Climate Letter #1981

The cloud albedo effect on surface temperatures is stronger than I once thought it would be. Yesterday’s letter provided the kind of evidence that leads to a conclusion of great strength in the selected examole, especially well-confirmed on the map of actual temperatures in the region. The 15 degree difference in temperature compared with surrounding territory is a real stunner. It makes you wonder what the maximum possible difference might be, for clouds delivering the worst possible rainstorm event. This one might be close! I have been checking out other situations that have a similar combination of circumstances and am left with no doubt that the potential effect of cloud albedo must be accounted for when making any full analysis of the cause of a daily temperature anomaly. This is especially true for days when the sun is high overhead and the effect is maximized.

To do this accounting properly we also need to think about days like this when the sky is perfectly clear and there is no possibility of any cooling due to cloud albedo. The total effect of all factors that cause warming, like the greenhouse energy effect of precipitable water (PW), can then be expressed more clearly, with one major offset out of the way. But that’s not quite true. Based on historical averages for both, I like to compare an increase in regional PW values with any increase in temperature reported as an anomaly for the same region and same period. The trouble is that the baseline average for temperatures will include a number of days when there was cloud cover, some lighter and some heavier, as well as some when the sky was clear. This would end up by netting out the actual impact of an average amount of daily cooling, entirely because of the days of cloud coverage. That number could be significant—I am thinking of as much as two or three degrees as a possibility. Whatever the real number might be, a complete unknown, it should properly be subtracted from the current reported anomaly in order to even things up. The greenhouse energy production of PW, as described in part 1 of Carl’s theory, differs in the sense of being largely unaffected by any amount of cloud formation.

Another consideration that should be taken into account is the likelihood that the cloud albedo effect is subject to seasonal adjustments, marked by declines when the days are shorter and the sun is lower in the sky (or disappears completely). Such results can be looked for by making careful observations on the weather maps in coming months. I expect that clouds in the NH will soon begin to lose most of their current albedo power, but let’s wait and see for sure.

Carl’s theory, part 1, claims that any doubling of PW value (by weight) in the atmosphere, relative to any non-tropical location, will cause surface temperatures at that location to increase by 10C, plus or minus an allowance of no more than 2 degrees as a margin of error. The possibility of the greenhouse effect being altered by cloud formation, and larger particles that follow, cannot be dismissed, which is the main reason for having the margin of error. If anything, I would guess that the effect would be lowered a bit, especially by larger particles, but have no evidence or solid argument to offer. What’s most important about the claims made in part 1, in my mind, is that the greenhouse effect of PW is so very powerful, even if or when it is limited to only 8C when its value is doubled, along with observations of an extended sequence of actually realized short-term doublings reaching from the polar zones in mid-winter all the way to the borders of the tropical belt.

Carl