Turn Do wn t he H e at: W h y a 4 ° C War m e r Wor ld Mu st B e Avoided
absent, will become common, especially in the tropics and in the Northern Hemisphere (NH) mid-latitudes during summertime. According to the authors’ analysis, the most pronounced warming will occur over land (see Figure 33, top row). Monthly mean temperatures over oceans will increase between 0°C and 4°C and over continents between 4°C and 10°C. Warming over continental regions in the tropics and in the Southern Hemisphere (SH) is distributed rather evenly without strong spatial and seasonal variations. The only exception is Argentina, which is expected to see less wintertime (JJA) warming. In the NH, much stronger spatial and seasonal variations in continental warming patterns are observed. During the boreal winter, strong warming in the near Arctic region is observed due to the so-called “arctic amplification” effect, resulting in temperature anomalies of over 10°C. Two NH regions can be identified that are expected to see more warming in summertime than in wintertime: The subtropical region consisting of the Mediterranean, northern Africa, and the Middle East, as well as the contiguous United States, are likely to see monthly summer temperatures rise by more than 6°C. All land areas show a mean warming of at least 1-sigma above the present-day mean and most land areas (greater than 80 percent) show warming of at least 2-sigma. Roughly half of the land
area will likely experience a mean warming of more than 3-sigma during the boreal winter and more than 4-sigma during the boreal summer. This seasonal difference is due to enhanced warming over NH mid-latitudinal land areas during the boreal summer.
Shifts in Temperature by Region In the authors’ analysis, a 4°C warmer world will consistently cause temperatures in the tropics to shift by more than 6 standard deviations for all months of the year (Figure 33 bottom panels). Particularly, countries in tropical South America, Central Africa, and all tropical islands in the Pacific will see unprecedented extreme temperatures become the new norm in all months of the year. In fact, a temperature shift of 6 standard deviations or more implies a new climatic regime with the coolest months in 2080–2100 being substantially warmer than the warmest months in the end of the 20th century. In the SH mid-latitudes, monthly temperatures over the continents by the end of the 21st century lie in the range of 2- to 4-sigma above the present-day mean in both seasons. Over large regions of the NH mid-latitudes, the continental warming (in units of sigma) is much stronger in summer,
Figure 33: Multimodel mean of monthly warming over the 21st century (2080–2100 relative to present day) for the months of JJA (left) and DJF (right) in units of degrees Celsius (top) and in units of local standard deviation of temperature (bottom). The intensity of the color scale has been reduced over the oceans for distinction.
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