2 billion people could face chaotic and 'irreversible' shift in rainfall patterns if warming continues

Nearly 2 billion people could face wild disruptions in water availability if the planet continues to warm — and the change could be irreversible, new research suggests.

Earth's average surface temperature is already about 2.1 degrees Fahrenheit (1.2 degrees Celsius) higher than pre-industrial levels, and with 2024 the hottest year on record, the future forecast is not promising.

The new study, published May 14 in the journal Earth's Future, looked at what would happen should global temperatures swell to 2.7 F (1.5 C), even for just a few decades.

Such an increase in global temperature could have a permanent impact on the intertropical convergence zone (ITCZ), a region near the equator where trade winds from the northern and southern hemispheres meet, the study found.

"These impacts that we quantify here will be there for the long term," said lead author Norman Steinert, senior climate researcher at the Center for International Climate Research in Norway.

The ITCZ has a heavy influence on rainfall patterns, and the increase in global temperatures could cause it to shift south, changing the length and intensity of wet and dry seasons, especially in parts of Africa, the Amazon and Southeast Asia. Too much rain in some areas and not enough in others could have dire effects on agriculture, ecosystems and water availability for a major portion of the planet.

Related: The decline of key Atlantic currents is underway, and it's been flooding parts of the US for 20 years

Several factors affect this wide band of clouds, including the ocean's largest conveyor belt, a network of currents known as the Atlantic Meridional Overturning Circulation (AMOC). Emerging research suggests this conveyor belt is weakening, largely due to climate change.

The researchers looked at two different scenarios run by eight different Earth System Models — powerful climate simulation tools. One "idealized" scenario analyzed how precipitation patterns might change if atmospheric CO2 increased at a rate of 1% per year for 140 years, then decreased at the same rate for another 140 years — it's a "clean," way to assess the impact of a rise and fall in global temperatures, if unrealistic, Steinert said.

The researchers also looked at data showing a potentially more realistic scenario, where emissions increase until the year 2040, which is followed by aggressive mitigation efforts to bring the global temperature back down.

The assumption is "that we won't be able or won't like to live in a warmer world, and would make actual efforts to bring temperatures down again at some point," Steinert told Live Science.

Most of the projections resulted in little or no shift in the intertropical convergence zone. But in one of the idealized scenarios and two of the more realistic scenarios, the zone shifts significantly, causing potentially major upheaval to rain patterns for much of the world.

Based on the number of models predicting different outcomes, the paper describes the ITCZ shift as “unlikely.” But given the already weakened response of the AMOC and a time lag between when the climate warms and when the ocean heats up, the researchers argue a shift in the ITCZ may be more likely than the new study suggests.

Central and West Africa and parts of Southeast Asia could face reduced rainfall, whereas northeast Brazil would be inundated. The timing and intensity of weather patterns could disrupt the lives of billions, as well as complicate agriculture that relies on consistent weather patterns. In total, 23% of the world population and more than 12% of the global land area could be impacted.

The likelihood of this scenario playing out is "a low probability, but plausible outcome," Steinert said, and the models suggest the worst impacts would take decades, at a minimum, to play out. In multiple scenarios, the damage was permanent, at least at human-time scales.

"It's an important study," Richard Allan, a professor of climate science at the University of Reading in the U.K., who was not involved in the research, told Live Science.

Allan pointed out water availability is more complicated than what the study considered, because the simulations don't take into account the amount of water and moisture in the ground or how much water is flowing in the rivers, for example.

Still, "this storyline could play out in the future," Allan said. "Because it has such big possibilities for regional water availability, this has got to be taken seriously."

In terms of future research, Steinert says it would be helpful to look at the local, specific outcomes for places that might be impacted by shifting weather patterns due to a warming climate. But the best way to avoid these risks is straightforward.

"I mean, that's very clear," Steinert said. "Cut emissions as soon as possible."