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This is not a very long, or complicated, paper. It’s written by Grant Foster and Stefan Rahmstorf. They set out to remove the effect of three large natural influences on global temperature variability, El Nino, volcanic and solar effects, to find the “real global warming signal.” I’m unconvinced that they find it.

Grant Foster and Stefan Rahmstorf, are both well known commentators on climate change who, if I may have the temerity to point out, are not well known for their flexible or generous approach to the various shades of opinion expressed on climate change that they don’t adhere to. Much of this research is reproduced in the website run by the anonymous Tamino, whom I have noticed does have a tendency to draw a straight lines through everything, even when it is not appropriate.

Reading the paper I felt that the introduction went a little to far. I don’t think that the ‘widespread temperature increase’ is corroborated by shrinking mountain glaciers, accelerating ice loss from ice sheets in Greenland and Antarctica, Arctic ice loss and sea level rise. I think that list covers a multitude of causes and uncertainties.

The authors refer to ‘possible recent changes in warming rates’ by which they mean post-2001. This has now an extensive bibliography on that effect so to challenge the opinion that there has been a flatlining in temperature in the past ten years or so will take a convincing analysis. Foster and Rahmstorf address the post-2001 data problem by examining data from five global temperature datasets over the period 1979 -2010.

They say that the warming trend since 1979 has been approximately linear. Now there is a statement that begs the question, and stretches the definition of the word ‘approximately.’ It is an assumption, imposed on the data.

There are clearly major parts of some of these data sets that are unchanging straight lines. I will give two examples, but there are more. Such as the UAH lower troposphere from the start of satellite data in 1978 to 1995.

Don’t forget the debate about whether the warming seen in HadCrut3 since 1995 is statistically significant. In 2010 Prof Phil Jones said no, then changed his mind in 2011 to yes (marginally) because of the relatively warm 2010. Presumably, now that 2011 seems likely to be quite a cool year the post 1995 warming will revert to insignificance. In any case, the post-2001 data is consistent with an unchanging line, and there is no scientific case, from the data alone, to introduce an additional, un-needed variable.

I said from the data alone, but this paper however is not just about the data alone. Given what I have said about obvious standstills in some of these datasets the assumption that a straight line regressed through the 1979 – 2010 data is not warranted. It comes from the theory that greenhouse gas forcing is increasing monotonically. Drawing such a straight line obliterates the information in the data contained in the standstill. I don’t think that information in the real world data, which opposes the straight line assumption, should be discarded in favour of what the authors think the data should show.

The authors make the contentious statement that removing the effects of El Nino, volcanic and the solar influence, will reveal the real global warming signal. I don’t think that is the case. There are other factors, stratospheric and oceanic, that are significant, and anyone who has been paying attention to recent developments must realise that the solar equation is rather more complex than just consideration of the Total Solar Irradiance.

Beware The Flaw Of Averages

They look at a given month’s temperature in five global temperature datasets, GISS, NCDC, CRU, RSS and UAH, and calculate an average for that month and then look at the differences. To my mind this is a tricky exercise. As we see in previous GWPF articles that look at months of the year compared to the same months in previous years, even in a warm year there can be many cool months, showing that warm years are frequently due to a few warm months and not a general uplift in temperatures. Beware averaging such data into yearly bins.

In addition, time lags between some of the natural effects have to be introduced and the ‘best’ time lag pulled out from the calculations that best fits the data. Although these time lags seem consistent with other studies selecting ‘best’ results does concern me.

Looking at their figure showing global temperatures with the El Nino, volcanic and solar effects removed I must say that I don’t think their removal has been very good, as many of the features associated with El Nino and volcanic effects are still very obvious in the processed data. What’s more, these effects are large, tenths of a degree to many tenths of a degree, and I have no great confidence that inadequately removing them to reveal a tiny ‘natural’ trend of between 0.014 deg C and 0.018 deg C a year is a robust result.

Curiously, the authors’ analysis shows that in their adjusted data set 2009 and 2010 are the hottest years. 2010 was warmer (though not statistically significantly so) than some previous years because of an El Nino. Taking it away (and adding a linear trend) seems to have made it even warmer!

In conclusion, the authors say that, regarding the global temperature of the past three decades there is “no indication of any slowdown or acceleration, beyond the variability induced by these known natural factors.”

And yet, it is still flat over timescales longer than El Nino, in the face of no recent large volcanic effects and uncertain solar effects.

Still, the results of this study are testable. “The unabated warming is powerful evidence that we can expect further temperature increase in the next few decades, emphasising the urgency of confronting the human influence on the climate,” lead author of the study, Grant Foster is reported to have said.

Putting to one side the call for political action and concentrating on the science, let’s wait and see.

Finally, they average the five adjusted global temperature datasets and say this is the “true global warming signal.” I don’t think it’s appropriate to add and average them, but by the time I had got to the end, I almost didn’t care anymore.