September 2, 2008
Scientists Back Up Climate ‘Hockey Stick’ Graph
A new study suggests the controversial 1998 "hockey stick" graph was correct.
A team led by Michael Mann analyzed 2,000 years worth of data. They found Northern Hemisphere temperatures are now "anomalously warm".
The report was published in Proceedings of the National Academy of Sciences.
The 1998 hockey stick was a series of discussions over man-made global warming. The graph was featured prominently in the Intergovernmental Panel on Climate Change's (IPCC) 2001 assessment.
The graph reveals that Northern Hemisphere temperatures were constant for 1,000 years before turning abruptly upwards in the industrial age.
Academics questioned the graphs methodology and conclusions. One U.S. politician demanded to see financial and research records from the scientists who conducted the research.
A 2006 report from the National Research Council (NRC), commissioned by the US Congress, broadly endorsed its conclusion.
Ever since, a number of research groups have produced "proxy records" of temperatures from the centuries before thermometers were common.
These proxies include the growth patterns of trees and coral, the contents of ice cores and sediments, and temperature changes in boreholes.
Dr. Mann's group combined the use of more than 1,200 proxy records. The majority was from the Northern Hemisphere, and used different statistical methods to analyze their findings.
"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," Mann said.
"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."
Researchers found the analytical methods produced graphs very close to the original hockey stick. But this time around the graph started further back in time, and now the "shaft" extends back to about 700 AD.
"I think that having this extra data and using more methods to analyze it makes the conclusions more robust," noted Gabi Hegerl from the University of Edinburgh, UK, who was not involved in the research.
Opponents of the idea of man-made climate change argue that conditions 1,000 years ago were warmer than today.
However, the findings dispute that notion.
One of the analytical methods used in the study shows that temperatures in the Mediaeval Warm Period could have been no higher than they were in about 1980. Another method suggests they were no higher than those seen 100 years ago.
Dr Hegerl said, "The whole line of argument [about whether temperatures have been as high in the past as they are now] is not very relevant."
Researchers say Earth's climate has always responded to factors such as changes in solar activity or volcanic eruptions; 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."
Michael Mann's conclusion from ten years ago is still far from being broken, "The hockey stick is alive and well".
However, the Penn State University researcher noted, "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."
Image 2: Graph which shows the global temperature variation during the last 1000 years.
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