Very Wet Day Precipitation (R95p) Projections under 1.5°C, 2°C, and 3°C Global Warming Levels: A CMIP6 Multi-Model Analysis for the Rapti River Basin

Rain does not fall evenly. In many parts of the world, especially those dependent on the monsoon, the total amount of rain a year is less important than how that rain is delivered. A gentle drizzle over a month allows the soil to drink and rivers to flow steadily. But when that same amount of water arrives in a few violent bursts, the ground cannot absorb it, and the result is a flood. As the planet warms, the atmosphere holds more moisture, and physics dictates that this extra water tends to concentrate into fewer, more intense storms. Scientists call the days with the heaviest rainfall "very wet days," and they measure the total rain that falls on these specific days to understand how much of a region's annual water comes from these dangerous extremes. This is not just a matter of weather statistics; it is a question of survival for millions of people living in river basins where a single week of heavy rain can determine the fate of a harvest or a city.

Researchers recently turned their attention to the Rapti River Basin, a region straddling the border of India and Nepal that supports eight to ten million people. This area is already prone to flooding, with a history of catastrophic inundations following just a few days of exceptionally heavy rain. To understand what the future holds, a team of scientists used advanced computer models to simulate the climate under three different global warming scenarios: 1.5 degrees Celsius, 2 degrees Celsius, and 3 degrees Celsius above pre-industrial levels. These are the specific temperature targets and limits discussed in international climate agreements. The team did not look at the average rain for the whole year; instead, they focused exclusively on the "very wet days," calculating the total rainfall that falls on the top five percent of the wettest days in a year. They ran these simulations using four of the most sophisticated climate models available, each built by different research institutions around the world, to see how the intensity of these extreme events might change as the world gets hotter.

The results paint a picture of a future where the most dangerous rain becomes significantly more intense, but the exact amount depends heavily on which model is used. In the simulations, the amount of rain falling on these very wet days increases as the global temperature rises. At a warming level of 1.5 degrees, the models suggest that the total rain from these extreme days could range from 800 to 2,200 millimeters across the basin. As warming reaches 2 degrees, this total climbs to between 900 and 2,600 millimeters. By the time warming hits 3 degrees, the projections show a further jump, with some models predicting totals as high as 2,800 millimeters just from these few intense days. One specific model, which includes complex interactions between plants and the atmosphere, consistently projected the highest amounts of rain, suggesting that the very wet days could contribute nearly 2,000 millimeters of water in a single year under the 3-degree scenario. This is a staggering amount of water, comparable to or even exceeding the total annual rainfall some parts of the basin currently receive, delivered in just a handful of days.

The study also identified a critical tipping point in how these changes become detectable. At the 1.5-degree warming level, the models showed mixed signals, with some suggesting an increase in extreme rain and others showing no clear trend or even a slight decrease. The changes were not strong enough to be distinguished from the natural ups and downs of the weather. However, once the simulations reached the 2-degree threshold, the picture changed. Three out of the four models began to show a clear, statistically significant increase in the intensity of these very wet days. This suggests that 2 degrees is a threshold where the influence of human-caused warming on extreme rainfall becomes undeniable, moving beyond the noise of natural variability. The researchers found that the 1.5-degree target might be the difference between a future where extreme rain is still somewhat unpredictable and one where a dramatic intensification is locked in.

Perhaps the most striking finding is how unevenly this danger is distributed across the landscape. The simulations revealed a sharp divide between the northern highlands and the southern plains. The mountainous regions in the north, where the air is forced upward by the terrain, are projected to receive two to three times more rain on these very wet days than the flat southern plains. This gap widens as the world gets hotter. While the southern areas will still face increased risks, the northern sub-basins could see rainfall totals from extreme days that are vastly higher, demanding completely different engineering solutions for flood protection. A levee or drainage system designed for the flat plains would be woefully inadequate for the steep, rain-soaked hills, and vice versa. The study argues that treating the entire river basin as a single unit for planning purposes is no longer sufficient; adaptation strategies must be tailored to these specific local realities.

For the farmers and communities living in the Rapti Basin, these findings translate into a fundamental shift in how water must be managed. The rain is not just getting heavier; it is becoming more concentrated. Instead of a steady soaking that helps crops grow, the water arrives in overwhelming surges that wash away seeds, erode soil, and flood fields for days. The study suggests that the portion of the year's total rainfall coming from these extreme days could rise from about 35 to 45 percent today to as much as 50 to 65 percent in a 3-degree world. This means that even if the total amount of rain stays the same, the way it falls becomes more destructive. The researchers emphasize that while the models disagree on the exact numbers, they all agree on the direction: the extremes are getting worse. This uncertainty does not mean inaction; rather, it calls for flexible strategies that can adapt as the climate changes, such as building drainage systems that can be expanded and creating flood plans that prepare for scenarios far beyond what history has ever recorded. The difference between limiting warming to 1.5 degrees and allowing it to reach 3 degrees is not just a number on a thermometer; it is the difference between a manageable challenge and a catastrophic flood risk for millions of people.