The causes, effects and feedbacks of global climate change

Scientists who study global climate change, says Benjamin Felzer, confront a complex array of causes, effects and feedbacks that are triggered by natural phenomena and by human activity.
The age-old flux of air and water through the environment, he says, is affected by the more recent human enterprises of agriculture, forestry and industry.

Felzer, an assistant professor of earth and environmental sciences, develops numerical models of the terrestrial ecosystem and the biogeochemical relationships involved in the cycling of carbon, nitrogen and water among land, oceans and atmosphere. He is affiliated with the university’s STEPS (Science, Technology, Environment, Policy and Society) initiative.

Carbon dioxide released into the atmosphere by human activity, he says, causes global temperatures to rise while also promoting more photosynthesis, through which plants take in CO₂ and give off oxygen. This CO₂ eventually returns to the atmosphere as plants respire and organic carbon in soil decomposes, and those processes are enhanced by warmer temperatures. Warmer temperatures also cause more evaporation, which results in drier climates and soils, especially in continental interiors.

Factoring in ozone, nitrogen and the carbon sink

Meanwhile, about half the CO₂ produced by human activity is sequestered—accumulated and stored—by the world’s land surface and oceans. This phenomenon, called the “carbon sink,” is affected by agriculture and forestry practices. More carbon sequestration occurs in young growing forests, Felzer says, than in older, mature woodlands.

Other factors are at play. Ozone from air pollution damages photosynthesis and reduces crop yields significantly in industrial areas. Nitrogen in pollutants like nitrates and ammonia offsets some effects of the ozone by helping plants grow. But it also promotes the growth of algae and other organisms in water bodies, which leads to eutrophication, reduces oxygen supply and kills fish.

“Nitrogen pollution comes from fertilizer runoff and as a product of fossil-fuel combustion,” says Felzer. “It counteracts the effect of ozone; ozone hurts plants while nitrogen helps them.

“My models have to take phenomena like this into account. If you model one effect without the other, you can be misled. You have to constantly be aware of other processes to capture all the feedbacks.”

Felzer came to Lehigh two years ago from the Ecosystems Center of the Marine Biological Laboratory (MBL) in Woods Hole, Mass., where he helped researchers Jerry Melillo and David Kicklighter develop the terrestrial ecosystem model (TEM).

He is continuing his collaboration with MBL and also working with the Massachusetts Institute of Technology to revise the TEM, and he is incorporating the TEM into MIT’s Integrated Global System Modeling framework, which analyzes interactions between humans and climate, the causes of global climate change, and its social and environmental consequences.

“Our TEM is the terrestrial component of the IGSM,” says Felzer.

Simulations at the global and continental scales…

Felzer’s models incorporate data from the IGSM, as well as the Intergovernmental Panel on Climate Change (IPCC), the U.S. Geological Survey, and other government and academic organizations.

Felzer uses Lehigh’s Beowulf cluster to run simulations of the National Center for Atmospheric Research’s 3-D, fully coupled Community Earth System Model.

“We model at the continental and global scales. We look at the fluxes of carbon, nitrogen and water between the atmosphere, vegetation and soils to determine how plants will grow, and how much carbon plants are taking out of the environment and releasing back to the atmosphere.”

To improve the accuracy of his models and projections, Felzer and his colleagues analyze the various parts of a plant to assess the interactions between carbon and water.

…and at the level of stems, leaves and roots

“Before, vegetation was [treated as] one monolithic model. Now, we look at stems, leaves and roots. We have to understand these individually to accurately assess carbon-water linkings. These are dynamic relationships based on physics. They require numerical models—a series of differential equations that are highly nonlinear.”

Given that most temperate and high-latitude forests are nitrogen-limited, says Felzer, understanding the behavior of nitrogen is critical to making accurate assessments about carbon.

“A lot of northeast U.S. forests are naturally nutrient-limited,” he says. “TEM was one of the first models to deal with these limitations.

“Our goal is to understand how ecosystems respond to changes in carbon dioxide or to climate. We’re trying to understand why things occurred historically and how sensitivity to various disburbances will affect future climate feedbacks.”

Felzer has funding from the U.S. Department of Energy and MIT, and from Lehigh.