rather than car. This requires building new public transit capacity while also overcoming political, sociocultural, economic, and technical hurdles. Second is improving the emissions performance of public transit networks through electrification and efficiency improvements. We accommodate the latter in this solution by assuming that all shifted trips to buses are electric buses.

These two processes are linked in complex ways. For example, construction of the new public transit networks needed to accommodate additional demand creates an opportunity to install low-carbon vehicles and infrastructures, and bringing additional passengers onto an underused public transit network generates close to zero additional GHG emissions. However, since these complexities are difficult to calculate, we assume that all increases in public transit ridership are supported by a linear increase in capacity.

Buses, trains, streetcars, subways, and other public-transit vehicles predate cars. During the 19th century, most cities developed complex and efficient networks of streetcars and rail that carried large numbers of passengers (Norton, 2011; Schrag, 2000). As a result, it’s clear that a good public transit network can provide for the basic mobility needs of most people, and can therefore substitute for most – if not all – transportation that fossil fuel–powered cars currently provide. Today, public transit networks worldwide already collectively deliver trillions of pkm, not only in big cities but also in small towns and rural areas. 

We identified several different types of public transit:

Buses

Low-capacity vehicles running on rubber tires on roads. Buses in the baseline are a mix of diesel and electric. For the purposes of this solution, we assume that all buses serving shifted trips are electric.

Trams or streetcars

Mid-capacity vehicles running on steel rails that for at least part of their routes run on roads with traffic, rather than in a dedicated rail corridor or tunnel.

Metros, subways, or light rail

High-capacity urban train systems using their own dedicated right-of-way that may or may not be underground.

Commuter rail

Large trains running mostly on the surface designed to bring large numbers of commuters from the suburbs into the core of a city that often overlap with regional or intercity rail.

Other modes

Ferries, cable cars, funiculars, and other forms of public transit that generally play a marginal role.

We assessed all modes together rather than individually because public transit relies on the interactions among different vehicles to maximize the reach, speed, and efficiency of the system. Public transit reduces emissions of CO₂,  methane, and nitrous oxide to the atmosphere by replacing fuel-powered cars, which emit these gases from their tailpipes. Some diesel-powered buses in regions that have low quality diesel emit black carbon. The black carbon global annual total emissions from transportation is negligible compared with carbon emissions and is therefore not quantified in our study. 

Impact Calculator

Adjust effectiveness and adoption using range sliders to see resulting climate impact potential.

Effectiveness

58.27
t CO2-eq/million pkm
25th
percentile
0.13
75th
percentile
106.7
58.27
median

Adoption

1.67×10⁷
million passenger kilometers (million pkm)
Low
2.2×10⁷
High
4.19×10⁷
1.67×10⁷
current
Achievable Range

Climate Impact

0.975
Gt CO2-eq/yr (100-yr)
06 Gt
1.649%
of total global emissions*
*59.09 Gt CO2-eq/yr (100-yr basis)

Maps

Public transit is most effective in urban areas with high population density, where buses, subways, trams, and commuter rail can efficiently carry large numbers of passengers. Electrified or low-emission transit modes achieve the greatest climate impact, especially in regions with clean electricity grids (Bloomberg New Energy Finance, 2018). However, even diesel-based public transit systems can outperform fossil fuel-powered cars on a per-pkm basis if they have high ridership and operate efficiently.

Socioeconomic and political factors, including investment capacity, institutional coordination, and public perceptions of reliability, safety, and comfort, highly influence the adoption and effectiveness of public transit. Regions with well-funded public infrastructure, integrated fare systems, and strong governance tend to have the highest adoption and climate benefits. Conversely, underinvestment, informal transit dominance, or poorly maintained systems can undermine public transit’s potential (Börjesson et al., 2020; Mees, 2010).

High public transit adoption is seen in Western and Northern Europe, Post-Soviet countries, East Asia (including Japan, South Korea, and China), and some Latin American cities, like Bogotá and Santiago. In contrast, many developing regions face barriers to public transit expansion, such as inadequate funding, urban sprawl, or a reliance on informal minibus systems. However, these same areas offer some of the highest potential for impact. Rapid urbanization, growing demand for mobility, and severe air quality challenges create strong incentives to expand and modernize transit networks.

Population (millions)
1
10
30
Active Mobility
Public Transport
Private Cars

Primary mode of transport

Mapping the primary mode of transportation reveals mobility patterns and opportunities to shift travel toward lower-emitting modes.

Prieto-Curiel, R. and Ospina, Juan P. (2024). The ABC of mobility [Data set]. Environmental International, https://doi.org/10.1016/j.envint.2024.108541. Retrieved May 9, 2025 from https://github.com/rafaelprietocuriel/ModalShare/tree/main

Population (millions)
1
10
30
Active Mobility
Public Transport
Private Cars

Primary mode of transport

Mapping the primary mode of transportation reveals mobility patterns and opportunities to shift travel toward lower-emitting modes.

Prieto-Curiel, R. and Ospina, Juan P. (2024). The ABC of mobility [Data set]. Environmental International, https://doi.org/10.1016/j.envint.2024.108541. Retrieved May 9, 2025 from https://github.com/rafaelprietocuriel/ModalShare/tree/main

The Details

Current State

Our calculations suggest that an efficiently designed public transit system using the best available vehicle technologies (especially battery-electric buses) would save 58.26 t CO₂‑eq /million pkm (0.000058 t CO₂‑eq /pkm) on a 100-yr basis compared with fossil fuel–powered cars, in line with the estimates by other large transportation focused organizations (International Transport Forum, 2020; US Department of Transportation, 2010). This number is highly sensitive to public transit vehicle occupancy, which we estimated using the most recent available data (American Public Transit Association, 2021). Increasing the number of trips taken via public transit would likely increase occupancy, although ideally not to the point of passenger discomfort. This elevated ridership would significantly reduce public transit’s pkm emissions.

To arrive at this figure, we first estimated the emissions of fossil fuel–powered cars as 115 t CO₂‑eq /million pkm (0.000115 t/pkm, 100-yr basis). We then separately calculated the emissions of commuter rail, metros and subways, trams and light rail systems, and electric buses. We used data on the modal share of different vehicles within public transit systems around the world (although much of the available data are biased towards systems in the United States and Europe) to determine what each transit system’s emissions would be per million pkm given our per-million-pkm values for different transit vehicles (UITP, 2024). The median of these city-level values is 58 t CO₂‑eq /pkm (0.000058 t/pkm, 100-yr basis). Subtracting this value from the per-pkm emissions for cars gives us the public transit GHG savings figure cited above. Note that none of these values includes embodied emissions (such as emissions from producing cars, buses, trains, roads, etc.), or upstream emissions (such as those from oil refineries).

