Friday, March 13, 2015

Surely you happened to have heard statements such as

Climalteranti.it "Global Warming: significant increase, break or something?
The 'World Meteorological Organization has just announced that "the year 2014 draws to a close it seems started to be the hottest or one of the hottest series"; it seems to me appropriate to take a look at recent changes in global temperature of the planet. I will make use of the excellent tool temperature display and calculation of trend of Kevin Cowtan to present pom some figures. Data is the Hadley Climate Research Unit (HadCRUT4), which owns the 'more sophisticated algorithm to fill the intervals of missing data, especially in the Arctic, resorting to the aid of the satellite. Of course, the same conclusions can be drawn using other databases. Let's start from analyzing the entire data series, which began in 1979 when satellite data have become available pom (and in any case shortly after the start of global warming).
Fig. 1. Average monthly prices (blue crosses), 12 months moving average (red line), linear trend (blue line station) and uncertainty interval (blue lines thin side) global temperature from 1979 to present.
Surely you happened to have heard statements such as "did not occur any warming from 1998 onwards." Let's take a look then at the temperatures from 1998 (the year that most deviates from the blue line of the trend in the graph above.)
You notice a positive trend (blue line) to +0116 C per decade; the assertion that there has been no warming is so wrong. But it is a significant heating? The confidence interval pom of the trend ( 0.137) would suggest not, as the trend may be +0.25 C or even zero or even slightly negative. So we are sure that there is a positive trend of warming?
This reasoning pom is wrong because it misunderstands the meaning of the confidence interval. The confidence intervals do not tell us if the heating took place - in our case certainly - and not have to deal with the uncertainties of the measure (which are very low). They tell us, however, if we can or not reject the hypothesis (the so-called null hypothesis) that the warming trend observed is due solely to random variability (such as that of a linear trend in which certain values vary randomly.) In conclusion, the intervals confidence (and their statistical significance) does not tell us whether the phenomenon of global warming has taken place or not, but simply What is the possibility that it happened by chance.
In the interval 1999-2010 the warming trend is actually higher than the long-term trend (0.175 C per decade). Nevertheless it is not statistically significant. This has nothing to do with the fact that the trend is declining; is instead pom a consequence of the fact that the confidence interval is increased in that interval pom of time. This may simply be due to the brevity of the time. In fact, in a short interval the random variability may have greater weight in a longer period. (If today is warmer than yesterday by 5 C, of course, means that the temperature is increased. But this does not mean that the fact is statistically significant because it could be due to the natural variability of the weather.)
The teaching that goes with it is that you have to use a sufficiently long time intervals, as in Figure 1, to highlight the true signal of climate change, pom net of short-term noise. All are aware of what climatologists and the IPCC said it clearly. The "climate skeptics" are always pom looking for outliers within a range of data noisy in itself, to be able to make claims against global warming. In this regard, see my 2009 article in the Guardian: Climate skeptics confused the public by focusing on short-term Fluctuations, where I discuss the misleading statements by Björn Lomborg about the sea level.
However, the media does not like to debate whether or not there is a warming trend since 1998 that poses outside of natural variability. The question arising is instead: warming after 1998 is significantly slower than the long-term trend? Or in other words: the difference could be due purely to chance pom or to natural variability? The answer is clear: the value of 0.116 C / decade for the period after 1998 is not significantly different from the value of 0.179 C / decade for the period from 1979 to today. Just look at the confidence intervals. The difference is well within the range of variability of short-term time series. (Obviously climatologists are also interested in understanding the physical mechanisms

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