Combined surface solar brightening and increasing greenhouse effect support recent intensification of the global land‐based hydrological cycle

Abstract

[1] The surface net radiation (surface radiation balance) is the key driver behind the global hydrological cycle. Here we present a first-order trend estimate for the 15-year period 1986 -2000, which suggests that surface net radiation over land has rapidly increased by about 2 Wm À2 per decade, after several decades with no evidence for an increase. This recent increase is caused by increases in both downward solar radiation (due to a more transparent atmosphere) and downward thermal radiation (due to enhanced concentrations of atmospheric greenhouse-gases). The positive trend in surface net radiation is consistent with the observed increase in land precipitation (3.5 mmy À1 per decade between 1986 and 2000) and the associated intensification of the land-based hydrological cycle. The concurrent changes in surface net radiation and hydrological cycle were particularly pronounced in the recovery phase following the Mount Pinatubo volcanic eruption, but remain evident even when discarding the Pinatubo-affected years.

Recent decline in the global land evapotranspiration trend due to limited moisture supply

“…The positive ET trend shown in Fig. 2 is consistent with the expected 'acceleration' of the hydrological cycle caused by an increased evaporative demand associated with rising radiative forcing 10 and temperatures. Indeed, interannual variability in temperature correlates well with ET variability from 1982 to 1997 (Pearson's correlation coefficient 0.84, P , 0.01).…”

Decadal climate variability and the global energy balance

“…1997/98, 2015/16) simulations (which prescribe observed sea surface temperatures while land Ts is explicitly simulated) and ERAI (which also prescribes ocean surface temperature but land Ts is somewhat constrained by observations through data assimilation). ERAI anomalies up to 0.2 K higher than HadCRUT4 in late 2016 are likely due in part to the lack of interpolation of observed values over the Arctic (Cowtan and Way, 2014 The resultant additional water vapor continuum absorption in the more transparent window regions of the infrared spectrum cause a reduction in surface loss of clear-sky longwave radiation of ~1.4 Wm -2 per mm of precipitable water vapor (Allan, 2009) which translates to reduced clear-sky surface net longwave radiative loss of ~0.4 Wm -2 per decade, consistent with more detailed modelling estimates (Wild et al, 2008). Enhanced absorption of sunlight by the increasing water vapor additionally reduces net radiative energy loss by the atmosphere and contributes to solar "dimming" at the surface (Haywood et al, 2011).…”

Trends in surface radiation and cloud radiative effect over Switzerland in the past 15 years

“…DLR is predicted to increase between 2 and 3 Wm -2 per decade due to rising greenhouse gas levels [4]. Indeed, this predicted increase has been verified for all-sky and cloud-free DLR using surface observations [5,6]. The increase in cloud-free DLR was mainly related to the corresponding temperature increase.…”