Cities are an often overlooked as being a major contributor to climate change. Yet their diversity has made it hard to assess how far they can cut their emissions.
Moreover, efforts to tackle their emissions, such as those from urban transport, often come with trade-offs in terms of costs and co-benefits from cleaner air or health.
In our new study, published in Nature Sustainability, we conducted a comprehensive analysis of transport policies in 120 cities, spanning five continents.
We found that the cities could cut carbon dioxide (CO2) emissions by a combined 22%, without cutting residents’ quality of life measured via an aggregate, monetised metric.
In individual cities, we found that a combination of policies, such as fuel taxes, public transport improvement and urban planning, could reduce transport CO2 by up to 31%.
Cities in climate policies
Cities bear a major responsibility for climate change, as they account for 70% of global emissions.
They can also play a key role in implementing climate action at the local level. Many cities have set ambitious climate goals, with members of the Global Covenant of Mayors city network, for instance, aiming to reduce their emissions by 66% by 2050.
Urban transport, in particular, is an important sector, representing 8% of global emissions alone. This also tends to be an area where cities have the ability to act.
Yet, quantifying the aggregated potential of cities to mitigate transport emissions has proven challenging. Indeed, the impacts of city-level policies depend on the unique characteristics of each city, such as urban spatial organisation and existing infrastructure.
On the other hand, detailed city models applied to case-study cities are difficult to generalise, since the scientific literature is fragmented and biased toward larger cities and developed countries.
Exploring strategies
Using an urban simulation model, we estimated the aggregate potential for 120 cities on five different continents to reduce their urban transport emissions.
We also considered the impact of such climate actions on inhabitants’ quality of life, through housing and transport prices, local taxes and health co-benefits related to transportation. This included those related to cleaner air, reduced noise, reduced traffic accidents and increased physical activity due to active transportation modes.
The 120 cities are home to 525 million inhabitants, or about 20% of the total global population that lives in cities larger than 300,000 people.
To calibrate the model on each city, we relied on spatially explicit socio-economic data that we collected through web-scraping of local websites, as well as data on local transportation systems that was provided by Open Street Map, Google Maps and Baidu Maps.
Our study explores four main types of complementary strategies that could help reduce transport-related emissions in cities: taxation of polluting vehicles; incentives to use vehicles that consume less fossil fuel; investment in public transport; and urban planning policies that restrict urban sprawl.
For each of these strategies, we examined examples of public policies that can be implemented at local level. For example, to improve public transport, one possibility is to set up a bus rapid transit system on dedicated lanes.
Other examples include that urban planning can involve limiting new construction away from public transport stations. Polluting vehicles can be taxed by raising fuel prices or by introducing local congestion charges.
Finally, the use of more efficient vehicles, such as electric cars, can be encouraged by a combination of subsidies and bans on the most polluting vehicles in urban centres.
Comparing local policies
Our findings suggest that a combination of these policies could reduce overall transportation GHG emissions by up to 31% in 15 years, across the 120 cities studied.
Policies implemented individually could mitigate emissions by 4% to 12%, depending on the policy considered.
These results are in line with the scientific and “grey” literature on the topic, which has shown that urban transport emissions could be mitigated by 20% to 25% through a combination of urban planning and technological solutions.
The impact of a given policy varies according to the city in which it is implemented. For example, in the majority of South American cities studied, the introduction of new public transport lines would be particularly beneficial, our results suggest.
Given the relatively high population density and underdeveloped public transport systems in these cities, our simulations indicate that the implementation of new public transport lines could potentially reduce emissions by up to 21% and 26% in Brazilian cities such as Goiânia and Belém, respectively.
In Europe, taxing fuel prices appears to be more effective, primarily due to the generally well-developed public transport networks available in European cities. For instance, a fuel tax would lead to a 7.5% reduction in transport-related emissions in Barcelona, we found, while the impact would be only 0.6% in Atlanta (USA), where alternatives to private cars are less readily accessible.
Simultaneous implementation of multiple policies can have a particularly significant impact, our results show. Combining taxes on polluting vehicles with public transport development could result in substantial emission reductions, as could promoting public transport alongside measures to control urban sprawl and increase population density near railway stations.
For instance, in Lille (France), implementing policies to control urban sprawl, tax polluting vehicles and develop public transport concurrently would potentially reduce transport-related emissions by almost 24%, compared to reductions of 9%, 4%, and 7%, respectively, if each policy was implemented individually.
