October 24, 2006

For Peat’s Sake: Warmer is Better

Imagine your dog digging a hole in the backyard, and somehow, you then have an intricate story to tell about the climate history of human civilization. This is somewhat the case from an article published recently in the journal (we doubt you read regularly) Palaeogeography, Palaeoclimatology, Palaeoecology. In the global change fraternity, it is referred to as “Palaeo.”

A team of scientists from China’s State Key Laboratory of Loess and Quaternary Geology and the State Key Laboratory of Environmental Geochemistry turned their attention to a peat bog at Hongyuan in central China; they extracted a 5 meter core from the peat that contained a climate signal of the past 6,000 years. The grasses that built the peat over the centuries preserved an oxygen 18 isotope (d18O) signal over the entire 6,000 year period. The team of Xu, Hong, Lin, Zhu, Hong, and Jiang note that “For plants using precipitation as source water, the isotopic signals of precipitation can be modulated and recorded into plant cellulose through series of plant physiological processes. Since both the relative humidity and the amount of precipitation vary mildly, the oxygen/hydrogen isotopic fractionations during the plant physiological processes should be relatively constant. Therefore, the variation of d18O in plant cellulose should mainly reflect that of the precipitation. Because source water of the grass in the studying peat land is predominantly meteoric water, the d18O of peat cellulose should be quantitatively correlated to that of the precipitation, and thereby to air temperature.” More simply, the peat can tell us about the temperature over the past 6,000 years.

Figure 1 (below) contains a lot of information, but if you focus on the top plot, you will see the d18O of peat cellulose from their Hongyuan study site. It shows tremendous variation over the past 6,000 years, with climate variability the rule, not the exception. There are many periods much warmer than today, and of course, periods much cooler as well. The patterns that show up from the Hongyuan site are largely seen at a Jinchuan site 2,400 km away. The focus of the Xu et al. article is largely about the Sun’s role in determining variations in climate, and Xu et al. write “Comparisons between temperature variations and solar activities indicate that both temperature trends on centennial/millennial timescales and climatic events are related to solar variability, suggesting that solar variability is possibly a primary driving force that influences temperatures.”


Figure 1. Comparison of general trends between temperature and solar activity during the past 6,000 years. The top plot is the d18O of peat cellulose from the Hongyuan study site. The second time series is another d18O dataset collected in Jinchuan in northeastern China. Higher d18O values are indicative of higher temperatures. The third time series represents atmospheric 14C values that are sensitive indicators of solar variability. The fourth time series represents modeled solar output values.

Xu et al. comment that “it was cold between AD 1250 and 1850, corresponding to the Little Ice Age. Considerable evidence suggests that cold climate during the Little Ice Age was related to solar activity. The low temperatures at both Hongyuan and Jinchuan during this time were likely induced by changes in solar activity.”

They then write “Other historical and human activity records support a relation between temperature changes and variations in solar activity. In the Sahara, nearly all of the freshwater lakes desiccated, and the ancient Saharan civilizations collapsed, about 4600~
4000 a BP. Cold climates may have forced the Mesopotamians to abandon their settlements in northern Mesopotamia, and the Akkadian Empire in south Mesopotamia collapsed. The ancient civilizations in the Indus River region and in Egypt also collapsed. To escape the cold climates, the Indo-Europeans moved to Greece, south Russia, Persia, India, and northwest China. Climates varied on a more favorable level during the interval from 4200 to 1500 a BP, corresponding to a time of relatively strong solar activity. Chinese civilization developed rapidly during this warm period. The birth of the ancient Chinese civilization has been assumed to take place about 4000 a BP.” Further, during the warm period “Europeans moved northwards into Scandinavia and engaged in agriculture. The Assyrian Empire, the Hittite Empire, China (Shang Dynasty), and Egypt prospered at that time.”

OK – there are multiple climate lessons from their work: (a) climate has varied quite a bit over the past 6,000 years, and the fluctuations can be rapid, (b) the variations are strongly controlled by our wonderful, but imperfect Sun, (c) the Little Ice Age is a clear signal in the data collected at two sites in China, (d) there are many times in the past when China was warmer than today, and (e) warmer times are better for civilizations than cooler periods. Warmer is simply better!

Reference:

Xu, H., Y. Hong, Q. Lin, Y. Zhu, B. Hong, and H. Jiang, 2006. Temperature responses to quasi-100-yr solar variability during the past 6000 years based on d18O of peat cellulose in Hongyuan, eastern Qinghai–Tibet plateau, China. Palaeogeography, Palaeoclimatology, Palaeoecology, 230, 155– 164.




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