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While training for the London Marathon in 2014, Sadiq Khan - then a British parliamentarian from the southern London district of Tooting - found himself gasping for air during runs and discovered he'd developed asthma for the first time in his life.
Since becoming the capital's mayor in 2016, the Labour Party politician has made cleaning up London's air a top priority, through measures from electrifying its pollution-belching diesel buses to installing an advanced air quality monitoring system.
This week, a Central London Ultra Low Emission Zone (ULEZ) Khan launched in 2019 expanded to 18 times its initial size, to cover large parts of London.
Drivers of older polluting cars, vans and motorcycles must pay £12.50 ($17.20) a day to enter the 140-square-mile (360-square-km) area - a powerful incentive to invest in greener forms of transport.
Two years after the first ULEZ came into effect, four out of five vehicles entering the original zone now meet cleaner standards, and roadside nitrogen dioxide, a major air pollutant, has fallen by more than 40%, the mayor's office said.
That is a change Khan - brought up on a council-run housing estate and the son of a Pakistani immigrant bus driver - wants to see happen across London.
Expanding the anti-pollution zone will allow 4 million more Londoners to breathe healthier air and help tackle climate change at the same time, he said.
"I've got skin in the game," Khan, a former human rights lawyer, told the Thomson Reuters Foundation in an interview.
London is part of the C40 Cities network, a global group of nearly 100 major metropolitan areas working to drive faster action on climate change and improve residents' lives in the process.
The cities have each committed to delivering climate action plans designed to spur uptake of clean energy, boost adaptation to climate threats and turn the 2015 Paris Agreement on climate change into an on-the-ground reality.
London - which has pledged to reach net-zero carbon emissions by 2030 - is undertaking a wide range of actions, from installing electric vehicle charging points to better preparing people and infrastructure to deal with worsening floods and heat.
England's capital now has the largest number of electric buses of any European city, according to the mayor's office, which pledged last month to have a fully zero-emissions fleet by 2034, three years earlier than first planned.
The Greater London Authority - which Khan heads - has also succeeded in divesting the city's pension funds from coal, oil and gas, with only about 0.5% of its investments still linked to those climate-damaging fuels.
"We're almost there, almost at zero," said Khan, who will become the new chair of C40 in December.
But as London rushes to slash emissions and clean up its air, much of what still needs to happen lies outside the mayor's control, which is limited to things like transport, policing and strategic planning.
Funding for badly needed retrofits for 3.8 million draughty London homes, for instance, is largely controlled by the central government.
In addition, much of the practical action needed to cut planet-heating emissions on the ground - from issuing planning permits to managing waste disposal - lies with the 32 local authorities, called boroughs, that make up Greater London.
Creating effective climate change policy across London, and putting it into action, involves a constant and complex set of negotiations with dozens of partners, often from different political parties - and it doesn't always work.
"It would help if the mayor of London had more power - he'd be able to do much more," said Deirdre Costigan, deputy leader of Ealing Council in west London, which declared a "climate emergency" earlier this year.
Ealing, one of London's larger boroughs with more than 340,000 residents, set new strategic priorities this year, including a focus on creating jobs and boosting equity for its residents, as well as addressing climate change.
As part of that push, the council has identified 5,000 social housing homes to retrofit with full exterior insulation - "like a tea cozy" - as well as triple-glazed windows, heat pumps and solar panels with battery systems, Costigan said.
Once finished - a process that takes just two weeks for each building - the homes are expected to be able to run without grid electricity, eliminating hefty power bills for the borough's most vulnerable.
Residents will, however, pay a small monthly energy fee to the council to help fund the next round of retrofits.
"Climate justice is not just about those suffering climate impacts but about the poorest not paying big bills," said Costigan, standing in front of one of the butter-coloured two-storey homes slated for work on the council's "Trees Estate".
Asma Yaqub, 42, is one of 68 occupants whose homes will get a full retrofit starting early next year, including an electric oven to replace the gas appliance she now uses to bake cakes for a living to support her children.
