The Amazon rainforest is a carbon sink, absorbing more CO2 than it emits (Picture: Getty)

Tropical forests lose their ability to absorb carbon when conditions become exceptionally hot and dry due to El Niño, research has revealed.

For decades, tropical forests have acted as a ‘carbon sink’ – taking more carbon out of the atmosphere than they release into it – a process that scientists say has moderated the impact of climate change.

But a new study has shown that in 2015-2016, when an El Niño climate event resulted in drought and the hottest temperatures ever recorded, South American forests were unable to function as a carbon sink.

El Niño occurs when sea-surface temperatures in the Pacific Ocean increase sharply, triggering a major shift in the world’s climate system.

In 2015-2016, the result was exceptionally hot weather for South America.

And a similar event is underway now.

‘Tropical forests in the Amazon have played a key role in slowing the build-up of carbon dioxide in the atmosphere,’ said study leader Dr Amy Bennett, a research fellow at the University of Leeds.

‘Scientists have known that the trees in the Amazon are sensitive to changes in temperature and water availability, but we do not know how individual forests could be changed by future climate change.

‘Investigating what happened in the Amazon during this huge El Niño event gave us a window into the future by showing how unprecedented hot and dry weather impacts forests.’

Humans rely on tropical rainforests to help absorb some of the carbon dioxide emitted by fossil fuels (Picture: Getty)

Dozens of short-term grants enabled more than 100 scientists to measure forests for decades across 123 experimental plots for the study, published in the journal Nature Climate Change.

The plots span Amazon and Atlantic forests as well as drier forests in tropical South America.

The tree-by-tree records showed that the majority of forests had acted as a carbon sink for most of the last 30 years, with tree growth exceeding mortality.

But when the 2015–2016 El Niño hit, the sink shut down because tree death increased with the heat and drought.

Professor Beatriz Marimon, of Mato Grosso State University in Brazil, said: ‘Here in the south eastern Amazon on the edge of the rainforest, the trees may have now switched from storing carbon to emitting it.

‘While tree growth rates resisted the higher temperatures, tree mortality jumped when this climate extreme hit.’

Of the 123 plots studied, 119 experienced an average monthly temperature increase of 0.5C. Ninety nine of the plots also suffered water deficits. Where it was hotter, it was also drier.

The researchers calculated that before El Niño, the plots were storing and sequestering around a third of a tonne of carbon per hectare per year.

That collapsed to zero with the hotter and drier El Niño conditions due to biomass being lost through the death of trees.

Fragmented areas of forest are more vulnerable to extreme temperatures (Picture: Getty)

The researchers say that the greatest relative impacts of the El Niño event were in forests where the long-term climate was already relatively dry.

The expectation was that wetter forests would be most vulnerable to the extreme drier weather, as they would be least adapted to such conditions.

However, the opposite was the case – forests more used to a drier climate at the dry periphery of the tropical forest biome turned out to be most vulnerable to drought.

That suggested some trees were already operating at the limits of tolerable conditions, according to the research team.

Professor Oliver Phillips, an ecologist at the University of Leeds who supervised the research, said the new findings offer hope about the resilience of the South American tropical nature.

‘The full 30-year perspective that our diverse team provides shows that this El Niño had no worse effect on intact forests than earlier droughts,’ said Professor Phillips. ‘Yet this was the hottest drought ever.

‘Where tree mortality increased was in the drier areas on the Amazon periphery where forests were already fragmented.

‘Knowing these risks, conservationists and resource managers can take steps to protect them.

‘Through the complex dynamics that happen in forest environments, land clearance makes the environment drier and hotter, further stressing the remaining trees.’

Professor Phillips added: ‘The big challenge is to keep forests standing in the first place.

‘If we can do that, then our on-the-ground evidence shows they can continue to help lock up carbon and slow climate change.’

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The great carbon sink stopped working in 2016.