The Physics of Climate Change Prof Tom Choularton

1. The Physics of Climate Change Prof Tom Choularton Everybody talks about the weather – but nobody does anything about it. Mark Twain

2. The Physics of Climate Change Prof Tom Choularton Why Bother? THE ONLY WAY GOVERNMENTS CAN MAKE INFORMED DECISIONS ON ENERGY USE, CO2 REDUCTION MITIGATION STRATEGIES IS TO HAVE CONTRETE ASSESSMENTS OF HUMAN INDUCED CLIMATE CHANGE THAT ARE BASED ON SOUND PHYSICAL SCIENCE

3. This talk will cover: • What science do we need to know? • What do we know about climate and how it changes? • How do we know this? • What remains uncertain? • What work is going on to reduce these uncertainties?

4. Surface area of Earth = 4πR2 Area of Earth normal to Solar Radiation = πR2 Solar Flux per unit area, S Background physics we need to know:

5. Background physics we need to know: Not all incoming radiation is absorbed by the surface, some is reflected back to space The fractional reflectance is known as the global mean planetary reflectance or albedo, A. The average planetary albedo, A, is around 0.3. So incoming irradiance absorbed by the Earth’s surface, Fs, is given by: and has a value of 240 W m-2. Fs must be balanced by the outgoing blackbody radiation of the Earth given by Te4, where Te is the effective blackbody temperature of the Earth-atmosphere system. Equating incoming and outgoing fluxes gives an expression for Te: an equilibrium temperature of 255 K, compared to 288 K, the average surface temperature of the Earth.

6. Max Planck Nobel Prize for Physics 1918 For developing a theoretical deduction of radiation from a black body cavity. The formula renounced classical physics and introducing the quanta of energy, a quantum mechanical concept. In December 1900 he presented the theory In doing so he rejected the accepted wisdom that the second law of thermodynamics was an absolute law of nature, and showed that Boltzmann’s interpretation that it was a statistical law were correct. In a letter written a year later Planck described proposing the theoretical interpretation of the radiation formula saying:- ... the whole procedure was an act of despair because a theoretical interpretation had to be found at any price, no matter how high that might be.

7. IR absorption So far we have assumed that the atmosphere acts simply to scatter and reflect incoming shortwave radiation and does not absorb light. However this is not the case. The atmosphere interacts with both incoming solar radiation and outgoing terrestrial radiation. The strength of the interaction as a function of wavelength is responsible for the heating of the lower atmosphere.

8. Increased KE Incoming Visible no absorption Upwelling IR absorbed by CO2, H2O, N2O, CH4, CFCs etc and re-radiated in all directions

9. Scattering Why is the sky blue and why are sunset’s red In 1899 Lord Raleigh used Maxwell’s new formulation of electromagnetism to explain why the sky is blue The intensity, I, of light of wavelength λ scattered through an angle θ by a sphere of radius a and refractive index m is: So as blue light is half the wavelength of red light it scatters 16 times more efficiently

10. Scattering by particles Rayleigh scattering works well for bodies that are much smaller than the radiation, ie molecules. Geometrical optics works well when the scatterer is much larger than the wavelength of light BUT when the size of the scatterer is the same order as the light then the scattering is much more complex: Mie scattering Most particles in the atmosphere are around the same size as the wavelength of light MiePlot simulation of scattering of sunlight from r = 4.8 µm water drops superimposed on a digital image of a glory taken from a commercial aircraft.

11. Our Climate We can measure the net short, long and global radiation entering and leaving the top of our atmosphere using satellite spectrometry, interferrometry and radiometry. These measurements allow us to determine change in planetary temperature These data are from the Earth Radiation Budget Experiment (ERBE)

12. Climate Change

13. What do we know is changing?

14. What do we know is changing?

15. JFM 2002 is clearly the warmest ever Temperature rise °C Global temperatures 1860-2001

16. Arctic sea ice extent, million km2 Hadley Centre Change in Arctic sea ice extent

17. ATMOSPHERE Terrestrial radiation Clouds Greenhouse gases and aerosol Solar radiation Ice- sheets snow Precipitation Sea-ice OCEAN Biomass LAND Hadley Centre The climate system

18. Met Office Hadley Centre Development of Hadley Centre climate models ATMOSPHERE OCEAN ICE SULPHUR CARBON CHEMISTRY LAND ATMOSPHERE OCEAN ICE SULPHUR 1999 CARBON LAND 1997 ATMOSPHERE OCEAN ICE SULPHUR LAND 1992 ATMOSPHERE OCEAN ICE LAND Component models are constructed off-line and coupled in to the climate model when sufficiently developed ATMOSPHERE OCEAN LAND ATMOSPHERE 1985 LAND ATMOSPHERE 1960s

19. 30km 19 levels in atmosphere 2.5 lat 3.75 long The HadleyCentrethirdcoupledmodel -HadCM3 1.25 1.25 20 levelsin ocean Hadley Centre -5km

20. Testing models on recent history

21. 700 600 500 400 350 Carbon dioxide concentration in the atmosphere, due to three emissions scenarios Hadley Box Model Business as usual emissions Constant 1990 emissions 50% reduction in emissions CO2 concentration ppm

22. CO2 in the SRES emissions scenarios A1FI A2 B2 B1

23. IPCC A1FI emissions A2 emissions B2 emissions B1 emissions Global temperature rise, degrees C Start to diverge from mid-century Hadley Centre Global temperature rise

