PAOC Spotlights

How Might a Steep Decline in Aerosols Impact the Climate?

Wed September 14th, 2016
Mark Dwortzan | MIT Joint Program on the Science and Policy of Global Change

EAPS PhD student Daniel Rothenberg targets one of climate modeling’s biggest unknowns.

Air pollution from power plants, internal combustion engines and other artificial sources impacts not only human health but also the global climate. The more particles we emit into the atmosphere, the more water droplets are likely to form around those particles inside clouds. Clouds with more droplets are thicker and brighter, so they reflect more solar radiation, thereby cooling the climate system in a process called the aerosol indirect effect. While much is known about the physics of how aerosols impact cloud formation, it’s hard to measure just how big a role the indirect effect plays in offsetting global warming. Today’s estimates of the magnitude of the indirect effect are highly uncertain; it may well have masked as much as 80 percent of warming during the 20th century due to carbon dioxide (CO2) emissions alone.

EAPS PhD student Daniel Rothenberg targets one of climate modeling's biggest unknowns

Reducing that uncertainty will become critical throughout the 21st century as more and more countries significantly reduce their greenhouse gas emissions in pursuit of the Paris Agreement’s goal of capping the rise in global mean surface temperature since preindustrial times at 2 degrees Celsius. Any major cut in airborne particulates will reduce the indirect cooling effect considerably, resulting in additional warming that climate models will need to estimate as precisely as possible.

To help meet this challenge, Daniel Rothenberg, a Joint Program PhD student in the Department of Earth, Atmospheric and Planetary Sciences, has spent the past two years developing concepts and software aimed at reducing the uncertainty in the magnitude of the indirect effect.

“If the sensitivity of the indirect effect is strong, then you might expect a much stronger warming associated with aerosol reduction in the future, and that makes the 2°C goal even more difficult to achieve,” says Rothenberg. “That our current models haven’t been able to reduce the uncertainty is a specter hanging over the climate community’s head, so it’s important to determine how strong these effects might be.”

Modeling Droplet Activation

Toward that end, Rothenberg has incorporated a sub-model of the physics of aerosol-cloud interaction into a widely used climate model called the Community Earth Systems Model (CESM). This sub-model represents droplet activation, the process by which individual aerosol particles become cloud droplets, the building blocks out of which clouds are formed. Just as dew condenses on grass and leaves on a cold morning, water vapor in the atmosphere condenses onto airborne particulates. Drawing on research that pinpoints which particles tend to form cloud droplets, the sub-model projects how many cloud droplets form based on the level and type of ambient pollution.

“Rather than relying primarily on approximations used in previous aerosol-cloud interaction models, the method that Daniel developed is much more physically-based and thus the best choice now for studying the climate response to the aerosol indirect effect,” says Joint Program Senior Research Scientist Chien Wang, Rothenberg’s advisor.

Supported by funding from the National Science Foundation and developed in consultation with Wang and other prominent atmospheric scientists, Rothenberg’s droplet activation sub-model is described in a paper he co-authored with Wang in the Journal of the Atmospheric Sciences. It’s unique among aerosol-cloud physics models in its focus on droplet activation and its impact on climate.

“How you represent how aerosols influence clouds—the fundamental droplet-activation process—contributes a huge amount of uncertainty to the aerosol indirect effect,” says Rothenberg. “This ends up being a very big lever you can pull on your model, to make it either much more or less sensitive to aerosol pollution.”

Upon graduation this year, Rothenberg aims to pursue postdoctoral studies centered on the potential impact of aerosols on the climate in coming decades. By collaborating with experts on policy being considered in China, India and other countries, identifying likely emissions-reduction policy scenarios, and running simulations of those scenarios with his customized version of the CESM, he hopes to provide a more accurate read on the likely climate response to such policies.

Science, software and policy

Rothenberg’s career aspirations and foundational work at MIT combine three interests—science, software and policy—that he has cultivated from an early age.

Shortly before starting first grade in Louisville, Kentucky, a severe thunderstorm sent him beneath the bedcovers, where he fixated on each rolling detail of a local TV meteorologist’s live report. By the time he reached high school, his fear of extreme storms morphed into a passion to learn more about the weather, a stint as a volunteer at the local office of the National Weather Service, and a decision to study meteorology at Cornell University. Guided by Professor Natalie Mahowald (one of Joint Program Co-Director Ronald Prinn’s PhD students in the 1990s), Rothenberg completed a senior research thesis on how volcanoes impact the carbon cycle and the climate, and graduated magna cum laude with a B.S. degree in Atmospheric Science. He also emerged intent on learning more about aerosols and other components of the climate system whose impact on the climate are highly uncertain. Graduate studies in MIT’s Program in Atmospheres, Ocean and Climate (PAOC) provided a natural pathway to dig deeper.

Rothenberg’s interest in software was kindled by high school computer science courses that showed him how fun, interesting and impactful computer programming could be, and introduced him to the concept of open-source software development. At MIT he developed an open-source, modular modeling framework for studying droplet activation from diverse aerosol populations.

Rothenberg’s policy acumen taps into a lifelong fascination with politics, activism as president of the Young Democrats in high school, and insights from a Cornell course in environmental governance. At MIT, he served on the MIT Science Policy Initiative (SPI) executive board as Liaison to the National Science Policy Group. The SPI, through visits to congressional offices and federal agencies, trains graduate students on how to advance policies that support science and use science to inform policy.

To wind down from a typical day’s work in science, software and policy, Rothenberg turns to the violin. A classically trained violinist, he has participated in ensembles ranging from the Louisville Youth Symphony Orchestra to the MIT Symphony Orchestra, contributed to recordings of pop and indie rock musicians, participated in community musical theater productions and accompanied touring musicians.

“Playing the violin is fun and challenging, but in a different way than my research,” says Rothenberg. “I get to exercise a different part of my brain.”

For more information about Daniel Rothenberg, visit

This article originally appeared in the Summer 2016 issue of Global Changes, a triennial publication of the MIT Joint Program on the Science and Policy of Global Change. 


Photo: Lauren Wadsworth Photography