Carbon Sequestration to Mitigate Climate Change 

Human activities, especially the burning of fossil fuels such as coal, oil, and gas, have caused a substantial increase 

in the concentration of carbon dioxide (CO2 ) in the atmosphere. This increase in atmospheric CO2 —from about 280 to 

more than 380 parts per million (ppm) over the last 250 years—is causing measurable global warming. Potential adverse 

impacts include sea-level rise; increased frequency and intensity of wildfires, floods, droughts, and tropical storms; changes 

in the amount, timing, and distribution of rain, snow, and runoff; and disturbance of coastal marine and other ecosystems. 

Rising atmospheric CO2 is also increasing the absorption of CO2 by seawater, causing the ocean to become more acidic, with 

potentially disruptive effects on marine plankton and coral reefs. Technically and economically feasible strategies are needed 

to mitigate the consequences of increased atmospheric CO2. The United States needs scientific information to develop ways 

to reduce human-caused CO2 emissions and to remove CO2 from the atmosphere. 

What is carbon sequestration? 2.0

The term “carbon sequestration” is used to describe both natural and deliberate 

 CARBON, IN GIGATONS PER YEAR 1.5 

Fossil-fuel 

CO2 

release 

forest 

Net 

uptake 

Net 

non-forest 

uptake 

Net CO2 

release

0 

NET FLUX OF CARBON AS CO2 

processes by which CO2 is either removed from the atmosphere or diverted from 

emission sources and stored in the ocean, terrestrial environments (vegetation, soils, 

and sediments), and geologic formations. Before human-caused CO2 emissions began, 

the natural processes that make up the global “carbon cycle” (fig. 1) maintained 

a near balance between the uptake of CO2 and its release back to the atmosphere. 

However, existing CO2 uptake mechanisms (sometimes called CO2 or carbon 

“sinks”) are insufficient to offset the accelerating pace of emissions related to human 

activities. Annual carbon emissions from burning fossil fuels in the United States 

are about 1.6 gigatons (billion metric tons), whereas annual uptake amounts are only 

about 0.5 gigatons, resulting in a net release of about 1.1 gigatons per year (fig. 2). 

Oceans

Vegetation

Soils

Oxidation

Natural Anthropogenic

Atmosphere

Fossil fuels

7 2

Photosynthesis

Enhanced gas

exchange

Sea-surface

gas exchange

Burning Deforestation

fossil

fuels

~120 ~120 3 20 ~70 ~70 22

Geologic formations

Uptake by 

plants

and soils

CARBON FLUX, IN GIGATONS PER YEAR

~120 20

Figure 1. The global carbon cycle. Carbon naturally moves, or cycles, between the atmosphere 

and vegetation, soils, and the oceans over time scales ranging from years to millennia and longer. 

Human activities, primarily the burning of fossil fuels and clearing of forests, have increased the 

transfer of carbon as CO2 to the atmosphere. Although some of this anthropogenic CO2 is removed 

from the atmosphere by the natural uptake processes (“sinks”) of the carbon cycle, much of it 

remains in the atmosphere and causes rising CO2 concentrations. The goal of deliberate carbon 

sequestration is to decrease the net flux of CO2 to the atmosphere by sequestering carbon in the 

oceans, vegetation, soils, and porous rock formations. Fluxes shown are approximate for the period 

2000–05, as documented by the Intergovernmental Panel on Climate Change. 

1.0 

0.5 

Figure 2. Estimated annual net CO2 

emissions and uptake in the United States in 

2003, according to the U.S. Climate Change 

Science Program. U.S. fossil-fuel CO2 

emissions accounted for more than 20 percent 

of the global total in 2003. Net uptake fluxes 

(photosynthesis minus oxidation) are shown 

for forest and non-forest areas. The largest net 

U.S. CO2 uptake was associated with regrowing 

forests and harvesting wood products. 

Non-forest net carbon uptake—including 

accumulation in shrubs, agricultural soils, 

wetlands, rivers, and reservoirs—was smaller 

and more uncertain. 

Controlling atmospheric CO2 will require 

deliberate mitigation with an approach 

that combines reducing emissions and 

increasing storage. 

Scientists at the U.S. Geological 

Survey (USGS) and elsewhere are 

working to assess both the potential 

capacities and the potential limitations 

of the various forms of carbon 

sequestration and to evaluate their 

geologic, hydrologic, and ecological 

consequences. The USGS is providing 

information needed by decision makers 

and resource managers to maximize 

carbon storage while minimizing 

undesirable impacts on humans and their 

physical and biological environment. 