Pessimistic assumptions regarding the emissions and occupancy of public transit vehicles, and optimistic assumptions about emissions from cars, can suggest a much more marginal climate benefit from public transit (see the 25th percentile row in Table 1). In most cases, however, well-managed public transit is likely to produce a meaningful climate benefit. Such an outcome will depend on increasing the average occupancy of vehicles, which faces a challenge because transit has seen declining occupancies since the COVID-19 pandemic (Qi et al., 2023). For this reason, encouraging additional use of public transit networks without expanding these networks can have an outsized impact because it will allow the substitution of fossil fuel–powered car trips by trips on public transit vehicles for which emissions would not change meaningfully as a result of adding passengers.

Table 1. Effectiveness at reducing emissions.

Unit: t CO₂‑eq/million pkm, 100-yr basis

25th percentile 0.127
mean 61.76
median (50th percentile) 58.27
75th percentile 106.7

The extremely large range of values between the 25th and 75th percentile is the result of 1) the large diversity of public transit systems in the world and 2) multiplying multiple layers of uncertainty (e.g., varying estimates for occupancy, energy consumption per vehicle kilometer (vkm), percent of pkm reliant on buses vs. trains).

Under present-day public transit costs and revenues, it costs the transit provider US$0.23 to transport a single passenger one kilometer. In comparison, travel by car costs the consumer US$0.42/pkm. On a per passenger basis, for the transit provider, public transit is almost 50% cheaper than car transportation, costing US$0.20/pkm less. Combined with the emissions reductions from using public transit, this means that the emissions reductions from shifting people out of cars onto public transit has a net negative cost, saving US$3,300/t CO₂‑eq mitigated (Table 2). 

This figure includes all relevant direct costs for travel by public transit and by car, including the costs of infrastructure, operations, vehicle purchase, and fuel. It does not include external costs, such as medical costs resulting from car crashes. Capital costs (i.e., the large fixed costs of building public transit infrastructure) are accounted for via the annualized capital costs listed in public transit agencies’ financial reports. 

A very large proportion of the total costs of providing public transit is labor (e.g., wages for bus drivers and station attendants). This cost is unlikely to come down as a result of technological innovations (Bloomberg New Energy Finance, 2018).

For an individual passenger, however, the marginal costs of public transit (i.e., the fares they pay) can sometimes be higher than the marginal costs of driving. This is in large part due to many external costs of driving which are borne by society at large (Litman, 2024). However, increasing the public transit availability would likely increase occupancy, which would in turn drive costs down.

Table 2. Cost per unit of climate impact.

Unit: 2023 US$/t CO₂‑eq , 100-yr basis

median -3300

Transit provider cost, not passenger cost.

Public transit is a largely mature technology with limited opportunities for radical cost-saving innovation. While our research did not find any papers reporting a learning curve in public transit as a whole, battery-electric buses are in fact subject to many of the same experience effects of other battery-electric vehicles. Although there are no studies assessing declines in the cost of electric buses as a whole, there are studies assessing learning curves for their batteries, which is the most costly component. The cost of batteries used in battery-electric buses has declined 19.25% with each doubling of installed capacity (Table 3).

Table 3. Learning rate: drop in cost per doubling of the installed solution base.

Unit: %

25th percentile 18.63
mean 19.25
median (50th percentile) 19.25
75th percentile 19.88

This applies only to the cost of batteries in electric buses, not to public transportation as a whole.

Speed of action refers to how quickly a climate solution physically affects the atmosphere after it is deployed. This is different from speed of deployment, which is the pace at which solutions are adopted.

At Project Drawdown, we define the speed of action for each climate solution as gradual, emergency brake, or delayed.

Enhance Public Transit is a GRADUAL climate solution. It has a steady, linear impact on the atmosphere. The cumulative effect over time builds as a straight line.

Adoption

In cities around the world surveyed over the last 15 years, public transit has an average modal share of approximately 26.2% of trips. In comparison, fossil fuel–powered cars account for 51.4% of all trips, while nonmotorized transportation accounts for 22.4% (Prieto-Curiel & Ospina, 2024). The 26.2% of trips taken via public transit corresponds to approximately 16.7 trillion pkm traveled on public transit in cities every year (Table 4).

These numbers are calculated from modal share data (i.e., the percentage of trips in a given city that are taken via various modes of transportation). We estimated total pkm traveled by assuming a global average daily distance traveled, using travel surveys from the United States as well as several European countries (Christensen & Vázquez, 2013; Department for Transport, 2024; Ecke, 2023; Federal Highway Administration, 2022; Statistics Netherlands, 2024). We used Prieto-Curiel and Ospina’s (2024) global population-weighted mean modal share as our global adoption value. The other statistical measures in Table 4 reflect the distribution of estimates drawn from the literature, most of which do not account for population, and therefore give too much weight to small cities, skewing the results. 

We assumed that Prieto-Curiel and Ospina’s data refers only to urban modal share. While the database does include some small towns and rural areas, most of the modal share data we found comes from cities. Public transit can be useful in rural areas (Börjesson et al, 2020), but we did not attempt to estimate rural public transit adoption in this assessment .

Table 4. Current (2024) adoption level.

Unit: million pkm/yr 

25th percentile 512,900
Population-weighted mean 16,720,000
median (50th percentile) 5,106,000
75th percentile 15,080,000

We used the population-weighted mean calculated by Prieto-Curiel and Ospina (2024) as our authoritative estimate to carry forward to other calculations.

Based on data from Prieto-Curiel and Ospina (2024) and the UITP (2024) for 1,097 cities worldwide, the rate of adoption of public transit has not changed since 2010, with the median annual growth rate equal to 0 (Table 5). This was calculated using all of the cities in Prieto-Curiel and Ospina’s (2024) database for which modal share data exist.

Despite the lack of a global trend in public transit use, some cities, including Amsterdam, Edinburgh, and Leeds, report double-digit growth rates in the use of public transit.

Table 5. 2023–2024 adoption trend.

Unit: million pkm/yr

25th percentile -697,100
mean 71,490
median (50th percentile) 0.00
75th percentile 1,791,000

Public transit could theoretically replace all trips currently undertaken by fossil fuel–powered cars. This would amount to 75 trillion pkm on public transit annually, worldwide (Table 6). This would not be feasible to achieve in practice, as it would require construction of new public transit vehicles and infrastructure on an unfeasibly large scale, and massive changes to living patterns for many people. It would also be much more expensive than we calculated above, because such a change would require extending public transit coverage into areas where it would be highly uneconomic. Public transit is capable of providing a good transportation option in rural areas, but there is a limit to its benefits when population densities are low even by rural standards. Even in cities, this scenario would require a radical redesign of some neighborhoods to prioritize public transit. Such large public transit coverage would also inevitably shift other modes of transportation, such as pedestrian travel and cycling, leading to an even higher pkm total than that suggested by current adoption of fossil fuel–powered cars.

Table 6. Adoption ceiling.

Unit: million pkm/yr

median (50th percentile) 75,000,000

The achievable range of public transit adoption is 22.2–41.9 trillion pkm traveled by public transit in cities globally.

