Various aspects of "projected" climate change in the 21th century

Temperature

• "In the long run" (century-scale trend)
  • Temperature in the stratosphere will be lower than 20C everywhere.
  • Temperature in the troposphere and (land and sea) surface will be higher than 20C probably everywhere.
  • The amount of warming (T(future) - T(present)) will be particularly larger in the lower part of the troposphere in cold regions. ("polar amplification"). Its (partial) causes are as follows:
    • Ice-albedo feedback. In the present climate, snow and sea ice is widespread in high latitudes. In warmer climate, their extent will be smaller. It will result in change of absorption of solar radiation, and it firstly affects the region where the snow and ice cover changes (though it affects global average energy balance as well).
    • Change in vertical temperature profile in the troposphere. In cold regions, particularly in winter, the land surface becomes very cold, accordingly convection in the atmosphere near the surface is suppressed, and the layer of the atmosphere very near to the surface becomes very cold but the temperature of the air above does not change so much. In warmer climate, this feature concentrated near the surface is weakened. So, the difference of temperature between warmer and colder climates is large near the surface in the cold region.
• Year-to-year variation
  • During the course of warming, "cold air outbreaks" and heavy snow events may occur.
    • Atmospheric circulation in the mid-latitude has troughs and ridges. Sometimes a trough becomes very strong, and bring cold air from higher latitudes.
    • Temperature may be high enough to enhance evaporation from sea surface, but low enough to cause snowfall rather than rainfall.

Water vapor, precipitation, hydrological cycle

• Specific humidity (concentration of water vapor in air) in the atmosphere will increase very roughly in proportional to saturation specific humidity determined by (changing) temperature (see [Figure]). In other words, relative humidity will remain very roughly constant. There is no reason that it will be exactly so, however.
• Global mean precipitation and global mean evaporation will be nearly in balance, so the climate is in a quasi-steady state. (The balance is not exact, since the stock of water vapor in the atmosphere very likely increase.) Both the terms will be larger in warmer climate. But they do not increase as strongly as specific humidity does, because evaporation from the surface requires energy input, and the increase of net radiation at the surface is not so strong as to support increase of evaporation proportional to saturation specific humidity.
• Therefore, the answer to "Will the hydrological cycle accelerate in warmer climate" depends on the definition of "faster".
  • In terms of mass flow per unit time, it will accelerate. (e.g. 1000 mm/year to 1100 mm/year.)
  • In terms of mean residence time of water vapor in the atmosphere, it will decelerate. (e.g. 9 days to 10 days.)
  • An example from a "projection"-type simulation [Figure (graphs)] (Global average values according to a "projection" by the climate model "MIROC 3.2 high resolution version" with concentration scenario "SRES A1B". Data are part of CMIP3. Graphs were drawn by Masuda.)
• Local and short-duration rainfall rate can increase according to increase of specific humidity.
• Therefore, precipitation will be more concentrated in time and in space.
• "With global warming, both floods and droughts will increase." sounds contradictory, but it is a reasonable. At such places where precipitation is concentrated, the amount of precipitation is likely to be larger in warmer climate. At other places, precipitation does not increase, but potential evaporation (amount of evaporation expected in the condition where the surface is wet) increases, so the stock of water at the land surface is likely to decrease.
• Projection of meridional (north-south) distribution of zonal (east-west) average temperature seems qualitatively robust. The basic structure (maximum near the equator, minimum in the subtropics, maximum in the middle latitudes) will remain, and becomes stronger. It will increase where it is large now, and it will decrease where it is small now. (In the tropics, the Hadley circulation will become stronger, with both its upward and downward branches. In the middle latitudes, activities of extra-tropical cyclones may not change so much or becomes a little weaker, but the amount of water vapor will increase.)
  • (However, it is not robust where (what longitude) within the same latitude zone will get more precipitation and where will get less.)
  • Example: [Figure] (Emori & Brown 2005: Analysis of many model projections used in IPCC AR4)
• Tropical storms. Strong tropical storms (tropical cyclones, typhoons, hurricanes) will be stronger in warmer climate (with higher sea surface temperature). However, the number of tropical storms including relatively weaker ones will decrease, according to projection simulations.

Snow and ice

• Sea ice will decrease in warmer climate.
• Snowfall and snowpack will decrease in the middle latitudes and in polar regions in summer (in the long run). But it is likely to increase in polar regions in winter, because of larger supply of water vapor due to higher temperature and lower sea ice cover.
  • Example: [Figure] (Hosaka et al. 2005: Projection by Meteorological Research Institute (Japan) before IPCC AR4)
• Mountain glaciers will shrink (melting faster than accumulation), and some of them will disappear.
• The Greenland ice sheet will shrink, but uncertainty is large about the rate.
• The situation of the Antarctic ice sheet is complicated. Some parts of it is likely to collapse (by processes combined with sea level change), and some parts of it will grow somewhat (because of increased supply of water vapor).

Sea level

• With global warming, the sea level will rise because:
  • Migration of mass of water from land-based ice (mainly due to melting of glaciers (including ice sheets)) to the ocean. (Note: melting of sea ice does not affect the sea level.)
  • Change of volume of sea water according to temperature (thermal expansion).

References (sources of figures)

• S. Emori and S. J. Brown, 2005: Dynamic and thermodynamic changes in mean and extreme precipitation under changed climate.Geophysical Research Letters, 32: L17706. doi: 10.1029/2005GL023272.
• M. Hosaka, D. Nohara and A. Kitoh, 2005: Changes in snow cover and snow water equivalent due to global warming simulated by a 20km-mesh global atmospheric model. SOLA, 1: 93-96. doi: 10.2151/sola.2005-025 (SOLA is an on-line academic journal published by the Meteorological Society of Japan.)
• CMIP (Coupled Model Intercomparison Project): See https://www.wcrp-climate.org/wgcm-cmip . "CMIP3" was used in IPCC AR4, "CMIP5" in AR5, "CMIP6" in AR6 draft.