New study assesses when climate change in tropical expansion may be detectable

Pacific Ocean and clouds (Credit: Xiao-Wei Quan, CIRES)
Pacific Ocean and clouds (Credit: Xiao-Wei Quan, CIRES)

Much of the world’s weather and climate is affected by what happens in the tropics. Scientists have discovered that the boundaries of the tropics have been expanding, which is associated with a poleward shift of mid-latitude storm tracks and an increased risk of droughts in the subtropics in each hemisphere. The tropical width is measured by the distance between the latitudes where air in tropical rainfall systems descends in the Northern and Southern subtropics (referred to as the Hadley Circulation). The increase in human-induced greenhouse gases has been found to be a main driver of tropical expansion. While scientists recently suggested that the observed expansion since 1979 has been largely a reflection of natural variability, an important question is whether and, if so, when the impact of climate change may be detectable. In a new study to be published in the Journal of Climate, CIRES and NOAA researchers from the Physical Sciences Division and a University of Hawaii collaborator address this question using large sets (or ensembles) of climate models that simulate the climate’s response to changes due to both human activity and natural variability. By assessing the tropical expansion due to global warming with more certainty and comparing it to the expansion due to year-to-year natural variability, the researchers were able to estimate the time when the climate change signal becomes detectable.

The researchers found that a 1°C increase in the global surface temperature will cause the tropics to widen by approximately 50 km. They also found that the magnitude of year-to-year variability is about 110 km. Using the projections of global surface warming, they estimate that the tropical expansion due to global warming will likely be larger than the year-to-year variability and be detectable by the middle of the 21st Century.

This study provides new information that may be useful for those trying to understand and prepare for droughts in highly-populated subtropical regions. The importance of a drying trend in many subtropical regions is underscored by larger, more intense, and more frequent forest fires and severe water shortages that lead to food insecurity in many areas within this region. Results of the study showed that the time of emergence of a change in the width of the tropics that could be distinguished from the natural variability tropical width (about one-degree of latitude) will occur within a span of a couple of decades, centered around the year 2058.

Posted: June 28, 2018

Authors of "On the time of emergence of tropical width change" are: Xiao-Wei Quan, Martin Hoerling, and Judith Perlwitz of the ESRL Physical Sciences Division, and Henry Diaz of the University of Hawaii.