Atmospheric rivers (AR) are large trails of condensed of water vapor that stretch for thousands of miles in the atmosphere. At any moment, there are between three to five atmospheric rivers in the Earth’s atmosphere, transferring water and bringing precipitation in regions far away from the AR’s source. They are usually found on the boundaries of neighboring weather zones.
The sources of atmospheric rivers are large forests, tropical rainforests in particular. Every tree in a forest acts like sort of a fountain. It pulls water from the ground and then releases the excess water through the pores in its foliage – in the form of water vapor.
With their millions of trees, it turns out that “rainforests” shouldn’t be called like that only because they thrive in the rainy tropical climate, but because they actually help deliver rain to the other parts of the world. Knowing that fact, it might seem like common sense to conclude that cutting down large tracts of forests can influence the atmospheric rivers and cripple their possibilities to transfer water vapor.
However, deforestation and changes in atmospheric rivers have been strangely overlooked in the conversation about climate change and increasing droughts worldwide.
A recent study by Michael Wolosin from the Forest Climate Analytics think-tank and Nancy Harris of the World Resources Institute warns that “tropical forest loss is having a larger impact on the climate than has been commonly understood”. Their study claims that large-scale deforestation in the biggest tropical forest zones: Amazon, Congo basin and southeast Asia, could disrupt the water cycle so much that it would affect agriculture in faraway places – including agricultural parts of the U.S., India, and China, the world’skey grain-producing regions.
In the background study prepared for the thirteenth session of the United Nations Forum on Forests titled “Forests and Water”, David Ellison touches upon the same issue, plus considers the local effects of deforestation.
It is well known that forests keep local environments cool; trees do this by casting shade on the ground, and also by releasing moisture from the leaves through the process of transpiration. Transpiration requires energy which is extracted from the air, thereby cooling it. A single tree can transpire hundreds of liters of water per day, and every hundred liters has a daily cooling effect equivalent to two domestic air conditioners.
We must also consider the fact that healthy forests are significant carbon dioxide sinksand the broader cooling effect ofvolatile organic compoundsreleased by the trees. When all these impacts are added, Wolosin and Harris conclude that the contribution of deforestation to global warming since 1850 is about 40%. A calculation by Natalie Mahowald of Cornell University and her team concludes present-day tropical deforestation could add 1.5° C (2.7°F) to global temperatures by 2100, even if we would manage to immediately switch from fossil fuels to other sources of energy.
It seems that along with forests, we are cutting down the global climate stability – and along with it our own food production, economy, and the civilization that depends on it.
Harris, N. Wolosin M. 2018. Tropical Forests And Climate Change: The Latest Science – Working Paper. World Resources Institute https://wriorg.s3.amazonaws.com/s3fs-public/ending-tropical-deforestation-tropical-forests-climate-change.pdf
Elison, D. 2018. Forests and Water. Background study prepared for United Nations Forum on Forests April 2018. https://www.un.org/esa/forests/wp-content/uploads/2018/04/UNFF13_BkgdStudy_ForestsWater.pdf
Mahowald N. et al. 2017. Are the impacts of land use on warming underestimated in climate policy? Environmental Research Letters. IOP Science https://iopscience.iop.org/article/10.1088/1748-9326/aa836d/meta
Pearce, F. 2018. Rivers in the Sky: How Deforestation Is Affecting Global Water Cycles. Yale Environment 360 https://e360.yale.edu/features/how-deforestation-affecting-global-water-cycles-climate-change
Welch, C. 2018. How Amazon forest loss may affect water—and climate—far away. National Geographic https://www.nationalgeographic.com/environment/2018/11/how-cutting-the-amazon-forest-could-affect-weather/