Temperature Uptick In Last 100 Years Study Questioned as Some Claim Flaws in Study Methodology

Scientists published a paper in March with paleotemperature data they believe shows an unnatural “uptick” in the last 100 years. (Image: Marcott et al./Science/AAAS)

In early March, the Associated Press, the New York Times and many other media outlets reported about a new study that claimed to have reviewed the past 11,000 years of global temperatures and deemed the heat wave in the 20th century as unnatural and unlike anything seen prior to this time.

Now though, others who are considered “climate deniers” are saying they have found flaws in the study and are calling its conclusions “groundless.”

The study published in the reputable journal Science in the first week of March led by Shaun Marcott with Oregon State University used marine fossils to reconstruct global temperatures beyond the historical record to the end of the last ice age. What they found was a trend of cooling for thousands of years interrupted by an abrupt shift toward warming in recent history.

“In 100 years, we’ve gone from the cold end of the spectrum to the warm end of the spectrum,” Marcott said, according to the Associated Press. “We’ve never seen something this rapid. Even in the ice age the global temperature never changed this quickly.”

Here’s more from the Associated Press report of the study when it was released last month:

Marcott’s data indicates that it took 4,000 years for the world to warm about 1.25 degrees from the end of the ice age to about 7,000 years ago. The same fossil-based data suggest a similar level of warming occurring in just one generation: from the 1920s to the 1940s. Actual thermometer records don’t show the rise from the 1920s to the 1940s was quite that big and Marcott said for such recent time periods it is better to use actual thermometer readings than his proxies.

Before this study, continuous temperature record reconstruction only went back about 2,000 years. The temperature trend produces a line shaped like a “hockey stick” with a sudden spike after what had been a fairly steady line. That data came from tree rings, ice cores and lake sediments.

[...]

Marcott’s research finds the climate had been gently warming out of the ice age with a slow cooling that started about 6,000 years ago.

Then the cooling reversed with a vengeance.

The study shows the recent heat spike “has no precedent as far back as we can go with any confidence, 11,000 years arguably,” said Pennsylvania State University professor Michael Mann, who wrote the original hockey stick study but wasn’t part of this research. He said scientists may have to go back 125,000 years to find warmer temperatures potentially rivaling today’s.

However, another outside scientist, Jeff Severinghaus of the Scripps Institution of Oceanography thinks temperatures may have changed even more dramatically 12,000 years ago, at least in Greenland, based on research by some of his colleagues.

Ross McKitrick on April 1 published an opinion piece in the Financial Post (via Watt’s Up With That) called “We’re not screwed?” countering the research in the study. McKitrick, it should be noted, is not a climate scientist but is an economics professor at the University of Guelph.

McKitrick wrote that the researchers “quietly admitted over the weekend, their new and stunning claim is groundless.”

Calling up what was noticed earlier by Steven McIntyre, a Canadian mathematician who maintains what would also be considered a “climate denier” blog Climate Audit, McKitrick pointed out what they believe are flaws in the Marcott et al. methodology.

The analysis of temperatures was based on 73 proxies established by other research prior to the study. Thirty-one of these proxies were established by dating an organic compound produced by microorganisms that have properties that will correlate to the temperature at the time. The problem is though, according to McIntyre and McKitrick, is that Marcott et al. dated the cores of the organic compounds at an area subject to more variability. The original research for the proxies, McKitrick wrote, stated that core tops varied in age and are more likely to be disturbed in the drilling process. McKitrick said taking measurements further down the core could yield more reliable measurements.

“Had Marcott et al. used the end dates as calculated by the specialists who compiled the original data, there would have been no 20th-century uptick in their graph, as indeed was the case in Marcott’s PhD thesis,” McKitrick wrote. “But Marcott et al. redated a number of core tops, changing the mix of proxies that contribute to the closing value, and this created the uptick at the end of their graph.”

With McIntyre and others pointing out that this was disclosed in the research, Marcott et al. published an FAQ on Real Climate for clarification over this past weekend. Here’s what they wrote about the temperature uptick (emphasis added):

Our global paleotemperature reconstruction includes a so-called “uptick” in temperatures during the 20th-century. However, in the paper we make the point that this particular feature is of shorter duration than the inherent smoothing in our statistical averaging procedure, and that it is based on only a few available paleo-reconstructions of the type we used. Thus, the 20th century portion of our paleotemperature stack is not statistically robust, cannot be considered representative of global temperature changes, and therefore is not the basis of any of our conclusions. Our primary conclusions are based on a comparison of the longer term paleotemperature changes from our reconstruction with the well-documented temperature changes that have occurred over the last century, as documented by the instrumental record. Although not part of our study, high-resolution paleoclimate data from the past ~130 years have been compiled from various geological archives, and confirm the general features of warming trend over this time interval (Anderson, D.M. et al., 2013, Geophysical Research Letters, v. 40, p. 189-193; http://www.agu.org/journals/pip/gl/2012GL054271-pip.pdf).

Real Climate, which published the full FAQ, wrote that it considers the accusations against the study “from the usual suspects” “characteristically misleading.”

“Our view is that the results of the paper will stand the test of time, particularly regarding the small global temperature variations in the Holocene. If anything, early Holocene warmth might be overestimated in this study,” Real Climate wrote.

Think Progress too wrote that “ disinformer blogs” attacking the proxy flaws they believe to identify in the study is “anti-scientific” because the uptick in temperatures within the last 100 years mentioned in the study “just happens to match the uptick in the heavily documented and independently verified instrumental record.”

McIntyre wrote though that using this logic shouldn’t fly. Here’s why:

The modern record is sampled continuously and as a result is able to register short-term trends and variability. The proxy model, by the authors’ own admission, is heavily smoothed and does not pick up fluctuations below a time scale of several centuries.

Overall, McIntyre wrote that he believes recent, high-profile climate studies have shown that some scientists “rely on unseemly tricks, fudges or misleading analysis.” He continued saying when these things are exposed, people calling them out are labeled “deniers.”

“There’s denialism going on all right — on the part of scientists who don’t see that their continuing defence of these kinds of practices exacts a toll on the public credibility of their field,” McIntyre writes, ending his post.

Read McIntyre’s full analysis in his opinion piece here.