A tipping point for the Amazon rainforest?

Could human impacts be about to overtake nature in a forest the size of the continental U.S.?

The Amazon rainforest faces two major threats, 1) continued deforestation and 2) climate change. Researchers have long been aware of the potential for these two factors to reinforce one another (1), but a recent review in Nature (2) suggests that the Amazon may be nearing a tipping point after which the combined human impacts of deforestation/forest degradation and a warming climate will supersede natural environmental cycles in determining the structure of the Amazon ecosystem.

When we think of the Amazon rainforest we tend to imagine only the very wet, “high” rainforest or Hylea, but the drainage basin of the Amazon River is vast (6,916,000 km2) and covered by a variety of plant communities, adapted to differing climatic conditions. Broadly speaking annual precipitation follows a gradient from the constantly wet areas in the north-west to the drier and more seasonal south and east (Fig 1). The south-east region experiences an annual dry season and natural droughts occur as a consequence of the El Nino/ENSO climatic effects. The tree species here tend to be adapted to the lack of water during the dry season, but drought experiments conducted on forest plots have shown that several years of successive drought will overcome those adaptations and eventually lead to tree death (3). Over the next century the IPCC climate models predict a rise of 3°C in Central and northern South America and along with this an increase in the frequency and length of droughts, which may have grave consequences for Amazonian ecosystems and waterways.

Nature, 481: 321-328.

Figure 1. Precipitation gradient in Amazon basin. Adapted from

In fact we have already seen the damage that more frequent and severe drought can...