Earth has probably never warmed as fast as it has in the past 30 years. Now a study published on page 1179 of this issue tells us that it has probably never been as wet, at least not in the past millennium. Kerstin Treydte of the Swiss Federal Institute for Forest, Snow and Landscape Research in Birmensdorf, Switzerland, and her colleagues got their information from a knowledgeable source: trees that have been around long enough to witness the change.

“Our first question was: can we get precipitation information from tree rings?” says Treydte, who started working on the project in 1999, as a PhD student at the Research Center Jülich in Germany. Several studies had used tree rings to gain information about changes in temperature from year to year. But a relatively new technique of analysing oxygen isotopes in the rings promised information about the amount of snow or rainfall — a wetter year leads to wood that has less oxygen-18.

For three months in 2000, Treydte and colleagues camped out in the Karakorum and Himalayan mountains in northern Pakistan, an area known to contain old juniper trees. Using local shepherds as guides, they climbed different mountains and bored hundreds of pencil-like rods (called cores) through the trunks of trees. “We had worried a lot about where to look for trees and what instruments to use, but what turned out to be really critical to our work was to have local people helping us,” says Treydte.

The work was physically demanding. “You really have to love the mountains,” says Treydte. “I liked the place very much but in the end I was glad to go back.” Back home, the hard work was not over. In the lab, she spent two years analysing all the samples. For each core, she counted the number of rings, cross-checking with other cores from the same tree. She cut individual tree rings from sufficiently old trees and extracted cellulose from them, which she then analysed to determine what ratios of oxygen isotopes they contained. “It was not technically difficult work, but you had to be very accurate,” says Treydte. To her surprise, trees from one of the sites they sampled were more than 1,000 years old. “That was very exciting,” she recalls.

The following year was spent trying to make sense of the data, but it was not until she left for her postdoc in Switzerland that she started to apply the necessary statistical analysis. By obtaining records from several local weather stations, she and her colleagues were able to correlate precipitation records spanning 30 to more than 100 years with the tree-ring isotope data. They then extrapolated weather information for the entire millennium.

This study is the first millennium-long log of precipitation. It finds that the latter part of the twentieth century was by far the wettest period, which suggests that humans may play a role in the climate of the world.

But according to Treydte, one of the biggest pay-offs of the study was getting to know people from a different culture. “One day the locals had a party for us and gave us permission to collect a sample from a very large juniper tree in the centre of the village, which has a very special, spiritual meaning for them,” she says. When the team counted the tree rings, it turned out to be only 500 years old.

“We did not know how to tell them that, to us, this was a young guy,” says Treydte.