Missing Pieces

Editors must inevitably choose to draw the line somewhere in order to balance the scope and focus of a book. The decisions made here reflect partly the expertise and interests of the authors involved, and partly our own biases on the most relevant and scientifically mature topics. However, we wish to point out many less mature topics that may prove important in the final analysis. For instructors who plan to use this book as a text, we encourage instructors planning to use this book as a text to supplement it with current papers on these topics.

Water Resources for Irrigation Impact assessments for irrigated regions often assume that water supply will be unaffected by climate change. Although this is a reasonable starting point (in order to focus first on the more direct effects of temperature and precipitation on crops), indirect effects through changes in regional water resources may be important. Studies that do link regional hydrology models to crop models, in order to simultaneously treat both supply and demand for irrigation water, have shown that local impacts and adaptation responses can be constrained by water supplies (Thomson et al. 2005).

Irrigation is currently practiced on roughly 17% of global cropland, with these systems contributing 40% of global food production (FAO 2002). Most of global irrigation water is applied in Asia, and therefore it is in this region that consideration of changes in water resources is most urgently needed. For example, it is widely acknowledged that much of the irrigation water in India and Pakistan originates as meltwater from Himalayan glaciers, that these glaciers are rapidly melting, and that summer streamflow may be significantly reduced within a few decades (Singh and Bengtsson 2004; Barnett et al. 2005; Rees and Collins 2006). Yet the implications of these limited water supplies on agriculture in general, and on the ability to adapt to climate change in particular, have to our knowledge only been superficially addressed.

Sea Level Rise Little work has considered the direct impacts of rising sea levels on agricultural production. Increases over the next few decades will likely be too small to have a major impact on agricultural production, but increases of more than 1 m, which are possible by the end of the century (Rahmstorf 2007), could result in the inundation of large tracts of low-lying coastal agriculture throughout Asia. Even smaller rises in the near-term could have strong local effects related to saltwater intrusion, with three particularly vulnerable sectors suggested in a recent FAO report: vegetable production, which tends to be irrigated in coastal regions, low lying aquaculture, and coastal fisheries (Bruinsma 2003).

Pest and Pathogens Farmers are constantly faced with the prospects of yield losses from weeds, animal pests, fungal and bacterial pathogens, and viruses. By one estimate, roughly 30-40% of global production for the major food crops is lost to these factors each year (Oerke 2005). Although climate change will undoubtedly modify pest dynamics, current understanding of these changes is quite limited (Easterling et al. 2007). With a few exceptions (Aggarwal et al. 2006), crop models in common use today do not include treatment of weeds, pests, or pathogens. Approaches to modeling responses to climate change include models that explicitly simulate weed competition or predator-prey interaction as well as simpler projections that use thresholds to define pest ranges. Nearly always the effects of temperature and CO2 changes have been considered separately, although interactions between the two may prove important (Fuhrer 2003). Pests and pathogens can not only impact yields, but also the nutritional quality and health impacts of many crops. For example, carcinogenic aflatoxins are commonly found in maize and groundnuts and are most prevalent in hot and dry conditions (Chauhan et al. 2008).

Livestock and Fisheries This book focuses mainly on food crops, but meat, poultry, dairy, and fish are important sources of calories, protein, and income for many, including the food insecure. Livestock is a particularly important means of risk management (i.e. mixed crop-livestock systems) and adaptation to drought throughout much of the tropics (Thornton et al. 2007). Livestock systems broadly fit into two classes: those fed on grains or managed pasture grasses, such as in intensive feedlot systems common in developed countries, and those based mainly on grazing of wild grasses such as those common in poor countries with large malnourished populations. For the former, the main effects of climate change may be via crop yield and price changes discussed in this book, though higher temperatures will also present a challenge to management of heat stress and disease among animal populations. In pasture and rangeland systems, direct effects of heat on animals will be complemented by effects on forage quantity and quality. Pasture grasses in many temperate locations show yield increases for moderate warming, but also exhibit significant declines in nutrient content with higher CO2 (Easterling et al. 2007).

