Now Guido Risaliti of the Harvard-Smithsonian Center for Astrophysics and colleagues have looked at markedly higher energies - less subject to absorption in those gas layers - using Europe's XMM-Newton telescope and the recently launched Nustar telescope.
Stacking up the numbers, he realised that human activities were adding CO2 to the atmosphere in a quantity and rate equivalent to all natural processes of absorption or emission of the gas.