Widespread wildfires in places such as the Amazon Valley change large areas of one-time rainforest from carbon-dioxode “sinks” (absorbers) to “sources” that add more of this greenhouse gas to the atmosphere. Jennifer Balch describes this transformation in “Atmospheric Science: Drought and Fire Change Sink to Source,” as does R.J.W. Brienen et al. in “Long-term Decline of the Amazon Carbon Sink.” Essentially, once the ground dries, pollen and other noxious lifeforms may assume a larger role. NASA Earth Observatory has reported on this change in Australia as well in “Bushfires Menace Towns in Western Australia,” as has Robert Draper in his 2009 article, “Australia’s Dry Run.”
Rivers and lakes face loss of water because of increasing drought at their headwaters, among other factors. As precipitation patterns have become more extreme, monsoon rains have become more erratic, risking the economic livelihood of millions of farmers, especially in India and other nearby countries. This change is explored by Stefan Rahmstorf et al. in “Exceptional Twentieth-Century Slowdown in Atlantic Ocean Overturning Circulation” and by M.A. Srokosz and H. L. Bryden in “Observing the Atlantic Meridional Overturning Circulation Yields a Decade of Inevitable Surprises.” Similarly, “Increasing CO2 Threatens Human Nutrition,” by Samuel S. Myers et al.; “The Nile Delta’s Sinking Future,” by John Bohannon; “Anthropogenic Aerosols and the Weakening of the South Asian Summer Monsoon,” by Massimo A. Bollasina et al.; and “Aerosols Implicated as a Prime Driver of Twentieth-Century North Atlantic Climate Variability,” by Ben B. B. Booth et al. all explore variability in modern precipitation patterns.
Changes in the Arctic Oscillation in the North Atlantic Ocean may have particularly dramatic effects on the climate of nearby landmasses, as Chris Mooney describes in “Global Warming Is Now Slowing Down the Circulation of the Oceans—with Potentially Dire Consequences.” Region-specific analyses of this phenomenon include “The Weakening Summer Circulation in the Northern Hemisphere Mid-latitudes” by Dim Coumou et al. and “Increased Frequency of Extreme Indian Ocean Dipole Events Due to Greenhouse Warming” by Wenju Cai et al.
Other sources of change in ocean circulation that can have very severe influences on land climates are El Niño and La Niña, both of which change water temperatures in the equatorial Pacific Ocean. These fluctuations usually provoke changes in wind direction over the very high and very sharp Andes between Peru, the equator, northern Chile, and the Amazon Valley. El Niño favors heavy rain in the Amazon (where wind flows east to west) and a persisting dry wind over the Andes, which maintains a desert on South America’s west coast. La Niña changes the wind pattern west to east across the Andes, causing damaging rains to the west of the Andes and a drought-inducing, down-slope wind from the mountains to the coast. These changes can ruin the agricultural economies of both areas.
As Wenju Cai et al. examine in “Increasing Frequency of Extreme El Niño Events due to Greenhouse Warming,” these oscillations are themselves accelerating, which in turn amplifies their effect on the environment. Thiago F. Rangel looks more closely at the effects this has on animal and plant life in the Amazon in “Amazonian Extinction Debts,” while Magdalena Balmaseda et al. discuss this effect on overall ocean temperature in “Distinctive Climate Signals in Reanalysis of Global Ocean Heat Content.” Changing perspective somewhat, Nate Cohn considers the political implications in “How El Niño Might Alter the Political Climate.”