Some of the rainiest places on Earth could see their annual precipitation nearly halved if climate change continues to alter the way ocean water moves around the globe.
In a new University of Colorado Boulder-led study published July 30 in Nature, scientists revealed that even a modest slowdown of a major Atlantic Ocean current could dry out rainforests, threaten vulnerable ecosystems and upend livelihoods across the tropics.
"That's a stunning risk we now understand much better," said lead author Pedro DiNezio, associate professor in CU Boulder's Department of Atmospheric and Oceanic Sciences, adding that parts of the Amazon rainforest could see up to a 40% reduction in annual precipitation.
The study leverages climate records from 17,000 years ago to inform computational models capable of predicting how climate patterns could change in the future.
"This study is an excellent example of how we can use paleoclimate data to better understand how the Earth system works," said study co-author Chijun Sun, an assistant professor of earth and planetary sciences at the University of California, Davis. "Improving our knowledge of these potential changes is crucial to preparing for the future."
The ocean conveyor belt
The Atlantic Meridional Overturning Circulation (AMOC) is a massive system of ocean currents that moves water through the Atlantic Ocean, transporting warm, salty water from the tropics to the North Atlantic. The AMOC plays an important role in regulating the climate by redistributing heat from the southern to the northern hemisphere. It also makes sure the tropical rain belt, a narrow band of heavy precipitation near the equator, stays north of it.
As the climate warms, melting polar ice and increasing rainfall will dilute the ocean's surface waters, making them less dense and potentially slow down the circulation. The impact of a weakened AMOC on the tropics remains uncertain, because scientists have been monitoring the system directly for only two decades.
As a technician at a National Oceanic and Atmospheric Administration (NOAA) lab in Miami in 2005, DiNezio helped calibrate some of the earliest measurements of AMOC. At the time, he had no idea that he'd be studying that very same system two decades later.
"A few years ago, this monitoring system recorded signs of a decline in the AMOC, but it later rebounded. So we weren't sure if it was just a fluke. The problem is, we haven't been measuring the ocean long enough to detect meaningful long-term change," DiNezio said.
While scientists are uncertain whether the AMOC has already begun to decline, climate models predict the system will eventually weaken because of climate change.
Predicting the future
DiNezio and his team set out to explore how a future slowing of these critical ocean currents could influence global precipitation patterns.
"Changes in rainfall are very difficult to predict, because so many factors are involved in making rain, like moisture, temperature, wind and clouds. Many models struggle to predict how the pattern will change in a warming world," DiNezio said.
The team turned to climate records from about 17,000 years ago, when the AMOC last slowed down significantly due to natural causes. Evidence of precipitation preserved in cave formations, as well as lake and ocean sediments, revealed how rainfall patterns responded to the slowdown during that period.
Sun helped build a global database of these paleoclimate records.
"We were stunned by the dramatic climate shifts recorded in several paleoclimate records from tropical sites during this period," Sun said. "Yet how a slowdown of the AMOC - a change originating in the North Atlantic - could trigger such global-scale climate responses remained poorly understood."
Drawing on that data, DiNezio's team identified the computer models that best captured those ancient rainfall shifts and used them to predict how the patterns could change in the future.
Their best models predict that as the AMOC weakens and cools the northern Atlantic, this temperature drop would spread toward the tropical Atlantic and into the Caribbean. This change, on top of rising global temperatures, will lead to significant reductions in precipitation over Central America, the Amazon and West Africa.
"This is bad news, because we have these very important ecosystems in the Amazon," said DiNezio. The Amazon rainforest contains almost two years of global carbon emissions, making it a major carbon sink on Earth. "Drought in this region could release vast amounts of carbon back into the atmosphere, forming a vicious loop that could make climate change worse."
While DiNezio said the AMOC is unlikely to stop completely, even a small reduction in its strength could lead to changes across the entire tropical region, increasing the risk of reaching a tipping point. But how fast and how much it slows depends on the degree of future climate change.
"We still have time, but we need to rapidly decarbonize the economy and make green technologies widely available to everyone in the world. The best way to get out of a hole is to stop digging," DiNezio said.
Sun's research group at UC Davis is expanding upon the research by investigating whether similar changes occurred during other periods of AMOC weakening.
"We are also investigating how AMOC changes may have contributed to some of the most extreme climate fluctuations observed in the southwestern U.S. over the past 100,000 years," he said.
Additional co-authors include: Timothy Shanahan and Xian Wu, University of Texas; Tianyi Sun, Environmental Defense Fund; Allison Lawman, Colorado College; David Lea, UC Santa Barbara; Masa Kageyama, l'Environnement/Institut Pierre-Simon-Laplace; Ute Merkel and Matthias Prange, University of Bremen; Bette Otto-Bliesner, National Center for Atmospheric Research; and Xu Zhang, British Antarctic Survey.
The research was supported by the National Science Foundation, the French National Centre for Scientific Research and the German Federal Ministry of Education and Research.