U.S. Department of the Interior Fact Sheet 2008-3097 Printed on recycled paper U.S. Geological Survey Page 1 December 2008 
Page 2 

Oceanic carbon sequestration 

The world’s oceans are the primary long-term sink for human-caused CO2

emissions, currently accounting for a global net uptake of about 2 gigatons of 

carbon annually (fig. 1). This uptake is not a result of deliberate sequestration, 

but occurs naturally through chemical reactions between seawater and CO2 in 

the atmosphere. While absorbing atmospheric CO2, these reactions cause the 

oceans to become more acidic. Many marine organisms and ecosystems depend 

on the formation of carbonate skeletons and sediments that are vulnerable 

to dissolution in acidic waters. Laboratory and field measurements indicate 

that CO2-induced acidification may eventually cause the rate of dissolution of 

carbonate to exceed its rate of formation in these ecosystems. The impacts of 

ocean acidification and deliberate ocean fertilization on coastal and marine 

food webs and other resources are poorly understood. Scientists are studying 

the effects of oceanic carbon sequestration on these important environments. 

Terrestrial carbon sequestration 

Terrestrial sequestration (sometimes 

termed “biological sequestration”) is typically 

accomplished through forest and soil conservation 

practices that enhance the storage of carbon (such 

as restoring and establishing new forests, wetlands, 

and grasslands) or reduce CO2 emissions (such 

as reducing agricultural tillage and suppressing 

wildfires). In the United States, these practices are 

implemented to meet a variety of land-management 

objectives. Although the net terrestrial uptake 

fluxes shown in figure 2 offset about 30 percent 

of U.S. fossil-fuel CO2 emissions, only a small 

fraction of this uptake results from activities 

undertaken specifically to sequester carbon. The 

largest net uptake is due primarily to ongoing 

natural regrowth of forests that were harvested 

during the 19th and early 20th centuries. 

Existing terrestrial carbon storage is 

susceptible to disturbances such as fire, disease, 

and changes in climate and land use. Boreal forests 

and northern peatlands, which store nearly half 

the total terrestrial carbon in North America, are 

already experiencing substantial warming, resulting in large-scale thawing of permafrost 

and dramatic changes in aquatic and forest ecosystems. USGS scientists have estimated 

that at least 10 gigatons of soil carbon in Alaska is stored in organic soils that are 

extremely vulnerable to fire and decomposition under warming conditions. 

The capacity of terrestrial ecosystems to sequester additional carbon is uncertain. 

An upper estimate of potential terrestrial sequestration in the U.S. might be the amount of 

carbon that would be accumulated if U.S. forests and soils were restored to their historic 

levels before they were depleted by logging and cultivation. These amounts (about 32 and 

7 gigatons for forests and soils, respectively) are probably not attainable by deliberate 

sequestration because restoration on this scale would displace a large percentage of U.S. 

agriculture and disrupt many other present-day activities. Decisions about terrestrial 

carbon sequestration require careful consideration of priorities and tradeoffs among 

multiple resources. For example, converting farmlands to forests or wetlands may increase 

carbon sequestration, enhance wildlife habitat and water quality, and increase flood 

storage and recreational potential– but the loss of farmlands will decrease crop production. 

Converting existing conservation lands to intensive cultivation, while perhaps producing 

valuable crops (for example, for biofuels), may diminish wildlife habitat, reduce water 

quality and supply, and increase CO2 emissions. Scientists are working to determine the 

effects of climate and land-use change on potential carbon sequestration and ecosystem 

benefits, and to provide information about these effects for use in resource planning. 

The term “carbon 

sequestration” is 

used to describe 

both natural 

and deliberate 

processes by 

which CO2 is 

either removed 

from the 

atmosphere or 

diverted from 

emission sources 

and stored in the 

ocean, terrestrial 

environments 

(vegetation, soils, 

and sediments), 

and geologic 

formations. 

CROPLAND

TRADEOFFS

FORESTS WETLANDS
Geologic carbon sequestration 

Geologic sequestration begins with 

capturing CO2 from the exhaust of fossilfuel power plants and other major sources. 

The captured CO2 is piped 1 to 4 kilometers below the land surface and injected 

into porous rock formations (fig. 3). 

Compared to the rates of terrestrial carbon 

uptake shown in figures 1 and 2, geologic sequestration is currently used to 

store only small amounts of carbon per 

year. Much larger rates of sequestration 

are envisioned to take advantage of the 

potential permanence and capacity of 

geologic storage. 

The permanence of geologic sequestration depends on the effectiveness of 

several CO2 trapping mechanisms. After 

CO2 is injected underground, it will rise 

buoyantly until it is trapped beneath an 

impermeable barrier, or seal. In principle, 

this physical trapping mechanism, which 

is identical to the natural geologic trapping of oil and gas, can retain CO2 for 

thousands to millions of years. Some of 

the injected CO2 will eventually dissolve in ground water, and some may 

be trapped in the form of carbonate 

minerals formed by chemical reactions 

with the surrounding rock. All of these 

processes are susceptible to change over 

time following CO2 injection. Scientists 

are studying the permanence of these 

trapping mechanisms and developing 

methods to determine the potential for 

geologically sequestered CO2 to leak 

back to the atmosphere. 

