Climate Modeling Dr. Jehangir Ashraf Awan Pakistan Meteorological Department

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1 Climate Modeling Dr. Jehangir Ashraf Awan Pakistan Meteorological Department Source: Slides partially taken from A. Pier Siebesma, KNMI & TU Delft

2 Key Questions What is a climate model? What types of climate models are there?

3 What is a climate model? A mathematical representation of the many processes that make up our climate. Requires: Knowledge of the physical laws that govern climate Mathematical expressions for those laws Numerical methods to solve the mathematical expressions on a computer (if needed) A computer of adequate size to carry out the calculations

4 Important climate model components Radiation as it drives the system each climate model needs some radiation Dynamics description of the exchange of shortwave and longwave the movement of energy in the system both in the horizontal and vertical (winds, ocean currents, convection, bottom water formation) Surface processes the exchange of energy and water at the ocean, sea-ice and land surface, including albedo, emissivity, etc. Chemistry chemical composition of the atmosphere, land and oceans as well as exchanges between them (e.g., carbon exchanges)

5 Model Types Depending on our question we need to decide how to divide the Earth in our model and how often we need to calculate the state of the system. Choices in space are 0-d (point) 1-d (e.g., 1 vertical column) 2-d (1 vertical layer, latitude and longitude), and 3-d (many layers, lat and lon) Examples: 0-d : A global energy balance model treats the Earth as one point and has no time resolution

6 2. The Simplest Climate Model: 0-dimensional energy balance model

7 Energy Absorbed by the Atmosphere (1) 1) How much energy is reaching the top of the atmosphere from the sun? The solar flux received at the top of the atmosphere from the sun depends on the distance of the Earth from the sun. The average value of this flux is called the solar constant, S0, and has a value of 1367 Wm -2. Note that this value varies as the orbit of the Earth around the sun is not a perfect circle Wm -2 S0

8 Energy Absorbed by the Atmosphere (2) 2) How much energy is directly reflected back to space? Some of the solar flux arriving on Earth is directly reflected back to outer space by clouds and the Earth surface. Clouds have a very high albedo * (up to 0.8). Taking all reflectors (clouds, ground, sea) together, the Earth has an albedo of approximately 0.3. Hence only 70 % of the solar flux arriving on earth is available to the system. AE*S0 S0 Albedo comes from a Latin word for whiteness

9 Energy Absorbed by the Atmosphere (3) 3) What is the total energy absorbed by the Earth? The flux we used so far describes the energy per unit area, hence we now know how much energy per square meter is available to the Earth from solar radiation. To calculate the total energy absorbed we need to multiply the flux with the area that intercepts that radiation. As we can see, that area (the shadow area) is a disk with the radius of the Earth: 2 R e

10 Energy Absorbed by the Atmosphere (4) E in 2 S 0 AS 0 R E Total energy absorbed Reflected flux Solar flux at TOA Area of a disk with radius of the earth or after some minor rearrangement: E in S A R E

11 Energy Emitted by the Atmosphere (1) 1) How much energy is emitted per unit area from the Earth? For a good estimate of this number, we can assume that the Earth is a blackbody. By making that assumption we can now use the Stefan- Boltzmann law to calculate the flux of longwave (infrared) radiation as: F E T E W m 2 K 4 where σ is the Stefan Boltzmann constant and T E the temperature at which the Earth emits radiation.

12 Energy Emitted by the Atmosphere (2) 2) How much energy is emitted in total from the Earth? Again, to find the total amount of energy emitted by the Earth we need to multiply the flux with the area over which energy is emitted. Longwave radiation is emitted from the entire Earth surface and hence: E out T E 4 A E T E 4 4 R E 2

13 Earth s Radiative Balance (1) On average the energy absorbed and emitted by Earth have to balance, as otherwise the system would heat or cool indefinitely. We can calculate the temperature the Earth emits at by assuming a balance of incoming and outgoing energy: E in E out

14 Earth s Radiative Balance (2) S 0 E in E out 1 A R 2 E T 4 2 E 4 R E S0 4 1 A) T 4 T 4 E S E 1 A 0 4 T E S 0 1 A 4 255K 18 C 4 The world s simplest climate model

15 Remarks We calculated that the temperature at which the Earth emits radiation is about -18 o C. If the Earth had no atmosphere, this would be the mean temperature at the surface. We know the observed mean surface temperature is about +15 o C. Hence the presence of the atmosphere increases the surface temperature by 33 o C. This is due to the Earth greenhouse effect, the magnitude of which can be calculated as: T g T S T E 15 C 18 C 33 C

16 The Greenhouse Effect How does it work? The atmosphere contains gases that absorb the infrared radiation emitted from the surface and then re-emit it from the atmosphere in all directions. Some of this radiation will therefore be emitted downwards and be an additional source of energy at the surface, which leads to a warming at the surface! Source: IPCC, 2007

17 General Circulation Models

18 Global climate models (GCMs) General Circulation Models (GCMs) are computer-driven models for weather forecasting, understanding climate and projecting climate change. There are both atmospheric GCMs (AGCMs) and ocean GCMs (OGCMs). An AGCM and an OGCM can be coupled together to form an atmosphere-ocean coupled general circulation model (AOGCM).

