Half-a-Degree Matters for Reducing and Delaying Global Land Exposure to Combined Daytime-Nighttime Hot Extremes

The Paris Agreement has motivated rapid analysis differentiating changes in frequency/intensity of weather and climate extremes in 1.5 versus 2 degrees C warmer worlds. However, implications of these global warming levels on locations, spatial scales, and emergence timings of hot spots to extremes are more relevant to policy-making but remain strikingly under-addressed. Based on a bivariate definitional framework, we show that compared to 2 degrees C, the 1.5 degrees C target could avoid a transition of prevailing type of summertime hot extremes from daytime-/nighttime-only events to combined daytime-nighttime hot extremes in approximately 18% of global continents and protect 14-26% of land areas from seeing over threefold-to-tenfold increases in occurrence of combined hot extremes. This half-a-degree reduction also matters for around 21% of global lands, mostly within the tropics, in constraining historically unprecedented combined hot extremes from becoming the new norm within just one to three decades ahead. By contrast, previous analyses based on univariate-defined hot days substantially underestimate the magnitude, areal extent, and emergence rate of 0.5 degrees C-caused aggravation of summertime hot extremes. These projected changes of bivariate-classified hot extremes, therefore, underline not only the imperative but also the urgency of striving for the lower Paris target. Plain Language Summary Since the milestone Paris Agreement, policymakers are eager to know the extent to which and the time when their own regions will suffer from consequences associated with pertinent global warming levels (1.5 and 2 degrees C). We show that even the aspirational 1.5 degrees C target would still be translated into huge threats from summertime hot extremes to the tropics, as manifested by a type transition toward health-damaging daytime-nighttime combined heat and threefold-to-tenfold increases in its occurrence. Further rise to 2 degrees C would put an extra 20% of global continents at risk from similar transitions and increases. Unexpectedly, despite involving substantial reduction in carbon emissions, the 2 degrees C-compatible development pathway would bring about a rapidly emerging novel era of hot extremes for most tropical countries, where historically unprecedented daytime-nighttime combined heat will be registered every other year within the next one to three decades and thereafter. Our results highlight that the extra 0.5 degrees C of global warming, from 1.5 to 2 degrees C, would impose the earliest and severest heat-related consequences on the least-developed regions, whose adaptive capacity is unfortunately very limited.