Global Circulation Models the basis for climate change

Global Circulation Models – the basis for climate change science

Weather prediction l l l l Vilhelm Bierknes claimed we can predict weather by calculations. 7 equations that predicted "largescale atmospheric motions. ". Weather processes were too complex for calculation. Computers became essential for this process. NWP (National Weather rediction) first carried out by the Royal Swedish Air Force Weather Service in Stockholm (December 1954). Forecasts were three times a week. NWP was soon available in most western countries.

NWP vs climate models l l l NWPs are designed to predict regional weather conditions in the short (1 -3 days) and medium (4 -10 day) term. Climate models are derived from these models, but are designed to predict weather conditions years into the future. Given the moderate accuracy of models in the short term, how is it feasible to predict weather conditions so far into the future? The answer is: STATISTICS. Climate models are not designed to give an accurate forecast on a daily basis, but rather to predict means and variability in climatic indicators – to give a statistically accurate picture of CLIMATIC, not WEATHER conditions.

NWP vs climate models (cont) Contrasts NWP Goal to predict weather Spatial coverage regional or global Temporal range days Spatial resolution variable (20 -100 km) Relevance of: Initial conditions high Clouds/radiation low Surface (land/ocean/ice) low Ocean dynamics low Model stability low Time dimension essential Climate models to predict climate global years usually coarse low high ignored

How does the climate work? l l l The global climate system is a result of the link of atmosphere, oceans, the ice sheets (cryosphere), living organisms (biosphere) and the soils, sediments and rocks (geosphere), each of which will be considered in greater detail after this. Each of these systems is integrally connected to the others, and energy exchanges between and within systems, as well as other interactions (such as the provision of nuclei for rain droplet formation) determine climatic condtions. However, despite the interconnectedness, an explanation must clearly focus on aspects separately, and the linkages between these systems. Climate models allow us to study these aspects of the systems independently. GCMs stitch these individual models together through a process of linkages, the development of which has taken many years and a great degree of understanding of the climate

The atmosphere I: vertical structure l l The lowest level of the atmosphere (the troposphere) is where the majority of weather processes take place. It contains 75% of the gases and almost all water vapour and aerosols. (Barry & Chorley, 1992) l l l The tropopause marks the upper limit of the troposphere. Temperature change is due to absorption of UV by the ozone layer. Consequently very stable. The atmosphere above this level is mostly irrelevant in terms of weather. The action and feedbacks associated with clouds are still poorly understood, and only recently have models begun to incorporate cloud cover in any comprehensive detail. Source: Barry & Chorley, 1992

The atmosphere II: energy budget l l l 1368 Wm-2 of incoming radiation hits the top of the atmosphere. A black body would reflect all the radiation, although some would be absorbed and re-radiated as longer wavelengths (dotted lines). However, atmospheric gases absorb some of the radiation, reducing the radiated energy (grey area). Atmospheric gases/aerosols also scatter incoming radiation. Although 30% of incoming energy is reflected, little of the remainder escapes directly. The atmosphere consequently heats up (greenhouse effect). Source: IPCC Third Assessment Report

The atmosphere III: Horizontal transfers l Because of the earth’s curvature, more radiation falls in equatorial regions than at the poles. (Trewartha & Horn, 1980). l l To restore equilibrium, an interchange of heat from tropics to poles occurs through movement of air masses. (Barry & Chorley, 1992) This latitudinal transfer of energy occurs in several ways: - movement of sensible heat - movement of latent heat - ocean circulation Source: NASA For each packet of air that moves polewards, a similar quantity moves towards the tropics, setting up circulation cells (also affected by the coriolis forces of the earth’s rotation). These energy fluxes are the principal components of the climate – therefore actions which interfere with the fluxes necessarily affect the climate.

The oceans l l l It is divided into two distinct layers: - The upper, seasonal layer of warm mixed water that stretches up to 100 m deep in the tropics, and interacts with the atmosphere. - The lower deeps, which contain more than 80% of the water in the oceans. The ocean holds more energy than the atmosphere because: - Heat capacity is 4. 2 times higher - Density is 1000 times higher. Heat is transferred to the atmosphere by evaporation of water vapour, which passes on its energy to the atmosphere when it condenses into clouds or precipitates. Vertical energy transfer at the poles – freezing oceans become more saline, and the water sinks. The world therefore has extensive global thermohaline circulation, which warms polar regions and transfers nutrients to the tropics. .

