240-Month HadCrut trends as of April month-end 2011
Posted by The Diatribe Guy on June 24, 2011
Today I present the 240-month charts using the most recent HadCrut data.
The 240-month trend line is positive, with a slope of 0.1399, which translates to a temperature change of approximately 0.34 degrees Celsius over the course of 20 years. This is fairly significant warming over that period. Since the 15-year chart shows less than a third of that amount in total warming, the majority of the slope value is attributable to the lower anomalies in the first five years. The current slope value of the 20 year trend line is at about the same level as it’s been for the last year and a half or so. It would certainly appear that if temperatures do not dramatically increase, the slope value will decrease as some of those lower anomalies drop off. Every single anomaly for the next three years is lower than every anomaly since mid-2001. However, since no anomalies have since exceeded the peak value in 1998, the trend line certainly looks to have a high probability of decreasing in slope.
CORRECTION ON CURRENT FLAT/COOLING TREND
As I was reviewing the above chart, and eyeballing the data since 1997, I was a bit surprised that there wasn’t a trend line that was flat or negative going back to 1997/98. So, I went back to the data and am embarrassed to say that there is. In my first post back after a long hiatus, I didn’t offer my best work, apparently.
Extending a trend line backwards from April month-end, the best fit trend line does in fact switch from negative to positive between July and August 2000. What I missed was that there is a stretch from May 1997 to November 1997 where the best fit trend line is negative. In other words, we can extend a negatively sloped trend line back in the HadCrut data to May 1997. That is exactly 14 years (which better explains why it looks like the 15-year trend line will go negative within the next year).
I know that there are issues with linear fit and all that stuff, but when the current line goes back 14 years, you’d think that would make some people pause a bit and wonder where this exponential warming is.
My apologies for the erroneous statement in my initial post of this series. I suppose I’ll need to update that chart with May month-end data.
In the chart below, we see the latest series of 20-year slope values since 2004. There was a steady decline in the slope value until late 2009, at which point it’s vaccilated around where the current value is. Unless temperature anomalies start to meet and exceed the 1998 peak, we can expect a continued decline in the slope value. Since longer-period slopes take a longer time to adjust, we will need to see anomalies around current averages for the next 5 years or so before we’d see an actual declining trend line. If temperatures drop, this would obviously accelerate. Likewise, if temperatures spurt upwards, we may not see a negative trend line at all in the foreseeable future. Even under a very aggressive cooling scenario, it would take nearly three years to see a negative trend line. So, like it or not, global warming proponents will be able to use 20+ year trend lines to make their point for a few years.
The entire history of 240-month slopes is shown in the chart below, with the following observations:
There has not been a negative trend line in the 20-year charts since February 1979. Now, more than 32 years later, we will not reasonably expect any possibility of seeing a negative slope for at least another 3 years.
Looking at the history, the last prolonged stretch above zero trend started in October 1919 and lasted until August 1950. That was a stretch of almost 31 years, a length of time we’ve already passed. It will be interesting to see if a downward plunge is in store for us, as a cyclical mindset might think is the case. However, even with a cyclical mindset, it has to be recognized that we’re comparing to exactly one previous similar cycle, so it’s hardly a credible sample.