Via +Ken Burnside.
Someone on Facebook asked me today how we can be sure that human emissions are actually important factors in driving the observed warming trends rather than inconsequential in the face of natural variation.  Here's the answer I gave, archived for posterity.

OK, so let's see what I can say here. First, I agree that trying to tease this stuff apart is really tricky. The global climate is a monstrously complex system, and attributions of single causes are notoriously tricky in cases where there are a lot of interlinking feedbacks operating on multiple different scales. However, there have been some efforts to quantify the kind of thing you're interested in, I think.

Jim Hansen pioneered this back in the mid-90s, but our models have improved A LOT since then, so let's stick to more recent stuff. One of the more prominent recent pieces looking at this is "Combinations of natural and anthropogenic forcings in twentieth-century climate" by Gerald Meehl et. al. in the Journal of Climate in 2004 (http://journals.ametsoc.org/.../1520-0442%282004%29017<3721%3ACONAAF>2.0.CO%3B2). They used an ensemble of CGCMs to try to hindcast the observed temperature trends of the 20th centuries using four different scenarios. The first three included the various natural forcings only (volcanic aerosols, varying insolation, &c.), while the last included both the natural variations and anthropogenic GHG emissions. They were unable to reproduce the observed trends without accounting for anthropogenic emissions, and including those emissions resulted in ensemble predictions that matched the observed data almost perfectly. I've got the graph attached here.

We're able to discern anthropogenic GHG emissions from natural emissions of the same compounds by tracking the relative prevalence of different isotopes of carbon in CO2 molecules in the atmosphere. Because fossil fuels are composed of decomposed organic matter, they have a distinctive ratio of carbon-13 to carbon-12 that's not found in other sources. By tracking changes in the observed ratio in the atmosphere, we can get a fairly good sense of what proportion of CO2 is coming from us and what proportion is coming from other sources, giving the picture that Meehl found in his study.

Climate models also predict that if a large portion of the shifts in temperature ranges at the regional level are due to anthropogenic GHG emissions, we should also see a reduction in the difference between maximum and minimum daily temperatures, a value called the "diurnal temperature range" (DTR). We wouldn't expect to see a significant shift in the DTR if warming trends were the result of natural forcings alone, as natural forcings would alter temperatures more-or-less uniformly or (depending on the forcing) would have a much larger positive effect on the maximum temperature. Multiple data sets have shown that observed DTR in the last 50 years has narrowed significantly, and that there's been a relatively larger increase in minimum temperatures than in maximum temperatures, a fact which again can't be explained without reference to the impact of anthropogenic GHG. This is a good roundup of those studies: http://onlinelibrary.wiley.com/doi/10.1029/2004GL019998/epdf

One of the most important thing about the DTR as a fingerprint is that variations of DTR are more-or-less independent of variations in global mean temperature, as increases in the global mean could be accounted for by a lot of different combinations of changes in the DTR in different regions (a large increase in daily maximums in an isolated region will artificially inflate the global mean, for instance). However, we see a fairly constant increase in daily minimum temperatures in geographically disparate regions, and a consequent shrinking of the average DTR. This can't be accounted for without including anthropogenic GHGs, and is an important independent confirmation of humanity's impact.

Yet another independent measure is the variation in the observed wavelengths of incoming radiation. Because the physical basis of the greenhouse effect is (as I'm sure you know) quantum mechanical, different molecules absorb and reradiate energy at distinctive frequencies because of differences in their structure. By using a spectrometer, we can figure out how much of the incoming radiative forcing is due to insolation itself and how much is due to the reradiative effect of different GHGs. If anthropogenic emissions were a major factor in planetary warming, we'd expect to see a significant contribution to overall radiative forcing coming from the molecular constituents of anthropogenic GHGs. This is exactly the observed result. In "Measurements of the radiative surface forcing of climate," (2006), Evans and Puckrin observed that anthropogenic GHGs were responsible for a radiative flux increase of 3.52 W/m^2, which is hugely significant (and in line with model predictions). I can't find a copy of the paper that isn't behind a paywall, so I put it up on my Google drive here: https://drive.google.com/.../0B4wR7F7Si7hsaUE3elB.../view...

Those are three independent measures for estimating the human emission impact on observed temperature trends, and all three agree in their attribution. There are other independent measures as well that I can talk about if you want (atmospheric temperature gradient, wavelength flux in upward IR radiation) that also agree, but I think I've said enough here already, and I'm hungry. 

All that said, I also agree with you that what exactly should be done about this is a much more challenging question to answer than is the question of whether or not human activity is a major driver in climate change. I think reasonable people can have reasonable disagreements about what the optimal policy strategy is, and I also agree that some proposals for dealing with it are likely to do more harm than good, at least by some measures. However, that's a completely independent question from the question of whether or not anthropogenic GHGs are actually major factors in climate change, which has been definitively answered in lots of different ways.

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