Clouds and Global Warming

How do clouds affect global warming? This is an important, urgent and fascinating question! Clouds are involved in a number of physical and chemical processes and are surprisingly complex. Their effect on atmospheric temperatures is both warming and cooling. The question we have to answer is: Do they warm more than they cool or is it the other way around?  

Cloud structure is probably a little different from what you imagine. Clouds are not made up of water vapor floating in the air, but instead are concentrations of tiny droplets of liquid water so light that the air pressure from below keeps them afloat. Water vapor is constantly evaporating from the Earth’s surface, especially the surface of oceans, to rise until they can condense as water droplets around “condensation nuclei”. These condensation nuclei are microscopic particulates of various types suspended in air. Without the condensation nuclei acting as a catalyst, the water vapor density required for condensation would be several hundred times what is required now. As you will see soon, this dependence on condensation nuclei ties cloud formation with cosmic radiation.

As it turns out water is a far more potent greenhouse gas interacting with infrared radiation than CO₂ is. Water probably accounts for about 60% of the total greenhouse effect. This does not necessarily mean that it is responsible for observed global warming periods (more on this later), but it does mean that bulk water in the form of clouds can act as reflectors of infrared radiation. The collisions of infrared photons with water will by far overshadow the collisions of photons with CO₂. A fraction of the radiation entering a cloud will be reflected back by the same collisions with greenhouse gas molecules (this time of H₂O) that create the diffusion of infrared photons through the troposphere. This fraction will increase with the thickness of the cloud.

Consider what this means for the infrared radiation reflected from the surface of the Earth. If the surface is overshadowed by cloud, the cloud will reflect much of the radiation back toward the Earth, thereby creating a local heating effect. Of course eventually if the cloud stayed put, the radiation would diffuse through the cloud much as it does through CO₂ to the top of the troposphere. However, clouds do not stay put for long and once they do move, the uncovering of the area underneath will release any radiation still “trapped”. Also radiation will escape around the edges of the cloud, The “trapping” is imperfect, local, and temporary. This is the warming effect of clouds.

The effect of a cloud on incident radiation from the Sun is exactly the mirror image of the cloud’s effect on radiation from the Earth’s surface. The higher the altitude of the cloud, the closer it is to the top of troposphere and the larger the fraction of reflected infrared radiation that is radiated immediately back into space. This is the cooling effect of clouds.

So which effect is stronger: the warming or the cooling? It is easy to make a picture where the net effect is cooling. It is much harder to make a case for a net warming effect. I will first paint a picture for a net cooling effect, and then we will make an appeal to measured data. First, we need to look at the spectral content of the incident solar radiation. This is given in the figure below taken from the website blog.spotter.org.

As you can see, the power density of solar radiation in watts per meter-squared per unit of wavelength is graphed versus wavelength. The longer the wavelength the redder the radiation is. You can see the diagram’s demarcations between visible light and the infrared, as well as between the ultraviolet (UV) and the visible. By visual examination you should be able to see that there is approximately as much power (area under the curve) in the visible wavelengths as in the infrared, with the infrared being perhaps a tad stronger.

The first encounter of the radiation with a cloud will be on the space side where the radiation is reflected back to space. The infrared radiation that actually reaches the Earth’s surface will be a fraction of the radiation that was incident from the Sun. The thicker the cloud, the more radiation is reflected back to space on the high-altitude side of the cloud; and the less that reaches the surface, and then is reflected to arrive at the cloud’s low-altitude side. As a first approximation we can assume the cloud to be symmetrical in altitude, so that the same fraction of incident radiation from the lower side passes through the cloud as passed through it initially from the upper side. Using these assumptions it is relatively easy to build a model using infinite series that shows net cooling due to the cloud.

That is what I would expect, but what does measured data tell us? Enter Dr. Roy Spencer of the University of Alabama, Huntsville. You will remember meeting him in the post on Global Warming Data as the head of the group that maintained the satellite-measured UAH dataset on average global temperatures. He too came to the conclusion that clouds have a net cooling effect, and as a result he endured a large number of ad hominem attacks and ridicule from the Anthropogenic Global Warming (AGW) enthusiasts, as noted in a preface to Spencer’s popular book, The Great Global Warming Blunder. See reference [S1]. In his introduction to the book, Spencer wrote

I will advance the argument that natural, internally generated cloud variability is responsible for most of the climate change we have seen up to the present and will likely see in the future. And contrary to the claims of some scientists that recent warming is unprecedented, the warming we experienced through the twentieth century is not much different from that experienced during other centuries over the last 2,000 years. 

Also he wrote

The most obvious way for warming to be caused naturally is for small, natural fluctuations in the circulation patterns of the atmosphere and ocean to result in a 1% or 2% decrease in global cloud cover. Clouds are the Earth’s sunshade, and if cloud cover changes for any reason, you have global warming — or global cooling.

In 2012 a paper was published  in the Journal of Climate by Ryan Eastman and Stephan G. Warren entitled A 39-Year Survey of Cloud Changes from Land Stations Worldwide 1971-2009: Long-Term Trends, Relation to Aerosols, and Expansion of the Tropical Belt. In that paper the authors examined archives of land-based, surface-observed cloud observations that covered all continents and spanned 39 years from 1971 to 2009. Using that data they calculated surface averages of cloud cover as a function of year. The major results are shown below. Click on the image for a better resolution view.

Their conclusions: Global cloudiness decreased over those years between 0.9% to 2.8% depending on the  continent. They also said, “Global average trends of cloud cover suggest a small decline in total cloud cover, on the order of 0.4% per decade.” Over the 39 years of the record, that amounts to a decrease of 1.56%, closely agreeing with what Spencer said was required for global warming to be explained by cloud physics.

We still have to find the reason why global cloudiness would have varied over those 39 years. In the next post of this series, we will examine the connections between solar power variability, incident cosmic rays on the atmosphere, and cloud formation.

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