1. CO2 and Long-Term Climate • What has moderated Earth surface temperature over the last 4.55 by so that • All surface vegetation did not spontaneously catch on fire and all lakes and oceans vaporize? • All lakes and ocean did not freeze solid?

2. Greenhouse Worlds • Why is Venus so much hotter than Earth? • Although solar radiation 2x Earth, most is reflected but 96% of back radiation absorbed

3. Energy Budget • Earth’s temperature constant ~15C • Energy loss must = incoming energy • Earth is constantly receiving heat from Sun, therefore must lose equal amount of heat back to space • Heat loss called back radiation • Wavelengths in the infrared (long-wave radiation) • Earth is a radiator of heat • If T > 1K, radiator of heat

4. Energy Budget • Average Earth’s surface temperature ~15C • Reasonable assumption • Surface of Earth radiates heat with an average temperature of 15C • However, satellite data indicate Earth radiating heat average temperature ~-16C • Why the discrepancy? • What accounts for the 31C heating?

5. Energy Budget • Greenhouse gases absorb 95% of the long-wave, back radiation emitted from Earth’s surface • Trapped radiation reradiated down to Earth’s surface • Accounts for the 31C heating • Satellites don’t detect radiation • Muffling effect from greenhouse gases • Heat radiated back to space from elevation of about 5 km (top of clouds) average 240 W m-2 • Keeps Earth’s temperature in balance

6. Greenhouse Worlds • Why is Venus so much hotter than Earth? • Although solar radiation 2x Earth, most is reflected but 96% of back radiation absorbed

7. What originally controlled C? • In solar nebula most carbon was CH4 • Lost from Earth and Venus • Earth captured 1 in 3000 carbon atoms • Tiny carbon fraction in the atmosphere as CO2 • 60 out of every million C atoms • Bulk of carbon in sediments on Earth • CaCO3 (limestone and dolostone) and organic residues (kerogen) • Venus probably had similar early planetary history • Most carbon is in atmosphere as CO2 • Venus has conditions that would prevail on Earth • All CO2 locked up in sediments were released to the atmosphere

8. Earth and Venus • Water balance different on Earth and Venus • If Venus and Earth started with same components • Venus should have either • Sizable oceans • Atmosphere dominated by steam • H present initially as H2O escaped to space • H2O transported "top" of the Venusian atmosphere • Disassociated forming H and O atoms • H escaped the atmosphere • Oxygen stirred back to surface • Reacted with iron forming iron oxide

9. Planetary Evolution Similar • Although Earth and Venus started with same components • Earth evolved such that carbon safely buried in early sediments • Avoiding runaway greenhouse effect • Venus built up CO2 in the atmosphere • Build-up led to high temperature • High enough to kill all life • If life ever did get a foothold • Once hot, could not cool

10. Why Runaway Greenhouse? • Don't know why Venus climate went haywire • Extra sunlight Venus receives? • Life perhaps never got started? • No sink for carbon in organic matter • Was the initial component of water smaller than that on Earth? • Did God make Venus as a warning sign?

11. Early Earth: Faint Young Sun • Solar Luminosity 4.55 bya 25% lower than today • Faint young Sun paradox • If early Earth had no atmosphere or today’s atmosphere • Radiant energy at surface well below 0°C for first 3 billion years of Earth history • No evidence in early Earth rock record that planet was frozen

12. Early Earth: A Greenhouse World • Early Earth was more Venus-like • Models indicate that greenhouse required • Several greenhouse gases • H2O, CO2, CH4, NH3, N2O • H2O and CO2 most likely • 102-103 x PAL CO2

13. Early Earth Atmosphere • Faint young Sun paradox presents dilemma • 1) What is the source for high levels of greenhouse gases in Earth’s earliest atmosphere? • 2) How were those gases removed with time? • Models indicate Sun’s strength increased slowly with time • Geologic record strongly suggests Earth maintained a moderate climate throughout Earth history (i.e., no runaway greenhouse like on Venus)

14. Source of Greenhouse Gases • Input of CO2 and other greenhouse gases from volcanic emissions • Most likely cause of high levels in early Earth

