Southwest’s monsoon season may heat up with the climate

The summer monsoon that dumps rain on an otherwise-arid American Southwest may grow stronger as the climate warms, suggests a study of the region’s monsoon patterns of the last millennium.  

Across the Northern Hemisphere, monsoons — winds that change directions seasonally, altering rainfall — could intensify, the team reports May 27 in the Proceedings of the National Academy of Sciences.

The results complement recent observations and simulations of monsoon activity, says Pang-chi Hsu, a climate scientist at the University of Hawaii at Manoa, who was not involved in the work. “We do see enhanced Northern Hemisphere monsoons over the recent decades, from the 1970s.”

In many regions, farmers and others depend on summer monsoons to deliver more than half the year’s rain. The researchers can’t say whether stronger monsoons will result in more water overall for these areas, says coauthor Yemane Asmerom, a geochemist at the University of New Mexico in Albuquerque. Warmer air holds more moisture, which leads to more rain. But, he says, warmer temperatures also increase evaporation in dry places such as the Southwest.

One outcome of stronger monsoons could be fewer but heavier fits of rain, which could unleash flash floods, says climate scientist Andy Turner of the University of Reading in England. But local factors will probably cause regional differences in monsoon. For example, he says, the concentration of soot and other aerosols or land surface features might weaken monsoons in some areas, offsetting some of the strengthening caused by warmer temperatures.

In the new study, Asmerom and colleagues analyzed a stalagmite from Bat Cave in New Mexico’s Carlsbad Caverns. Such cave formations are archives of past climate. New layers grow each year as water dripping into the cave evaporates, allowing dissolved minerals to crystallize. In arid regions, the thickness of these layers corresponds with how much moisture was present, Asmerom says. The chemistry in the layers of these cave formations also reveals whether a year was relatively wet or dry.

To see how the North American monsoon over the last 1,400 years compared with monsoons elsewhere, the team looked at other cave formations and sediments that record changes in the East Asian and West African monsoons over the same period. Then they matched up monsoon activity with records of temperature in the Northern Hemisphere and North Atlantic Ocean. A striking pattern emerged: Over long timescales of multiple decades and centuries, all three monsoons behaved similarly. Warm temperatures made the monsoons wetter while cold temperatures dried them out. During an extended cold period known as the Little Ice Age, for instance, the entire Northern Hemisphere appears to have suffered a “superdrought” that lasted from 1350 to 1650, the team found.

But on shorter timescales, monsoon activity was more erratic from year to year. That’s because of regional differences and natural patterns of short-term climate variability, Asmerom says.

The researchers also found a connection between solar activity and the monsoons. Weaker monsoons and droughts often corresponded with periods of when the amount of solar radiation reaching the planet was low. The relationship between solar output, temperature and monsoon strength is probably a complicated one, Asmerom says. “How that whole thing works, we don’t know.” But, he says, compared with the sun’s activity, warming caused by high concentrations of greenhouse gases will likely have a larger effect on future monsoons.