Warming link to cholera outbreaks

And the effect is getting stronger, they say, suggesting that climate change in recent years is having an effect.

The work is based on Bangladesh, where there is the clearest association between weather and disease levels - but scientists are hopeful that in future, they will be able to predict epidemics far more accurately.

The so-called El Nino Southern Oscillation (ENSO) happens approximately once every four years.

Warm waters

El Nino is a warming of water temperatures in the eastern Pacific Ocean which has a knock-on effect on weather patterns across large areas of the globe.

It can often bring disruptive weather - the 1997 to 1998 El Nino caused the disappearance of the monsoon in southeast Asia, and severe flooding in East Africa.

Scientists do not yet understand what drives this four-year cycle.

However, a team of researchers is now convinced that, particularly in recent decades, the arrival of ENSO also heralds an increase in cholera cases in Bangladesh.

This diarrhoeal disease is a leading cause of death in the country.

Strong links

The researchers, from the universities of Barcelona and Michigan, US, joining forces with others in Dhaka, Bangladesh, looked at weather patterns from 1893 onwards.

They found only weak links between El Nino and recorded cholera outbreaks up to 1980, but in the past two decades, the association has been "strong and consistent".

"This climate phenomenon accounts for over 70% of disease variance," they wrote.

Dr Jonathan Patz, from Johns Hopkins University in Baltmore, said that the study probably represented the first evidence that "warming trends" over the last century were affecting human disease.

Experts say it is hard to predict the severity of outbreaks of diseases such as malaria and cholera, because there are so many other factors apart from rainfall and temperature which might be having an influence.

However, experts say that in future, efforts to look ahead might be more successful.

Dr Menna Bouma, of the London School of Hygiene, told BBC News Online: "If you know that you are going to have an increased risk of disease, you can at least make sure that your systems are in place for dealing with that.

"Those sort of preparations save lives."