Exceeding 1.5C of global warming will have "irreversible" consequences for glaciers, sea ice and the Earth's frozen regions, two UN reports warned on Monday, calling for urgent action.
The landmark 2015 Paris Agreement commits the world to limiting warming to well below 2C and pursuing efforts to hold it at 1.5C to try to avoid the worst effects of climate change.
One of the reports said overshooting the 1.5C target "will bring large-scale and in many cases irreversible impacts" for the cryosphere -- Earth's ice in all its forms.
Melting of glaciers and sea ice, the thawing of permafrost and the accelerating rise in sea levels, will have widespread repercussions, from flooding to water shortages.
Some of the effects of the rise in temperature are already visible, such as the 20-percent loss in glacial mass in Switzerland.
But even if the 1.5C target remains relevant, it is already too high for the cryosphere, as current levels of warming were "already unsafe" for the Arctic, Antarctic, mountain glaciers and polar oceans, the UN warned.
"Although returning to 1.5 C will slow ice loss, a truly 'safe' temperature for Earth's ice is now thought to be closer to 1C or even below over the coming centuries," it added.
Global warming has already reached 1.37C above pre-industrial levels and according to the European Copernicus network, the 1.5C threshold could be exceeded around 2030.
"There are no good outcomes in exceeding 1.5C," said glaciologist James Kirkham, lead author of one of the reports, entitled "Overshoot and the Cryosphere", which was written with 80 scientists.
"To contain the global fallout and damage from ice loss in the world's polar and mountain regions, overshoot needs to be kept to the absolute minimum," he said.
"Every extra fraction of a degree and every year spent in overshoot means more devastating impacts."
- Urgency -
Scientists warn in particular about the risks of crossing certain tipping points and say that, even if temperatures were to return to below the 1.5C threshold, recovery would be very slow.
Reducing warming to under 1.5C and even 1C could see some glaciers partially regenerate, while there could also be benefits for sea ice.
But Matthew England, an oceanographer and climatologist at the University of New South Wales in Australia, said recovery could take "decades to centuries".
The second report -- "State of the Cryosphere 2026. The Cost of Overshoot" -- shows that exceeding 2.8C into the next century would have significant consequences.
Sea levels could rise by up to five centimeters a year by 2150, compared to the current 0.45 centimeters while the thawing of permafrost could release as much carbon dioxide each year as the United States, the world's second-largest polluter.
A worst-case but not-so-improbable scenario is that the world is heading towards warming of 2.6C by 2100 even if countries meet their climate commitments.
"Temporarily exceeding 1.5C does not mean that the goals of the Paris Agreement are irrelevant but instead makes them more urgent," argued Kirkham.
"Returning to 1.5C or below by 2100 remains possible with sufficient political ambition and urgency, specifically by speeding the transition away from fossil fuel use to reach net zero by mid-century," he said.
"Such urgent action can prevent and even reverse some cryosphere loss."
Gunjan Silwal, a Nepali glacier researcher, said decisions made in the next few years "will lock in the scale of the damage", warning that August's mountain and glacier collapse that killed more than 1,400 and left over 5,000 missing, was "only a foretaste".