To estimate the upper bound of achievable adoption, we assumed that urban trips taken by fossil fuel–powered car (currently 51.4% of trips globally) can be shifted to public transit until public transit increases to 76.6% of trips (the current highest modal share of public transit in any city with a population of more than 1 million) or until car travel decreases to 12.0% of trips (the current lowest modal share of fossil fuel–powered cars in any city with a population of more than 1 million). This equals a shift of 25.2 trillion pkm from fossil fuel–powered car travel to public transit, which, added to present-day public transit trips (16.7 trillion trips/yr), equals 41.9 trillion pkm/yr (Table 7).

To set the lower bound, we performed the same calculation as above, but on a regional basis, adding up all the resultant modal shifts to get a global figure. For example, every northern European city might reach the public transit modal share of London (44.5% of trips), while every South Asian city might reach that of Mumbai (52.0% of trips). Having done that, we then added together the public transit adoption rates from all world regions, apart from three (Polynesia, Micronesia, and Melanesia) for which we did not find any modal share data. This corresponds to a shift of 5.5 trillion pkm/yr from cars to public transit, and a total achievable public transit adoption rate of 22.2 trillion pkm/yr.

Achieving both of these levels of adoption would require not only major investments in expanding public transit networks, but also major changes in how cities are planned so as to allow more areas to be effectively served by transit. These levels of adoption would also require overcoming cultural and political resistance to abandoning cars in favor of public modes. However, unlike the scenario discussed under Adoption Ceiling, these scenarios are feasible, since they are based on real achievements by cities around the world.

Table 7. Range of achievable adoption levels.

Unit: million pkm/yr

Current Adoption 16,720,000
Achievable – Low 21,980,000
Achievable – High 41,910,000
Adoption Ceiling 75,000,000

Impacts

If all public transit trips were taken by fossil fuel–powered cars instead of by public transit, they would result in an additional 0.97 Gt CO₂‑eq/yr of emissions (Table 8).

The global potential climate impact of enhancing public transit, if all car trips were shifted onto public transit systems, is 4.37 Gt. As discussed under Adoption Ceiling, this is an unrealistic scenario.

In a more realistic scenario, if every city in the world shifted car traffic onto public transit until it reached the public transit modal share of Hong Kong (i.e., the high estimate of achievable adoption), it would save 2.44 Gt CO₂‑eq/yr globally. Meanwhile, if every city shifts car trips to public transit until it reaches the car modal share of the region’s least car-dependent city (i.e., the low estimate of achievable adoption), it would save 1.28 Gt CO₂‑eq/yr.

Table 8. Climate impact at different levels of adoption.

Unit: Gt CO₂‑eq/yr, 100-yr basis

Current Adoption 0.97
Achievable – Low 1.28
Achievable – High 2.44
Adoption Ceiling 4.37

Air Quality

GHG emissions from transportation are often emitted with other harmful air pollutants. Consequently, reducing fuel consumption by replacing transport by fossil fuel–powered cars with public transit can lead to cleaner air. The scale of this benefit varies by location and is influenced by differences in emission levels between private and public transit travels and the relative demand substitutability between modes (Beaudoin et al., 2015). For U.S. cities, significant investment in public transit could cut pollution around 1.7% on average (Borck, 2019). The benefits are more significant in low- and middle-income countries, where fossil fuel–powered cars are more polluting due to lenient air quality regulations (Goel & Gupta, 2017; Guo & Chen, 2019).

Health Benefits

Improved air quality due to enhanced public transit has direct health benefits, such as lowering cardiovascular disease risk, and secondary health benefits, such as increased physical activity (Xiao et al., 2019), fewer traffic-related injuries, lower rates of cancer, and enhanced access to health-care facilities and nutritious food (Gouldson et al., 2018; Health Affairs, 2021).

Equality

Limited access to transportation restricts labor participation, particularly for women. Expanding public transit can foster gender equity by improving women’s access to employment opportunities. For example, in Peru expansion of public transit has led to improvements in women’s employment and earnings (Martinez et al., 2020). Similarly, in India, the extension of the light rail system in Delhi has increased women’s willingness to commute for work (Tayal & Mehta, 2021).

Public transit enhances community connectivity by providing accessible transportation options. Expanded mobility allows individuals to reach employment, health-care, education, and recreational sites with greater ease, heightening social inclusion. The social equity benefits of public transit are especially significant for low-income people in terms of time and cost savings and safety and health benefits (Serulle & Cirillo, 2016; Venter et al., 2017). 

Income and work

Investment in enhancing public transit can also generate substantial economic returns. The APTA estimated that each US$1 billion invested in transit can create 49,700 jobs and yield a five-to-one economic return (APTA, 2020). According to another study, shifting 50% of highway funds to mass transit systems in 20 U.S. metropolises could generate more than 1 million new transit jobs within five years (Swanstrom et al., 2010). 

Nature protection

An indirect benefit of enhanced public transit is its contribution to reducing resource consumption, such as the minerals used in manufacturing personal vehicles. Enhanced public transit can also improve land-use efficiency by curbing urban sprawl, which helps reduce pollution and limit biodiversity loss (Ortiz, 2002). 

Other

Public transit competes for passengers not just with cars, but also with other transportation modes – some of which have lower emissions on average. If an increase in public transit’s modal share comes at the expense of nonmotorized transportation (i.e., pedestrian travel or cycling), or electric bicycles, this will result in a net increase in emissions. Similarly, public transit could generate additional trips that would not have occurred if the public transit network those trips were taken on did not exist. Under this scenario, a net increase in emissions would occur; however, these new trips might bring additional social benefits that would outweigh these new emissions.

Low occupancy could also diminish the climate benefit of enhancing public transit. While it is certainly possible to build effective and efficient public transit networks in suburban and rural areas, there is a risk that such networks could have high per-pkm GHG emissions if they have low average occupancy (Mees, 2010). It is therefore important to efficiently plan public transit networks, ensure vehicles are right-sized and have efficient powertrains, and promote high levels of ridership even in rural areas to maximize the climate benefit of these kinds of networks.

Upscaling public transit networks – and, crucially, convincing more motorists to use them – is an enduring challenge that faces cultural resistance in some countries, issues with cost, and sometimes a lack of political will. Successfully enhancing public transit will require that these hurdles are overcome.

If expanded service on high-quality public transit systems replaced journeys from nonmotorized transportation or electric bicycles rather than from cars – or if expanded service on high-quality public transit systems generated journeys that would not have otherwise happened – this will have a net-negative climate impact, since public transit has higher per-pkm GHG emissions than electric bicycles or not traveling (International Transport Forum, 2020). 

There may be cases where public transit networks cannot be implemented efficiently enough to provide a meaningful benefit compared to fossil fuel–powered cars in terms of GHG emissions. This would occur in places where there are so few potential riders that most trips would have a very low occupancy. The result would be a much higher rate of emissions per pkm. However, effective public transit networks can be built in suburban and even rural areas (Börjesson et al., 2020; Mees, 2010).

