Download WB_Turn_Down_Heat_Report

Figure 2.6:A good way to gain appreciation of the warming is to compareit to the historically observed natural year-to-year temperaturevariability (Hansen et al. 2012). The absolute warming is thusdivided (normalized) by the local standard deviation (sigma),which represents the normal year-to-year changes in monthlytemperature because of natural variability (see the box 2.2). Anormalized warming of 5-sigma, therefore, means that the averagechange in the climate is five times larger than the currentnormal year-to-year variability. In the tropics, natural variabilityis small (with typical standard deviations of less than 1°C), so thenormalized warming peaks in the tropics (Figure 2.6), althoughthe absolute warming is generally larger in the Northern Hemisphereextra-tropics. Under a high-emissions scenario (RCP8.5),the expected 21 st century warming in tropical regions in Africa,South America, and Asia shifts the temperature distribution bymore than six standard deviations (Fig. 2.2.1.2). A similarly largeshift is projected for some localized extra-tropical regions, includingthe eastern Mediterranean, the eastern United States, Mexico, andparts of central Asia. Such a large normalized warming implies atotally new climatic regime in these regions by the end of the 21 stcentury, with the coldest months substantially warmer than thehottest months experienced during 1951–80. The extent of theland area projected to shift into a new climatic regime (that is,a warming by six standard deviations or more) is dramaticallyreduced when emissions are limited to the RCP2.6 scenario.Under such a low-emissions scenario, only localized regionsin eastern tropical Africa and South East Asia are projected tosee substantial normalized warming up to about four standarddeviations. In some regions, non-linear climate feedbacks seem toplay a role in causing warming under RCP8.5 to be much largerthan under RCP2.6. The eastern Mediterranean region illustratesthis situation. It warms by ~3°C (or ~2 sigma) under the lowemissionsscenario compared to ~8°C (or ~6 sigma) under thehigh-emissions scenario.Projected Changes in Heat ExtremesA thorough assessment of extreme events by the IPCC (2012) concludesthat it is very likely that the length, frequency, and intensityof heat waves will increase over most land areas under futureclimate warming, with more warming resulting in more extremes.The following quantifies how much a low emission scenario(RCP2.6) would limit the increase in frequency and intensity offuture heat waves as compared to RCP8.5.Several studies have documented the expected increase inheat extremes under a business-as-usual (BAU) emissions scenario