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This chapter assesses and quantifies projections of possible future climate change from global climate models. It discusses global climate models, global mean response, patterns of future climate change, and more.
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Myneni L31: Projections of Future Climate Change Apr-22-05 (1 of 13) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Further Reading: Detailed Notes Posted on Class Web Sites Outline • global climate models • global mean response • patterns of future climate change • - summary of changes
Myneni L31: Projections of Future Climate Change Apr-22-05 (2 of 13) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Introduction The purpose of this chapter is to assess and quantify projections of possible future climate change from climate models. The Climate System
Myneni L31: Projections of Future Climate Change Apr-22-05 (3 of 13) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University 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).
Myneni L31: Projections of Future Climate Change Apr-22-05 (3a of 13) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Global Climate Models
Myneni L31: Projections of Future Climate Change Apr-22-05 (4 of 13) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Global Mean Response-1 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%.
Myneni L31: Projections of Future Climate Change Apr-22-05 (5 of 13) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Global Mean Response-2 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
Myneni L31: Projections of Future Climate Change Apr-22-05 (6 of 13) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Global Mean Response-2 Projections from Forcing Scenarios-2 • The time evolution of the globally averaged precipitation change relative to the years (1961- • 1990). 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
Myneni L31: Projections of Future Climate Change Apr-22-05 (7 of 13) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University 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.
Myneni L31: Projections of Future Climate Change Apr-22-05 (8 of 13) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University 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.
Myneni L31: Projections of Future Climate Change Apr-22-05 (9 of 13) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University 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.
Myneni L31: Projections of Future Climate Change Apr-22-05 (10 of 13) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University 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.
Myneni L31: Projections of Future Climate Change Apr-22-05 (11 of 13) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University 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
Myneni L31: Projections of Future Climate Change Apr-22-05 (12 of 13) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Summary of Changes-2
Myneni L31: Projections of Future Climate Change Apr-22-05 (13 of 13) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Summary of Changes-3