CARBON CYCLE 343 circulation Although the coral reef hypothesis seems unlikely to explain the entire CO2 shift between glacials and interglacials, the same mechanism would have been more effective before the introduc- tion of shelly plankton and before the establishment of a CCD The causes of CH4 changes are unlikely to be the same as for CO2 changes The largest sources of CH4 are the methane clathrate reservoirs that form beneath cold ocean floors at depths of several hundreds of metres (Figure 7); on land, the CH4 reservoirs are in deep lakes or deep beneath the tundra floor in Siberia, Greenland, and North America (see Petroleum Geology: Gas Hydrates) Rising CH4 levels could represent a response to global warming in the form of decomposition of these volatile methane reservoirs, but whether on land, beneath the seafloor, or beneath retreating glaciers is unclear Because CH4 is unstable in the presence of oxygen and has a very short half-life in the atmosphere (about 10 years), any increase in CH4 levels would translate eventually into a more sustained increase in the CO2 content of the atmosphere Anthropogenically Induced CO2 Increase and Future Predictions Figure A qualitative comparison of CaCO3 deposition and erosion to and from carbonate reef platforms from interglacial to glacial episodes Reprinted from Milliman JD (1974) Marine carbonates In: Milliman JD, Mueller G, and Foerstner U (eds.) Recent Sedimentary Carbonates, vol Berlin: Springer Verlag Atmospheric CO2 levels have been measured directly since 1957 at Mauna Loa, Hawaii; these and other data reveal that PCO2 has risen by nearly 40% since 1800, most of half of this rise occurring during only the past half century (Figure 10) Ice-core Figure 10 The ‘Keeling’ curve Measurements of monthly average atmospheric CO2 concentrations at the top of Mauna Loa, Mauna Loa Observatory, Hawaii Source: CD Keeling and TP Wharf, Scripps Institute of Oceanography, La Jolla, California, USA (available on the Internet at http://cdiac.esd.ornl.gov/trends/co2/graphics/mlo144e.pdf)