An example of the differing impact of public transport development on CO2 emissions after 15 years can be seen in the map below, with purple circles showing cities that could achieve a more than 10% saving, blue showing 5-10%, green 1-5% and yellow less than 1%.
Variation in transport-related CO2 emissions
Inhabitants’ quality of life
Assuming that these policies are fully financed locally by a tax, our study also estimates their impact on the material conditions of residents and on their health.
We analysed the impacts of urban transport emission reduction policies on a range of factors linked to quality of life. In terms of income, for example, building locally financed public transit lines increases local taxes, while taxing polluting vehicles reduces them. We also looked at transportation costs, average housing prices, air quality, noise pollution, road accidents and the health benefits associated with “active” mobility (walking or cycling instead of driving).
Depending on the city, these impacts can be positive or negative overall. A fuel tax or the opening of new public transport lines would be expected to improve air quality, reduce noise pollution and the number of road accidents. More efficient vehicles improve air quality and reduce the household transport budget – even if their impact in terms of road accidents remains unchanged.
On the other hand, urban planning that limits urban sprawl can contribute to higher housing prices and the introduction of public transport lines can sometimes prove extremely costly.
To facilitate comparison, we expressed these variations in monetary terms, creating a composite indicator of welfare that encompasses all dimensions of residents’ quality of life mentioned earlier, as shown in the figure below.
Our study reveals that, in all cities examined, there are policy combinations that can effectively reduce emissions while improving overall well-being.
Most importantly, if, in each of the 120 cities of our sample, instead of applying all the policies that we considered, we choose to apply only policy combinations which do not reduce our monetised measure of welfare, we find that we can reach in total a 22% reduction in urban transportation GHG emissions in 15 years.
This means that most of the emission reductions that we simulate in our study can be reached without affecting residents’ quality of life in any of the cities that we considered.
While 22% is not sufficient, in itself, to reach carbon neutrality, we only analysed four simple and generic policies. Specifically designed and optimised policy portfolios for each city could reach larger emission reductions.
Climate governance and research
As numerous protests worldwide have demonstrated, public policies aimed at reducing emissions must also positively impact residents’ quality of life to gain acceptance.
In all the cities that we studied, we found that it is possible to combine reduction of GHG emissions and the enhancement of quality of life, through well-adapted policy choices.
In order to achieve this, the set of policies needs to be tailored to each city’s specificities, however, with strategies which cannot necessarily be directly transposed from one city to another. In our city sample, the emission reduction that can be reached – even with a generic policy toolkit – is also significant.
Cities are frequently overlooked in international climate discussions, in part because of the diversity of their characteristics. This diversity does make it difficult to assess the potential of urban policies to contribute to global climate goals.
With the recent increases in local urban data becoming available, however, our study shows that it is now possible to explicitly model and assess the consequences of climate strategies over a wide range of cities.
Important research gaps still remain. We could not, for instance, include any African cities in our sample due to challenges accessing reliable and comparable data. If current trends in data availability continue, this and other issues could be solved over the coming years.
The post Guest post: How 120 of the world’s major cities could cut transport CO2 by 22% appeared first on Carbon Brief.
Guest post: How 120 of the world’s major cities could cut transport CO2 by 22%
Seven Pacific island nations say they will demand heftier levies on global shipping emissions if opponents of a green deal for the industry succeed in reopening negotiations on the stalled accord.
The United States and Saudi Arabia persuaded countries not to grant final approval to the International Maritime Organization’s Net-Zero Framework (NZF) in October and they are now leading a drive for changes to the deal.
In a joint submission seen by Climate Home News, the seven climate-vulnerable Pacific countries said the framework was already a “fragile compromise”, and vowed to push for a universal levy on all ship emissions, as well as higher fees . The deal currently stipulates that fees will be charged when a vessel’s emissions exceed a certain level.
“For many countries, the NZF represents the absolute limit of what they can accept,” said the unpublished submission by Fiji, Kiribati, Vanuatu, Nauru, Palau, Tuvalu and the Solomon Islands.
The countries said a universal levy and higher charges on shipping would raise more funds to enable a “just and equitable transition leaving no country behind”. They added, however, that “despite its many shortcomings”, the framework should be adopted later this year.
