Climate Change and the Amazon Rainforest

Climate change may have a significant impact on the Amazon, according to a number of studies conducted since the mid-1990s.

Of particular concern is the link between sea temperatures in the tropical Atlantic and drought in the Amazon. As the tropical Atlantic warms, the large parts of the Amazon may see higher temperatures and less rainfall. The changes could have cascading effects on the region's ecosystem, killing trees and leaving forests more vulnerable to fire. Some models forecast a transition towards seasonal forests and savannas toward the end of the century, as climate warms.

2005: a peak at the future?

2005 is viewed by some researchers as a precursor to the impact climate change could have in the Amazon. In 2005 the Amazon experienced the worst drought in memory. As rivers dried up, remote communities were isolated while commerce slowed to a standstill. Thousands of square kilometers of land burned for months on end, releasing more than 100 million metric tons of carbon into the atmosphere.

At the time of the drought scientists observed an apparent correlation between sea surface temperatures in the Atlantic Ocean and precipitation in the Amazon. In 2008 Dr. Jose Marengo and colleagues from Brazil's Instituto Nacional de Pesquisas Espaciais (INPE) and the Instituto de Aeronáutica e Espaço (IEA), confirmed that "the 2005 drought was linked not to El Niño as with most previous droughts in the Amazon, but to warming sea surface temperatures in the tropical North Atlantic Ocean."

Rainforest in the Peruvian Amazon. Photo by Rhett A. Butler

How drought affects the Amazon

In 2009 a team of 68 researchers across 13 countries and 40 institutions presented an analysis of the 2005 Amazon drought. They found that rainfall-starved tropical forests lose massive amounts of carbon due to reduced plant growth and dying trees. The drought—and associated fires—resulted in a net flux of 5 billion tons of carbon dioxide (CO2) into the atmosphere — more than the combined annual emissions of Japan and Europe.

Analyzing data from 100,000 trees in 100 forest plots, the scientists found that a 100-millimeter (4 inch) increase in water deficit triggers the loss of 2.7 tons of aboveground forest carbon per hectare. Drought also affected the species composition of the forest. Some species, especially fast-growing, light-wooded trees, are particularly vulnerable to reduced rainfall.

"Amazon drought kills selectively and therefore may also alter species composition, pointing to potential consequences of future drought events on the biodiversity in the Amazon region," the authors wrote.

Small fires, big impact

Drought greatly increases the incidence of fire in the Amazon rainforest, an ecosystem unaccustomed to burning. Under dry conditions, small surface fires set by landowners clearing brush and vegetation can easily spread into surrounding forests areas, burning leaf litter and seedlings. While flames from these fires rarely reach above knee-height, they inflict considerable damage. Research led by Jos Barlow and Carlos A. Peres has shown that forests affected by these small fires on more than one occasion can experience almost a complete turnover in their species composition.

Future projections

Future projects paint a dire outlook for the Amazon between the effects of climate change and continuing deforestation. In 2008 Dan Nepstad and colleagues laid out a bleak scenario with 31 percent of the Amazon rainforest deforested and 24 percent damaged by drought or logging by the year 2030. The researchers estimated that a 10 percent drop in rainfall will result in drought damage to an additional 4 percent of the forests.

"The economic, ecological and climatic systems of the Amazon may be interacting to move the forests of this region towards a near-term tipping point," Nepstad and colleagues wrote. "In this scenario, the growing profitability of deforestation-dependent agriculture and cattle ranching provides an expanding frontier of forest fragmentation and ignition sources that inhibits rainfall as forests are replaced by fields and pastures and as fires fill the late dry season atmosphere with aerosols."

"Forests damaged by drought, logging, fragmentation and previous fire burn repeatedly as tall canopy tree species are gradually replaced by coppicing trees, grasses and other high-biomass plants. These local and regional processes are exacerbated when sea surface anomalies and extreme weather events cause severe drought episodes and the burning of vast forested landscapes. Global warming reinforces these trends by elevating air temperatures, increasing dry season severity and increasing the frequency of extreme weather events."