‘Cool’ossol Dilemma: Aerosols Explained
- Scales and States Team
- Jul 14, 2020
- 4 min read
“We already have planet-cooling technology. The problem is, it’s killing us.”
Eric Holthaus, a meteorologist and columnist for Grist, covering climate science, policy, and solutions refers to aerosols as “devil’s bargain”. When most people hear “climate change”, they think of greenhouse gases overheating the planet. But there’s another product of industry changing the climate that has received scant public attention: aerosols. Tiny particles floating in the atmosphere have a much bigger impact on the planet than you might think, and human activity plays a role.
Atmospheric aerosols are tiny solid/liquid/mixed particles suspended in the air originating from natural or anthropogenic sources. With a typical lifetime of days to weeks in the troposphere and about a year in the stratosphere, aerosol size ranges from a few nanometers to several tens of micrometers. Fundamentally, aerosol particles absorb and scatter incoming solar radiation modifying the global and regional radiative budget. Non-absorbing aerosols like sulfate, nitrate, and sea spray scatter shortwave radiation back to space leading to a net cooling of the climate system while absorbing aerosols produce the opposite effect. Although depending on the local emissions and transport processes, regionally, the anthropogenic aerosol radiative forcing can be either negative or positive; it is well established that globally, the radiative effect of anthropogenic aerosols produces cooling of the planet (IPCC 2013). The non-uniform distribution of aerosols in the atmosphere creates uneven atmospheric heating and surface cooling patterns, which drive changes in atmospheric circulation and regional rainfall. On longer timescales, the declining trend in ISM precipitation post-1950s has been linked to the rising anthropogenic aerosol burden over various regions.
According to the first Assessment of Climate Change over the Indian Region, Aerosol loading over India has substantially increased during the recent few decades. The annual mean 500 nm aerosol optical depth (AOD) from ground-based observations shows an overall increasing trend of 2% year-1 during the last 30 years (high confidence). The aerosol concentrations over the subcontinent are dominated by wind-driven desert dust, biomass burning, industrialization, agricultural activities, etc. Rapid growth in population, industrialization, and urbanization—over South, East, and Southeast Asia—has contributed to the significant rise in emissions producing different types of aerosol over the region. The associated increase in anthropogenic aerosol loading in recent decades (Satheesh et al. 2017) has led to increased reduction of surface insolation, contributing to solar dimming over the Indian landmass, affecting the energy balance at the surface (Ramanathan et al. 2005; Soni et al. 2012). The high aerosol burden has also been linked to changes in the hydrological cycle of the region. The long-term decline in southwest monsoon precipitation has been associated with anthropogenic aerosol forcing over South Asia.
Quantifying the effects of anthropogenic aerosols on regional climate cannot be done based on observations alone. Modeling studies are needed to enumerate the aerosols–climate interactions and prediction of future climate change. Aerosol-cloud-precipitation-meteorology interaction is one of the most challenging scientific issues requiring intensive observational and modeling with focused research from the climate science community. The complexity in the aerosolcloud interaction arises from variations in dominant phase changes and microphysical and dynamical processes associated with different types of clouds.
What’s clear is the net cooling effect that they have. If we magically transformed the global economy overnight, and air pollution fell to near zero, we’d get an immediate rise in global temperatures of between 0.5 and 1.1 degrees Celsius, according to the new study. “It is well understood that [aerosols’] presence is masking a substantial amount of greenhouse gas warming,” says Cat Scott, a research fellow at the University of Leeds whose own work has helped scientists understand the cooling effect of aerosols. This puts our increasingly interdependent global civilization in a tough bind. Get rid of carbon emissions to fight global warming and you get rid of aerosols, pushing temperatures back up. The only choice might be to engage in a delicate and risky gamble. It would involve gradually eliminating pollution from factories and tailpipes; replacing them with artificial aerosols in the upper atmosphere where they’re much less likely to damage human health.
Previous attempts at removing harmful aerosols have proven largely successful, especially in the United States and Europe. The U.S. Clean Air Act, one of the most important fruits of the 1970s environmental movement, led to a sharp and nearly immediate fall in air pollution. Not coincidentally, global temperatures began climbing in the late 1970s after the Clean Air Act was passed, ending a relatively stable 30-year period of global temperatures. Coal-powered China’s rapid economic rise over recent decades, and the resulting aerosol emissions have blackened skies in Shanghai, Beijing and other megacities — and probably contributed to a brief slowdown in the rate of global warming. China has responded to public outrage over the country’s airpocalypse by putting pollution controls in place. And there’s initial evidence that they’re beginning to work.
Embarking on a planetary-scale aerosol geoengineering project would produce “a wide range of unintended regional consequences,” Samset says. One of the biggest risks is that the cooling would work too well, producing shifts in ecosystems at “unprecedented speeds,” according to the Nature Ecology and Evolution study. That could be a fatal shock to animals and plants already stressed by decades of warming. There’s overwhelming consensus among scientists and policy experts that humanity is not doing enough to address climate change. After 25 years of global negotiations, greenhouse gas emissions are still rising. Extreme weather is now considered the biggest risk to the world economy.
Aerosols have a complicated suite of different effects on the planet, but humans have directly impacted their presence, abundance, and distribution. And while the climate effects are complex, the health impacts are clear: more fine material in the air hurts human health.
by - Shailja Singh
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This story is part of a five part series on the Climate Change Impact Assessment Report by the Ministry of Earth Sciences.
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Have a look at this piece about the report, or reach out to us at submissions.scalesandstates@gmail.com for feedback, queries, and sharing your thoughts.
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