19 General circulation models Processes to include

20 Atmospheric model Component E-W wind N-S wind vertical balance mass Temperature Ideal Gas 6 equations for 6 unknowns (u,v,w,t,p,ρ) - Moisture often added as 7th equation

21 Two types of predictions Edward N. Lorenz ( ) Predictions of the 1 st kind Initial-value problem Weather forecasting Lorenz: Weather forecasting fundamentally limited to about 2 weeks Predictions of the 2 nd kind Boundary-value problem IPCC climate projections (century-timescale) No statements about individual weather events Initial values considered unimportant; not defined from observed climate state Climate modeling 22

22 Weather Forecasting Weather (atmospheric) prediction is essentially a initial value problem: timescale boundary conditions >> timescale prediction period (15 days) e.g. Continents, Glaciers, Atmospheric Composition, vegetation, solar constant, ocean temperatures can be kept constant! Atmosphere loses its memory after two weeks

23 Predictions at a seasonal scale Extension beyond the 15 days predictability horizon is possible through the thermal inertia of oceans, snow, soil Requires coupling of the atmosphere with the ocean (which is the most important source of inertia) So far only somewhat successful in the tropics. Outside the tropics the coupling between atmosphere and ocean is weak. In Europe there is little skill on the seasonal scale Note that the problem is slowly shifting from a initial value problem (weather prediction) to a boundary condition (climate prediction) problem Long lasting sea surface temperature (SST) anomalies: El Nino on timescale of seasons to years

24 Decadal to Centennial Projections decadal (10yrs) to centennial is possible through changes of the boundary conditions of the atmosphere: through the ocean (1 to 10 year), through change in greenhouse gases (10+ years)

25 Climate Change Projections

26 Concentration of Greenhouse gases In order to determine the impact of climate change in the future, we need to have an idea of the concentrations of greenhouse gases and other pollutants in the atmosphere to which climate is sensitive, in the years to come. These concentrations depend on their emissions from various sources, natural as well as man-made.

27 The natural greenhouse gases The two main gases responsible for the greenhouse effect (and not only its recent increase) are : Water vapour (H2O) Carbon dioxide (CO2) Beyond water and CO2, the other important natural greenhouse gases are Methane (CH4), which is nothing else than the cooking gas we use in our stoves, Nitous oxide (N2O), the scholarly name for.. laughing gas (which is not so much amusing here), ozone (O3), which molecule comprises 3 oxygen atoms (the molecules of the regular oxygen gas have only 2 atoms of oxygen). The industrial greenhouse gases The main industrial greenhouse gases are halocarbons (Hal represents any halogen) CFCs (for chloro-fluro-carbons) are well-known halocarbons 28 Sulfur hexafluoride (SF6) used in transformers

28 human induced greenhouse effect CO2 55% of the human induced greenhouse effect. fossil fuel use (coal, oil, natural gas) main part industrial processes little part Deforestation negligible Methane 15% biomass combustion cattle raising rice paddies Halocarbons 10% fluids for filling fridges, air con. Emissions due to leaks industrial processes (manufacturing of plastic foams, semiconductors). Nitrous oxide (N2O) 5% fertilizers in agriculture, chemical industries Ozone (O3) 10% Stratospheric ozone: naturally through the interaction of solar ultraviolet (UV) radiation with oxygen (O2). Tropospheric ozone photochemical reactions between air pollutants, volatile organic compounds (VOC) and nitrogen oxides (NOx). 29

29 How long do greenhouse gases stay in the atmosphere? Gas Period CO2 100 years Methane (CH4) 12 years Nitrous Oxide (N2O) 120 years Halocarbons (CnHalp) from several weeks up to years 30