In fisheries, interannual climatic variations, most notably related to the El NiƱo Southern Oscillation, lead to wide fluctuations in fish stocks. However, the net effects of future climate changes on fisheries are currently very uncertain, aside from a likely northward shift of many fish populations (Brander 2007). In addition to effects of warming, aquatic food webs could be as or more impacted by increased acidity resulting from oceanic CO2 uptake (Easterling et al. 2007).

Mitigation in Agriculture Though this book focuses on the impacts of climate change on agriculture and food security, the role of agriculture in mitigating climate change is an important related topic. It has long been recognized that agriculture is a significant contributor to global greenhouse gas emissions, in terms of CO2 and especially methane and nitrous oxide (Rosenzweig and Hillel 1998). Major reductions in emissions of these gases from agricultural activities could thus contribute to climate mitigation, and a myriad of technologies offer promise in this respect. For more information, a good starting point is the periodic reports of the Intergovernmental Panel on Climate Change.

Mitigation could even present an opportunity to adapt to climate impacts. For example, the prospect of a global emissions trading market will make it possible to generate rural income from either reducing emissions or providing renewable fuels. Such income, for example through biofuel production in poor oil-importing nations, may be an important means of income generation and represent a possible adaptation to declining staple crop production. Other synergies between adaptation and mitigation have been argued in the literature, such as the potential of conservation tillage practices to both sequester carbon in the soil and improve soil moisture needed in dry years (Lal 2004).

Policy Responses Despite the many uncertainties in physical and biological aspects of food security response to climate change, much of the inability to project future impacts relates to the simple fact that we cannot predict how humans will respond. Put differently, the severity of future impacts will depend in large measure on whether humans can effectively adapt. This book deals extensively with models of how rational farmers and regional economies might respond to climate change, but it should be clear that, like most other assessments, we implicitly assume that government policies that influence these behaviors remain fairly stable.

Any significant shifts in policy could dramatically affect the capacity of economies to cope with climate change, either for better or worse. One particularly important set of policies relates to long-term investments in the types of institutions and technologies that are needed to adapt to climate change, such as agricultural research or extension activities and emergency relief organizations. Funding for these activities has fluctuated in recent years and it is difficult to predict the future trajectory of overall policy support for agricultural development. Although many have argued convincingly that these investments offer high returns even in current climate (Alston et al. 2000), it can be difficult to prioritize long-term investments in public goods, particularly in poor countries.

Standing as a complement to these decisions about longer-term institutional investments are policies that deal with the short-term supply shocks that occur in years of bad harvests, shocks that may become more frequent and widespread with climate change. The recent experience with rapid price changes in 2008 provides a clear example. Many governments instituted new policies aimed at stabilizing local markets, including price controls, import tariffs, and export restrictions (FAO 2008). Yet the effect of these inward looking policies was often to destabilize global markets even further, causing rapid spikes in food prices and declines in food access in many rice importing countries.

How will governments respond if a year with extreme heat waves reduces global cereal harvests by 10% in 2020? Will they preserve existing policies and increase support of famine relief organizations, or will they embark on politically popular but potentially harmful protectionist policies? Though the recent experience of 2008 provides a cautionary note, perhaps it was a good learning experience that will lead to improved coordination during a future crisis.

There are many difficulties in predicting the future course of human decisions, not the least of which is that human behavior is not necessarily rational. As Bertrand Russell wrote: "It has been said that man is a rational animal. All my life I have been searching for evidence which could support this." Progress in anticipating future policy responses will therefore likely be slow.

The only certainty is perhaps that good policies, or the absence of bad policies, will be critical to maintaining food security in a changing climate. Identifying what these particular policies should be, and how to implement them, is beyond the scope of our book, but a topic that surely deserves much study in years ahead.

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