Trends of surface solar radiation in unforced CMIP5 simulations

Abstract

We consider decadal scale trends of annual mean all‐sky surface solar radiation (SSR) that occur solely because of internal variability of the climate system. We give statistical estimates of their magnitude and probability of occurrence. The estimates are based on 43 preindustrial control (piControl) experiments of the Coupled Model Intercomparison Project phase 5 (CMIP5). Trends are found to depend strongly on geographical region and on whether they are quantified in absolute units or relative to the long‐term mean SSR. We find it to be sufficient to provide one map for absolute and one for relative trends, as approximate analytical relations are shown to hold between trends of different length and likelihood and the standard deviation of the underlying SSR time series. We estimate that a positive trend over 30 years and with 25% chance of occurrence (75th percentile of all possible trends) has a magnitude between 0.15 and 1.7 W/m2/decade or 0.11 and 1.4% of long‐term mean SSR per decade, depending on geographical location. Comparison with present‐day observations and intermodel spread suggests an average uncertainty of these estimates of about 30%. Intermodel spread suggests that regional uncertainties can be up to about 3 times larger or smaller. We give examples of how these results may be used to obtain statistical estimates of how (un)likely it is that observed SSR trends or part thereof are due to internal variability alone.

Internal Variability of All‐Sky and Clear‐Sky Surface Solar Radiation on Decadal Timescales

“…To examine trends caused by internal variability only, we use an analytical model, described in Thompson et al ( 2015 ) and applicable to data that possess an approximately Gaussian distribution and are stationary in time, and applied to SSR by Folini et al ( 2017 ). The model links the standard deviation of the distribution of all possible N-year trends σ N to the standard deviation of the underlying annual time series σ ts through 𝐴𝐴 𝐴𝐴𝑁𝑁 ≈ √ 12𝑁𝑁 −3∕2 𝐴𝐴𝑡𝑡 𝑡𝑡 ( Hinkelman et al, 2009;Nishizawa & Yoden, 2005;Tiao et al, 1990;Weatherhead et al, 1998 ).…”

“…The current work employs the same methodology as used in Folini et al ( 2017 ) but goes beyond the results presented there in that here we consider not only CMIP-Phase 5 data ( Taylor et al, 2012 ) but also CMIP6 data ( Eyring et al, 2016 ), examine not only all-sky SSR but also clear-sky SSR, and explore some potential causes for the observed model spread. Clear-sky is of interest as there are no cloud effects, thus a major source of internal variability ( noise ) is absent, suggesting that any anthropogenic contribution should be more easily identifiable than under all-sky conditions.…”

From Point to Area: Worldwide Assessment of the Representativeness of Monthly Surface Solar Radiation Records

“…The β G is similar to the measure used in Hakuba et al, (, ) except that we use the standard deviation instead of the mean absolute deviation ( ) as measure for the subgrid variability. This choice was made as one can easily recalculate the β G for any other significance level using z‐values when assuming that the β P values inside a grid box are normally distributed (as done, e.g., in Folini et al, ).…”

“…as measure for the subgrid variability. This choice was made as one can easily recalculate the G for any other significance level using z-values when assuming that the P values inside a grid box are normally distributed (as done, e.g., in Folini et al, 2017).…”

Changes in atmospheric shortwave absorption as important driver of dimming and brightening

“…A substantial body of literature exists, which investigated potential causes of the changes in surface solar radiation. Scientific consensus about the relative role of different forcing agents as well as the role of internal variability has, however, not yet been reached [37,1,38]. This is also because most previous observational analyses studied dimming and brightening from a surface perspective only, as long-term high-quality satellite observations of TOA fluxes and surface albedo were not yet available.…”