Increased concentrations of atmospheric greenhouse gases have led to a global mean surface temperature 1.0°C higher than during the pre-industrial period. We expand on the recent IPCC Special Report on global warming of 1.5°C and review the additional risks associated with higher levels of warming, each having major implications for multiple geographies, climates, and ecosystems. Limiting warming to 1.5°C rather than 2.0°C would be required to maintain substantial proportions of ecosystems and would have clear benefits for human health and economies. These conclusions are relevant for people everywhere, particularly in low- and middle-income countries, where the escalation of climate-related risks may prevent the achievement of the United Nations Sustainable Development Goals.
Risk is the focal topic in the management of many activities and technologies. For that management to be successful, an explicit and accepted definition of the term "risk" is essential. Creation of that definition is a political act, expressing the definers' values regarding the relative importance of different possible adverse consequences for a particular decision. Those values, and with them the definition of risk, can change with changes in the decisionmaker, the technologies considered, or the decision problem. After a review of the sources of controversy in defining risk, a general framework is developed, showing how these value issues can be systematically addressed. As an example, the approach is applied to characterizing the risks of six competing energy technologies, the relative riskeness of which depends upon the particular definition used.
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