Analyzing tree rings and matching them with previous work, they found what they consider to be compelling evidence a long dry spell in the Southwest stretching through the first four centuries of the Christian era and punctuated by more acute dry stretches.
Scientists already knew the Southwest was prone to long, severe megadroughts far worse than anything that has happened there since European settlement of the region began. Some of the same scientists involved in this work helped document such a drought in the medieval era. And some previous research had already pointed toward the likelihood of a second-century megadrought. But with the fresh evidence, the Arizona scientists believe the case is nailed.
Evidence of the causes is of course quite thin, but the scientists speculate that one factor may have been a broad pattern of warmth in the Northern Hemisphere, possibly caused by an uptick in the sun’s energy output. This warm spell is often called the “Roman Warm Period,” and while it has not been definitively proven to exist, evidence for the idea is growing.