  While it is well known that water availability controls vegetation growth and
soil microbial activity, how aridity affects ecosystem carbon patterns is not
completely understood. Towards a more quantitative assessment of terrestrial
carbon stocks, here we uncover a remarkable self-similar behavior of the global
carbon stock. Using international survey and remote sensing data, we find that
the key statistics (e.g., mean, quantiles, and standard deviation) of carbon
stock tend to scale with the hydrological regime (i.e., aridity) via a
universal exponent. As a result, when normalized by its averages in the
corresponding hydrological regime, the carbon stock distributions collapse onto
a single curve. Such a scaling reflects the strong coupling between
hydrological cycle and biogeochemical process and enables robust predictions of
carbon stocks as a function of aridity only.
