Over the past 50 years, average global temperatures have been rising at the fastest rate ever recorded. Scientists have focused on modelling the future impacts of this strong warming, but when it comes to rainfall patterns the future gets harder to predict. Where is it going to get wetter? Where will it get drier? These are big climate change questions we need to answer. Jessica Conroy doesn’t yet have all the answers, but she’s working on it. Jessica is an Assistant Professor in the geology department at University of Illinois Urbana Champaign and also has an appointment in plant biology.
Her lab’s work focuses on the last 10,000 years and in particular the last 2,000 years (which is relatively recently geologically speaking). Within this time period are two century-scale periods of climate variability called the Little Ice Age (c. 1500 - c. 1850) and the Medieval Climate Anomaly (c. 900 - c. 1300). They weren’t forced by human activity, so they give a taste of the natural variability of the climate system. Despite how recent they are, these periods are not well understood.
She has used sediment coring to shed light on monsoon patterns in the Tibetan plateau. The cores came from a lake in the southern part of the plateau, a region that gets a lot of monsoon precipitation. The monsoon is obviously important for the rainwater that it brings and agriculture in the area depends on it. However, there’s a lot of uncertainty about the future of the monsoon. Is it going to get stronger or weaker as we continue to emit CO2? Will there be more or less rainfall? The monsoon has proven difficult to model and rainfall records only cover the last 60 years or so, but paleoclimatology can provide vital evidence for how the monsoon has varied historically on a longer timescale. Jessica’s group studied the sediment cores of the particular lake, looking for micron-scale changes in median particle size. They were able to line up the changes in grain size with fluctuations in the 50 years of rainfall data from the region. That strong monsoon signal meant that they were able to interpret the 4,000 years in the sediment record more confidently.
“I’m very proud that we have this very long monsoon record from that region,” says Jessica. “The key thing is that we do see this relationship with modern precipitation which makes it more robust in terms of interpreting it.” The record doesn’t show a sharp increase in the monsoon in the last century. It is going up, meaning it’s getting wetter now, but the variability that they saw in the past was stronger. There was a 1000-year period where the monsoon was much weaker are the area was dry. It increases towards present. It was wettest around the mid 1800s and then dropped off again in the 20th century. “When you think about something like temperature, usually the 20th century is this crazy anomaly. But when it comes to precipitation, it’s not as clear yet,” says Jessica. The natural variability of the monsoon precipitation is what makes it so difficult to figure out what’s going to happen in the future.
At the moment, Jessica’s lab had been working on Kiritimati (or Christmas Island) in the central tropical Pacific. It’s an area that has only limited paleoclimate records because of how difficult it is to get there. But it’s vital that they understand the last 1000 years of climate variation since what happens in the central pacific influences global climate. For example, the effects of tropical Pacific phenomena like El Niño and La Niña ripple across the atmosphere and then ends up influencing climate patterns over North America. “The big question is how is this part of the world going to change with climate change, because that has implications for everywhere else,” explains Jessica. “We’re hoping to have a really important piece of the puzzle completed with this work on Christmas Island.”