The big melt: new data on ice caps

Loss of ice in Antarctica and Greenland has contributed about 11mm to the rise in global sea level since 1992 – about 20% of the overall figure

Meltwater rushes through a channel on the Greenland ice sheet, which is losing mass at an increasing rate

The past few weeks have seen the publication of several important studies about global warming, timed partly to coincide with the UN climate change talks in Doha. Most significant from the scientific point of view, was the achievement of a consensus on one of the most contentious issues in climatology: what is happening to the polar ice sheets.

It turns out that, taken as a whole, the Antarctic and Greenland ice caps are losing mass at an increasing rate, now equivalent to 344 billion tonnes of ice per year. The loss has contributed about 11mm to the rise in global sea level since 1992 – about 20 per cent of the overall figure. (The main contributor is the expansion of seawater due to warming.)

The consensus, achieved through an international collaboration between 26 laboratories and published in the journal Science, provides a firm basis for monitoring future ice melt and sea-level rise, which may accelerate further in the years ahead.

Greenland ice was already known to be melting before this Ice Sheet Mass Balance Inter-comparison Exercise (Imbie), funded jointly by the European Space Agency and its US counterpart Nasa. The big unknowns were in Antarctica, where some studies showed rapid ice loss while others suggested that increasing snowfall could be building up the ice sheet.

The view now is that the huge East Antarctic ice sheet is indeed growing slowly as more snow settles there – but that this growth is more than offset by losses in West Antarctic and the adjacent Antarctic Peninsula.

The Imbie effort reconciles three different ways in which scientists have measured change in ice sheets. Two use satellites: one by bouncing a radar or laser signal off the ice to measure its height; the second by measuring the gravitational pull of the ice mass to calculate its size. The third is an accounting method, which combines regional climate models and observations (for example, of flowing glaciers) to estimate gains from snowfall and losses from melting.

One big source of error and uncertainty, which the study ironed out, was “postglacial rebound” – the tendency of the earth to rise as less ice weighs down on it. Satellite observations must be corrected to take account of the fact that the underlying polar landscape is still rising by as much as a centimetre a year in places, following the end of the great ice ages many thousands of years ago.

“This project is a spectacular achievement,” comments Richard Alley, a climatologist at Penn State University, who was not involved in Imbie. “The data will lead to a better understanding of how sea-level change may depend on the human decisions that influence global temperatures.”


A guide to the coolest places on Mercury

Extraterrestrial ice has turned up in a surprising place: Mercury, the planet closest to the sun. Although most of the surface is extremely hot, a Nasa spacecraft has discovered hundreds of billions of tonnes of ice in Mercury’s shady polar craters.

Instruments on the Messenger orbiter, which reached Mercury last year, suggest that almost pure ice covers some of the coldest craters near the planet’s north pole. In slightly warmer areas, where ice would be unstable on the surface, larger glacial expanses are buried beneath an unusually dark material believed to include a mixture of complex organic compounds.

The Nasa researchers propose that both the ice and the dark covering came to Mercury during tens of millions of years of bombardment by asteroids and comets. A similar delivery mechanism may have brought water to Mars and to Earth’s moon, where spacecraft have also discovered icy deposits in the shade of craters near the lunar poles (although less than on Mercury).

Planetary scientists suggested decades ago that ice might be trapped at Mercury’s poles and in 1991 the Arecibo radio telescope in Puerto Rico detected bright patches that might have been ice – though they could also have been shiny minerals.

It took Messenger’s neutron spectrometer to confirm their glacial nature. In addition, the light and dark areas correlate with the planet’s topography and surface temperature, with ice clinging to the coldest spots.

Altogether, Mercury’s ice would cover an area the size of Washington DC with a layer two miles thick, estimates Messenger scientist David Lawrence.

“For more than 20 years the jury has been deliberating whether the planet closest to the sun hosts abundant water ice in its permanently shadowed polar regions,” says his colleague Sean Solomon. “Messenger has supplied a unanimous affirmative verdict.”


On your marks: a pygmy mole cricket prepares to leap

The pygmy mole cricket – a tiny African insect about the size of a rice grain – has evolved a unique way of jumping out of water, using specialised paddles on its hind legs.

On land, the cricket’s powerful hind legs can propel it more than a metre. But, as Cambridge university scientists have discovered, something more interesting happens when it finds itself in water and needs to escape quickly to avoid drowning or becoming fish food.

As its legs push down, spring-loaded paddles and spurs fan out – more than doubling the legs’ surface area. The paddles then snap shut to reduce drag and the insect jumps out of the water. If an aquatic leap 3cm long and 10cm high is not enough to reach safety, it can jump again.

“Other [aquatic] animals use surface tension, keeping a layer of air between their feet and the water. However, if their feet get wet, they are pulled into the water and drown,” says Cambridge zoology professor Malcolm Burrows. “Pygmy mole crickets turn the stickiness of water to their advantage and use this property to enable jumping.”

The study, published in Current Biology, started when Burrows was eating lunch near a Cape Town pond – and heard strange noises coming from the water. He saw insects jumping in a way new to him in 48 years of research. After collecting a few mole crickets and analysing them back in the lab with colleague Dr Gregory Sutton, he discovered their method of water leaping.


The body’s early warning system

Geneticists have known for decades that we all carry many potentially damaging mutations in our genome. Yet, most of them cause little or no ill effect. Now researchers at the Wellcome Trust Sanger Institute near Cambridge have listed and quantified all the harmful human genetic variants discovered so far.

On average, a normal healthy adult carries about 400 potentially damaging DNA variants. The vast majority of these are single “recessive” mutations that are unlikely to cause trouble, because a carrier only suffers when he or she inherits two faulty copies, one from each parent.

The study, published recently in the American Journal of Human Genetics, shows that about one person in 10 can expect to suffer some ill effect from known genetic variants – either by inheriting two copies of a specific recessive gene or from a single copy of a “dominant”gene.

The researchers anonymised the genetic samples selected for the study, so that participants would receive no personal risk information. But this raises an increasingly pressing issue for researchers in medical genetics, as they discover more disease-causing genes.

“Should incidental findings be fed back to people who have volunteered their sample to a study? There is no clear answer to this question,” says Chris Tyler-Smith, the lead author of the new study. “Early warning could be useful, but might still come as an unwelcome surprise to the participant.”

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