The daily weather maps that I constantly refer to for temperature anomalies have a baseline period of 1979-2000, which is only three decades, using 1990 as the mid-point of the averages. During those three decades we know from the consistently low daily anomalies that the SH as a whole has warmed up only slightly, the NH has warmed by a full degree and the tropics by about one-half of one degree. For just the last few days the Antarctic zone has been running a fever while the Arctic shows a net cooling. Consistently, day after day, the two are reversed, with the Arctic up by maybe a couple of degrees and the Antarctic more often down than up. They both tend to be highly erratic from any one day to the next, unlike the other regional measures or the globe as a whole. The reading for the full globe just summarizes the mean of the two hemispheres and ends up at close to +0.5C day after day. There is a set of charts on Hansen’s website that covers all these regions using monthly summaries of temperature data instead of daily, and extending back in time as far as 1880. (Baselines become irrelevant because anomalies can—and must—all be calculated from the temperature data.) These charts tell us that the daily anomaly summaries we look at all provide good results for the three decades they cover, consistent with results from other methods. These charts also tell us that prior to 1990 the two hemispheres were much more alike in their temperature trends. Everything changed around 1990.
We certainly want to know everything that may have contributed to the divergence between the SH and NH since the 1990s. It has clearly affected the mid-latitude regions in the NH to almost the same extent as the polar zones, which looks unusual from a historical standpoint. Something fairly radical must have happened at around 1990 that serves as an effective cause, and it would apparently have historical significance. If you look closely at the two mid-latitude hemisphere trendlines you will see that the trend in the SH did not shift much at all after 1990, while mid-latitudes in the NH have kind of gone crazy. This trend has emulated the extremely fast trend of the Arctic zone, but only about half as fast in rate of gain. Both of these are very worrying. The big question is this: how far into the future will they stay on this track, which gives no current sign of diminishing?
I want to show another chart today, comparing global land and ocean surface temperature anomalies since 1880. We can see how their trends pretty much stayed on the same track prior to 1980. Since then the oceans, on average, have gained about one-half a degree while land surfaces have gained about 1.4 degrees. For oceans, the trend for the entire last century has hardly shifted, while land, by itself, has done all the accelerating. Again, looking ahead to the future, how well locked-in are these two trends?
There is still one more map that should be displayed today, in conjunction with the images above, because it contains some interesting and quite unusual information that may or may not help to explain everything else that is going on. I want to think about it for awhile before trying to reach any sort of conclusions. This map shows sea surface temperature anomalies over the past 3 1/2 decades, from a 1971-2000 baseline period centered in 1985. The numbers at the bottom tell us that sea surface temperatures in the SH have risen an average of 0.2C during this period, which is at least as much as the SH as a whole has gained—including land. This is in spite of the cold anomaly in the Pacific, currently burdened by a large body of intensely cold surface water occasioned by episodically strong La Nina conditions, and also by a growing amount of cold meltwater and chunks of floating ice coming off the coasts of the Antarctic continent. The NH is quite different—sea water, +0.5C, has warmed up only half as fast as all NH surfaces, +1.0C, during this brief period. The North Atlantic surface, by itself, seems all out of proportion with its increase of +0.8C in just 3 1/2 decades. All of these things are puzzling, and call for a correct explanation.
Carl