Dust in the Wind
EAS Prof. Natalie Mahowald studies the complex impact of atmospheric aerosols on climate
By Lauren Cahoon
We all know dust can be a pain; it makes you sneeze, collects under your bed, and then there’s the way it can mess with geological climate change patterns and models. Didn’t know about that last one? You’re not alone. “Even people in the scientific community overlook it,” says Natalie Mahowald, an associate professor in Cornell Engineering’s Earth and Atmospheric Sciences department. She studies how dust—both man-made and natural—affects the planet’s ecosystems and climate, revealing multilayered feedback systems that, despite their broad-ranging effects, have largely been ignored.
Carbon dioxide is the primary culprit when it comes to climate change, but, thanks to Mahowald and her collaborator’s, dust and aerosols are beginning to get attention. In fact, Mahowald will serve as a lead author on the next edition of the International Panel on Climate Change (IPCC) report. She will work with 250 other experts in Working Group I, which assesses the physical, scientific aspects of the climate system and climate change. Mahowald will write the introduction and contribute to chapters on paleoclimate and the impacts of clouds and aerosols—a topic that didn’t get much play in the last report.
|Natalie Mahowald works with research assistant Aaron Perry '11.|
“That was one of the big uncertainties that came from the 4th assessment report,” says Pauline Midgley, head of the technical support unit for IPCC Working Group I. For this iteration, Midgley noted, there was a “large enough nucleus of work” for an entire chapter to be dedicated to clouds and aerosols.
Dust’s role in climate change is “really complicated,” Mahowald says. In fact, one of its most significant effects on the climate is actually a cooling one. Dust affects the ”radiative budget,” or the ratio of incoming solar radiation to the radiation that is reflected from the earth. CO2, for example, traps only outgoing radiation—thus the term “greenhouse effect.” Dust, however, can alter the heat that both enters and leaves the planet’s atmosphere. “Dust acts like a greenhouse gas,” says Mahowald, “but it reflects solar energy too. We think that in the net, it actually cools.”
A dramatic example of this effect is illustrated in global climate data from the 1950s to the 1980s, a period in which North Africa experienced massive droughts, turning once-lush regions into parched desert. With deserts came the dust, so much that by 1980, there was four times as much dust pouring into the atmosphere as there was in the ’50s. The climate turned cooler by 0.1 degrees Celsius—a seemingly small change, but a significant one. More important, Mahowald says, is that many scientists don’t understand that the change was due to dust. “It’s important to understand what’s going on,” she says. “Even people in the scientific community are ignoring mineral dust, especially when looking at what’s happened in the past.”
While a cooling effect sounds like a good thing when it comes to droughts, Mahowald and her colleagues argue that the ’80’s drought was partly due to the dust. As the tiny particles reflect back heat and cool the air, the air sinks, preventing precipitation (warm, rising air is the kind that breeds rain clouds) and exacerbating the drought. It’s subtle phenomena like this that drive Mahowald to increase awareness and understanding of desert dust’s effects on climate change.
In an effort to do this, Mahowald and her collaborators have compiled a series of data sets that aggregate dust-relevant information—including data from satellites, wind observations, deposition data, as well as paleoarchives such as ice cores and marine, terrestrial, and lake sediments. This information is combined to create climate models that simulate atmospheric dust’s effect on the climate for every year between 1870 and 2000. Those datasets, and similar datasets for paleoclimate time periods (e.g., last glacial maximum), are “available to anyone who wants to use them,” Mahowald says. As a result, the model data have become the gold standard of dust data for climate researchers worldwide. “That’s our claim to fame,” says Mahowald. A key finding from this wealth of data is a stunning one; the amount of atmospheric dust has doubled over the 20th century.
Mahowald clearly enjoys being a teacher as well. She raves about a climate change class that she co-taught at Cornell with a philosophy professor, and is looking forward to teaching classes on climate change and how it impacts humans. “ Teaching is really hard, a whole lot of work,” she says, “but students make you think so much more creatively.”
