Monday, 5 September 2016

How climate change in changing cloud cover

Climate change is messing with clouds – and it's a really big deal



24 August, 2016

Scientists have found that a warmer Earth is indeed pushing clouds upward and poleward – a response to global warming that climate models have predicted for some time, but we had difficulty detecting until now.

New research published last month in the journal Nature revealed the changes in cloud elevation and coverage after analyzing 30 years of satellite data.

A warmer Earth elevates clouds because the troposphere, the lowest layer of our atmosphere where weather occurs, can extend higher with a hotter surface. Warming also moves clouds poleward because circulation patterns in the tropics are expanding, pushing storms north and south.

But there’s a bigger issue at play here: These perturbed clouds may cause further warming, triggering a vicious cycle of increasingly rising global temperatures. That critical detail was often glossed over as news of the cloud changes spread last month.

As clouds move higher, they trap more heat

While clouds strongly reflect sunlight, as indicated by their bright white color, they also absorb the heat that radiates from Earth’s surface.

Anything that absorbs energy must also re-emit energy. How much is released depends on the temperature of the object.

Heat absorbed and then re-emitted by low clouds that are close to the ground is similar to the heat emitted by the surface because the temperature of the ground and the cloud are similar.

But the higher the cloud is in the sky, the colder it is. So when these high clouds absorb Earth’s heat, they re-emit it at a much lower temperature, forming a blanket that traps heat in the climate system similar to how greenhouse gases trap heat.

If climate change is causing clouds to form at higher elevations, as the science suggests, this phenomenon may thus be causing even more warming of the climate system.

Earth gets more sun as clouds move poleward

While high clouds trap heat, low clouds block sunlight from reaching Earth’s surface, keeping us cool.

The tropics get the most sunlight because of Earth’s orientation. As we move towards the poles, there is less and less sunlight reaching the surface.

Clouds are now moving poleward because of a northward shift in the storm tracks due to the expansion of circulation patterns in the tropics. As a result, these clouds are reflecting less sunlight back out to space than they did at lower latitudes because less sunlight is hitting them when farther north.

There is also more sunlight reaching the mid-latitudes, heating up the surface. This raises temperatures, especially in some arid parts of the world – causing additional warming of the climate system.

There’s only one conclusion we can draw from these alarming findings: We need to curb climate change, and fast.



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