The albedo effect still acts as a positive feedback to warming and melting. In fact the general circulation model study shows Arctic sea ice melting will speed up over recent years. From 2020 to 2030 more than 2.5 million square kilometres of ice will be lost. Although this decline is much faster than the current rate of melting, it would be more or less linear with temperature rise and time.
According to the study that was published in Nature last week, albedo effects would not lead to a runaway melting phase beyond any threshold. Reason for this is the pile-up of Siberian winter sea ice on the Canadian Arctic shores, with increased ice dynamics as the ice sheet gets thinner and wind fetch increases over spots of open water.
If GHG emissions are unabated, the study shows, only a tiny bit of sea ice [less than 0.1 million square kilometres] could survive the coming decades. This would not be enough to sustain the iconic Arctic species like seals, walruses and polar bears.
However, as sea ice loss would act in a linear fashion to temperature rises, once these are slowed down and controlled to no more than 2 degrees of warming on average [the official UN climate target], part of the Arctic summer sea ice could recover.
As there is such a large thermal inertia at play in the Earth’s climate system [delay between emitting a CO2 molecule and optimum of warming suggested to be more than 40 years] emission reduction would have to be implemented fast, to achieve any results.
In case of achieving the 450 Scenario, that for instance requires all industrialised nations to reduce their GHG emissions by 25-40 percent between 1990 and 2020, Arctic summer sea ice would recover from around 2035 -quite sharply- and establish a new equilibrium state at around 2.5 million square kilometres of ice, still a loss of almost 2 million compared to the current situation.
It is yet unclear whether such an ice extent could sustain the Arctic ecosystem. For polar bears much depends on having excess to the summer ice from a solid coastline. Different populations could still become isolated with increased risk of extinction over time.
It is also important to note that whereas albedo feedback was not presumed to cause a melting tipping point, others were not examined, most notably the Arctic feedbacks that act via the biosphere and the global climatic energy balance, like tundra methane emissions and taiga biomass degradation. Add a dramatic increase of CO2 and methane emissions to the albedo declines of sped up Arctic ice and snow cover losses and you may still witness a runaway situation.
© Rolf Schuttenhelm | www.bitsofscience.org