Worsening droughts in the Amazon - dubbed the 'lungs of the world' - are speeding up climate change, scientists have warned.

Trees are absorbing up to a tenth less carbon dioxide from the atmosphere during droughts and the forests actually emit more carbon than they capture.

The last dry spell in 2010 saw the forest release eight billion tonnes - as much as the annual emissions of China and Russia combined.

Worsening droughts in the Amazon - dubbed the 'lungs of the world' - are speeding up climate change, scientists have warned. Pictured is an view of a river boat trapped on the Lago do Cristo Reis during one of the worst droughts ever recorded on October 21, 2005 in the Amazon region in Brazil

In an average year the basin absorbs about 1.5 billion tonnes of CO2 from the atmosphere but prolonged dry spells are due to become more common.

The Oxford University study provides the first direct evidence of the rate at which individual trees in Earth's greatest rainforest 'inhale' carbon from the atmosphere during a severe dry spell.

It found photosynthesis - the process by which trees convert the greenhouse gas into energy - slowed down by 10 per cent over six months.

Dr Christopher Doughty said: 'Tropical rainforests have been popularly thought of as the 'lungs' of the planet.

The latest findings published in Nature confirm fears that the days of the legendary rainforest curbing the impact of rising emissions could be coming to an end. A researcher is pictured here measuring tree diameters in Tambopata National Park, Peru, soon after the 2010 drought

'Here we show for the first time during severe drought the rate at which they 'inhale' carbon through photosynthesis can decrease.'

The 2005 drought in the Amazon was described as a 'once a century' event - but five years later an even worse one followed raising concerns about the region's capacity to continue soaking up carbon dioxide.

The latest findings published in Nature confirm fears that the days of the legendary rainforest curbing the impact of rising emissions could be coming to an end.

The 2010 drought saw the Amazon River at its lowest levels for half a century with several tributaries completely dry and more than 20 municipalities declaring a state of emergency.

Both droughts were associated with unusually warm seas in the Atlantic Ocean off the Brazilian coast - the suspected result of manmade emissions - and suggesting drought years in the Amazon could become more common.

Some computer models of climate change project more droughts across the region as the planet warms - and a diminishing capacity to absorb CO2.

HOW THE SAHARA FERTILISES THE AMAZON WITH 182M TONNES OF DUST Last month, a Nasa satellite revealed the stunning link between the Sahara desert and the Amazon rainforest. For the first time it showed vast dust clouds travelling high in the atmosphere. Scientists have not only measured the volume of dust, they have also calculated how much phosphorus – remnant in Saharan sands from part of the desert's past as a lake bed – gets carried across the ocean from one of the planet's most desolate places to one of its most fertile. The data show that wind and weather pick up on average 182 million tons of dust each year and carry it past the western edge of the Sahara at longitude 15W. This volume is the equivalent of 689,290 semi trucks filled with dust. The dust then travels 1,600 miles across the Atlantic Ocean, though some drops to the surface or is flushed from the sky by rain. Near the eastern coast of South America, at longitude 35W, 132 million tons remain in the air, and 27.7 million tons – enough to fill 104,908 semi trucks – fall to the surface over the Amazon basin. About 43 million tons of dust travel farther to settle out over the Caribbean Sea, past longitude 75W. Advertisement

The three year study measured the growth and photosynthesis rates of trees at 13 rainforest plots of up to 500 trees across Brazil, Peru and Bolivia - comparing those affected by the strong drought of 2010 with unaffected plots.

The Amazon drought of 2010 occurred right in the middle of the study but only affected some parts of Amazonia.

Researchers found that while the rate of photosynthesis was constant among trees on plots unaffected by drought, rates on the six drought-affected plots dropped significantly - as compared with before the 2010 drought.

There was up to a trebling in some sites in numbers of trees dying in the years after the drought - making them no longer available to absorb CO2 from the air.

Dr Doughty said: 'This decreased uptake of carbon does not decrease growth rates but does mean an increase in tree deaths.

'As trees die and decompose the concentrations of carbon dioxide in the atmosphere will increase - potentially speeding up climate change during tropical droughts.'

The results support the findings of a paper published last year in Nature published last year based on aircraft data.

Professor Yadvinder Malhi added: 'These plots are our canaries in the climate change coal mine. As this study demonstrates they can give us important early insight into the actual mechanisms of how these complex forests are responding to extreme climates.