There is a recent post on Watts Up With That (WUWT) called are we in a pause or a decline (now includes at least April data). The answer, in my scientific opinion, is neither. I don’t quite know what argument is being made in this post but it seems to be that the author thinks we could fit a sine wave to the temperature data and hence the fit suggests we’re about to enter a cooling phase. Of course, even underneath this sine wave is an underlying warming trend, but there’s no mention of that in the post.
I was going to address one particular thing. The post goes on to say
For RSS the warming is not significant for over 23 years.
For RSS: +0.123 +/-0.131 C/decade at the two sigma level from 1990
For UAH the warming is not significant for over 19 years.
For UAH: 0.142 +/- 0.166 C/decade at the two sigma level from 1994
For Hadcrut3 the warming is not significant for over 19 years.
For Hadcrut3: 0.092 +/- 0.112 C/decade at the two sigma level from 1994
For Hadcrut4 the warming is not significant for over 18 years.
For Hadcrut4: 0.093 +/- 0.108 C/decade at the two sigma level from 1995
For GISS the warming is not significant for over 18 years.
For GISS: 0.103 +/- 0.111 C/decade at the two sigma level from 1995
For NOAA the warming is not significant for over 18 years.
For NOAA: 0.085 +/- 0.104 C/decade at the two sigma level from 1995.
Notice that the statement made is “not significant“. This statement is simply wrong. The above numbers are from linear regression of the various temperature anomaly data sets. In each case, the best-fit line has a trend of close to 0.1oC per decade. However, this data is quite noisy and so there is an uncertainty in the trend and what is presented above is the 2σ errors on the trend line. This means that there is a 95% chance that the actual trend lies between trend-error and trend+error.
That for all the intervals considered above (which aren’t the same for each dataset) the 2σ errors are larger (in magnitude) than the magnitude of the trend, means that we can’t rule out (at the 95% level) that there hasn’t been cooling. However, we also can’t rule out that the actual trend isn’t twice as big as the best-fit trend. If the author of this post thinks they can use this analysis to claim there’s been no warming, then I could use exactly the same argument to suggest that it’s been warming very rapidly. Neither is the correct interpretation of this analysis.
This analysis does not tell us that the warming is “not significant“, it tells us that it is “not statistically significant“. These are very different things. The term “significant” implies there has been no warming. The term “statistically significant” means that we can’t say, with certainty, what it is. The data, however, suggests that it is most likely close to 0.1oC per decade. This is also not surprising. You can go to the Skeptical Science trend calculator and see how many years of data are typically required for a statistically significant trend to be determined. It’s typically in excess of 10 years and often close to 20 years. That the trend for the last 18/19 years is not statistically significant does not mean that there is a good chance that there’s been no warming. It’s simply a consequence of the requirement that we need to consider sufficiently long time intervals if we wish to accurately determine the trend in the temperature anomaly data.