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Carbon capacity

Unearthing nature's climate controls

Winter/Spring 2013 | By Meghan Lepisto

In the exchange that is the carbon cycle, people giveth and land and oceans taketh away.

The outcome of this transaction is a critical factor determining the future climate. 

As humans produce carbon emissions, primarily by burning fossil fuels, about half currently remains in the atmosphere, intensifying the greenhouse effect and driving global warming. 

The other half, in roughly equal parts, is taken up by oceans and land. These natural sinks of carbon serve as a buffer against climate change. 

But in recent years, the percentage of carbon emissions remaining in the atmosphere is increasing. A warming climate has made land or ocean systems possibly less productive at absorbing carbon. This warms the climate even more. 

At the Nelson Institute Center for Climatic Research (CCR), scientists are studying the processes behind atmospheric carbon content and the role of land and water. Their findings could help improve predictions of climate change and provide critical information for decision makers.

Ankur Desai and Galen McKinley, both associate professors of atmospheric and oceanic sciences and CCR affiliates, are examining two complementary pieces of this global carbon cycle puzzle. Desai studies carbon exchange between the atmosphere and terrestrial ecosystems. McKinley is an expert in large aquatic bodies. 

“Over the last 20 or 30 years, one of the things we’ve discovered is how uniquely sensitive the global biosphere is to climate variability, and that strongly influences how fast atmospheric carbon dioxide grows,” says Desai. 

In projections of climate scenarios, researchers can confidently predict a variety of feedbacks, including how the climate responds to greenhouse gases. Carbon cycle feedbacks, however, are more of a puzzle. 

“One of our big motivating factors is trying to reduce that uncertainty, because it really makes it harder to project how a change in emissions affects temperature change,” says Desai. “That’s an open policy question that’s made very much dependent on the basic science of this interaction between the biology and atmosphere.”

“Over the next 100 years, are the land and ocean sinks going to get stronger or weaker?,” he continues. “That turns out to not be a trivial question to answer and is important for any discussion regarding future regulation of greenhouse gas emissions.”

Land-atmosphere interactions

Desai’s research is aimed at untangling the two-way exchange between how ecosystems on the ground interact with the atmosphere and climate, and vice versa. 

On one hand, the atmosphere and climate determine the availability of resources that are fundamental to life – things like light, carbon dioxide, precipitation and moisture – which can determine the type of vegetation in an ecosystem, Desai explains. 

On the other hand, as plants use resources like light and moisture, they alter the atmosphere around them. For example, the darkness or reflectivity of a landscape affects how much light and heat is reflected into the atmosphere. 

“A lot of what we do is try to figure out, if we know something about the landscape, how does a climate system respond?” Desai explains. “And if a climate system responds, how does the landscape change?”

With a range of student and staff expertise that he says mimics a United Nations of ecology, Desai’s lab specializes in bridging the gap between small-scale ecosystem studies and global climate studies. 

“What happens at the small scale influences what happens at the large scale,” he says. “There’s a meeting place where the biology-climate interaction is strongest.” 

Forest feedbacks

For the past decade, Desai has been working with collaborators from across the country to study the uptake and emission of carbon in northern Wisconsin’s forests, wetlands and lakes. 

The region is home to a band of temperate hardwood forests and a significant accumulation of peat bogs. “It’s a combination where you have a lot of carbon in the system and you have very productive organisms that can cycle carbon,” he explains. 

Desai’s research combines observational and modeling techniques. He’s placed a network of towers – ranging from 30- to 1,200-feet tall – throughout the region to capture continuous atmospheric measurements. By taking readings as many as 20 times per second, the towers infer the amount of carbon dioxide and methane – another important greenhouse gas – going in or out of the system. 

Desai’s lab, which is a leader in the use of this technology, has discovered that the region is quite sensitive to climate extremes, though with lags in the system. For instance, the drought that affected much of Wisconsin in the summer of 2012 was delayed in reaching the northern part of the state. But once the drought hit, it shut off production in some of the forests, in a way that was far stronger than expected.

Wisconsin forests are not typically limited by moisture, so they lack adaptation to long periods of drought. “They turn out to be pretty sensitive,” Desai says. 

Desai’s observations are helping to refine models of future climate feedbacks. “If, as we suspect – and as projections indicate – Wisconsin is getting drier in the summer and warmer in the winter, then that has huge ecosystem implications for the productivity of forests,” he says. 

The work also has value outside of the state – some of Desai’s measurements are being shared with global data networks to be used by ecologists and climate scientists from across the world. 

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