Atmospheric Chemistry and Climate in the Anthropocene By Paul Crutzen (with model results from Phil Rasch, NCAR)

1. Atmospheric Chemistry and Climate in the Anthropocene By Paul Crutzen (with model results from Phil Rasch, NCAR)

2. Since the beginning of the 19th Century, by its own growing activities, Mankind opened a new geological era: the Anthropocene It is clearly affecting climate and is even able to deliberately do so.

3. During the past 3 centuries human population has increased tenfold to 6000 million and fourfold in the 20th century • Cattle population increased to 1400 million (one cow/family); by a factor of 4 during the past century • Urbanisation grew more than tenfold in the past century; almost half of the people live in cities and megacities • Industrial output increased 40 times during the past century; energy use 16 times • Almost 50 % of the land surface has been transformed by human action • Water use increased 9 fold during the past century to 800 m3 per capita; 65 % for irrigation, 25 % industry, ~10 % households

5. Human appropriation of terrestrial net primary productivity ~ 30%, but with large uncertainties 3-39%, Vitousek et al., Science, 494, 1997; 10-55%, Rojstaczer et al., Science, 2549, 2001 • Fish catch increased 40 times • The release of SO2 (160 Tg/year) by coal and oil burning is at least twice the sum of all natural emissions; over land the increase has been 7 fold, causing acid rain, health effects, poor visibility, and climate changes due to sulfate aerosols • Releases of NO to the atmosphere from fossil fuel and biomass burning is larger than its natural inputs, causing high surface ozone levels over extensive regions of the globe • Several climatically important ”greenhouse gases” have substantially increased in the atmosphere, eg. CO2 by 30 %, CH4 by more than 100 %.

6. The great acceleration

7. Humanity is also responsible for the presence of many toxic substances in the environment and even some which are not toxic at all, but which have, nervetheless, led to the ozone hole. • Among the „greenhouse gases“ are also the almost inert CFC (chlorofluorocarbon) gases. However, their photochemical breakdown in the stratosphere gives rise to highly reactive chlorine atoms, which destroy ozone by catalytic reactions. As a consequence UV-B radiation from the sun increases, leading for instance to enhanced risk of skin cancer.

8. E.O. Wilson “Before humans existed, the species extinction rate was (very roughly) one species per million species per year. Estimates for current species extinction rates range form 100 to 10,000 times that, but most hover close to 1,000 times prehuman levels (0.1% per year) • In an article with title “Humans as the World‘s Greatest Evolutionary Force“, Palumbi (Science, 7 September 2001) mankind also effects evolutionary change in other species, especially in commercially important, pest, and disease organisms, through antibiotica and pesticides. This accelerated evolution costs at least $33 billion to $50 billion a year in the United States.

9. Man the Eroder • Sedimentary rock formation over 500 million years corresponds to an erosion rate of 24 meters per million years. • Man-caused erosion (crop tillage, land conversion for grazing and construction): 15 times natural erosion • At current rate anthropogenic soil erosion would fill the Grand Canyon in 50 years. • According to Wilkinson (Geology) March 2005.

10. Vitousek (1994)

16. Solar “constant“ 340 W/m2 Heat release from earth 0.087 W/m2

17. “The balance of evidence suggests a discernable human influence on global climate“ (IPCC, 1995) „There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities“ (IPCC, 2001) Average Global Temperature Rise: 1.4 – 5.8 °C from 1999 to 2100 (includes cooling effects by sulfate aerosol) Sea level rise: + 9 – 88 cm until 2100. + 0.5 – 10 m until ~ 3000. Redistribution of precipitation Enhanced risk for extreme weather (flooding, desertification)? Increase in heat waves in Europe, as in the summer of 2003? Too rapid climate changes, so that ecosystems cannot adapt.

21. Climate of the polar regions is most sensitive Model calculated temperature charges for a doubling of atmospheric CO2 content

22. „New studies indicate that the Arctic oceans ice cover is about 40% thinner than 20-40 years ago“. Levy, Physics Today, January 2000. • There is dramatic climate change happening in the Arctic, about 2-3 times the pace for the whole globe: Robert Corell, Chairman of the Arctic Climate Impact Assessment, November 2004. • Melting of permafrost, causing releases of CO2 and CH4.

23. Can we do something against greenhouse gas emissions? • Reduce the emissions of greenhouse gases!! • Energy savings / Renewable energy / Nuclear energy / Wind- and solar energy / CO2 sequestration • Climate cooling by depositing sulfur in the stratosphere • H2S → SO2 → H2SO4 → Sulfate particles, which reflect sunlight. • Mount Pinatubo (1991) 10 Tg S injected in the stratosphere. Average 6 Tg S in the stratosphere for first year. Radiative cooling was 4.5 W/m2 (0.5°C global temperature drop) 1 Tg S in stratosphere causes negative radiative forcing of 0.75 W/m2. • If all SO4= air pollution disappears → Climate warming by 1.4 W/m2. To counteract we need a stratospheric sulfate loading of 1.4 / 0.75 = 1.9 Tg S. ΔT global ≈ 1°C • Assuming 1 or 2 year lifetime of SO4= in the stratosphere, requires an injection of 1 – 1.9 Tg S / year. • To counteract global warming due to a doubling of CO2 (4 W/m2) requires injection of 2.6 – 5.3 Tg S / year

