
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?
Greenhouse Worlds • Why is Venus so much hotter than Earth? • Although solar radiation 2x Earth, most is reflected but 96% of back radiation absorbed
Energy Budget • Earth’s temperature constant ~15C • 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 > 1K, radiator of heat
Energy Budget • Average Earth’s surface temperature ~15C • Reasonable assumption • Surface of Earth radiates heat with an average temperature of 15C • However, satellite data indicate Earth radiating heat average temperature ~-16C • Why the discrepancy? • What accounts for the 31C heating?
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 31C 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
Greenhouse Worlds • Why is Venus so much hotter than Earth? • Although solar radiation 2x Earth, most is reflected but 96% of back radiation absorbed
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
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
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
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?
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
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
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)
Source of Greenhouse Gases • Input of CO2 and other greenhouse gases from volcanic emissions • Most likely cause of high levels in early Earth
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
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?
Removal of Atmospheric CO2 • Slow chemical weathering of continental rocks balances input of CO2 to atmosphere • Chemical weathering reactions important • Hydrolysis and Dissolution
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)
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
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
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
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
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
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
Chemical Weathering: Earth’s Thermostat? • Chemical weathering can provide negative feedback that reduces the intensity of climate warming
Chemical Weathering: Earth’s Thermostat? • Chemical weathering can provide negative feedback that reduces the intensity of climate cooling
Greenhouse vs. Faint Young Sun • Cold surface temperatures created by the faint young Sun compensated by stronger atmospheric CO2 greenhouse effect
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
Greenhouse vs. Faint Young Sun • When solar luminosity strengthen, chemical weathering increased and helped transfer atmospheric CO2 into sediments
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
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
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
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
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
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
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
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
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
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
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
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