1 Carbon - I This figure from IPCC, 2001 illustrates the large variations in atmospheric CO2 (a) Direct measurements of atmospheric CO2 concentration, and O2 from 1990 onwards. O2 concentration is the change from an arbitrary standard. (b) CO2 concentration in Antarctic ice cores. Recent atmospheric measurements at Mauna Loa are shown for comparison. (c) CO2 in the Taylor Dome Antarctic ice core. (d) CO2 concentration in the Vostok Antarctic ice core (e) Geochemically inferred CO2 concentrations. (f) Geochemically inferred CO2 concentrations: colored bars represent different published studies.2 The time scale matters. On tectonic time scales, volcanism and the weathering of carbonate rocks has a strong influence on atmospheric CO2. On glacial-interglacial timescales (and longer), the whole ocean and the ocean sediments can come into equilibrium with the atmosphere and subtle changes in ocean biology or alkalinity may drive the observed variability. The recent, fossil fuel and land use change alteration of the carbon cycle may be unprecedented in the speed at which atmospheric CO2 is being driven. Because of the fast rate of change, disequilibria are the rule rather than the exception. Although the surface ocean is at close equilibrium with the atmosphere, the terrestrial biosphere (probably) and the deep ocean (certainly) are important sinks at present. In this lecture, we will talk about long time scale controls on CO2 and then next lecture we will discuss the modern forcing. http://www.carleton.edu/departments/geol/DaveSTELLA/Carbon/long_term_carbon.htm3 Weathering Chemical weathering of minerals exposed at the surface is important for setting the amount of CO2 present in the near surface reservoirs. Although the weathering of limestones does indeed involve the transfer of carbon, it has no net effect on the amount of CO2 in the atmosphere. Weathering silicate minerals consumes acid (CO2): CaSiO3 + 3H2O + 2CO2 = Ca2+ + 2(HCO3-) + H4SiO4 [1] wollastonite + water + carbon dioxide = calcium + bicarbonate + silicic acid The products of this reaction are all carried away in solution, ending up in the oceans. The silicic acid eventually is used by siliceous planktonic organisms; the calcium and bicarbonate ions will be combined by other organisms in the oceans, forming calcite and some dissolved carbon dioxide, which can escape back to the atmosphere: Ca 2+ + 2(HCO3-) = CaCO3 + H2O + CO2 [2] One of the CO2 molecules is returned to the atmosphere in this step; the other is sequestered in the form of limestone (CaCO3). Silicate weathering [1] thus represents an important feedback mechanism in the long-term carbon cycle (probably negative feedback). Like most chemical reactions, the process of dissolution described in equation [1] above is sensitive to the temperature; the hotter it is, the faster the dissolution. Thus, if the climate is warm (high atmospheric CO2) then weathering will occur faster, removing CO2 from the atmosphere, cooling the climate. This negative feedback mechanism is potentially one of the most important long-term stabilizers in Earth's climate system. Berner RA, Lasaga AC, Garrels RM. 1983. The carbonate-silicate geochemical cycle and its effect on atmospheric carbon dioxide over the past 100 million years. Am. J. Sci. 283:641–83 [BLAG Hypothesis]: Surface area of the land or continental area - depends on sea level and extent of continents. Function of the sea floor spreading rate. Also includes estimate of exposed limestones versus that of silicate rocks Run-off – warmer = more precipitation => weathering increases Atmospheric CO2 - higher => more acidic, more active biosphere Continental elevation - physically stripping off weathered stuff exposes fresh new rocks to be weathered. Unfortunately, it is not so simple alas ….. plants (high CO2 at roots), ice (which fractures rock and thus increases surface area) are all likely important and difficult to model. Nevertheless, on long timescales, CaCO3 sediments are recycled through the mantle or outgas during subduction.4 Variation between glacial and interglacial CO2 As we have seen, orbital forcing of climate at the 41,000 year obliquity period dominates the climate record until ~700,000 years ago when the glacial/interglacial timing appeared to become paced at ~100,000 years. This period corresponds to the changes in eccentricity of Earth’s orbit, but the magnitude of the 100,000 year orbital forcing is generally considered too small to drive the climate system alone. In other words, it is hard to understand how this forcing could become so dominant and what would cause the shift to 100,000 years ~0.5 million years ago. The remarkable record of atmospheric trace gas composition obtained from ice cores shows that the Earth’s carbon-cycle was also paced at ~100 kYr. CO2 rises and fall in step with the changes in temperature recorded in the isotopic composition of the ice [K. Cuffey et al., 2001, Nature]. Although association does not imply causality, it is clear that the changes in CO2 do produce a significant radiative forcing to climate that is either a direct forcing responsible for the glacial/interglacial climate shift, or more likely acts as a powerful (and positive) feedback on some other forcing. The glacial / interglacial changes in CO2 alone exert a direct radiative forcing of about -2 W/m2 (vs +1.5 W/m2 preindustrial -> today). In addition, the water vapor feedback will act to amplify this cooling. Most evidence suggests that the carbon signal lags the thermal signal. In this figure from Cuffey et al., comparison of carbon dioxide and reconstructed Southern Hemisphere temperature, ΔTH illustrate that strong covarience of CO2 and temperature using a new proxy for temperature, excess deuterium (see paper for more detail). Large changes in methane have also been observed, but the forcing appears more consistent with the orbital timescales (particularly 23,000 years) and less driven at 100,000 years. It has been argued that insolation driven changes in the monsoon may be responsible for the changes in methane. The strength of the monsoon, it is suggested, is determined by the instantaneous summer insolation forcing (precession). ). The most recent instance of summer insolation considerably higher than at present was 10,000 years ago. Much of subtropical North Africa (and indeed LA) were considerably moister at this time. Many dry lake beds of