The word "hysteresis" does not immediately look threatening; he hints in the "history" and "thesis" portraits ̵
Earth's climate system can give examples. Take the water circulation in the conveyor belt in the Atlantic Ocean. Looking back on the past, you can see times that the circulation seems to have turned an alternative model to the climatic consequences around the North Atlantic. Moving from one model to another makes a significant boost, but it's hard to reverse – like climbing to the top of the ridge and rolling in the next valley.
A new study, led by Tappi Schneider of Caltech, may have identified a disturbing hysteresis in Earth's climate – a change in the patterns of clouds in response to warming that can rapidly warm the planet. If we continued to emit more and more greenhouse gases, we would finally have done this experiment. (Let's not, please.)
The center of this drama is a kind of cloud. Slightly cloudy clouds usually cover about a fifth of low-ocean oceans. Most clouds are formed because the air, heated by the earth (or abused above the mountains), cools when it rises, condensing the water vapor to the droplets of clouds. The convection that raises their moisture is not driven by warming the bottom, but by cooling at the top.
The water in this cloud platform absorbs much of the infrared radiation radiating upward from the warm surface. The class of the cloud re-emits a certain radiance down, while others – in the cosmic space. The air above these clouds is drier and absorbs much less than the outgoing energy passing through it. This means that you can think of these clouds as the cooling fins of the radiator. They throw more heat up than they get from the atmosphere above them, allowing them to cool down from top to bottom. The cold air at the top of the clouds sinks, creating a convection contour that leads the water vapor from the sea surface to the deck of the cloud.
So what happens to this unique process in a warmer world? it was hard to say. The key processes in these cloud clouds happen on a much smaller scale than a cellular network in global climate models, so their physics is not directly simulated. Instead, we have a simplified mathematical position for their overall behavior. There is good reason to believe that this does not allow us to understand the details of how they will react to the ongoing global warming.
Nothing but a blue sky
To deal with this, Schneider and his colleagues turned things around. They use a model that can simulate these clouds in a small part of the atmosphere – considering a simplified version of the world around them. In particular, they simulated a stretch of subtropical ocean with straw-cloud clouds and a neighboring stretch of tropical ocean responding to global warming. They have done this for different greenhouse gas concentrations equivalent to 400 parts per million CO 2 (similar to today's) to 1600 parts per million.
Up to about 1,000 parts per million, there were no big surprises. Things got warmer about 4 ° C and numbers change for things like water vapor and cloud height. But the cloud deck seemed familiar.
At about 1,200 parts per million, however, the simulated clouds suddenly dissipated. And without this shadow reflecting sunlight, the world warmed up another 8 ° C .