Tropical Rain Belt Might First Widen, Then Contract in Response to Climate Change
Violent thunderstorms are a daily occurrence in the tropics. The intense heat in the equatorial region and the convergence of trade winds from the northern and southern hemispheres cause warm, moist air masses to rise. As this air rises and cools, the moisture is released in the form of intense rainfall. About one third of the world’s precipitation falls within this rain belt, called the Intertropical Convergence Zone (ITCZ). Changes in the ITCZ’s position or extent could have far-reaching consequences—for the weather and the drinking water supply in the tropics, as well as for the global climate.
According to long-term climate projections, the ITCZ is expected to narrow as a result of human-induced global warming. However, a tendency toward expansion has been observed for some time now. In a new study, researchers at the Max Planck Institute for Meteorology (MPI-M) have resolved this seemingly contradictory issue. They show that, as the concentration of carbon dioxide (CO₂) in the atmosphere increases, the ITCZ initially expands poleward and precipitation decreases. Later, the ITCZ contracts, becoming narrower than before, and rainfall intensifies—much like squeezing a sponge.
Virtual experiment
MPI-M researchers Jiayu Zhang and Sarah Kang identified these two phases through a virtual experiment. Using the MPI-ESM climate model, they quadrupled the atmospheric CO2 concentration abruptly to elicit a pronounced fast response, separating it from the slow response. As a result, the ITCZ expands rapidly to the north and south. After the first decade, it starts to slowly contract. Qualitatively, the two phases also emerge in a more realistic scenario in which the CO₂ concentration increases gradually rather than abruptly. In this simulation, the transition from expansion to contraction occurs when the CO₂ concentration in the atmosphere is slightly more than twice the pre-industrial level, provided other factors are neglected. This occurs approximately 80 years after the CO₂ increase begins.
Connection with ocean warming pattern
This finding aligns with recent evidence that climate patterns can first evolve in one direction and later reverse. For example, researchers in Kang’s team previously demonstrated that the eastern part of the tropical Pacific initially cools down in response to increasing CO₂ concentrations, but later warms up even more than the western part. The new findings are consistent, as there is an established link between sea surface temperatures and the extent of the ITCZ: Temperatures in the tropical Pacific that are cooler relative to the background warming of the tropics lead to an expansion, while warmer temperatures lead to the predicted “deep tropics squeeze”.
With regard to human-induced climate change, the findings imply that the current situation, in which the ITCZ appears to be widening, could persist for several more decades. The contraction of the ITCZ and intensification of precipitation in the tropical rain belt will be more likely to occur towards the end of the century. In the next step, the researchers plan to investigate the causes of inter-model uncertainty in this response.
Original publication
Jiayu Zhang, Sarah M. Kang, Fast expansion and slow contraction of the ITCZ in response to CO2 forcing. Sci. Adv. 12, eaef2833(2026). DOI: 10.1126/sciadv.aef2833
Contact
Dr. Jiayu Zhang
Max Planck Institute for Meteorology
jiayu.zhang@mpimet.mpg.de
Prof. Dr. Sarah Kang
Max Planck Institute for Meteorology
sarah.kang@mpimet.mpg.de