There was an item on BBC news last night about the Met Office’s revisal of the the temperature trend for the next few years. They were predicting that the period between 2012 and 2016 would be higher than the long-term mean by 0.54oC, with a range of 0.36-0.72oC. It has now been revised to 0.43oC with a range of 0.28-0.59oC. What was a little frustrating was a comment made by David Shukman on the BBC news last night that this would mean 20 years of no significant warming. This is, I imagine, based on the claim that there has been no significant warming for the last 16 years.
As I pointed out in in an earlier post, there is a difference between the trend not being statistically significant and there being no significant warming. Given that the trend appears to be between 0.1 and 0.2oC per decade and the scatter in the anomaly data is about 0.1oc, the error in the trend over a period of about 15 years is likely to be about 0.15oC per decade. It will be similar to, or bigger than, the trend and hence, if one only consider a 15 year period, the trend will not be statistically significant. This doesn’t mean that there isn’t a warming trend. It just means that we can’t yet measure it. We would need to probably consider 20 years or longer to get a statistically significant result. In 4 years time, if we were to consider the previous 20 years, the error will be smaller than it currently is when we only consider 16 years of data, and the trend will probably be statistically significant. We don’t know this for sure, but we do know that 16 years is too short a time to determine a statistically significant trend.
I thought I would show this by considering the data that exists for the last 132 years. The data I’m using is the GISS Surface Temperature Analysis data from NASA. This gives the monthly temperature anomaly since 1880. Although I’ve written a code of my own to analyse this data, for this quick test I’ve used the Skeptical Science Trend Calculator. I’ve considered a series of 15 year intervals, starting in 1885 and progressing through to today. The values are shown in the table below. With the exception of 1985-2000, the trend in every interval is statistically insignificant. These are 2σ errors which means that there is a 95% chance that the trend lies between the (mean value – error) and the (mean value + error). Given that the error is bigger than the trend, we can’t rule out – over each of these 15 year time periods – that that the global surface temperature didn’t decrease. Even the 1985-2000 interval produces a trend that is only marginally significant.
Therefore if, at any time, we were only to consider the previous 15 years (I know this isn’t 16, but it’s close enough) we would always conclude that there is no statistically significant warming trend. The bottom row of the above table, however, shows the warming trend from 1880-2012. It is 0.064oC per decade with an error of 0.007oC per decade. This is clearly statistically significant. Below I repeat the figure that I included in my previous post which shows the anomaly data from 1880-2012 together with the best-fit trend line. It’s very clear that there has been warming since 1880 and that the mean surface temperature is about 0.85oC warmer today than it was in 1880 (which is the same as 0.064oC per decade over a period of 13 decades).
What I’m trying to get at here is firstly that just because something is not statistically significant, does not mean that it is not significant. It just means that we don’t yet know with any certainty. Secondly, if one is going to make claims about the significance of global warming one needs to use a time interval over which a statistically significant trend can be determined. If we only ever consider the past 16 years then the measured trend will always be statistically insignificant even if there is real, long-term, warming trend. It’s a very important issue and I’m more than happy to discuss and debate this with others. I’m just not willing to do so with those who misuse, or don’t understand, the data or the anaylsis.