By Richard Black
Environment Correspondent, BBC News website
Rising temperatures may partly explain increasing cases of malaria in regions of Africa, new research suggests.
The incidence of malaria is rising in parts of the East African highlands
Temperatures in East African highlands have risen by half a degree Celsius in the last 50 years, scientists found.
Writing in Proceedings of the National Academy of Sciences (PNAS), they say this small rise may have doubled the number of malaria-carrying mosquitoes.
Malaria has recently emerged in parts of the highlands, with climate change one possible explanation among many.
The new research relies on a fresh analysis of temperature data for four highland locations in western Kenya, southwestern Uganda, southern Rwanda and northern Burundi.
"Previous researchers had analysed the same sites and not found evidence for warming," said Mercedes Pascual from the University of Michigan in Ann Arbor, US.
"So we revisited it with a longer data set and longer analysis up to 2002," she told the BBC News website.
"We found there has been a rise of about half a degree Celsius over 50 years, but we see it mainly from the end of the 1970s to the present."
Fitting the pattern
In recent years malaria has emerged or re-emerged in several parts of the East African highlands, where temperatures are many degrees cooler than in low-lying areas.
Factors considered as possible explanations include increasing movement of people, rising resistance to anti-malarial drugs, and decreasing quality of health care.
Climate change has also been proposed as something that could affect the Anopheles mosquitoes which transmit the malaria parasite, either through higher temperatures or increased rainfall.
The Intergovernmental Panel on Climate Change, the global body which collates scientific information on the extent of climatic shifts and their impacts, said in its 2001 report: "Future climate change may increase transmission in some highland regions, such as in East Africa."
The new research integrates temperature changes into a computer program which models the mosquito population.
For a half-degree rise, the model predicts that mosquito numbers would rise by between 30% and 100%.
"The impact comes from the effect of temperature on their time to development," said Dr Pascual, "and then more weakly on the lifetime of the adults - but it's mainly the accelerated rate of development."
In lowland areas where mosquitoes are abundant and malaria endemic, such an increase would probably have a marginal effect on rates of infection.
But in highland areas, where the insects are much scarcer, it is likely to be a key factor affecting transmission rates.
"Our results do not mean that temperature is the only or the main factor driving the increase in malaria," concluded Dr Pascual, "but it is one of many factors that should be considered."