July 15, 2010

Solar Control on Tibet’s Climate

We see many articles like the following two that show more evidence of a solar control on climate even at the regional scale.

In the first article published recently in Geophysical Research Letters, four scientists from Pennsylvania’s Lehigh University, China’s Lanzhou University, and the University of Minnesota turned their attention to Hurleg Lake located in the Qaidam Basin (QB) on the northeastern portion of the Tibetan Plateau (see Figure 1); the research was funded by the National Science Foundations of both China and the USA.


Figure 1. Map of the location of Hurleg Lake (from Zhao et al., 2009)

Zhao et al. begin stating “Documenting the effect of large-scale forcing to regional climate is an important first step in understanding the underlying mechanisms of climate change.” To that end, they extracted a core from the bottom of the Hurleg Lake in the summer of 2006 and used a variety of analytical methods to determine the carbonate percentage and ostracode abundance throughout the datable core. Ostracodes are tiny, swimming, crustaceans (same family as the lobster, crab and shrimp), and their shells are preserved at the bottom of the lake for thousands of years.

Zhao et al. note that “The carbonate percentage and ostracode abundance at Hurleg Lake reflect changes in depositional environment and in particular water depth.” Once again, Mother Nature has provided a way for us to peer into the past in terms of water levels of a lake. The authors find “The carbonate percentage and ostracode abundance show a consistent pattern with ~200 year moisture oscillations during the last 1000 years.” The variations appear to be related to periodicities of solar output – Zhao et al. conclude “Higher solar output corresponds to a stronger monsoon, which intensifies the uplift of air mass on the high Tibetan Plateau and strengthens the subsidence of air mass over the QB. The reverse is true during the period of lower solar output. Thus, high solar activity is correlated with dry climate in QB and increased precipitation in monsoonal areas.”

Note that the solar control is strong enough that it can be identified by patterns of shells in the bottom of a lake in the Tibetan Plateau. Arguing against a strong solar control of climate seems silly in light of the evidence for solar control found throughout the world.

This same basic team reshuffled the deck in terms of authors and focused on another set of measurements of materials available in cores from Hurleg Lake and nearby Toson Lake. Rather than focus on carbonate and ostracode material, they turned their attention to pollen spores well-preserved in the core. Plants throw out pollen spores throughout their lives, and the pollen spores give away the identity of the plants that must have populated the study area over the past few centuries.

In the second article, Zhao et al. begin stating “Central Asia in the interior Eurasian continent has experienced increasing aridity and desertification during the 20th century. However, the causes of these moisture changes, which have been attributed to changes in regional human activities, human-induced global warming, or natural climate variability, are still poorly understood.” Can you believe the number of times we report that something about the climate system is “still poorly understood”? The Zhao et al. group better get the memo that any changes found in Tibet darn well better be attributed to global warming … period!

The cores from the two lakes contain all sorts of pollen spores, but two end up taking center stage. The first are Chenopodiaceae spores that make up approximately 40% of all spores from the cores and the second are the Artemisia spores that make up 30–35% of the total spore count. Chenopodiaceae are flowering plants that tend to dominate the area during dry periods while Artemisia are hardy herbs and scrubs with fern-like leaves that dominate during moist periods. Zhao et al. develop a ratio between the two (the A/C ratio) that varies throughout the length of the cores.

The bottom line is seen in the slice below from one of Zhao et al.’s most important figures (Figure 2). In describing the A/C ratio time series, the team reports “In the past 1700 years, A/C ratio at Hurleg Lake shows high values at AD 390–460, 550–650, 790–860, 950–1050, 1120–1180, 1230–1320, 1400–1500, 1620–1700, and 1760–1800, suggesting the vegetation was more “steppe-like” under a relatively moist climate; while during the intervening periods, Chenopodiaceae dominates with low A/C ratio, representing more “desert-like” vegetation under a relatively dry climate. After AD 1700, A/C ratios decrease from >0.8 to <0.6, following a long-term trend which started at AD 1150, suggesting a drying climate. Visually A/C ratios show 150–200 yr oscillations at Hurleg Lake.”


Figure 2. A/C ratio curves (three-point moving average) from Hurleg Lake (black line) and from Toson Lake (gray line) (from Zhao et al., 2010)

As was the case in the first article, the 200 year quasi “cycle” is again linked to a similar cycle in solar activity. Zhao et al. speculate that “The ~200-yr time spacing between wet and dry climate periods indicated by the A/C ratio variations suggests a possible solar forcing of effective moisture changes in the region.” The authors also note that the basin was especially dry “around AD 1600 during the first few centuries of the Little Ice Age”. So much for any argument that the Little Ice Age was somehow confined to Europe – it’s a long flight from Europe to Tibet.

Certainly there is a strong solar control on the climate system, and at the timescales from decades to centuries, the solar influence on global and regional climate can be profound. Clear examples of this can be founding the results from the Zhao et al. teams, who headed off to Tibet, cored a couple of lake bottoms, analyzed carbonate levels, shells from crustaceans, and pollen spores from plants, and what jumped out at them was the solar control on the climate of their study area.

References:

Zhao, C., Z. Yu, Y. Zhao, and E. Ito. 2009. Possible orographic and solar controls of Late Holocene centennial-scale moisture oscillations in the northeastern Tibetan Plateau. Geophysical Research Letters, 36, L21705, doi:10.1029/2009GL040951.

Zhao, Y., Z. Yu, X. Liu, C. Zhao, F. Chen, and K. Zhang. 2010. Late Holocene vegetation and climate oscillations in the Qaidam Basin of the northeastern Tibetan Plateau. Quaternary Research, 73, 59–69.




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