Washington: A NASA study has found that a single season of drought in the Amazon rainforest can reduce its capacity to absorb carbon dioxide for years after the rains return.

The study, published in the journal Nature, is the first to quantify the long-term legacy of drought in Amazon, the largest tropical forest on Earth.

Researchers from NASA's Jet Propulsion Laboratory (JPL) in the US and other institutions used satellite data to map tree damage and mortality caused by a severe drought in 2005.

In years of normal weather, the undisturbed forest can be a natural carbon ‘sink’, absorbing more carbon dioxide from the atmosphere than it puts back into it.

However, starting with the drought year of 2005 and running through 2008 - the last year of available data - the Amazon basin lost an average of 270 million metric tonnes per year of carbon, with no sign of regaining its function as a carbon sink.

At about 2.3 million square miles, the Amazon is the largest tropical forest on Earth.

Scientists estimate that it absorbs as much as one-tenth of human fossil fuel emissions during photosynthesis. "The ecosystem has become so vulnerable to these warming and episodic drought events that it can switch from sink to source depending on the severity and the extent," said Sassan Saatchi of JPL, who led the study.

If droughts continue to occur with the frequency and severity of the last three events in 2005, 2010 and 2015, Saatchi said, the Amazon could eventually change from a rainforest to a dry tropical forest.

That would reduce the forest's carbon absorption capacity and its biological diversity. "These are rainforests; the trees almost always have leaves. So the loss of leaves is a strong indication the forest is stressed," said Saatchi.

Even if trees eventually survive defoliation, this damages their capacity to absorb carbon while under stress.

Observers on the ground also notice that droughts tend to disproportionately kill tall trees first.

Without adequate rainfall, these giants cannot pump water more than 100 feet up from their roots to their leaves. They die from dehydration and eventually fall to the ground, leaving gaps in the forest canopy far overhead.

However, an observer on the ground can monitor only a tiny part of the forest.

There are only about hundred plots used for research and a few tower sites for long-term monitoring of the Amazon forests.

The research team used high-resolution maps derived from the Geoscience Laser Altimeter System aboard the Ice, Cloud, and land Elevation Satellite (ICESat).

These data reveal changes in canopy structure, including leaf damage and gaps. The researchers found that following drought, fallen trees, defoliation and canopy damage produced a significant loss in canopy height.

The most severely impacted region declined an average of about 0.88 metres in the year after the drought, they said. Less severely affected regions of the forest declined less, but all continued to decline steadily throughout the remaining years of the data record.