Forcing or Feedback?

Guest Post by Willis Eschenbach

I read a Reviewer’s Comment on one of Richard Lindzen’s papers today, a paper about the tropics from 20°N to 20°S, and I came across this curiosity (emphasis mine):

Lastly, the authors go through convoluted arguments between forcing and feed backs. For the authors’ analyses to be valid, clouds should be responding to SST and not forcing SST changes. They do not bother to prove it or test the validity of this assumption. Again this is an assertion, without any testable justification.

Now to me, showing that clouds respond to the tropical surface temperatures, either on land or sea, is a no-brainer, I’m surprised that the Reviewer would question it. But before I discuss that, let me digress for a moment to say that I think there are two simple changes in the reviewing process that would help it immensely.

1. Double blind reviewing. Right now, reviewing is just single blind, with the reviewer knowing the identity of the author but not the other way around. This is a huge, huge violation of the scientific norms, and it leads and has led to problems. A recent proposal to use double-blind reviewing in the awarding of grants by the National Science Foundation makes for interesting reading … http://www.nsf.gov/nsb/publications/2011/meritreviewcriteria.pdf This would put the focus back onto the ideas rather than the author.

2. Publish the reviews and the reviewer’s names along with the paper when it is published. This sunshine, this transparency, will have several benefits.  First, if a reviewer still has strong reservations about the paper, or if their objections have led to improvements or changes in the paper, this will be made visible when the reviews are published (presumably online) along with the paper. Second, we get to see who agrees that the paper is valid science, and who might not. Third, in the fullness of time, we will have a record of who was right and who was wrong. But I digress … the anonymous Reviewer wanted Lindzen to show that clouds respond to SST.

Not having read Lindzen’s paper, I don’t know what he said about the question. But for me, Figure 1 should convince even the most alarmist reviewer that clouds are responding to the surface temperature, both on land and over the ocean.

Figure 1. Monthly albedo averages from ERBE. Images show the N/S range of the geographical tropics (~ 23.5° N/S). Thin black horizontal line shows the Equator.

The upper panel shows the albedo for August (northern hemisphere summer) and the lower panel for February (southern hemisphere summer). The brighter areas show the regions with higher albedo, because there are more clouds. There are some things of note in the figure.

For example, look at Brazil and the Amazon in the lower panel (the bright spot in the lower left of the panel, below the Equator.). In February, in the southern summer, the heat creates lots of clouds. But in the upper panel, the southern hemisphere winter, Brazil has a much lower level of clouds.

The same thing is true about southern Africa. In the southern summer (lower panel) the albedo is high in southern Africa, with lots of clouds. In the southern hemisphere winter (upper panel), on the other hand, southern Africa loses its clouds, and above the Equator where it is summer we see increased albedo in the northern part of Africa.

Next, tropical Asia in August, when it is warm, has lots of clouds. But when winter comes, the clouds move with the summer down to the southern hemisphere.

Similar changes are generally occurring over the oceans as well, but in a more muted fashion. Again, this difference in effect shows that surface temperature is the driving factor, since the land temperatures change more with the seasons than do the ocean temperatures.

Now, we have three choices here:

1. Tropical clouds are increasing and decreasing  in response to the changes in surface temperature resulting from the sun’s motion, which alternately warms the areas north of and south of the Equator, or

2. The clouds are just coincidentally in sync with the sun by random chance, or

3. The clouds are making the sun move north and south of the Equator.

Your choice, but to me, that should satisfy even the most recalcitrant reviewer. It’s clear that the clouds are responding to the variations in surface temperature, with more clouds forming as the surface warms and less clouds when the surface is cooler.

w.

PS: Since we’re discussing clouds as forcing and feedback, you’ll excuse me if I take this opportunity to re-post the following from my “Thermostat Hypothesis” paper. In it I am describing how the clouds and thunderstorms work as a governor to stabilize the temperature:

The problem with my thought experiment of describing a typical tropical day is that it is always changing. The temperature goes up and down, the clouds rise and fall, day changes to night, the seasons come and go. Where in all of that unending change is the governing mechanism? If everything is always changing, what keeps it the same month to month and year to year? If conditions are always different, what keeps it from going off the rails?

In order to see the governor at work, we need a different point of view. We need a point of view without time. We need a timeless view without seasons, a point of view with no days and nights. And curiously, in this thought experiment called “A Day In the Tropics”, there is such a timeless point of view, where not only is there no day and night, but where it’s always summer.

The point of view without day or night, the point of view from which we can see the climate governor at work, is the point of view of the sun. Imagine that you are looking at the earth from the sun. From the sun’s point of view, there is no day and night. All parts of the visible face of the earth are always in sunlight, the sun never sees the night-time. And it’s always summer under the sun.

If we accept the convenience that north is up, then as we face the earth from the sun, the visible surface of the earth is moving from left to right as the planet rotates. So the left hand edge of the visible face is always at sunrise, and the right hand edge is always at sunset. Noon is a vertical line down the middle. From this timeless point of view, morning is always and forever on the left, and afternoon is always on the right. In short, by shifting our point of view, we have traded time coordinates for space coordinates. This shift makes it easy to see how the governor works.

The tropics stretch from left to right across the circular visible face. We see that near the left end of the tropics, after sunrise, there are very few clouds. Clouds increase as you look further to the right. Around the noon line, there are already cumulus. And as we look from left to right across the right side of the visible face of the earth, towards the afternoon, more and more cumulus clouds and increasing numbers of thunderstorms cover a large amount of the tropics.

It is as though there is a graduated mirror shade over the tropics, with the fewest cloud mirrors on the left, slowly increasing to extensive cloud mirrors and thunderstorm coverage on the right.

After coming up with this hypothesis that as seen from the sun, the right hand side of the deep tropics would have more cloud than the left hand side), I though “Hey, that’s a testable proposition to support or demolish my hypothesis”. So in order to investigate whether this postulated increase in cloud on the right hand side of the earth actually existed, I took an average of 24 pictures of the Pacific Ocean taken at local noon on the 1st and 15th of each month over an entire year. I then calculated the average change in albedo and thus the average change in forcing at each time. Here is the result:

Figure 2. Average of one year of GOES-West weather satellite images taken at satellite local noon. The Intertropical Convergence Zone is the bright band in the yellow rectangle. Local time on earth is shown by black lines on the image. Time values are shown at the bottom of the attached graph. Red line on graph is solar forcing anomaly (in watts per square meter) in the area outlined in yellow. Black line is albedo value in the area outlined in yellow. 

The graph below the image of the earth shows the albedo and solar forcing in the yellow rectangle which contains the Inter-Tropical Convergence Zone. Note the sharp increase in the albedo between 10:00 and 11:30. You are looking at the mechanism that keeps the earth from overheating. It causes a change in insolation of -60 W/m2 between ten and noon.

Now, consider what happens if for some reason the surface of the tropics is a bit cool. The sun takes longer to heat up the surface. Evaporation doesn’t rise until later in the day. Clouds are slow to appear. The first thunderstorms form later, fewer thunderstorms form, and if it’s not warm enough those giant surface-cooling heat engines don’t form at all.

And from the point of view of the sun, the entire mirrored shade shifts to the right, letting more sunshine through for longer. The 60 W/m2 reduction in solar forcing doesn’t take place until later in the day, increasing the local insolation.

When the tropical surface gets a bit warmer than usual, the mirrored shade gets pulled to the left, and clouds form earlier. Hot afternoons drive thunderstorm formation, which cools and air-conditions the surface. In this fashion, a self-adjusting cooling shade of thunderstorms and clouds keeps the afternoon temperature within a narrow range.