Finally, expanding public transit networks has proven very difficult in recent years. Entrenched preferences for car travel, reluctance on the part of governments to invest heavily in new transit infrastructure, and local political challenges over land use, noise, gentrification, and similar issues are all obstacles to increased public transit use. Public transit expansion has faced stronger headwinds in recent years in particular, due to both the impact of the COVID-19 pandemic and competition from new (and mostly less sustainable) mobility services, such as app-based ride-hailing (Shaller, 2017).

Public transit vehicles are sometimes unsafe, particularly for vulnerable groups such as women (Loukaitou-Sideris, 2014). In some circumstances – although this remains controversial – new public transit routes can also lead to gentrification of neighborhoods, forcing people to move far away from city centers and use cars for travel (Padeiro et al., 2019). 

Expansion of public transit networks could also have negative consequences in areas directly adjacent to transit infrastructure. Diesel buses create air pollution (Lovasi et al., 2022), and public transit networks of all types can create noise pollution (Hemmat et al., 2023).

Reinforcing

For people living without cars, public transit provides a crucial service that is hard to replace for certain kinds of trips, such as trips over long distances, with small children, or carrying large objects. As a result, public transit plays a large role in making it more viable for people to live without owning a car (Brown, 2017). Research suggests that the key to a low-carbon mobility system is to reduce the need for people to own cars altogether (Van Acker & Witlox, 2010).

Public transit requires a lot less space than cars. Some of this space could be reallocated to ecosystem conservation through revegetation and other land-based methods of GHG sequestration (Rodriguez Mendez et al., 2024).

Competing 

Electric cars and public transit compete for pkm. Consequently, increased use of public transit could reduce kilometers traveled using electric cars. 

Consensus of effectiveness in reducing transportation emissions: High

Experts agree that public transit usually produces fewer GHG/pkm than fossil fuel–powered cars (Bloomberg New Energy Finance, 2018; Brunner et al., 2018; Ilie et al., 2014; International Transport Forum, 2020; Kennedy, 2002; Kuminek, 2013; Lim et al., 2021; Mahmoud et al., 2016; Rodrigues & Seixas, 2022; Sertsoz et al., 2013). There is also consensus on two points: First, shifting people from cars to public transit even under status-quo emissions levels will reduce transport emissions overall; second, opportunities exist to decarbonize the highest-emitting parts of public transit systems through electrification, especially buses (Bloomberg New Energy Finance, 2018).

According to the Intergovernmental Panel on Climate Change (IPCC, 2023), public transit can help decrease vehicle travel and lower GHG emissions by reducing both the number and length of trips made in fossil fuel–powered cars (medium confidence). Adjustments to public transportation operations – such as increasing bus stop density, reducing the distance between stops and households, improving trip duration and frequency, and lowering fares – can encourage a shift from fossil fuel–powered car use to public transit.

Bloomberg New Energy Finance (2018) provides a good overview of the state of electric buses – a technology crucial to reduce the public transit fleet’s fossil fuel consumption, and help transition these fleets entirely to electric power. It determined that electric buses have significantly lower operating costs and can be more cost-effective than conventional buses when considering total ownership costs.

Litman (2024) found that “High quality (relatively fast, convenient, comfortable, and integrated) transit can attract discretionary passengers who would otherwise drive, which reduces traffic problems including congestion, parking costs, accidents, and pollution emissions. This provides direct user benefits, since they would not change mode if they did not consider themselves better off overall.”

The results presented in this document summarize findings from 28 reviews and meta-analyses and 23 original studies reflecting current evidence from 32 countries, primarily the American Public Transit Association (APTA, 2020), Bloomberg New Energy Finance (2018), International Transport Forum (2020), and UITP (2024). We recognize this limited geographic scope creates bias, and hope this work inspires research and data sharing on this topic in underrepresented regions.

Take Action

Looking to get involved? Below are some key actions for this solution that can get you started, arranged according to different roles you may play in your professional or personal life.

These actions are meant to be starting points for involvement and are not intended to be prescriptive or necessarily suggest they are the most important or impactful actions to take. We encourage you to explore and get creative!

Lawmakers and Policymakers

  • Use public transit and create incentive programs for government employees to use public transit.
  • Improve and invest in local public transit infrastructure, increasing routes and frequency while improving onboard safety, especially for women.
  • Electrify public buses, vans, and other vehicles used in the public transit system.
  • Implement the recommendations of transit-oriented development, such as increasing residential and commercial density, placing development near stations, and ensuring stations are easily accessible.
  • Provide online information, ticketing, and payment services.
  • Implement regional or nationwide public transit ticketing systems.
  • Consider a wide range of policy options that include demand-side options, such as free fare or fare reductions, and that are informed by citizen-centered approaches.
  • Create dedicated coordinating bodies across government agencies, businesses, and the public to develop public transit.
  • Disincentivize car trips in areas serviced by public transit through reduced access, increases in parking fares, congestion charges, taxes, or other means.
  • Incorporate social signaling in public transit information and signage, such as smiley faces and “sustainable transport” labels.
  • Develop public transit awareness campaigns – starting from early childhood – focusing on internally motivating factors such as money saved, health benefits, reduced pollution, free time while traveling, and lifestyle sustainability.

Further information:

Practitioners

  • Use public transit and create incentive programs for government employees to utilize public transit.
  • Increase routes and frequency while also improving onboard safety, especially for women.
  • Electrify public buses, vans, and other vehicles used in the public transit system.
  • Incorporate social signaling in public transit information and signage, such as smiley faces and “sustainable transport” labels.
  • Provide online information, ticketing, and payment services
  • Implement regional or nationwide public transit ticketing systems.
  • Consider a wide range of policy options that include demand-side options, such as free fare or fare reductions, and that are informed through citizen-centered approaches.
  • Create dedicated coordinating bodies across government agencies, businesses, and the public to develop public transit.
  • Develop public transit awareness campaigns – starting from early childhood – focusing on internally motivating factors such as money saved, health benefits, reduced pollution, free time while traveling, and a sustainable lifestyle.

Further information:

Business Leaders

  • Use public transit and encourage employees to do so when feasible.
  • Encourage public transit use for company purposes.
  • Offer employees who agree to forego a free parking space the annualized cash value or cost of that parking space as a salary increase.
  • Incorporate company policies on public transit use into company sustainability and emission reduction initiatives and communicate how they support broader company goals.
  • Ensure your business is accessible via public transit and offer information on nearest access points both online and in person.
  • Offer employees pre-tax commuter benefits to include reimbursement for public transit expenses.
  • Create and distribute educational materials for employees on commuting best practices.
  • Partner with, support, and/or donate to infrastructure investments and public transit awareness campaigns.
  • Advocate for better public transit systems with city officials.

