Responding to the CO2 Coalition's "Facts #13 and #16" on Whether Current Warming is Unprecedented

CO2 Coalition's "Fact #13" and  "Fact #16" are essentially identical. Fact 13 claims that temperatures changed dramatically before human activity, and Fact 16 claims current warming isn't unprecedented. Both use the same graph. Fact 16 claims to be the first of three parts, but these parts aren't given sequentially. The other two of the three parts are "Fact #17" and "Fact #26." These two "facts" are (like many others) lifted from Gregory Wrightstone's self-published book, which I've reviewed elsewhere. The text of this "Fact 16" is not identical to the book, but the content is basically the same, and some of it directly quotes the book. We're told that the current warming period is "very similar to nine other warming trends of the last 10,000 years. For more than 6,100 years (or 60%) of the current interglacial warm period, the temperature was warmer than it is today. Of the nine earlier significant periods of warming since the end of the last ice age, five had higher rates of temperature increase and seven had larger total increases in temperature. Moreover, each of the previous warming cycles experienced significantly higher temperatures than today." This is the graph that CO2 Coalition thinks supports their claim.

Astute observers will note that this time series is the GISP2 ice core published by R.B. Alley[1], which is a proxy that approximates the temperature at the Greenland summit. It does not represent global temperatures at all. It also isn't adjusted for elevation changes. The "warming periods" in this graph are not global warming periods at all. They actually virtually disappear even if you use a multiproxy reconstruction of Greenland. Vinther's reconstruction used multiple ice cores to reconstruct temperatures across Greenland, and all these "warming periods" essentially disappeared. What remains is a warm HTM about 8000 years ago and slow cooling following insolation changes due to Earth's orbital cycles. So these warming periods are not valid globally; they aren't even valid for Greenland as a whole.   

Reconstructions of global temperatures show current warming to be essentially unprecedented for the last 10K years or more. There is some disagreement between reconstructions using only proxy data and those that use reanalsyses constrained by proxy data (referred to as data assimilation or DA). Proxy-only reconstructions tend to show a warmer HTM than DA reconstructions, but even in reconstructions that show the warmest HTM, like Marcott 2013[3], current warming exceeds anything seen during the Holocene. In another post, I go into the various reasons why we can be confident that current global warming is indeed unprecedented for the Holocene. But you can see how current warming compares to Marcott's reconstruction for the Holocene with 1-sigma confidence intervals below.

This should let us see pretty clearly that CO2 Coalition's claim that the rate of increase is higher than current warming in 5 of the 9 previous warming trends is completely made up. These warming increases are localized to the Greenland Summit and are not even remotely consistent with changes in global temperatures.

CO2 Coalition's Misuse of GISP2

We should note one other failure in these graphs. Gregory Wrightstone plotted "current" temperatures from a study (Box 2009), which Wrightstone misplotted. As explained here, the 2000-2009 average temperature at the GRIP site was -27 C, and that was 1.44 C warmer than the 1850-1859 average, according to Box 2009. The problem is that there’s a mismatch between the Alley’s 2004 temperature for 1855 and the the one reported by Box. But even adjusting for that mismatch, the 2000-2009 average is still warmer than what is reported in Wrightstone’s graph. Besides, even we were to take the warming rates in Wrightstone's graph at face value for the first 9 warming periods, warming rates are always less than 0.1 C/decade. Current warming rates globally exceed 0.2 C/decade with the Arctic warming exceeding 3x faster than the global average. In other words, even if we were to take at face value Wrightstone's calculations of warming rates from the GISP2 ice core, they are all less than half of the current global warming rate.

CO2 Coalition's claim to "fact" here is wrong on two counts. First, they confused local with global temperatures (a common trick among contrarian). And second, they didn't even evaluate Greenland Summit temperatures compared to current temperatures accurately. A proper analysis of global temperatures shows that current warming is happening at rates unprecedented during the Holocene, and the 21st century is now likely experiencing warmth greater than the warmest years of the Holocene.

References:

[1] Alley, R. B. (2000). Ice-core evidence of abrupt climate changes. Proceedings of the National Academy of Sciences, 97(4), 1331 LP-1334. http://doi.org/10.1073/pnas.97.4.1331

[2] Vinther, B. M., et al. (2009). Holocene thinning of the Greenland ice sheet. Nature, 461, 385. https://doi.org/10.1038/nature08355

[3] Marcott, Shaun et al. “A Reconstruction of Regional and Global Temperature for the Past 11,300 Years.” Science 339 (2013): 1198-1201. http://shpud.com/Science-2013-Marcott-1198-201.pdf