Clouds could be contributing to the shrinking of the Greenland ice sheet, a new study suggests. The melting of this ice sheet, which is about 1.7 million square kilometers in size, has contributed significantly to the rise in global sea levels in recent years.
Scientists monitored the effects of cloud cover on the melting and refreezing of the Greenland ice sheet using both satellite data and ground-based observations from 2007 to 2010. Special tools aboard the satellites were used to measure what is known as the “cloud radiative effect,” which is the difference in radiation present in cloudy skies compared to clear skies.
The team concluded that cloud cover significantly decreases the amount of radiation loss over the Greenland ice sheet meaning that more heat is being trapped. There wasn’t a noticeable difference in the amount of snow and ice that melted on a cloudy day compared to a clear one, but at night, the presence of clouds decreases the refreezing of this snow and ice. Normally, night is when melted water refreezes back into ice, but the heat trapped by clouds can prevent this. The water can then flow into the ocean increasing sea levels. A total increase of 2 to 3 degrees is occurring at night because of thick cloud cover.
“At night, clear skies make a large amount of meltwater in the sponge refreeze. When the sky is overcast, by contrast, the temperature remains too high and only some of the water refreezes. As a result, the sponge is saturated more quickly and excess meltwater drains away,” said Kristof Van Tricht, lead author of the study.
Under clear skies, about 58% of melt water refreezes. This number falls to 45% when cloud cover is present. This may not seem like a dramatic change but when considering the size of the Greenland ice sheet, the amount of water entering instead of refreezing the ocean can be large.
As climate change proceeds, the use of accurate models that can predict factors such as rising temperature and sea levels is becoming increasingly important. By better understanding the effect of cloud cover on the refreezing of water, more accurate models on the change in sea level can now be made. Without this knowledge, sea level models may have underestimated the rise in sea level in the future.
“With climate change at the back of our minds, and the disastrous consequences of a global sea level rise, we need to understand these processes to make more reliable projections for the future,” said Van Tricht.
The input of more greenhouse gases can also affect the composition and thickness of clouds, which could potentially increase the cloud radiative effect and further decrease the amount of water that refreezes at night.