Dr. Curry has a new very informative post (here) on clouds and climate, including links to several studies recently announced from CERN and others. It reminded me of Joni Mitchell’s song Both Sides Now:
Bows and flows of angel hair
And ice cream castles in the air
And feather canyons everywhere
I’ve looked at clouds that way
But now they only block the sun
They rain and snow on everyone
So many things I would have done
But clouds got in my way
I’ve looked at clouds from both sides now
From up and down and still somehow
It’s clouds’ illusions I recall
I really don’t know clouds at all
– Joni Mitchell – Both Sides Now Lyrics
The above chorus could serve as an anthem for climate modelers. Clouds are arguably the least understood and most unpredictable of factors in climate change. We are getting much better at the weather connection between storms and cloud formation. But the long-term effects of clouds and cloudiness are still uncertain. Dr. Curry helpfully separates the cloud problem into two issues: cloud microphysics and cloud dynamics. She observes that the latter is much more difficult and also has much more impact on climate.
Some things are known and described in textbooks of Atmospheric Physics. In introducing Chapter 9: Aerosols and Clouds in his updated volume, Murray Salby (here) suggests the complexities involved:
Radiative transfer is modified importantly by cloud. Owing to its high reflectivity in the visible, cloud shields the Earth-atmosphere system from solar radiation. It therefore introduces cooling in the SW energy budget of the Earth’s surface, offsetting the greenhouse effect. Conversely, the strong absorptivity in the IR of water and ice sharply increases the optical depth of the atmosphere. Cloud thus introduces warming in the LW energy budget of the Earth’s surface, reinforcing the greenhouse effect. We develop cloud processes from a morphological description of atmospheric aerosol, without which cloud would not form. The microphysics controlling cloud formation is then examined. Macrophysical properties of cloud are developed in terms of environmental conditions that control the formation of particular cloud types. These fundamental considerations culminate in descriptions of radiative and chemical processes that involve cloud.
The microphysics is mostly related to how clouds form, and the role of aerosols. Even though clouds can form simply from enough water vapor, in practice the required conditions for such “homogenous” formation are higher than those needed for “heterogenous” formation from ever-present aerosols, termed CCN. From Salby (pg. 272):
The simplest means of forming cloud is through homogeneous nucleation, wherein pure vapor condenses to form droplets. . . Yet, the formation of most cloud cannot be explained by homogeneous nucleation. Instead, cloud droplets form through heterogeneous nucleation, wherein water vapor condenses onto existing particles of atmospheric aerosol. Termed cloud condensation nuclei (CCN), such particles support condensation at supersaturations well below those required for homogeneous nucleation.
Cloudiness Impact on Radiative Balance
The extent of cloudiness varies a lot, as shown by measures of OLR (Outgoing Longwave Radiation) by satellites above TOA (h/t greensand). Notice that the scale has a range of 100 W m^2 compared to estimated CO2 sensitivity of ~4 W m^2.
OLR or ‘Cloudiness’ at the equatorial dateline 7.5S – 7.5N, 170E – 170W (large sea surface area) has been below norm for 15/16 months. Below average OLR is the result of increased cloud cover, which in turn = reduced insolation, less incoming solar energy. Yet as Salby says, cloud tops can reflect SW solar energy away while the cloud mass absorbs IR from the surface, delaying cooling. Different types of clouds have different impacts on radiative forcing. Not to mention water changing between all 3 phases inside.
Therein lies the cloud conundrum: How much warming and how much cooling from changes in cloudiness?
Clouds Complicating Climate
A quantitative description of how cloud figures in the global energy budget is complicated by its dependence on microphysical properties and interactions with the surface. These complications are circumvented by comparing radiative fluxes at TOA under cloudy vs clear-sky conditions. Over a given region, the column-integrated radiative heating rate must equal the difference between the energy flux absorbed and that emitted to space.
Shortwave cloud forcing represents cooling. It is concentrated near the Earth’s surface, because the principal effect of increased albedo is to shield the ground from incident SW. Longwave cloud forcing represents warming. It is manifest in heating near the base of cloud and cooling near its top (Fig. 9.36b).
That radiative forcing depends intrinsically on the vertical distribution of cloud. For instance, deep cumulonimbus and comparatively shallow cirrostratus can have identical cloud-top temperature, yielding the same LW forcing of the TOA energy budget. However, they have very different optical depths, producing very different vertical distributions of radiative heating. The strong correlation between water vapor and cloud cover introduces another source of uncertainty.
Since 90% of water in the atmosphere comes from the ocean, clouds are another way that Oceans Make Climate. And as Roger Andrews demonstrates (here) cloudiness correlates quite positively with SSTs.
Bottom Line: Any CO2 effect is lost in the Clouds
Globally averaged values of CLW and CSW are about 30 and −45 W m−2, respectively. Net cloud forcing is then −15 W m−2. It represents radiative cooling of the Earth atmosphere system. This is four times as great as the additional warming of the Earth’s surface that would be introduced by a doubling of CO2. Latent heat transfer to the atmosphere (Fig. 1.32) is 90 W m−2. It is an order of magnitude greater. Consequently, the direct radiative effect of increased CO2 would be overshadowed by even a small adjustment of convection (Sec. 8.7).