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| Last Updated:17/03/2020

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Amazon forests soak up more carbon than they emit, new Nasa study finds

NEW DELHI: A long-standing debate was resolved yesterday when a new Nasa-led study confirmed that natural forests in the Amazon remove more carbon dioxide from the atmosphere than they emit.

This means that the 5.5 million square kilometer forest is a crucial factor in reducing global warming. 

Living trees take carbon dioxide out of the air as they grow, and dead trees put the greenhouse gas back into the air as they decompose. Since there was no estimate of how many trees die of natural causes in the Amazon forest, scientists were unclear of the whole equation. The new study, published in Nature Communications on March 18, is the first to measure tree deaths caused by natural processes throughout the Amazon forest, even in remote areas where no data have been collected at ground level. 

The seven-year study was led by Fernando Espirito-Santo of Nasa's Jet Propulsion Laboratory and included scientists from US, UK, Brazil, Peru and Australia. 

Using new techniques to analyze satellite and other data, they found that each year, dead Amazonian trees emit an estimated 1.7 billion metric tons of carbon to the atmosphere. To compare this with Amazon carbon absorption, the researchers used censuses of forest growth and different modeling scenarios that accounted for uncertainties. In every scenario, carbon absorption by living trees outweighed emissions from the dead ones, indicating that the prevailing effect in natural forests of the Amazon is absorption. 

Until now, scientists had only been able to estimate the Amazon's carbon balance from limited observations in small forest areas called plots. On these plots, the forest removes more carbon than it emits, but the scientific community has been vigorously debating how well the plots represent all the natural processes in the huge Amazon region. That debate began with the discovery in the 1990s that large areas of the forest can be killed off by intense storms in events called blowdowns. 

Correlating satellite and airborne-instrument data with ground observations, Espirito-Santo and his colleagues devised methods to identify dead trees in different types of remotely sensed images. For example, fallen trees create a gap in the forest canopy that can be measured by lidar on research aircraft, and dead wood changes the colors in a satellite optical image. The researchers then scaled up their techniques so they could be applied to satellite and airborne data for parts of the Amazon with no corresponding ground data. 

This study looked only at natural processes in Amazonia, not at the results of human activities such as logging and deforestation, which vary widely and rapidly with changing political and social conditions.

The Times of India (19-03-2014)