Posted: December 20, 2018

Researchers develop first carbon budget for North American coastal waters

By Matt Carroll and Jeff Mulhollem

UNIVERSITY PARK, Pa. - Penn State researchers were among a team of international scientists whose work on a new climate assessment of North America provides a better understanding of the carbon cycle.

The Second State of the Carbon Cycle Report (SOCCR2) offers the first carbon budget for North American coastal waters and new information on how factors such as rivers and livestock impact concentrations of greenhouse gasses in the atmosphere.

"Carbon dioxide and methane are big drivers of climate change," said Raymond Najjar, professor of oceanography in the College of Earth and Mineral Sciences. "If we want to do something about it, we need to manage carbon sources and sinks."

The report found that the burning of fossil fuels continues to be the largest source of atmospheric carbon dioxide. The planet's ecosystems, such as forests and oceans, remain a net sink, removing and storing almost half the carbon dioxide produced each year.

"This report sets the baseline for where we are now and suggests areas where we might exert some control on the carbon cycle to reduce emissions or enhance sinks," said Najjar, who was one of five science leads on the report, organizing chapters on wetlands, inland waters and coastal waters.

Alex Hristov, professor of dairy nutrition in the College of Agricultural Sciences, served as lead author on the report's agriculture chapter. Also contributing to the report from Penn State were Kenneth Davis, professor of atmospheric and climate science, and Maria Herrmann, assistant research professor, both in the Department of Meteorology and Atmospheric Science.

New findings for coastal, inland waters

The report provides the first carbon budget for North American coastal waters. Scientists found that these waters are an important carbon sink, removing large amounts of carbon dioxide from the atmosphere.

"It turns out coastal waters are taking up a significant amount of carbon," Najjar said. "It's less than what the land surface is taking up, but it's considerable. And it's the first time it's been estimated."

While they make up a small overall percent of ocean surface area, coastal waters are thought to contribute disproportionally to the burial of carbon in ocean sediments.

Aquatic plants such as plankton and sea grass use photosynthesis to absorb carbon dioxide. When the organic material dies, some amount makes it way to the ocean floor. There it can be stored for millions of years, eliminating it from contributing to the greenhouse effect.

Najjar also oversaw the chapter focusing on inland waters such as rivers and lakes that included a surprising finding - the waters are a large net source of carbon dioxide. This finding differs from the first State of the Carbon Cycle Report, published in 2007, that estimated inland waters were a net sink.

The new report revealed that inland waters do store some carbon, burying it in reservoirs and river and lake beds and transporting it to the open ocean, but they release even more into the atmosphere as carbon dioxide.

"There's got to be something that's feeding those losses in order for the system to be balanced," Najjar said. "There must be quite a lot of carbon coming in. Working to better understand that process is going to be a very active area of research moving forward."

Livestock plays important role

Agricultural production is a huge enterprise in North America with major environmental influence. It is a fundamental activity conducted on 45 percent of the U.S. land area, 55 percent of Mexico and 7 percent of Canada.

Because of this vast expanse and the strong role that land management plays in how agricultural ecosystems function, agricultural lands and activities represent a large portion of the North American carbon budget.

Hristov, who has published a number of studies in recent years quantifying the amount of greenhouse gases emitted by livestock, made certain that livestock emissions were emphasized in the agriculture chapter, since they did not receive as much attention in the first SOCCR, he noted.

The North American livestock sector represents a significant source of greenhouse gas emissions, generating many tons of methane, nitrous oxide and carbon dioxide throughout the production process. Livestock contributions to greenhouse gas emissions occur either directly from enteric fermentation and manure management or indirectly from feed-production activities.

Emissions from livestock represent 3.8 percent of total U.S. greenhouse gas emissions. Most of those emissions come from beef and dairy cattle and from manure-management activities associated with dairy cattle. "But significantly, there is an opportunity for a 30- to 80-percent mitigation in greenhouse gases emitted by the livestock sector resulting from feed additives and manure management," Hristov said.

The chapter warns that climate change will increase methane emissions from manure.

"Manure management will become more challenging because warmer conditions will speed biological processes," Hristov said. "This process is largely temperature dependent."

But potential effects of climate change on agricultural soil carbon stocks are difficult to assess, the report states, because they will vary according to the nature of the change, onsite ecosystem characteristics, production system and management type. There is uncertainty in those predictions, Hristov explained, because no "conclusive science" exists to gauge those influences.

Agricultural regional carbon budgets and net emissions are directly affected by human decision-making, the chapter states. Trends in food production and agricultural management, and thus carbon budgets, can fluctuate significantly with changes in global markets, diets, consumer demand, regional policies and incentives.