by Kevin Birdwell
Recent reports show a discrepancy between model-estimated cloud cover and actual cloud observations. Considering the role of cloud cover in regulating Earth’s temperature, it’s important to correct models so that they provide more accurate estimates.
The clouds in the sky often help us decide if we should grab an umbrella or a picnic basket for the day. Cloud cover also represents a significant component of the Earth’s climate regulation system. Without clouds, Earth’s atmosphere and oceans would become significantly warmer. Proper estimations of cloud cover are important to understanding the nature and extent of climate change—a topic with potent implications.
Clouds influence Earth’s radiation budget by affecting atmospheric circulation, the water cycle, and exchanges of energy in the atmosphere and at the planet surface.1 They reflect about 20 to 25 percent of the incoming radiation our planet receives from the Sun, while absorbing only 3 percent of that radiation.
Yet, much uncertainty remains about the intensity of the cloud feedback.2 In 2005, researchers3 noted that most climate models varied by a factor of four with respect to high cloud estimates (typically a warming feedback). In addition, models underestimated middle clouds by 30 to 40 percent, and about half of the models underestimated low clouds (together, low and middle clouds tend to favor cooling feedbacks). In 2009, researchers K. D. Williams and M. J. Webb4 added that most of the cloud errors in climate models resulted from problems resolving observations of low clouds (up to 65 percent).
A recent paper published in Atmospheric Research5 suggests that 20 of the 21 global climate models (GCMs) used by the United Nations Intergovernmental Panel on Climate Change (IPCC) may underestimate cloud cover percentages over the Earth’s surface. Such a discrepancy implies an overall warm temperature bias in the models. The paper authors operated these 21 GCMs for the years 1984 to 1999 and compared the model-estimated cloud cover to actual observations derived from the International Satellite Cloud Climatology Project (ISCCP). Largely ignoring polar clouds, which are less accurately measured, the authors found that all but one of the 21 GCMs underestimated annual cloud cover amounts between 1 to 19 percent.
Although the models were qualitatively correct in terms of the cloud cover’s geographic distribution , the modeled cloud cover averaged 7 percent less than observations recorded during the 15-year sample period. Model performance was somewhat better in the tropics (30°N to 30°S), but exhibited more error in the mid-latitudes (30°N to 60°N and 30°S to 60°S). In addition, all of the GCMs underestimated the clouds’ seasonal variability.
Researchers tend to create ensembles or averages of multiple climate projections in the hope of reducing error. However, the cloud cover differences suggested in the Atmospheric Research paper would not be eliminated by model averaging since nearly all of the models predicted less cloud cover than was observed.
Even so, not all of the cloud cover difference would result in cooling effects since some of the cloud differences correspond to high clouds. As noted by Williams and Webb,6 the majority of these differences are probably related to low clouds. This means that the dominant effect of the model vs. observation discrepancies would result in a cooler real climate when compared to climate model projections.
Given that Earth’s temperature is quite sensitive to cloud cover changes of even a few percent, the 7 percent variations between the GCMs and observed clouds is potentially significant. Although our planet has been observed to warm during most of the twentieth and early twenty-first centuries, the differences between modeled and observed clouds suggest that projected future warming could proceed more slowly than some climate models suggest.
The potentially enhanced cooling effects of clouds with respect to the actual climate vs. the GCMs infer one possible answer to a 2010 Journal of Climate article7 that asked, “Why hasn’t the Earth warmed as much as expected?” In this paper it’s noted that, according to assumed estimates of sensitivity,8 Earth’s climate system would have already warmed about 2.1 to 2.4°C during the industrial age — yet only 0.8°C of warming has actually been observed. From these values, these authors pointed out that, depending on the uncertain magnitude of aerosol effects, the planet-heating rate may be less than half of what has been assumed by some. At the presumed reduced heating rate (0.37 W/m2), overall warming of the atmosphere should be around +2°C during the twenty-first century.9 However, significant improvement in our understanding of the water cycle, the radiative effects of aerosols, and land cover effects (albedo) will be required to better estimate real twenty-first century warming. These authors cautioned that even at the reduced heating rate, unacceptable levels of green house gas increases could still be reached well before the end of the century if current rates of increase continue.
Job 37:16 says, “Do you know about the layers of the thick clouds, the wonders of One perfect in knowledge?” Climate warming will continue to be a concern, and while there are still large uncertainties. it is truly wonderful to observe the stabilizing feedbacks that our Creator has provided in the design of the climate system, including clouds, according to His perfect knowledge.
- P. Probst et al., “Total Cloud Cover from Satellite Observations and Climate Models,” Atmospheric Research 107 (2012): 161–70.
- S. Solomon et al., eds., Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007 (Cambridge, UK: Cambridge University Press, 2007).
- M. H. Zhang et al., “Comparing Clouds and Their Seasonal Variations in 10 Atmospheric General Circulation Models with Satellite Measurements,” Journal of Geophysical Research 110 (2005).
- K. D. Williams and M. J. Webb, “A Quantitative Performance Assessment of Cloud Regimes in Climate Models,” Climate Dynamics 33, no. 1 (July 2009).
- Probst et al., “Total Cloud Cover:” 161–70.
- Williams and Webb, “A Quantitative Performance Assessment:”
- Stephen E. Schwartz et al., “Why Hasn’t the Earth Warmed as Much as Expected?” Journal of Climate 23, no. 10 (May 2010): 2453–64.
- Climate sensitivity, a measure of the climate system’s temperature response to changes in radiative forcings, is dependent on a number of warming or cooling feedbacks that serve to either enhance or reduce changes in the Earth’s temperature (about 0.8°C in the last 100+ years).
- S. Schwartz et al., “Why Hasn’t the Earth Warmed as Much as Expected?:” 2453–64.