Yakutsk, Russia: Eduard Romanov points to a spot on a block of flats where a major supporting beam has sagged and begun to crack, destabilising the nine storeys of apartments above.
In Russia's Siberian city of Yakutsk, one of the coldest on Earth, climate change is causing dangerous melting of the frozen ground, or permafrost, on which the buildings stand.
"Since the year before last, the building has started to list and has tilted about 40 centimetres," says Romanov, a construction worker and environmental activist.
"There is a danger that it will tilt even more," he says, as labourers perform emergency welding on the structure, the temperature around minus 35 degrees Celsius.
Average temperatures in Yakutsk have risen by 2.5 degrees Celsius over the past decade, say scientists at the Melnikov Permafrost Institute located here, the world's only such research centre.
Most Soviet-era apartment blocks in Yakutsk are made of concrete panels and stand on stilts to ventilate the building's underside and prevent it heating the permafrost, a layer of soil cemented together with water that is only stable as long as it stays frozen.
Rising summer temperatures can destroy the solid permafrost. As the ice melts, the clay or sand simply sinks together with whatever is on top of it - a road, a building, a lake or a layer of fertile "black earth" for agriculture.
300000population of Yakutsk
Permafrost covers almost the whole of Yakutia - a northeast Siberian region bordering the Arctic Ocean, an area five times the size of France.
In total, around 65 per cent of Russian territory is covered by permafrost.
With a population of about 300,000 Yakutsk is the world's largest city built on permafrost, and it could be especially in danger from the melting that Romanov and many residents fear.
Older buildings were not constructed with a warming climate in mind. In the 1960s, the norm was to drive stilts six metres (20 feet) deep into the solid permafrost, which is no longer sufficient today as the surface warms, Romanov says.
Some buildings in Yakutsk have already had to be demolished while others are full of cracks.
"All of Yakutsk is in danger. The owners face losing their property, and nobody is ready for this," Romanov says.
"These problems will multiply in the future, so we need to start addressing this today."
As an Arctic country, Russia is warming about two-and-a-half times faster than the rest of the world.
In Yakutsk, locals say that two decades ago schools would be closed for weeks on end when temperatures dropped to minus 55 Celsius but such spells of extreme cold are now rare.
Russia's environment ministry said in a report this year that deterioration of permafrost poses many risks to people and nature.
It affects water, sewage and oil pipes as well as buried chemical, biological and radioactive substances, the report said.
65%of Russia is covered in permafrost.
Melting permafrost enables any pollutants to spread faster and more widely, seeping through previously solid ground, the report said.
Mikhail Grigoryev, deputy director of the Permafrost Institute, says so far the warming is "not critical" locally but could endanger the city if it continues over decades.
He is most concerned about permafrost's southernmost boundary, in areas such as oil-rich western Siberia.
There, permafrost is not as cold, consistent or thick, and warming can "lead to the deformation of buildings, to disasters".
"Nothing that was built on permafrost was built with the expectation that it would melt," Grigoryev says.
"We must prepare for the worst."
Protecting the frost
In the Permafrost Institute's underground labs - a network of ice-covered tunnels and rooms dug in the permafrost - scientists and engineers develop improved construction techniques and ways to keep the ground frozen as the atmosphere warms.
One method that is already available involves burying vertical metal tubes filled with a non-freezing agent like freon or kerosene in the ground with a part sticking out around or near a building. In winter the agent condenses in the cold atmosphere and drops below the ground surface to keep it cold.
2metresof coastline each year lost by Yakutia because of melting permafrost
Yet the technology is costly and its use in construction is not required by the law, which has not adapted to the warming climate, says a lawmaker in Yakutia's regional parliament, Vladimir Prokopyev.
Melting permafrost accelerates erosion of Russia's Arctic coast, and Yakutia is losing about two metres of coastline every year, he said.
The region this year became the first in Russia to pass a permafrost protection law and is lobbying Moscow to take measures on a national level.
The law calls for the monitoring and prevention of irreversible loss of permafrost, but Prokopyev laments that Moscow has been hesitant to treat it as a priority.
"We need a national law if we want to conserve permafrost and prevent serious harm to the environment of the Russian North and Siberia," he says.
Why melting permafrost could kill you
Yakutsk, Russia: The Earth's vast tracts of permafrost hold billions of tonnes of planet-heating greenhouse gases that scientists warn will be released by global warming, along with diseases long locked into the ice.
Here is some background.
What is it?
Permafrost is soil that is frozen, although not necessarily permanently as it name implies. It is found mostly in the Northern Hemisphere, where it covers about a quarter of exposed land and is generally thousands of years old.
It covers a wide belt between the Arctic Circle and boreal forests, spanning Alaska, Canada, northern Europe and Russia.
Permafrost exists to a lesser degree in the Southern Hemisphere where there is less ground to freeze, including in the South American Andes and below Antarctica.
It can vary in depth from a few metres to more than 100.
Locked into the permafrost is an estimated 1.7 trillion tonnes of carbon in the form of frozen organic matter - the remains of rotted plants and long-dead animals trapped in sediment and later covered by ice sheets.
When permafrost thaws, this matter warms up and decomposes, eventually releasing the carbon that it holds as carbon dioxide (CO2) and methane, gases which have a greenhouse warming effect on the planet.
1.7trtonnes of carbon trapped in permafrost
Permafrost soils contain roughly twice as much carbon - mainly in the form of methane and CO2 - as Earth's atmosphere.
Most of the carbon stocks are thought to reside fairly close to the surface.
Vicious circle of warming
The release of greenhouse gases threatens a vicious circle in the warming of the Earth, jeopardising the objective set in the 2015 Paris Agreement to strive to limit the rise in temperature to 1.5 degrees Celsius (2.7 degrees Fahrenheit) above pre-industrial levels.
CO2 is the most abundant greenhouse gas blamed for global warming but methane is 25 times more efficient at trapping heat.
Adding these into the atmosphere will spur further warming and ice melt, which will in turn cause more thawing of the permafrost and free up more locked-up carbon.
25xMethane is more efficient at trapping heat than CO2
Even if global warming were stabilised at around 2 C, research points to a 30-per cent loss of permafrost by 2100, researcher Susan Natali of the Woods Hole Research Center told 2015 climate talks in Bonn.
This could reach up to 70 per cent assuming emissions continue on their current trajectory, her research said, warning that "emissions from permafrost could lead to out-of-control global warming".
The thawing of the permafrost also threatens to unlock disease-causing bacteria and viruses long trapped in the ice.
There have already been some cases of this happening. In 2016 a child died in Russia's far northern Siberia in an outbreak of anthrax that scientists said seemed to have come from the corpses of infected reindeers buried 70 years before but uncovered by melting permafrost.
Released from the ice, the anthrax seems to have been passed to grazing herds.
Scientists have also warned that other dormant pathogens entombed in frozen soil may be roused by global warming, such as from old smallpox graves.
In 2014 scientists revived a giant but harmless virus, dubbed Pithovirus sibericum, that had been locked in the Siberian permafrost for more than 30,000 years.
Risks to roads, pipelines
A permafrost thaw could be a boon for the oil and mining industries, providing access to previously difficult-to-reach reserves.
But it also presents a serious and costly threat to infrastructure, risking mudslides and damage to buildings, roads and oil pipelines.
A Greenpeace report published in 2009 said thawing soil in Russia's permafrost zones caused buildings, bridges and pipelines to deform and collapse, costing up to 1.3 billion euros (nearly $1.5 billion) a year in repairs in western Siberia.