February 20, 2007

More Ice for Your Shasta

Filed under: Glaciers/Sea Ice

Warming means less ice. Simple. Intuitive. Untrue. An appealing poster child for the global warming crusade is starting to take some heat…

Spatial extents of snow cover and sea ice and the locations of glacial margins have been championed as monitors of global and local climate change with the improvement of observations over the last half of the 20th century. One of the most common images that come to mind when hearing the term “global warming” is melting ice and snow. If you happen to picture something else, don’t worry - climate change alarmists will help cultivate that image for you with an avalanche of propaganda. What many in the global warming debate have failed to be mindful of is the fact that changes in snow cover extent and glacial mass are a product of several climate variables. In regions where temperatures are near or above freezing for a portion of the year, yes, temperature variations are critical to the stability of perennial ice. However, accumulation and ablation of ice is also driven by variation in precipitation. In the past year, we have highlighted research that shows increases in snow cover extent and glacial mass in regions where the atmosphere has warmed. In each case, the warming of the still-cold climate was accompanied by an increase in precipitation. Twentieth century retreats of the glaciers on Africa’s Mount Kilimanjaro were recently attributed to variables other than temperature, since the glaciers remain at elevations above the mean freezing level and much of the recession occurred early in the century and prior to the warming of the last few decades. Turns out that humidity and cloudiness are primary controllers of glacial variability in the tropics.

We just found another report of a group of glaciers that are not cooperating with the alarmists’ cause. The January 2007 issue of Climate Dynamics contains a research article that documents glacial expansion in the face of a warming atmosphere. Ian Howat of the Department of Earth Sciences at the University of California, Santa Cruz and four of his colleagues report on their study of the 110-year history of the glaciers of a northern California (USA) mountain in “A precipitation-dominated, mid-latitude glacier system: Mount Shasta California.” Part of the Cascade Mountain chain, Mount Shasta holds the largest volume of glacial ice in the state of California, including the glacier of greatest spatial extent. The glaciers of Mount Shasta did not retreat during the 20th century despite a warming of several degrees Celsius over the last half of the century. In fact, two of the mountain’s glaciers, including the largest, expanded over the second half of the century. The expansion came on the heels of a rapid decline in the glaciers, when over one-half of the ice volume was lost between 1920 and 1940, which was well before the global temperature increase of the latter portion of the century. As Howat et al. state, “the continued growth and stability of Mount Shasta’s glaciers suggests that temperature may not always be the dominant control on changes in the size of glaciers in temperate regions, as often assumed in assessing the potential response of glaciers to future climate change.” The researchers parenthetically refer to the Intergovernmental Panel on Climate Change (IPCC) when raising the issue of this poor assumption. They further note the far-reaching implications of the poor assumption: “This would have implications for paleoclimate studies that use ice volume changes to infer regional climate conditions. Also, due to the currently poor simulation of precipitation in mountainous regions, it may suggest that climate model predictions of the impact of warming on snow and ice may be inaccurate.”

Howat et al. used numerical models of Shasta’s glaciers and popular reanalysis climate data to reconstruct variability in two of the largest glaciers over the past 110 years and to test the sensitivity of the glaciers to climate forcing. The researchers found the size of the glaciers to be much more strongly correlated to precipitation than to temperature, and that the extensive glacier retreat in the early twentieth century and subsequent expansion was primarily driven by inter-decadal swings in precipitation that are linked to oscillatory modes of Pacific Ocean climate. Despite regional warming over the past few decades, Shasta’s glaciers did not recede because of increased winter snow accumulation and a strengthened correlation with wet El Niño phases. In terms of sensitivity, Howat et al. found that a 20% increase in precipitation would offset a 1oC increase in temperature.

The research group also used experimental future climate change scenarios to drive the numerical models of Shasta’s glaciers to assess the impacts of potential climate change. They applied two long-term climate change trends to the glacier models with climate oscillations built into the trends as determined from historic climate data. They also employed random application of monthly climate anomalies based on a probability distribution constructed from historical data. As a control case they used 110-year climate trends in temperature (+0.01oC/year) and precipitation (+2 mm/year). Under this scenario the glaciers decline until about 2040, at which time rapid expansion begins and glacier volume reaches upwards of 140% of current volume by year 2100 (Figure 1). The year 2040 represents the approximate time of a climate shift that stems from the oscillatory nature of the regional climate. Next, they used a long-term climate projection from a regional climate model that represents the long-term greenhouse-driven climate change trend of a popular IPCC carbon dioxide scenario. For Mount Shasta, the model predicts a temperature increase of 2.6oC and precipitation increase of 17% for the period 2040-2059 over the period 1980-1999. Under this scenario, the glaciers rapidly lose volume, with one of the two largest glaciers disappearing entirely by 2065 (Figure 1).


Figure 1. Numerical model predictions for the volume change (as a percent of the modeled 2003 value) of two (top and bottom) of Mount Shasta’s largest glaciers under trends derived from the observed 110-year trend (left) and regional climate model predictions (right). Gray lines show the output of 500 individual probabilistic model runs with inter-annual random variability. Solid black lines show the mean volume values for each model year. Dashed lines show the 55, 75 and 95% confidence intervals. (Taken from Howat et al. (2007))

Since precipitation is a much greater controller of the variability in the size of the Shasta glaciers, and Howat et al. clearly state that “due to the currently poor simulation of precipitation in mountainous regions…climate model predictions of the impact of warming on snow and ice may be inaccurate,” we ask the question: Why are we even looking at the right side of Figure 1? Are we really being asked to believe projected impacts on the glaciers of Mount Shasta from poor simulations of the most critical variable controlling the volume of Shasta’s glaciers? Isn’t the continuation of the climate trend over the past century a more believable scenario? Shouldn’t this be the case in any mountainous region given the poor model simulation of precipitation?

Maybe many climate change alarmists focus only on temperature trends when it comes to changes in snow and ice extent because they realize that global climate models, the tip of their spear, tend to struggle mightily in simulating precipitation. To us, accounting for one-half of the equation is not good enough…and Mount Shasta’s glaciers back us up.

Reference:

Howat, I. M., Tulaczyk, S., Rhodes, P., Israel, K., Snyder, M, 2007. A precipitation-
dominated, mid-latitude glacier system: Mount Shasta California. Climate Dynamics, 28, 85-98.




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