Over long periods of time (~Ma), we may consider the oceans, atmosphere and biosphere as a single exospheric reservoir for CO₂. The geological carbon cycle describes the inputs to this exosphere from mantle degassing, metamorphism of subducted carbonates and outputs from weathering of aluminosilicate rocks (Walker et al. 1981). A feedback mechanism relates the weathering rate with the amount of CO₂ in the atmosphere via the greenhouse effect (e.g., Wang et al. 1976). An increase in atmospheric CO₂ concentrations induces higher temperatures, leading to higher rates of weathering, which draw down atmospheric CO...

We present a reliable methodology to estimate the energy associated with the subaerial diffuse degassing of volcanic‐hydrothermal fluids. The fumaroles of 15 diffuse degassing structures (DDSs) located in eight volcanic systems in the world were sampled and analyzed. Furthermore, each area was measured for soil temperature gradients and for soil CO₂ fluxes. The results show that each hydrothermal or volcanic system is characterized by a typical source fluid which feeds both the fumaroles and diffuse degassing through the soil. Experimental data and the results of physical numerical modeling of the process demonstrate that the heat released by condensation of steam at depth is almost totally transferred by conduction in the uppermost part of the soil. A linear relationship is observed between the log of the steam/gas ratio measured in the fumaroles and the log of the ratio between soil thermal gradient and soil‐gas flux. The main parameter controlling this relation is the thermal conductivity of the soil (K_c). For each area, we computed the values of K_c which range from 0.4 to 2.3 W m⁻¹ °C⁻¹. Using the CO₂ soil fluxes as a tracer of the deep fluids, we estimated that the total heat released by steam condensation in the systems considered varies from 1 to 100 MW.