Through their reflection of sunlight and absorption/re-emission of thermal radiation, clouds regulate Earth’s energy balance. But it remains uncertain, in particular, how the fraction of sunlight reflected by clouds will change as greenhouse gas concentrations rise. Projections differ widely among climate models, and differences in the solar reflection by low clouds over tropical oceans account for much of the spread in climate projections across current models. We investigate to what extent this uncertainty can be reduced through the use of observations from space.
A convenient yardstick to measure how sensitive the climate system is to increases in the concentration of greenhouse gases is the equilibrium climate sensitivity (ECS)—the surface warming eventually reached after a sustained doubling of carbon dioxide concentrations. ECS ranges from 2.1 to 4.7 K across current climate models (IPCC AR5). More than half of the ECS variance across models can be traced to differences in the reflection of sunlight by tropical low clouds (TLCs) (Bony and Dufresne 2005; Vial et al. 2013). Neither the sign nor the strength of this TLC feedback are well constrained. Yet constraining the TLC feedback is essential for narrowing the wide range of ECS projected by current models.
A number of observational studies points to a weakening of solar reflection by TLCs under warming (Clement et al. 2009; Dessler 2010, 2013; Zhou et al. 2013), suggesting a positive TLC feedback. Other studies indicate that models with strongly positive low-cloud feedback are more consistent with observations than models with weakly positive or negative feedback (Qu et al. 2014, 2015b, Myers and Norris 2016). This is in line with other model–observation comparisons that also point to higher ECS (Fasullo and Trenberth 2012; Sherwood et al. 2014; Tian 2015). By contrast, studies focusing on Earth’s energy budget generally point to a lower ECS (Otto et al. 2013), albeit with large uncertainties that still allow a high ECS. In Brient and Schneider (2016), we show how space-based observations can be used to robustly constrain the TLC feedback and constrain ECS.
First, we identify TLC regions as the 25% of the tropical ocean area (30°N–30°S) with the lowest mid-tropospheric (500-hPa) relative humidity (Fig. 1a, b). This gives moving TLC regions, in observations and in climate models, that follow regions of mid-tropospheric dryness. The regions in which monthly means meet the mid-tropospheric dryness criterion frequently broadly correspond to regions with frequent low-cloud cover, both in observations (Fig. 1a) and in climate models. We chose to identify TLC regions in this way to be able to follow low-cloud regions as they shift seasonally and under climate change—an effect that may be missed when considering fixed regions.
Then we calculate the TLC reflection from the insolation and the amount of solar radiation reflected by clouds back to space. Space-based measurements indicate that on average 8.95% of the incoming solar insolation is reflected by clouds in the TLC regions. But the TLC reflection varies around this average with the underlying surface temperature. Our analysis of observations shows that TLC reflection robustly weakens when the underlying surface warms, for example, by -(0.96 +/- 0.22) % per Kelvin surface warming for deseasonalized temperature and cloud variations. In the intraannual, seasonal, and interannual frequency bands, TLC reflection likewise weakens robustly when the surface warms, and the weakened reflection arises primarily because the low-cloud cover decreases. Thus, observations point to a positive low-cloud feedback on warming.
The observations can be compared with what current (CMIP5) climate models produce. In simulations of the warmer climate reached after quadrupling carbon dioxide concentrations, higher-sensitivity (HS) models project a reduction of TLC reflection, whereas lower-sensitivity (LS) models project less change or even an increase. The models’ ECS correlates strongly (r=-0.73) with changes in TLC reflection (Fig. 2a). Additionally, changes in TLC reflection under global warming in the models correlate strongly with variations in TLC reflection inferred from temporal variations in simulations of the present climate alone (Fig. 2b). That is, as also seen in previous studies (Qu et al. 2014; Zhou et al. 2015), how TLC reflection covaries with temperature in simulations of the present climate is a strong indicator of a model’s TLC feedback under global warming.
Because of the strong correlation between the TLC reflection feedback and ECS in models, these results imply that, remarkably, almost half of the ECS variance across models can be accounted for by simulations that do not involve any perturbation of the atmospheric greenhouse gas concentrations (Fig. 3). Therefore, we can use the covariation of TLC reflection with temperature obtained from observations of the present climate to constrain model projections of ECS. We estimate a posterior ECS from an average of climate models in which each model is weighted according to how well its simulation of the present climate reproduces the observed deseasonalized covariation of TLC reflection with temperature. This information-theoretic model averaging generally assigns greater weight to HS models because they are more consistent with the observations (Fig. 3a). It yields a posterior probability density function (PDF) of ECS with the median and mode at 4.0 K, and with a 90% confidence interval of [2.3 K;5.0 K] (Fig. 3b). That is, ECS most likely lies in the upper half of current estimates; values below 2.3 K are very unlikely.
Of course, these posterior ECS estimates are still conditional on the range of ECS simulated by current climate models. They merely indicate which ECS in the model range are more likely than others, given the observations. They do not rule out ECS entirely outside the range indicated by current climate models; all models may be wrong. But accepting the climate models as our currently best representation of the climate system, the observations unmistakably point to higher ECS being more likely, and a substantially higher ECS than previously thought as most likely—though the range of possible ECS obtained in this way is still wide, still indicating large uncertainties.
In our paper, we also investigate how other environmental factors besides the surface temperature may influence TLCs. Recent articles have highlighted the inversion strength as a factor controlling cloud cover (Qu et. al 2015a, Myers and Norris 2016). Using a bivariate regression to quantify the influences of both surface temperature and inversion strength on TLC reflection, we found that temperature exerts the primary control. Estimates of the TLC feedback on warming and of ECS are not substantially affected by including inversion strength as a predictor in the analysis.
Thus, our results robustly indicate that TLC reflection of sunlight feeds back positively onto warming. They also point to higher ECS within the range simulated by current climate models as being more likely. The consistent covariance of TLC reflection with surface temperature on timescales from seasonal to interannual and under global warming in climate simulations indicates that temperature is a key factor controlling TLC cover, and that similar processes likely govern the TLC response to warming across the timescales. So a process-oriented analysis of low-cloud variations in the present climate has the potential to lead to improvements in the representation of the low-cloud response to climate changes in models. Such as process-oriented analysis can also shed light on the mechanisms responsible for this low-cloud response—a crucial question our paper leaves unanswered.