24. Phil Rasch and Paul Crutzen (with thanks to D.B. Coleman) Geo-Engineering Climate Change with Sulfate Aerosols • What would be the impact of injecting precursors of sulfate aerosols into the middle atmosphere, where they would act to increase the planetary albedo, and thus counter some of the effects of greenhouse gas forcing? • Follow up to a study by Crutzen (Climatic Change, 2006, in press) • Back of the envelope calculation • This study uses a relatively sophisticated General Circulation Model for a somewhat more quantitative, and comprehensive look at the problem, but it is still far too simple to be usable for a believable characterization

25. An incomplete history of Geo-engineering • Concept goes back • Budyko (1977) • NAS (1992) • More recent studies • Teller (1997) • Govindaswami and Caldeira (2000, 2002, 2003)reduced solar flux by 1.8% (4.2W/m2) • Crutzen (2006, in press)Theoretical, Back of the envelope

26. Experimental Setup (part 1) • The General Circulation model used is a version of the Community Atmosphere Model (CAM), a component of the more comprehensive Climate System Model (CCSM). • This version includes a relatively comprehensive “physical” characterization of the atmosphere and land, but: • No Biogeochemistry (particularly as it contributes to Carbon and Nitrogen Cycles) • Prescribed Ocean and Sea Ice Dynamics (but does include thermodynamics) --- So called “slab ocean model” + “thermodynamic sea ice model” • No Aerosol/Cloud Microphysical formulations relevant to the “indirect aerosol forcing effect”

27. Experimental Setup (part 2) • Photochemistry includes only that relevant to the oxidation of DMS and SO2 –> SO4 • Oxidants are prescribed • Model is configured to produce a reasonable representation of feedbacks and climate system response associated with “direct” radiative forcing • Because the problem requires a reasonable dynamical characterization of the middle atmosphere we work in a so-called “middle atmosphere configuration” • Model top at about 80km, 52 layers • 2x2.5 Degree Horizontal resolution • Slab Ocean “Q fluxes” (ocean horizontal heat fluxes) are prescribed to reproduce a reasonable “present day” climate simulation using present day anthropogenic forcing by Greenhouse gases and aerosols. Assumption is ocean dynamics wont change. This provides a crude estimate of the upper ocean response to aerosol forcing

28. Experimental Setup (part 3) • Four Simulations performed • Fixed aerosol and greenhouse forcing at present day values (Control) • Doubled CO2 at beginning of simulation (2XCO2) • Injection of 1 Tg S/yr as SO2 at 25km between 10N and 10S (Geo-sulfate) • Doubled CO2 + Injection of SO2 (2XCO2 + Geo-Sulfate)

29. Global Annual Averaged Surface temperature response to forcings

30. Global Annually average precipitation responses (mm/day) to CO2 and aerosol forcing

31. Burden and Lifetime of Geo-Sulfate Residence time ~ 4 years TgSulfate (3x S) Residence time ~ 3 years Residence time longer than usually estimated from volcanic aerosol (1-2 years) Residence time depends on State! controlled by: - cross tropopause exchange - hydrologic cycle Residence time = Burden / (source or Sink)

32. Latitude Height Distributions of Source and Aerosol Heating rates associated with Geo-engineering aerosol

33. Geo-Sul Optical Depth

34. Annual Averaged Change (Geo-Sul/2xCO2 – Control) Precip (mm/day)Change currently cannot be distinguished from noise

35. Annual Average Surface Temperature 2xCO2 - Control Geo-SO4/2xCO2 - Control

36. Seasonal Surface Temperature Changes DJF JJA

37. Volcanic injections  aerosol loading cools earth Models can reasonably simulate climate response to substantial aerosol loading in stratosphere(e.g. Soden et al 2003)

38. Closing thoughts • With respect to processes included in the model the signal is the anticipated one, and there are no big surprises, although exploration very cursory at this time • Required injections on low end of initial estimates • Lifetime of aerosol likely to be larger than volcanic aerosol • Changes to transport circulation unplanned • Many unresolved issues • Particle size, effect on longwave • Chemistry (Ozone, through CLO,CL reactions at least) • What happens to sulfate entering upper troposphere (effect on cirrus?) • Ameliorates only some of the effects of CO2, • Ocean PH • The commitment is long term (continuous injections for more than a century) • Should not effect our resolve to reduce emissions

39. There are obviously very important Moral, Ethical, Legal issues to be considered here • Arguments have been made that we shouldn’t even be considering these types of “solutions” • That dialogue itself could easily take the whole week, much less the time allocated for this presentation. • There are some extremely insightful commentaries appearing in the same Climatic Change issue as the Crutzen (2006) paper regarding these issues. Worth Reading… Cautions