In the pre-industrial era volcanic CO2 emissions played a major role in controlling atmospheric CO2 concentrations, balanced by the effects of weathering which removes the CO2 from the atmosphere. Together, these two processes determined the CO2 concentrations for billions of years and therefore also the global temperature. Over the last 600 million years the CO2 concentration in the atmosphere has been relatively stable.
Recent work supports the hypothesis that CO2 concentrations were controlled by an efficient system whereby the rate of CO2 removal from the atmosphere by weathering was itself a function of temperature, controlled by CO2 concentration. Thus, a low CO2, cooler planet would have a lower weathering rate. Volcanic emissons of CO2 would build up, increasing the CO2 concentration until temperatures rose and weathering was fast enough to maintain a steady concentration. This stability allowed life to flourish on Earth for the last 20 million years, enjoying relatively stable temperatures notwithstanding changes in solar insolation due to orbital variations.
The rapid increase in CO2 emissions from human activity has broken down this equilibrium as weathering rates have not increased quickly emough to draw down the CO2 concentrations. In order to understand this geochemical cysle we must quantify the input from volcanism, metamorphism and the output from weathering.
Magmatic degassing has been the primary source of CO2 to the Earth, via de-gassing at mid-ocean ridges, hotspots and arc volcanism. Current estimates of the weathering removal rate of CO2 vary widely, ranging from 380 Mt/yr (Gaillardetet al., 1999) to 880Mt/yr (varekamp and Thomas, 1998). There is clearly a lot of uncertainty about the global geochemical cycle. Hard data is either difficult to collect directly (in the case of global weathering rate) or have been produced by inferences and assumptions. The situation is particularly egregious for the volcanic emissions. Of approximately 500 active volcanoes in the world at any one time, passively or explosively releasing CO2 into the atmosphere, only -10 have had direct measurements of their CO2 flux determined by measuring CO2/SO2 ratios in the plume at the same time as the SO2 flux has been measured. This is currently the onlu quantitative methodology we have a t our disposal for the determination of CO2 fluxes from degassing volcanoes.
This enormous uncertainty in our knowledge of a critical input into the global geochemical cycle let Berner and Lagasa (1989) to state that it is the most vexing problem facing us in understanding that cycle. On the contrary however, if we were to finally derive a quantitative measurement of the volcanic CO2 flux we would be able to place the whole geochemical cycle on a much sounder footing, converting uncertainty and guesswork about one of the most fundamental aspects of our planet's behaviour into a solid scientific framework. In turn, this would allow us to better understand the likely impact large eruptions have had in Earth's history, by comparing them with annual background degassing amount. We may also ascertain the true natural contribution compared to the anthropogenic CO2 flux.