Update on Climate Change and Global Crop Productivity The In fl uence of Climate Change on Global Crop Productivity 1

Climate trends over the past few decades have been fairly rapid in many agricultural regions around the world, and increases in atmospheric carbon dioxide (CO2) and ozone (O3) levels have also been ubiquitous. The virtual certainty that climate and CO2 will continue to trend in the future raises many questions related to food security, one of which is whether the aggregate productivity of global agriculture will be affected. We outline the mechanisms by which these changes affect crop yields and present estimates of past and future impacts of climate and CO2 trends. The review focuses on global scale grain productivity, notwithstanding the many other scales and outcomes of interest to food security. Over the next few decades, CO2 trends will likely increase global yields by roughly 1.8% per decade. At the same time, warming trends are likely to reduce global yields by roughly 1.5% per decade without effective adaptation, with a plausible range from roughly 0% to 4%. The upper end of this range is half of the expected 8% rate of gain from technological and management improvements over the next few decades. Many global change factors that will likely challenge yields, including higher O3 and greater rainfall intensity, are not considered in most current assessments. Many factors will shape global food security over the next few decades, including changes in rates of human population growth, income growth and distribution, dietary preferences, disease incidence, increased demand for land and water resources for other uses (i.e. bioenergy production, carbon sequestration, and urban development), and rates of improvement in agricultural productivity. This latter factor, which we define here simply as crop yield (i.e., metric tons of grain production per hectare of land), is a particular emphasis of the plant science community, as researchers and farmers seek to sustain the impressive historical gains associated with improved genetics and agronomic management of major food crops. Sources of growth in agricultural productivity are also multifaceted and include levels of funding for public and private research and development, changes in soil quality, availability and cost of mineral fertilizers, atmospheric concentrations of CO2 and ozone (O3), and changes in temperature (T) and precipitation (P) conditions. This Update focuses on changes in weather, CO2, and O3 in agricultural areas and how that has affected and will affect crop productivity. In doing so, we recognize that this is only part of the fuller story on crop productivity, which in turn is only part of the fuller story on future food security. For example, this Update is silent on the many ways that global change can influence food security via pathways other than agricultural productivity, such as by influencing human disease incidence or income growth rates. The main question of interest here is the following: how important will climate change and CO2 be in shaping future crop yields at the global scale, relative to the many other factors that influence productivity? This question helps to set the challenge of climate adaptation in context. We are less concerned, for example, with whether impacts are statistically distinguishable from zero than with whether they are costly enough to justify a major acceleration of investment in agriculture in order to reach target growth rates. Two spatial scales are of primary interest when discussing impacts of climate change on food security. One is the global scale, because most major sources of human calories (e.g. maize [Zea mays] or wheat [Triticum aestivum]) are international commodities whose prices are determined by the balance of global supply and demand. In this context, individual regions are only of interest to the extent that they contribute to global supply. However, it is equally true that not all areas are fully integrated into global markets. In fact, many of the poorest and most food-insecure areas currently lack the infrastructure and institutions needed to fully participate in global (and sometimes even regional) markets. Although most of these areas are more integrated into global markets than they used to be, and will be even more so over the next few decades, it is important that assessments of global food security consider local and regional impacts in addition to those at the global scale. If nothing else, transport costs will always make local supply more closely tied than global supply to local prices. For brevity and focus, this Update discusses mainly global-scale issues. Similarly, climate impact assessments must make choices about which crops to consider. By far, the most 1 This work was supported by the Rockefeller Foundation. * Corresponding author; e-mail dlobell@stanford.edu. www.plantphysiol. org/cgi/doi/10.1104/pp.112. 208298