The University of Arizona

Sequestering Greenhouse Gases in Soils, Trees, and Underground

By Joe Abraham | The University of Arizona | June 01, 2009

There are several ways to remove greenhouse gases from the atmosphere, both natural and engineered. Plants naturally absorb carbon dioxide (CO2) from the atmosphere as part of the process of photosynthesis. While some CO2 is released back to the atmosphere through plant respiration and decomposition, a significant amount of carbon can be stored or “sequestered” in soils and trees over time.

photo of a cotton field

Agriculture operations like this cotton field near Las Cruces, New Mexico, can play an important role in reducing atmospheric greenhouse gas concentrations.
Credit: ©M. Moritz, istockphoto.com

Also, scientists and engineers are working on technologies that capture CO2 shortly after it is produced by large industrial sources like coal-fired power plants and sequester it in underground geologic formations. If widely used, these technologies would help society meet future energy needs while transitioning to more low-carbon and renewable energy.

Thus, solutions for sequestering atmospheric CO2 are proceeding along three general trajectories:

Options for sequestering CO2 discussed below are based on state plans created between 2005 and 2007 in Arizona, New Mexico, Utah, and Colorado to help achieve statewide greenhouse gas emission reductions. Gubernatorial executive orders instigated the planning processes in Arizona, New Mexico, and Utah, while a coalition of stakeholders (including the state of Colorado) led by a regional non-profit organization initiated the Colorado plan. Each planning process produced a set of recommended options for reducing emissions.

Sequestering atmospheric CO2 in agricultural land and preserving soil carbon content

A significant amount of land is used to grow crops in the Southwest. Farmers typically till their land to prepare it for the next season’s crops. Tilling the soil has many benefits but can also leave the soil vulnerable to erosion from wind and water. “Conservation tillage” is an agricultural practice that reduces soil disturbance, stabilizes carbon in the soil, and reduces the release of CO2 to the atmosphere.1 Depending on the particular method, conservation tillage can also reduce fuel-based emissions during tilling, improve soil quality and water infiltration, and reduce dust. Recent research, however, suggests in some cases conservation tillage may increase the release of other, more potent greenhouse gases, potentially offsetting any CO2 sequestration benefits.2

Graph of carbon sequestration recommendations

Figure 1. Options from state climate action plans for sequestering atmospheric CO2 greenhouse gas emissions.
| Enlarge This Figure |
Credit: Joe Abraham and Rebecca Macaulay, CLIMAS, The University of Arizona

Regardless, both the New Mexico and Colorado climate action plans identified conservation tillage as an important carbon sequestration strategy. The Colorado plan recommended the amount of acres farmed in the state using conservation tillage be increased by 50 percent by 2020. The New Mexico plan recommended a nearly five-fold increase in land farmed with conservation tillage methods. Estimates of emission reductions for each state differ significantly (Figure 1), with Colorado estimating a net savings of approximately $4 per ton of carbon sequestered from 2007 through 2020, and New Mexico estimating a net cost of $15 per ton. Differences in emission reduction and cost estimates reflect various assumptions and factors discussed in appendices of the New Mexico3 and Colorado4 state plans.

Related to conservation tillage, New Mexico’s climate action plan recommends increasing organic farming in the state to include approximately 70 percent of all vegetable and field crop production acreage by 2050. Organic farming generally results in higher levels of carbon sequestered in the soil. It also diminishes the use of petroleum-based fertilizers that may be shipped long distances.5 New Mexico’s estimate of emission reductions from increased organic farming (Figure 1), however, only considers emission reductions from carbon sequestered in the soil.3

Read about the Foodprint NM project that is helping reduce the carbon footprint of food production in New Mexico.

Converting marginal farmland from annual crop production to permanent grass or forested cover is another strategy for sequestering carbon in soils. It would help reduce the loss of existing soil carbon to the atmosphere from tillage and, over time, also would help build up carbon sequestered in the soil. One policy option for converting marginal farmland involves financial incentives for landowners based on estimated carbon sequestration potential (Colorado). Another option is paying owners of marginal farmland that is registered with the federal Conservation Reserve Program (CRP) to refrain from farming the land once it expires from the program (Colorado, New Mexico). Approximately 145,000 acres in New Mexico alone will no longer qualify for the CRP program by 2010. Soil carbon loss from the first two years of tilling these lands was estimated at more than 15 tons per acre3, but was estimated to cost $7 per ton of CO2 sequestered in New Mexico and almost $30 per ton in Colorado for 2007 through 2020.

