Rain, Drought, Extreme Weather
A warming atmosphere also shifts storm tracks, altering rainfall, drought, and other weather patterns. For example, a 2004 study by the National Center for Atmospheric Research found that the percentage of Earth’s land experiencing serious drought had more than doubled since the 1970s.
From Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. FAQ 3.2, Figure 1, Cambridge University Press.
While observations are limited over much of Central and South America, the data indicate precipitation has increased in the southern part of the continent (with the exception of the west coast).
Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental. Panel on Climate Change. Figure 3.13, Cambridge University Press.
However, drier conditions have occurred in eastern Brazil and parts of Colombia, Ecuador, and Venezuela. During the past century the Amazon experienced slightly less rain, particularly in the north. This may be bad news for the health of the forests.
What do you think?
Did global warming contribute to the Amazonian drought of 2005? (Choose the best answer.)
The best answer is “Maybe.”
It generally isn’t possible to attribute any single weather event to one cause. Climate change is more easily detected over long periods using lots of data. However, we can say that all weather today is happening in the background of a warming world and increasing droughts are consistent with what might be expected.
Researchers found that the 2005 drought stressed the Amazon forests, killed trees and released a lot of carbon into the atmosphere as a result. In normal times, the Amazon absorbs about 2 billion tons of carbon dioxide, but in 2005, the forests lost more than 3 billion tons, leaving a net of 5 billion extra tons of CO2 in the atmosphere—more than the European Union emits from all sources.
An experiment in Tapajó National Forest, in Pará State, Brazil, confirmed these results. Scientists there covered a section of the forest with plastic panels for three years and found the soil dried down to 11 meters. It also killed many big trees, exposing the center of the forest to further heat and drying, and reduced wood production by one ton per hectare per year. Holes in the forest encourage the growth of plants that prefer more light.
Together with drought, this increases the risk of fires, which releases carbon to the atmosphere, opens the forest canopy even more, and can reduce rainfall nearby.
In some places, the forest becomes savannah with plants that are not as good at absorbing carbon dioxide. If such conditions become more frequent due to climate change throughout the Amazon basin, rainforest trees will make much less wood from atmospheric carbon dioxide, seriously worsening global warming.
Paradoxically, more water vapor held by a warmer atmosphere also leads to heavier rains and more snowfall. In particular, heavy rains are increasing in temperate zones.
Heavier rains increased significantly in eastern South America between 1900 and 2005, particularly in southern Brazil, Paraguay, Uruguay, the Argentine Pampas, and some parts of Bolivia. These increases have reduced crop yields and promoted floods. For example, there has been a 10% increase in flood frequency in the Amazon River at Obidos and increases of up to 50% in stream flow in the rivers Uruguay, Paraná, and Paraguay. Scientists have also measured more deaths and illness due to flooding, landslides, and storms in Bolivia.
From Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Figure 3.39, Cambridge University Press.
In addition, South America has seen highly unusual extreme weather in recent years—droughts in the Amazon, hail storms in Bolivia and Argentina, and the unprecedented Hurricane Catarina in the South Atlantic in 2004. There were also more forest fires and more and longer-lasting El Niños. Scientists have calculated that climate-related disasters have doubled in the last several decades.
