The trees’ lessons: climate records are written in tree rings

I study the relationship between global warming and rainfall. We can learn so much from trees about how the climate is changing. With the right technology, we can hear what they are telling us about the climate thousands of years ago.

Climate change will almost certainly alter the annual monsoons that three billion people in Asia rely on for their food production. Climate change is expected to intensify monsoons, increasing flooding, and lead to fluctuations in when they start: a delay of even two weeks can hugely affect crop yields. My goal is to learn as much as possible from past monsoons by studying oxygen isotopes — different forms of the element — in tree rings. Oxygen isotopes allow us to determine how much rainfall a tree experienced during the growing season. As a result, we can identify periods of heavy rainfall or drought throughout time.

Collection: Fieldwork

We need evidence to ‘crossdate’, or verify, the age of each tree ring. Tree-ring widths are affected by a variety of environmental factors. In northern latitudes, scientists can crossdate tree rings on the basis of their width, because cold temperature limits tree growth. But in tropical areas, which have warmer temperatures and ample rainfall for tree growth, tree-ring widths do not reflect monsoon precipitation, so we use oxygen isotopes to track monsoon rainfall patterns.

So far, I have built a 700-year record of droughts in southwest China. Before global warming started in the mid-nineteeth century, drought were very similar to each other. But once the planet started to warm, notably in the past 50 years, there have been bigger and more-frequent droughts.

In the above image, I’m in the forest in southwest China, extracting cores from trees to analyse their rings’ oxygen isotopes. Sampling conifers is quick and easy, but hardwood oaks and other trees require many people. Luckily, when I’m in a forest, surrounded by trees, I feel very comfortable and relaxed.