Featured Research

Will Global Change Jeopardize the US Forest Carbon Sink?

For the past 50 years forests in the United States have been a net carbon sink—absorbing more carbon than they release. This sequestration of carbon in forests has provided important benefits for the global climate system and for the United States, offsetting an estimated 10-15% of annual gross U.S. greenhouse gas (GHG) emissions.

Recent projections from the U.S. Forest Service, however, suggest that the ability of the nation’s forests to sequester carbon may diminish over the coming decades, in part due to the impacts of climate change, including increasing temperatures and changes in the frequency and intensity of disturbances like forest fires, pest infestations, and severe storms. At the same time, climate change impacts on agricultural crop yields may in turn further impact forests through cropland area expansion.

Important drivers of change include technological progress in agriculture, biofuel production, the future state of the climate and its impacts on agriculture and forestry, all of which are uncertain. As the U.S. looks for climate change strategies and policies for reducing GHGs, a better understanding of these drivers and their interactions is essential.

With funding from the USDA/AFRI, Professor Tomas Hertel and Alla Golub, Agricultural Economics, along with collaborators Brent Sohngen, Ohio State University, and Yongyang Cai and Ken Judd, Stanford University Hoover Institution, will advance understanding of the impacts of climate change, technological progress in agriculture, and production of biofuels on U.S. forest carbon sequestration. Their analysis will factor in the stochastic nature of these drivers and their potential impacts, the global scope of changes, the irreversibility of decisions, and the degree of risk aversion in decision making.

Interactions of Clouds with Smoke Particles from Fires Burning in Southern Africa

Aerosol-cloud interactions are recognized in the scientific community as the largest uncertainty in estimates of the Earth’s changing energy budget. In the context of these interactions, the unique situation in the South East (SE) Atlantic arising due to seasonal burning (July-October) of agricultural residue in the southwestern African savannah merits special attention—this region is the world’s largest source of biomass-burning aerosols.

A significant portion of the dense, smoky aerosol layer originating from the continent moves westward towards the ocean and initially overlies vast stretches of subtropical stratocumulus cloud decks. However, farther offshore, as the marine boundary layer thickens, it gets mixed into the clouds. Thus, during the course of this transport, aerosol-cloud interactions include changes to aerosol-induced solar heating and microphysical effects.

To improve our predictions of future changes to the Earth’s climate system, we must better understand aerosol-cloud interactions in this important region. Professor Harshvardhan and graduate student Sampa Das, Earth, Atmospheric, and Planetary Sciences, are analyzing the interactions of smoke particles with clouds by comparing simulations of the vertical distribution of smoke-aerosol extinction (a fundamental aerosol property) from several chemical transport models including the Goddard Chemistry Aerosol Radiation and Transport (GOCART) and the Community Atmosphere Model (CAM) with observations from the Cloud-Aerosol Lidar with Orthoganol Polarization (CALIOP) sensor onboard the CALIPSO satellite.

Building a Global Perspective on Drought

Under climate change, precipitation regimes are expected to shift, with the climates of some regions becoming drier and others becoming wetter. At the same time, the precipitation arriving in a given region is expected to come in fewer, larger events. This suggests that, even in areas of the planet that will become wetter, intervals between precipitation events will get longer. This, along with the warmer temperatures, is expected to produce more extreme regional droughts.

To better understand the potential implications of more extreme drought around the world, an international network called Drought-Net was started in 2014. Among this network’s activities is the development of the International Drought Experiment (IDE). This “coordinated distributed experiment” will use a simple methodology to impose the equivalent of a local 100-year drought at each of dozens of sites around the world, and study the resilience of ecosystems in different regions. Professor Jeffrey Dukes, Forestry and Natural Resources and Biological Sciences, serves on the Drought-Net steering committee and his laboratory has started an IDE site at the Purdue Wildlife Area (PWA), located nine miles west of campus.

In Dukes’s laboratory, Ph.D. student Laura Ploughe, Biological Sciences, has focused her research on drought responses and led the installation of the IDE site in 2015. At the same time, she has developed additional experiments that manipulate precipitation at the PWA. Her work ultimately seeks to link changes in soil moisture and nutrient cycling with changes in plant growth and species composition in prairie and forest understory. With three experiments underway, and more to come, many new insights about the local effects of drought and their relationship to the sensitivities of other ecosystems and regions are sure to follow.

What Can Shrinking Ice Caps Tell Us About Climate Change?

Professor Nat Lifton, Earth, Atmospheric and Planetary Sciences, graduate student (and 2012 PCCRC Fellow) Casey Beel, and collaborators from the University of Colorado and the University of Buffalo are studying Holocene records of the fluctuations of small ice caps in eastern Baffin Island and western Greenland. The ultimate goal of their research, funded by the National Science Foundation, is to evaluate whether currently observed Arctic warming is outside the range of long-term natural variability.

