Rainforests can make their own rain

Evaporating water from the southern Amazon makes the wet season begin earlier than it otherwise would

Clouds over the Amazon.
Clouds over the Amazon.
Center for International Forestry Research

Chances are that at some point in a school geography class you were told that rainforests only grow in tropical climates, but a recent investigation of water evaporation patterns in the southern Amazon provides evidence that the relationship goes both ways.

This discovery is important because it explains why the Amazon rainforest receives such an early wet season south of the equator, which is an enigma that has perplexed environmental scientists for years.

While there are two seasonal forms of wind that typically control the onset of the wet season in tropical regions (monsoonal winds and the Intertropical Convergence Zone), neither one affects the southern Amazon until months after the beginning of the wet season, which is currently in mid-October.

The new research, led by Jonathon Wright from Tsinghua University in China, provides an alternative explanation, suggesting that the onset of the wet season in the southern Amazon is caused by transpiration, the process of water evaporating from the leaves of plants.

This theory, published in Proceedings of the National Academy of Sciences, is supported by evidence in the form of water vapour data that was picked up by the Tropospheric Emission Spectrometer (TES) on NASA’s Aura satellite.

Specifically, the TES was used to examine the proportion of deuterium – a heavier isotope of hydrogen – in water vapour as the amount of this material is lower in vapour from the ocean than it is in the byproduct of transpiration.

The results of this analysis show that during the dry season, water from vegetation in the southern Amazon is pumped into the middle troposphere, where it can turn into rain and thus instigate an earlier wet season.

The work of Wright and his colleagues further highlights the tight-knit relationship between rainforests and climate, indicating that their fates are intertwined and that activities like deforestation could have a more widespread impact than previously estimated.

  1. http://www.pnas.org/content/early/2017/07/19/1621516114.abstract
  2. http://www.pnas.org/content/early/2017/07/19/1621516114.abstract
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