Crown copyright Met Office Essentials of climate modelling PRECIS Workshop, University of Reading, 22 nd 26 th August 2011 Crown copyright Met Office Essentials of Climate Modelling The goal of this session is a brief introduction to: the climate system drivers of climate climate variability modelling of the climate system projection of future climate Crown copyright Met Office The climate system Crown copyright Met Office Components of the climate system Crown copyright Met Office Planetary energy balance A planetary object intercepts a circle (of radius R) of incoming solar energy S as S R 2 A (for Albedo) of which is reflected back into space. Energy absorbed is balanced by radiation to space. Hence, S R 2 (1-A) = 4 R 2 T 4 4 therefore T = [ S(1-A)/4 ] T = Temperature = Stefan-Boltzmann constant See the Stefan-Boltzmann Law for more information Crown copyright Met Office Planetary energy balance For the Moon: S = 1365 Wm -2 (Watts/metre 2 ) and A = 0.1 (a constant) which predicts T = 272 K (degrees Kelvin) Crown copyright Met Office Planetary energy balance For the Earth: S = 1365 Wm A = 0.3 predicts T = 255 K (~ -18 C) In fact, the mean surface temperature of the Earth is T = 287 K (~ +14 C) What accounts for this? Crown copyright Met Office The Greenhouse Effect Crown copyright Met Office The Greenhouse Effect Visible energy from the sun passes through the glass and heats the ground Infra-red heat energy from the ground is partly reflected by the glass, and some is trapped inside the greenhouse Crown copyright Met Office The Greenhouse Effect Some solar radiation is reflected by the earths surface and the atmosphere EARTH Some of the infrared radiation is absorbed and re-emitted by the greenhouse gases. The effect of this is to warm the surface and the lower atmosphere Most solar radiation is absorbed by the surface, which warms Infrared radiation is emitted from the earths surface Crown copyright Met Office Climate sensitivity Crown copyright Met Office Changes in certain components of the climate system perturb (i.e. cause a change to) the radiative energy budget of the system, providing a radiative forcing. Examples include: the concentration of radiatively active species changes in the solar irradiance incident upon the planet changes affecting radiation absorbed by the surface Human-induced perturbations include: composition of the atmospheric gases increases in atmospheric aerosols land-use change (agriculture, deforestation, reforestation, afforestation, urbanisation, traffic, ) Radiative forcing Crown copyright Met Office External radiative forcings, e.g.: solar radiation (output & Milankovitch) volcanic eruptions Internal climate variability, e.g.: ENSO (El Nio Southern Oscillation) NAO (North Atlantic Oscillation) and other leading modes of variability Natural variability of climate Crown copyright Met Office The effect of the Mt. Pinatubo eruption (June 1991) on global temperature Crown copyright Met Office Perturbations of the atmospheric composition - the enhanced greenhouse effect Effect of aerosols: direct effect (scattering of incoming solar radiation) indirect effect (affecting the radiative properties of clouds) Land-use change (agriculture, deforestation, reforestation, afforestation, urbanisation, traffic, ) Human-induced climate variations Some sunlight reflected More sunlight reflected cooling effect Brighter cloudsNormal clouds Relatively clean lower atmosphere Polluted lower atmosphere The indirect effect of aerosol Greenhouse species Various trace gases and aerosols intercept reflected longwave radiation and re-emit in all directions Water vapour is the biggest contributor (~30 of ~33 K) Other important gases are CO 2 (~2 K), CH 4 and N 2 O (total ~1 K) Contribution depends on absorption spectrum, concentration and residence time Aerosols (including cloud droplets) do similar 350 ppm Crown copyright Met Office The Enhanced Greenhouse Effect Solar (S) and longwave (L) radiation in Wm 2 at the top of the atmosphere Crown copyright Met Office Radiative Forcings Crown copyright Met Office Indicators of the human influence on the atmosphere during the industrial era Crown copyright Met Office What are the processes which feedback on the climate Source: Intergovernmental Panel on Climate Change (IPCC), WG1-AR3, Ch 1. Processes which represent feedbacks are in red boxes Crown copyright Met Office The response of the climate system to these forcing agents is complicated by: feedbacks the non-linearity of many processes different response times of the different components to a given perturbation The only means available to calculate the response is by using numerical models of the climate system. How do we quantify the response of the climate? Crown copyright Met Office Climate models Crown copyright Met Office General overview A global climate model (GCM) is a model of the climate system, including the atmosphere, oceans, land-surface and more. The advective (relating to motion) and thermodynamical (relating to heat) evolution of atmospheric pressure, winds, temperature and moisture (prognostic variables) are