Further information:

Nonprofit Leaders

  • Use public transit and encourage staff to do so when feasible.
  • Offer staff pre-tax commuter benefits to include reimbursement for public transit expenses.
  • Offer employees who agree to forego a free parking space the annualized cash value or cost of that parking space as a salary increase.
  • Expand access to underserved communities by providing fare assistance through microgrants and/or public-private partnerships.
  • Create, support, or partner with existing public transit awareness campaigns that – starting from early childhood – focus on internally motivating factors such as money saved, health benefits, reduced pollution, free time while traveling, and a sustainable lifestyle.
  • Ensure your office is accessible via public transit and offer information – online and in person – on the nearest access points.
  • Advocate to policymakers for improved infrastructure and incentives for riders.
  • Advocate for infrastructure improvements and note specific locations where improvements can be made.
  • Encourage local businesses to create employee incentives.
  • Host or support community participation in local public transit infrastructure design.
  • Join public-private partnerships to encourage, improve, or operate public transit.

Further information:

Investors

  • Use public transit and encourage staff to do so when feasible.
  • Encourage public transit use for company purposes.
  • Invest in electric battery and component suppliers for public buses and vehicle fleets.
  • Deploy capital to efforts that improve public transit comfort, convenience, access, and safety.
  • Seek investment opportunities that reduce material and maintenance costs for public transit.

Further information:

Philanthropists and International Aid Agencies

  • Use public transit and encourage staff to do so when feasible.
  • Award grants to local organizations advocating for improved public transit and services.
  • Expand access to underserved communities by providing fare assistance through microgrants and/or public-private partnerships.
  • Improve and finance local infrastructure and public transit capacity.
  • Build local capacity for infrastructure design, maintenance, and construction.
  • Assist with local policy design or provide means for assessments, data collection, citizen participation, and other steps in the policymaking process.
  • Create, support, or partner with existing public transit awareness campaigns that – starting from early childhood – focus on internally motivating factors such as money saved, health benefits, reduced pollution, free time while traveling, and a sustainable lifestyle.

Further information:

Thought Leaders

  • Lead by example and use public transit regularly.
  • Create, support, or partner with existing public transit awareness campaigns that – starting from early childhood – focus on internally motivating factors such as money saved, health benefits, reduced pollution, free time while traveling, and a sustainable lifestyle.
  • Share detailed information on local public transit routes.
  • Assist with local policy design or provide means for assessments, data collection, citizen participation, and other steps in the policymaking process.
  • Advocate to policymakers for improved infrastructure, noting specific locations that need improvements and incentives for riders.

Further information:

Technologists and Researchers

  • Use public transit and encourage your colleagues to use public transit when feasible.
  • Improve electric batteries and electrification infrastructure for public buses and vehicles.
  • Develop models for policymakers to demonstrate the impact of public transit policies on pollutant emissions, health, and other socioeconomic variables.
  • Conduct randomized control trials and collect longitudinal data on the impacts of interventions to increase public transit usage.
  • Innovate better, faster, and cheaper public transit networks – focusing on infrastructure, operations, and public transit vehicles.

Further information:

Communities, Households, and Individuals

  • Use public transit and encourage your household and neighbors to use public transit when feasible.
  • Share your experiences with public transit, as well as tips and reasons for choosing this mode of transportation.
  • Advocate to local officials for infrastructure improvements and note specific locations where improvements can be made.
  • Advocate to employers and local businesses to provide incentives and start local initiatives.

Further information:

“Take Action” Sources

References

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American Public Transit Association. (2021). National Transit Database Tables. American Public Transportation Association. https://www.apta.com/research-technical-resources/transit-statistics/ntd-data-tables/

Beaudoin, J., Farzin, Y. H., & Lin Lawell, C.-Y. C. (2015). Public transit investment and sustainable transportation: A review of studies of transit’s impact on traffic congestion and air quality. Research in Transportation Economics, 52, 15–22. https://doi.org/10.1016/j.retrec.2015.10.004

Bloomberg New Energy Finance. (2018). Electric buses in cities: Driving towards cleaner air and lower CO₂ .

Börjesson, M., Fung, C. M., & Proost, S. (2020). How rural is too rural for transit? Optimal transit subsidies and supply in rural areas. Journal of Transport Geography, 88, 102859. https://doi.org/10.1016/j.jtrangeo.2020.102859

Borck, R. (2019). Public transport and urban pollution. Regional Science and Urban Economics, 77, 356–366. https://doi.org/10.1016/j.regsciurbeco.2019.06.005

Brown, A. E. (2017). Car-less or car-free? Socioeconomic and mobility differences among zero-car households. Transport Policy, 60, 152–159. https://doi.org/10.1016/j.tranpol.2017.09.016

Brunner, H., Hirz, M., Hirschberg, W., & Fallast, K. (2018). Evaluation of various means of transport for urban areas. Energy, Sustainability and Society, 8(1), 9. https://doi.org/10.1186/s13705-018-0149-0

Christensen, L., & Vázquez, N. S. (2013). Post-harmonised European National Travel Surveys. Proceedings from the Annual Transport Conference at Aalborg University, 20(1), Article 1. https://doi.org/10.5278/ojs.td.v1i1.5701

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Ecke, L. (2023, December 19). German Mobility Panel—Startseite (KIT) [Text]. Lisa Ecke. https://mobilitaetspanel.ifv.kit.edu/english/

Federal Highway Administration. (2022). Summary of Travel Trends: 2022 National Household Travel Survey. US Department of Transportation. https://nhts.ornl.gov/assets/2022/pub/2022_NHTS_Summary_Travel_Trends.pdf

Goel, D., & Gupta, S. (2017). The Effect of Metro Expansions on Air Pollution in Delhi. The World Bank Economic Review, 31(1), 271–294. https://doi.org/10.1093/wber/lhv056

Gouldson, A., Sudmant, A., Khreis, H., & Papargyropoulou, E. (2018). The Economic and Social Benefits of Low-Carbon Cities: A Systematic Review of the Evidence. https://urbantransitions.global/en/publication/the-economic-and-social-benefits-of-low-carbon-cities-a-systematic-review-of-the-evidence/

Guo, S., & Chen, L. (2019). Can urban rail transit systems alleviate air pollution? Empirical evidence from Beijing. Growth and Change, 50(1), 130–144. https://doi.org/10.1111/grow.12266

Health Affairs. (2021). Public Transportation in the U.S. RWJF. https://www.rwjf.org/content/rwjf-web/us/en/insights/our-research/2021/07/public-transportation-in-the-us-a-driver-of-health-and-equity.html

Hemmat, W., Hesam, A. M., & Atifnigar, H. (2023). Exploring noise pollution, causes, effects, and mitigation strategies: A review paper. European Journal of Theoretical and Applied Sciences, 1(5), Article 5. https://doi.org/10.59324/ejtas.2023.1(5).86

Ilie, N., Iurie, N., Alexandr, M., & Vitalie, E. (2014). Rehabilitation of the tram DC traction with modern power converters. 2014 International Conference and Exposition on Electrical and Power Engineering (EPE), 704–709. https://doi.org/10.1109/ICEPE.2014.6970000

