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Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
L31: Projections of Future Climate Change
Further Reading: Detailed Notes Posted on Class Web Sites
Outline
- global climate models
- global mean response
- patterns of future climate change
- summary of changes
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Introduction
The purpose of this chapter is to assess and quantify projections of possible future climate
change from climate models.
The Climate System
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Global Climate Models
• Global climate models (GCMs) include as central components atmospheric and ocean
general circulation models, as well as representation of land surface processes, sea-ice and
all other processes shown in the previous slide.
• Models and their components are based upon physical principles represented by
mathematical equations that describe the atmospheric and ocean dynamics and physics.
• Such equations are solved numerically at a finite resolution using a three-dimensional grid
over the globe.
• Typical resolutions used for simulations are about 250 km in the horizontal and 1 km in the
vertical.
• Because of such coarse spatial resolution, many of physical processes cannot be properly
resolved, and one resorts to including their average effect through parametric representations
(parameterization).
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Global Climate Models
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Global Mean Response-1
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1% CO2 Experiments
• The time evolution of the globally averaged (a) temperature change relative to the years
(1961-1990) of the CMIP2 simulations (degrees C). (b) same for precipitation (%).
• At the time of CO2 doubling at year 70, the 20 year average (years 61-80) global mean
temperature change for these models is 1.1 to 3.1C with an average of 1.8C and a standard
deviation of 0.4C.
• Likewise, at the time of CO2 doubling at year 70, the 20 year average (years 61-80)
percentage change of the global mean precipitation for these models ranges from -0.2% to
5.6% with an average of 2.5% and a standard deviation of 1.5%.
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Projections from Forcing Scenarios-1
• The time evolution of the globally averaged temperature change relative to the years
(1961-1990). G: greenhouse gas only (left), GS: greenhouse gas and sulfate aerosols
(right). The observed temperature change (CRU) is indicated by the black line. Used IS92a
(business-as-usual) type forcing.
• The temperature change for the 30 year average 2021-2050 for GS compared to 1961-90 is
+1.3C with a range of +0.8C to +1.7C as opposed to +1.6C with a range of +1.0C to 2.1C for
GHG only
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Global Mean Response-2
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Projections from Forcing Scenarios-2
• The time evolution of the globally averaged precipitation change relative to the years (19611990). G: greenhouse gas only (left), GS: greenhouse gas and sulfate aerosols (right).
Used business-as-usual scenario
• The globally averaged precipitation response for 2021-2050 for GHG plus sulfates is
+1.5% with a range of +0.5% to +3.3% as opposed to +2.3% with a range of +0.9% to +4.4%
for GHG only
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Patterns of Future Climate Change-1
• Multi-model annual mean zonal temperature change (degrees C).
• There is consistent mid-tropospheric tropical warming and stratospheric cooling.
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Patterns of Future Climate Change-2
• The multi-model ensemble annual mean change of the temperature (color shading) and its
range (isolines) (degrees C) at the time of CO2-doubling.
• The model experiments show maximum warming in the high latitudes of the NH and a
minimum in the Southern Ocean (due to ocean heat uptake)
• Land warms more rapidly than ocean almost everywhere.
Natural Environments: The Atmosphere
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Patterns of Future Climate Change-3
• Change in sea-ice thickness between the periods 1971-1990 and 2041-2060 as simulated by
four of the most recent coupled models. The left panels show thickness changes in the
northern hemisphere, the right panels show changes in the southern hemisphere. The
color bar indicates thickness change in meters - negative values indicate a decrease in
future ice thickness.
• The large warming in high latitudes of the Northern Hemisphere is connected with a
reduction in the snow (not shown) and sea-ice cover.
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Patterns of Future Climate Change-4
• The multi-model ensemble annual mean change of the precipitation (colour shading) and
its range (isolines) (%) at the time of CO2-doubling.
• Models in all categories shows a general increase in the tropics (particularly the tropical
oceans and parts of northern Africa and south Asia) and the mid and high latitudes, while
the rainfall generally decreases in the subtropical belts.
• Areas of decrease show a high inter-model variability and therefore little consistency.
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Summary of Changes-1
• As the radiative forcing of the climate system changes, the land warms faster than the
ocean. The cooling effect of tropospheric aerosols moderates warming both globally and
locally.
• As the climate warms, Northern Hemisphere snow cover and sea ice extent decreases.
The globally averaged precipitation increases.
• Most tropical areas, particularly over ocean, have increased precipitation, with decreases
in most of the subtropics, and relatively smaller precipitation increases in high latitudes.
• The signal to noise ratio (from the multi-model ensemble) is greater for surface air
temperature compared to precipitation.
• The geographic details of various forcing patterns are less important than differences
among the models' responses. This is the case for the global mean response as for patterns of
climate response. Thus, the choice of model makes a bigger difference to the simulated
response than the choice of scenario
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