The “hockey stick” graph is a reconstruction of Northern Hemisphere temperatures over the past thousand years. It showed a sharp rise starting about a century ago. Global warming deniers and doubters have long attacked the graph asserting that we were as warm if not warmer hundreds of years ago. But a 2006 National Academy of Sciences report largely reaffirmed the analysis.

A new peer-reviewed study by climatologists and earth scientists Michael Mann, Zhihua Zhang, Malcolm Hughes, Raymond Bradley, Sonya Miller, Scott Rutherford, and Fenbiao Ni now extends the reconstruction back nearly 2,000 years:


Here is link to the Proceedings of the National Academy of Sciences study, “Proxy-based reconstructions of hemispheric and global surface temperature variations over the past two millennia.” The Supplemental Material is here [PDF]. I have also taken some of the PDF figures and turn them into JPEGs. For the first ever, I believe, the authors did a multi-proxy reconstruction of the Southern Hemisphere for the past 1,500 years (see figure at end).

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A key advance of the new work is that it derives historical temperature through multiple, overlapping proxy records, including “the growth patterns of trees and coral, the contents of ice cores and sediments, and temperature fluctuations in boreholes.” Proxies are used because modern scientific instruments were available for only a small and recent part of Earth’s climatic history:

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“Ten years ago the estimates for earlier centuries were really primarily reliant on just one sort of information: tree ring measurements,” said Mann of Pennsylvania State University.

“To satisfy the critics, we now have enough other sources that we can achieve meaningful reconstructions back a thousand years without tree ring data, and we get more or less the same answer” — that global warming is not mainly due to natural variability.

Some 1,200 proxy records were used, mostly from the Northern Hemisphere. This figures shows the spatial and temporal distribution of the nine different kind of proxy records used:

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What are the paper’s main conclusions?

Following the suggestions of a recent National Research Council report [NRC (National Research Council) (2006) Surface Temperature Reconstructions for the Last 2,000 Years (Natl Acad Press, Washington, DC).], we reconstruct surface temperature at hemispheric and global scale for much of the last 2,000 years using a greatly expanded set of proxy data for decadal-to-centennial climate changes, recently updated instrumental data, and complementary methods that have been thoroughly tested and validated with model simulation experiments. Our results extend previous conclusions that recent Northern Hemisphere surface temperature increases are likely anomalous in a long-term context. Recent warmth appears anomalous for at least the past 1,300 years whether or not tree-ring data are used. If tree-ring data are used, the conclusion can be extended to at least the past 1,700 years, but with additional strong caveats. The reconstructed amplitude of change over past centuries is greater than hitherto reported, with somewhat greater Medieval warmth in the Northern Hemisphere, albeit still not reaching recent levels.

And what does the Southern Hemisphere reconstruction look like? The figures below are from the supplemental material, comparing the Northern Hemisphere reconstruction (top) with the Southern Hemisphere reconstruction (middle) — and a total planetary reconstruction is also thrown in (bottom). The Southern Hemisphere does not appear to show much of a Medieval warm period, based on admittedly much less data.


(As a side note, these figure match the overall shape of the “first must-have slide,” though it looks a bit to me like that figure is more the Northern Hemisphere than the whole Earth.)

The NAS’s National Research Council report in 2006 was essentially a vindication of the original hockey stick, as evidenced by the Nature article on the report, “Academy affirms hockey-stick graph” and by the BBC article on the new study:

… a 2006 report from the National Research Council (NRC), commissioned by the US Congress, broadly endorsed its conclusion that Northern Hemisphere temperatures in the late 20th Century were probably warmer than at any time in the previous 400 years, and perhaps at any time during the previous 1,000 years.

But the 2006 report recommended using more and different proxy records, which the new study has. The bottom line from Mann:

You can go back nearly 2,000 years and the conclusion still holds — the current warmth is anomalous. The burst of warming over the past one to two decades takes us out of the envelope of natural variability.

This updated result will no doubt be widely attacked by the deniers, much as the original result was. But given the greater extent and diversity of the evidence, coupled with the superior analytical methods applied, I suspect this will be a definitive work for quite some time.

The planet is warming anomalously and dangerously thanks primarily to human greenhouse gas emissions, the bulk of which come from burned fossil fuels. The time to act is yesterday.

This post was created for, a project of the Center for American Progress Action Fund.