International Transport Forum. (2020). Good to Go? Assessing the Environmental Performance of New Mobility (Corporate Partnership Board). OECD. https://www.itf-oecd.org/sites/default/files/docs/environmental-performance-new-mobility.pdf

IPCC. (2023). Renewable Energy Sources and Climate Change Mitigation—IPCC. https://www.ipcc.ch/report/renewable-energy-sources-and-climate-change-mitigation/

Kennedy, C. A. (2002). A comparison of the sustainability of public and private transportation systems: Study of the Greater Toronto Area. Transportation, 29(4), 459–493. https://doi.org/10.1023/A:1016302913909

Kuminek, T. (2013). Energy Consumption in Tram Transport. Logistics and Transport. https://www.semanticscholar.org/paper/Energy-Consumption-in-Tram-Transport-Kuminek/2aa2d97130a8e51ea7f64913c2065e8437126774

Lim, L. K., Muis, Z. A., Hashim, H., Ho, W. S., & Idris, M. N. M. (2021). Potential of Electric Bus as a Carbon Mitigation Strategies and Energy Modelling: A Review. Chemical Engineering Transactions, 89, 529–534. https://doi.org/10.3303/CET2189089

Lovasi, G. S., Treat, C. A., Fry, D., Shah, I., Clougherty, J. E., Berberian, A., Perera, F. P., & Kioumourtzoglou, M.-A. (2023). Clean fleets, different streets: Evaluating the effect of New York City’s clean bus program on changes to estimated ambient air pollution. Journal of Exposure Science & Environmental Epidemiology, 33(3), 332–338. https://doi.org/10.1038/s41370-022-00454-5

Litman, T. (2024). Evaluating Public Transit Benefits and Costs.

Loukaitou-Sideris, A. (2014). Fear and safety in transit environments from the women’s perspective. Security Journal, 27(2), 242–256. https://doi.org/10.1057/sj.2014.9

Mahmoud, M., Garnett, R., Ferguson, M., & Kanaroglou, P. (2016). Electric buses: A review of alternative powertrains. Renewable and Sustainable Energy Reviews, 62, 673–684. https://doi.org/10.1016/j.rser.2016.05.019

Martinez, D., Mitnik, O., Salgado, E., Yãnez-Pagans, P., & Scholl, L. (2020). Connecting to Economic Opportunity: The Role of Public Transport in Promoting Women’s Employment in Lima | Journal of Economics, Race, and Policy. https://link.springer.com/article/10.1007/s41996-019-00039-9

Mees, P. (2010). Transport for Suburbia: Beyond the Automobile Age. Earthscan.

Norton, P. D. (2011). Fighting Traffic: The Dawn of the Motor Age in the American City. MIT Press.

Ortiz, F. (2002). Biodiversity, the City, and Sprawl. Boston University Law Review, 82(1), 145–194.

Padeiro, M., Louro, A., & da Costa, N. M. (2019). Transit-oriented development and gentrification: A systematic review. Transport Reviews, 39(6), 733–754. https://doi.org/10.1080/01441647.2019.1649316

Prieto-Curiel, R., & Ospina, J. P. (2024). The ABC of mobility. Environment International, 185, 108541. https://doi.org/10.1016/j.envint.2024.108541

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Rodrigues, A. L. P., & Seixas, Sonia. R. C. (2022). Battery-electric buses and their implementation barriers: Analysis and prospects for sustainability. Sustainable Energy Technologies and Assessments, 51, 101896. https://doi.org/10.1016/j.seta.2021.101896

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Serulle, N. U., & Cirillo, C. (2016). Transportation needs of low income population: A policy analysis for the Washington D.C. metropolitan region. Public Transport, 8(1), 103–123. https://doi.org/10.1007/s12469-015-0119-2

Schaller, B. (2017). Unsustainable? The Growth of App-Based Ride Services and Traffic, Travel and the Future of New York City. Schaller Consulting.

Schrag, Z. M. (2000). “The Bus Is Young and Honest”: Transportation Politics, Technical Choice, and the Motorization of Manhattan Surface Transit, 1919-1936. Technology and Culture, 41(1), 51–79.

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Xiao, C., Goryakin, Y., & Cecchini, M. (2019). Physical Activity Levels and New Public Transit: A Systematic Review and Meta-analysis. American Journal of Preventive Medicine, 56(3), 464–473. https://doi.org/10.1016/j.amepre.2018.10.022

Credits

Lead Fellow

  • Cameron Roberts

Contributors

  • Ruthie Burrows

  • James Gerber

  • Yusuf Jameel 

  • Daniel Jasper

  • Heather Jones

  • Alex Sweeney

Internal Reviewers

  • Aiyana Bodi

  • Hannah Henkin

  • Ted Otte

  • Amanda Smith

  • Tina Swanson

Methods and Supporting Data

Methods and Supporting Data

Comments / Questions

  • Greenhouse gas quantity expressed relative to CO₂ with the same warming impact over 100 years, calculated by multiplying emissions by the 100-yr GWP for the emitted gases.

  • Greenhouse gas quantity expressed relative to CO₂ with the same warming impact over 20 years, calculated by multiplying emissions by the 20-yr GWP for the emitted gases.

  • Reducing greenhouse gas concentrations in the atmosphere by preventing or reducing emissions.

  • The process of increasing the acidity of water or soil due to increased levels of certain air pollutants.

  • Benefits of climate solutions that extend beyond their ability to reduce emissions or store carbon (e.g., benefits to public health, water quality, biodiversity, advancing human rights).

  • The extent to which emissions reduction or carbon removal is above and beyond what would have occurred without implementing a particular action or solution.

  • An upper limit on solution adoption based on physical or technical constraints, not including economic or policy barriers. This level is unlikely to be reached and will not be exceeded.

  • The quantity and metric to measure implementation for a particular solution that is used as the reference unit for calculations within that solution.

  • Farming practices that work to create socially and ecologically sustainable food production.

  • Addition of trees and shrubs to crop or animal farming systems.

  • Spread out the cost of an asset over its useful lifetime.

  • A crop that live one year or less from planting to harvest; also called annual.

  • black carbon

  • Made from material of biological origin, such as plants, animals, or other organisms.

  • A renewable energy source generated from organic matter from plants and/or algae.

  • An energy source composed primarily of methane and CO₂ that is produced by microorganisms when organic matter decomposes in the absence of oxygen.

  • Carbon stored in biological matter, including soil, plants, fungi, and plant products (e.g., wood, paper, biofuels). This carbon is sequestered from the atmosphere but can be released through decomposition or burning.

  • Living or dead renewable matter from plants or animals, not including organic material transformed into fossil fuels. Peat, in early decay stages, is partially renewable biomass.

  • A type of carbon sequestration that captures carbon from CO₂ via photosynthesis and stores it in soils, sediments, and biomass, distinct from sequestration through chemical or industrial pathways.

  • A climate pollutant, also called soot, produced from incomplete combustion of organic matter, either naturally (wildfires) or from human activities (biomass or fossil fuel burning).

