SCIENCE FOR SHARING
Forests, wetlands, and farms in the eastern United States sequester approximately 300 million tons of carbon a year, an amount comparable to the annual greenhouse gases emitted by all passenger cars in the U.S., according to a new USGS report.
What, exactly, is carbon sequestration? Why is it a good thing?
Two types of carbon sequestration
Carbon sequestration is the process by which carbon dioxide is removed from the atmosphere and stored. There are two main types of carbon sequestration: biologic and geologic.
We’ll explain biologic carbon sequestration below. USGS also studies geologic carbon sequestration, the storing of carbon in underground rock formations, and has published a national assessment of storage potential. This 2013 report estimates a mean of 3,000 metric gigatons of potential carbon dioxide (CO2) storage in the United States.
Biology at work
Carbon is a part of every living thing. For instance, the human body is about 18% carbon. Carbon is also a part of many non-living things, too — like air, soil, rocks, the oceans and, famously, pencils and soft drinks.
Carbon moves in natural processes called the carbon cycle between the atmosphere (air), the biosphere (living things), and the geosphere (land and water). Biologic carbon sequestration is a more specific part of the carbon cycle in which plants naturally take in CO2 from the atmosphere by way of photosynthesis. Here’s how it works.
As plants grow, they accumulate, or sequester, carbon in various ways: in living biomass, in litter on the ground when leaves fall, and in soils when organic material decomposes. At the same time, plants also release carbon as carbon dioxide to the atmosphere. While plants live, these carbon releases occur through natural respiration and, when they die, either by natural means (e.g. fire or decomposition) or by human causes (e.g. clear cutting).
When the amount of carbon that is accumulated in ecosystems is greater than the amount removed either as emissions to the atmosphere or by physical removal (e.g. for wood products), we refer to this situation as a carbon sink because the rate of sequestration is greater than the rate of removal. The term carbon source applies to the opposite situation when the rate of carbon removal from the ecosystem is greater than the rate of carbon sequestration.
Science to support climate change policies
The President’s Climate Action Plan (June 2013) notes the importance of biological carbon sequestration: “America’s ecosystems are critical to our nation’s economy and the lives and health of our citizens. These natural resources can also help ameliorate the impacts of climate change, if they are properly protected.” Congress, too, has recognized its importance, mandating a national inventory of carbon sequestration and greenhouse gas fluxes under section 712 of the Energy Independence and Security Act of 2007.
The goals of the USGS Land Carbon project are to implement the 2007 Congressional directive and to improve the scientific understanding of carbon sequestration and greenhouse gas fluxes in and out of ecosystems related to land use.
The USGS regional report on eastern ecosystems released June 25 covers an area of 1.18 million square miles in the eastern United States — from northwest Minnesota to the tip of Florida, from east Texas to northernmost Maine, completing the resource assessment of biologic carbon sequestration in the conterminous United States. The first report, the Great Plains, was released in 2011. The second report on Western Ecosystems was released in 2012. Reports on Alaska and Hawaii will follow in 2015.
Eastern ecoregion findings
This regional assessment shows that the East stores more carbon than all of the rest of the lower 48 states combined even though it has just under 40 percent of the land base. Forests, which occupy about half the land in the East, accounted for more than 80 percent of the region’s estimated carbon sequestered annually. They are the largest carbon-storing pools, and have the highest rate of sequestration of the different ecosystem types.
Wetlands, including coastal ones, which comprise only about 9 percent of the land cover in the East, account for nearly 13 percent of the region’s estimated annual carbon storage. They also have the second-highest rate of sequestration of all ecosystem types. Nutrients and sediments in rivers and streams flowing from terrestrial environments contribute significantly to the storage of carbon in eastern coastal sediments and deep ocean waters.
The eastern United States is projected to continue to be a carbon sink (absorbs more carbon than it emits) through 2050, increasing the carbon stored by as much as 37 percent. However, the rate of sequestration is projected to slow by up to 20 percent, primarily because of decreases in the amount of forest cover.
The national perspective
Based on studies done to date on the lower 48 states, national ecosystems are projected to continue to absorb CO2 through 2050. However, the overall rate of carbon storage could drop significantly, up to 25%, largely due to projected increases in urbanization and agriculture and an increased demand for forest products.
Under many climate change scenarios, wildfires in the West are expected to increase in number, scope, and intensity, with results that could impact western ecosystems and alter the carbon storage potential. Projections based on IPCC scenarios show greenhouse gas emissions from wildfires in the West could increase 28-56% by 2050.
In addition, in nearly every scenario global greenhouse gas emissions are projected to continue increasing. Consequently, USGS projections based on future land-use scenarios consistent with the IPCC, show that, by 2050, our nation’s lands would have less sequestration ability to offset increasing greenhouse gas emissions.
Visualizing carbon storage
A new web tool, the “Land Carbon Viewer,” allows users to see the land carbon storage and change in regional ecosystems between 2005 and 2050 in the lower 48 states. Developed by U.S. Geological Survey, in collaboration with the University of California-Berkeley Geospatial Innovation Facility, this visualization tool gives the public an easily accessible look at the capacity of land-based ecosystems to store, or sequester, carbon. A simple tutorial on how to use the tool is available here.
Start with science
In addition to the Land Carbon assessment, USGS studies climate and land use change effects across the country. The resulting research and products add to the scientific foundation upon which policymakers, natural resource managers, and the public can make informed decisions about the management of the nation’s natural resources.
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