June 1, 2007

Warmer world gets wetter

Filed under: Global Warming — rdrutherford @ 5:45 pm

Warmer world gets wetter
Global warming will increase worldwide precipitation by three times the amount predicted by current climate models, according to a study based on two decades’ worth of satellite observations.

The discrepancy between the models and the data might mean that the models are wrong. Or it might be that two decades is not long enough to test their predictions. But researchers believe that the work is a step towards understanding the thorny issue of how global temperatures affect rainfall.

Warmer air holds more water. Satellite observations and climate models agree that each rise of 1 °C in global temperatures increases the amount of water vapour in the atmosphere by about 6.5%.

But climate models project that global warming will also bring weaker winds, leading to less water evaporating from the ocean and counteracting the effect of warming. Models predict that worldwide precipitation — which must match the amount of evaporation — will increase by only 1-3% for each degree of future global warming.

Uncertain forecast

To see how rainfall had changed with the 0.4 °C warming of the past 20 years, Frank Wentz and his colleagues at Remote Sensing Systems in Santa Rosa, California, analysed data collected by US weather satellites from 1987 to 2006. The satellites measure atmospheric water vapour, surface winds and precipitation.

They report in Science that the amount of water in the atmosphere, evaporation and precipitation all increased at the same rate, by about 1.3% per decade1 — or about 6.5% for every degree of warming. Surface winds increased, not decreased, with warming.

It is currently impossible to predict where additional precipitation will fall, says Wentz. Wet areas may get wetter, but drought-plagued regions might also get some relief.

The study is “a good first step” in the debate over future precipitation, says Gerald North of Texas A&M University, College Station, who studies rainfall patterns. But, he adds, the single 20-year data set is not enough to get a definite answer.

He also notes that it is difficult to measure evaporation from space — and even from the ground. “The satellite-inferred trends are not a measurement but an estimate with unknown and subtle error characteristics,” he says.

Wentz agrees that two decades’ worth of data are not definitive — but “it is all we have”. The accuracy of climate models’ predictions regarding precipitation has not been tested before, he adds.

John Bates, a remote-sensing expert at the US National Climatic Data Center in Asheville, North Carolina, says studying the global water budget is “notoriously difficult”, but the latest climate models may be better able to account for precipitation.

He is looking forward “to seeing how those models compare with the ever improving satellite observations”.


February 23, 2007

Antarctic waterworks revealed

Filed under: Global Warming — rdrutherford @ 12:34 am

Large lakes in Antarctica speed up ice flow to the ocean.
Olive Heffernan
There are more than 150 lakes under the Antarctic ice sheet.
Lakes under the Antarctic ice pack lubricate the flow of ice off the continent and into the ocean, researchers have found. The discovery has implications for our understanding of how the southern ice sheet will respond to climate change, and how this will contribute to sea-level rise.

Robin Bell of the Lamont-Doherty Earth Observatory in Palisades, New York, and her colleagues discovered four lakes beneath the ice in a remote part of eastern Antarctica. The lakes sit near the head of the Recovery ice stream, which every year pours 35 billion tonnes of ice from Antarctica’s interior to the Weddell Sea.

Upstream of the lakes, the ice flows at only 2-3 metres per year. Downstream, this leaps to roughly 30 metres per year, reaching hundreds to thousands of metres per year by the time it reaches the sea. The liquid water makes the ice slip faster over the rock beneath, the researchers report in this week’s Nature1.

“These lakes are like an engine battery driving the machine that moves ice to the ocean,” says Bell.

Icy landscapes

Bell’s team discovered the lakes and measured the rates of ice flow using data from lasers fired from satellites and radar measurements taken from the ground by an expedition that visited the area in 1964-66, the last researchers to do so.

Over the past 20 years, researchers have discovered more than 150 lakes beneath the Antarctic ice pack. The largest, Lake Vostok, has an area of more than 15,000 square kilometres, which is similar to that of Lake Ontario in Canada.

Two of the new lakes are the second and third largest known, and the four have a combined area similar to that of Lake Vostok. But lakes such as Vostok seem to be static and isolated from ice flow.

Watery ends

Meanwhile, a team led by glaciologist Helen Fricker of Scripps Institution of Oceanography in La Jolla, California, has found two large lakes beneath the Whillans and Mercer ice streams2. The streams carry ice from the West Antarctic Ice Sheet to the Southern Ocean.

Fricker’s team used laser measurements from satellites to find the lakes and track changes on the ice sheet’s surface from 2003 to 2006.

They were, she says, “amazed” to see that in the time the height of the ice sheet decreased by about 9 metres — about the height of a three story building.

The changes were caused by lakes filling and draining, as water flowed beneath the ice sheet from one area to another. Full lakes show up as huge, unusually flat blisters on the surface of the ice sheet.

“This is the first time that an interconnected system of reservoirs has been discovered in Antarctica underneath the ice stream,” says Fricker. The system of cascading pools and flowing rivers is an active part of the ice stream, which moves about 500 metres a year in this region.

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The presence of so much liquid water under the ice sheets could change climate predictions, says Martin Siegert, of the University of Edinburgh in Scotland. “We need to reconcile this with our climate models to understand how the Antarctic will behave in the future,” he says. “The Antarctic is hugely important and virtually unknown. We need to understand how water under the ice increases ice flow.”

If the Antarctic ice sheet melted completely, the sea level would rise by 70m. The recent report from the Intergovernmental Panel on Climate Change emphasized the importance of rising sea level with global warming. It also highlighted how far we are from predicting this accurately.

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