â€œThese future droughts are not only going to be bad compared to what weâ€™ve experienced over the historical period, but also really bad compared to the past millennium,â€ says Benjamin Cook, a drought researcher at NASAâ€™s Goddard Institute for Space Studies in New York City, who led the work. â€œItâ€™s going to be a pretty much fundamental shift.â€
Much of North America has a long and detailed climate history, thanks to tree rings that preserve records of temperature and rainfall. Many scientists have used these to piece together the story of decades-long droughts, like one that gripped the US Southwest in the thirteenth century and probably contributed to the disappearance of ancient Pueblo peoples. Others have used global climate models to study the regionâ€™s future, and found that it may already be transitioning to a fundamentally drier state.
Cookâ€™s team aimed to bridge past and present. The scientists compared 1,000 years of North American climate history with future projections from 17 different climate models â€” â€œas many as we could get our hands on that gave us the data we neededâ€, Cook says.
Among other metrics, the researchers looked at a measure known as the Palmer Drought Severity Index, which is an indicator of soil moisture. Some scientists criticize the Palmer index because it can overestimate future drying if it is calculated on the basis of temperature projections alone. To get around this problem, Cookâ€™s team used a different method of calculating the index, one that incorporates humidity and energy from sunlight.
Kevin Anchukaitis, a palaeoclimatologist at the Woods Hole Oceanographic Institution in Massachusetts, says that the revised method gives a much more accurate projection of how dry things will really get. â€œThis is the first convincing demonstration Iâ€™ve seen that it is both possible to seamlessly connect past, present and future, and to then be confident that they are on comparable scales,â€ he says.