  • High-latitude (>50°N or >50°S) climate regions characterized by short growing seasons and cold temperatures.

  • The components of a building that physically separate the indoors from the outdoor environment.

  • Businesses involved in the sale and/or distribution of solution-related equipment and technology, and businesses that want to support adoption of the solution.

  • A chemical reaction involving heating a solid to a high temperature: to make cement clinker, limestone is calcined into lime in a process that requires high heat and produces CO₂.

  • A four-wheeled passenger vehicle.

  • Technologies that collect CO₂ before it enters the atmosphere, preventing emissions at their source. Collected CO₂ can be used onsite or in new products, or stored long term to prevent release.

  • A greenhouse gas that is naturally found in the atmosphere. Its atmospheric concentration has been increasing due to human activities, leading to warming and climate impacts.

  • Total GHG emissions resulting from a particular action, material, technology, or sector.

  • Amount of GHG emissions released per activity or unit of production. 

  • A marketplace where carbon credits are purchased and sold. One carbon credit represents activities that avoid, reduce, or remove one metric ton of GHG emissions.

  • A colorless, odorless gas released during the incomplete combustion of fuels containing carbon. Carbon monoxide can harm health and be fatal at high concentrations.

  • Activities or technologies that pull CO₂ out of the atmosphere, including enhancing natural carbon sinks and deploying engineered sinks.

  • Long-term storage of carbon in soils, sediment, biomass, oceans, and geologic formations after removal of CO₂ from the atmosphere or CO₂ capture from industrial and power generation processes.

  • carbon capture and storage

  • carbon capture, utilization, and storage

  • A binding ingredient in concrete responsible for most of concrete’s life-cycle emissions. Cement is made primarily of clinker mixed with other mineral components.

  • methane

  • Gases or particles that have a planet-warming effect when released to the atmosphere. Some climate pollutants also cause other forms of environmental damage.

  • A binding ingredient in cement responsible for most of the life-cycle emissions from cement and concrete production.

  • carbon monoxide

  • Neighbors, volunteer organizations, hobbyists and interest groups, online communities, early adopters, individuals sharing a home, and private citizens seeking to support the solution.

  • A solution that potentially lowers the benefit of another solution through reduced effectiveness, higher costs, reduced or delayed adoption, or diminished global climate impact.

  • A farming system that combines reduced tillage, cover crops, and crop rotations.

  • carbon dioxide

  • A  measure standardizing the warming effects of greenhouse gases relative to CO₂. CO₂-eq is calculated as quantity (metric tons) of a particular gas multiplied by its GWP.

  • carbon dioxide equivalent

  • The process of cutting greenhouse gas emissions (primarily CO₂) from a particular sector or activity.

  • A solution that works slower than gradual solutions and is expected to take longer to reach its full potential.

  • Microbial conversion of nitrate into inert nitrogen gas under low-oxygen conditions, which produces the greenhouse gas nitrous oxide as an intermediate compound.

  • Greenhouse gas emissions produced as a direct result of the use of a technology or practice.

  • Ability of a solution to reduce emissions or remove carbon, expressed in CO₂-eq per installed adoption unit. Effectiveness is quantified per year when the adoption unit is cumulative over time.

  • Greenhouse gas emissions accrued over the lifetime of a material or product, including as it is produced, transported, used, and disposed of.

  • Solutions that work faster than gradual solutions, front-loading their impact in the near term.

  • Methane produced by microbes in the digestive tracts of ruminant livestock, such as cattle, sheep and goats.

  • environmental, social, and governance

  • exchange-traded fund

  • A process triggered by an overabundance of nutrients in water, particularly nitrogen and phosphorus, that stimulates excessive plant and algae growth and can harm aquatic organisms.

  • The scientific literature that supports our assessment of a solution's effectiveness.

  • A group of human-made molecules that contain fluorine atoms. They are potent greenhouse gases with GWPs that can be hundreds to thousands times higher than CO₂.

  • food loss and waste

  • Food discarded during pre-consumer supply chain stages, including production, harvest, and processing.

  • Food discarded at the retail and consumer stages of the supply chain.

  • Combustible materials found in Earth's crust that can be burned for energy, including oil, natural gas, and coal. They are formed from decayed organisms through prehistoric geological processes.

  • greenhouse gas

  • gigajoule or billion joules

  • The glass layers or panes in a window.

  • A measure of how effectively a gas traps heat in the atmosphere relative to CO₂. GWP converts greenhouse gases into CO₂-eq emissions based on their 20- or 100-year impacts.

  • A solution that has a steady impact so that the cumulative effect over time builds as a straight line. Most climate solutions fall into this category.

  • A gas that traps heat in the atmosphere, contributing to climate change.

  • metric gigatons or billion metric tons

  • global warming potential

  • hectare

  • household air pollution

  • Number of years a person is expected to live without disability or other limitations that restrict basic functioning and activity.

  • A unit of land area comprising 10,000 square meters, roughly equal to 2.5 acres.

  • hydrofluorocarbon

  • hydrofluoroolefin

  • Particles and gases released from use of polluting fuels and technologies such as biomass cookstoves that cause poor air quality in and around the home.

  • Organic compounds that contain hydrogen and carbon.

  • Human-made F-gases that contain hydrogen, fluorine, and carbon. They typically have short atmospheric lifetimes and GWPs hundreds or thousands times higher than CO₂. 

  • Human-made F-gases that contain hydrogen, fluorine, and carbon, with at least one double bond. They have low GWPs and can be climate-friendly alternatives to HFC refrigerants.

  • internal combustion engine

  • Greenhouse gas emissions produced as a result of a technology or practice but not directly from its use.

  • Device used to power vehicles by the intake, compression, combustion, and exhaust of fuel that drives moving parts.

  • The annual discount rate that balances net cash flows for a project over time. Also called IRR, internal rate of return is used to estimate profitability of potential investments.

  • Individuals or institutions willing to lend money in search of a return on their investment.

  • internal rate of return

  • A measure of energy

  • International agreement adopted in 2016 to phase down the use of high-GWP HFC F-gases over the time frame 2019–2047.

  • A measure of energy equivalent to the energy delivered by 1,000 watts of power over one hour.

  • kiloton or one thousand metric tons

  • kilowatt-hour

  • A land-holding system, e.g. ownership, leasing, or renting. Secure land tenure means farmers or other land users will maintain access to and use of the land in future years.

  • Gases, mainly methane and CO₂, created by the decomposition of organic matter in the absence of oxygen.

  • leak detection and repair

  • Regular monitoring for fugitive methane leaks throughout oil and gas, coal, and landfill sector infrastructure and the modification or replacement of leaking equipment.

  • Relocation of emissions-causing activities outside of a mitigation project area rather than a true reduction in emissions.

  • The rate at which solution costs decrease as adoption increases, based on production efficiencies, technological improvements, or other factors.

  • Percent decrease in costs per doubling of adoption.

