Transcript Ch 3
CO2 and Long Term Climate GEOL 1130 Spring 2007 Earth-Venus contrast • Which planet receives more incoming solar radiation? • Which planet absorbs more solar radiation? • Which planet has a bigger greenhouse effect? • Which planet is warmer? • How important are greenhouse gas concentrations in determining the surface temperature of a planet? Faint Young Sun Paradox • Sun 25% weaker • Climate models show that even with today’s level of greenhouse gases, the planet would have been very frozen • Yet, even the earliest geologic deposits reflect evidence of erosion through liquid water?! • What is the solution? If no C was returned from the sediments and rocks to the atmosphere, how long would it take to draw down the C in the atmosphere, all of the surface reservoirs, and then the deep ocean? 4000 yrs, 24,700 years, 278,000 years Long Term CO2 • How do we know that there is a balance between sources and sinks for CO2? – Without a pretty close balance, the CO2 concentrations would vary wildly • What are the sources of CO2 to the atmosphere? – (Surface exchanges) and volcanoes • What are the sinks of CO2 from the atmosphere? – (Surface exchanges), weathering and sediments Two Types of Weathering: one is a CO2 sink, the other is not • Hydrolysis: Acidic rain water converts minerals into ions bound to H+ – Acidic rainwater: H20 (atm) + CO2 (atm) -> H2CO3 (atm) – Example: CaSiO3 (rock) + H2CO3 (atm) -> Ca2+ (ocean) + H2SiO4 (ocean) + HCO3- (ocean) • Dissolution: Acidic rainwater dissolves limestone – CaCO3 (rock) + H2CO3 (atm) -> Ca2+ (ocean) + HCO3- (ocean)+ H2O (ocean) + CO2 (atm) (what happens when you put acid on limestone?!) • How do we tell which one is the sink? – Trace the atmospheric CO2 HYDROLYSIS Atmospheric CO2 transferred to ocean and then stored in sediments Hydrolysis and Climate • Now that we’ve got a sink for CO2, we need it to be connected to climate for this to work as a feedback • How is weathering affected by climate? Where is chemical weathering most intense? The tropics Why is chemical weathering most intense in the tropics? Most chemical reactions go faster at higher temperatures Earth’s Thermostat: The negative feedback between atmospheric CO2 levels and climate New evidence that the atmospheric CO2 3.75 billion years ago was much higher than today: marine iron carbonates. Iron isotopes show the carbonate was marine, and marine iron carbonate requires “far higher levels of CO2 than are found in the atmosphere today” Big Swings • Before 550 Ma, there is evidence that the Earth experienced bigger climate swings than it does today: – The Snowball Earth Hypothesis • “Freeze-Fry World” EVIDENCE Some glacial sediments have paleomagnetic orientations consistent with tropical latitudes What evidence recently convinced many scientists that the low latitude Orientations were not the result of later “resetting” of the magnetism? More Evidence • Re-emergence of iron formations during glacial periods • Flip from glacial sediments to hundreds of meters of carbonate rock (“cap carbonates” with very sharp contact • Carbon isotopes in rocks leading up to glaciation get lighter and lighter, reach their lightest value early in the cap carbonate deposit, and then get progressively heavier