Wider study shows spike in warming – last decade hottest in 1,300+ years

posted Wednesday, February 18, 2009        

By Tony Davis, published September 2, 2008, Arizona Daily Star

Data from tree rings, other sources indicate last decade was hottest in
1,300-1,700 years

The decade ending in 2006 was the warmest such period in the Northern Hemisphere for at least the last 1,300 years and possibly longer, says a new study written by a University of Arizona Regents Professor and six other researchers.

The study, is the latest of three reports by some of these researchers contrasting recent temperatures with those of hundreds or more years ago.

Because this study used a broader array of research data, the scholars say they have more confidence in their findings than in the earlier studies. Those studies, involving what was called a “hockey stick” diagram showing a dramatic rise in recent temperatures, brought them worldwide scientific fame and recognition by an international climate-change panel that studies global warming. But they also sparked controversy and a congressional investigation.

The new study concluded that the average Northern Hemisphere temperature over that recent decade was at least a bit more than half a degree Fahrenheit warmer than the historical, decade-long averages dating back 1,300 to 1,700 years. More likely, they were close to a full degree warmer than historical averages, the study found.

The paper is “one brick in the wall” of the case for saying that human-induced causes such as greenhouse-gas emissions are raising temperatures, said Malcolm Hughes, a dendrochronologist at the UA’s Laboratory of Tree-Ring Research.

Other pieces of evidence are more important, including the atmosphere’s basic physics and what is already known about the amounts of gases such as carbon dioxide, methane and nitrous oxide that humans have been putting into the atmosphere, Hughes said.

“It is not the foundation stone. We are not claiming that,” Hughes said of the latest study’s contribution to theories of human-caused global warming. “We are part of the supporting cast of this case. The results that we have are consistent with what we know about how climate is controlled, as consistent as they can be given the uncertainties.”But this study also offers much more reliable conclusions than the earlier studies, because they relied much more heavily on tree-ring records for data, said another researcher who like Hughes worked on both this study and the earlier ones. The new study made much more use of other sources, including data from sediments, stalactites and stalagmites from caverns and other things.

The original studies by Hughes and his colleagues led to a congressional investigation in 2005. That led directly to a National Academy of Sciences investigation.

In the academy’s 2006 report, it agreed with a critic of Hughes’ research that it is hard to quantify that one decade is hotter than any going back 1,000 years. But it said there is high confidence that surface temperatures during the late 20th century were warmer than any comparable period over the past 400 years.

The problems with going back 1,000 years, the academy review panel said at the time, was a lack of evidence besides tree rings such as boreholes, ice cores and retreating glaciers.

By broadening data-gathering, the latest study sought to address the academy’s concerns. The academy, by concluding that the earlier research was overly dependent on one type of data, had put “sort of an asterisk” on the earlier research, said Michael Mann, a Penn State University meteorology professor and colleague of Hughes on both the older studies and the new one.

“We sort of removed that asterisk,” because the new study’s conclusion didn’t depend as much on one kind of data, said Mann, director of Penn State’s Earth System Science Center.

The two earlier studies, published in 1998 and 1999, compared recent temperatures to those dating back first 600 years and then 1,000 years.The latest study will be published Sept. 9 in the Proceedings of the National Academy of Scientists.

In a diagram in the new study showing temperature changes over the centuries, a bar graph shows only relatively minor ups and downs from average temperatures until recently. But starting in the late 1980s or the early 1990s, the bar shoots almost straight up from the rest of the graph.

That diagram is similar to those from the earlier studies done by Hughes and other researchers. At the time of the original research, it was called a “hockey stick” diagram.

The original diagram made its way into the 2001 report of the International Panel of Climate Change on global warming, and gave Hughes and his colleagues huge exposure to their work outside scientific circles.

But it also brought controversy from those who felt that theories of global warming had been overblown. As part of an investigation, in June 2005, Rep. Joe Barton, a Texas Republican, requested data, documentation of the research methods, the location of all relevant data archives and computer codes pertaining to the past records of Hughes, Mann and a third colleague who worked on the earlier studies.Barton also sought records of all grants and contracts and the agreements for use of grant money that the three researchers had received. Barton at the time chaired the House Energy and Commerce Committee.

In his letter to the researchers, Barton referred to a Wall Street Journal article that said questions had been raised about the significance of methodological flaws and data errors in the researchers’ two studies on temperatures and climate change. He also pointed to work by two Canadian researchers and several articles in scientific journals questioning this work.

“As these researchers find, based on the available information, the conclusions concerning temperature histories – and hence whether warming in the 20th century is actually unprecedented – cannot be supported by” the two previous studies done by Hughes and others, Barton wrote at the time.
But the investigation drew sharp criticism from many scientists and other congressmen, including the Republican head of the House Science Committee. “My primary concern about your investigation is that its purpose seems to be to intimidate scientists rather than to learn from them, and to substitute congressional political review for scientific review,” wrote then Rep. Sherwood Boehlert of New York.

Hughes and his colleagues turned over some of the information, told them that other information was in the public record and “that is the last we ever heard from them,” Hughes recalled this week.

Recalling the congressional probe, Mann said that theirs wasn’t the only research on the subject of sharply higher recent temperatures that the IPCC, the climate-change panel, relied on.

“Three other studies came to that same conclusion at the time the IPCC report was published in 2001. By 2005, more than a dozen studies concluded that,” Mann said. “They tried to make it about one seven-year-old paper. It is much easier to attack a straw man.”

The National Academy of Sciences had put “sort of an asterisk” on the earlier research. “We sort of removed that asterisk” with the new work. Michael Mann, Penn State Earth System Science Center

How they arrived at conclusion
Data records used in the three studies by Malcolm Hughes and his colleagues:

  • 1998 study: 373 total records, including 42 not from from tree rings.
  • 1999 study: 39 total records, including 5 not from tree rings.
  • New study: 1,209 total records, including 173 that weren’t from tree rings.

What the researchers found
Hughes, of the University of Arizona, and six other researchers concluded in their latest study that:

  • The average Northern Hemisphere temperature during the decade ending in 2006 has been at least slightly more than half a degree Fahrenheit warmer than the warmest decade over the past 1,300 years – if data coming from tree ring records are excluded.
  • That same gap between the recent decade and historical temperatures dates back 1,700 years, when researchers added tree ring records to their database.
  • The most likely difference between the recent decade and historical temperatures is actually larger -nearly one degree Fahrenheit. That compares to overall figures of a range of 1.8 to 2.16 degrees in all average Northern Hemisphere temperatures over the past 1,300 years and 2.16 degrees over the past 1,700 years.

Contact reporter Tony Davis at 806-7746 or tdavis@azstarnet.com.

Leave a comment