All other things equal, developed land sequesters and stores less carbon than farmland, grasslands, or forested land. Climate action plans from Utah, Colorado, New Mexico, and Arizona recommended policies that reduce the rate of development of farmland and forest (Figure 1). A range of policy options was identified by the state plan participants, including targeted financial incentives, conservation easements, and help for older landowners in transferring land to young farmers and ranchers. Costs for avoiding development are relatively high because they reflect financial incentives that would be required to equal the profitability of developing the land.

Sequestering atmospheric CO2 in trees and forests

photo of downtown Tucson trees

Trees in downtown Tucson, Ariz., sequester atmospheric CO2, provide shade, and help reduce the urban heat island effect.
Credit: ©Scott Wood, istockphoto.com

In addition to reducing the development of forested lands, Arizona and Colorado identified reforestation and urban forestry to increase carbon sequestration in natural, rural, and urban environments. While the amount of carbon stored based on the proposed options is small relative to other options for sequestering carbon (Figure 1), reforestation and urban forestry have important co-benefits, including protecting watersheds and mitigating urban heat islands. Arizona, for example, has several hundred thousand acres of forestland recently affected by wildfire and insects that could be reforested, but doing so would require significant investment. Focusing on urban forestry, the Colorado plan recommended planting more than three million new trees in the state’s urban areas by 2025. While relatively expensive at $79 per ton of CO2 sequestered, urban forestry can help reduce cooling costs and improve the natural quality of urban settings.

Sequestering carbon dioxide emissions underground

illustration of geological storage options

Figure 2. Methods for storing CO2 in deep underground geological formations.
| Enlarge This Figure |
Credit: Intergovernmental Panel on Climate Change

The gap between projected energy demand and renewable energy supply suggests society’s reliance on fossil fuel energy is likely to continue for some time. Researchers are developing and testing technologies, including some in the Southwest that would remove greenhouse gases from large fossil fuel projects and energy sources (e.g., coal-fired power plants) and store those emissions underground in geologic formations, including oil and gas reservoirs and coal beds.

Capturing greenhouse gas emissions and storing them underground is appealing to many policymakers because it relies on existing energy infrastructure and domestic fossil fuels that are generally abundant, and it has the potential to significantly reduce emissions (Figure 1). While the technology and policy to implement geologic sequestration of greenhouse gases is not yet widely available or in place, state climate action plans are identifying policies, practices, and strategies that can be planned and engineered now. For example, pipelines will need to be built between current fossil fuel power plants and geologic formations; ownership of underground sequestration resources will need to be clarified; long-term liability against CO2 leakage will need to be articulated; and potential adverse environmental impacts will need to be managed with regulations.

Related Links

U.S. Environmental Protection Agency web pages on carbon sequestration in Agriculture and Forestry
| http://www.epa.gov/sequestration/faq.html |

United Nations Food and Agriculture Organization page on organic agriculture and greenhouse gas emissions:
| http://www.fao.org/docrep/005/y4137e/y4137e02b.htm#TopOfPage |

City of Albuquerque’s urban forest program
| http://www.cabq.gov/albuquerquegreen/green-goals/trees/urban-forest |

U.S. National Energy Technology Laboratory carbon sequestration FAQ information portal
| http://www.netl.doe.gov/technologies/carbon_seq/faqs.html |

The Southwest Regional Partnership for CO2 Sequestration
| http://www.southwestcarbonpartnership.org/AboutSWP.aspx |

References

  1. Follett, R.F., S.R. Shafer, M.D. Jawson and A.J. Franzluebbers. 2005. Research and implementation needs to mitigate greenhouse gas emissions from agriculture in the USA. Soil and Tillage Research, 83(1): 159–166
  2. Li, C., S. Frolking, and K. Butterbach-Bahl. 2005. Carbon sequestration in arable soils is likely to increase nitrous oxide emissions, offsetting reductions in climate radiative forcing. Climatic Change, 72(3): 321–338.
  3. Mexico Climate Change Advisory Group. 2006. Final Report, Appendix J.
  4. Colorado Climate Action Panel. 2007. Final Report, Appendix H.
  5. Pimentel, D., P. Hepperly, J. Hanson, D. Douds, and R. Seidel. 2005. Environmental, energetic, and economic comparisons of organic and conventional farming systems. Bioscience, 55(7): 573–582.

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