October 17, 2006

Another Swipe at the Hockey Stick

We have covered the “Hockey Stick” controversy many times at World Climate Report, but an article appeared recently in the Proceedings of the National Academy of Sciences that further buries the “Hockey Stick” depiction of planetary temperature.

For over a century, climatologists have collected evidence of the existence of substantial variations in the temperature of the Earth since the end of the last major ice age 10,000 years ago (Figure 1). Evidence from throughout the planet shows that 1,000 years ago, the Earth (or at least the Northern Hemisphere) was in the “Medieval Warm Period”. However, approximately 450 years ago, the Earth plunged into the “Little Ice Age” during a time when the Sun had very few sunspots, and its output had decreased. Fortunately, the luminosity of the Sun increased, and by 1900, the Earth was clearly rebounding from hundreds of years of cold temperatures.

Figure 1. Global temperature reconstruction for the past 10,000+ years.

As we look at Figure 1, there is nothing unusual about the warming of the 20th century; in fact, the warming is unimpressive compared to other events in the last 10,000 years. However, five years ago, the United Nations Intergovernmental Panel of Climate Change (IPCC) issued a report featuring a new plot of the Northern Hemisphere’s temperature over the past 1,000 years (Figure 2). The Medieval Warm Period and Little Ice Age vanished, and now the warming of the past 100 years looked completely unlike anything observed for 1,000 years. The IPCC plot looked like a hockey stick, and scientists have been fighting about the shape of the curve since the day the IPCC released the report. Congressional hearings have been held, reports have been written about how the hockey stick was determined, accusations have flown back and forth about errors in various analyses, data sets seem to have vanished, and on and on. Nonetheless, the “Hockey Stick” has its fan base (of course, it was featured in the Gore film), and the IPCC leaders appear ready to defend the “Stick” to the grave. However, the paleoclimate literature is full of articles clearly documenting the existence of the Medieval Warm Period and the Little Ice Age. The real gems in this literature are those reporting results away from Eurasia and/or the mid-to-high latitudes of North America.

Figure 2. IPCC Northern Hemispheric temperature reconstruction for the past 1,000 years.

The latest evidence comes from lake sediments in the Venezuelan high Andes. A team of scientists from the United States, Venezuela, and Spain write in their opening paragraph “During the past millennium, significant climatic fluctuations have occurred. Prominent among these is the Little Ice Age (LIA), recognized in historical records and documented in proxy climate records from many locations. Although the LIA was a significant global event, its causes and regional differences in the timing and climatic response remain unclear.” That is hardly a ringing endorsement of the “Hockey Stick” preferred by the global warming crusade.

Paleoclimatologists are very clever at finding environmental signals and remnants that record long-term variations in local, regional, hemispheric, and even global climate. The Polissar et al. team who did the work in Venezuela note that “Tropical glaciers respond rapidly to precipitation and temperature variations and, hence, are faithful recorders of climatic variability.” Obviously, the glaciers of the past need to leave something behind that tells us of their existence and in some ways preserves the story of their climatic history.

In the case of Venezuelan glaciers, the team reported “The presence of glaciers in the L. Mucubajı´ watershed increased the flux of inorganic sediment to the lake, producing a continuous lake-sediment record of glacier activity.” At a nearby site “the flux of inorganic sediment to the lake increases during wet periods, producing a distinctive signature in the sediments that contrasts with drier periods.” Furthermore, they found that “Increased catchment glacierization enhances clastic sedimentation in proglacial lakes, leading to higher concentrations of fine-grained magnetic minerals that can be identified visually by color changes and quantified by magnetic susceptibility (MS) measurements.”

Polissar et al. also analyzed pollen spores from nearby sites that preserve hard evidence of past changes in vegetation. They used accelerator mass-spectrometry radiocarbon analyses to put dates on the lake sediments, they used some mathematical wizardry to determine some key environmental variables from the past, and they put the entire story together about the Little Ice Age (LIA) and 1,500 years of past climate in Venezuela.

Data from the lake sediments “and pollen estimates indicate that during the LIA the Venezuelan Andes were both cooler (-3.2°C) and wetter (+208 mm yr-1, +22%) than present.” They analyzed solar radiation data and concluded “The data presented here suggest that solar activity has exerted a strong influence on century-scale tropical climate variability during the late Holocene, modulating both precipitation and temperature.”

If one looks at the environmental variables they collected over the past 1,500 years, the Little Ice Age stands out clearly; the Medieval Warm Period of 1,000 years ago stands out as well! Defenders of the “Hockey Stick” argue that the Little Ice Age was some local phenomena that occurred in Europe and possibly in North America. The tropical Andes of Venezuela is hardly a mid-to-high latitude location of the Northern Hemisphere, and yet, the Little Ice Age shows up clearly in the lake sediments from that equatorial setting.

The “Hockey Stick” must be harder to defend given these latest results.


Polissar, P.J. , M. B. Abbott, A. P. Wolfe, M. Bezada, V. Rull, and R. S. Bradley. 2006. Solar modulation of Little Ice Age climate in the tropical Andes. Proceedings of the National Academy of Sciences, 103, 8937-8942.

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