US allies want exemption for ‘transition fuels’
The previous attempt to adopt the framework failed after governments narrowly voted to postpone it by a year. Ahead of the vote, the US threatened governments and their officials with sanctions, tariffs and visa restrictions – and President Donald Trump called the framework a “Green New Scam Tax on Shipping”.
Since then, Liberia – an African nation with a major low-tax shipping registry headquartered in the US state of Virginia – has proposed a new measure under which, rather than staying fixed under the NZF, ships’ emissions intensity targets change depending on “demonstrated uptake” of both “low-carbon and zero-carbon fuels”.
The proposal places stringent conditions on what fuels are taken into consideration when setting these targets, stressing that the low- and zero-carbon fuels should be “scalable”, not cost more than 15% more than standard marine fuels and should be available at “sufficient ports worldwide”.
This proposal would not “penalise transitional fuels” like natural gas and biofuels, they said. In the last decade, the US has built a host of large liquefied natural gas (LNG) export terminals, which the Trump administration is lobbying other countries to purchase from.
The draft motion, seen by Climate Home News, was co-sponsored by US ally Argentina and also by Panama, a shipping hub whose canal the US has threatened to annex. Both countries voted with the US to postpone the last vote on adopting the framework.
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IMO head: Shipping decarbonisation “has started” despite green deal delay
Arsenio Dominguez said the shipping industry’s net zero emissions goal is not in dispute although countries have stalled on adopting a framework to reach it -
US-led alliance wins a year’s delay in adoption of green shipping deal
The IMO’s Net-Zero Framework will be up for approval again in October 2026, after the US and Saudi Arabia persuaded countries not to vote on it as planned
The IMO’s Panamanian head Arsenio Dominguez told reporters in January that changes to the framework were now possible.
“It is clear from what happened last year that we need to look into the concerns that have been expressed [and] … make sure that they are somehow addressed within the framework,” he said.
Patchwork of levies
While the European Union pushed firmly for the framework’s adoption, two of its shipping-reliant member states – Greece and Cyprus – abstained in October’s vote.
After a meeting between the Greek shipping minister and Saudi Arabia’s energy minister in January, Greece said a “common position” united Greece, Saudi Arabia and the US on the framework.
If the NZF or a similar instrument is not adopted, the IMO has warned that there will be a patchwork of differing regional levies on pollution – like the EU’s emissions trading system for ships visiting its ports – which will be complicated and expensive to comply with.
This would mean that only countries with their own levies and with lots of ships visiting their ports would raise funds, making it harder for other nations to fund green investments in their ports, seafarers and shipping companies. In contrast, under the NZF, revenues would be disbursed by the IMO to all nations based on set criteria.
Anais Rios, shipping policy officer from green campaign group Seas At Risk, told Climate Home News the proposal by the Pacific nations for a levy on all shipping emissions – not just those above a certain threshold – was “the most credible way to meet the IMO’s climate goals”.
“With geopolitics reframing climate policy, asking the IMO to reopen the discussion on the universal levy is the only way to decarbonise shipping whilst bringing revenue to manage impacts fairly,” Rios said.
“It is […] far stronger than the Net-Zero Framework that is currently on offer.”
The post Pacific nations want higher emissions charges if shipping talks reopen appeared first on Climate Home News.
Pacific nations want higher emissions charges if shipping talks reopen
Doubts over whether governments will maintain ambitious targets on boosting the use of sustainable aviation fuel (SAF) are a threat to the industry’s growth and play into the hands of fossil fuel companies, investors warned this week.
Several executives from airlines and oil firms have forecast recently that SAF requirements in the European Union, United Kingdom and elsewhere will be eased or scrapped altogether, potentially upending the aviation industry’s main policy to shrink air travel’s growing carbon footprint.
Such speculation poses a “fundamental threat” to the SAF industry, which mainly produces an alternative to traditional kerosene jet fuel using organic feedstocks such as used cooking oil (UCO), Thomas Engelmann, head of energy transition at German investment manager KGAL, told the Sustainable Aviation Fuel Investor conference in London.
He said fossil fuel firms would be the only winners from questions about compulsory SAF blending requirements.
The EU and the UK introduced the world’s first SAF mandates in January 2025, requiring fuel suppliers to blend at least 2% SAF with fossil fuel kerosene. The blending requirement will gradually increase to reach 32% in the EU and 22% in the UK by 2040.
