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In the last decade, climate mitigation policy has galvanized around staying below specified thresholds of global mean temperature, with an understanding that exceeding these thresholds may result in dangerous interference of the climate system. United Nations Framework Convention on Climate Change texts have developed thresholds in which the aim is to limit warming to well below 2 degrees C of warming above preindustrial levels, with an additional aspirational target of 1.5 degrees C. However, denoting a specific threshold of global mean temperatures as a target for avoiding damaging climate impacts implicitly obscures potentially significant regional variations in the magnitude of these projected impacts. This study introduces a simple framework to quantify the magnitude of this heterogeneity in changing climate hazards at 1.5 degrees C of warming, using case studies of emergent increases in temperature and rainfall extremes. For example, we find that up to double the amount of global warming (3.0 degrees C) is needed before people in high-income countries experience the same relative changes in extreme heat that low-income nations should anticipate after only 1.5 degrees C of warming. By mapping how much warming is needed in one location to match the impacts of a fixed temperature threshold in another location, this temperature of equivalence index is a flexible and easy-to-understand communication tool, with the potential to inform where targeted support for adaptation projects should be prioritized in a warming world.

Plain Language Summary While the threshold of global mean warming which results in damaging climate impacts is thought to differ significantly between different locations, quantifying these differences has proven difficult for the scientific community. This paper introduces a simple tool, called the temperature of equivalence index, which maps how much global warming is needed for one location to match the effects of reaching a specific warming threshold in another. As an illustration of the framework, we find changes to the severity of extreme heat events for low-income nations after 1.5 degrees C of warming would not be seen for other regions of the world until after a global temperature rise twice as high. By aggregating the temperature of equivalence index for previously incompatible measures of the impacts of climate change, future work could enable a more holistic understanding of which nations will have shared experiences in a warmer world, with potential benefits for adaptation planning as a consequence.