Sunday, 1 January 2012

Foster and Rahmstorf 2011 - The True Global Warming Signal

Everyone knows that the temporary flickers in the global temperature curve are the marks of El Ninos, La Ninas and volcanic explosions. The slower variations are the symptoms of changes in the overall forcing due to the vagaries of solar output, accumulations of greenhouse gases in the atmosphere and others, or they arise as natural variations internal to the climate system.
Disentangling these effects is not easy. In fact it is not at all clear that they can, or want to be disentangled. What if high latitude aerosol forcing caused a slowdown in the meridional overturning circulation that projected on to patterns of internal variability such as the Atlantic Multidecadal Oscillation (AMO)? Would it be correct to attribute such a drop to internal variability or to anthropogenic forcing? Likewise should one separate out the thermodynamic effects of large volcanoes from the dynamic effects such as the hypothesised link with El Nina and the North Atlantic Oscillations? These questions cannot be answered categorically because each interpretation presupposes a different set of questions. On a basic level, if El Nino increases global temperature, what does global temperature minus the effects on it of the El Nino Southern Oscillation (ENSO) actually measure? What does it represent?

FR11 preform a multiple regression of a linear trend, annual cycle, ENSO, Total Solar Irradiance (TSI) and Aerosol Optical Depth (AOD) on global mean temperature and, having removed the ‘exogenous’ effects of ENSO, TSI and AOD claim - and claim here is the key word - that what remains is the “true global warming signal”.
On the plus side, they consider a range of different data sets - GISS, NCDC, HadCRUT, RSS and UAH - as well as looking at alternative measures of ENSO. They use MEI for their main analysis but also looked at SOI. They also looked at alternative measures of TSI (sunspots as opposed to TSI) and volcanism (Ammann as opposed to Sato). In principle such switches explore the sensitivity of their results to such choices. In this case “None of these substitutions affected the results in a significant way, establishing that this analysis is robust to the choice of data to represent exogenous factors”.

However, once started down that road it doesn’t pay to stop too soon. Why, for example were the lags of each predictor allowed to be decided by taking the value that gave the best fit to the data? If each GMT data set is a fair measure of global temperature then this ought to give some idea of how uncertain the lags are and one ought to play mix and match – take the lag that give the best fit for GISS and use it for HadCRUT for example. What of other measures of ENSO such as those provided by tropical Pacific SSTs or that used by Thompson et al. (2008 and later) (similar series to SOI, MEI etc) and Compo and Sardeshmukh (very, very different). Or other measures of TSI reconstructed from the same satellite data which show different mid to long-term trends.

One might plausibly argue for or against certain choices, but to do so would require a greater understanding of what the resulting signal is intended to measure but this is not provided. The considerations are further muddled by combining surface temperatures with tropospheric temperatures. They are quite different things.

A clue to what FR11 intended to extract is seen in their interpretation of the results. They claim that

The resultant adjusted data show clearly, both visually and when subjected to statistical analysis, that the rate of global warming due to other factors (most likely these are exclusively anthropogenic) has been remarkably steady during the 32 years from 1979 through 2010.

This suggests that FR11 have some concept of an underlying global temperature trend, largely anthropogenic, that might be revealed if we could somehow contrive to run the latter half of the twentieth century over again and again to obtain some kind of ensemble average. This hypothetical – and unmeasurable – standpoint stands in opposition to the somewhat more literal interpretation that global warming is simply a rise in global mean temperature. Other definitions – many definitions – have been offered. The FR11 interpretation is roughly in accord with the mental model shared by many climate scientists, that global temperatures are essentially on the up, but overlain on this monotonic rise are more rapid fluctuations associated with a variety of factors.

Taking too narrow a view can potentially make some apparent headway however, in this case, what to make of :

There is no indication of any slowdown or acceleration of global warming, beyond the variability induced by these known natural factors.

This latter question of whether or not there has been a slowdown in warming has mildly exercised all manner of minds over the past few years and it rather depends on precisely what is meant by global warming. The two factors discussed above prevent FR11 from definitively answering this question. First what does this index measure and second, how well does it measure it?

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