There's been another breakthrough in the study of the Greenland ice sheet, whose increasing melt rate and growing contribution to global sea-level rise has captured the attention and concern of climate scientists in recent years. While changes in air temperature, water temperature and precipitation are known to influence melting events on the ice sheet, a new study has identified another, perhaps less obvious culprit: clouds.
It's a finding that should be reflected in current climate models to help scientists make more accurate predictions about future Greenland melt and could become even more important in the coming years if cloud cover over the ice sheet were to increase as a result of climate change.
The study, published Tuesday in the journal Nature Communications, concludes that cloud cover can actually increase the amount of meltwater that runs off the surface of the glacier. Clouds have the effect of trapping heat on Earth; they can cause local temperatures to be warmer, so one would imagine that clouds might increase the amount of ice that actually melts during the day. But it turns out that the influence of cloud cover is strongest after the sun goes down. At night, the clouds actually prevent temperatures from cooling as much as they would on clear nights and keep already-melted ice from refreezing. This liquid water then pools on the surface of the ice and can be lost as runoff.
Currently, "even the best models out there disagree on cloudiness over Greenland and its effect on the ice sheet," said the new study's lead author, Kristof Van Tricht of the University of Leuven in Belgium, in an email to The Post. Until now, few studies have managed to address the effects of clouds on a Greenland-wide scale.
There are a number of difficulties that have prevented such research until now, said Ralf Bennartz, a professor of environmental studies at Vanderbilt University and co-principal investigator on the ICECAPS project in Greenland, who was not involved with this study. Trying to observe clouds over an ice sheet by satellite can be difficult because the clouds and the surface of the ice tend to look somewhat similar by air. Even using infrared sensing techniques has been challenging in the past because both clouds and ice cover in Greenland have similar temperatures.
"Over the last couple of years, there have been new observation techniques using RADAR and LIDAR," Bennartz said. "They give us a much clearer view. That is really what this article is using, these new and very accurate types of observation."
The new study combines satellite measurements with on-the-ground observations of clouds over Greenland. Then, the researchers put all the data into models to run simulations of how the ice sheet responds to different types of cloud cover.
The models suggest that both ice clouds and clouds containing liquid water have similar effects in that they both reduce heat loss over the ice sheet, causing a local warming effect when they are present. However, they also found that the amount of meltwater produced during the day is essentially the same whether it's cloudy or clear. It's what happens at night that really counts.
At night, when temperatures cool back down, a certain amount of the water that melted during the day tends to refreeze. According to the simulations, cloudy conditions keep the temperature from cooling quite as much, preventing as much water from refreezing as would happen under clear skies. The study suggests that in clear conditions, about 58 percent of meltwater refreezes at night, but only 45 percent does so when clouds are present.
The liquid water that remains can be partially recovered by the ice sheet by trickling into pores in the snow on the surface of the ice (although recent research has suggested that the availability of this pore space may be decreasing). The rest is lost as runoff. Altogether, recent observations of Greenland combined with the information from the models suggests that clouds may enhance meltwater runoff in Greenland by about a third annually.
That's already a substantial influence from the cloud cover currently observed over Greenland. But this amount could increase if nighttime cloud cover were to become more common in the future as a result of climate change. Recent research has suggested that warming patterns in the Arctic over the past decade have caused more water to evaporate into the air, leading to greater moisture content in the atmosphere and more cloud cover in the fall and winter, although the researchers also observed reduced cloud cover in the summer.
It's unclear whether such patterns will continue to become more pronounced in the future and they may not be the same in all regions of the Arctic. Currently, the authors of the new paper note that mean annual cloud cover over the Greenland ice sheet is about 67 percent.
Regardless, the results open up new doors for future research, the authors note. The findings are not necessarily that surprising "if you think about the process," Bennartz said. "I'm not sure though if anyone has ever articulated the process the way the authors have done this," he added. "And they also use this. . . model that substantiates their findings in a very nice way."
Now that scientists know clouds are such an important influence on the ice, one big problem is that this information is not fully reflected in current climate models, as Van Tricht noted. By conducting more studies on cloud cover over Greenland and applying the information to climate models, scientists can come up with more accurate predictions about how the ice sheet will respond under future conditions.
"This study shows very clearly for the case of the Greenland ice sheet that if we are able to better understand how clouds change as climate changes, we will be able to get much tighter constraints on Greenland surface melt and ultimately sea-level rise," Bennartz said.