Update on What’s Going on in the Tropics – RSS
Posted by The Diatribe Guy on September 8, 2009
This is actually a fairly interesting region to observe. This, along with the Antarctic(ish) region was a contributor to the higher July anomaly.
The July anomaly was 57.90 units (where 1 unit = 0.01 degrees Celsius). This was a pretty warm anomaly, ranking 14th out of 367 total observations in the data set, and ranking tops as the warmest July on RSS record (31 years of records). The increase over last July was 54.10 units, and the increase over June 2009 was 36.1 units. It would have been a great time for your equatorial vacation if you like running around in a Speed-o (note to readers: I do not wear a Speed-o).
It should be noted that the overall average anomaly in the data set is actually 6.27, and not zero. So, if one considers the overall average to be the point of departure from which an anomaly should be measured, the values are slightly overstated. Not as much as in the Arctic, but overstated nonetheless. This may seem conniving, but I don’t read much into it. Restating all historical anomalies every month to consider the latest average makes it difficult for the casual reviewer to come to grips with trends, and since all values would be changing, some may question whether or not the change is anything more than a readjustment to baseline. No, it probably is best to make only the occasional adjustment if and when the average drifts too far from zero. In this case, I probably would leave the Tropics as is. One could probably argue that the Arctic baseline should be changed and the anomalies adjusted, but it all really only matters if you let the perception of a high number cloud the issue. Unfortunately, too many people – including those who probably know better – will tend to look at the magic number of 100.00, for example, and freak out about the high number.
One of the interesting things I noted in the anomaly record was the tendency of this region to have some persistency in relative temperature. Starting with April 2004, we saw a stretch of 9 consecutive months where the anomaly was cooler than previous year. That was followed by a stretch of 9 consecutive months with anomalies warmer than previous year. After that, we had a stretch of 12 consecutive months where the anomaly was cooler than previous year. In the next year, 10 of the 12 months were warmer than previous year. Then, we saw 12 consecutive months of anomalies that were cooler than previous year. We are now in the midst of a stretch of warmer-than-previous-year anomalies, with July being the 10th consecutive such month.
Thanks to the last couple higher anomalies, the 12-month average is 15.3, which is the highest average since the year ending January 2008. The 12-month average reached a low of -13.1 10 months ago and has steadily increased since then to current level.
The overall trend line shows an increase in warming at a pace of 1.42 degrees Celsius per Century. I’ve alluded to this in the past, but it’s worth pointing out again while looking at this longer-term chart that there appears to be a “step” that occurred around 1997/98. From 1979 to that point, there’s basically a flat trend line, then from that point to current there’s another flat trend line. So, while the trend line is positively sloped, a single linear trend line doesn’t really tell the whole story here. It’s not so much that we’ve steadily warmed as much as it shows a significant warming event that had long-lasting effects. These effects seem to be slowly fading, though.
To demonstrate the above point, the chart above shows the flat line generated by the last 13+ years of data. The Tropics simply are not heating up. The average level took a jump 12/13 years ago and we see activity above and below the line, but there is no warming trend evident, and certainly no an acceleration in the warming trend. The latest anomaly was a warm one, but if you look at its place relative to the red line, you’ll see that it’s not particularly out of place in the big scheme of things.
To get an idea of the short-term temperature activity, we look at the 60-month slopes. Such a short-term measure cannot be extrapolated far into the future, but it does give us kind of a snapshot of current conditions. At the very least, we can tell easily enough from this whether or not warming is accelerating or decelerating. If the trend line of the slopes is up, it’s accelerating. If down, it’s decelerating. The trend can be up and the slope still positive – which means that we’re still showing a warming trend, though not as dramatic as it used to be. Also, we could be negative with a positive trend line, which would say that we are cooling, but not as much as before.
However, above, we show that the trend line slopes have gone from highly positive (about 1.29, or over 15 degrees per Century warming rate) to very negative (a low of -0.81, or 10 degrees cooling per Century). We’re still pretty low at the moment, so the current state of affairs – despite the current warm anomaly – is that we’ve cooled in the tropics quite a bit over the last 6 years, at least from a trend line perspective.
I’m going to leave the narrative at that. Here’s a handful of other charts to show perspective on the longer time frames. It’s along the lines of the other charts: longer trend lines still show positive slopes to varying degrees. All these slopes are declining over time. Keep in mind there are limited data points on the longest time frames.