Climate change has the potential to be a source of increased variability if crops are more frequently exposed to damaging weather conditions. Yield variability could respond to a shift in the frequency of extreme events to which crops are susceptible, or if weather becomes more variable. Here we focus on the United States, which produces about 40% of the world’s maize, much of it in areas that are expected to see increased interannual variability in temperature. We combine a statistical crop model based on historical climate and yield data for 1950–2005 with temperature and precipitation projections from 15 different global circulation models. Holding current growing area constant, aggregate yields are projected to decrease by an average of 18% by 2030–2050 relative to 1980–2000 while the coefficient of variation of yield increases by an average of 47%. Projections from 13 out of 15 climate models result in an aggregate increase in national yield coefficient of variation, indicating that maize yields are likely to become more volatile in this key growing region without effective adaptation responses. Rising CO₂ could partially dampen this increase in variability through improved water use efficiency in dry years, but we expect any interactions between CO₂ and temperature or precipitation to have little effect on mean yield changes.

An important source of uncertainty in anticipating the effects of climate change on agriculture is limited understanding of crop responses to extremely high temperatures. This uncertainty partly reflects the relative lack of observations of crop behaviour in farmers’ fields under extreme heat. We used nine years of satellite measurements of wheat growth in northern India to monitor rates of wheat senescence following exposure to temperatures greater than 34 °C. We detect a statistically significant acceleration of senescence from extreme heat, above and beyond the effects of increased average temperatures. Simulations with two commonly used process-based crop models indicate that existing models underestimate the effects of heat on senescence. As the onset of senescence is an important limit to grain filling, and therefore grain yields, crop models probably underestimate yield losses for +2 °C by as much as 50% for some sowing dates. These results imply that warming presents an even greater challenge to wheat than implied by previous modelling studies, and that the effectiveness of adaptations will depend on how well they reduce crop sensitivity to very hot days.

Crop models predict that recent and future climate change may have adverse effects on crop yields. Intentional deflection of sunlight away from the Earth could diminish the amount of climate change in a high-CO₂ world. However, it has been suggested that this diminution would come at the cost of threatening the food and water supply for billions of people. Here, we carry out high-CO₂, geoengineering and control simulations using two climate models to predict the effects on global crop yields. We find that in our models solar-radiation geoengineering in a high-CO₂ climate generally causes crop yields to increase, largely because temperature stresses are diminished while the benefits of CO₂ fertilization are retained. Nevertheless, possible yield losses on the local scale as well as known and unknown side effects and risks associated with geoengineering indicate that the most certain way to reduce climate risks to global food security is to reduce emissions of greenhouse gases.

Perennial crops are among the most valuable of California’s diverse agricultural products. They are also potentially the most influenced by information on future climate, since individual plants are commonly grown for more than 30 years. This study evaluated the impacts of future climate changes on the 20 most valuable perennial crops in California, using a combination of statistical crop models and downscaled climate model projections. County records on crop harvests and weather from 1980 to 2005 were used to evaluate the influence of weather on yields, with a series of cross-validation and sensitivity tests used to evaluate the robustness of perceived effects. In the end, only four models appear to have a clear weather response based on historical data, with another four presenting significant but less robust relationships. Projecting impacts of climate trends to 2050 using historical relationships reveals that cherries are the only crop unambiguously threatened by warming, with no crops clearly benefiting from warming. Another robust result is that almond yields will be harmed by winter warming, although this effect may be counteracted by beneficial warming in spring and summer. Overall, the study has advanced understanding of climate impacts on California agriculture and has highlighted the importance of measuring and tracking uncertainties due to the difficulty of uncovering crop-climate relationships.

The four-volume Encyclopedia of Global Studies covers the field of global studies and subjects related to it, such as globalization, transnational activity and themes of global society. This encyclopedia is written for the educated general reader as well as students and professionals working in the field of global studies. It is the first encyclopedia of its kind, and aims to become the internationally-recognized reference work for academics, policymakers, and practitioners interested in the various dimensions of globalization. It provides succinct summaries of concepts and theories, definitions of terms, biographical entries, and organizational profiles; offers a guide to sources of information; and establishes an overview of Global Studies in different parts of the world and across cultures and historical periods.  The wide range of subjects covered include the following:
            - intellectual approaches, such as global sociology, political economy, world systems theory, peace and conflict studies, and communications;
            - global and transnational topics, such as cross-border conflicts and terrorism, worldwide health crises and climate disruption, the planetary immigration patterns and new cultural diasporas, and the seemingly boundless global market, rapid communications, and transnational cyberspaces devised by technology and new media.