Science themes

The earth system is made up of many components that all interact with each other. To understand the importance of all these processes and their interactions, we need to put them in a computer model and simulate the climate.

Atmospheric circulation

The atmosphere can be divided into several distinct layers, as shown in the diagram. The troposphere is the part of the atmosphere where weather systems occur. The troposphere extends from the Earth's surface to the tropopause (about 10-16 km in altitude, depending on latitude), and is generally characterised by temperatures which decrease with height. The stratosphere extends from the tropopause to the stratopause, at about 50 km, and is characterised by a high static stability associated with an overall increase of temperature with height. One important feature of the stratosphere is the ozone layer, which protects the Earth by absorbing much of the ultraviolet radiation from the Sun. The mesosphere extends from the stratopause to about 90 km; together the stratosphere and mesosphere make up the middle atmosphere.

Physical Oceanography

The oceans and sea-ice play an important part in the global climate system. In particular, the large thermal inertia of the oceans, and the consequent long timescales of adjustment, means an accurate representation of ocean processes is critical for realistic climate simulation. For many years the relatively coarse horizontal ocean resolution used has meant these models have had to adjust the heat and freshwater fluxes at the ocean surface to produce realistic simulations. In the latest models, heat and freshwater transport processes are better represented than before. This has been a major factor in the ability of the model to run without the need for flux adjustments.

Land- and sea-ice

Sea-ice cover dramatically effects ocean-atmosphere exchanges of heat and water. The rejection of brine during freezing and the transport of fresh water as ice are key parts of the fresh water budget of the polar regions and have an influence on the formation of deep oceanic waters. A sea-ice model has two distinct components, a thermodynamic model and a dynamic model. The thermodynamic model calculates melting and freezing rates to achieve a local balance of heat and water fluxes. It treats the ice and the snow above it as a single layer of variable thickness with partial areal coverage. Some recent developments include changes to the sea-ice/snow albedo parametrization and the calculation of the basal heat flux from the ocean. The dynamic model calculates ice velocity by balancing forcing from windstress, ocean drag and sea-surface tilt with internal ice stresses.

Carbon cycle

Carbon dioxide in the atmosphere is a 'greenhouse gas' and contributes significantly to global warming. The amount of carbon dioxide in the atmosphere depends on human activities, such as burning fossil fuel, and also the behaviour of the ocean and terrestrial biosphere. Understanding how the global carbon cycle works is essential for us to predict how it, and the climate system, may behave in the future. To address this issue we include models of the carbon cycle within the existing climate model.

Land carbon cycle

Carbon dioxide from the atmosphere is utilised by plants by photosynthesis. The carbon they absorb is allocated within the plant to make up its roots, wood and leaves. Some of this carbon is then lost - either when the leaves drop, or when the plant dies - and becomes soil carbon. Microbes within the soil breakdown this carbon and release it back to the atmosphere as respiration, in the form of carbon dioxide. This is the terrestrial carbon cycle on a small scale (i.e. on the scale of individual plants). On a larger scale (i.e. across geographical regions), the distribution of vegetation is important in the carbon cycle. Different plant types store different amounts of carbon, but they grow at different speeds and favour different conditions. For example trees can store more carbon than grass (per unit area of land covered), but they take a lot longer to grow. So if a previously barren area of land becomes fertile for some reason then grasses will grow first, but trees may take over later. The local climatic conditions, and how they change over time, determine which type of plant dominates in any given location. Human activity also changes the land use, and hence the carbon stored by the biosphere - cutting down trees removes a potentially large absorber of carbon dioxide and if the wood is burnt, or left to decay, then the carbon is released back to the atmosphere. Disturbance of vegetation also affects the soil - deforestation can also lead to large amounts of carbon being lost from the soil. This has an impact on the fertility of the ground and may affect future vegetation growth in the area. Such changes in land use (predominantly in the tropical forests) accounted for the most significant part of anthropogenic carbon dioxide release during the 19th Century. It was not until about 1950 that fossil fuel emissions became significantly larger than the source from land use change. Present day emissions due to anthropogenic land use change still amount to around 1 GtC per year.

Ocean carbon cycle

Carbon dioxide from the atmosphere dissolves in the surface waters. On entering the ocean, carbon dioxide undergoes rapid chemical reactions with the water and only a small fraction remains as carbon dioxide. The carbon dioxide and the associated chemical forms are collectively known as dissolved inorganic carbon or DIC. This chemical partitioning of DIC ('buffering') affects the air-sea transfer of carbon dioxide, as only the unreacted carbon dioxide fraction in the sea water takes part in ocean-atmosphere interaction. The dissolved inorganic carbon (DIC) is transported by ocean currents. Near the poles, cold dense waters sink towards the bottom of the ocean and subsequently spread through the ocean basins. These waters return to the surface hundreds of years later. As more carbon dioxide can dissolve in cold water than in warm, these cold dense waters sinking at high latitudes are rich in carbon and act to move large quantities of carbon from the surface to deep waters. This mechanism is known as the 'solubility pump'. As well as being transported around the ocean, dissolved inorganic carbon is also used by ocean biology. In the surface waters, drifting microscopic oceanic plants known as phytoplankton grow. As with land based plants, phytoplankton take in carbon dioxide during growth and convert it to complex organic forms. The phytoplankton are eaten by drifting oceanic animals known as zooplankton, which themselves are preyed upon by other zooplankton, fish or even whales. During these biological processes, some of the carbon taken in during growth of the phytoplankton is broken down from the organic forms of the biology back to inorganic forms (DIC). If between the carbon uptake by phytoplankton and the subsequent return of the carbon to DIC, the biological material has been transported to depth, for example by the sinking of large biologically formed particles, there is a net transfer of carbon from the surface to depth. This process is termed the 'biological pump'. The carbon can also sink as skeletal structures of the biology which is known as the 'carbonate pump'.

Atmospheric chemistry

Most of the ozone in the atmosphere exists as a layer in the stratosphere at a height of between 8 and 30 kilometres as shown in the diagram on the main stratospheric page. Stratospheric ozone acts as a shield by absorbing potentially harmful ultraviolet (UV) radiation which would otherwise reach the earth's surface. Stratospheric ozone should not be confused with tropospheric ozone. Part of this is a pollutant formed in the lower atmosphere (the troposphere) by the action of sunlight on other compounds such as those from car exhausts.

Vegetation and Crops

A large fraction of the world's food is grown as rainfed annual crops in the tropics, where climate variability plays a key role in determining productivity. Asia alone has more land under cultivation than all of the industrialised nations taken together and 80% of the cultivated area in developing countries is rainfed. Hence understanding and predicting the impacts of climate variability on crops in the tropics is a key research issue, which can inform assessments of crop productivity across a range of timescales.

Land surface

Search in the technical reports of the Hadley Centre:

Literature

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