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Aim As the demands for food, feed and fuel increase in coming decades, society will be pressed to increase agricultural production -whether by increasing yields on already cultivated lands or by cultivating currently natural areas -or to change current crop consumption patterns. In this analysis, we consider where yields might be increased on existing croplands, and how crop yields are constrained by biophysical (e.g. climate) versus management factors.Location This study was conducted at the global scale.Methods Using spatial datasets, we compare yield patterns for the 18 most dominant crops within regions of similar climate. We use this comparison to evaluate the potential yield obtainable for each crop in different climates around the world. We then compare the actual yields currently being achieved for each crop with their 'climatic potential yield' to estimate the 'yield gap' . ResultsWe present spatial datasets of both the climatic potential yields and yield gap patterns for 18 crops around the year 2000. These datasets depict the regions of the world that meet their climatic potential, and highlight places where yields might potentially be raised. Most often, low yield gaps are concentrated in developed countries or in regions with relatively high-input agriculture.Main conclusions While biophysical factors like climate are key drivers of global crop yield patterns, controlling for them demonstrates that there are still considerable ranges in yields attributable to other factors, like land management practices. With conventional practices, bringing crop yields up to their climatic potential would probably require more chemical, nutrient and water inputs. These intensive land management practices can adversely affect ecosystem goods and services, and in turn human welfare. Until society develops more sustainable high-yielding cropping practices, the trade-offs between increased crop productivity and social and ecological factors need to be made explicit when future food scenarios are formulated.

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The COVID-19 pandemic will be an unprecedented test of governments' ability to manage compound risks, as climate hazards disrupt outbreak response around the world. Immediate steps can be taken to minimize climate-attributable loss of life, but climate adaptation also needs a long-term strategy for pandemic preparedness.

The Reasons for Concern (RFC) framework communicates scientific understanding about risks in 1 relation to varying levels of climate change. The framework, now a cornerstone of the IPCC 2 assessments, aggregates global risks into five categories as a function of global mean temperature 3 change (GMT). We review the RFC's conceptual basis and the risk judgments made in the most recent 4 IPCC report, confirming those judgments in most cases in the light of more recent literature and 5 identifying their limitations. We point to extensions of the framework that offer complementary 6 climate change metrics to GMT and better account for possible changes in social and ecological 7 system vulnerability. Further research should systematically evaluate risks under alternative scenarios 8 of future climatic and societal conditions. 9The RFC framework was developed in the IPCC Third Assessment Report (TAR) to inform discussions 10 relevant to implementation of Article 2 of the UN Framework Convention on Climate Change (UNFCCC). 11Article 2 presents the Convention's long-term objective of avoiding "dangerous anthropogenic 12 interference with the climate system." The RFC framework and the associated "Burning Embers" 13 diagram illustrating authors' risk judgments have since been widely discussed and used to inform policy 14 decisions. For example, they informed a recent dialog between Parties to the UNFCCC and experts 1, 2 on 15 the adequacy of the long-term goal of avoiding a warming of 2°C relative to pre-industrial, contributing 16 to a strengthening of that goal in the recent Paris Agreement 3 . Elaborations of the Burning Embers have 17 been used to represent climate impacts and risks at the regional level 4 and for specific systems (e.g., 18ocean systems 5 ). 19This article reviews the conceptual basis for the RFCs (Box 1) and offers an explanation of the reasoning 20 behind associated risk judgments that is complementary to, but goes beyond, the treatment in the IPCC 21Fifth Assessment Report 6 . We focus explicitly on the evidence base for transitions from one risk level to 22 the next, incorporate post-AR5 literature in those discussions, and offer thoughts about limitations of 23 the subjective judgments behind each RFC. We also improved the synthesis of RFC-related material 24 across AR5, and in turn provide both a clearer connection to evidence from AR5 that supports the RFC 25 judgments, as well as a comparison of the RFCs to similar approaches employing metrics other than 26 GMT for characterizing risk. Perhaps most importantly, we consider improvements in the framework, 27 particularly emphasizing the dynamic nature of exposure and vulnerability, two key components of risk 28 not sufficiently covered in the current approach. 29 TEXT BOX 1: Conceptual Basis 30The Reasons for Concern (RFCs) reported in AR5 are: 31 Types of risk included in each category are discussed in the next section. The categories share an 37 emphasis on going beyond changes in biophysical systems to possible consequences for society and 38...

Do the wet savannahs and shrublands of Africa provide a large reserve of potential croplands to produce food staples or bioenergy with low carbon and biodiversity costs? We find that only small percentages of these lands have meaningful potential to be low-carbon sources of maize (∼2%) or soybeans (9.5-11.5%), meaning that their conversion would release at least onethird less carbon per ton of crop than released on average for the production of those crops on existing croplands. Factoring in land-use change, less than 1% is likely to produce cellulosic ethanol that would meet European standards for greenhouse gas reductions. Biodiversity e ects of converting these lands are also likely to be significant as bird and mammal richness is comparable to that of the world's tropical forest regions. Our findings contrast with influential studies that assume these lands provide a large, low-environmental-cost cropland reserve.

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Complex interactions in the climate system can give rise to strong positive feedback mechanisms that may lead to sudden climatic changes. The prolonged Sahel drought and the Dust Bowl are examples of 20th century abrupt climatic changes that had serious effects on ecosystems and societies. Here we analyze global historical rainfall observations to detect regions that have undergone large, sudden decreases in rainfall. Our results show that in the 20th century about 30 regions in the world have experienced such changes. These events are statistically significant at the 99% level, are persistent for at least ten years, and most have magnitudes of change that are 10% lower than the climatological normal (1901–2000 rainfall average). This analysis illustrates the extent and magnitude of abrupt climate changes across the globe during the 20th century and may be used for studying the dynamics of and the mechanisms behind these abrupt changes.