July 16, 2004

Cold, Hard Facts

Filed under: Glaciers/Sea Ice

Geologic history reveals that Earth’s glaciers, ice caps, and sea ice advance and retreat in natural cycles—not because of greenhouse forcing. Montana’s glaciers began to retreat long before the atmospheric concentration of greenhouse gases began to change very much. The Pacific Decadal Oscillation (PDO) has been identified as the potential driving force behind snowpack variability in the western United States.

Are glaciers, ice caps, and sea ice are melting worldwide because human industrial activity is causing global warming? Geologic history says otherwise.

Ice ages have come and gone for millennia in the absence of anthropogenic greenhouse gas forcing. In the Northern Hemisphere, for example, a Medieval Warm Period (800–1300 A.D.) gave way to a Little Ice Age (1300–1900 A.D.), from which we began to emerge only a century ago. The temperature and ecosystem changes they triggered are vastly greater than anything observed in recent decades.

Natural variability is critical to glacial fluctuation, according to a new paper in Geophysical Research Letters by Montana State University researcher Greg Pederson and colleagues.

At Glacier National Park in Montana, the research team established long-term chronologies using both tree rings and National Park Service observations to determine the glaciers’ terminus positions since the 1850s. They also used tree-ring data to reconstruct long-term drought variability, or a Mean Summer Deficit (MSD), that incorporates the region’s temperature and moisture dynamics. Rigorously tested, this MSD reconstruction, which is on the conservative side, served as a proxy for summer melting of the glaciers back to 1540 A.D.

The resulting 461-year MSD time series uses “wavelet analysis” — a statistical method that simultaneously decomposes a time series into time-and-frequency space in order to determine two things: 1) the amplitude of “periodic” signals within a time series and 2) how that amplitude varies over time.

Analyzing the MSD, the researchers found that drought in Glacier National Park exhibited distinct multi-decadal variability during the past 461 years. More important, the periodic signals coincide with periods of glacier variability.

Throughout the 19th and 20th centuries (when glacier data are available), variations in drought and precipitation correspond with observed variability in the glaciers. Similar observations were made of glaciers half a world away, on Africa’s Mt. Kilimanjaro (see: http://www.co2andclimate.org/wca/2004/wca_14a.html for further details about glacial behavior in Mt. Kimilmanjaro).

During the late 19th century, a shift in MSD from cool and rainy conditions to sustained drought coincides with the onset of glacial retreat from the Little Ice Age’s maximum. Extreme drought between 1917 and 1941 coincides with rapid glacial recession in Glacier National Park.

Therefore, fluctuations in glaciers are linked with naturally occurring, multi-decadal patterns of drought variability. Furthermore, as appears to be the case at Mt. Kilimanjaro, Montana’s glaciers began to retreat long before the atmospheric concentration of greenhouse gases began to change very much.

Pederson et al. searched for a cause for historical moisture and temperature fluctuations in Glacier National Park and their impact on glacier dynamics. They identify the Pacific Decadal Oscillation (PDO) as the potential driving force behind snowpack variability in the western United States. The PDO is a dominant pattern of Pacific-wide sea-surface temperature variations that is believed to fluctuate on interdecadal time scales. Using a reconstructed PDO index that extends back to 1700 A.D., they found the positive phase of the PDO (characterized by cool conditions in the North Pacific) contributes to high winter snowpack. Its opposite (negative) phase is linked with lower snow accumulations.

Coupling the PDO-related wintertime snowpack accumulation with their MSD drought reconstruction, the researchers uncovered a convincing and complete picture of glacier dynamics linked to natural climate variability.

From approximately 1770 to 1790 and again from 1800 to 1830, the PDO experienced a positive phase. Similarly, the MSD indicates cool, moist conditions. This coincides with the peak of the Little Ice Age in Glacier National Park.

After the 1850s the PDO and MSD overlap again in a way that favors glacial retreat. It coincides with the end of the Little Ice Age and the onset of glacial retreat in Glacier National Park (Figure 1). Between 1917 and 1941 (when the MSD indicates severe drought), the PDO was in a strong and prolonged negative phase. That coincides with the time of greatest glacial retreat in Glacier National Park — when ice was retreating at a rate greater than 100 meters per year in some glaciers. Again after the Pacific Climate Shift in the mid-1970s, the PDO’s positive phase is associated with the modest retreat.

Glacier trends

Figure 1. Profiles of the two glaciers investigated by Pederson et al. Elevation and terminus positions are plotted over time and key dates are indicated (in black) with rates of glacial retreat measured in meters per year (blue). Yellow shading outlines the onset of glacial retreat following the Little Ice Age. Red lines and shading denote the period of severe recession in the early 20th century. (From Pederson et al., 2004.)

Because the correspondence between PDO and MSD is so good, Pederson et al. derive a combined 300-year index called “mass balance potential,” which captures the large-scale glacier fluctuations. This coupled variable provides a precise history of glacial dynamics (Figure 2) that illustrates how two related, natural quantities (drought and snow pack accumulation) account for observed glacier variability. The Little Ice Age’s glacial advance is accurately captured from the late 1700s to the mid 19th century, as is the subsequent glacial retreat and severe ablation between 1917 and 1941.

Mass balance

Figure 2. The glacial mass balance index derived from the PDO and MSD. Blue shading indicates periods of accumulation. Red shading indicates periods of glacial retreat (ablation) in the proxy data. The dotted black line represents the instrument record.

The fluctuation of Montana’s Glacier National Park’s glaciers is the result of unique interactions between summer drought and winter snow accumulation. While it is difficult to tease out the potential impact of rising CO2 concentrations on these interactions in recent decades, this research illustrates that natural interactions have resulted in glacial retreat and advance glaciers for hundreds of years in the absence of anthropogenic greenhouse gas forcing. Similarly, ice around the world has fluctuated for millennia as a result of natural cycles. The worldwide decline of ice sheets and glaciers is, in large part, their response to earth’s emergence from the last glacial maximum 150 years ago.


Pederson, G. T., D. B. Fagre, S. T. Gray, and L. J. Graumlich, 2004: Decadal-scale climate drivers for glacial dynamics in Glacier National Park, Montana, USA. Geophysical Research Letters, 31, L12203, doi:10.1029/2004GL019770.

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