300-Month HadCrut trends as of April month-end 2011
Posted by The Diatribe Guy on June 28, 2011
Today I present the 300-month charts using the most recent HadCrut data.
The 300-month trend line is positive, with a slope of 0.1311, which translates to a temperature change of approximately 0.39 degrees Celsius over the course of 25 years. It’s kind of interesting to see the breakdown of the warming by looking at the different trend-lines. The 15-year chart ending in the current period attributes 0.10 degrees in temperature change in total over the last 15 years. The 20-year trend line tells us that total warming over the last 20 years equals 0.34 degrees. So, in that five year period from 1991-1996, we see a period of anomalies that were lower than both preceding and succeeding periods, which drives the slope higher on that trend line and tells us that 0.24 degrees of the 0.34 degrees can be blamed on that time period’s lower anomalies. The current slope value of the 25 year trend line adds 0.05 degrees to the total warming, so the 5 year period from 1986-91 only accounts for a total of about 0.01 degrees Celsius per year.
In the chart below, we see the latest series of 25-year slope values since 2007. There has been a steady decline in the slope value, with only a short lull (flat) in 2008. In the last half-year, the rate at which the trend line is decreasing has accelerated as some older, lower anomalies drop off the front end. The last time the slope value has reached current levels was January 2002.
In all likelihood, the 25-year trend line will remain positive for the next few years to come. In a scenario of just using a 12-month average going forward, the trend line would not become negative until 2020. Obviously, if temperature readings decline that would accelerate. If temps drift upward, it’s possible that we won’t see a 25-year trend line reach zero or negative at all.
The entire history of 300-month slopes is shown in the chart below:
There has not been a negative trend line in the 25-year charts since March 1969. Now, more than 42 years later, we will not reasonably expect any possibility of seeing a negative slope for at least another 5 years or more. Things got real close in 1976, but at its low point the trend stayed slightly positive.
Looking at the history, the last prolonged stretch above zero trend started in January 1922 and lasted until July 1954. That was a stretch of 32.5 years, so we are well past that at this point. We are in a period where the current retracement is the largest since 1976, but despite that we are still well above the zero line.