Saturday round-up: Southwest Western Australia suffering unrivalled drought?

Tas van Ommen collecting an ice core at Law Dome in Antarctica Credit: Joel Pedro

Tas van Ommen collecting an ice core at Law Dome in Antarctica Credit: Joel Pedro

Australian researchers say that snowfall records in Antarctica show that the current drought south of Perth is unlikely to have been matched in 750 years, and possibly even longer. To reach this conclusion, Tas van Ommen and Vin Morgan used measurements of cylindrical samples of ice drilled from the Law Dome. This small icecap on the coast of East Antarctica is south of the south-west Western Australia (SWWA) region that has been affected by a 15-20% decline in winter rainfall since 1970. It’s hard to understand the drought’s full historical significance, due to the relatively small amount of direct rainfall information, say van Ommen and Morgan in a Nature Geoscience paper published online last Sunday. “Local records commence only around the start of the twentieth century,” they write.

Analysing ice core information to give weather patterns showed that declining rainfall in SWWA corresponded with increasing snowfall at the Law Dome. There is also a clear link between smaller changes in rain and snow at the two locations, the Australian Antarctic Division and Antarctic Climate and Ecosystems Cooperative Research Centre scientists say. So, the ice core data can tell us how unusual the SWWA weather is. The increase in snowfall at the Law Dome during the period of the drought is the largest in 750 years, van Ommen and Morgan find. They calculate that if environmental conditions had stayed the same, such a change in rain and snowfall should happen just once in at least 5,400 years. “The recent post-1970 anomaly in Law Dome precipitation lies outside the envelope of natural variability and supports the hypothesis of [man-made climate change],” the scientists write.

Meanwhile, amid controversy about how fast glaciers are receding elsewhere, another Nature Geoscience paper, published in the February issue, gives an improved estimate of the status of glaciers in Alaska. The collaboration of scientists from France, the USA and Canada suggests they are melting more slowly than previously thought, contributing around a third less to sea level rises than previously estimated. They say that thinning in other glacial regions could have been been overestimated by a similar amount.

The team’s calculations are based on the height of the glaciers, comparing data from 1962 maps with satellite maps from 2006. They say that using satellite data, rather than manual surveys as other groups have, gives them a more detailed picture of the situation. It also shows that if glaciers are covered with debris, they melt more slowly, as they are insulated from the warmth of the atmosphere. This is a factor overlooked by previous studies, they write. Their estimates suggest that between 1962 and 2006, Alaskan glaciers lost around 42 cubic kilometres of water per year, which raised sea levels by around 0.12 mm per year.