120-Month HadCrut trends as of April month-end 2011
Posted by The Diatribe Guy on June 22, 2011
Today I present the 120-month charts using the most recent HadCrut data.
While 10 years still doesn’t have the significance that longer term trends will have, there is some value in looking at the last decade, at least in the sense that if temperatures don’t appreciate significantly for a 10 year stretch or more, it allows for a bit more of a skeptical eye towards claims of future temperature increases of 1, 2, or 5 degrees by the end of the century. I already showed in my “Overall” post that temperatures have been flat for almost 11 years.
The 10-year trend line is negative. Again, this is a shorter term line that can adjust between positive and negative, so fluctuations are to be expected, and a negative trend line isn’t at this point indicative of a trend that we could feel confident projecting forward. Whereas the trough in the anomaly measure at the end of 2007 helped spur on a positive trend in the 60-month line, the same phenomenon coupled with lower anomalies very recently are helping to pull that best-fit trend line down.
Much more significant in a longer term view of what’s been happening over the last few years with the 10-year trend lines is the next chart, which plots the slope vlaues since 2002. Even though the 10-year trend line has been positive for most of that time period, the slope value has steadily declined (very steady, in fact, shown by the very high R-squared value). So, even though (until a few months ago) the trend line showed warming, it showed warming at continually lower rates. This is quite different from the view of acceleration in the rate of warming. This indicates the exact opposite.
Over the last few months, the trend line took another plunge into negative territory. As those troughs in the chart above move to the left side of the chart, we can expect the trend line to flatten, and unless there are continually lower troughs on the way, the trend will likely be positive again within the next couple years. The question is whether that will be the start of a continually increasing slope or if that will be another soft bump on the way to lower trend readings.
Finally, I present the entire history of the slope values to put the current downtrend into perspective. The current value of -0.0578 is a bit difficult to see on the right side because I have a thicker border obfuscating that, but there are some interesting observations to be made:
1) The last negative slope reading occurred with the period ending May 1997. This was a very short-lived stay in negative territory. The 10-year slope was only there for 6 months and never reached a value below -0.01.
2) The last time the slope was as negative as it is now occurred with the period ending April 1977 – exactly 34 years ago. Once that stretch of negative trend lines ended in August 1979, the period of almost unbroken strings of positive 10-year trends continued with that brief exception noted in (1) until November 2010. That’s a period of 31 years, 4 months.
3) A similar period began in January 1915. With the exception of a brief and shallow period of negative trend lines in the 1920s (lasted 5 months) there was continued positive 10-year trend lines through July 1946. That is a period of 31 years, 7 months.
4) At the end of the period in (3) we saw the steepest decline in temperatures (on a 10-year trendline basis) in the history of the chart going back to 1850. The negative trend lines continued through October 1957, a period of about 11 years. I have no basis to say that history will repeat itself, but i do find the parallels in the chart interesting.