La Paz, Bolivia (Photo Credit: Aldo C. P. G.)
Catastrophic drought is imminent for the capital city of Bolivia, according to new research into the historical ecology of the Andes from Florida Tech (USA) and The Open University (UK) published in the Global Change Biology journal. If temperatures rise more than 1.5-2 oC (3-5 oF) above modern averages, the Altiplano of Peru/Bolivia will become a desert-like setting which, in terms of the water supply and agricultural capacity for the two million inhabitants of La Paz, would be disastrous.
Led by Mark Bush, a climatologist at Florida Tech, the research team investigated a 370,000 year record of climate and vegetation change in Andean ecosystems, using fossilized pollen trapped in the sediments of Lake Titicaca, Peru/Bolivia. His team found that during two of the last three interglacial periods, which occurred between 130,000-115,0000 years ago and 330,000-320,000 years ago, Lake Titicaca shrank by as much as 85% and adjacent shrubby grasslands were replaced by desert conditions.
During each past warming event trees migrated upslope, just as they are doing today. However, as the climate kept on warming the system suddenly flipped from becoming wooded to becoming a desert.
The post-doctoral researcher on the project, and now lecturer at The Open University, William Gosling said, “the evidence from the fossil pollen is clear, in the past, when global temperatures were just a few degrees warmer than today, conditions around Lake Titicaca suddenly changed to a much drier state”
If current warming trends result in a similar pattern of vegetation and climate change the implications would be profound for the two million inhabitants of the Altiplano. Severe drought, and a loss of stored water in lakes, would reduce yields from an important agricultural region and threaten drinking water supplies. The research suggests that limiting wildfire would help to delay the worst effects of the drought.
Evidence for the flip is also documented in work by collaborators. Diatomologist, Dr. Sherilyn Fritz (UN-L) showed that during these warm episodes the algae living in the lake shifted from freshwater species to ones tolerant of salty water, while Dr. Paul Baker (Duke University) identified peaks of carbonate deposition. Both of these strands of evidence point to a sudden shallowing of the lake due to evaporative loss.
The environmental reconstruction shows that with moderate warming forests moved upslope, but as that warming continued a climatic tipping point was exceeded throwing the system into a new, drier state that halted forest expansion. The tipping point is caused by increasing the evaporative loss from Lake Titicaca. As the lake contracts the local climatic effects attributable to a large lake, a local doubling of rainfall, being the most important, would be lost. Such tipping points have been postulated by other studies, but this study actually allowed the researchers to define when the system will change. Based on the growth limits of Andean forests, the researchers were able to define that the tipping point was exceeded within a 1.5-2oC warming above modern conditions. Given an observed rate of warming in the Peruvian Andes of about 0.3-0.5oC per decade, the tipping point would be reached by c. A.D. 2040-2050.
For the full journal article http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2486.2010.02203.x/pdf
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