24. Hadley Centre Components of sea-level rise

25. 0.5 0.4 0.3 0.2 0.1 A1FI A2 B1 B2 Sea-level rise (m) Thermal expansion + glacier melt + Greenland + Antarctica (land movement not included) Hadley Centre 1990 2000 2020 2040 2060 2080 2100 Sea-level rise HadCM3

27. Met Office / Hadley Centre A1FI emissions scenario 0 1 2 3 4 5 6 Pattern of annual temperature changes2080s relative to present day

28. Met Office / Hadley Centre A1FI emissions scenario – 3 – 2 – 1 – 0.5 – 0.25 0 0.25 0.5 1 2 3 Pattern of annual precipitation changes2080s relative to present day

29. What is driving climate change?

30. The uncertainties – aerosols and clouds The Direct Effect Higher aerosol loadings in the atmosphere typically reflect more aerosol back to space and so reduce the amount of radiation that reaches the Earth’s surface Increased aerosol leads to a larger optical depth, making it hazier So far the IPCC has assumed that all man made (or anthropogenic) aerosol are sulphate, this is not the case

31. The role of organic material These laboratory measurements and predictions show that significant amounts of organic can reduce aerosol size appreciably and hence reduce the scattering efficiency of the sulphate aerosol Is this important in the atmosphere?

32. Examples from ADRIEX • (background) A view of the polluted lower Po valley. Polluted stratified layers were observed under anticyclonic conditions. The layers most likely arise from nocturnal inversions cutting off the surface layer from the residual pollution from the previous day. Once the surface warms in the morning, a new polluted layer is formed and begins to fill the boundary layer The FAAM G-LUXE aircraft on the ground at Treviso airport. The Low Turbulence Inlet is visible above the front right hand side door. • The AMS rack fitted to the G-LUXE aircraft during ADRIEX

33. E A C D B Examples from ADRIEX D D • The flight track of 29/08/04 is shown. A mapping study of the Po valley was conducted, performing the zigzag pattern twice to observe the boundary layer development. • Time series of mass loadings (at 30 sec resolution) are highly variable and show evidence for very enhanced concentrations of NH4NO3 and organics (> 8 µg m-3) in some plumes. • The plumes are highest in northern side of the valley (C and E) and are larger and more widespread in the later run (10:30 UT) onwards • The data shown can be retrieved every 30 secs in real time on the aircraft via the G-LUXE LAN. A B C D E C B A B C D E C B A • The blue box marked in the time series identifies an example plume close to point E on the second run.

34. Worse carbonaceous material in the atmosphere may absorb! Mixing (internal vs external) BC can absorb 2x as much light as inclusions in scattering particles (e.g. Fuller et al (1999). ? Factor of 2 in forcing calculations (Haywood and Shine, 1995) Obs? • Global Climate Models treat this poorly • Extra BC absorption sometimes implicit in empirically determined light abs co-efficients. • Size distribution is usually fixed (maybe in a number of bins or modes)

35. Dust also affects radiation balance

36. Dust also affects radiation balance Dust is not simply a natural phenomenon It may be mixed with biomass burning, changing its absorption efficiency Its emission may change with desertification and changes in land use, largely brought about through changes in agricultural practices and irrigation

37. Clouds – The Indirect Effect Ship tracks Forest fire plume (direct aerosol scattering and absorption) Tropical cyclone

38. Clouds – The Indirect Effect Low clouds change the surface reflectivity and so reflect considerable radiation back to space, increasing aerosol increases their reflectivity, however they are at the same temperature as the surface so they do not affect the LW radiation much. A NET COOLING Cold high clouds, are optically much thinner and so they don’t scatter as much incoming sunlight BUT they are much colder than the surface so they absorb outgoing IR and re-radiate at a colder temperature. A NET WARMING

39. Other Effects – As yet even more uncertain The Semi Direct Effect • Absorbing aerosols may reduce low cloud cover • This would warm the climate as low clouds scatter solar radiation back to space. • Aerosols absorb solar • radiation • Evaporation of the cloud! Absorbing aerosols in and around a cloud

40. Other Effects – As yet even more uncertain The vertical distribution of cloud and aerosol Absorbing aerosols above cloud increase warming as reflecting surface below increases flux into absorbing layer NET WARMING Absorbing aerosols below a reflecting cloud have a smaller effect than they would otherwise as less radiation reaches the layer NET COOLING

41. How we study clouds and aerosols

42. UK FAAM G-LUXE BAeS 146-300 Aircraft Installation ADA-100 CPI

43. EMERALD-1 Cirrus Missions 19/09/01 EM09 Adelaide. CPI Ice Crystal Measurements Egrett flight profile Ice crystal scale 200m Altitude (km) 180o turn of both aircraft 180o turn of both aircraft Time UTC Flight leg 4 Flight leg 5 RHice measured by Egrett (%) CPI Ice Crystal Data courtesy UMIST Lidar data courtesy J,Whiteway, Clive Cook, University of Aberystwyth Temperature measured by Egrett oC

45. Martin Gallagher and Paul Connelly, UMIST, Imperial, and Aberystwyth

46. Field Tests of PDPA ADA-100 at the Sphinx Observatory Jungfraujoch, Swiss Alps 12,000 ft Microphysics Platform

47. Hadley Centre Deg C Change in surface temperature with forced THC collapse, but without change in greenhouse gases

48. Hadley Centre Circulation strength (Sverdrups) No change SRES A1FI SRES B2 SRES B1 SRES A2 North Atlantic Ocean circulation

49. winter summer Hadley Centre Hadley Centre °C Temperature rise Medium-high emissions scenario, 2080s

50. Medium-high emissions scenario, 2080s winter summer Hadley Centre Hadley Centre Change in precipitation %