Stochastic Approach to the Evolution of the Global Water Cycle: Results of Historical Experiments on the CMIP-6 Models

Changes in the integral characteristics of the global water exchange, at climatic time scales, are considered as random functions (processes). “Trajectories” obtained as the results of numerical calculations on various, from 34 to 43, climate models (participating at the CMIP-6 “historical” experiment covering the period from 1850 through 2014) are taken as realizations of these processes. Temporal variations of following annually averaged parameters are studied: (1) average evaporation from the ocean surface, (2) precipitation over the ocean, (3) “effective evaporation” from the ocean (difference “evaporation minus precipitation”, on average equal to the water transport from the ocean to land), (4) precipitation over land, (5) evaporation (evapotranspiration) from the land surface, (6) “effective precipitation” over land (or “climatic runoff”: precipitation minus evaporation), and (7) river runoff. It is shown that precipitation over the ocean and evaporation from land largely suppress the monotonous trends in the mean values of evaporation from the ocean and precipitation over land, respectively, at secular time scales. At the same time, this damping does not extend to the trends of the last few decades, which may be due to a combination of a sharp increase in global temperature with explosive volcanic eruptions that preceded this period. An analysis of the time divergence in the model trajectories of each of the components of the global water exchange, as well as the very existence of such divergences, indicates an increase in the uncertainty of processes that is not associated with anthropogenic impact on the climate system.