Are we headed for runaway climate change? Is the future a planet like Venus?

Dr Jonathan Chenoweth gives an overview of the side-event that he attended this morning at COP21.


I have just been to another fascinating parallel session at COP21, this time focusing upon the risks of irreversible cryosphere climate change, with this session chaired by the International Cryosphere Climate Initiative. The Cryosphere refers to the ice covered parts of the planet – the polar icecaps, sea ice, permafrost and mountain glaciers. Climate change has already had a big impact on these areas, with the rate of change likely to accelerate over the course of the coming century. As the first speaker in the session pointed out, while you can negotiate over carbon emissions and many other aspects of climate change, you cannot negotiate with the temperature at which ice melts. This is set by the laws of physics.

In the last 100 years sea levels have risen by about 20 cm. This has already caused major problems, leading to cities like London to invest in the Thames Barrier to protect its valuable real estate. If the Eastern Antarctic Ice Sheet were to ever melt, it would raise sea levels by about 50 metres. Even the much smaller Western Antarctic Ice Sheet raise sea levels by several metres. The ice sheets are held back on the land by the ice shelves which are adjacent. Once these are lost the whole ice sheet behind becomes destabilised and its loss becomes almost inevitable with time. However, Antarctica is made up of several ice sheets, and so the destabilisation and eventual melting of the Western Antarctic Ice Sheet does not make the loss of the other ice sheets inevitable.

The world has approximately 200,000 mountain glaciers. If they all melted sea levels would rise about half a metre. However, unlike with the Antarctic and Greenland ice sheets, these glaciers are responding rapidly to global warming. If global temperatures rise by 2.5 degrees by 2100, then all the world’s mountain glaciers will be lost soon afterwards. Even with a 1.5 degree global temperature rise, most glaciers will be lost next century.

The loss of sea ice over the Arctic is another cause of concern. Since 1979 the area of sea ice at the end of each summer has halved in area and thickness, so 75% of the volume of the ice has been lost. This loss of ice is having big impacts on weather systems, including areas much further south such in heart of Europe. Ice covered sea reflects sunlight whereas open ocean absorbs much of the sun’s energy. If global temperatures rise by more than 1.5 degrees, it is likely that no sea ice will remain in the Arctic each year at the end of the summer, when it is measured.

Permafrost is the other major cryospheric landscape, making up 25% of the land area of the northern hemisphere. 30% to 70% of this will probably be lost – 30% if humans achieve a low level of carbon emissions and 70% if we end up with a high level of emissions. The loss of permafrost leads to the release of carbon trapped in the permafrost. There is currently about 1,500 billion tonnes of carbon in the permafrost – more carbon than is present in the atmosphere. The release of some of the carbon as a result of climate change would amplify climate change by itself causing further climate change.

So with all of these changes, is our future on Earth to have a climate like Venus, as all sea ice is lost and the oceans warm, as the ice caps melt and seas levels rise dozens of metres and as all the carbon stored in the permafrost is released and itself further amplifies climate change? According to one of the presenters in this session in response to a question from the audience, if carbon emissions are at the upper limits of what is currently being discussed, the Earth’s climate is likely to return to a state similar to what it was 35 million years ago. But will we get run away climate change leading to an environmental state like Venus? No. There is nothing to suggest that a future like Venus awaits. At least that’s good to know.


This talk was based on the report, “Thresholds and Closing Windows: risks of irreversible cryospheric climate change”, which can be found at