May 28, 2017, by Tim Radford
The distant summit of Mount Vinson in Antarctica, rising to 4,892 metres, is higher than Mont Blanc (4,810 metres) in the European Alps. Image: Torch Magazine via Flickr
Computer simulation shows that Antarctica is warming far slower than the Arctic region because of its much greater height.
LONDON, 28 May, 2017 – Scientists believe they have settled one of the great polar puzzles − why Antarctica is warming at a rate so much slower than the Arctic region. And the answer is a simple one: Antarctica is so much higher.
To ram the point home, they used a computer simulation to hammer the entire southern continent until it was no more than a metre above sea level. At which point, in their simulation, warming at the South Pole became much more dramatic.
The two poles are very different: the Arctic is an ocean almost entirely surrounded by land, while Antarctica is a vast continent entirely surrounded by frozen ocean.
Landmass of ice
As the north polar ocean ice melts, dark seas begin to absorb more radiation. In the southern hemisphere, the landmass of ice reflects radiation back into space to insulate the continent and keep its temperatures far below freezing.
But the new study published in Earth System Dynamics journal shows that what makes the biggest difference is the elevation of the surface. Antarctica is not just an enormous continent: thanks to many millions of years of snowfall at very low temperatures, it is banked high with ancient ice.
Its average elevation is 2,500 metres − far higher than the highest peaks in the UK, for example − and its highest mountain, Mount Vinson, reaches 4,892 metres, which is higher than any alpine peak in Europe.
Evidence from the distant past and climate models both show that, in a warming world, the poles should warm faster than the rest of the planet. But while the Arctic is warming at twice the rate of the rest of the globe, change in Antarctica has been much more sluggish.
“On average, warming for the entire Antarctic continent has been much slower than Arctic warming so far,” says Marc Salzmann, a researcher at the Institute for Meteorology at the University of Leipzig in Germany, and author of the study.
“I wondered why some of the reasons to explain
Arctic warming have not yet caused strongly
amplified warming in all of Antarctica as well”
“Moreover, climate models suggest that, by the end of this century, Antarctica will have warmed less compared with the Arctic.
“I wondered why some of the reasons to explain Arctic warming have not yet caused strongly amplified warming in all of Antarctica as well. I thought that land height could be a game changer that might help explain why the Arctic has thus far warmed faster than Antarctica.”
Research like this is basic, rather than practical. Were Antarctica to melt at the rate of the Arctic, sea levels would rise dramatically and overwhelm all the world’s great coastal cities.
But this won’t happen in a hurry, because the southern continent is the coldest place on Earth, with a mean temperature in the interior of minus 57°C.
The icy surface, even at higher altitudes, does melt in summer, but freezes again. Although ice shelves may be responding to attrition from a gradually warming southern ocean, they still keep the land-based glaciers moving only very slowly.
But scientists don’t like it when the real world behaves in ways not matched in perfect theory. They itch to explain anomalies, even when the differences between the two poles are so obvious.
They have so far tried to establish a basic understanding of the astronomical principles behind climate change over the aeons − whether the Earth could never freeze entirely, and whether the greenhouse effect could make the planet intolerably hot, or even boil the oceans dry.
For Dr Salzmann, the difference between rates of change at the two poles was an anomaly that needed to be settled at a theoretical level.
He began his experiment with a computer model of the Earth’s system of heat distribution, and tested the impact of the kind of global warming that would follow if the ratios of carbon dioxide in the atmosphere doubled.
For most of human history, these have hovered around 280 parts per million, but in the 200 years since modern economies began exploiting coal, oil and natural gas, they have risen to 400 ppm, and global average temperatures have crept up by almost 1°C.
Dr Salzmann then tweaked his model to flatten Antarctica to a height of one metre above sea level, to match the elevation in the Arctic. In response, the simulated warming at the southern pole of the globe accelerated, and the difference between warming at the two poles diminished.
Significantly, too, the same change meant a shift in the way air is transported from the tropics to the far south.
“Assuming a flat Antarctica allows for more transport of warm air from lower latitudes,” Dr Salzmann says, “this is consistent with the existing view that when the altitude of the ice is lowered, it becomes more prone to melting.”
So, in the long term, this entirely theoretical study contributes to a better understanding of real world global warming. If ice on the southern continent melts and flows away, however slowly, then the average altitude of the continent will begin to drop.
In response, warming rates in the southern ocean will begin to accelerate, to increase the melt rate and amplify global warming yet further.
This would take thousands of years, but it could be the start of processes that would be very difficult to reverse. – Climate News Network
Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.