What a stalagmite from a Grand Canyon cave might tell us about climate change and groundwater

Summer monsoons in the Southwest are difficult to forecast with total accuracy, but the future of the temperamental rainstorms under climate change is an even bigger mystery.

The North American Monsoon, a regional weather system ranging from Central America to the southwestern United States, is an important contributor to water budgets in regional deserts, when torrential rains in the midst of the hottest point of the year sustain ecosystems. 

“The Southwest monsoon is really one of the hardest seasonal patterns to predict,” says Dan McEvoy, a regional climatologist for the Desert Research Institute, who specializes in extreme weather in the West.

“There can be a lot of variability from year to year. We’ve seen some very sharp extremes in the monsoon over the last five years. There isn’t a very clear picture on how it will change going into the future,” McEvoy said.

Some studies suggest the North American Monsoon will intensify, but may not necessarily come with an increase in total summer precipitation. Other studies suggest the region’s monsoon season is actually weakening overall.

Climate scientists say it’s likely summer rainfall events throughout the U.S. will intensify as the atmosphere warms due to increased greenhouse gas concentrations, but what the distinct regional manifestations of monsoons will be is still a big question.

Now a research team — led by UNLV paleoclimatologist and Geology professor Matthew Lachniet — is working on forecasting the future of the North American Monsoon by studying nature’s past.

In a study published last week in Nature Geoscience, researchers examined an ancient stalagmite from the floor of an undisturbed Grand Canyon cave, and found that increased levels of water seeped into the cave between 8,500 and 14,000 years ago, when temperatures rose throughout the region following the last Ice Age.

In short, the research results suggest that during this period of rapid warming, there were also greater amounts of groundwater recharge in the Grand Canyon than today.

Researchers concluded that during this period, known as the early Holocene, fast melting snow packs and rapid evaporation created more intense and widespread summer monsoon rains powerful enough to infiltrate the surface into caves and aquifers.

“The climate must have been significantly different eight and a half thousand years ago, so that the summer rainfalls were either more intense or more frequent, or there was something different about the vegetation that allowed that rain to soak into the ground in a way that it doesn’t today,” Lachniet said.

Today, most groundwater recharge in the region comes from winter snow melt. By studying the stalagmites geochemistry, researchers found that during the early Holocene — when temperatures were only slightly warmer than today — both summer and winter moisture contributed to groundwater recharge in the region.

Paradoxically, future aridification and a hotter climate in the Southwest may also come with fiercer and more frequent monsoon storms strong enough to penetrate high-altitude plateaus like the Grand Canyon.

Lachniet says there’s a “big uncertainty” about how climate change will affect the North American Monsoon. But there are clues. Warmer air can hold more water, and while it may not rain as much in the future, the monsoons that make it to the region will potentially be more intense. 

“If we look at the past as a guide, the answer is yes, warmer conditions can produce either a stronger monsoon or more effective groundwater infiltration,” Lachniet said.  “It’s an intensity that might have implications for water infiltrating into groundwater.”

Still, it’s hard to say whether the past is an exact reflection of what the future of climate change will look like. Climate history can show researchers “what nature is capable of doing,” but isn’t necessarily a  crystal ball, Lachniet said.

“There were a lot of differences 8,000 years ago compared to what we’re looking at today,” Lachniet said. “What we’re seeing is that nature is capable of making stronger monsoons under warm conditions. So it gives us one possibility when we’re looking to the future. Did it happen in the past? Yes. Could it happen again, in the future? Possibly.”

University of Kentucky researcher Ben Tobin set out to study the stalagmites at the Grand Canyon National Park after serving as a case specialist with the U.S. National Park Service. During a backcountry expedition documenting caves at the park, Tobin realized the mineral deposits in one remote cave were extremely well preserved. He soon contacted Lachniet about collaborating on a study.

“I had never been into a Grand Canyon cave before,” said Lachniet. “I’ve been in many other caves, but this involved hiking down the Colorado River and a 4,000 feet of altitude drop carrying heavy packs and everything. It was a lot of work to get down in there.”

Stalagmites are common cave formations that grow as mineral-rich waters seep through the ground over thousands of years, creating layers like tree rings. Researchers can accurately record rainfall history in an area through chemistry analysis.

The sample the researchers collected proved to be a good one, allowing the scientists to pull vital information. Over three years, the research team studied the oxygen isotope composition of the stalagmites to determine how much water reached the cave in the summer and the winter.

“It tells us what the climate system, as a whole, is capable of doing. And it’s not necessarily only a story about drying in the future,” Lachniet said.