C L IMATEwatch

Decades of drought predicted for southwest USA

THE SOUTHWEST USA could be experiencing decades-long periods of drought in the not-too-distant future, according to new research led by scientists at the University of Arizona.

By reviewing previous research into the historical climate of the southwest USA, Connie Woodhouse and her colleagues were able to model future climate in the region and identify a plausible worst-case scenario that includes a 60-year drought. ‘We’re not saying future droughts will be worse than what we see in the palaeo record, but we are saying they could be as bad,’ Woodhouse said. ‘However, the effects of such a worstcase drought, were it to recur in the future, would be greatly intensified by even warmer temperatures.’

The worst warm-climate drought in the southwest within the past 1,200 years was 60 years long and occurred during the mid-12th century, according to research by one of Woodhouse’s co-authors, David Meko, and others. ‘Even without warming, if you had one of those medieval droughts now, the impact would be devastating,’ Woodhouse said. ‘Our water systems aren’t built to sustain us through that length of drought.’

The flow of the Colorado River (above) recorded at Lees Ferry between 2000 and 2009 is the lowest on record, and according to Woodhouse, the current drought could be part of a longer dry period. Other research has predicted that changes in atmospheric circulation will reduce the amount of winter precipitation that falls in the southwest, she said.

■ Greek reptiles offer glimpse of warming’s possible impact: A wave of reptile extinctions that took place on the Greek islands over the past 15,000 years may provide information about how plant and animal populations will respond to future global warming, according to a University of Michigan ecologist and his colleagues.

As sea levels rose in response to warming at the end of the last ice age, parts of the Greek mainland were cut off to form islands. The changing climate also caused forests to shrink as the area became more arid. Many of the reptile populations on the newly formed islands perished as a result of the restriction of their ranges and the changes in vegetation and climate.

In a paper published in American Naturalist, Johannes Foufopoulos and his colleagues used the presence or absence of 35 reptile species on 87 Greek islands that were once connected to the mainland to calculate the rates of extinction of the different species. They found that populations disappeared from the smallest islands – where habitat choices would be most limited – first, and that ‘habitat specialists’ were especially hard hit. Those whose distributions tended to be in cooler, moister, more northerly regions also tended to show higher extinction rates.

The researchers warned that the increasing fragmentation of natural habitats could drive a similar pattern of extinctions in the future. ‘The widespread fragmentation of natural habitats greatly exacerbates the effects of climate change and undermines the ability of species to adapt to new conditions,’ Foufopoulos said.

SHUT TERSTOCK

12 www.geographical.co.uk FEBRUARY 2011 C l imatewatch n Cloud feedback identified: Global warming will cause clouds to trap more heat, amplifying warming, according to a new study by a Texas A&M University researcher.

sing a decade’s worth of data from the Clouds and the Earth’s Radiant Energy System instrument on NASA’s Terra satellite, as well as meteorological analyses provided by NASA’s Modern Era Retrospective Analysis for Research and Applications and by the European Centre for Medium-Range Weather Forecasts, Professor Andrew Dessler calculated the amount of energy trapped by clouds as the climate varied.

e found evidence of a positive feedback, whereby for every 1°C of warming, clouds trapped an additional 0.5 watts per square metre. ‘It’s a vicious cycle – warmer temperatures mean clouds trap more heat, which in turn leads to even more warming,’ Dessler explained.

he study is the first to look at real-world observations of global clouds at low and high altitudes. The results were in broad agreement with those from most climate models.

‘Based on my results, I think the chances that clouds will save us from dramatic climate change are pretty low,’ Dessler said. ‘In fact, my work shows that clouds will likely be amplifying the warming from human activities.’

n Coral study reveals current changes: A study of growth rings in deep-sea corals has revealed drastic changes in oceanic currents in the western North Atlantic since the 1970s.

ater masses carry different ratios of two stable isotopes of nitrogen, depending on their origins. These signatures are recorded in the skeletons of deep-sea corals, and individual samples can be dated because the corals produce clear growth rings, providing a record of the currents flowing above and around the corals over time.

he results of the present study, which appeared in the Proceedings of the National Academy of Sciences, revealed that during the early 1970s, the North Atlantic Oscillation (NAO), a periodic variation of atmospheric pressure differences between the Azores and Iceland that influences oceanic currents in the North Atlantic, entered a ‘warm water mode’. In this mode, the influence of the cold Labrador current is diminished and the warmer Gulf Stream becomes dominant.

he change coincided with the recent rise in global temperatures and the researchers who carried out the study suspect that global warming is to blame. A similar analysis of fossil deep-sea corals from the same region revealed that the ocean currents had remained practically unchanged over the past 2,000 years, suggesting that a change in currents on this scale is unprecedented in recent times.

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DSplashdown

Extreme weather, not gradual warming, melting Greenland ice

The melting of Greenland’s ice sheets is apparently being driven by sudden changes in the volume of meltwater, rather than the slow increase in temperature, according to a University of British Columbia study.

Since the 1990s, the Greenland ice sheet has been losing about 100 billion tonnes of ice a year, a process that most scientists agree is accelerating. ‘The conventional view has been that meltwater permeates the ice from the surface and pools under the base of the ice sheet,’ said Christian Schoof, the study’s author. ‘This water then serves as a lubricant between the glacier and the earth underneath it, allowing the glacier to shift to lower, warmer altitudes where more melt would occur.’

owever, when Schoof modelled the complex fluid dynamics that occur at the interface of glacier and bedrock, he found that a steady supply of meltwater is well accommodated and drained through water channels that form under the glacier.

‘Sudden water input caused by short term extremes – such as massive rain storms or the draining of a surface lake – however, can’t easily be accommodated by existing channels. This allows it to pool and lubricate the bottom of the glaciers and accelerate ice loss,’ he said.

In brief

USA An 18-year survey of several hundred plant species in the Shaker Trace Wetlands in southwestern Ohio has revealed that 39 per cent are blooming earlier. The mean annual temperature during the survey periods has increased by almost 2°C over roughly a decade.

USA Forests in the southwestern USA are likely to suffer more severe and more frequent fires, higher tree death rates, heavier insect infestations and weaker trees if current climate predictions hold true, according to a study led by University of California, Santa Barbara researchers. The study of tree rings suggested that forests in the region were more sensitive to changes in temperature than those in other parts of the USA.

ARCTIC Stopping greenhouse gas levels increasing by 2020 could be enough to save the polar bear, according to a new study published in Nature. The modelling study suggested that mitigating emissions would lead to sea ice cover shrinking until 2020 and then recovering, and polar bear numbers declining only slightly.

GLOBAL A NASA modelling study has revealed a potential negative feedback effect in the Earth’s climate resulting from the increased growth of plants as CO2 levels increase. The cooling effect caused by plants growing more leaves would be –0.3°C if CO2 levels doubled.

INDIA Local experts and growers have warned that global warming is having a detrimental effect on the yields and flavour of Assam’s famous tea. According to the Tea Board of India, yields have dropped from 512,000 tonnes in 2007 to an estimated 445,000 tonnes in 2009, while growers complain that the tea has lost its ‘creamy and strong’ flavour.

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