More than ever, we understand that environmental changes in one part of the world can have ripple effects far away—and in ways which might seem unexpected. To wit: atmospheric scientists have just demonstrated that extreme deforestation in the Amazon rainforest could create droughts and massive reduction in snow pack across the western United States.
That's the bleak finding just published in the American Meteorological Society’s Journal of Climate. The researchers, led by David Medvigy of Princeton University, set out to uncover what might happen if the Amazon was “stripped bare.” They found that high levels of deforestation could lead to 20 percent less rainfall, not in South America, but in many regions of the United States. Washington, Oregon, Idaho, and California would all suffer—and we'd see a 50 percent reduction in snow pack in the Sierra Nevada. This would be on top of other changes in precipitation in the region caused by climate change.
Should this happen, the water availability in critical agricultural areas would be severely limited. After all, the prime agricultural land in the central valley of California is only as fertile as it is because of irrigation bringing in water from melting spring snow originating in the mountains to the east.
Such dramatic reductions in water would be the result of dry air over the Amazon being sucked northward by the same high-altitude winds that cause shifts in the El Niño system. The researchers explain:
[These] Rossby waves are instrumental forces in the Earth’s weather than move east or west across the planet, often capturing the weather of one region and transporting it to another. Because the Amazon pattern forms several thousand miles to the southeast from El Niño, the Rossby waves that put the rainy side of El Niño over southern California would instead subject that region to the dry end of the Amazon pattern. The pattern’s rainy portion would be over the Pacific Ocean south of Mexico.
The scientists clearly point out, though, that this is one possible outcome.
Medvigy says, “We don’t know what the world will be like without the Amazon. We know exactly what happens with El Niño. It’s been studied extensively. Our intention with this paper was to identify an analogy between El Niño and Amazon deforestation. There’s good reason to believe there will be strong climatic similarities between the two.”
It might seem like an exaggeration to model what might happen as a result of the Amazon becoming completely deforested. It is true that, while deforestation across the region does continue, clearing of land in Amazonia has slowed significantly in recent years, with the Brazil government in particular making great strides in reducing tree-felling.
As the climate continues to change, however, these scientists point out that some 85 percent of the Amazon forest could disappear, transitioning over years from the lush forest it is today into savannah.
This was the conclusion of the 2007 IPCC report on climate change, which said, “By mid-century, increases in temperature and associated decrease in soil water are projected to lead to gradual replacement of tropical forest by savannah in eastern Amazonia.”
More recent research mollifies this scenario though, showing that while rainfall patterns across this large swath of South America are likely to change over the 21st century, the changes will not be enough to create grassland where forest once was. Rather, what may happen is that the wet and dry seasons will become more pronounced than they are today—a clear-cut Amazon might very well mean a drier America.