Climate Scenario 3 Catastrophic Climate Change

Between 2040 and 2100 the impacts associated with climate scenario 2 progress and large-scale singular events of abrupt climate change occur. The average global temperature rises to 5.6°C (10.1 °F) above 1990 levels, with greater warming over land masses and at higher latitudes. Because of continued acceleration of dynamical polar ice sheet changes, global mean sea level rises 2 meters (6.6 feet) relative to 1990, rendering low-lying coastal regions uninhabitable, including many large coastal cities. The large fertile deltas of the world become largely uncultivable because of inundation and more frequent and higher storm surges that reach farther inland. The North Atlantic MOC collapses at mid-century, generating large-scale disruption of North Atlantic marine ecosystems and associated fisheries. Northwestern Europe experiences colder winters, shorter growing seasons, and lower crop yields than those of the twentieth century.

Outside northwestern Europe and the northern North Atlantic Ocean, the MOC collapse increases average temperatures in most regions and reorganizes precipitation patterns in unpredictable ways, hampering water resource planning around the world and drying out existing grain-exporting regions. Southern Europe and the Mediterranean region remain warmer than the twentieth-century average and continue to experience hotter, drier summers with more frequent and longer heat waves, more frequent and larger wildfires, and lower crop yields. Agriculture in the traditional breadbaskets is severely compromised by alternating persistent drought and extreme storm events that bring irregular severe flooding. Crops are physiologically stressed by temperatures and grow more slowly, even when conditions are otherwise favorable. Even in many regions with increased precipitation, summertime soil moisture is reduced by increased evaporation. Breadbasket-like climates shift strongly northward into subarctic regions with traditionally small human populations and little infrastructure, including roads and utilities. Furthermore, extreme year-to-year climate variability in these regions makes sustainable agriculture difficult on the scale needed to feed the world population.

Mountain glaciers are virtually gone and annual snowpack dramatically reduced in regions where large human populations have traditionally relied on glaciers and annual snowfall for water supply and storage, including Central Asia, the Andes, Europe, and western North America. Arid regions expand rapidly, overtaking regions that traditionally received sufficient annual rainfall to support dense populations. The dry subtropics—including the Mediterranean region, much of Central Asia, northern Mexico, much of South America, and the southwestern United States—are no longer inhabitable. Not only is the area requiring remote water sources for habitability dramatically larger than in 1990, but such remote sources are much less available because mountain glaciers and snowlines have retreated dramatically. Half of the world's human population experiences persistent water scarcity.

Locally devastating weather events are the norm for coastal and mid-latitude continental locations, where tropical and mid-latitude storm activity and associated wind and flood damage becomes much more intense and occurs annually, leading to frequent losses of life, property, and infrastructure in many countries every year. Whereas water availability and loss of food security disproportionately affect poor countries at lower latitudes, extreme weather events are more or less evenly distributed, with perhaps greater frequency at mid-latitudes because of stronger extratropical storm systems, including severe winter storms.

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