Over the last half century, extreme droughts and heavy rain have become increasingly common in the countries that hug the Andes. The mountain chain, which stretches along the western coast of South America, blocks incoming moisture and wind from ocean currents. This drenches the land on the seaside and leaves the inland regions in long dry spells. We know that climate change may affect these currents, and we want to prepare for the shifts in weather. Computer models can help predict future events, but data about past fluctuations can make the models more robust and accurate. So in 2011, my team started creating an atlas of the region's floods and dry spells going back 600 years. We collected weather station logs, historical newspapers, and additional archival documents, but there were many gaps. So we went to nature's own database: We analyzed the rings of more than 13,000 trees from the Andes, drilling tiny cylindrical holes into the trunks and extracting samples of their cores. Each layer represents one year, and its size can tell us about conditions at the time. Wet seasons allow for growth spurts that create wider bands, whereas dry stretches produce much thinner ones.
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