The Paris Agreement has set the goal to limit global warming to a maximum of 1.5 ˚C above pre-industrial temperatures. It is up to scientists to evaluate what sort of effects different temperature increases can have on the Earth and whether these goals are actually safe limits. Two studies published in Nature Climate Change have both reached the same conclusion; if we keep the temperature increase down to 1.5 ˚C it will be much better for keeping an icy Arctic than even just half a degree more.
In the past forty years, the amount of sea ice in the Arctic has dropped dramatically. The minimum ice extent is observed annually in September, and now this amount is 40% less than in the 1980s. The ice in the Arctic is important for reflecting heat from the earth, providing habitat for arctic species such as polar bears, and keeping sea levels from rising too much. With global temperatures that keep increasing, an ice-less Arctic is a true possibility.
The two research groups evaluated the probability of an ice-less Arctic with a 1.5˚C, 2 ˚C, and 3 ˚C temperature increase. With a 1.5˚C increase, ice-free conditions could be expected every 40 years, whereas with 2 ˚C, they could be expected every 3-5 years, and with 3 ˚C, they would be expected almost every year. Both studies agreed on these points, with very similar probabilities, although they employed different models. Sigmond et al. used an Earth System Model to project sea ice under global warming. Jahn used the Community Earth System Model. Sigmond et al. and Jahn both report a similarly low probability (2.4% and 2.5%, respectively) of ice-free conditions in September if the global average temperature becomes stable at 1.5 °C above pre-industrial levels. Though the very exact probabilities should be taken with a grain of salt, because the sensitivity of sea ice to global warming is very complex and hard to predict completely, the general conclusion is robust.
Both groups show for the first time that even if global warming is stopped after reaching 1.5˚C above pre-industrial levels, the probability of an ice-free Arctic occurring at least once increases over time due to natural variability. It’s like rolling a die, the more times that it is rolled, the more accurate its forecasted probability will be. Although ice-free conditions could occur even when limiting warming to 1.5˚C, it does seem to be enough to avoid ice-free conditions in months other than September. Two degrees, on the other hand, could have us looking at an ice-less August as well. Several months a year of ice-free conditions could promote heating even more, causing a positive feedback loop.
Current pledges to reduce carbon dioxide emissions put us on a course to reach 3˚C warming by 2100. This would lead to ice-free summers every year. However, sea ice could recover if the global temperature cools. It would be harder to decrease global temperature than it is to increase it—CO2 concentrations would need to be reduced more.
It is critically important that countries stick to the goals of the Paris Climate Agreement as they can keep sea ice from reaching dangerously low levels.
Journal references:
1. Sigmond et al. 2018. Ice-free Arctic projections under the Paris Agreement. Nature Climate Change.
10.1038/s41558-018-0124-y
2. Jahn. 2018. Reduced probability of ice-free summers for 1.5 °C compared to 2 °C warming.
3. Screen. 2018. Arctic sea ice at 1.5 and 2 °C. Nature Climate Change.10.1038/s41558-018-0127-8
