By Galen Rende
As countries within the United Nations scramble to decrease greenhouse gas emissions in support of the historic Paris Climate Agreement signed last year, researchers have taken aim at discovering just how impactful these potential pollution reductions will be. Even as scientists come to understand the atmosphere with more precision and detail, there are still questions left unanswered. One of the greatest sources of uncertainty lies in the influence of aerosols.
Aerosols are small particles suspended in the atmosphere. The product of many direct and indirect sources, including the burning of fossil fuels, biomass burning, volcanoes, and atmospheric chemical reactions, aerosols impact climate in a number of ways.
Dr. Ross Salawitch, a Professor with joint appointments across the University of Maryland (UMD) Departments of Atmospheric and Oceanic Science, Chemistry and Biochemistry, and the Earth System Science Interdisciplinary Center, explains that some aerosols exhibit a cooling effect on the atmosphere by acting as “little mirrors.”
“Those little mirrors reflect sunlight. In particular sulfate aerosols impact the climate by reflecting sunlight,” Salawitch says. “But there’s another class of aerosols, called black carbon, that heat the atmosphere by both absorbing sunlight and absorbing infrared energy emitted by earth.”
The dual nature of aerosols makes their impact very difficult to quantify, and makes future projections more uncertain.
“We understand with a large amount of certainty how much climate has warmed over the past couple hundred years due to rising greenhouse gases. That’s known rather well,” Salawitch explains. “We don’t really know how much that warming has been offset by aerosol cooling. It could be a lot, or it could be very little. So while we understand the mechanisms behind everything, the uncertainty on the aerosol side of the ledger is quite large.”
Apart from the direct heating and cooling effects, aerosols also have an indirect effect on the atmosphere through the ability to alter cloud formation and precipitation. To make matters more complicated, these indirect effects can feed back into the reflection and absorption rates of the atmosphere.
Salawitch explains, “You can have situations where aerosols suppress precipitation because they cause the cloud droplets to be smaller and the cloud doesn’t precipitate as efficiently. So whatever radiative effect the cloud had, be it heating or cooling, just persists for longer.”
While researchers are trying to pinpoint the net effect of aerosols, they also understand that drastic reductions in aerosols over the past decades will soon lead to a drastic decline in their overall impact. As their influence weakens, it will become more and more evident just how important the cooling or heating effect has been.
Salawitch and his team of climate researchers have composed two projections: one where aerosols have had little overall effect on the atmosphere due to competing heating and cooling effects, and one where the cooling effect has dominated. In the first scenario, future warming is much more modest, though warming does occur. But in the second scenario, Salawitch says, the absence of climate-cooling sulfate aerosols will lead to even more accelerated warming in the future.
“In my second scenario, aerosols have offset a lot of the greenhouse gas warming,” Salawitch contends. “Very strong climate feedback has caused greenhouse gas warming to increase over time despite this cooling effect. If I assume that climate feedback persists in the absence of aerosols, I get much more warming than in the first scenario.”
While Salawitch cannot state which scenario is more likely, he notes, “This is why it’s so important to actually try to define what aerosols are doing to climate today in a comprehensive manner.”
With time running out before the impacts become self-evident, Salawitch and his research group are actively examining the impacts of aerosols, with the hope of getting publications out this summer.
