More common, and already debunked, climate change denial "arguements";
"The alleged "hockey stick" graph of temperatures over the past 1,600 years has been disproved. It doesn't even acknowledge the existence of a "medieval warm period" around 1000 A.D. that was hotter than today is. Therefore, global warming is a myth.
It is hard to know which is greater: contrarians' overstatement of the flaws in the historical temperature reconstruction from 1998 by Michael E. Mann and his colleagues, or the ultimate insignificance of their argument to the case for climate change.
First, there is not simply one hockey-stick reconstruction of historical temperatures using one set of proxy data. Similar evidence for sharply increasing temperatures over the past couple of centuries has turned up independently while looking at ice cores, tree rings and other proxies for direct measurements, from many locations. Notwithstanding their differences, they corroborate that Earth has been getting sharply warmer.
A 2006 National Research Council review of the evidence concluded "with a high level of confidence that global mean surface temperature was higher during the last few decades of the 20th century than during any comparable period during the preceding four centuries"—which is the section of the graph most relevant to current climate trends. The report placed less faith in the reconstructions back to 900 A.D., although it still viewed them as "plausible." Medieval warm periods in Europe and Asia with temperatures comparable to those seen in the 20th century were therefore similarly plausible but might have been local phenomena: the report noted "the magnitude and geographic extent of the warmth are uncertain." And a new research paper by Mann and his colleagues seems to confirm that the Medieval Warm Period and the “Little Ice Age” between 1400 and 1700 were both caused by shifts in solar radiance and other natural factors that do not seem to be happening today.
After the NRC review was released, another analysis by four statisticians, called the Wegman report, which was not formally peer reviewed, was more critical of the hockey stick paper. But correction of the errors it pointed out did not substantially change the shape of the hockey stick graph. In 2008 Mann and his colleagues issued an updated version of the temperature reconstruction that echoed their earlier findings.
But hypothetically, even if the hockey stick was busted... What of it? The case for anthropogenic global warming originally came from studies of climate mechanics, not from reconstructions of past temperatures seeking a cause. Warnings about current warming trends came out years before Mann’s hockey stick graph. Even if the world were incontrovertibly warmer 1,000 years ago, it would not change the fact that the recent rapid rise in CO2 explains the current episode of warming more credibly than any natural factor does—and that no natural factor seems poised to offset further warming in the years ahead.
Seven Answers to Climate Contrarian Nonsense - Scientific American