11,000 Year Temperature Trend Study Not All It Is Claimed to Be?


Melting less
Credit: Dreamstime

Back in early March, a team of climate researchers led by Shaun Marcott from Oregon State University published an intriguing study in Science looking at global average temperature trends using proxy data. As I reported the study found:

Our results indicate that global mean temperature for the decade 2000–2009 has not yet exceeded the warmest temperatures of the early Holocene (5000 to 10,000 yr B.P.). These temperatures are, however, warmer than 82% of the Holocene distribution as represented by the Standard5×5 stack, or 72% after making plausible corrections for inherent smoothing of the high frequencies in the stack. In contrast, the decadal mean global temperature of the early 20th century (1900–1909) was cooler than >95% of the Holocene distribution under both the Standard 5×5 and high-frequency corrected scenarios. Global temperature, therefore, has risen from near the coldest to the warmest levels of the Holocene within the past century, reversing the long-term cooling trend that began ~5000 yr B.P.

They illustrated their point using this hockey-stickesque graph:

the new hockeystick
Credit: Roger Pielke

 The National Science Foundation press release about the study stated:

Peter Clark, an OSU paleoclimatologist and co-author of the Science paper, says that many previous temperature reconstructions were regional and not placed in a global context.

"When you just look at one part of the world, temperature history can be affected by regional climate processes like El Niño or monsoon variations," says Clark.

"But when you combine data from sites around the world, you can average out those regional anomalies and get a clear sense of the Earth's global temperature history."

What that history shows, the researchers say, is that during the last 5,000 years, the Earth on average cooled about 1.3 degrees Fahrenheit--until the last 100 years, when it warmed about 1.3 degrees F.

Now it seems that the conclusion is a bit less robust than has been claimed. Various climate change watchdogs of a skeptical bent have pointed out that the the proxies used by the researchers for the more recent decades do not in fact show the big jump in temperatures in the 20th century at the end of the time series graph. in other words, their data do not produce a blade at the end of this particular hockeystick. Consequently, the researchers have backed off a bit on the claim that the proxies used in their study show that the earth has warmed 1.3 degrees Fahrenheit over the past 100 years. See, for example, their FAQ response to various critcisms over at Real Climate. In particular: 

Q: What do paleotemperature reconstructions show about the temperature of the last 100 years?

A: 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.

The critical phrase is …

… 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.

Somehow this subtlety got missed in the NSF press release. The researchers do, however, further note:

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;

The last bit is a reference to a GRL article that found that data derived from some 170 temperature proxies basically matched the upward trend in global average temperatures as measured by thermometers over the past century.

Nevertheless, it's pretty clear that the NSF press release overstated what could be concluded about recent temperature trends from the study itself. University of Colorado environmental science studies professor Roger Pielke, Jr. is recommending that the authors retract the figures showing the 20th century uptick in temperatures and issue a correction. Readers wishing to go deeper into the weeds over this particular study, please take a look at Steve McIntyre's analyses over at his Climate Audit site and Tamino's take over at his Open Mind site.