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C l imatewatch n Drought looms for La Paz: Results from a study investigating a 370,000-year record of climate and vegetation change in the Andes suggest that the Bolivian capital, La Paz, could suffer catastrophic drought within a few decades.

hen scientists affiliated with the Florida Institute of Technology analysed fossilised pollen trapped in sediments from Lake Titicaca, they found that during two of the past three interglacial periods, the lake shrank by as much as 85 per cent. In each case, steady warming caused trees to migrate upslope, as they are doing today, and then suddenly die out, with desert replacing the woodland. ‘The evidence is clear that there was a sudden change to a much drier state,’ said climatologist Mark Bush.

urther evidence from algae found in the lake and carbonate deposits also pointed to a sudden shallowing of the lake due to evaporative loss. An environmental reconstruction showed that evaporation of the lake was behind the sudden shift from woodland to desert. As the lake contracts, local climatic effects – notably a doubling of rainfall – that are the result of the presence of a large lake are lost.

he researchers used the growth limits of Andean forests to estimate that the sudden shift to desert would take place when temperatures were around 1.5–2°C warmer than modern conditions. As the Peruvian Andes are warming at around 0.3–0.5°C per decade, that suggests that the climate tipping point could be reached by 2050 or earlier. The resulting drought would be disastrous for the water supply and agricultural capacity of La Paz’s two million inhabitants, according to the scientists.

n Water warms ice sheets: Melt water flowing through ice sheets can greatly speed up the rate at which they heat, according to a new study.

onventional thermal models of ice sheets don’t take into account the warming action of water as it flows through the sheet, instead primarily considering heating via warmer air on the ice sheet surface. This sort of warming tends to be relatively slow, often requiring centuries to significantly warm the ice sheet.

ut as ice sheets flow towards the coast, they grate on the bedrock, causing crevasses and fractures to form in the sheet’s upper 30 metres. According to the new study, as melt water flows through these openings, it can create ‘caves’ and networks of ‘pipes’ that carry water through the ice and spread warmth.

In order to quantify this phenomenon, the scientists modelled the effect on an ice sheet’s temperature of water flowing through it for eight weeks every summer – the length of a typical active-melt season. The results suggested that the presence of water flowing through an ice sheet could significantly speed up warming. ‘We are finding that once such water flow is initiated through a new section of ice sheet, it can warm rather significantly and quickly, sometimes in just ten years,’ said the study’s lead author, Thomas Philips of the University of Colorado at Boulder.

everal factors contributed to the effect, including the fact that the water can remain in liquid form throughout the winter, which leads to a reduction in seasonal cooling.

Clouds get clearer new study has taken scientists a step closer to solving one of the central mysteries of future climate change: the behaviour of clouds.

he response of clouds to global warming is one of the most significant unknowns when it comes to modelling future climate change. Some models predict that increasing temperatures will result in an increase in global mean cloud cover, which will limit warming as increasing amounts of solar radiation reflect sunlight back into space. Other models predict a reduction in cloudiness and magnified warming.

In research that has been published in the Journal of Climate, researchers at the University of Hawaii at Manoa assessed the performance of current global models in simulating clouds. ‘All of the global climate models we analysed have serious deficiencies in simulating the properties of clouds in present-day climate,’ said the paper’s lead author, Axel Lauer. ‘It is unfortunate that the global models’weakness may be in one aspect that is most critical for predicting the magnitude of global warming.’

he researchers then created a model that represented a limited region of the atmosphere over the eastern Pacific Ocean and adjacent land areas. Clouds in this region are known to have a significant effect on present climate but have been poorly simulated up to now.

he model successfully simulated several key features of the clouds’ responses to present day climatic change, such as the influence of El Niño. When the researchers then looked at the model’s response to predicted global warming, they found that the tendency of the cloud cover to thin was more pronounced in their model than in any of the current global models.


shut terstock

14 january 2011

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