Biosphere l l l The biosphere is the living component of the world. It affects many aspects of the climate: Plants absorb more light than bare ground, reducing albedo (coniferous forest: 0. 09 -0. 15; cf. bare ground: 0. 3). The biosphere also affects the fluxes of certain greenhouse gases: - Terrestrial plants fix CO 2 in their structure. - Oceanic plankton remove CO 2 from the atmosphere as shells when they fall to the ocean bottom. The biosphere also generates large amounts of aerosols such as spores, viruses, dust, bacteria and pollen that scatter and reflect incoming radiation. Primary productivity in the oceans also generates dimethyl sulphides (DMS), which act as nuclei for cloud formation.

The geosphere l l l This is the physical structure of the earth, from the soils and rocks of the continental shelf to the planet’s core The internal energies of the earth can cause climate change over extremely long periods. Plate tectonics change the shape of the surface, and transform ocean basins or mountains, affecting energy transfers between coupled systems. The structure of soil can affect both its The Mahameru volcano on the interaction with the air (in terms of gas island Java of Indonesia. Photo by Jan-Pieter Nap fluxes) and its water retention for biological processes. Volcanism can emit vast quantities of CO 2 from single events, as well as putting large amounts of aerosols into the atmosphere, which can reduce incoming radiation for several years. (Sear et al. , 1987).

Different types of climate models l l l It is often convenient to regard climate models as belonging to one of four main categories: - energy balance models (EBMs) - one dimensional radiative-convective models (RCMs); - two-dimensional statistical-dynamical models (SDMs) - three-dimensional general circulation models (GCMs). It is not always necessary to use the most complex model. Using a simpler model allows more runs to be carried out as sensitivity tests to assess the accuracy of modelling assumptions.

Energy balance models l These simple models only really concern themselves with two things: - Radiation balance (between incoming solar radiation and heat loss) - Latitudinal energy transfer l l EBMs may be 0 -D, in which case latitudinal characteristics are ignored. In 1 -D models, the dimension included is latitude. Temperature for each latitude band is calculated using the appropriate latitudinal value for the various climatic parameters

Radiative-convective models l l These models are generally 1 -D or 2 -D, with height always present as a dimension. They model: - Radiative transformations as energy is absorbed, emitted and scattered. - The role of convection and vertical energy transfer through atmospheric motion. By considering surface albedo, cloud amount and atmospheric turbidity, it calculates the heat absorption in various atmospheric layers. If the heating in a layer exceeds a certain value (the lapse rate), it will convect into the layer above, transferring heat energy. The tropical Hadley cells models are an example of this type of model. Source: http: //www. newmediastudio. org/Data. Discovery/Hurr_ ED_Center/Easterly_Waves/Trade_Winds/Trade_Wi nds_fig 02. jpg

Statistical-dynamical models l l l These are generally 2 -D, with one horizontal and one vertical dimension (although there are some models with two horizontal dimensions). They combine the horizontal energy transfer of EBMs with the radiative-convective functions of RCMs. However, the equator-pole transfer is more accurately simulated than in EBMs, based on theoretical and empirical relationships of the cellular flow between latitudes. Energy diffusion is simulated using the laws of motion. Statistical relationships define the windspeed and wind direction within the models. These models are useful for simulating and studying horizontal energy flows, and processes that disrupt them. SOURCE: http: //www. newmediastudio. org/Data. Discovery /Hurr_ED_Center/Easterly_Waves/Trade_Win ds/Trade_Winds_fig 01. jpg

Global circulation models l l l GCMs “…are the only credible tools currently available for simulating the response of the global climate system to increasing greenhouse gas concentrations” (IPCC-TGCIA, 1999). The first GCM was a very simple 2 layer, hemispheric, quasigeotrophic computer model, developed in the 1950’s by Norman Philips. Such early GCMs involved several atmospheric layers and a very simple oceanic model. The model was run to equilibrium with a set CO 2 level (such as 300 ppm) and then the CO 2 level was increased. Contemporary models are considerably more complex, and are capable of being run in a transient mode. They are 3 -D, and may comprise thousands of individual cells

Contemporary GCMs: an outline l l l l The most complex current models are known as coupled atmospheric ocean general circulation models (AOGCMs). They have between 10 and 20 layers in the atmosphere, and as many as 30 layers in the ocean. Contemporary AOGCMs have a horizontal resolution of between 250 km and 600 km. For local planning, this is a very coarse scale, and the underlying topography is poorly represented. However, taken over the whole globe, this resolution results in an extremely large number of individual cells. For a given time step, calculations are carried out for each of these cells over the whole globe, including energy exchanges between each of the 26 adjacent cells. Clearly this is very computationally intensive, and it is no surprise that atmospheric predictions have been at the forefront of computer development since the early 1950 s.