Clouds halve the climatic effect of bare ocean
Dwindling sea ice in the Arctic is given a key role in climate change and is feared as a driver of global warming. New research in Tromsø shows that clouds halve the climatic effect of the disappearing ice.
Climate scientist Stephen Hudson at the Norwegian Polar Institute has studied the net effect of the severe retreat of the ice around the North Pole.
The basic principle is that the snow cover on the ice reflects between 80% and 90% of sunlight, while the dark ocean without ice cover reflects only 7% of the light. This gives a nine-fold increase in the energy that is absorbed by the sea if the ice disappears.
The ability to reflect sunlight is called albedo. When sea ice is replaced by open water, the albedo is reduced. The increased warming will lead to the melting of more ice, which further reduces the albedo in the Arctic Ocean. This situation is defined as a self-reinforcing feedback and is referred to by researchers as the SIAF (sea-ice-albedo feedback).
Hudson told NTB, “This effect is often mentioned by the media and in books that try to explain climate change. It can come across as the strongest driver of global warming since it is so often mentioned. Therefore, I wanted to find out if the SIAF really is so important for the climate, or if it is just really easy to explain.”
The SIAF is the only feedback mentioned by former vice president Al Gore in his well known climate book and film, An Inconvenient Truth, from 2006.
In an article in the reputable scientific publication Journal of Geophysical Research, Stephen Hudson calculates how large of an effect the albedo feedback mechanism has on global warming. Here he has presented a model that also accounts for the fact that cloud cover reflects more light than the black ocean that is taking over previously ice-covered areas.
Hudson’s calculations consider an Arctic Ocean that is ice-free for one month in summer and has less ice than today for the rest of the year. He compares this with ice conditions in the 1970s and 80s.
“My calculations show that the warming driven by the disappearance of the ice corresponds to 0.3 Watts per square metre, if you spread it evenly over the whole planet. If you do not consider the cloud cover that reflects more sunlight than dark ocean, the effect is nearly 0.6 Watts per square metre,” said Stephen Hudson.
Five times more from CO2
With that, the SIAF contributes about half as much (corrected for clouds) or the same amount (without correcting for clouds) to global warming as emissions of the potent greenhouse gas methane.
When the same calculations are done for the human emissions of CO2 up to now, we find that they have a warming effect of 1.6 Watts per square metre. In other words, CO2 emissions give 5.3 times more warming than a melting Arctic that makes the Arctic Ocean ice-free for one month in summer.
This has become a more and more likely scenario. But sea ice expert Stephen Hudson finds it difficult to predict when it will happen, telling NTB, “the biggest problem is that climate models have not predicted that the sea ice would retreat as quickly as it has.”