A new index for measuring climate change

Bob Henson | 9 December 2009  •  When assessing the health of the U.S. stock market, most observers focus on collective measures such as the Dow Jones or S&P 500 indexes, rather than the performance of a single stock. Likewise, a group of scientists has put forth a new tool that combines four distinct measures of climate change into a single number—and the trend is clearly in one direction.

The IGBP Climate-Change Index was unveiled on Monday at a Copenhagen press conference by the International Geosphere-Biosphere Programme. Formed in 1987 by the International Council on Science, the IGBP’s wide-ranging activities involve scientists from 74 nations.

"We felt people outside global-change research are not clear about the scale of the changes scientists are witnessing. The index is a response to these concerns,” said IGBP executive director Sybil Seitzinger in an IGBP news release.

IGBP Climate-Change Index graphic

The IGBP’s new index (shown at right; larger version here) draws on the annual changes observed in four different variables:

The yearly change in each of these four variables is converted into an annual number from -100 to 100, with the endpoints representing the largest changes recorded in each direction since 1980. The four annual numbers are combined to produce each year’s index. It serves as a stand-alone value and can also be combined with previous years’ values to provide a running total. Positive values correspond to an overall warming trend.

As shown in the IGBP chart, the index has been positive each year since 1980 except for three: 1982, 1992, and 1996. Each of those years saw major volcanic eruptions, which are known to cool global temperatures for a year or two.


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As a paleoclimatologist, NCAR scientist Bette Otto-Bliesner values the ability of the new index to demonstrate long-term change.



When average temperature is not enough

One motive for creating the Climate-Change Index is persistent public confusion that arises when global average temperatures don’t rise in a straight line in sync with steady carbon dioxide increases. Part of the apparent mismatch is that the gradual increase in radiation trapped by carbon dioxide doesn’t simply heat up Earth’s atmosphere; some of it also goes into warming the oceans and melting sea ice. Any given year may feature a different mix of these factors, thrown into the hopper with natural cycles such as El Niño and La Niña. All of which means we shouldn’t expect a step-by-step rise in annual global temperature.

Obviously, there’s no cookbook formula for choosing what should go into a climate change index, just as Dow Jones could build its index from any number of stocks. In fact, the IGBP team debated over whether to include carbon dioxide, and it doesn't yet have a method to measure changes in vegetation and other land features.

"We did not identify any good land-surface variable, because no good standard exists," says Steven Running (University of Montana). “But some day we may have annual albedo or land-cover change.”

NCAR’s Bette Otto-Bliesner, co-chair of the IGBP’s PAGES (Past Global Changes) project, is enthusiastic about the new tool. “The IGBP index is an important new measure for communicating global 
change that will not only be updated annually but can also be 
extended back in time to give a longer-term context,” she says.

Meanwhile, the World Meteorological Organization has put recent trends into a fresh context. Although global temperatures have not yet matched the peaks they set in 1998 and 2005, the first decade of the 2000s will end up topping the 1990s, which in turn topped the 1980s, as noted in a news release issued Sunday by the WMO.

Bob HensonBob Henson, a writer/editor in UCAR Communications, is the author of The Rough Guide to Climate Change.

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