Clouding the forecast: Why so many climate models are wrong about rate of Arctic warming - Phys.org - Earth Sciences

The Arctic is one of the coldest places on Earth, but in recent decades, the region has been rapidly warming, at a rate three to four times faster than the global average. However, current climate models have been unable to account for this increased pace. Now, researchers from Kyushu University have reported in a study, published April 29 in Ocean-Land-Atmosphere Research, that clouds may be to blame. Credit: 2012 RUSALCA Expedition, RAS-NOAA | Kate Stafford

The Arctic is one of the coldest places on Earth, but in recent decades, the region has been rapidly warming, at a rate three to four times faster than the global average. However, current climate models have been unable to account for this increased pace.

Now, two researchers from Kyushu University—graduate student Momoka Nakanishi, from the Interdisciplinary Graduate School of Engineering Sciences, and her advisor, Associate Professor Takuro Michibata, from the Research Institute for Applied Mechanics—have reported in a study, published in Ocean-Land-Atmosphere Research, that clouds may be to blame.

The most common clouds found in the Arctic are mixed-phase clouds, which contain both ice crystals and supercooled liquid water droplets. In the Arctic summer, when the sun shines around the clock, these clouds act like a parasol, reflecting sunlight back into space and providing a cooling effect.

But in the long, dark Arctic winter, when there's no sunlight to reflect, these clouds act more like a blanket, trapping heat radiated from Earth's surface and sending it back down to the Arctic's surface.

"However, how well these mixed-phase clouds trap heat depends on their ratio of ice to liquid," explains Nakanishi. "The more liquid water the clouds contain, the better they are at trapping heat. But many climate models have a large bias in representing this ratio, causing incorrect predictions."

In this study, Nakanishi and Michibata analyzed 30 climate models and compared them to satellite observations of clouds in the Arctic during winter over the last decade. They found that 21 of the 30 models significantly overestimated the fraction of ice to liquid in wintertime Arctic clouds.

"These ice-dominant models are not properly accounting for the present-day warming potential of the clouds during the winter," says Nakanishi. "That's why they cannot account for the rapid warming we are currently seeing." 

In the present climate, mixed-phase clouds have a slightly larger ice-to-liquid ratio. As the climate warms, more ice shifts to liquid, increasing the amount of heat emitted from the clouds, leading to an increased warming effect. However, in the future, clouds will contain enough liquid that the heat emitted reaches a maximum, stabilizing the warming effect. Credit: Takuro Michibata/ Kyushu University

However, every cloud has a silver lining. While climate models are underestimating the rate of global warming in the present day, they are overestimating the rate of global warming in the future.

The errors in future projections are due to a process called "cloud emissivity feedback." In a nutshell, as the Arctic warms, clouds shift from containing mostly ice to more liquid, which increases their ability to trap heat, further warming the Arctic and creating a positive feedback loop.

But importantly, this feedback loop has a time limit. Once clouds become so rich in liquid that they behave like blackbodies—fully absorbing and re-emitting heat—further warming has less effect.

However, because many climate models underestimate how much liquid is already present in today's clouds, they assume a larger shift still lies ahead. As a result, they overestimate how much extra heat-trapping will occur in the future, and predict the feedback effect will last longer than reality suggests.

Moving forward, the study's findings could be used to refine climate models so that they provide a more accurate representation of the ice-to-liquid ratio within clouds and better predictions of current and future rates of Arctic warming.

Since the Arctic's climate also plays a key role in shaping weather patterns further south, these findings could also lead to more accurate forecasts of extreme weather in mid-latitude regions.

"The biggest uncertainty in our forecasts is due to clouds," concludes Michibata. "Fixing these models is essential not just for the Arctic, but for understanding its impact on weather and climate change across the globe." 

by Kyushu University

edited by Gaby Clark, reviewed by Robert Egan

Source: Clouding the forecast: Why so many climate models are wrong about rate of Arctic warming