I was very interested to read the latest post over on Bob Tisdale’s blog on the abject failure of the IPCC’s models in relation to sea surface temperatures. His point is well made, so I won’t belabour it here. My purpose with this post is to spark a discussion about that most contentious of issues: the question of whether or not most of the warming of the late C20th was due not to rising co2 levels, but to a reduction in cloud cover, letting more solar radiation warm the upper ocean.
Here is Bob’s Figure 4 from his post:
Bob says of this plot:
“I used the data through the KNMI Climate Explorer so that I could change the base years for anomalies to 1995-2011. This helped to reduce the strong seasonal signal that appears in the data of some ocean basins. The North Pacific (0-65N, 100E-90W) sea surface temperature anomaly data from NOAA [which uses a 1971-2000 baseline climatology], for example, has a very strong seasonal component, as shown in Figure 4. Using the base years of 1995-2011, also illustrated, the seasonal component is drastically reduced.”
The question I want to explore here is why the strong seasonal signal is drastically reduced by shifting the baseline climatology.This is a question Bob doesn’t explore in his post, but I think it is an important question.
It seems to me that the most plausible explanation is that cloud cover was reduced over the earlier climatology period of 1971-2000, compared to the later period of 1995-2011. This would cause the ocean surface to warm more in summer during the earlier period as the brown curve on the plot indicates. That, I believe is the reason why the earlier baseline climatology shows a stronger seasonal signal. Both the ISCCP tropical low cloud cover data and the Earthshine project of Palle et al show reduced cloud albedo from the start of their series followed by an increase after 1997. This later increase would explain the reduction in seasonal variation using the later climatology baseline. However, this tentative conclusion will be affected by the question of how, and from what areas the baselines are calculated, so I hope Bob drops by to tell us more.
We have seen in earlier posts on this blog that varying sunshine hours are correlated more closely to surface temperature anomalies than co2 is. The latest was Doug Proctor’s excellent study on the issue. My earlier post on Willie Soon’s far east study is worth a look too. Tim C has hinted recently in comments that he has a major post in preparation on the same issue.
I think we are getting close to determining the real cause of the late C20th surface warming, and that this seasonal variation difference between the two climatology periods is further supporting evidence for our finding. The question then becomes: what caused the changes in cloud cover? Strong late C20th Sun reducing GCR’s a la Svensmark’s hypothesis? Latitudinal shifts in the climate circulation zones a la Stephen Wilde’s hypothesis?
A bit of both? Or something else? Please post your thoughts in comments below.