October 20, 2006

Diatom Diatribe

If you have followed World Climate Report over the past few years, you are aware that we have taken countless swings at the “Hockey Stick” depiction of planetary temperature. The “Stick” is popular with the global warming crowd for it wipes out the “Medieval Warm Period” of 1,000 years ago and the “Little Ice Age” that began 450 years ago and thankfully ended around 1900. The “Stick” makes the warming of the 20th century look incredible, disturbing, and completely unmatched over the past 1,000 years. The only explanation for the recent warming must be the dreaded buildup of greenhouse gases.


Figure 1. IPCC Northern Hemispheric temperature reconstruction for the past 1,000 years.

Don’t look now, but a new study has been published in Quaternary Research with amazing evidence for the existence of both the Medieval Warm Period and the Little Ice Age. A team of scientists from Finland headed north in their country to Lake Tsuolbmajavri located in the Finnish Lapland at nearly 70°N latitude. Weckström et al. note that the lake “has been shown to contain a representative undisturbed and continuous Holocene stratigraphical record. Due to its location in the climatically sensitive northern treeline area and its relatively uniform sedimentation rates, the lake is extremely suitable for paleoclimatic studies.”

The lake contains, and has always contained, diatoms that are a major group of algae and are one of the most common types of phytoplankton. Most diatoms are unicellular, although some form chains or simple colonies. A characteristic feature of diatom cells is that they are encased within a unique cell wall made of silica. These walls show a wide diversity in form, but usually consist of two symmetrical sides with a split between them, hence the group name. Diatoms are wonderful for climate reconstruction because the silica outer shell preserves them for long periods of time. To make matters even better, there are many taxa and genera of diatoms, and some flourish in warm conditions and others flourish in cool conditions. Weckström et al. note that “The rapid development of quantitative paleoecology has increased the use of sedimentary diatoms as tools in reconstructing past environments. Especially for the last ca. 1000 yr, the record seemed to mimic well the known climatic events, such as the Medieval Warm Period (MWP), the Little Ice Age (LIA), and the post-industrial climate warming.”

The team of scientists went to Lake Tsuolbmajavri in March of 2001 to core the sediment in the lake, identify and date the diatoms, and convert the information into a temperature reconstruction. Their core was 27.5 cm long and when analyzed, they found 160 taxa of diatoms representing 29 genera. The 800 year long temperature reconstruction (Figure 2) looks nothing like the “Hockey Stick” whatsoever – the plot is a nightmare for the global warming crusade for it shows that the study area was much warmer before the Little Ice Age than it is today. The study site is in the Arctic area where the “global warming” signal should be loud and clear, but the final results show a much warmer Lapland prior to the Little Ice Age.


Figure 2. Diatom-based temperature reconstruction over the last eight centuries in northern Fennoscandia. The actual reconstructed temperatures are shown by asterisks in the upper panel of the graph. Three smoothed curves reflecting millennial, centennial and decadal timescales (yellow, purple and blue lines, respectively)(from Weckström et al., 2006)

The scientists write “We associate the warmth in the 13th century with the termination phase of the MWP and the rapid post-AD 1920 temperature increase with the industrially induced anthropogenic warming. It is interesting to note that at centennial timescale, the 20th century warming is comparable to the warming trend in the late 14th and early 15th centuries.” It certainly appears that this team accepts the reality of the Medieval Warm Period so mysteriously missing on the Hockey Stick.

The reconstructed temperatures certainly show the Little Ice Age as well, and in this regard, Weckström et al. write “Pronounced coolness occurred between AD 1600 and 1920, reflecting the LIA. Traditionally, AD 1550–1850 has been assigned for the duration of the LIA, but glacier-margin variations and temperature records from Scandinavia suggest that the LIA lasted until ca. AD 1920.” Further, they write “The centennial time scale curve of the reconstructed temperatures shows the short-term temperature variations and suggests that the climate was cool throughout the LIA.” It appears this team believes in the Little Ice Age as well as the Medieval Warm Period.

In a final blow to the Hockey Stick, the team reported that the warmest 10 to 30 year period occurred from 1470-1500. Had they discovered that the warmest period in the last 800 years had been 1970-2000, they would have made front page news the world over. Instead, you had to come to World Climate Report to learn what the diatoms of the Lapland had to tell us about the climate of the last 800 years.

Reference:

Weckström, J., A. Korhola, P. Erästö, and L. Holmström. 2006. Temperature patterns over the past eight centuries in Northern Fennoscandia inferred from sedimentary diatoms. Quaternary Research, 66, 78–86.




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