The team will use powerful datasets derived from radiocarbon-dated vegetation preserved beneath cold-based ice caps for centuries to millennia, but now being exposed annually by current ice-margin retreat across northeastern Canada and West Greenland. These chronologies define the pattern and timing of abrupt summer coolings in the recent past and place current warming in a millennial context. 14C dating of vegetation will be complemented by measuring in situ cosmogenic 14C inventories in recently exposed rock surfaces, providing essential time constraints on the duration of ice-covered and ice-free conditions throughout the Holocene.

The longer-lived in situ cosmogenic 10Be and 26Al pair is also being used to assess exposure and cover histories under cold-based ice (ice that is frozen to the bedrock all year round) at longer (i.e., glacial-interglacial) time scales. Early results for 10Be/26Al from several deeply weathered high plateau surfaces in western Greenland indicate only minor ice cover over the last 1-2 glacial cycles, while 10Be/26Al results from several other ice cap margins in the area indicate long periods of burial by cold-based ice, a more typical Arctic pattern both elsewhere in western Greenland and on Baffin Island. The team suggests that these surprising results are due to differences in the marine- or land-terminating nature of adjacent fjord glaciers and corresponding differences in basal shear stress. A reduction in shear stress at the terrestrial-marine boundary would be expected to cause a lowering of the ice surface profile down-fjord, thus potentially enabling adjacent high-altitude plateaus to remain exposed while surrounded by glacial ice.

Arctic Warming Also Impacts Midlatitude Weather and Climate

The Arctic is warming faster than the rest of the globe; this phenomenon is generally referred to as “Arctic amplification.” There is increasing evidence that Arctic amplification might strongly impact both weather and climate, not only in the Arctic region, but also in regions farther away, such as the Northern Hemisphere midlatitudes. A recent study by Professor Yutian Wu, Earth, Atmospheric and Planetary Sciences and Dr. Karen Smith (Lamont-Doherty Earth Observatory, Columbia University) looked at the impacts of Arctic warming on the Northern Hemisphere midlatitude circulation pattern using a simple atmospheric general circulation model.

They found that, as a result of Arctic warming, the midlatitude tropospheric jet stream shifts toward the equator and the stratospheric polar vortex becomes weaker. Wu and Smith also conducted experiments to further explore the role of the stratosphere-troposphere coupling, or the stratospheric pathway, in linking the Arctic warming to the midlatitude circulation. They found that the Arctic warming could excite more planetary-scale waves to propagate upward into the stratosphere and weaken the stratospheric polar vortex. This signal in the stratosphere could further migrate downward back into the troposphere and the surface and modulate the tropospheric jet stream.

Their study, published in Journal of Climate, demonstrates, for the first time, that the stratospheric pathway plays a significant role in linking the Arctic warming to the Northern Hemisphere midlatitude circulation. Many of the currently used climate models, however, utilize a “low-top” atmospheric model with a poorly resolved stratosphere. The findings suggest that use of “high-top” models which have a fully resolved stratosphere with a model top above the stratopause is necessary to fully simulate how atmospheric circulation responds to climate change.

Transformation of Organic Sulfur in Natural Waters: Implications to the Global Radiation Budget

Coastal waters such as the Chesapeake Bay estuary are strong sources of a range of volatile sulfur-containing gases like carbon disulfide (CS2) and carbonyl sulfide (COS). Once in the atmosphere, these gases are readily oxidized to sulfate aerosols that affect radiative forcing directly (they can scatter and absorb both solar and terrestrial radiation), as well as indirectly (by influencing cloud formation). Professor Amisha Shah, Civil Engineering and Environmental and Ecological Engineering, is studying the key reaction pathways through which dissolved organic sulfur compounds such as the amino acid cysteine are converted to COS and CS2 when exposed to sunlight, with an ultimate goal of improving our understanding of the global sulfur budget.

Shah and collaborator Michael Gonsior, University of Maryland, have built a photochemical reactor that allows them to run experiments over different sunlight exposures at controlled temperatures. Early results from their work indicate that water type (freshwater vs. seawater) is an important factor in the conversion of cysteine to CS2 and COS, with a predominance of CS2 formation over COS. Additional work is underway examining samples collected from Chesapeake Bay.

Degradation of Global Grasslands: A Soil Organic Matter Perspective

Grasslands cover approximately 35% of the Earth’s land surface. These highly dynamic ecosystems provide important services such as provisioning of food, maintaining biodiversity and pollination, providing flood control, and supporting nutrient cycling and soil formation. Globally, however, grasslands are among the most degraded ecosystems, threatened by land use and land cover change, nitrogen deposition, fire suppression, and climate change.

