The cost of allowing global temperature to rise to 2C, rather than capping warming at 1.5C, is an area of permafrost the size of Mexico, according to new research.

The study, carried out by a team of scientists from Sweden, Norway and the UK, is the first to work out what the ambitious targets contained in the Paris Agreement mean for permafrost loss.

While warming of 2C would ultimately see permafrost-covered land shrink by more than 40%, stabilising at 1.5C would “save” approximately 2m square km, says the new study.

The paper, published today in Nature Climate Change, warns that thawing permafrost unlocks large amounts of CO2 and methane, which could potentially be released to the atmosphere.

Thawing out

Permafrost is the name given to frozen soil that has been at a temperature of below 0C for at least two consecutive years. Currently covering an area of approximately 15m square km in high latitudes, permafrost accounts for 24% of exposed land in the northern hemisphere.

But permafrost is also known to be sensitive to rising temperatures. With the Arctic warming twice as fast as the rest of the world, large areas of permafrost are already starting to thaw.

With roughly 35 million people living in the permafrost zone, the thawing landscape threatens communities by putting roads and buildings at risk of collapse.

Added to that, there are potentially serious consequences for the climate. Permafrost is estimated to contain more carbon than currently exists in the atmosphere, some of which can be released as CO2 and methane as the frozen soils begin to thaw.

As Dr Christina Schaedel, a research associate in effects of warming on permafrost at Northern Arizona University, wrote in a recent guest post for Carbon Brief:

“As the soils thaw, the microbes they contain are woken from their ice-induced hibernation. The microbes feed on the organic carbon, converting it into carbon dioxide and methane, which is released into the atmosphere.”

While climate models all predict permafrost thaw as high northern regions warm, they differ on how severe the impacts are likely to be, the paper explains. For example, model simulations of past changes predict a present day permafrost cover of anywhere between 0.1 and 1.8 times the size of that actually observed.

The authors set out to examine how much permafrost would be lost with each degree of additional warming. First, the team looked at how the current distribution of permafrost changes in relation to air temperature. Then, they used this relationship to infer the likely pattern under specific warming levels, specifically 1.5C and 2C above pre-industrial times.

Permafrost is not exclusively determined by air temperature, with topography, soil properties and snow depth all playing a role, the authors acknowledge. Nevertheless, air temperature offers a good indication of the probability of finding permafrost in a region, the paper explains.

Shrinkage

According to the results, the area covered by carbon-rich frozen ground in the Arctic is expected to shrink by 4m square km for every extra degree that global average surface temperature rises.

Even accounting for uncertainties in air temperature, snow and other factors, this suggests permafrost is around 20% more susceptible to warming than previously thought, the paper says.

For 2C warming, the paper suggests 6.6m square km of permafrost would be lost, compared with a 1960-1990 baseline. This corresponds to over 40% of today’s permafrost area, leaving just over 8m square kilometres intact. Capping warming at 1.5C would likely “save” about 2m square kilometres of permafrost, leaving the total extent around the 10m square kilometre mark.

The figure below from the paper compares the maximum permafrost extent (red solid line) at 1.5C (left) and 2C (right). The shading represents permafrost cover during the 1960-1990 reference period, the orange squares indicate three cities build on continuous permafrost, which will face varying risks to their infrastructure depending on the level of warming.

Permafrost thaw wouldn’t happen immediately, the authors note. There would likely be a lag as soil temperatures caught up with air temperatures. A recent study suggests the result is likely to be a gradual release of greenhouse gases over many decades, rather than an abrupt pulse.

While the effects wouldn’t be felt as soon as we pass the 1.5C or 2C threshold, the new results are very relevant for climate discussions surrounding the Paris Agreement, say the authors. The Intergovernmental Panel on Climate Change (IPCC) will produce a Special Report in 2018, for example, specifically designed to address the question of relative impacts at 1.5C vs 2C.

The new paper looks at other temperature levels, too. With 5C of warming (similar to the Intergovernmental Panel on Climate Change’s RCP8.5 scenario) by 2100, the vast majority of permafrost would disappear, leaving just 0.3-3 square kilometres unaffected, say the authors.

Carbon losses

While today’s paper is a first for comparing the extent of permafrost with 1.5C and 2C of warming, it doesn’t go as far as working out the likely consequences for carbon emissions.

With more than 1000bn tonnes of carbon estimated to be locked up in permafrost soils, the impacts for climate could be very significant, says Dr Sarah Chadburn, a specialist in Arctic permafrost modelling at the University of Exeter and lead author on the new paper. She tells Carbon Brief:

“It’s very likely that there will be carbon emissions from permafrost, but difficult to say how much.”

This is because converting permafrost loss into carbon emissions is far from trivial, says Prof Pierre Friedlingstein, a professor of global carbon cycling at the University of Exeter and co-author on the new research. There are big uncertainties over how much of the soil carbon will decompose, he tells Carbon Brief:

“Thawing of permafrost does not mean all permafrost carbon is eventually lost. A larger (deeper) amount of carbon will be accessible to decomposers, but certainly not the entire soil column.”

A further complication is that greenhouse gases released by permafrost could create warmer conditions and trigger the remineralisation of nutrients, says Friedlingstein. Both of these could help support vegetation growth, potentially locking some of the carbon back up in the soil.

Together with their colleagues, Chadburn and Friedlingstein have a paper currently in review that aims to throw some light on the consequences of permafrost thaw on future climate. The study examines permafrost carbon emissions in various climate models and under different scenarios, finding that the extra boost to warming from thawing permafrost could be 0.2-12 % of the change in global mean temperature.

The team also have a separate project, called Climate feedbacks from wetlands and permafrost thaw in a warming world (CLIFFTOP), which aims to quantity the amount of methane likely to be released from thawing permafrost methane emissions under 1.5C and 2C scenarios. This is one of several short projects funded in 2016 by the Natural Environment Research Council, designed to feed directly into for the IPCC’s special report on 1.5C.