15. Is Volcanic CO2 Earth’s Thermostat? • If volcanic CO2 emissions provided the early Earth greenhouse, has volcanic activity continuously slowed through geologic time? No, but… • Carbon input balanced by removal • Near surface carbon reservoirs • Stop all volcanic input of CO2 • Take 270,000 years to deplete atmospheric CO2 • Surface carbon reservoirs (41,700 gt) divided by volcanic carbon input (0.15 gt y-1) • Rate of volcanic CO2 emissions have potential to strongly affect atmospheric CO2 levels on billion-year timescale

16. Volcanic CO2 inputs? • No geologic, geophysical or geochemical evidence indicates that rates of tectonism decreased slowly through Earth history • Rates of volcanic CO2 input did not change slowly with time • Volcanic CO2 emissions did not moderate Earth climate through geologic time • If not inputs, what about a change in removal rate of atmospheric CO2?

17. Removal of Atmospheric CO2 • Slow chemical weathering of continental rocks balances input of CO2 to atmosphere • Chemical weathering reactions important • Hydrolysis and Dissolution

18. Acid Rain • Both natural and anthropogenic processes cause acidification of precipitation. • Natural: CO2 —> H2CO3 (carbonic acid); acids from volcanoes (CO2 and H2S) • Anthropogenic: Oxides of N are by-products (nitric acid)

19. Acid Rain • Since the 1950s the pH of precipitation in eastern N. America has decreased (i.e., more acidic) significantly over a large area • Other regions effected include eastern Europe and China • Consequences of more acidic precipitation include problems for fish and other wildlife and increased chemical weathering, especially of regions with lots of carbonate rocks

20. Before

21. After

22. Hydrolysis • Main mechanism of chemical weathering that removes atmospheric CO2 • Reaction of silicate minerals with carbonic acid to form clay minerals and dissolved ions • Summarized by the Urey reaction • CaSiO3 + H2CO3 CaCO3 + SiO2 + H2O • Atmospheric CO2 is carbon source for carbonic acid in groundwater • Urey reaction summarizes atmospheric CO2 removal and burial in marine sediments • Accounts for 80% of CO2 removal

23. Dissolution • Kinetics of dissolution reactions faster than hydrolysis • Dissolution reaction neither efficient nor long term • Dissolution of exposed limestone and dolostone on continents and precipitation of calcareous skeletons in ocean • CaCO3 + H2CO3 CaCO3 + H2O + CO2 • Although no net removal of CO2 • Temporary removal from atmosphere

24. Atmospheric CO2 Balance • Slow silicate rock weathering balances long-term build-up of atmospheric CO2 • On the 1-100 million-year time scale • Rate of chemical hydrolysis balance rate of volcanic emissions of CO2 • Neither rate was constant with time • Earth’s long term habitably requires only that the two are reasonably well balanced

25. What Controls Weathering Reactions? • Chemical weathering influenced by • Temperature • Weathering rates double with 10°C rise • Precipitation • H2O is required for hydrolysis • Increased rainfall increases soil saturation • H2O and CO2 form carbonic acid • Vegetation • Respiration in soils produces CO2 • CO2 in soils 100-1000x higher than atmospheric CO2

26. Climate Controls Chemical Weathering • Precipitation closely linked with temperature • Warm air holds more water than cold air • Vegetation closely linked with precipitation and temperature • Plants need water • Rates of photosynthesis correlated with temperature

27. Chemical Weathering: Earth’s Thermostat? • Chemical weathering can provide negative feedback that reduces the intensity of climate warming

28. Chemical Weathering: Earth’s Thermostat? • Chemical weathering can provide negative feedback that reduces the intensity of climate cooling

29. Greenhouse vs. Faint Young Sun • Cold surface temperatures created by the faint young Sun compensated by stronger atmospheric CO2 greenhouse effect

30. Volcanism & Weathering • Volcanism on early Earth probably produced more atmospheric CO2 • Counteracted lower radiant energy and warmed our planet • Volcanism did not slow at same rate as Sun increase in strength • Early Earth probably still cold • Weathering slow • Continents small • Continental crustal rocks silica-poor (basaltic) • Stoichiometry of Urey reaction different • Less efficient CO2 removal from atmosphere

