I’ve moved to WordPress. This post can now be found at Dip and Rebound################
More than two decades ago, the hypothesis of Anthropogenic Global Warming (AGW) seemed logical to me. I bit into it hook, line, and sinker. (See following note) Until recent times, most articles and papers presented both sides of the AGW debate. The one sentence used by skeptical authors or resources back then that always seemed like a way to avoid a concrete data-based statement was something to the effect of “The change in global temperature is well within the realm of Natural Variability.” I have mixed feelings about the term “Natural Variability” even to this day, yet I will conclude this post with it.
Note: My understanding of and position on AGW changed more than 10 years ago. I would now be classified as a full-fledged, bona fide, card-carrying AGW skeptic. Well, I’d have a card if one was available.
THE DIP AND REBOUND IN LATE 19th TO EARLY 20th CENTURY SST ANOMALY
Figure 1 illustrates Global SST anomalies from Jan 1854 to October 2008. The data is raw and smoothed with an 11-year (133-month) filter. (The reason for the 11-year filter: I was plotting Monthly Sunspot Numbers for an upcoming post and I liked the way it smoothed global SST anomaly data. No other reason.) I’ve also circled the period from 1870 to 1940, during which SST anomalies dipped and rebounded.
As shown in Figure 2, which isolates the data over the period of January 1870 to December 1939, the variation is more than 0.3 deg C. I have yet to find an explanation for the dip and rebound in any IPCC or CCSP report. Are the IPCC and CCSP taking advantage of a natural decline in Global SST anomaly to reinforce or amplify their claims of an unprecedented rise in 20th Century Global Temperatures caused by anthropogenic forcings? Of course, they are.
Figure 3 contains the same data as Figure 2, except the data has been inverted. It’s supplied simply as a reference for the next illustration.
In Figure 4, I took the inverted 1870-to-1939 data and shifted it 90 years. I also shifted the temperature range a few tenths of a degree (I did not scale it) so that there was agreement in the starting points of the rises in the two curves after 1960. Note how the two curves have very similar trends from 1960 to 2000.
Note: I do not intend the broken extension of the red curve from present to the year 2030 as a prediction of future events. It simply seemed appropriate to include it. We’ll just have to wait and see what the future holds.
Using the unexplained 1870 to 1939 dip and rebound as reference, as of now, the rise in Global SST over the term of the instrument temperature record is well within the range of Natural Variability.
Sea Surface Temperature Data is Smith and Reynolds Extended Reconstructed SST (ERSST.v2) available through the NOAA National Operational Model Archive & Distribution System (NOMADS).
There's been several studies examining why climate changed so dramatically in the early 20th century when forcing from greenhouse gases was low. Eg - Tett 2002 and Reid 1997. The dip and rebound was largely due to volcanic and solar forcing. Eg - strong volcanic activity in the late 19th century had a strong cooling effect coupled with a drop in solar activity. In the early 20th Century, volcanic activity dropped and solar activity rose.
However both these factors have had negligible effect over the past 30 years. Volcanic activity has been relatively high and solar activity has shown little to no trend over the past 30 years (if anything, a slight long term cooling).
More info on early 20th century warming here...
JC: Thanks for the links, but a decline in solar forcing can’t account for the decrease in SST anomalies--not with the present opinions about climate sensitivities to changes in TSI. For SST’s to drop more than 0.3 deg C over 40 years, TSI would have had to have dropped more than 3 times the average of the trough-to-peak range of the last 3 solar cycles. That didn’t happen.
The effects of volcanic aerosols last only for a few years and would not be able to cause the decrease in SST over the 40 years illustrated in the ERSST.v2 version of Global SST anomalies. Note the timing of the decreases in global mean forcings illustrated in Figure 1 of the Hadley Centre paper you linked. They do not coincide with the dip and rebound. Therefore, volcanic aerosols do not work. Combining solar and volcanic aerosols won’t work either, because most of the change occurs in the Northern Hempisphere, confirming that it is not related to solar or volcanic aerosol forcings.
Again, the dip and rebound remains unaccounted for by the IPCC and CCSP. Now I can add the Hadley Centre to the list. Thanks.
JC: Sorry. I missed your SkepticalScience.com link. It doesn't address the questions raised in my post. You missed the point. Try again.
Your SkepticalScience link also fails to address the contributions of natural oceanic oscillations from the 1970s to present. These include the North Pacific Residual, the Atlantic Multidecadal Oscillation, and the excessive frequency and magnitudes of El Nino events (versus La Ninas). This failing is typical of AGW proponent websites.
This dip and rebound isn't that difficult to explain. The sea level has increased over every >20-years periods since 1870 and maybe longer. This means the radiation imbalance must have remainded quite stable too, the ice-melt probably has not changed much.
The only possible explanation is that during periods of increasing surface temp the system is in a phase which favours warming of the NH while during periods of decreasing surface temp the system is in a phase which favours warming of the SH. Because of all the water and ice in SH the globe will not warm much in that phase, but there is plenty of ocean so the ocean heat content keeps rising at almost the same rate and thereby also the sea level.
That leaves the long trend to be explained and GCR seems a pretty good candidate.
lgl: I didn't say it wasn't explainable. I said, "I have yet to find an explanation for the dip and rebound in any IPCC or CCSP report. Are the IPCC and CCSP taking advantage of a natural decline in Global SST anomaly to reinforce or amplify their claims of an unprecedented rise in 20th Century Global Temperatures caused by anthropogenic forcings? Of course, they are."
I can explain it as a function of Thermohaline Circulation/Meridional Overturning Circulation in the North Atlantic and North Pacific. Again, the IPCC and CCSP don't address it. They use it to their advantage to inflate the trend in global temperature anomaly. And if they use an inflated trend for the past century as a basis for GCM tuning, no wonder actual temperatures are falling short of those projected by GCMs.
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