Not all grasslands are equally vulnerable to such stresses. Those facing some of the biggest challenges are often defined by coarse, sandy, nutrient-poor soils, like the grasslands found across wide spans of the North American South and West and in Inner Mongolia, China. Research out of Professor Timothy Filley’s group, Earth, Atmospheric and Planetary Sciences, studies how these important ecosystems are changing, both above and below ground, and what that means to our understanding of the regional controls on soil organic matter stability and its resilience to environmental stress.

At field sites in Texas and Arizona the group is investigating how woody plant encroachment into grass-dominated ecosystems is altering the above- and belowground storage and cycling of carbon and affecting soil organic matter stability. Work led by Courtney Creamer (PhD 2012; now at USGS- Menlo Park) studied the encroachment of honey mesquite trees into grasslands impacted by over a century of overgrazing and fire suppression. They found that the grass-to-woody transition changes the source, rate and chemistry of microbial decomposition of organic carbon below ground; and that groupings of trees coincided with a suppressed microbial use of woody-plant carbon in the soil.

Parallel work in the arid grasslands of New Mexico, in collaboration with Heather Throop and Jennie DeMarco (visiting scholar in Filley lab), New Mexico State University, found that woodland replacement of degraded grasslands increased the proportion of mineral-stabilized soil carbon, which was shown to have a greater than anticipated resistance to microbial decay even after 50 years post-tree death by intentional woodland removal. These two studies are helping improve representation of plant-soil interactions in global carbon cycle models.

A separate study led by Ruzhen Wang, a U.S.-China Ecopartnership Visiting Scholar, looked at the coupled impacts of nitrogen (N) addition and changing rainfall patterns in a 9-year long manipulation experiment in a semi-arid grassland in Inner Mongolia, China. The team found that microbes housed within different soil particle size classes have distinct responses to N input and greater water availability. As soil carbon accessibility and concentration varied across the particles, the team was able to demonstrate the spatial heterogeneity of microbial-controlled soil response to environmental change.

Prices a Poor Metric for Food Security Outcomes in Climate Change Analysis

The implications of climate change for food security were recently the focus of an IPCC Expert Meeting convened in May 2015 in Dublin, Ireland to discuss assessment options. Food prices seem an obvious metric—and they have been the historical focus in the literature—but, as Professor Thomas Hertel, Agricultural Economics, points out in his recent Commentary in the journal, Nature Climate Change, this linkage is misleading at best, and altogether wrong in some cases.

Because climate change will have differential impacts on income, a broader measure of household well-being, such as changes in absolute poverty, is needed to assess the impact of climate change on food security. Agriculture-dependent households can benefit from higher food prices as long as they sell more than they buy; for example, in countries where poverty is concentrated among agricultural households, such as Bangladesh or Uganda, higher food prices should lead to improved incomes and food security for the poor. Urban households, on the other hand, are likely to feel the full impact of rising food prices.

Hertel and post-doctoral researcher Dr. Uris Baldos have also explored the role of international trade in managing food security risks from climate change. This work, published in the journal Food Security, finds that free trade and strongly linked global food markets could help counterbalance projected climate change impacts on agricultural production, both from single events such as a severe drought in the U.S. Midwest, as well as more gradual declines in crop yields, particularly in the tropics.

With current trade policies, the number of people in South Asia suffering from malnutrition would rise 120 percent in 2050 under the worst-case climate change scenario. Economic models indicate that fully integrated world markets would dramatically stem these effects, offering "insurance" against these worst-case outcomes.

Frames Describing Local Climate Change Impacts Affect Public Opinion in Distinct Ways for Democrats and Republicans

States and sub-national governments have taken the lead on climate change mitigation policy in many parts of the world, including the United States. In the U.S., however, explaining the variations among states—why some states facing serious threats from climate change have taken little to no action on the issue, while others have moved aggressively to reduce emissions—remains a challenge.

In a study led by graduate student Sara Wiest and Professors Leigh Raymond and Rosalee Clawson, Political Science, laboratory experiments were used to test the influence of different frames presenting projected local versus global climate impacts on individual perceptions of the severity of climate change, behavioral intentions to address the issue, and attitudes toward different climate change policies. The results indicated that frames stressing local impacts from climate change increased both perceptions of severity of the problem and support for local policy action on the issue among all subjects, as well as behavioral intentions among only Republicans. Interestingly, the study also found that presenting individuals with projected benefits as well as losses from climate change weakens perceptions of problem severity for all subjects, but decreases support for local policy action among Democrats only.

Overall, these results are consistent with policy research suggesting that perceptions of local vulnerability are an important factor in the adoption of sub-national climate change policies. The findings also imply that the effectiveness of particular climate change impact frames will vary from one state to another depending on a state’s partisan leanings.

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