360-Month HadCrut trends as of April month-end 2011
Posted by The Diatribe Guy on June 29, 2011
Today I present the 360-month charts using the most recent HadCrut data.
The 360-month trend line is positive, with a slope of 0.1356, which translates to a temperature change of approximately 0.49 degrees Celsius over the course of 30 years.
Now that I’ve looked at all the different charts and see warming of 0.49 degrees over a 30 year period, here’s how it appears to break down by period:
May 1981 – Apr 1986: +0.10 degrees Celsius contribution to increase
May 1986 – Feb 1991: +0.05 degrees Celsius contribution to increase
May 1991 – Apr 1996: +0.24 degrees Celsius contribution to increase
May 1996 – Apr 2001: +0.17 degrees Celsius contribution to increase
May 2001 – Feb 2007: -0.18 degrees Celsius contribution to increase
Mar 2007 – Apr 2011: +0.11 degrees Celsius contribution to increase*
* – Due to an original error in the presented 60-month chart only being 50 months, the dates do not represent an equal 5-year span. The actual 5 year span was near zero. The previous 5 year period would adjust accordingly.
I’ve discussed the “step function” that seems to have taken place in 1996-97. While not presented here, a much better fit to describe the “trend” in global temperature, simply by observation, is to fit a line from 1981 to 1997-ish and then from that point forward. The best fits of those independent lines would show a very notable jump occurring at that point.
In the chart below, we see the latest series of 30-year slope values since 2003. There was a steady decline in the slope value into 2009 that met the .1300 line, but has since increased to the current levels. The trend line is responding to a “fulcrum” of sorts. What I mean by that is that, even though lower anomalies are dropping off the left side of the chart (which normally would imply a lowering of the trend line) this is outweighed by anomalies on the right side of the chart staying up around the last decade’s average, and this is because of that step function that is now reaching the mid-point of the chart. If future anomalies stay around where they are at now, then when that fulcrum point moves to the left side of the chart, we will see that trend line slope value decline. We wouold not expect to see the 30 year trend line go negative unless there is a prolonged stretch of declining anomalies. Even in that scenario, it will take years to move that line around.
The entire history of 360-month slopes is shown in the chart below:
There has not been a negative trend line in the 30-year charts since the period ending November 1972. We won’t reasonably be expecting a negative trend line for another few years, even under a cooling scenario. What we can probably expect to see, however, is the slope in the trend line start to decrease soon. The slope will probably be fairly steady over the next 3-5 months, and then we’ll see some lower values emerge. In my estimation (assuming average 12-month anomaly values) we’ll see the slope on the trend line fall below 0.13 around march 2012, thus continuing the downtrend and slope values we’ve seen since 2003. We haven’t seen a slope under 0.13 in the 30-year line since April 1999.