Global ground heat flux from remote sensing data: preliminary results and evaluation

Ground heat flux constitutes the conductive component of the surface energy budget. Quantifying this energy component is important to close the surface energy balance and to understand the energy exchanges between the lower atmosphere and the shallow subsurface. Furthermore, ground heat storage accounts for approximately 90 % of the continental heat storage, and 4-5 % of the total Earth heat storage. Therefore, monitoring changes in ground heat flux at global scale is of critical importance to quantify and understand the evolution of the Earth heat inventory, and thus climate change. However, the main sources of information about past and present ground heat flux are measurements of subsurface temperature profiles and micrometeorological observations, which are incomplete records biased towards northern extratropical latitudes.

Here, we present preliminary estimates of global ground heat flux derived from remote sensing products from the European Space Agency (ESA) Climate Change Initiative (CCI). Estimates from four land surface temperature (LST) products (MODIS-Terra, MODIS-Aqua, ENVISAT-AATSR, and SSMI-SSMIS) are evaluated against FLUXNET observations, obtaining a range of root mean squared errors from 3.8 to 5.2 W m^-2 at monthly resolution. Nevertheless, there are some spatial inconsistencies among estimates from different LST products, as well as in long-term trends during the period 2003-2013. Several factors affecting the estimated ground heat flux are analyzed, with soil water and land cover having the largest effect on the retrieved values. These results suggest that land surface temperature from satellite observations may be able to provide global long-term ground heat flux estimates, although some issues still need to be solved.