Another case of diluted green rules?
Speaking at the World Economic Forum in Davos in January, CEO of French oil and gas company TotalEnergies Patrick Pouyanné said he would bet “that what happened to the car regulation will happen to the SAF regulation in Europe”.
The EU watered down green rules for car-makers in March 2025 after lobbying from car companies, Germany and Italy.
“You will see. Today all the airline companies are fighting [against the EU’s 2030 SAF target of 6%],” Pouyanne said, even though it’s “easy to reach to be honest”.
While most European airline lobbies publicly support the mandates, Ryanair Group CEO Michael O’Leary said last year that the SAF is “nonsense” and is “gradually dying a death, which is what it deserves to do”.
EU and UK stand by SAF targets
But the EU and the British government have disputed that. EU transport commissioner Apostolos Tzitzikostas said in November that the EU’s targets are “stable”, warning that “investment decisions and construction must start by 2027, or we will miss the 2030 targets”.
UK aviation minister Keir Mather told this week’s investor event that meeting the country’s SAF blending requirement of 10% by 2030 was “ambitious but, with the right investment, the right innovation and the right outlook, it is absolutely within our reach”.
“We need to go further and we need to go faster,” Mather said.
SAF investors and developers said such certainty on SAF mandates from policymakers was key to drawing the necessary investment to ramp up production of the greener fuel, which needs to scale up in order to bring down high production costs. Currently, SAF is between two and seven times more expensive than traditional jet fuel.
Urbano Perez, global clean molecules lead at Spanish bank Santander, said banks will not invest if there is a perceived regulatory risk.
David Scott, chair of Australian SAF producer Jet Zero Australia, said developing SAF was already challenging due to the risks of “pretty new” technology requiring high capital expenditure.
“That’s a scary model with a volatile political environment, so mandate questioning creates this problem on steroids”, Scott said.
Others played down the risk. Glenn Morgan, partner at investment and advisory firm SkiesFifty, said “policy is always a risk”, adding that traditional oil-based jet fuel could also lose subsidies.
Asian countries join SAF mandate adopters
In Asia, Singapore, South Korea, Thailand and Japan have recently adopted SAF mandates, and Matti Lievonen, CEO of Asia-based SAF producer EcoCeres, predicted that China, Indonesia and Hong Kong would follow suit.
David Fisken, investment director at the Australian Trade and Investment Commission, said the Australian government, which does not have a mandate, was watching to see how the EU and UK’s requirements played out.
The US does not have a SAF mandate and under President Donald Trump the government has slashed tax credits available for SAF producers from $1.75 a gallon to $1.
Is the world’s big idea for greener air travel a flight of fancy?
SAF and energy security
SAF’s potential role in boosting energy security was a major theme of this week’s discussions as geopolitical tensions push the issue to the fore.
Marcella Franchi, chief commercial officer for SAF at France’s Haffner Energy, said the Canadian government, which has “very unsettling neighbours at the moment”, was looking to produce SAF to protect its energy security, especially as it has ample supplies of biomass to use as potential feedstock.
Similarly, German weapons manufacturer Rheinmetall said last year it was working on plans that would enable European armed forces to produce their own synthetic, carbon-neutral fuel “locally and independently of global fossil fuel supply chain”.
Scott said Australia needs SAF to improve its fuel security, as it imports almost 99% of its liquid fuels.
He added that support for Australian SAF production is bipartisan, in part because it appeals to those more concerned about energy security than tackling climate change.
The post Doubts over European SAF rules threaten cleaner aviation hopes, investors warn appeared first on Climate Home News.
Doubts over European SAF rules threaten cleaner aviation hopes, investors warn
Climate Change
Southern Right Whales Are Having Fewer Calves; Scientists Say a Warming Ocean Is to Blame
After decades of recovery from commercial whaling, climate change is now threatening the whales’ future.
Southern right whales—once driven to near-extinction by industrial hunting in the 19th and 20th centuries—have long been regarded as a conservation success. After the International Whaling Commission banned commercial whaling in the 1980s, populations began a slow but steady rebound. New research, however, suggests climate change may be undermining that recovery.
Southern Right Whales Are Having Fewer Calves; Scientists Say a Warming Ocean Is to Blame
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