NEW YORK, NEW YORK—Tree ring chronologies have been used to create a drought atlas of the Old World that reaches back more than 2,000 years. When combined with drought atlases of North America and Asia, also created by the Tree Ring Lab at Columbia University’s Lamont-Doherty Earth Observatory, scientists will be better able to pinpoint causes of drought and extreme rainfall in the Northern Hemisphere. “Climate variability tends to occur within patterns that span the globe, creating wet conditions somewhere and dry conditions somewhere else. By having tree ring-based hydroclimate reconstructions for three northern hemisphere continents, we can identify the responsible modes of variability,” climate modeler Richard Seager said in a press release. This information can help scientists understand climate conditions during historic famines, such as in 1741, when rainfall was well below normal during the spring and summer in Ireland, England, and Wales. It had been thought that an unusually cold winter and spring were to blame. Excessive rains beginning in 1314 also led to famine. To read about how climate change may have impacted Iron Age cultures in Wales, go to "Hillforts of the Iron Age."