From NASA's Clouds and radiation factsheet:
The study of clouds, where they occur, and their characteristics, play a key role in the understanding of climate change.
Clouds exist. Before we worry about marginal changes, it's good to understand how clouds anchor sea level air temperature at average 288K.
The Earth's climate system constantly adjusts in a way that tends toward maintaining a balance between the energy that reaches the Earth from the sun and the energy that goes from Earth back out to space.
Correct and agreed.
Energy goes back to space from the Earth system in two ways: reflection and emission.
Part of the solar energy that comes to Earth is reflected back out to space in the same, short wavelengths in which it came to Earth. The fraction of solar energy that is reflected back to space is called the albedo. Different parts of the Earth have different albedos. For example, ocean surfaces and rain forests have low albedos, which means that they reflect only a small portion of the sun's energy. Deserts, ice, and clouds, however, have high albedos; they reflect a large portion of the sun's energy.
Over the whole surface of the Earth, about 30 percent of incoming solar energy is reflected back to space.
So clouds exist for the purpose of calculating albedo and are considered part of the surface. They said it themselves. Good. The 30% reflected is the weighted average of the two-thirds of the surface covered by clouds with albedo 40% and one third cloud-free oceans/land with albedo 10%. This leaves an average of 238 W/m2 being absorbed by clouds and cloud-free ocean/land. All coherent so far.
The top of the cloud is usually colder than the Earth's surface. Hence, if a cloud is introduced into a previously clear sky, the cold cloud top will reduce the longwave emission to space, and (disregarding the cloud albedo forcing for the moment) energy will be trapped beneath the cloud top. This trapped energy will increase the temperature of the Earth's surface and atmosphere until the longwave emission to space once again balances the incoming absorbed shortwave radiation.
Clouds still exist. The conclusion is broadly correct, but the explanation is poor or downright misleading. Clouds are emitting what LW they can. The upper layers that emit to space are average 255K (sea level temp 288K minus 5km altitude x 6.5 K/km lapse rate) and, given their emissivity of 70%, emit 168 W/m2. Note that the word or concept "emissivity" is not mentioned in the article. As they say themselves, there is a separate system that operates as between clouds and sea level; what we are looking at is the 238 W/m2 from the Sun and back out to Space.
So for every three square metres of Earth (two of clouds, one of cloud-free) total outgoing LW has to be 3 x 238 W/m2 = 714 W total. The two m2 covered by clouds are emitting 2 x 168 W = 336 W/m2. That means the remaining 1 m2 of cloud-free oceans/land has to be emitting 714 - 336 = 378 W.
[Analogy: that's like air escaping from an inflatable mattress which you are still inflating. Cloud-free areas are the puncture. The smaller the puncure a) the higher the air pressure in the mattress and b) the faster the air will leak through the puncture.]
Ocean/land emissivity is 96%, so working backwards from 378 W/m2, the temperature of ocean/land has to be 388K to bring up the overall average LW emitted to space of 238 W/m2. 168 + 168 + 378 = 714. The upwards LW absorbed by clouds and LW reflected or re-emitted down again by clouds need not be taken into account again - we have an answer that ties in with actual observed temperatures and the gravity-induced lapse rate of 6.5 K/km (assuming average cloud-top altitude to be 5km, which seems about right). The reflected and re-emitted LW is already part of that overall balance.
And now, having shown that the entire Greenhouse Effect can be attributed to clouds (once you factor in emissivity, which Climate Scientists seldom do), clouds leave the stage and The Villain makes a surprise entrance:
However, a significant fraction of the longwave radiation emitted by the surface is absorbed by trace gases in the air. This heats the air and causes it to radiate energy both out to space and back toward the Earth's surface.
So, let's ascribe the effect of clouds to 'trace gases', shall we? Ignore the big white fluffy things that cover two thirds of the surface and reflect and absorb significant amounts or radiation in both directions and provably cause the entire Greenhouse Effect? Let's focus on invisible trace gases?
To ram the deception home, they conclude with this:
The overall effect of all clouds together is that the Earth's surface is cooler than it would be if the atmosphere had no clouds.
That is quite simply untrue, they've provided all the evidence to show that clouds have - surprisingly perhaps - a warming effect at sea level. See the calculation a few paragraphs above. Which is why the Greenhouse Effect is much smaller in cloud-free deserts, non-existent on Mars (a few scattered dust clouds) and very high on Venus (100% covered in very thick, very high clouds). But let's not drag real life into this, eh? Let's live in our logic-free fantasty world?
Conclusion: the Climate Science argument is there is a 33 degree Greenhouse Effect (sort of true) and that this is down to 'trace gases'. Therefore, more trace gases = more Greenhouse Effect. But the Greenhouse Effect is actually a measurement problem - on closer inspection either there isn't one at all and/or it is down to clouds. Therefore trace gases have zero impact, therefore any change in trace gas levels can't have any effect either. I have no strong opinion on what causes small fluctiatons in surface temperatures over longer periods, but it sure as heck ain't changes in the amount of trace gases.
Debate Magazine
