Denniston awarded NSF grant to study climate change

The National Science Foundation (NSF) has awarded a nearly half-million-dollar research grant to Cornell College Professor of Geology Rhawn Denniston and a team of researchers to study climate variability.

The research will utilize stalagmites from a cave in Nepal to understand variations in Indian monsoon rainfall over the last 4,000 years. 

The Indian Summer Monsoon is critical to the lives of the 2 billion people living in South Asia. In fact, it provides more than three-quarters of the annual precipitation in this region, including snow to Himalayan glaciers and rains to farmers’ fields. Even one year of above or below average monsoon rainfall can have major consequences, which is why Denniston says understanding how and why the monsoon varies is critically important. 

Scientists, however, currently only have about 150 years of instrumental records of Indian monsoon rainfall to study. During that time, the monsoon has experienced numerous short-term droughts and exceptionally wet years, but there’s evidence in natural records of rainfall that in the past these episodes have lasted much longer.

“We want to use the geological, paleoclimate record within these stalagmites to see how often these large-scale rainfall fluctuations happened,” Denniston said. “If instead of a really dry year, you get a really dry decade, then that could be a huge deal.”

In order to conduct this research, the scientists will cut the stalagmites in half and explore each tiny layer that has been perfectly preserved over time.

“The basis for this work is that monsoon rainwater contains chemical signals,” Denniston said. “Water is hydrogen and oxygen, and the ratios of different isotopes of oxygen correspond to how hard and how much monsoon rain is falling. You can actually get a sense of how much rain falls in any given month by looking at the oxygen isotopes of the rain that fell in that month.”

Once the rain soaks into the ground, it works its way into underlying caves and eventually the isotopes of oxygen become part of the stalagmites.

“It’s really amazing that the stalagmites are basically like little rain gauges sitting down there in caves over thousands of years, in total quiet and total darkness, just slowly recording how wet it is on the land above them,” Denniston said.

Denniston first collected stalagmite samples from Nepal as a graduate student in 1995. Little did he know, he was just starting what has become his life’s passion–using stalagmites to understand ancient monsoon rains. 

“It was the very first thing I ever did with stalagmites and now 25 years later I’m going back, and it’s really exciting,” Denniston said. “But I am only able to do it now because of the collaborators I have.”

On this new award, as with previous grants, he’s working with Dr. Alan Wanamaker from Iowa State University, Dr. Yemane Asmerom of the University of New Mexico, and Dr. Caroline Ummenhofer of the Woods Hole Oceanographic Institution on Cape Cod.

The work will not only involve the oxygen isotope analyses but will also include measuring radioactive isotopes in the stalagmites to determine when each layer was formed. There will be some fieldwork in Nepal, as well. The last big piece of this research will be to use cutting-edge climate model simulations to help understand why each of the droughts and wet periods occurred. 

Each step of this research will be performed by undergraduates working closely with one of these team members. Denniston has already recruited several Cornell geology and environmental studies students to take part in this three-year study. Denniston says the work is important for students because they’re not only learning what others have done before, but they are discovering and adding new knowledge to a critically important topic. 

Denniston, the William Harmon Norton Professor of Geology and chair of the Environmental Studies program, has now been a part of 11 NSF grants to study climate change and has served as lead principal investigator on eight of them.

“Understanding how the monsoon rains have changed in the past can help us predict how they might change in the future, and in South Asia, the monsoon is everything,” Denniston said. “So to me, it’s just really important and meaningful research. And it’s made even more so by the fact that I get to train such bright and engaged students as part of the project.”