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EOS Study Ends Decades-Long Controversy Over Amazon Climate

12 Jan 2017

An Earth Observatory of Singapore study published in Nature ends a long-standing debate among climate scientists about the Amazon rainforest’s rainfall over the past 45,000 years. The study also gives a glimpse of the Amazon’s future in the face of a changing climate. 

The largest rainforest on Earth, the Amazon basin is twice the size of India and hosts some of the richest biodiversity on the planet. The Amazon is an important player in climate: its trees cycle through billions of tonnes of atmospheric-warming carbon each year. Besides helping to mitigate global warming, the rainforest also helps supply water to Brazil and other countries in the region. But as the Amazon shrinks from logging and other human activities, climate scientists wonder: if it survives us, will it also survive climate change? Or will it turn into grassland and add more carbon to the air than it breathes in?

Map of the Amazon Basin and Caverna Paraíso, also known as Cave Paradise, the cave site that Asst. Prof Wang Xianfeng and his colleagues studied. Located between Belém and Manaus, in the middle of Amazon jungles, the research team used the speleothem samples collected from the cave to rebuild the Amazon's rainfall history. (Source: Wang Xianfeng)

Because of ever-rising concentrations of greenhouse gases in the atmosphere, it’s a given that the tropics will become warmer in the future. But it’s still hard to predict how global warming will affect rainfall. By looking at markers of past climate such as ice cores, climate scientists hope to better predict future climate. However, the Amazon poses a challenge because there aren’t as many obvious signs of climate change over its long history.

With the help of a cave deep in the Amazon, Assistant Professor Wang Xianfeng and his colleagues could start learning about the past. Analysing cave deposits called speleothems, Asst Prof Wang, the lead author of the study, measured isotopes of oxygen and carbon to rebuild the histories of rainfall as well as vegetation in the Amazon lowlands. The deposits provided the first detailed history of rainfall during the past 45,000 years, in periods of relative cooling and warmth.

The Amazon rainforest has thrived in the Amazon Basin for millions of years, surviving the harsh conditions during the glacial times. The fate of the rainforest is, unfortunately, now under threat because of the impact from human activities. (Source: Augusto S. Auler)

Asst Prof Wang found that during the last ice age (around 21,000 years ago), rainfall was only about 60% of today’s rainfall. Yet, the Amazon remained a forest, aided by a cool temperature. He also found that during a warmer period about 5,000 to 6,000 years ago, rainfall was about 40% higher. In general, changes in rainfall largely tracked with temperature change: higher temperatures had more precipitation, and vice versa.

According to the accompanying News & Views article about the study, these results are consistent with previous climate data from Amazon lake sediments. But Asst Prof Wang’s new research fills in the very first dated records from the heart of the Amazon, as well as the gaps in its climate record. It also shows once and for all that the Amazon remained a living forest during the last ice age despite lower rainfall.