She was initially sceptical about changing the systems she relies on.
"I'm not a very green person. I don't traditionally recycle and when I have I've found it's been a headache," she admitted.
But watching climate change and nature documentaries helped her see that "what's happening is just astonishing, and to think I'm part of that is not acceptable", she said.
"As a Muslim, I have an obligation to look after the planet and think about future generations and protect them from the mistakes we're making right now," she added. "If that means I have to be more vigilant, that's what I have to do."
She does like the idea of an expected "massive difference" in her energy bills, as well as reduced noise from the planned triple-glazed windows on her home, which faces a busy road.
Making retrofits work takes upfront investment to persuade the community the disruption is worthwhile, Costigan said.
"A lot of it is about engaging with residents, explaining it will be better and getting them to agree to come with us," she added.
Costigan said Ealing Council has 5,000 council homes ready to retrofit - but so far has cobbled together money for only 68, using leftover borough maintenance funds and some legacy funding from the European Union, which Britain has now left.
As London seeks to make its homes greener, finding far bigger, consistent and secure investment streams will be key, instead of applying for small amounts each year, she added.
"If we had a proper multi-year funding plan we could deliver this," she said.
Claire Holland, leader of Lambeth Council in south London, said the British government needed a better strategy to pay for green changes to meet its ambitious goals of cutting planet-heating emissions by 78% by 2035 and to net-zero by 2050.
"The way government funds net-zero is in small piecemeal funds that everyone has to bid for," requiring capacity that could be better deployed on building sustainable supply chains or technical know-how, she said.
"Then you have a time limit in which you have to spend that money and it's very short. There's no long-term plan around this," she added.
In both Lambeth - the first London council to declare a climate emergency - and Ealing, training locals for new green jobs and ensuring there are enough skilled workers to handle the scale of required home retrofits are a priority.
Lambeth estimates it needs 11,000-12,000 people to deliver the low-carbon changes planned in the borough, and is starting to work with local schools to prepare them.
"Lots of students can’t afford to go to university. We need to create a pathway for them that is not to zero-hour jobs but to careers" in things like retrofitting and clean energy installation, Holland said.
Lambeth Council also has invested £1.5 million ($2 million) into Sustainable Ventures, an incubator for clean-tech start-ups based in an old county hall building near the River Thames.
The project's website says it has supported the development of more than 250 green firms since 2011 under a mission to back 1,000 such companies by 2025.
While progress is being made in pockets, in London - and across the country - efforts to build the businesses needed to deliver net-zero emissions are only just getting started.
By 2035, about 11 million of Britain's existing homes will need better insulation and new low-carbon heating and energy systems to meet emissions goals, according to the Climate Change Committee, an independent advisory body.
The government this month said it would begin providing British homeowners with £5,000 grants to install electric heat pumps, aiming to add 90,000 of the low-energy home heating devices as the country phases out new gas boilers by 2035.
By 2028, Britain needs to install 600,000 heat pumps a year to meet its climate change goals, the government said in 2020.
But today there are only 950 accredited installers, compared to nearly 100,000 fitters of gas boilers and other fossil-fuel-powered heating systems, according to Ashden, a London-based green nonprofit that supports efforts to train workers.
"It’s hard enough in London to find a plumber, much less one of these (heat pump) installers," said David Pierpoint, founder of The Retrofit Academy, a social enterprise that trains project coordinators.
Cara Jenkinson, cities manager at Ashden, said the lack of skilled workers is hampering the push to expand retrofits.
"Getting local authorities carbon-literate, retrofit-literate and then capable of developing really big projects is important - but the management skills are not always there," she said.
The Greater London Authority runs an accelerator that aims in particular to help social housing providers ramp up green renovation work, she added. Other organisations like The Retrofit Academy are also stepping into the void.
Delivering on the 2035 national emissions target will require 300,000-400,000 extra people to retrofit homes, but the government lacks a plan to make that happen, said Pierpoint.
New businesses have launched each time a fresh pot of retrofit money appears, then failed as it dries up, leaving many wary of starting again, he and others said.
