Historical Climate Change

Long-term Climate Change We are in a period of relative warmth right now but on the time scale of the Earth’s history, the planet is cold. Long-term Climate Change The Archean is thought to have been warmer, or at least as warm, as it is now. But the sun was a lot fainter. This is the “Faint Early Sun Paradox”. 1 Faint early sun paradox About 4.6 billion years ago the sun was roughly 30% less luminous than today, but yet the oceans were not frozen. Why not? Faint early sun paradox Need another ~70 W/m2 heat 4 bya to prevent freezing of oceans. Possible sources: • additional geothermal flux from interior (No - currently 0.06 W/m2, 4 bya would have been about 0.3 W/m2) • albedo/ice cover (Unlikely – albedo currently is 0.3, would have needed to be almost 0.) • much larger G-H effect (CO2 concentration was probably much higher than now.) 2 Faint early sun paradox How much CO2 is necessary to keep the earth from freezing? About 1000x today, or 0.3 bar Faint early sun paradox How much CO2 is necessary to keep the earth from freezing if CH4 is also present? 3 Faint early sun paradox CH4 comes from methanogenic bacteria. These convert H2 from early atmosphere into CH4 by way of: CO2 + 4H2 -> CH4 + 2H2O CH4 is 30x as potent a G-H gas as CO2. The early earth is likely to have been kept warmer by a combination of much higher CO2 and CH4 concentrations. Long-term climate change How is the climate of the earth controlled over long time scales? Climate is controlled by a trade-off between solar luminosity and G-H gases – most importantly CO2. On a short time scale (less than a century) atm. CO2 is controlled by respiration, photosynthesis and decomposition. On a long time scale CO2 is largely controlled by geological processes. 4 Carbon reservoirs Short-term Organic Carbon Cycle inflow to atm: 30 Gt/yr from respiration + 30 Gt/yr from decomposition outflow: 60Gt/yr from photosynthesis 5 Marine Organic Carbon Cycle Biological pump Deep ocean much richer in CO2 than surface waters. Marine Organic Carbon Cycle CZCS shows areas of primary productivity in ocean. 6 Long-term Organic Carbon Cycle Human burning of fossil fuels is essentially the weathering process speeded up by a factor of a million or so. 7 Inorganic Carbon Cycle Flow of CO2 is down the concentration gradient. Ocean is source of atm CO2 where ocean CO2 is higher than atm, as in regions of upwelling. Ocean is sink for atm CO2 where primary productivity is high and CO2 has been removed from surface water. Inorganic Carbon Cycle The ocean’s ability to take up excess atm CO2 is limited. 1. carbon dioxide dissolves in water to produce carbonic acid (H2CO3). CO2 + H 2O → H 2CO3 2. carbonic acid molecules dissociate in water to produce bicarbonate ions (HCO3-). H 2CO3 ↔ H + + HCO3− 3. bicarbonate ions dissociate to produce carbonate ions (CO32-). HCO3− ↔ H + + CO32− 4. But this reaction can proceed in either direction. The amounts of bicarbonate and carbonate ions is adjusted according to the pH of the water. If the pH is below neutral (acidic), the reaction proceeds to the left. If it is above neutral, the reaction proceeds to the right. 8 Inorganic Carbon Cycle The ocean’s ability to take up excess atm CO2 is limited. 1. CO2 dissolves -> carbonic acid -> bicarbonate + H+ CO2 + H 2O → H 2CO3 ↔ H + + HCO3− 2. Now pH is lowered, so H+ reacts with carbonate ion -> bicarbonate ion HCO3− ← H + + CO32 − 3. Now the net result is: CO2 + H 2O + CO32− → H + + HCO3− + CO32− → 2HCO3− 4. there are 2 more bicarbonate ions and 1 less carbonate ion 5. This can only continue as long as there are carbonate ions in the ocean – But there are a lot more fossil fuels than carbonate ions so ocean’s ability to take up excess CO2 is limited. Chemical Weathering 1. atm CO2 dissolves in rainwater to produce carbonic acid (H2CO3) 2. Carbonate weathering: carbonic acid dissolves calcium carbonate to produce calcium ions + bicarbonate ions. CaCO3 + H 2CO3 → Ca 2+ + 2HCO3− 3. Silicate weathering: carbonic acid dissolves silicates (eg. wollastonite) to produce calcium ions, bicarbonate ions, silica and water. CaSiO3 + 2 H 2CO3 → Ca 2+ + 2 HCO3− + SiO2 + H 2O 4. RHS gets transported via rivers to ocean Note: Silicate weathering uses twice as much atm CO2 as carbonate weathering. 