If only there were some way to distinguish between the internal variability of the climate system and external forcings, and moreover to discriminate between different types of natural variability and external forcings in order to identify specific drivers of specific aspects of the climate system. Some sort of detection and attribution, if you will. Oh, wait!The climate system responds to changes in variables like solar variability, greenhouse gases, surface reflectivity/aerosol loading, orbital mechanics, etc. There is also a significant degree of internal variability driven by coupled ocean-atmospheric dynamics (think El Niño). These things can change the globally averaged surface temperature. That there are many possible drivers of surface temperature change might lead people to believe that the "why" of the current multidecadal warming trend is unsettled or in doubt.But this fails from two different directions:First, we know what these variables are doing with a surprisingly high degree of confidence. We've got satellites monitoring solar activity, we can calculate our orbital variations, we can differentiate between anthropogenic and natural sources of GHGs, etc. We can observe that these other drivers of climate are not responsible for the warming because they're of the wrong sign, magnitude, or both. Solar activity contributed to a moderate of the warming in the first half of the 20th century, but all solar trends have been flat or in opposition to temp in recent decades. Orbital forcing not only operates on timescales orders of magnitude too slow to be driving the current warming, it should be cooling the high latitude Northern Hemisphere, which it had been prior to anthropogenic warming. We have a good record of what many of these drivers have been doing not only during the period of instrumental observation, but well before.Secondly, good science is predictive, and climate science is no exception (despite what AM radio would have one believe). If enhanced greenhouse warming were taking place, this creates testable predictions; enhanced greenhouse warming should have a different "signature" than other drivers. Under enhanced greenhouse warming, we would expect the surface to warm, but not the atmosphere all the way through, as we would under increased solar warming. We'd expect a raising of the tropopause, stratospheric cooling, and contraction of the upper atmosphere. 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Courtesy of Andrew Snow