The capacity for geologic carbon 

sequestration is constrained by the 

volume and distribution of potential 

storage sites. According to the U.S. 

Department of Energy, the total storage 

capacity of physical traps associated 

with depleted oil and gas reservoirs in 

the United States is limited to about 38 

gigatons of carbon, and is geographically 

distributed in locations that are distant 

from most U.S. fossil-fuel power plants. 

The potential U.S. storage capacity of 

deep porous rock formations that contain 

saline ground water is much larger 

(estimated by the U.S. Department of 

Energy to be about 900 to 3,400 gigatons 

of carbon) and more widely distributed, 

but less is known about the effectiveness 

of trapping mechanisms at these sites. 

Unmineable coal beds have also been 

proposed for potential CO2 storage, 

but more information is needed about 

the storage characteristics and impacts 

of CO2 injection in these formations. 

Scientists are developing methods to 

refine estimates of the national capacity 

for geologic carbon sequestration. 

To fully assess the potential for 

geologic carbon sequestration, economic 

costs and environmental risks must be 

taken into account. Infrastructure costs 

will depend on the locations of suitable 

storage sites. Environmental risks may 

include seismic disturbances, deformation of the land surface, contamination 

of potable water supplies, and adverse 

effects on ecosystems and human health. 

Scientists are pioneering the use of new 

geophysical and geochemical methods 

that can be used to anticipate the potential costs and environmental effects of 

geologic carbon sequestration. 

 

Injection of CO2 into Pipeline transporting CO2 Offshore natural gas 

Depleted hydrocarbon reservoir

Deep coal

seam

Reservoir trap/seal

Formation 

containing 

saline water

from power plants to 

injection site

geologic reservoirs

Natural-gas reservoir Formation 

Coalbed 

methane 

production

production with CO2

separation and sequestration

containing

EXPLANATION saline water

CO2 injection Enhanced fossil-fuel recovery Conventional fossil-fuel recovery

Oil and gas reservoirs Formations containing saline water Unmineable coal beds 

Advantages: • Well-characterized volume

• Known seal

• Potential fuel recovery to offset cost

• Largest capacity

 (~250 to 900 gigatons carbon)

• Wide distribution

• Adjacent to many large power plants

 (CO2 source)

• Potential fuel (methane) recovery to offset cost

Disadvantages: • Smallest capacity (~25 gigatons carbon)

• Limited in number

• Requires infrastructure to transport CO2

• Poorly characterized

• Greatest geologic uncertainty

• Unknown seal effectiveness

• Poorly characterized

• Difficult to define "unmineable" coal

• Potential coal resources may be rendered unusable

Environmental issues: Potential for mobilization of ground-water contaminants; leakage of CO2 and CO2-saturated saline water; induced seismicity

Regulatory issues: Determination of rules affecting injection wells; multiple regulatory jurisdictions (State, Federal, local); post-injection ownership and liability

Figure 3. Types of geologic CO2 sequestration, their advantages and disadvantages, and potential environmental and regulatory issues. 

Offshore natural-gas production and CO2 sequestration are currently occurring off the coast of Norway, where the gas produced contains a 

high concentration of CO2 that is removed and injected into a nearby formation containing saline water. 

Page 3 
Mitigation 

by all other 

methods 

Mitigation by 

geologic 

sequestration 

Soils 

Vegetation 

3,400 (maximum) 

200 

100 

0 

150 

50

 CARBON, IN GIGATONS

Depleted oil 

and gas 

reservoirs 

Formations 

containing 

saline water 

TERRESTRIAL 

(MAXIMUM) 

GEOLOGIC 

(POTENTIAL) 

EMISSIONS REDUCTION 

AND SEQUESTRATION 

A Modeled U.S. mitigation by 2100 B Estimated U.S. sequestration capacity 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 4. Estimated U.S. atmospheric CO2 (as carbon) mitigation needs and potential sequestration 

capacities: (A) Cumulative U.S. CO2 emissions reduction and sequestration needed by 2100 to help stabilize 

atmospheric CO2 at 550 parts per million (model results from U.S. Climate Change Science Program); (B)

Estimated U.S. CO2 sequestration capacity. (The estimate of ~3,400 gigatons carbon for potential geologic 

storage is equivalent to ~12,600 gigatons CO2, as estimated by the U.S. Department of Energy. Terrestrial 

sequestration is not expected to approach the estimate shown. Uncertainties in estimated terrestrial and 

geologic sequestration are substantial.) 

