For decades loggers and ranchers have been clearing land in the Amazon rain forest and the Brazilian government has been carving it up with roads. As a result the forest has begun to dieback, which means the area of lush, green, wet tropical forest has begun to shrink.
Rain forests are always pretty wet, but they have dry seasons, and those dry seasons turn out to be a limiting factor on the survival of the plants and animals. As loggers reduce the number of trees that produce moisture to feed the gathering rains, the drier the dry seasons get and the longer they last. Lately dry seasons in the Amazon have gotten more severe and have put a crimp on the survival of many of the trees that form the forest canopy, which is the backbone of the rain-forest ecosystem.
As the dry season continues to lengthen, the plants draw more and more water from the soil, which eventually begins to dry out. The trees get stressed and begin to die. There is more fodder on the forest floor for wildfires. We saw this during the estimated twelve thousand wildfires that occurred in the Amazon during the drought of 2010. As the forest loses more and more trees, it loses its ability to feed the weather patterns with warm, moist air.
The tipping point comes when we can no longer stop this trend, and the rain forest trees reach a critical point where the entire region turns into something else entirely---something other than rain forest. What would that be, maybe a drier, deciduous forest, with a whole different set of flora? Or possibly the rain forest will turn into an open savanna, with mainly grasslands and sparse trees. Whichever state the Amazon flips into, it would be drier.
Scientists have known from research done in the 1980s and 1990s that the Amazon has a big effect not only on local climate but also on circulation patterns that reach far and wide. The Amazon is basically a big spot of wet tropics. Knock out the trees and lose that moist air, and the regional circulation pattern changes as well.
Air circulation is complex, but one basic fact of moisture is simple to understand; take moisture out of one part of circulation, and you increase it somewhere else. The Amazon supplies big masses of warm, moist air, moving upwards. Take that out of the map and replace it with a dry land surface and you're weakening the current flow pattern.
When you think about having to map out all the movng air on the planet in such a way, with land masses getting in the way and altering where the flow goes, and add in a jet stream, and storm systems, you begin to get a feeling for how interconnected the whole thing is.
What many scientists fear is a complete die-off of the forests and, in their place, new grasslands or praire.
In research meant to highlight how the destruction of the Amazon rainforest could affect climate elsewhere, Princeton University-led researchers report that the total deforestation of the Amazon may significantly reduce rain and snowfall in the western United States, resulting in water and food shortages, and a greater risk of forest fires.
An Amazon stripped bare could mean 20 percent less rain for the coastal Northwest and a 50 percent reduction in the Sierra Nevada snowpack, a crucial source of water for cities and farms in California. Previous research has shown that deforestation will likely produce dry air over the Amazon. Using high-resolution climate simulations, the researchers found that the atmosphere's normal weather-moving mechanics would create a ripple effect that would move that dry air directly over the western United States from December to February.
The rainforest serves as the source of one-fifth of all fresh water on the planet. Water condensation, evaporation, and transpiration over the Amazon are key drivers of the global atmospheric circulation, affecting precipitation across much of the Northern Hemisphere. Recent climate models indicate that deforestation has also had the effect of reducing precipitation as far afield as the lower Midwest of the United States.
Through an extraordinary process, recently unravelled by climatologists in the United States, Brazil and Britain, we now know that what falls as rain over the Amazon Basin is paralleled, three to four months later, in rain falling over the US corn belt during its spring and summer. 'Teleconnection' is the name given to the process that transfers energy and rain to the United States from Amazonia. Relatively slow-moving moist masses of air, taking some months to complete their journey, seek conduits in the atmospheric circulation system, pushing their way through mass-circulation systems such as the Hadley Cell and the high flying Easterlies. These waves of Amazonian air, fuelled by the water vapour they carry, then release their rain en route over the corn-belt regions of the US.
Let the forest wither away, or just cut it and burn it down, and the US as well as the world will suffer like no-one had ever imagined they would.
