Outline Further Reading: Detailed Notes Posted on Class Web Sites Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni L31: Projections of Future Climate Change Apr (1 of 13) - global climate models - global mean response - patterns of future climate change - summary of changes

The purpose of this chapter is to assess and quantify projections of possible future climate change from climate models. Introduction Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni L31: Projections of Future Climate Change Apr (2 of 13) The Climate System

Global Climate Models Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni L31: Projections of Future Climate Change Apr (3 of 13) 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).

Global Climate Models Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni L31: Projections of Future Climate Change Apr (3a of 13)

Global Mean Response-1 Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni L31: Projections of Future Climate Change Apr (4 of 13) 1% CO2 Experiments The time evolution of the globally averaged (a) temperature change relative to the years ( ) 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%.

Global Mean Response-2 Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni L31: Projections of Future Climate Change Apr (5 of 13) Projections from Forcing Scenarios-1 The time evolution of the globally averaged temperature change relative to the years ( ). 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 for GS compared to 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

Global Mean Response-2 Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni L31: Projections of Future Climate Change Apr (6 of 13) Projections from Forcing Scenarios-2 The time evolution of the globally averaged precipitation change relative to the years ( ). G: greenhouse gas only (left), GS: greenhouse gas and sulfate aerosols (right). Used business-as-usual scenario The globally averaged precipitation response for 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

Patterns of Future Climate Change-1 Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni L31: Projections of Future Climate Change Apr (7 of 13) Multi-model annual mean zonal temperature change (degrees C). There is consistent mid-tropospheric tropical warming and stratospheric cooling.

Patterns of Future Climate Change-2 Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni L31: Projections of Future Climate Change Apr (8 of 13) 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.

Patterns of Future Climate Change-3 Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni L31: Projections of Future Climate Change Apr (9 of 13) Change in sea-ice thickness between the periods and 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.

Patterns of Future Climate Change-4 Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni L31: Projections of Future Climate Change Apr (10 of 13) 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.

Summary of Changes-1 Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni L31: Projections of Future Climate Change Apr (11 of 13) 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

Summary of Changes-2 Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni L31: Projections of Future Climate Change Apr (12 of 13)

Summary of Changes-3 Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni L31: Projections of Future Climate Change Apr (13 of 13)