Amazon droughts reducing forest’s CO2 absorption: NASA

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

• 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 in the US, and other institutions used satellite data to map tree damage and mortality caused by a severe drought in 200s. 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. Scientists estimate that it absorbs as much as one-tenth of human fossil fuel emission during photosynthesis.

Vulnerable Ecosystem

• 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.

• If droughts continue to occur with the frequency and severity of the last three events in 2005, 2010 and 2015, 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.

• Even if trees eventually survive defoliation, this damages their capacity to absorb carbon while under stress. Observers on the ground also notice that drought 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, any observer on the ground can monitor only a tiny part of the forest. These are only about hundred plots used for research and a few tower sights for long-term monitoring of the Amazon forests.

• The research team used high-resolution maps derived from the Geoscience Laser Altimeter Systems aboard the Ice, Cloud, and Land Elevation Satellite. 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.

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