9 Mineral Deposition 1. marine organisms use calcium ions and bicarbonate ions in seawater to produce their shells (calcite = CaCO3) 2. Carbonate precipitation: Ca 2+ + 2 HCO3− → CaCO3 + H 2CO3 3. When these organisms (foraminifera, corals, shellfish) die, their shells sink to the bottom of the ocean. They arrive intact where the ocean depth is less than 4 km (shallow tropical waters and midocean ridges). They dissolve before reaching the bottom of the deep ocean basins. 4. At mid-ocean ridges they are buried by sediments and carried across the ocean by seafloor spreading. 5. When tectonic uplift occurs (after millions of years) the limestones are exposed at the Earth’s surface and the cycle continues. Inorganic Carbon Cycle (Summary) Carbonate precipitation is just the reverse of carbonate weathering. In silicate weathering however, there is a net removal of CO2 from the atmosphere because of the extra carbonic acid molecule used. Net result: CaSiO3 + CO2 → CaCO3 + SiO2 The rate of conversion of atm CO2 to limestone by silicate weathering is small (0.03Gton(C)/yr) but must be balanced otherwise the atmospheric supply of CO2 would be depleted in a million years (using the ocean to supply the atm first). This is short on geological time scales. 10 Inorganic Carbon Cycle (Summary) Carbonate-Silicate Geochemical Cycle Plate tectonics returns the CO2 to the atmosphere via metamorphism and volcanoes. Carbonate metamorphism: CaCO3 + SiO2 → CaSiO3 + CO2 11 Carbonate-Silicate Geochemical Cycle Together silicate weathering, carbonate precipitation and formation of carbonic acid in air are the reverse of carbonate metamorphism. Over the history of the earth these have balanced out (unlike Mars, which has lost its CO2-rich atm, and flowing water). Feedback loops help to keep things in balance: Long-term Climate Change 12 Earth’s Historical Climate Change 5 main periods of glaciation. Why? 2.3 bya (Huronian) Rise in atm. O2 reacted with CH4. CH4 greatly reduced. Temp. cooled. Recovery? silicate weathering reduced by ice cover or more methanogenesis. Earth’s Historical Climate Change 0.6 – 0.8 bya (late Proterozoic) “Snowball Earth” – even tropics were glaciated. Why? All landmasses joined at equator – lots of silicate weathering. Brings down atm. CO2. Earth cools. Polar ice sheets advance to lower latitudes. At ~ 30o latitude, positive (albedo) feedback becomes so strong that system is unstable. Oceans freeze. Earth freezes. Global temp -50oC. Albedo > 0.6. 13 Earth’s Historical Climate Change Recovery from late Proterozoic “Snowball Earth” Silicate weathering stopped (earth covered in ice). CO2 builds up in atm. from volcanic emissions. (At modern rates of outgassing, CO2 would reach 0.1 bar (300 x today’s) in 10 m.y.) Temp. increases, ice melts, positive feedback sets in. All ice could melt over a few thousand years. CO2-rich atm. low albedo. surface T: 50 – 60oC Lots of silicate weathering. CO2 decreases Climate restored. Earth’s Historical Climate Change 3 more glaciations in Phanerozoic Eon (544 mya – present). Ordovician is poorly understood. In Carboniferous the organic carbon burial rate doubled as a result of the formation of large coal beds. This led to a marked decrease in CO2 levels. 14 Earth’s Historical Climate Change Mesozoic was warm. 2 – 6 oC warmer at equator. 20 – 60 oC warmer at poles. CO2 levels were 4 x as high. Why? Sea-floor spreading rates were high -> increased carbon metamorphism and volcanism. Earth’s Historical Climate Change Cooling occurred during Cenozoic. Why? Decrease in mid-ocean spreading rates + India collided with Asia (40 mya). Created massive uplifted terrain (Tibetan Plateau) – more weathering surface and created Asian monsoon rain. Weathering rates increased and CO2 decreased. 15 Earth’s Historical Climate Change Thus Earth’s climate on a long time scale has largely been controlled by G-H gases – primarily CO2 and stabilized by silicate weathering. 16