31. Greenhouse vs. Faint Young Sun • When solar luminosity strengthen, chemical weathering increased and helped transfer atmospheric CO2 into sediments

32. Volcanism & Weathering • As solar luminosity increased • Earth warmed and became wetter • Chemical weathering increased • CO2 levels dropped • Continental crust began to grow ~2 bya • Became more siliceous (granitic) • Slow warming of Earth • Caused changes in chemical weathering • Moderated Earth’s climate

33. Tectonic Carbon Cycling • Carbon cycles continuously between rock reservoir and atmosphere • CO2 removed from atmosphere by chemical weathering, deposited in ocean sediments, subducted and returned by volcanism

34. Tectonic Carbon Cycling • Carbon cycles continuously between rock reservoir and atmosphere • Plate tectonic processes dependant on presence of water • Acts as a lubricant for plate subduction

35. Organic Carbon Burial Affect CO2 • If the rate of organic carbon burial increases, less organic matter available for decomposition and less carbon returned to the atmosphere as CO2 • Atmospheric CO2 reservoir shrinks

36. Organic Carbon Burial Affect O2 • If the rate of organic carbon burial increases, less organic matter available for decomposition and less oxygen is used during decomposition • Atmospheric O2 reservoir grows

38. Importance of Solar Irradiance • Affect planetary climate development • Earth vs. Venus vs. Mars • Faint young Sun paradox • Glacial-interglacial variations in solar irradiance • Milankovitch Cycles • Global Dimming • Affect of aerosols on cloud albedo

39. The Climate System

40. Global Dimming • Describes the gradual reduction in the amount of total solar radiation at the Earth surface since 1950’s • Recognized since 1989 (Ohmura 1989, Russak 1990, Stanhill and Moreshet 1992) • 2-3% reduction per decade • Largest reduction in northern hemisphere • Not due to reduction in strength of Sun • Experiments in the Maldives (INDOEX) • Linked global dimming to pollution

41. Effect of Pollution on Indian Ocean • Pollution from India and Asia carried into Indian Ocean by winter monsoon winds • Brown haze from aerosols surface to 3 km altitude • Soot, sulfates, nitrates, organic particles, fly ash and mineral particles • Haze particles scatter solar radiation reflecting sunlight • In the polluted INDOEX region, haze particles reduce the solar radiation absorbed the ocean surface by ~10% • South of the ITCZ, the sky was clear

42. INDOEX • Airborne particles over the Indian Ocean are different from those over North America and Europe • Airborne particles over the northern Indian Ocean are unusually dark • Contain large amounts of soot and other materials from incompletely burned fuels and wastes • Dark aerosols lead to increased absorption of solar radiation • Advanced pollution control technologies remove the dark material and yield particles that are relatively "white" • Thus the impact of Asian pollution particles on climate processes appears fundamentally different from that of American and European pollution particles

43. Effect of Contrails • The potential of condensation trails (contrails) from jet aircraft to affect regional-scale surface temperatures has been debated for years • Difficult to verify until an opportunity arose as a result of the three-day grounding of all commercial aircraft in the United States in the aftermath of the terrorist attacks on 11 September 2001 • An anomalous increase in the average difference between the daytime maximum and night-time minimum temperatures for the period 11–14 September 2001 • Persisting contrails can reduce the transfer of both incoming solar and outgoing infrared radiation and so reduce the daily temperature range

44. Contrails

45. Contrails

46. And now, some good news • Global dimming has reversed since 1990 • Studies of the amount of sunlight making it through the atmosphere suggest that our air is getting cleaner • Reduced industrial emissions and the use of particulate filters • ‘Global dimming’ has been noticed since measurements began in the late 1950s, but consensus that it was a global phenomenon was reached only since 2003 • Studies published in 2005 indicate dimming replaced by brightening since 1990

47. Global Dimming & Brightening

48. Global Dimming & Brightening

49. Global Dimming & Brightening

50. From Dimming to Brightening • Decline in solar radiation at land surfaces documented in observational records up to 1990 • 24 stations distributed globally show statistically significant decrease • New data from 1990 to the present (mostly from the Northern Hemisphere) show that the dimming did not persist into the 1990s • Data from 300 stations available • New data show widespread brightening since the late 1980s • But only up to values found at ~1960