In December 2015, 174 nations signed the Paris climate agreement to stop pumping carbon into our atmosphere. Eighteen months later, to international condemnation, President Trump announced the United States’ decision to withdraw from the Accord. Neither the question of how to prevent humanity committing climate suicide, nor the huge opportunities to raise citizens’ quality of life through clean, low-carbon development, merited any serious discussion in the US elections, or indeed in the French, Dutch or British polls.

Trump’s announcement followed, first, NASA’s confirmation that 2016 was the hottest year globally since records began (as 2015 and 2014 were before it); and second, his decision to slash the agency’s world-leading climate science programme.

Fortunately, city leaders have not lost focus, and tackling pollution is at the top of mayoral agendas from London to Paris, Beijing to Los Angeles. It is partly the nature of the job that compels mayors to be more active. The urban philosopher, activist and visionary, Benjamin Barber – who sadly died this year – was fond of quoting former New York mayor Fiorello La Guardia: “There is no Democratic or Republican way of fixing a sewer.” Mayors are judged on their ability to get stuff done and, thankfully, can leave the treaties and wars to others.

City mayors – at least the ones I work with in C40 – tend to have worked out that businesses want to invest in places where talented people want to live, and that clean air, extensive public transport and energy-efficient buildings help deliver strong economies.

Cities that adapt to the realities of a changing climate, pursuing ambitious emission-reduction goals as London is doing, will be the most economically successful, equal, and liveable cities of the future. The green spaces and urban gardens that protect cities against flooding and overheating make cities better places to live. When major roads are closed to traffic in order to tackle air pollution – on the right bank of the Seine in Paris, in the whole of Oslo’s city centre, and as planned for Oxford Street – pedestrians and cyclists reclaim the streets (and shoppers on foot spend more than those rushing back to their cars to beat the traffic warden). As cities halt suburban sprawl by shifting to compact centres connected by sustainable public transport, communities that may have been isolated and disadvantaged become vibrant and integrated once again.

But it is also true that the world’s big cities are already struggling with the negative impacts of a hotter planet and more chaotic weather.

Historic amounts of rainfall in Houston, Texas during 2016 saw more than 1,000 homes flooded and caused well over $1 billion in economic damage. A heatwave in Sydney this year brought temperatures to a peak of 47°C, triggering wildfires and blackouts as the power grid struggled in the soaring temperatures. Air pollution in cities, caused by many of the same emissions that contribute to global warming, is responsible for more than 3 million premature deaths per year worldwide.

The London Plan, City Hall’s long-term strategy, notes starkly that “[London] is vulnerable to flooding, overheating and drought conditions which can lead to water supply shortfalls. Climate change will increase the probability and severity of these effects through rising sea levels, heavier winter rainfall, higher tidal surges, hotter summers and less summer rainfall. The effects of climate change could seriously harm Londoners’ quality of life, particularly the health and social and economic welfare of vulnerable people.”

If global sea levels rise by more than one metre – a scenario that the UN’s climate change body, the IPCC, warns could be entirely possible this century without drastically curbed emissions – then we could expect the Thames barrier to be breached and London seriously flooded every 10 years. That is comparable to a risk of flooding once every 1,000 years at current sea levels.

Wealth does not provide London with a climate pass: look at what Hurricane Sandy did to New York, or Hurricane Katrina to New Orleans, where some neighbourhoods are still being rebuilt years later.

Mayors are leading climate action not least because they are good at working together. Climate change is a global problem: it needs global solutions. Intergovernmental decision-making has proved inadequate to the task (excepting the monumental achievement of the Paris Accord, and the underrated contribution of the scientific Intergovernmental Panel on Climate Change).

More than a decade ago, the then Mayor of London Ken Livingstone invited the mayors of G20 capital cities to discuss climate change, a topic that didn’t make it onto the agenda of their respective Presidents and Prime Ministers. Today, the C40 network that resulted from this initial gathering includes 91 members, representing 650 million urban citizens and more than a quarter of the global economy. When they get together, I rarely hear C40 mayors talking about climate change per se – the case for cutting pollution and accelerating prosperity through low-carbon investment stands on its own. They just want to swap ideas about how to get on with it.

London remains a global trailblazer. Mayor Khan is talking about cleaning up London’s air and tackling climate change. The policy prescriptions he is implementing will keep London at the forefront of providing sustainable urban transport with lower carbon emissions.

London’s congestion charge hasn’t been widely copied since its inception in 2003, but it undeniably set the tone for a global shift away from cities designed for cars. From this October, vehicles in central London will need to meet minimum exhaust emission standards or pay a daily £10 Emissions Surcharge (also known as the T-Charge). And when Sadiq Khan presses the button on the world’s first Ultra Low Emission Zone (ULEZ) in central London in 2019, mayors around the world will be watching and learning.

A couple of months ago, I stood in the wings as Mayor Khan joined the Mayor of Paris and Chair of C40, Anne Hidalgo, and the Mayor of Seoul, Park Won-Soon, to announce that cities were fed up with motor manufacturers fiddling emissions tests and so would be setting up (via C40) their own scoring system for car emissions. As a result, car buyers will have accurate data about the pollution that actually comes out of the exhaust pipe of the main car brands when they are driven on real roads. Consumers will be able to make informed choices about the impact of different cars on air pollution and climate change, and manufacturers will be held to account for continuing to produce vehicles whose fumes send millions of people to an early grave each year.

The C40 Clean Bus Declaration, spearheaded by London, has brought together 26 global cities in a shared commitment to reducing emissions and improving air quality by incorporating low- and zero-emission buses in their fleets. This reminder of the purchasing power of city halls has led bus manufacturers to accelerate R&D in the technology, driving down the cost of electric buses in cities around the world.

There is a lot happening at the national level on climate change that also deserves praise. China’s investment in renewable energy, for example, has made clean energy now largely cheaper than its dirty alternative. Yet many nations are dragging their feet when it comes to turning the aspirations of the Paris climate agreement into reality – and President Trump appears determined to take the USA backwards.

This is why it is so significant that mayors have upped their game. Since January this year, it has become a condition of membership of the C40 network that each mayor commits to setting out concrete plans for how they will deliver the goals of the Paris Agreement in their cities. If every city with a population of over 100,000 people cuts emissions, as the C40 mayors are committing to do, then they will deliver 40 per cent of the emissions reductions needed to meet the goals of the climate treaty.

Climate change is the most urgent crisis facing the great cities of the world, including London. It is also the greatest opportunity we have to transform our cities into places where people will want to live in the future.