Carbon capacity: Unearthing nature's climate controls
August 11, 2015
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.”
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.”
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.
Desai is also working in the central Rocky Mountains, a region plagued by persistent drought and a warming-induced spread in the frequency and intensity of pest outbreaks, particularly bark beetle infestations. The beetles have thrived through recent unusually mild winters, threatening the region’s evergreen forests.
With support from the National Oceanic and Atmospheric Administration, Desai is measuring the atmospheric carbon concentration on the tops of mountains to study how these forest stressors have impacted carbon absorption. The findings will help guide projections of how carbon uptake could change in a world with an increased frequency of drought or pest outbreaks, or a decreased frequency of either.
Galen McKinley, who studies how large bodies of water influence carbon cycling, is also investigating the impact of pest outbreaks on carbon uptake, but from an underwater perspective.
In collaboration with Harvey Bootsma of the UW-Milwaukee School of Freshwater Sciences, McKinley is trying to better understand how quagga mussels are modifying Lake Michigan.
The invasive species has blanketed the lake bottom at a rate of about 10,000 mussels per square meter. The trillions of mussels gorge on phytoplankton, removing the primary food source for fish.
McKinley and her lab of researchers are developing a hydrodynamic model that simulates circulation in the lake’s food web. By representing quagga mussels in the model, McKinley hopes to be able to demonstrate in more detail the mussels’ influence on nutrient and carbon cycling.
“Their impact is so important that you can’t simulate the rest of the system without them; it becomes part of the lake chemistry,” McKinley says. Once the mussels are integrated into the model, she says, “You can start asking ‘what if’ questions like ‘what if we get rid of them’ or ‘what if acidification hurts them? What might warming do?’”
Acidification – a decrease in water pH as a result of carbon uptake – is another area of study for McKinley in both the Great Lakes and the global oceans. “As we put more carbon dioxide in the atmosphere, we drive more carbon into the water and that acidifies the water,” she explains.
Ocean pH has fallen significantly since preindustrial times and continues downward, with a range of ecological impacts. McKinley suspects the same is true in the Great Lakes, but available datasets aren’t detailed enough to know for sure.
This lack of data isn’t limited to acidification; as a whole the Great Lakes are understudied, she says. With Nelson Institute Environment and Resources student Jennifer Phillips (M.S. ‘12), McKinley has worked to encourage better monitoring of pH and other factors.
In her research of the Great Lakes, McKinley draws from her extensive background in ocean physics. She’s been especially focused on how ocean carbon uptake is changing in the face of increasing atmospheric carbon. Her goal is to learn how much more carbon dioxide the oceans can pull from the air, particularly as the planet warms.
McKinley and colleagues recently published a nearly three-decade analysis of the rate at which the ocean is absorbing human-produced carbon by comparing the surface carbon content of the North Atlantic to atmospheric carbon trends from 1981-2009.
The report, funded by the National Aeronautics and Space Administration, provides some of the first evidence that the ocean is taking up less carbon because of climate change. Warm water can’t hold as much carbon dioxide, so rising temperatures weaken the ocean sink.
As is true with the Great Lakes, limitations in ocean data are a major obstacle to research. As more data becomes available, McKinley can expand her analyses, using her findings to refine predictive models and future data collection.
“That’s an area that a lot of people in CCR have worked on – trying to understand the future climate – but this brings in the carbon cycle component, which is still pretty nascent,” she says.
“As we better observe the ocean carbon cycle, see changes and get more data, there’s going to be a lot of new questions brought forward,” she says. “I don’t see any limitation on the questions that can be asked. That’s one of the reasons I love oceanography – it’s a brave new world.”