Climate change and pollution

1. Climate change and pollution Eleanor J Highwood Department of Meteorology, University of Reading MSc Intelligent Buildings April 2002

2. Outline: climate change • What is climate? • Has climate changed in the recent past? • If so has any change been unusual? • What might have caused climate to change? • Can we model climate change? • What might happen in the future? • What is there left to do?

3. What is climate? “Climate is what we expect, weather is what we actually get” A full description of climate includes: global means, geographical, seasonal and day-to-day variations of temperature, precipitation, radiation, clouds, snow cover etc.

4. Has climate changed in the recent past? • Temperature changes • Sea level rise • Precipitation changes • Mountain glaciers • Snow cover

5. Temperature changes 1 Global mean T of air at Earth’s surface has  by 0.6 +/- 0.2 C over the 20th century. IPCC 2001

6. Temperature changes: 2 • Regional changes can be much larger than global means; some places have also cooled: “global warming” is a misnomer. • Size of warming depends on time period considered and time of year considered.

7. Variation of warming with time period IPCC 2001

8. Seasonal variation in warming IPCC 2001

9. Temperature changes: 3 • Over the period 1950 to 1993, diurnal temperature range has reduced because the nights have warmed more than the days.

10. Sea level changes • Observed rise of 0.1 - 0.2m during 20th century. Rises are of order 2mm/year • Mostly due to thermal expansion of oceans

11. Precipitation changes •  over land in tropics and mid-latitudes and  in the subtropics. • NH mid-latitudes have seen an increase of 2-4% in frequency of heavy precipitation events

12. Mountain glaciers • Shrinkage of many glaciers since 1890. If it reaches the oceans this contributes to sea-level rise.

13. Snow cover • 10% reduction in NH snow cover between 1960s and present day

14. Sea ice • NH sea ice extent has decreased by 10-15% since 1950s

15. Have changes been unusual? Proxy records: • tree rings (past 100 years) • shallow ice cores • corals • deep sea sediments (past 10, 000 years) Natural variability: changes resulting from interactions between components of climate system

16. Changes over past 1000 years (from Mann et al 1999

17. Natural variability:1 There have been large changes in temperature in the past

18. Natural variability:2 Even a climate with no forcing has a lot of variability (IPCC 2001)

19. What might have caused these changes? • The balance of evidence suggests that there is a discernible human influence on global climate (IPCC, 1995) • There is new and stronger evidence that most of the warming over the past 50 years is attributable to human activities (IPCC 2001)

20. Fundamental processes • Many interacting components

21. Energy balance Solar energy absorbed by the Earth-atmosphere system Energy radiated from Earth- Atmosphere system to space = S0 (1- p) re2 = 4 re2  Te4 30% of incoming solar radiation reflected to space by clouds, surface, molecules and particles in the atmosphere (albedo).

22. Radiation and climate IPCC 2001

23. The “natural greenhouse effect” Ta4 Ts4 Ta4 Atmosphere absorbs radiation from ground and re-emits less radiation since it is colder (=0.77) Earth radiates to space Atmosphere traps radiation and warms surface so that life can exist.

24. Radiative forcing, F • Radiative forcing measures the change to the energy budget of the atmosphere. Positive  surface T  Negative  surface T  • Easier to calculate than change in temperature, but related to temperature change by T= F where  is the climate sensitivity.

25. Radiative forcing due to  in solar output ASR = OLR System in balance ASR > OLR OLR must increase to balance ASR, so system must warm up. F +ve

26. Radiative forcing due to  in carbon dioxide ASR = OLR System in balance OLR < ASR OLR must increase again to balance ASR, so system must warm up. F +ve CO2 raises  so more radiation comes from cold atmosphere so OLR increases

27. Possible causes of climate change

28. Natural climate change • Solar variability • Volcanic eruptions

29. Solar variability: 1 • Changes in the Suns strength • 11 year cycle with sunspots • small changes

30. Solar variability: 2 • Changes in Sun-Earth geometry • Sun-Earth distance, tilt of Earth and ellipse of orbit • act over very long timescales, many thousands of years • possibly play a role in inducing ice ages but not important on past 250 years time scale • at current time provides a cooling influence on climate

31. Volcanoes Large eruptions like Pinatubo (1991) put clouds of sulphur dioxide gas into stratosphere, above the weather.  cloud of sulphuric acid droplets scatter and absorb solar radiation  cooling of surface and warming of stratosphere But, aerosols only last a few years, so generally climate impact only lasts a few years (apart from cumulative effect? )

33. Observed effect on T IPCC 2001 El Chichon Pinatubo

34. Anthropogenic causes • Greenhouse gases • Ozone changes (stratospheric and tropospheric) • Tropospheric aerosols • Surface albedo changes • Heat pollution

35. Greenhouse gases: 1 • Water vapour is most important natural greenhouse gas, but we don’t usually change it directly • Strength of a greenhouse gas depends on • strength of absorption of infra-red radiation • overlap of absorption with other gases • lifetime in the atmosphere • amount added over given period of time

37. Greenhouse gases: 2 • CO2 (carbon dioxide) • CH4 (methane) • N2O (nitrous oxide) • CFCs/HCFCs/HFCs (chlorofluorocarbons/hydrochlorofluorocarbons/hydrofluorocarbons)

38. Strengths of greenhouse gases

39. CO2 :1 Risen by 31% since 1750, roughly in line with emissions from fossil fuel burning

40. CO2 :2 • Rate of recent increase has been unprecedented • Also increased by deforestation in the tropics and biomass burning • Lifetime of 200 years and is slow to respond to changes in emissions

41. Carbon cycle

42. CH4 :1 • Increased by 50% since 1750 and continues to increase.

43. CH4 • Current concentrations have not been exceeded in 420 thousand years • From rice-growing, domestic cattle, waste disposal and fossil fuel burning • 12 year lifetime (a quick-fix for “global warming”)

44. N2O • Increased by 17% • Unprecedented in past 1000 years • Half of current emissions are anthropogenic (fertilisers etc)

45. CFCs CFCs contain chlorine which damages the ozone layer in the stratosphere. They last 50 years or more and so built up in the atmosphere during 1970s/80s. Banned under Montreal ProtocolReplaced temporarily by HCFCs which still contain chlorine but break down in atmosphere much more quickly

46. HFCs • No chlorine (therefore don’t affect ozone layer)

47. HFCs • No chlorine (therefore don’t affect ozone layer) BUT • they are powerful greenhouse gases and very long-lived • Entirely anthropogenic in origin (and used in a variety of odd ways!) • Rising quickly in the atmosphere

48. Emission of CFCs etc

49. CF4 SF6

50. Ozone • Spatially non-uniform • Radiative forcing depends critically on level at which ozone changes: • troposphere: ozone has increased and produces a positive radiative forcing • stratosphere: ozone has decreased implying less absorption and re-emission of IR radiation producing a negative forcing (also small +ve forcing due to increased solar radiation reaching the surface)