October 8, 2010

Tibetan Snowpack Decreasing??

No global warming presentation is complete without some pictures of snowpacks and glaciers melting away in alpine environments. The world is warming and the warmth is melting snow and ice in mountainous areas – seeing is believing, and finding pictures of melting snow is rather easy (just wait until spring every year)!

We conducted a search on the internet for “Global warming and snowpack” and nearly 70,000 sites appear with nearly all of them insisting that snowpacks the world over are being devastated by ongoing warming. The Technical Summary of the IPCC states:

“Decreases in snowpack have been documented in several regions worldwide based upon annual time series of mountain snow water equivalent and snow depth. Mountain snow can be sensitive to small changes in temperature, particularly in temperate climatic zones where the transition from rain to snow is generally closely associated with the altitude of the freezing level. Declines in mountain snowpack in western North America and in the Swiss Alps are largest at lower, warmer elevations. Mountain snow water equivalent has declined since 1950 at 75% of the stations monitored in western North America. Mountain snow depth has also declined in the Alps and in southeastern Australia. Direct observations of snow depth are too limited to determine changes in the Andes, but temperature measurements suggest that the altitude where snow occurs (above the snow line) has probably risen in mountainous regions of South America.”

The IPCC is widely cited by the websites – clearly there is a consensus that snowpacks are declining, and who can argue with a consensus?

As with so many other variables, the observations of snowpacks are limited in time (rarely longer than 50 years), and the short records make any trends difficult to assess. A long-term record extending more than 100 years would be useful, and you guessed it, one has appeared recently in the scientific literature.

The paper of interest appeared in Theoretical and Applied Climatology and was produced by eight Chinese scientists all with the Chinese Academy of Sciences; their work was funded by “the Major State Basic Research Development Program of China and the Knowledge Innovation Project of Chinese Academy of Sciences, the Self-determination Project of State Key Laboratory of Cryospheric Sciences and the National Natural Science Foundations of China (apparently, the Chinese are very interested in their snowpacks). Liu et al. note in their introduction that “As a sensitive indicator of climate change, snow is a vital water resource in western China. Previous studies have suggested that an increase in the spring snow cover days on the Tibetan Plateau (TP) is closely associated with the variation in the East Asian summer monsoon”. This caught our eye – who would ever suggest that any increase in snow cover days is possible given all the hype about global warming and snowpacks all over the world. We suspected this would be interesting.

Liu et al. are in the tree ring business, and this time, they turned their attention of alpine fir trees (see below) growing in the Mt. Gongga area in the south-eastern fringe of the Tibetan Plateau (think south-central China). They found 20 different trees growing just over 11,000 feet and they extracted two or three cores from each tree. For each annual growth ring, they accurately measured both the width of the ring and the level of a carbon isotope in the cells that make up each ring. Both of these ring measurements were found to be highly related to temperature and precipitation of the area; obviously, temperature and precipitation are highly related to snowpack – you see where this is going.


Alpine fir trees (Abies fabri) used in the Liu et al. (2010) study

Liu et al. had snowpack data for their study area from 1988 to 2004, and as seen below, the tree ring measurements were relatively good at predicting variance in the snowpack measurements. As you look at the figure below, ask yourself if there is any evidence of a decline in snowpack in western China. Oh no, there is evidence of a decline, and we are amazed this finding hasn’t been reported already throughout the world. We suspect the reason why is because the tree rings allow for a reconstruction of snowpack extending back to 1890, and the longer term data tell a completely different story.


Figure 1. Observed and modeled snowpack in western China, 1988-2004 (from Liu et al., 2010)

The graph below certainly makes the downward trend from 1988 to 2004 look a whole lot differently, doesn’t it? Liu et al. describe the patterns noting “The most notable features of the reconstruction are the higher snow accumulation around 1990. The reconstruction also shows that the Gongga Mountain experienced some lower snow-depth episodes during 1910s, 1930s, 1950–1980, and later 1990s. The higher snow-depth intervals occurred during 1910s, 1940s, and the period around 1990 with the highest values during past 100 years. We defined extreme snowpack depth years as those years with values more than one standard deviation (plus or minus) from the average. Although there are several clusters of extreme years over the past century, the century is notable for the long period of snow-depth variations in a normal way except the higher snowpack depth values around 1990.”


Figure 2. Reconstructed snow depths in the Gongga Mountain over the period of 1890–2004.
The values of snow depth were transformed into the departure against the mean of 1890–2004. The thick line was smoothed to emphasize long-term fluctuations (from Kiu et al., 2010)

So, what we get here is a classic case of an apparent decline in snowpack from the late-1980s to the near-present, and someone might be tempted to suggest the decline is related to global warming. The Chinese scientists figured a way to reconstruct snowpack going back over 100 years, and that time series shows no evidence whatsoever of any downward trend (in fact, there appears to be a slight upward trend in the data).

So you thought snowpacks were declining everywhere in the world? Think again!

Reference:

Liu, X., L. Zhao, T. Chen, X. Shao, Q. Liu, S. Hou, D. Qin, and W. An. 2010. Combined tree-ring width and δ13C to reconstruct snowpack depth: A pilot study in the Gongga Mountain, west China. Theoretical and Applied Climatology, DOI 10.1007/s00704-010-0291-x.




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