In bringing together the host of expert chapters contained in this book, numerous linkages between the various sinks for carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) are apparent. The authors have attempted to identify these throughout, as well as the key cross-cutting issues that affect all of these sinks. Of particular note are the impacts of a changing global climate, driven by elevated greenhouse gas (GHG) concentrations in the atmosphere, on the capacity of these sinks. From drought-induced reductions in the terrestrial CO2 sink to temperature-dependent stratification of the oceans and consequent reduction in the oceanic CO2 sink, the feedbacks to the earth's climate through changes in its GHG sinks are of obvious importance and concern.
This chapter briefly reviews some of the emerging issues in GHG sink science and identifies some of the most pressing areas requiring further research to reduce uncertainties. Of particular interest is the impact of changing nitrogen deposition on GHG sinks and so this issue is covered in detail by De Vries et al. (Chapter 17, this volume).
Globally, sea level is expected to rise between 9 and 88 cm by the end of the 21st century. While in some areas, sea defences may be able to maintain existing coastlines, in most areas of the world substantial coastal retreat is inevitable. The impacts of such retreat on the various sources and sinks of CO2, CH4 and N2O may be substantial. Combined with this retreat is the recognition that wetlands creation and protection must form a central part of coastal defence strategies. As wetlands represent a strong CH4 source, increased CH4 emissions are likely.
Similarly, in tackling the growing problem of eutrophication of surface waters due to fertilizer runoff and leaching, interception strategies such as wetlands, buffer strips and ponds may increase N2O and CH4 emissions where they are used - swapping a nutrient pollution problem for a climate change issue.
Predicted climate change at high northern latitudes is expected to extend the northern ranges of forests. All other things being equal, this could be expected to increase the CO2 sink provided by vegetation at these latitudes. However, any increase in CO2 uptake by invading woody vegetation must be set against the reduction in albedo (land reflectance) that such northward spread would cause. While unforested tundra areas have snow cover for much of the year, and so a high albedo, forested areas have a much lower albedo and therefore absorb much more of the sun's energy as heat - increasing the warming at ground level. In addition to the direct forcing of climate from this warming, rapid rises in ground temperatures at these high latitudes may result in increased decomposition rates, and hence soil CO2 emissions in these areas. When such warming occurs in areas of CH4-rich permafrosts, there is the additional risk of elevated CH4 emissions.
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