simulated, while including the effects of many other physical processes. Other useful meteorological quantities (diagnostic variables) are derived consistently within the model from the prognostic variables, such as precipitation, evaporation, soil moisture, cloud cover and many more. Crown copyright Met Office Main components of global climate models Atmosphere and ocean dynamics Model grid Physical parameterizations Initial conditions of the model Boundary conditions (e.g. land sea mask, orographic height, vegetation and soil characteristics) Crown copyright Met Office Vertical exchange between layers of momentum, heat and moisture Horizontal exchange between columns of momentum, heat and moisture Vertical exchange between layers of momentum, heat and salts by diffusion, convection and upwelling Orography, vegetation and surface characteristics included at each grid box surface Vertical exchange between layers by diffusion and advection The three dimensional model grid 15 W 60 N 3.75 2.5 11.25 E 47.5 N Crown copyright Met Office Parameterization of physical processes Important processes occur in the atmosphere on scales smaller than those which are resolved by the grid of the dynamical part of the model. The effects of these unresolved (sub-grid scale) processes are deduced from the large scale state variables predicted by the model (wind, pressure, temperature, moisture). This procedure is called parameterization. Crown copyright Met Office Initial and boundary conditions All climate models require information about the initial state of the atmosphere at the beginning of the climate model experiment. These are the initial conditions of the model experiment. The three dimensional grid of a GCM has no lateral (North- South, East-West) boundaries. The upper boundary is the end of the atmosphere where it contacts outer space. The lower boundary is either the surface of the land or the bottom of the ocean. As such the GCM requires information about the topography of the Earths surface, called surface boundary conditions. Crown copyright Met Office Development of climate models Crown copyright Met Office Hadley Centre Global Climate Model FORTRAN program code Crown copyright Met Office 20 th Century climate Crown copyright Met Office Climate of the Recent Past The present-day climate and its implications for climate change in the future can only be properly appreciated when placed in the context of the climate changes that have taken place in the historical past. Crown copyright Met Office SPM 1a Variations of the Earths surface temperature for the past 140 years Crown copyright Met Office SPM 1b Variations of the Earths surface temperature for the past 1000 years Crown copyright Met Office Simulated annual global mean surface temperatures Crown copyright Met Office Predicting climate change Crown copyright Met Office Predicting Climate Change Studies of future climate change generally have the ultimate goal of trying to assess what the impacts of climate change will be on human life and the environment, as this information is of great interest to planners and decision-makers. These studies also need to assess what the level of risk associated with the various changes is, as some changes are more likely to occur than others. Crown copyright Met Office What will regional and local changes in climate, especially rainfall, be like? How will the incidence and characteristics of hazardous weather change? How will natural climate variability influence and be influenced by climate change? What will this mean for society and the natural environment? Predicting Climate Change Crown copyright Met Office Predicting Climate Change Range of emissions Range of global warming Range of regional climate change Range of impacts Range of concentrations The classic cascade of uncertainty Impact that we wish to avoid Regional climate change that may cause this impact Global climate change that may cause this range of regional climate change GHG concentrations that may cause this range of climate change Emissions that may lead to this range in concentrations Upper bound: very likely to lead to this impact Lower bound: very unlikely to lead to this impact Crown copyright Met Office Land areas are projected to warm more than the oceans with the greatest warming at high latitudes Annual mean temperature change, 2071 to 2100 relative to 1990: Global Average in 2085 = 3.1oC Crown copyright Met Office Some areas are projected to become wetter, others drier with an overall increase projected Annual mean precipitation change: 2071 to 2100 Relative to 1990 Crown copyright Met Office ... And in conclusion This presentation is intended as a brief overview of the climate system and climate modelling. For more in-depth training, consider registering for the free online course in the science of climate change and modelling atThis is a joint effort of the University of Oxford Continuing Education Department and the Met Office Hadley Centre consisting of 8 interactive online units intended for an educated (but non-scientist) audience. Crown copyright Met Office Questions