Plenary lecture: The use of iron and other trace element fertilizers in mitigating global warming

Abstract

If current trends continue, before the end of the next century atmospheric CO2 levels will be about double those of a century ago. Other greenhouse gases, including methane and chlorofluorocarbons, also are increasing at similar or even higher rates. The effects of such global atmospheric changes on climate, and, thus, on agriculture, forestry and natural ecosystems, are uncertain. Only the results of the planetary "experiment" itself will provide a definitive answer. Current projections range from catastrophic to favorable. Prudence suggests that steps be taken to slow down these rapid atmospheric changes, even in the absence of a definitive estimate of risks. Natural sinks for CO2 already account for the disappearance of as much as 60% of anthropogenic CO2 production due to fossil fuel combustion, deforestation, and soil organic carbon oxidation. Thus, current rates of increases in atmospheric CO2, about 0.4% per year, could be cut in half by reducing anthropogenic CO2 outputs by 20%, or increasing sinks by a similar amount. Such a reduction would significantly reduce the risks of major climatic changes. Programs to reduce greenhouse gas emissions are being instituted in Japan and Europe, but not in the U.S. or most other countries. Reduction of CO2 buildup must focus on the reduction of the major sources - fossil fuel usage, deforestation, soil carbon oxidation - and on increasing sinks. The only practical methods for increasing CO2 sinks are based on photosynthesis, primarily through increased C storage in standing tree biomass, the use biomass as substitutes for fossil fuels, and, possibly, through the stimulation of ocean primary productivity by iron fertilization. Fertilizing forests and oceans with iron and other trace elements to increase primary productivity and CO2 sequestration is an attractive concept. Application of small quantities of trace elements, could, in principle, result in the sequestration of many tons of CO2. Iron limits primary productivity in some ocean areas. How to practically fertilize the oceans with iron, the environmental impacts of such additions, and their effectiveness in reducing atmospheric CO2 concentrations are controversial issues. Long-term studies of ocean fertilization will be required, and such schemes can not be included in current global warming mitigation options. Iron and other trace elements are often limiting factors in agriculture, and many reports exist of such limitations in managed forestry. However, it is presently unclear how widespread such deficiencies are in managed forests, and global effects on CO2 balances can not be projected. Fertilization with major nutrients - P, K, and N - is often effective in stimulating forest productivity and could be an economical approach to large-scale CO2 sequestration. Increased forest fertilization with major nutrients will, in many cases, also require applications of some trace elements. In conclusion, forest fertilization with trace elements and even major nutrients could be an affordable and near-term method of reducing future global warming.