Why action is needed now 

Cumulative historical CO2 emissions 

from fossil fuels in the United States 

are equivalent to more than the total 

amount of carbon stored in U.S. forests. If 

current trends continue, cumulative U.S. 

emissions are projected to double by 2050 

and increase by a factor of three to four by 

2100. According to the Intergovernmental 

Panel on Climate Change Fourth 

Assessment Report of 2007, sequestration 

and reduction of emissions over the next 

two to three decades will potentially 

have a substantial impact on longterm opportunities to stabilize levels of 

atmospheric CO2 and mitigate impacts of 

climate change. 

Can sequestration control 

atmospheric CO2? 

Computer models of future CO2 

emissions and controls on atmospheric 

CO2 have been developed and summarized 

by the U.S. Climate Change Science Program (CCSP). These models indicate that 

projected annual global emissions during 

the next century would need to be reduced 

by more than 75 percent in order to stabilize atmospheric CO2 at about 550 ppm. 

This concentration would be about twice 

the level of CO2 in the pre-industrial 

atmosphere and about 45 percent higher 

than the atmospheric CO2 concentration in 

2007. According to the CCSP, stabilizing 

atmospheric CO2 would “require a transformation of the global energy system, 

including reductions in the demand for 

energy...and changes in the mix of energy 

technology and fuels.” 

The CCSP models have been used 

to evaluate scenarios for aggressive 

implementation of geologic carbon 

sequestration. As shown in figure 

4A, the estimated amount of geologic 

sequestration in the U.S. over the next 

century is projected in one model to be 

substantially smaller than the cumulative 

emission reductions anticipated from 

changes by all other methods. In this 

model, the needed amount of geologic 

sequestration would exceed U.S. capacity 

in depleted oil and gas reservoirs, 

implying the need to implement carbon 

storage in the Nation’s relatively 

unknown deep formations that contain 

saline water. In other models, predicted 

geologic sequestration needs are smaller 

as a result of different assumptions 

about global and national economic and 

technological trends. 

The CCSP model results have a 

large amount of uncertainty. The results 

shown in figure 4 do not take into account 

many of the uncertainties in costs and 

environmental risks of geologic carbon 

sequestration. Additional uncertainties 

prevent comparison of future oceanic and 

deliberate terrestrial sequestration. Future 

disturbances of vegetation and soils may 

add to future CO2 emissions and increase 

the amount of mitigation required to stabilize atmospheric CO2. For example, if 

a substantial portion of the carbon stored 

in Alaskan organic soils were converted 

to atmospheric CO2 as a result of climate 

change, the resulting emissions could offset or even exceed the likely magnitude 

of any deliberate U.S. terrestrial sequestration measures. 

The CCSP models illustrate the 

widely held view that sequestration 

is necessary but insufficient to 

control atmospheric CO2. Stabilizing 

atmospheric CO2 is likely to require 

substantial changes in energy sources 

and use as well as carbon management. 

Many of these changes will likely have 

significant, long-lasting impacts on land, 

water, and ecosystem resources. 

How the USGS is helping 

The USGS is committed to acquiring and communicating the scientific 

information needed to control atmospheric CO2 and minimize its harmful 

effects on natural resources. For many 

years, USGS scientists have contributed to the growing body of knowledge 

concerning climate change, the carbon 

cycle, and carbon sequestration. Building on its tradition of providing reliable, impartial scientific information 

to support the management of natural 

resources, the USGS provides information to support carbon management by: 

•	 applying the expertise of many 

technical disciplines to understand 

how to limit CO2 emissions and 

maximize carbon storage; 

•	 integrating information about the 

multiple resources that will be 

affected by carbon management 

decisions; and 

•	 communicating scientific information 

across the many jurisdictions and 

organizations responsible for and 

affected by carbon management. 

The USGS is providing information needed to assess the need for CO2 

mitigation, evaluate the potential for carbon sequestration to meet that need, and 

anticipate the potential environmental 

consequences of implementing the full 

portfolio of strategies for controlling 

atmospheric CO2. 

By Eric Sundquist, Robert Burruss, 

Stephen Faulkner, Robert Gleason, 

Jennifer Harden, Yousif Kharaka, 

Larry Tieszen, and Mark Waldrop 

Edited by Dale L. Simmons 

Graphic design by Christine T. Mendelsohn 

Figure 4 by Eric A. Morrissey and Sean Brennan 

For additional information about this report, 

please contact Eric Sundquist 

(esundqui@usgs.gov). 

For additional information about the 

U.S. Geological Survey, please visit our web site 

at http: //www.usgs.gov/. 

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