Just half a degree Celsius of global warming will determine whether ice-free Arctic Ocean conditions happen occasionally or frequently, two independent analyses show.
The studies, published yesterday in Nature Climate Change, are part of a broader effort to fill in what the world would look like under the emissions reduction goals and commitments outlined in the Paris Agreement.
Sea-ice is important for the survival of Arctic wildlife such as polar bears, preventing coastal erosion at high latitudes, and stabilizing the global climate system as a whole. As global average temperature has increased in recent decades, Arctic sea-ice has retreated rapidly.
The minimum extent of the sea-ice always occurs in September, at the end of the Arctic summer. The two new studies use similar methodology but different computer climate models to predict the probability of ice-free conditions in September under different temperature and emissions regimes.
The studies found that if global average temperature stabilizes at 2.0 °C above pre-industrial levels, the overall goal of the Paris Agreement, summer sea-ice will be absent from the Arctic every 3 to 5 years.
But if we reduce greenhouse gas emissions more sharply and hold the temperature increase to 1.5 °C, the Paris Agreement’s more ambitious goal, ice-free conditions happen only every 40 years or so.
The actual emissions reduction commitments countries have made under the Paris Agreement so far are predicted to lead to 3.0 °C of warming by the year 2100. These conditions will make absence of summer sea-ice the norm, occurring 2 out of every 3 years, according to the study by researchers from the Canadian Centre for Climate Modelling and Analysis1.
“Even when warming is limited to 1.5 °C, the Arctic summer sea-ice cover experiences significant reductions compared to today’s cover,” writes Alexandra Jahn, a researcher at the University of Colorado in Boulder and the author of the second study2.
For example, Jahn calculated that with 1.5 °C of warming, ice conditions comparable to the 2012 record minimum will occur about half the time. (If warming is 2 °C, those conditions will occur almost every year.)
Scientists don’t fully understand how sensitive sea-ice is to overall climate warming. Moreover, sea-ice conditions could continue to deteriorate even after global average temperature stabilizes at one level or another, because of delayed warming in the ocean. All of this suggests that the new studies are likely to understate rather than overstate effects on sea-ice.
On the plus side, Jahn’s analysis suggests that the loss of sea-ice need not be permanent. If global average temperature starts to decrease in the future, Arctic sea-ice will return along roughly the same relationship to temperature with which it disappeared.
However, to reduce global average temperature by a certain amount, we have to decrease the atmospheric carbon dioxide concentration by more than it increased to produce that warming in the first place. “Although sea-ice loss is generally reversible for decreasing temperatures, sea ice will only recover to current conditions if atmospheric CO2 is reduced below present-day concentrations,” she writes.
1. Sigmond M. et al. “Ice-free Arctic projections under the Paris Agreement.” Nature Climate Change. 2018.
2. Jahn A. “Reduced probability of ice-free summers for 1.5°C compared to 2°C warming.” Nature Climate Change. 2018.
Image: Excerpt from an animation showing Arctic sea-ice cover from 1979-2008. Credit: NASA Goddard Space Flight Center via Flickr.