Air Pollution as a Climate Forcing: A Workshop
Summary F. Agriculture and Ecosystems
This summary is based on the workshop presentations by Michael Bergin (aerosol effects on agriculture) and Denise Mauzerall (ozone effects on agriculture), and on the pre-workshop paper prepared by Hans Martin Seip and Frederic Menz (acid deposition effects on natural ecosystems). Both presentations and the paper are available on the workshop web site.
Aerosol effects on plant growth. Aerosols influence the uptake of CO2 by plants in several ways: (1) modification of the quantity and quality of radiation (direct and diffuse) reaching plant canopies, (2) alteration of climate, including temperature and precipitation, (3) acid deposition (see below), and (4) nutrient deposition, including N and Fe.
The aerosols have an impact on both crop production and terrestrial uptake of CO2. The mechanisms listed above include effects that would both increase and decrease crop production. Although it is likely that the net effect is negative in regions with heavy aerosol overburdens, more data and research are needed for quantitative assessment. The above mechanisms also have both positive and negative effects on terrestrial uptake of CO2, and the net effect is very uncertain.
Principal research needs are: (1) field studies linking CO2 fluxes with aerosol loading and other key variables (T, precipitation, nutrients), (2) laboratory studies to understand the linkages between radiation and photosynthesis, (3) more sophisticated and realistic models for prediction of the net CO2 exchange with crops and plants.
Ozone effects on agriculture and ecosystems. O3 exposure reduces crop yields. These yield reductions vary in size by crop and crop cultivar. In addition, the impact of O3 varies as a function of environmental conditions such as water availability, temperature and sunlight. O3 damages plants when it enters the leaves through the stomata. Hence environmental parameters which cause stomata to open (i.e. increasing light, temperature, humidity) increase the likelihood of damage. Environmental parameters which cause stomata to close (water stress, increased CO2 or other air pollutants) help protect the plant from damaging effects of O3. If O3 concentrations increase significantly above current levels, the yield reductions could become substantial.
The economic value of lost crops is not a large part of gross national product. However, in countries that are trying to maximize food production and which have severe and increasing air pollution, yield reductions could be large enough to be politically important.
Ozone also effects natural ecosystems, but less is known than is for crops. Concern exists in Europe that the growth of background O3 will lead to a reduction in the terrestrial uptake of CO2 in northern forests, thus increasing the growth rate of atmospheric CO2.
There are many uncertainties about impacts of O3 on crops and ecosystems, resulting in various research needs. The sensitivity of crops in developing countries to O3 is not well known, nor are the ambient O3 concentrations in those countries. The feasibility of developing crops that are resistant to O3, via traditional breeding or biotechnology, needs to be investigated, as well as the resulting trade-offs.
We need to learn, for the sake of understanding the climate change connection, how climatic parameters such as humidity, temperature and CO2 concentration influence O3 impacts. In addition we need to be able to relate ambient O3 concentrations to the flux into the plant, plant productivity, reduction in photosynthesis, detoxification, and ecosystem impacts.
We presently have a poor understanding of the impacts of O3 on natural ecosystems. The economic valuation of the impact of O3 on ecosystems is practically unknown.
Impacts of acid rain. A substantial amount is known about the effects of acid rain based on research over the past few decades. Acid rain, overall, is detrimental to natural ecosystems. The level of acidification of streams and lakes depends on the soil type where deposition occurs.
The economic valuation of the effects of SO2 and NOx on ecosystems is believed to be significant. However, it is very difficult to obtain quantitative credible valuations. This topic is discussed in the paper by Seip and Menz in this volume.