Global warming is pushing high altitude clouds towards the poles, Nasa study finds

The article below is a combination of fact and theory.  The factual finding is that high altitude clouds have moved polewards over the last 30 years -- something that would help explain why Arctic temperatures are so erratic and often out of step with the rest of the world.  But WHY the clouds have moved poleward nobody knows.  They speculate, predictably,  that it is due to global warming -- but since we have had so little warming for most of the period concerned, that is just a statement of faith.  Models don't you tell you anything about the world.  All they tell you about is the assumptions of the modeller.  I like the model below a  lot better

And I don't even know her name

A new Nasa analysis of 30-years of satellite data has revealed high altitude clouds shifting toward the poles are being moved by the expansion of the tropics.

The changes could dramatically affect the planet's climate, expert warn. 

Where clouds are absent, darker surfaces like the ocean or vegetated land absorb heat, but where clouds occur their white tops reflect incoming sunlight away, which can cause a cooling effect on Earth's surface. 

Understanding the underlying causes of cloud migration will allow researchers to better predict how they may affect Earth's climate in the future, the researchers say.

They say the changes are driven by the tropics effectively becoming larger as the planet warms. 

George Tselioudis, a climate scientist at NASA's Goddard Institute for Space Studies and Columbia University in New York City, was interested in which air currents were shifting clouds at high altitude - between about three and a half and six miles high - toward the poles.

The previous suggested reason was that climate change was shifting storms and the powerful air currents known as the jet streams - including the one that traverses the United States - toward the poles, which in turn were driving the movement of the clouds.

Researchers analysed the International Satellite Cloud Climatology Project data set, which combines cloud data from operational weather satellites, including those run by the National Oceanic and Atmospheric Administration, to provide a 30-year record of detailed cloud observations.

They combined the cloud data with a computer re-creation of Earth's air currents for the same period driven by multiple surface observations and satellite data sets.

What they discovered was that the poleward shift of the clouds, which occurs in both the Northern and Southern Hemispheres, connected more strongly with the expansion of the tropics, defined by the general circulation Hadley cell, than with the movement of the jets. 

'What we find, and other people have found it as well, is that the sinking branch of the Hadley cell, as the climate warms, tends to be moving poleward,' said Tselioudis. 

'It's like you're making the tropical region bigger.' 

That expansion causes the tropical air currents to blow into the high altitude clouds, pushing them toward the poles, he said.

The results were published in Geophysical Research Letters, a journal of the American Geophysical Union.

Scientists are working to understand exactly why the tropics are expanding, which they believe is related to a warming climate.

The poleward shift of high altitude clouds affects how much sunlight reaches Earth's surface because when they move, they reveal what's below.

'It's like pulling a curtain,' said Tselioudis. And what tends to be revealed depends on location - which in turn affects whether the surface below warms or not.

'Sometimes when that curtain is pulled, as in the case over the North Atlantic ocean in the winter months, this reduces the overall cloud cover' in the lower mid-latitudes, the temperate regions outside of the tropics, Tselioudis said. 

The high altitude clouds clear to reveal dark ocean below - which absorbs incoming sunlight and causes a warming effect.

However, in the Southern Ocean around Antarctica, the high altitude clouds usually clear out of the way to reveal lower altitude clouds below - which continue to reflect sunlight from their white tops, causing little effect on the solar radiation reaching the surface. 

That information is a new insight that will likely be used by the climate modeling community, including the scientists who contribute modeling expertise to the Intergovernmental Panel on Climate Change, said Lazaros Oreopoulos, a cloud and radiation budget researcher at NASA's Goddard Space Flight Center in Greenbelt, Maryland, who was not involved in the study.  


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