Studies conducted after those that contributed to the Intergovern-mental Panel on Climate Change (IPCC) Fourth Assessment Report (FAR) suggest that human society may be facing a very inconvenient truth—that emission reduction efforts alone are unlikely to stabilize greenhouse gas concentrations at levels low enough to prevent dangerous anthropogenic interference with the climate system. Here, we discuss reasons why the IPCC process is prone to underestimating the threats of global climate change. We then review some of the critical policy-relevant scientific findings that have emerged since the release of the IPCC FAR. Finally, we discuss how these new findings fundamentally transform the debate on efforts needed to prevent dangerous changes to our climate system. It now appears that to avoid such changes, society will likely need to adopt a mixed strategy of reducing greenhouse gas emissions and employing geoengineering approaches that extract carbon dioxide from the atmosphere and/or reduce the level of incoming solar radiation reaching Earth’s surface.

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