  • landfill gas

  • Greenhouse gas emissions from the sourcing, production, use, and disposal of a technology or practice.

  • low- and middle-income countries

  • liquefied petroleum gas

  • A measure of the amount of light produced by a light source per energy input.

  • square meter kelvins per watt (a measure of thermal resistance, also called R-value)

  • marginal abatement cost curve

  • Livestock grazing practices that strategically manage livestock density, grazing intensity, and timing. Also called improved grazing, these practices have environmental, soil health, and climate benefits, including enhanced soil carbon sequestration.

  • A tool to measure and compare the financial cost and abatement benefit of individual actions based on the initial and operating costs, revenue, and emission reduction potential.

  • A greenhouse gas with a short lifetime and high GWP that can be produced through a variety of mechanisms including the breakdown of organic matter.

  • A measure of mass equivalent to 1,000 kilograms (~2,200 lbs).

  • million hectares

  • Soils mostly composed of inorganic materials formed through the breakdown of rocks. Most soils are mineral soils, and they generally have less than 20% organic matter by weight.

  • A localized electricity system that independently generates and distributes power. Typically serving limited geographic areas, mini-grids can operate in isolation or interconnected with the main grid.

  • Reducing the concentration of greenhouse gases in the atmosphere by cutting emissions or removing CO₂.

  • Percent of trips made by different passenger and freight transportation modes.

  • megaton or million metric tons

  • A commitment from a country to reduce national emissions and/or sequester carbon in alignment with global climate goals under the Paris Agreement, including plans for adapting to climate impacts.

  • A gaseous form of hydrocarbons consisting mainly of methane.

  • Chemicals found in nature that are used for cooling and heating, such as CO₂, ammonia, and some hydrocarbons. They have low GWPs and are ozone friendly, making them climate-friendly refrigerants.

  • Microbial conversion of ammonia or ammonium to nitrite and then to nitrate under aerobic conditions.

  • A group of air pollutant molecules composed of nitrogen and oxygen, including NO and NO₂.

  • A greenhouse gas produced during fossil fuel combustion and agricultural and industrial processes. N₂O is hundreds of times more potent than CO₂ at trapping atmospheric heat, and it depletes stratospheric ozone.

  • Social welfare organizations, civic leagues, social clubs, labor organizations, business associations, and other not-for-profit organizations.

  • A material or energy source that relies on resources that are finite or not naturally replenished at the rate of consumption, including fossil fuels like coal, oil, and natural gas.

  • nitrogen oxides

  • nitrous oxide

  • The process of increasing the acidity of seawater, primarily caused by absorption of CO₂ from the atmosphere.

  • An agreement between a seller who will produce future goods and a purchaser who commits to buying them, often used as project financing for producers prior to manufacturing.

  • Productive use of wet or rewetted peatlands that does not disturb the peat layer, such as for hunting, gathering, and growing wetland-adapted crops for food, fiber, and energy.

  • A measure of transporting one passenger over a distance of one kilometer.

  • The longevity of any greenhouse gas emission reductions or removals. Solution impacts are considered permanent if the risk of reversing the positive climate impacts is low within 100 years.

  • A mixture of hydrocarbons, small amounts of other organic compounds, and trace amounts of metals used to produce products such as fuels or plastics.

  • Private, national, or multilateral organizations dedicated to providing aid through in-kind or financial donations.

  • An atmospheric reaction among sunlight, VOCs, and nitrogen oxide that leads to ground-level ozone formation. Ground-level ozone, a component of smog, harms human health and the environment.

  • passenger kilometer

  • particulate matter

  • Particulate matter 2.5 micrometers or less in diameter that can harm human health when inhaled.

  • Elected officials and their staff, bureaucrats, civil servants, regulators, attorneys, and government affairs professionals.

  • System in a vehicle that generates power and delivers it to the wheels. It typically includes an engine and/or motor, transmission, driveshaft, and differential.

  • People who most directly interface with a solution and/or determine whether the solution is used and/or available. 

  • The process of converting inorganic matter, including carbon dioxide, into organic matter (biomass), primarily by photosynthetic organisms such as plants and algae.

  • Defined by the International Union for the Conservation of Nature as: "A clearly defined geographical space, recognised, dedicated and managed, through legal or other effective means, to achieve the long-term conservation of nature with associated ecosystem services and cultural values". References to PAs here also include other effective area-based conservation measures defined by the IUCN. 

  • Very large or small numbers are formatted in scientific notation. A positive exponent multiplies the number by powers of ten; a negative exponent divides the number by powers of ten.

  • Small-scale family farmers and other food producers, often with limited resources, usually in the tropics. The average size of a smallholder farm is two hectares (about five acres).

  • soil organic carbon

  • Carbon stored in soils, including both organic (from decomposing plants and microbes) and inorganic (from carbonate-containing minerals).

  • Carbon stored in soils in organic forms (from decomposing plants and microbes). Soil organic carbon makes up roughly half of soil organic matter by weight.

  • Biologically derived matter in soils, including living, dead, and decayed plant and microbial tissues. Soil organic matter is roughly half carbon on a dry-weight basis.

  • soil organic matter

  • sulfur oxides

  • sulfur dioxide

  • The rate at which a climate solution physically affects the atmosphere after being deployed. At Project Drawdown, we use three categories: emergency brake (fastest impact), gradual, or delayed (slowest impact).

  • Climate regions between latitudes 23.4° to 35° above and below the equator characterized by warm summers and mild winters.

  • A polluting gas produced primarily from burning fossil fuels and industrial processes that directly harms the environment and human health.

  • A group of gases containing sulfur and oxygen that predominantly come from burning fossil fuels. They contribute to air pollution, acid rain, and respiratory health issues.

  • Processes, people, and resources involved in producing and delivering a product from supplier to end customer, including material acquisition.

  • metric tons

  • Technology developers, including founders, designers, inventors, R&D staff, and creators seeking to overcome technical or practical challenges.

  • Climate regions between 35° to 50° above and below the equator characterized by moderate mean annual temperatures and distinct seasons, with warm summers and cold winters.

  • A measure of how well a material prevents heat flow, often called R-value or RSI-value for insulation. A higher R-value means better thermal performance.

  • Individuals with an established audience for their work, including public figures, experts, journalists, and educators.

  • Low-latitude (23.4°S to 23.4°N) climate regions near the Equator characterized by year-round high temperatures and distinct wet and dry seasons.

  • United Nations

  • Self-propelled machine for transporting passengers or freight on roads.

  • A measure of one vehicle traveling a distance of one kilometer.

  • vehicle kilometer

  • volatile organic compound

  • Gases made of organic, carbon-based molecules that are readily released into the air from other solid or liquid materials. Some VOCs are greenhouse gases or can harm human health.

  • watt

  • A measure of power equal to one joule per second.

  • A subset of forest ecosystems that may have sparser canopy cover,  smaller-stature trees, and/or trees characterized by basal branching rather than a single main stem.

  • year

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