Climate 'hockey stick' is revived

Richard Black, BBC News 1 Sep 08;

A new study by climate scientists behind the controversial 1998 "hockey stick" graph suggests their earlier analysis was broadly correct.

Michael Mann's team analysed data for the last 2,000 years, and concluded that Northern Hemisphere temperatures now are "anomalously warm".

Different analytical methods give the same result, they report in Proceedings of the National Academy of Sciences.

The 1998 hockey stick was a totem of debates over man-made global warming.
The graph - indicating that Northern Hemisphere temperatures had been roughly constant for 1,000 years (the "shaft" of the stick) before turning abruptly upwards in the industrial age - featured prominently in the Intergovernmental Panel on Climate Change's (IPCC) 2001 assessment.



But some academics questioned its methodology and conclusions, and increasingly strident condemnations reverberated around the blogosphere.

One US politician demanded to see financial and research records from the scientists involved.

However, 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.

Twin study

Since then, a number of research groups have produced new "proxy records" of temperatures from the centuries before thermometers were widely deployed.

Such proxies include the growth patterns of trees and coral, the contents of ice cores and sediments, and temperature fluctuations in boreholes.



In their latest study, Dr Mann's group collated more than 1,200 proxy records - the majority from the Northern Hemisphere - and used different statistical methods to analyse their cumulative message.

"We used two different methods that are quite complementary in the assumptions they make about data, so that provides a test of the sensitivity of data to the methods used," he told BBC News.

"We also made use of a far wider network of proxy data than previously available.

"Ten years ago, the availability of data became quite sparse by the time you got back to 1,000 AD, and what we had then was weighted towards tree-ring data; but now you can go back 1,300 years without using tree-ring data at all and still get a verifiable conclusion."

Both analytical methods produced graphs similar to the original hockey stick, though starting further back in time. The "shaft" now extends back to about 700 AD.

The same basic pattern emerged when tree-ring data - whose reliability has been questioned - was excluded from the analysis.

"I think that having this extra data and using more methods to analyse it makes the conclusions more robust," commented Gabi Hegerl from the University of Edinburgh, UK, who was not involved in the research.

Past lessons

Critics of the idea of man-made climate change argue that conditions 1,000 years ago were as warm as, if not warmer than, they are today.

The new paper adds to the evidence against that notion. One of the analytical methods used suggests that temperatures in the Mediaeval Warm Period could have been no higher than they were in about 1980; the other suggests they were no higher than those seen 100 years ago.

In any case, said Dr Hegerl: "The whole line of argument [about whether temperatures have been as high in the past as they are now] is not very relevant."

The climate has always responded to factors such as changes in solar activity or volcanic eruptions, and always will, she said; the issue now is how it is responding to greenhouse gas emissions.

"In any case, the paper still comes to the firm conclusion that the most recent decades are unusual."

Ten years on from the study that provoked all the ire, Michael Mann's conclusion is that far from being broken, "the hockey stick is alive and well".

But, the Penn State University researcher added: "If we want to understand things like El Nino and how it relates to climate change, it's not enough to know just how anomalously warm the climate is today.

"We need to learn from the palaeoclimatic record. The science is not all done, there's still a lot of work to do; but what we are seeing now is definitely unusual in the context of the past."

A particular desire of scientists in the field is to increase the amount of data from the Southern Hemisphere. The majority of proxy records come from land rather than sea, and from land not covered in ice at that, which is in relatively short supply south of the equator.

Earth Hotter Now Than in Past 2,000 Years, Study Says
Mason Inman, National Geographic News 2 Sep 08;

The planet is hotter now than it has been for nearly the past 2,000 years, researchers report.

The new study is led by Michael Mann, a climatologist who helped develop the famous 1998 "hockey stick" graph—a reconstruction of Northern Hemisphere temperatures over the past thousand years showing a sharp uptick beginning around 1900.

In their new work, Mann and colleagues back up the hockey stick graph by citing other temperature indicators in the natural record.

The researchers analyzed coral reef skeletons, cores from glaciers and ice sheets and sea floor sediments, and stalagmites and stalagtites formed in caves—all of which trap chemicals that reveal what the temperatures were across past centuries.

"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.

Hot Anomaly

Measurements of the planet's temperature from reliable thermometers stretches back only about 150 years, and measuring temperatures of earlier centuries is quite a bit harder.

Taking the planet's temperature in, say, A.D. 1000, requires measuring tree rings, cores from ice sheets and glaciers, and other natural records that reveal, indirectly, how warm it was in a given year.

But in these reconstructions, "there was quite a bit of uncertainty," Mann said.

The climate has varied over the centuries, with warmer and cooler stretches, the study affirmed.

And yet, Mann said, "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."

The study will appears today in the journal Proceedings of the National Academy of Science.

Hockey Stick Graph

The hockey stick graph has become a lightning rod for criticism of the idea that the planet is warming mainly due to human-made greenhouse gases.

Many critics contend that tree rings are unreliable temperature gauges, because temperature is not the only factor that affects the rings.

The controversy led to hearings in the U.S. Congress over the methods Mann and colleagues used in the 1998 study.

However, a 2006 report from the National Research Council—a private, nonprofit scientific institution that advises the U.S. government—supported the hockey stick study while detailing the major uncertainties.

Pleasantly Surprised

In centuries past, isolated regions have warmed up from time to time, such as during the so-called Medieval Warm Period, when Europe experienced warmer temperatures from about A.D. 900 to 1400.

"But what's unique about modern warming is that essentially the whole globe is warming up in tandem," Mann said.

"The so-called hockey stick … it's alive and well."

Climatologist Gabriele Hegerl of the University of Edinburgh in Scotland is "pleasantly surprised" by the new study.

"Being able to get essentially [the] same result without tree ring data shows that what we are seeing is not something specific to tree rings," Hegerl said, "but a real temperature response."