On September 27 and 28, the airport in Kathmandu, Nepal, measured 240 millimeters of rain falling in twenty-four hours. A torrential episode of unprecedented intensity for more than twenty years, according to local observations reported by Agence France-Presse. More than 200 people have died due to floods in the country, according to the authorities’ provisional report on September 30.
These dramatic events are unfortunately not new. The torrential precipitation is characteristic of the monsoon, this rainy season that Nepal experiences every summer like the whole of South and Southeast Asia as well as part of Oceania, Africa and of America. Tropical or subtropical regions, all subject to the same phenomenon: the arrival of humid winds from the ocean, which generate clouds and then precipitation when they encounter warmer air masses on the continents.
The warmer the air, the more moisture it can contain and cause significant precipitation. And the higher the temperature contrast between the air masses that meet, the more intense the monsoon will tend to be. However, these two mechanisms are amplified by climate change. This partly explains why climate projections, summarized in the latest report of the Intergovernmental Panel on Climate Change (IPCC), anticipate an intensification of the Asian monsoons during the XXIe century. Another element, however, plays a crucial role in the future of these extreme events and complicates the equation: air pollution.
When aerosols weaken the monsoon
The fine particles released into the air by our polluting activities (via our transport and industrial activities in particular) have a cooling effect on the climate. On the one hand, because these particles directly reflect sunlight, on the other hand, because they can change the reflective properties and the lifespan of clouds.
So much so that these particles can ostensibly weaken the monsoons. The strong industrial development of South Asia, and particularly India, has been accompanied by very high atmospheric pollution. However, this period of development corresponds to a weakening of precipitation during the monsoon in the region, observed in the second half of the XXe century. Work on the subject shows that this weakening was indeed caused by the emission of anthropogenic aerosols, that is to say air pollution.
The same phenomenon also occurred elsewhere, notably in West Africa, recently affected by deadly floods and a monsoon, also destined to intensify. « Many studies show the effects of aerosols on monsoons. Particularly in West Africa, where it is increasingly accepted that the drought of the 1970s-1980s is linked to the increase in the concentration of aerosols. »underlines Benjamin Sultan, climatologist, research director at the Research Institute for Development (IRD).
Pollution and greenhouse gases: opposing effects
This period of weakened Asian and African monsoons seems well and truly behind us. Because not only will climate change accentuate its effects, but the fight against air pollution, which prematurely kills more than 4 million people per year, according to the World Health Organization (OMS), could further speed up the process.
This is notably the conclusion of a British study, led by the University of Reading (England) and published in 2020 in the journal Atmospheric Chemistry and Physics. The researchers predict that the reduction in air pollution, by leading to greater warming of the continent and therefore a greater temperature contrast between it and the ocean, will cause the occurrence of greater volumes of rain. during the monsoons in Asia.
The influence of these pollutants is such that, paradoxically, the Asian monsoon risks strengthening further in the very optimistic scenario where the world remains below 1.5°C of warming. Precipitation increases but relatively less, in comparison, in scenarios where global warming is greater. Why such a paradox ? Because the scenario in which we reduce our greenhouse gas emissions the most is also the one in which we reduce air pollution the most. Fewer greenhouse gases mean less warming, but less pollution means more warming at the local level…
The role of these fine particles, however, only comes into play in the medium term, specify the researchers. They influence changes in the monsoon by 2050, but at the end of the century, it is the climate’s response to greenhouse gases which definitively becomes dominant, according to the study. This is also confirmed by the latest IPCC report: precipitation from the South Asian monsoon is much more abundant in the long term (2081-2100) in the scenario leading to 3°C, and even more so in the scenario leading to 4° on average. C of global warming, only in one where we limit the rise in mercury to 1.5°C.
Beyond increases in average precipitation volumes during the rainy season, the occurrence of more short and extreme events is also expected. The latest climate models project an increase of 58 % of the intensity of extreme rainy days in South Asia over the period 2065-2100, compared to 1979-2014 in the median warming scenario, also note three American climatologists in a popular text published by Carbon Brief. As well as a three-fold increase in the frequency of these extreme rains on the Indian subcontinent from 1.5 to 2.5°C of warming.
The influence of aerosol pollutants in these mechanisms remains extremely complex to understand, recalls the study from the University of Reading. For example, it is possible that pollutant emissions continue to increase in South Asia but decrease in East Asia, leading to opposite effects on the monsoons. But as strong meteorological interactions exist between these two regions, this generates « high uncertainty in South Asian summer monsoon precipitation in the next 30 to 50 years »conclude the researchers.
« For an isolated event like what happened in Nepal, it is very complicated to quantify the responsibility of each factor. Climate change and aerosols play a role, but also natural climate variability, phenomena like La Niña also tend to strengthen the Indian monsoons, and variations take place during the monsoon itself. »adds Pascal Terray, researcher at the Oceanography and Climate Laboratory, at the Pierre-Simon Laplace Institute.
The only certainty: it is urgent to reduce all our polluting and greenhouse emissions, and to adapt to the already inevitable upheavals caused by the pharaonic volumes of these gases and particles already sent into the atmosphere.
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