The experts have uncovered a trend from 1979 to 2019. During this time, increases in greenhouse gases and reductions in aerosols triggered a delay in the rainy season over tropical land and the Sahel.
The postponement in rainfall could lead to many dire consequences, including delayed crop production, more intense heat waves, and more severe wildfires.
“The global warming we’ve seen has already been attributed to human activities with high confidence,” said study co-author Ruby Leung. “But, historically, we have not been very successful in pinpointing the footprint of human activity in the hydrological cycle.”
“This study shows that, yes, the later onset of monsoon rainfall, paired with future warming projected by climate models, has already emerged.”
The researchers explained that ironically, the rain delay is caused by an increasingly moist atmosphere. When heat-trapping greenhouse gases warm the surface of the Earth, more water vapor enters the atmosphere.
“When there is more water vapor in the atmosphere, it becomes more ocean-like,” said study first author Fengfei Song. “And we know the ocean takes longer to warm up than the atmosphere. More moisture means it will take longer for the atmosphere to absorb energy and produce rainfall.”
Leung noted that if aerosol concentrations continue to decline and greenhouse gas concentrations continue to rise, the delay will extend in the future.
The researchers project that by the end of the century, the rainy season could be further delayed by more than five days over northern tropical land and more than eight days over the Sahel.
“For monsoon regions, like India, with an agrarian economy, a delayed onset of summer rainfall could devastate crop production and jeopardize the livelihood of large populations, unless farmers recognize and adapt to the long-term changes amidst the highly variable monsoon onset date,” said Leung.
The study is published in the journal Nature Climate Change.