IPCC Report.pdf - Adam Curry

Changes in Climate Extremes and their Impacts on the Natural Physical EnvironmentChapter 3small-scale phenomena such as tornadoes and hail because of data inhomogeneities and inadequacies in monitoringsystems. There is medium confidence that since the 1950s some regions of the world have experienced a trend tomore intense and longer droughts, in particular in southern Europe and West Africa, but in some regions droughtshave become less frequent, less intense, or shorter, for example, in central North America and northwestern Australia.There is limited to medium evidence available to assess climate-driven observed changes in the magnitude andfrequency of floods at regional scales because the available instrumental records of floods at gauge stations are limitedin space and time, and because of confounding effects of changes in land use and engineering. Furthermore, there islow agreement in this evidence, and thus overall low confidence at the global scale regarding even the sign of thesechanges. It is likely that there has been an increase in extreme coastal high water related to increases in mean sealevel in the late 20th century. [3.2.1, 3.3.1, 3.3.2, 3.3.3, 3.4.4, 3.4.5, 3.5.1, 3.5.2, 3.5.3; Tables 3-1, 3-2]There is evidence that some extremes have changed as a result of anthropogenic influences, includingincreases in atmospheric concentrations of greenhouse gases. It is likely that anthropogenic influences have ledto warming of extreme daily minimum and maximum temperatures at the global scale. There is medium confidencethat anthropogenic influences have contributed to intensification of extreme precipitation at the global scale. It islikely that there has been an anthropogenic influence on increasing extreme coastal high water due to an increase inmean sea level. The uncertainties in the historical tropical cyclone records, the incomplete understanding of the physicalmechanisms linking tropical cyclone metrics to climate change, and the degree of tropical cyclone variability provideonly low confidence for the attribution of any detectable changes in tropical cyclone activity to anthropogenicinfluences. Attribution of single extreme events to anthropogenic climate change is challenging. [3.2.2, 3.3.1, 3.3.2,3.4.4, 3.5.3; Table 3-1]The following assessments of the likelihood of and/or confidence in projections are generally for the endof the 21st century and relative to the climate at the end of the 20th century. There are three main sources ofuncertainty in the projections: the natural variability of climate; uncertainties in climate model parameters andstructure; and projections of future emissions. Projections for differing emissions scenarios generally do not stronglydiverge in the coming two to three decades, but uncertainty in the sign of change is relatively large over this timeframe because climate change signals are expected to be relatively small compared to natural climate variability. Forcertain extremes (e.g., precipitation-related extremes), the uncertainty in projected changes by the end of the 21stcentury is more the result of uncertainties in climate models rather than uncertainties in future emissions. For otherextremes (in particular temperature extremes at the global scale and in most regions), the emissions uncertainties arethe main source of uncertainty in projections for the end of the 21st century. In the assessments provided in thischapter, uncertainties in projections from the direct evaluation of multi-model ensemble projections are modified bytaking into account the past performance of models in simulating extremes (for instance, simulations of late 20thcenturychanges in extreme temperatures appear to overestimate the observed warming of warm extremes andunderestimate the warming of cold extremes), the possibility that some important processes relevant to extremes maybe missing or be poorly represented in models, and the limited number of model projections and correspondinganalyses currently available of extremes. For these reasons the assessed uncertainty is generally greater than would beassessed from the model projections alone. Low-probability, high-impact changes associated with the crossing ofpoorly understood climate thresholds cannot be excluded, given the transient and complex nature of the climatesystem. Feedbacks play an important role in either damping or enhancing extremes in several climate variables.[3.1.4, 3.1.7, 3.2.3, 3.3.1, 3.3.2; Box 3-2]Models project substantial warming in temperature extremes by the end of the 21st century. It is virtuallycertain that increases in the frequency and magnitude of warm daily temperature extremes and decreases in coldextremes will occur through the 21st century at the global scale. It is very likely that the length, frequency, and/orintensity of warm spells or heat waves will increase over most land areas. For the Special Report on EmissionsScenarios (SRES) A2 and A1B emission scenarios, a 1-in-20 year annual hottest day is likely to become a 1-in-2 yearannual extreme by the end of the 21st century in most regions, except in the high latitudes of the NorthernHemisphere where it is likely to become a 1-in-5 year annual extreme. In terms of absolute values, 20-year extremeannual daily maximum temperature (i.e., return value) will likely increase by about 1 to 3°C by mid-21st century andby about 2 to 5°C by the late 21st century, depending on the region and emissions scenario (considering the B1, A1B,112