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.

Despite the fact that sub-Saharan Africa in 2012 contains much of the world’s unutilized and underutilized arable land, a significant and growing share of Africa’s farm households live in densely populated areas. Based on two alternative spatial databases capable of estimating populations at the level of one square kilometer and distinguishing between arable and non-arable land, we find that in at least five of the 10 countries analyzed, 25 percent of the rural population resides in areas exceeding 500 persons per square kilometer, estimated by secondary sources as an indicative maximum carrying capacity for areas of rain-fed agriculture in the region. The apparent paradox of a large proportion of Africa’s rural population living in densely populated conditions amidst a situation of massive unutilized land is resolved when the unit of observation is changed from land units to people.

A review of nationally representative farm surveys shows a tendency of (1) declining mean farm size over time within densely populated smallholder farming areas; (2) great disparities in landholding size within smallholder farming areas, leading to highly concentrated and skewed patterns of farm production and marketed surplus; (3) half or more of rural farm households are either buyers of grain or go hungry because they are too poor to afford to buy food; most households in this category control less than one hectare of land; and (4) a high proportion of farmers in densely populated areas perceive that it is not possible for them to acquire more land through customary land allocation procedures, even in areas where a significant portion of land appears to be unutilized.

Ironically, there has been little recognition of the potential challenges associated with increasingly densely populated and land-constrained areas of rural Africa, despite the fact that a sizeable and increasing share of its rural population live in such areas. Inadequate access to land and inability to exploit available unutilized land are issues that almost never feature in national development plans or poverty reduction strategies. In fact, since the rise of world food prices after the mid-2000s, many African governments have made concerted efforts to transfer land out of customary tenure systems (where the majority of rural people reside) to the state or to private individuals who, it is argued, can more effectively exploit the productive potential of the land to meet national food security objectives. Such efforts have nurtured the growth of a relatively well-capitalized class of “emergent” African farmers. The growing focus on how best to exploit unutilized land in Africa has arguably diverted attention from the more central and enduring challenge of implementing agricultural development strategies that effectively address the continent’s massive rural poverty and food insecurity problems, which require recognizing the growing land constraints faced by much of its still agrarian-based population. The final section of the paper considers research and policy options for addressing these problems.