May 11th, 2022
Key Findings
- Anthropogenic aerosols exert a substantial impact on the spatial occurrence of global tropical cyclones.
- The impact is somewhat different from the impact of green-house gases.
- Increased anthropogenic aerosols in China and India over the period 1980-2020 caused decreased typhoons in the western North Pacific.
- Decreased anthropogenic aerosols in Europe and the United States over the period 1980-2020 caused increased hurricanes in the North Atlantic and decreased tropical cyclones in the Southern Hemisphere.
- These changes in tropical cyclones are a result of changes in large-scale circulations as well as local cooling/warming induced by the changes in aerosol emissions.
Hiroyuki Murakami. Science Advances. DOI: 10.1126/sciadv.abn9493
Research reveals how pollution control measures in Europe and the United States over the past 40 years led to significantly decreased anthropogenic aerosols. During the same time period, economic and industrial growth in South and East Asia led to increased anthropogenic aerosols in those regions. The impact of these changes suggests effects on both the frequency of global tropical cyclones (hurricanes and typhoons), as well as a shift in the global wind circulation..
It is important to emphasize how the noted changes in anthropogenic aerosols, as well as greenhouse gases, delivers an important message to society regarding the seriousness of the impacts manmade greenhouse gas emissions are having on the global climate. The results suggest however society controls future anthropogenic aerosol emissions will ultimately exert a substantial impact on the world’s tropical cyclone activity.
Murakami’s work shows how decreased anthropogenic aerosols over Europe and the United States significantly reduced the number of tropical cyclones over the Southern Hemisphere. This also led to an increase of such storms over the North Atlantic. While the increases in anthropogenic aerosols over South and East Asia exerted substantial decreases in tropical cyclone activities over the western North Pacific. Future changes in tropical cyclones in the western North Pacific would largely depend on how the aerosol emissions are controlled in addition to the emission of the greenhouse gases.
Previous research showed the changes in the locations of global tropical cyclones over the past 40 years were attributable to the combined external forcing, including greenhouse gases, anthropogenic aerosols and volcano eruptions. Specifically, substantial decreases occurred in tropical cyclone activity over the North Atlantic when only CO2 was increased, while other external forces were fixed.
Murakami conducted a few climate modeling simulations in this research. Specifically, the GFDL Seamless System for Prediction and Earth System Research (SPEAR) was utilized. SPEAR consists of the new AM4-LM4 atmosphere and land surface model, the MOM6 ocean model and the SIS2 sea ice model. SPEAR then simulates the mass distribution of five aerosol types: sulfates, dust, black carbon, organic carbon and sea salt. To reveal the impacts of changes in the anthropogenic aerosols globally since 1980, model experiments imposed different spatial emission patterns including sulfate, black carbon, and organic carbon aerosols related to human activity.
The results suggest substantial changes in large-scale circulations caused by the changes in anthropogenic aerosols, which led to the changes in global spatial distribution of tropical cyclones. The decreased anthropogenic aerosols in Europe and the United States caused anomalous heating in the mid-latitudes of the Northern Hemisphere, thereby causing reduced meridional gradients of atmospheric temperature. This, in turn, led to a poleward shift in the subtropical jets, which led to reduced vertical wind shear in the tropical Atlantic, resulting in increased tropical cyclone activity in the North Atlantic.
The decreases in anthropogenic aerosols in Europe and the United States also exerted a warming in the Northern Hemisphere, causing anomalous upward air motion in the Northern Hemisphere and downward air motion in the Southern Hemisphere. This means convective activity is suppressed in the Southern Hemisphere, leading to fewer tropical cyclones being generated there.
The increases in anthropogenic aerosols from China and India led to a cooling over the Asian continent,which in turn led to a weakening of the Asian monsoon circulation in the boreal summer. This weakening of monsoon circulation caused a weakening of the monsoon trough in the tropical western North Pacific, leading to decreases in tropical cyclones there.
It is important to decrease aerosol emissions for human health. Meanwhile, the reduction is not always linked to the decreases in the risks of tropical cyclone hazards. This study underscores the importance of multiple consequences induced by anthropogenic activity.