“Whenever I’m meeting with Natalie to discuss research, she will often come up with entirely new ideas and directions for science,” says Daniel Ward, a postdoc in Mahowald’s lab. “This makes her a scientist that many people want to collaborate with … she shifts people’s viewpoints.”
Effects on Ecosystems
Part of Mahowald’s view-shifting work has looked at how dust’s effects can go beyond the atmosphere and affect other ecosystems, such as the oceans. Mahowald first began to unravel this connection more than a decade ago while working as an assistant professor at the University of California Santa Barbara’s Bren School of Environmental Science and Management. There, she collaborated with oceanographers such as Dave Siegel, now director of the Earth Research Institute at U.C.S.B. He recalls Mahowald’s novel perspective toward the relationship between dust and the living planet. “So much of our planning on how aerosols work has been on their roles in [the atmosphere],” says Siegel. “And what Natalie did was to say, wait a minute, there are ecosystem-based systems...that are playing a role.”
Siegel says that her ability to think outside the box has earned Mahowald significant recognition within the scientific community. “She is one of the most productive young scientists in this area,” says Siegel. “The work that she does is the best in the field.”
As Mahowald and her colleagues found, when desert dust lands in the ocean, it deposits key nutrients and minerals, such as iron. The iron is consumed by phytoplankton, the base of the ocean food chain. A higher abundance of iron yields greater blooms of phytoplankton, which then pull more CO2 out of the air as they photosynthesize, a phenomenon known as a biological pump. This effect, which reduces CO2, adds yet another complicated layer to dust’s impact on the climate. Adding to the complexity is the fact that man-made aerosols given off by pollution can chemically react with desert dust, making the iron more soluble, thus increasing the amount that enters the ocean. “We showed that humans have probably doubled the amount of soluble iron going into the oceans,” says Mahowald. Overall, she says that the iron deposition from desert dust since 1870 has resulted in the uptake of roughly 4 parts per million (ppm) of CO2.
Mahowald has also studied how aerosols affect terrestrial ecosystems. Desert dust can carry phosphorus, a limiting nutrient for many tropical forests. “These forests use all the phosphorus they can,” says Mahowald. Thus, she says any that comes via mineral dust is eagerly absorbed. As it turns out, “North Africa is probably fertilizing the Amazon, and Asian dust is fertilizing Hawaii.”
Even anthropogenic aerosols, typically viewed as having a negative impact on the environment, have a more nuanced role. These man-made pollutants can often carry nitrogen—another necessary nutrient for plant growth, a fact that has led to more interdisciplinary collaboration between Mahowald and other scientists. At Cornell, Mahowald has collaborated with Christine Goodale, a forest ecosystem ecologist in the Department of Ecology and Evolutionary Biology. “She’s greatly expanded the opportunities for me and my lab group,” Goodale says of Mahowald. While Goodale’s group typically looks at how nitrogen affects one particular section of land, Mahowald and her group have helped put that phenomenon into a larger, more comprehensive context. “She’s provided a door opening for people who work on plot scale to look at things on the global scale,” Goodale adds. “She’s amazingly collaborative and productive—I was at a conference last week and three colleagues and I were just saying how we wish we could be more like her,” she adds with a laugh.
As with ocean ecosystems, the overall effect of aerosols on forest ecosystems seems to also create a cooling effect on the climate, thanks to nutrient-rich aerosols that enhance plant growth and CO2 uptake.
Mahowald has summed up the myriad climate effects of aerosols, both natural and man-made, in a paper published this last November in Science, with a cautioning message: reducing aerosols, which has long been an environmental goal due to public health reasons, could exacerbate warming global temperatures. Mahowald estimates that this cooling effect is enabling the land and the ocean to take up an extra one to 50 ppm of CO2. As humans reduce aerosols for health reasons, they will also reduce the cooling and carbon uptake these aerosols indirectly provide.
“I’m thinking about the uncertainties,” says Mahowald. “Right now 50 percent of CO2 that humans are emitting is being taken up by land and ocean, which is unlikely to continue. That’s a huge negative feedback on the climate system and we poorly understand it. If we cut aerosols, that’s going to impact the carbon cycle—it will make things tougher.”