Reinforcement of Climate Hiatus by Decadal Modulation of Daily Cloud Cycle
Based on observations and climate model results, it has been suggested that the recent slowdown of global warming trends (climate hiatus), which took place in the early 2000s, might be due to enhanced ocean heat uptake. Here we suggest an alternative hypothesis which, at least in part, would relate such slowdown to unaccounted energy reflected or reemitted by clouds. We show that the daily cloud cycle is strongly linked to pacific decadal oscillation (PDO) and that its decadal variations during the climate hiatus have an overall cooling effect. Such an effect may have partially, and temporarily, counteracted the greenhouse warming trends.
The slowdown of global warming in the early 21th century, referred to as the climate hiatus, raises growing political and public concerns. Observations and climate modelling results suggest that such a phenomenon is caused by the compounding effects of inter-annual and decadal variations of ocean circulation, aerosols, volcanic eruptions, and variation of solar irradiance. While enhanced ocean heat uptake is regarded as one of its primary causes of the recent climate hiatus, there are still debates over which parts and depths of the ocean may be responsible for absorbing the imbalanced energy. Such uncertainties stem in part from the temporal interpolation method used for satellite calibration and the sparse spatial/temporal sampling of the ocean heat content measurement. It is thus logical to wonder whether the estimation of Earth’s energy balance might have missed some energy component linked to the finer temporal resolutions (e.g. sub-daily timescale).
While the importance of seasonal cycles of clouds and radiative fluxes is widely acknowledged, the impact of daily cycles on climate could be even stronger (see Supplementary Fig. 1). Thus, variations in the daily cycle of clouds (DCC) have the potential to affect the Earth’s energy balance and contribute to the climate variability. To test this hypothesis, we began by investigating the pacific decadal oscillation (PDO), which is widely regarded as an indicator of the climate hiatusand is strongly linked to the Earth’s mean surface temperature (see Supplementary Fig. 2). The PDO-temperature trends roughly show three periods of variations during the early 21th century. During the pre-hiatus (2000-2003) and post-hiatus (2013-present) periods, both Earth surface temperature and PDO increase; during the mid-hiatus (2003-2013) period, the PDO decreases while temperature keeps relatively constant. To explore whether DCC is also linked to the PDO, we used satellite observations of Single Scanner Footprint (SSF) from Clouds and the Earth’s Radiant Energy System (CERES)25 (see Methods), which can be used for long-term cloud trends analysis. We focus on the cloud fraction, one of the most important cloud properties that are critical to the Earth’s energy balance. In general, the daytime clouds tend to reflect more solar radiation and cool the Earth, while the nighttime clouds tend to keep longwave radiation and warm the Earth.