West Africa is highly vulnerable to climate hazards and better quantification and understanding of the impact of climate change on crop yields are urgently needed. Here we provide an assessment of near-term climate change impacts on sorghum yields in West Africa and account for uncertainties both in future climate scenarios and in crop models. Towards this goal, we use simulations of nine bias-corrected CMIP5 climate models and two crop models (SARRA-H and APSIM) to evaluate the robustness of projected crop yield impacts in this area. In broad agreement with the full CMIP5 ensemble, our subset of bias-corrected climate models projects a mean warming of +2.8 °C in the decades of 2031–2060 compared to a baseline of 1961–1990 and a robust change in rainfall in West Africa with less rain in the Western part of the Sahel (Senegal, South-West Mali) and more rain in Central Sahel (Burkina Faso, South-West Niger). Projected rainfall deficits are concentrated in early monsoon season in the Western part of the Sahel while positive rainfall changes are found in late monsoon season all over the Sahel, suggesting a shift in the seasonality of the monsoon. In response to such climate change, but without accounting for direct crop responses to CO2, mean crop yield decreases by about 16–20% and year-to-year variability increases in the Western part of the Sahel, while the eastern domain sees much milder impacts. Such differences in climate and impacts projections between the Western and Eastern parts of the Sahel are highly consistent across the climate and crop models used in this study. We investigate the robustness of impacts for different choices of cultivars, nutrient treatments, and crop responses to CO2. Adverse impacts on mean yield and yield variability are lowest for modern cultivars, as their short and nearly fixed growth cycle appears to be more resilient to the seasonality shift of the monsoon, thus suggesting shorter season varieties could be considered a potential adaptation to ongoing climate changes. Easing nitrogen stress via increasing fertilizer inputs would increase absolute yields, but also make the crops more responsive to climate stresses, thus enhancing the negative impacts of climate change in a relative sense. Finally, CO2 fertilization would significantly offset the negative climate impacts on sorghum yields by about 10%, with drier regions experiencing the largest benefits, though the net impacts of climate change remain negative even after accounting for CO2.

Abstract: A rapidly growing body of research examines whether human conflict can be affected by climatic changes. Drawing from archaeology, criminology, economics, geography, history, political science, and psychology, we assemble and analyze the 60 most rigorous quantitative studies and document, for the first time, a striking convergence of results. We find strong causal evidence linking climatic events to human conflict across a range of spatial and temporal scales and across all major regions of the world. The magnitude of climate’s influence is substantial: for each one standard deviation (1σ) change in climate toward warmer temperatures or more extreme rainfall, median estimates indicate that the frequency of interpersonal violence rises 4% and the frequency of intergroup conflict rises 14%. Because locations throughout the inhabited world are expected to warm 2σ to 4σ by 2050, amplified rates of human conflict could represent a large and critical impact of anthropogenic climate change.

Abstract: Are violent conflict and socio-political stability associated with changes in climatological variables? We examine 50 rigorous quantitative studies on this question and find consistent support for a causal association between climatological changes and various conflict outcomes, at spatial scales ranging from individual buildings to the entire globe and at temporal scales ranging from an anomalous hour to an anomalous millennium. Multiple mechanisms that could explain this association have been proposed and are sometimes supported by findings, but the literature is currently unable to decisively exclude any proposed pathway. Several mechanisms likely contribute to the outcomes that we observe.

In many discussions of climate change impacts in agriculture, the large magnitudes of expected impacts toward the end of the century are used to emphasize that most of the risks are to future generations. However, this perspective misses the important fact that demand growth for food is expected to be much slower after 2050 than before it, and that the next two decades represent the bulk of growth before 2050. Thus, impacts of smaller magnitude in the near-term can be as or more consequential for food prices or food security as larger magnitude impacts in the future. Here we estimate the risks that climate trends over the next 10 or 20 years could have large impacts on global yields of wheat and maize, with a focus on scenarios that would cut the expected rates of yield gains in half. We find that because of global warming, the chance of climate trends over a 20 year period causing a 10% yield loss has increased from a less than 1 in 200 chance arising from internal climate variability alone, to a 1 in 10 chance for maize and 1 in 20 chance for wheat. Estimated risks for maize are higher because of a greater geographic concentration than wheat, as well as a slightly more negative aggregate temperature sensitivity. Global warming has also greatly increased the chance of climate trends large enough to halve yield trends over a 10 year period, with a roughly 1 in 4 chance for maize and 1 in 6 chance for wheat. Estimated risks are slightly larger when using climate projections from a large ensemble of a single climate model that more fully explores internal climate variability, than a multi-model ensemble that more fully explores model uncertainty. Although scenarios of climate impacts large enough to halve yield growth rates are still fairly unlikely, they may warrant consideration by institutions potentially affected by associated changes in international food prices.

Stratospheric injection of sulphate aerosols has been advocated as an emergency geoengineering measure to tackle dangerous climate change, or as a stop-gap until atmospheric carbon dioxide levels are reduced. But it may not prove to be the game-changer that some imagine.

• Adaptation should be defined strictly as actions that reduce climate change impacts.

• Many studies that claim to show adaptation benefits do not satisfy this definition.

• Three main causes of “adaptation illusions” are discussed.

Projecting the impacts of climate change on agriculture requires knowing or assuming how farmers will adapt. But empirical estimates of the effectiveness of this private adaptation are scarce and the sensitivity of impact assessments to adaptation assumptions is not well understood. Here we assess the potential effectiveness of private farmer adaptation in Europe by jointly estimating both short-run and long-run response functions using time-series and cross-sectional variation in subnational yield and profit data. The difference between the impacts of climate change projected using the short-run (limited adaptation) and long-run (substantial adaptation) response curves can be interpreted as the private adaptation potential. We find high adaptation potential for maize to future warming but large negative effects and only limited adaptation potential for wheat and barley. Overall, agricultural profits could increase slightly under climate change if farmers adapt but could decrease in many areas if there is no adaptation. Decomposing the variance in 2040 projected yields and farm profits using an ensemble of 13 climate model-runs, we find that the rate at which farmers will adapt to rising temperatures is an important source of uncertainty. 

A key question for climate change adaptation is whether existing cropping systems can become less sensitive to climate variations. We use a field-level dataset on maize and soybean yields in the central United States for 1995 through 2012 to examine changes in drought sensitivity. Although yields have increased in absolute value under all levels of stress for both crops, the sensitivity of maize yields to drought stress associated with high vapor pressure deficits has increased. The greater sensitivity has occurred despite cultivar improvements and increased CO₂, and reflects the agronomic trend toward higher sowing densities. The results suggest that agronomic changes tend to translate improved drought tolerance of plants to higher average yields, but not to decreasing drought sensitivity of yields at the field scale. 

The full text of the article, abstract, and reprint are available via Science.