Now the government has offered a large amount of funding - but the supply chain and skilled installers, coordinators and assessors are lacking to do things at scale, Pierpoint noted.
Since late 2018, The Retrofit Academy has trained about 1,500 new coordinators, largely online because of the COVID-19 pandemic, with the largest number in London and Manchester.
Graduates include Sara Kamal, 28, a design and technical standards officer for the northeast London borough of Hackney, who finished a three-month course in September.
As Hackney works to reach net-zero emissions by 2040, "you need more than just skilled people to install things," she said. "It's about ... a whole system of coordination to achieve the end product."
That includes helping homeowners understand how to operate new and unfamiliar systems including heat pumps.
"You can't just hand over beautiful technology and leave residents with no idea how to use it to make their homes efficient," Kamal explained.
London's old draughty homes, which mainly run on fossil fuel gas, are not the city's only climate change challenge.
As global warming drives more extreme weather, the risks of flooding and heatwaves are growing, including for the capital's heavily used public transport systems.
Extended summer downpours closed 31 stations on the "tube" subway network this year, and researchers predict nearly 60 stations are at significant risk of flooding, including some of the busiest such as London Bridge and King's Cross.
According to the mayor's office, a quarter of London's rail stations also could face floods in the future, along with one in five schools and almost half of its hospitals.
Evelyn Kalmar, 64, who regularly travels from her home near Canada Water to her gym at London Bridge, said she could make the trip by foot if needed.
But if major stations like London Bridge were frequently closed by flooding, "it would make London very difficult to move around and cause a lot of unrest if people are fighting to get on transport", she predicted, standing outside the station.
While she does not think that is imminent, it is clear "climate change is coming to the top of the agenda" in London and nationally as warnings grow, she said.
The Greater London Authority is stepping up efforts to reduce the threats, from launching climate risk maps overlaid with social vulnerability data in March to piloting improved drainage systems and issuing heat alerts.
The Thames Barrier, a set of walls that can be deployed across the river to protect large parts of London from worsening tidal flooding, is also set to be replaced by 2070, according to the country's Environment Agency.
But flash floods caused by localised heavy rainfall are an additional - and growing - challenge.
Academic studies suggest London's subway network may also soon become too hot to use for up to a month's worth of days per year as global temperatures rise and Victorian-era tunnels are unable to accommodate air-conditioned trains.
That makes it crucial to act now to limit temperature hikes, said the city's mayor.
"If we don't hold down the temperatures, Londoners need to understand the consequences," he said. "This is not an issue we need to worry about in 10 or 20 years' time. This affects us now."
At Prior Weston, a Central London primary school, Khan last month stopped by to view a new chalk drawing by a local artist sketched out on the playground.
It depicts two future visions of London: a grim city of flooded roads, traffic congestion, dead trees against a smoggy sky and long lines outside a hospital; and a brighter one with solar panels, electric cars and people walking and cycling.
Head teacher Fiona MacCorquodale said her students were concerned "about the dark side of that picture".
Alusine Fane, 11, has tried to do his bit for the climate by walking and recycling more and adding plants in his garden.
"I worry about future generations even though it's not something I should have to worry about," he said.
Since Central London's ultra-low emissions zone was put in place, once-high pollution levels at Prior Weston have dropped to within legal limits, the mayor said.
But more such changes are needed to make London sustainable and livable into the future, Khan added - including more ambitious pledges of emissions cuts by national governments at the COP26 U.N. climate negotiations in Glasgow, starting Sunday.
"The choice is stark. It’s between two very different futures that young Londoners could inherit," he said.
"The first is the path of inaction which would see us sleepwalking to a climate catastrophe," he said.
"The other path is of action and courage, where we make the tough but necessary choices now. This second path is the one we must take."
Reporting for this story was supported by C40, a network of the world’s large cities committed to addressing climate change.
Reporter: Laurie Goering
Text editing: Megan Rowling
Photography: Dan Phillips and Cormac O'Brien
Producer: Amber Milne
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