30 GWP (Global Warming Potential) Measured relative to CO2. The GWP of a gas is therefore how much more (or less) it enhances the greenhouse effect over 100 years compared to a similar quantity of CO2 emitted at the same time. Gas Formula relative GWP / CO2 (100 years) Carbon Dioxyde CO2 1 Methane CH4 25 Nitrous Oxyde N2O 298 Perfluorocarbons CnF2n to Hydrofluorocarbons CnHmFp 120 to Sulfur hexafluoride SF * e.g. 1 kg of methane in the atmosphere, will increase the cumulated greenhouse effect, for the coming century, equal to 25 kg of CO2 31

31 CO2 Emissions Source: World Resources Time Magazine 9 April

32 Intergovernmental Panel on Climate Change (IPCC) International body for assessing the science related to climate change; not engage directly in climate research. IPCC was set up in 1988 by WMO and UNEP provide policymakers with regular assessments of the scientific basis of climate change, its impacts and future risks, and options for adaptation and mitigation CMIP (Coupled Model Intercomparison Project) CMIP, an international group coordinating the running of simulations of the global climate system Defines the set of standard experiments (with IPCC guidance) Played a key role in supporting IPCC in its previous Assessment Reports (e.g. AR5) and is now fully engaged in its sixth phase (CMIP6) to support the IPCC with the next Assessment Report (AR6) due to be released in

33 Intergovernmental Panel on Climate Change (IPCC) Panel of scientists that synthesizes published research on climate change including observations, modeling, and paleoclimate /14 First Assessment Report (FAR)Second Assessment Report (SAR) Third Assessment Report (TAR) CMIP1 & CMIP2 Fourth Assessment Report (AR4) CMIP3 Fifth Assessment Report (AR5) CMIP5

34 SRES Emission Scenarios CMIP2(IPCC TAR), CMIP3 (IPCC AR4) SRES (Special Report on Emissions Scenarios, 2000) Projections of climate change in IPCC TAR (2001), AR4 (2007) Based on assumptions about driving forces such as patterns of economic and population growth, technology development, and other factors, major families of SRES emissions scenarios are: A1 (A1FI, A1T, A1B) A2 B1 B2 For details

35 SRES Emission Scenarios A1 - a future world of very rapid economic growth, global population that peaks in mid-century and declines thereafter, and the rapid introduction of new and more efficient technologies. Three sub groups: fossil intensive (A1FI), non-fossil energy sources (A1T), or a balance across all sources (A1B). A2 - A very heterogeneous world. The underlying theme is that of strengthening regional cultural identities, with an emphasis on family values and local traditions, high population growth, and less concern for rapid economic development. B1 - a convergent world with the same global population, that peaks in mid-century and declines thereafter, as in the A1 storyline. B2 - a world in which the emphasis is on local solutions to economic, social and environmental sustainability.

36 Representative Concentration Pathways (RCP) CMIP5 (IPCC AR5) Consistent sets of projections of only the components of radiative forcing that are meant to serve as input for climate modelling. Conceptually, the process begins with pathways of radiative forcing, not detailed socioeconomic narratives or scenarios. Central to the process is the concept that any single radiative forcing pathway can result from a diverse range of socioeconomic and technological development scenarios.

37 RCP Four RCPs were selected, defined and named according to their total radiative forcing in RCP 8.5 Rising radiative forcing pathway leading to 8.5 W/m² in RCP 6 RCP 4.5 RCP 2.6/RCP 3-PD Stabilization without overshoot pathway to 6 W/m² at stabilization after 2100 Stabilization without overshoot pathway to 4.5 W/m² at stabilization after 2100 Peak in radiative forcing at ~ 3 W/m² by mid-century, returning to 2.6 W/m² by 2100

38 Climate Computing Climate modelling requires the use of the most powerful supercomputers on Earth, and even with those we have to simplify the models. Climate modelling is therefore constrained by the computer capabilities and will be for the foreseeable future. McGuffie and Henderson-Sellers, 2005

39 Climate Computing How many calculations does an atmospheric model alone have to perform: 2.5 x 2.5 degrees -> about 10,000 cells 30 layers in the vertical -> about 300,000 grid boxes At least 7 unknowns -> about 2.1 million variables Assume 20 calculations (low estimate) for each variable -> about 42 million calculations per timestep Time step of 30 minutes -> about 2 billion calculations per day 100 years of simulation -> 73 trillion calculations

40 Global Climate Models have their limitations GCMs have a coarse resolution (150~300 km) Land-sea mask Topography Convection, clouds, precipitation Land atmosphere interaction GCM RCM How can we increase the resolution?

41 Dynamical downscaling with regional climate models (RCMs) RCMs are GCMs, but: higher resolution (10km) limited domain Purpose: Better local representation RCM needs to be feeded at the boundaries with data from a GCM Acts like a looking glass. But.. which GCM should be used for downscaling????

42 Thank you