U.S. Senate Committee on Environment & Public Works
U.S. Senate Committee on Environment & Public Works
Hearing Statements
Date:   07/08/2003
 
Dr. Cynthia Rosenzweig
Research Scientist
Goddard Institute for Space Studies

Agricultural Sequestration of Carbon Dioxide

Agriculture and climate are mutually dependent. Their interactions involve temperature effects, water supply and demand, and fluxes of carbon through the processes of photosynthesis and respiration. Climate also affects the crop pests and predators. Climate is important not only in terms of average conditions, but also in regard to the frequency and intensity of extreme events, such as floods, droughts, and heat spells.

Agricultural soils can be both a contributor to and a recipient of the effects of a changing climate. In the past, land management has generally resulted in considerable depletion of soil organic matter and the release of carbon dioxide. Now, there is the potential to restore soil organic carbon through improved management techniques, enhancing soil structure and fertility and helping to counter climate change. An important caveat is that the capacity for agricultural soil carbon sequestration is constrained by the amount of carbon lost during the conversion of natural ecosystems to agriculture, so that its effectiveness as a mitigating activity for climate change is not unlimited.

After nearly two decades of research on potential impacts of climate change on agriculture (see Rosenzweig and Hillel, 1998), attention is now turning to mitigation and adaptation responses. Mitigation actions – such as carbon sequestration in agricultural soils – are aimed at reducing the atmospheric concentration of CO2 and other greenhouse gases, thereby countering climatic change. Adaptation actions – such as changes in crop types and management practices – are responses that optimize production under changing climate conditions. Research on these actions is proceeding on parallel tracks.

Here, we analyze these response actions and suggest that it is both useful and necessary for them to be considered jointly. A review of a combination of approaches, including field experiments, regression analyses, and modeling studies, leads to the following conclusions regarding how a changing climate may influence agriculture and how mitigation and adaptation responses may interact:

1) Agriculture regions will experience change over time. Effects on agricultural production systems will be heterogeneous across the nation and the world. Some regions will experience increases in production and some declines, due to the presence of minimum and maximum thresholds for crop growth. Adaptations, such as adjustments in planting dates, crop types, and irrigation regimes will likely be required. Geographic shifts in crop growing areas are likely to occur, with associated changes in production systems. Some production systems will likely expand while others contract. Although climate-influenced changes to agriculture are likely in the coming decades, the magnitudes and rates of these changes are uncertain at the regional scale, given the range of projected temperature and precipitation changes from global climate models and the unknown degree of manifestation of direct physiological effects of increasing CO2 on crops growing in farmers’ fields.

2) Agricultural production in developing countries is more vulnerable. Despite general uncertainties, studies have consistently shown that overall production in the mid- and high latitudes is likely to benefit in the near term (approximately to mid-century), while production systems in the low-latitudes are likely to decline. This finding has implications for world food security, since most developing countries are located in lower-latitude regions. The vulnerability of developing countries is related to the growth of crops under current climate conditions nearer their optimum temperature limits and the potential for greater increases in water stress under a warming climate. Developing countries also have fewer resources for development of appropriate adaptation measures to counter negative impacts.

3) Long-term effects on agriculture are negative. If climate change effects are not abated, agricultural production in the mid- and high-latitudes is likely to decline in the long term (approximately by the end of 21st century). These results are consistent over a range of projected temperature, precipitation, and direct CO2 effects tested. They are due primarily to detrimental effects of heat and water stress on crop growth as temperatures rise. Increased climate variability under climate change is also likely to negatively affect agriculture.

4) A changing climate will affect mitigation potential. Responses to a changing climate will contribute to determining which mitigation techniques are successful, and at what levels, over the coming decades. Because some carbon-sequestration projects have long durations (~40-50 years needed to accumulate carbon in agricultural soils in temperate regions), farmers may need to consider which sequestration techniques have the better chance to succeed under changing climatic regimes. Our research shows that the soil carbon sequestration potential of agricultural soils is likely to vary under changing climate conditions (Fig. 1). If changing climate is not taken into consideration, calculations of carbon to be sequestered may be in error.

5) Mitigation and adaptation responses are synergistic. Conversely, mitigation practices can also enhance the adaptation potential of agricultural systems. For example, carbon sequestration in agricultural soils leads to more stable soil-water dynamics, enhancing the ability of crops to withstand drought and floods, both of which may increase under changing climate conditions. In addition, many of the strategies proposed for reduction of greenhouse gas emissions from agriculture are “best practices,” i.e., they increase input efficiency while limiting environmental damage. For instance, use of tree shelterbelts can help to minimize soil erosion and stabilize soil carbon; mulches added between row crops help to conserve soil water, reduce erosion, and sequester carbon (Fig. 2).

6) Mitigation practices may help to make the U.S. agriculture sector “carbon-neutral.” A combination of management techniques, from reduced or no-tillage, to modified irrigation and fertilization application, has the potential to sequester ~50 million tons of carbon yearly, approximately matching yearly greenhouse gas emissions from the U.S. agricultural sector, estimated at ~50 million tons carbon (Fig. 3). Recall, however, the caveat that the capacity for agricultural soil carbon sequestration is constrained by the amount of carbon previously lost during conversion to agriculture, so that its effectiveness as a mitigating activity for climate change is not unlimited.

In conclusion, our research suggests that planning and implementation of mitigation and adaptation measures in response to the global climate change issue should be coordinated and proceed hand-in-hand. Investments in programs and research will be needed to assure effectiveness in both adaptation and mitigation activities for U.S. agriculture.

Key References

CAST (Council for Agricultural Science and Technology), 1992. Preparing U.S. Agriculture for Global Climate Change. Task Force Report No. 119, Ames, IA.

Fischer, G., Shah, M., van Velthuizen, H., and Nachtergaele, F.O., 2002. Global Agro-ecological Assessment for Agriculture in the 21st Century. International Institute for Applied Systems Analysis and U.N., Special Report 118, Laxenburg, Austria.

Rosenzweig, Cynthia and Daniel Hillel. 1998. Climate Change and The Global Harvest: Potential Effects of the Greenhouse Effect on Agriculture. Oxford University Press. New York. 324 pp.

Rosenzweig, C. and D. Hillel. 2000. Soils and global climate change: Challenges and opportunities. Soil Science 165(1):47-56.

Cynthia Rosenzweig

Dr. Cynthia Rosenzweig is a Research Scientist at the Goddard Institute for Space Studies, where she is the leader of the Climate Impacts Group. She is an Adjunct Senior Research Scientist at the Columbia University Earth Institute and an Adjunct Professor at Barnard College. A Fellow of the American Society of Agronomy, Dr. Rosenzweig’s research focuses on climate variability and change in relation to agriculture, at regional, national, and global scales. She has organized and led interdisciplinary national and international studies in this field, and published over 100 scientific articles and reports. She has developed methods for using remote sensing to identify agricultural areas in the U.S. Corn Belt sensitive to the El Niño Southern Oscillation phenomenon, and analyzed how climate affects crop production, plant diseases and pests, and soils. Dr. Rosenzweig is a recipient of a 2001 Guggenheim Fellowship.

Fig. 1. Interactions of a changing climate with soil carbon sequestration (Source: Tubiello, 2002; Scientific American).

Fig. 2. Potential synergies between adaptation and mitigation actions. (Source: GISS,

2002; CAST).

Fig. 3. Potential for carbon sequestration actions to contribute to a “carbon-neutral” U.S. agriculture sector (Source: GISS, 2002; CAST Report, 1992).