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Issue: October 2006
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Behind the Lines
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Cover Story
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Stories
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Crops |
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Letter From Europe
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Rural Roots
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How do we know that global warming isn’t just part of a natural trend?
Recent research and analysis of paleoclimate data, reviewed by the U.S. National Academies of Sciences, provide a strong indication that human activities have played a major role
by HENRY HENGEVELD
There are those who have argued for years that there was no evidence that our weather is getting warmer.
Now, faced with the accumulating evidence from all corners of the Earth that its surface and lower atmosphere are indeed heating up, these skeptics suggest that the recent warming is just part of a long-term, natural oscillation in global climate. Nothing to do with greenhouse gases or other human interference.
There is, of course, an element of truth to the arguments raised by those promoting the natural forcing theory. Although the Earth’s climate has been remarkably stable over the past 10,000 years, it has varied moderately on multi-century time scales, particularly at the regional level. European crop records and other historical data, for example, provide abundant evidence of a warm period in that region about 1,000 years ago (the so-called Medieval Warm Period), followed by a cold period (the Little Ice Age) that peaked about 400 years ago.
How do we know that the current warming isn’t just part of a natural trend from the Little Ice Age back to another multi-century warm spell like the Medieval Warm Period?
In themselves, meteorological records collected at weather stations aren’t much help in answering that question, since global coverage from these stations only goes back about 140 years.
However, the Earth itself provides a large variety of clues about how past climates have varied over thousands and even millions of years -- although it takes dedicated and well-trained paleoclimate scientists to undertake the detective work needed to calibrate and translate this information into something equivalent to real values for surface temperature and precipitation.
Tree rings provide one such indicator. In many locations, variations in both tree ring density and width are related to changes in the average temperatures during the growing season from one year to the next. However, these tend to be useful indicators only in land areas like Canada where the amount of tree growth during a summer season is primarily determined by temperature. In warm, dry regions, such growth may be much more dependent on precipitation amounts.
Another indicator is the assortment of vegetation pollen buried in layers of lake sediments. These tell us the combination of different plant species that surrounded that lake over time. Since each plant has its own range in growing season temperature and precipitation within which it can thrive, changes in the number and combination of pollens over time help define how the region’s climate may have varied.
In polar regions, where plants are scarce, it is glacial ice that tells its story. Both the ratios of oxygen isotopes 18 and 16 and of deuterium to hydrogen found in the ice are indicators of the air temperature at the time the snow that formed the ice originally fell. By examining how these vary with increasing depth in the glacier, experts can estimate changes in regional climate with considerable accuracy. In tropical ocean regions, experts use rings in ocean coral similar to those found in trees to estimate changes in regional ocean surface temperatures.
However, while each of these, as well as other paleoclimate indicators, tell part of the global climate story, none by themselves tell all of it. Nor, given the vastly different nature of each type of paleoclimate data, can experts simply average the results from different parts of the puzzle to get a large-scale picture of how temperatures have varied over past millennia.
So, while paleoclimatologists have provided interesting tidbits about regional changes in climates of the distant past, until recently they have failed to provide global or even hemispheric-scale reconstructions of past climate that could be usefully compared with weather station data of the past century.
That began to change about a decade ago. In the late 1990s, a team of experts led by Penn State University scientist Michael Mann developed methods for converting different types of paleoclimate data to a common scale. They then applied advanced statistical techniques to combine them into a single hemispheric record of climate variations for the past 1,000 years. When added to the data collected from weather stations during the past century, the resulting curve looked somewhat like a large hockey stick, gradually decreasing from warm climates about 1,000 years ago to the coldest period during the middle of the Little Ice Age between 1600 and 1800 AD. This slow decline represents the handle of the hockey stick.
During the past century, however, temperatures rose rapidly, forming the blade of the hockey stick. When Mann and colleagues published their first results in 1998, they argued that the 20th century now appeared to be the warmest of the past millennium, and the 1990s the warmest decade.
As to be expected with pioneering work of this kind, Mann’s conclusions have met with their share of criticism. Two years ago, two Canadians -- Steve McIntyre, a retired resource company executive, and Ross McKitrick, an economist from the University of Guelph -- published a critique claiming that Mann and colleagues had made errors in their calculations and had applied some questionable assumptions about which data to include in calculating the single hemispheric record. Subsequently, other paleo experts have cautioned that Mann’s results are contingent on the assumption that the past 100 years of climate data used to calibrate the paleoclimate record are adequate to represent the climate-proxy relationship for the entire millennium.
Over the past two years, both Mann’s team and a number of other European research teams have published further results that address many of these concerns. Some of these results indeed suggest that Mann’s original “hockey stick” may have underestimated the magnitude of natural climate variations over the past millennium.
With new results now extending back 2,000 years, the shape of the improved climate trend curve for the Northern Hemisphere looks more like an undulating serpent than a hockey stick. However, all of these studies appear to agree that the head of the serpent (the past 50 years of data) sticks well above the rest of the body. That is, it appears increasing unlikely that the recent warming is due to natural causes alone.
While no longer a significant issue within the science community, the political debate about the climate hockey stick still seems to be far from over. In recent months, U.S. Senate committees have been holding hearings, inviting both paleo experts and skeptics like McIntyre and McKitrick to testify. Republicans appear to be inclined to believe the skeptics, Democrats the experts.
Congress has also asked the U.S. National Academies of Sciences (NAS) to provide its advice. In response, the NAS has released a panel report arguing with high confidence that current climates are warmer than at any time during at least the past 400 years. It also suggests -- with somewhat less confidence -- that recent temperatures are unprecedented within at least the past millennium. Finally, it argues that paleo records “support the conclusion that human activities are responsible for much of the recent warming.”
Despite the NAS conclusions, it is safe to assume that the debate amongst politicians and within the media about the climate hockey stick is not yet over. Stay tuned for future developments. BF
Henry Hengeveld is Emeritus Associate, Science Assessment and Integration Branch/ACSD/MSC, Environment Canada
Results from a recent study by European paleoclimate experts show that average temperatures over the Northern Hemisphere, as reconstructed from paleoclimate records for the past 2,000 years (blue line), were consistently cooler than those measured with thermometers during the past 50 years (green line). This is in line with results from other similar studies.