While the IPCC is big on the idea that the warmth of the late 20th and early 21st century in the Northern Hemisphere is unprecedented in recent centuries, apparently that finding does not apply universally over longer timescales.
According to the Summary of Policymakers from the IPCC Fourth Assessmnet Report:
Average Northern Hemisphere temperatures during the second half of the 20th century were very likely higher than during any other 50-year period in the last 500 years and likely the highest in at least the past 1,300 years.
This is basically a verbal description of the “hockeystick”-like temperature progression of the past millennia or so.
How that representation came to be and just how scientifically accurate it is a story unto itself, and one which continues to be assessed and reassessed over at the Climate Audit website. An interesting discussion has been taking place there as to yet another methodological flaw in the mathematics involved in multiproxy reconstructions. And another oft-discussed issue there is the very selective use of only particular proxy temperature records which are combined to produce the now-too-familiar hockeystick shape.
One proxy record that most definitely was not included in the assembly of the hockeystick is a just-published proxy reconstruction of sea surface temperatures (SSTs) from the East China Sea.
The East China Sea SST reconstruction was developed by 5 researchers with various affiliations with several Chinese universities, led by Weichao Wu of Peking University. Wu and colleagues collected a sediment core from the sea floor in the Southern Okinawa Trough (SOT) over which the warm Kuroshio current flows (Figure 1). The Kuroshio Current is the Pacific Ocean’s analog of the Atlantic’s Gulf Stream moving warm water poleward and playing a significant role in the climate of eastern Asia.
Figure 1. General study region (from Wu et al., 2012).
The researchers analyzed the top 10 meters of the sediment core, corresponding to 2,700 years of sedimentation and from it were able to resolve 25-yr averages. To determine the SST, they used the “relative number of cyclopentane isoprenoid GDGTs in marine crenarchaeota (Thaumarchaeota)” which “increases with increasing growth temperature.” We didn’t know what that meant either! But after some digging, we translated that to mean that they identified a tracer in the sediment layers that is a measure of the relative abundance of a tiny marine organism whose number is sensitive to the sea surface temperature. The more crenarchaeota that are evident in the sediment layer, the higher the ocean temperature.
Their 2,700 year SST reconstruction from the East China Sea shows a significant degree of long-term temperature variability (Figure 2). And within this variability the authors were able to identify the well-recognized climate periods of the past several millennia, including the Little Ice Age (LIA), Medieval Warm Period (MWP), Sui-Tang dynasty Warm Period (STWP), Dark Age Cool Period (DACP), and the Roman Warm period (RWP) along with the Current Warm Period (CWP) beginning in the mid-19th century.
Figure 2. Proxy sea surface temperature from the East China Sea (see text for description of acronyms). The points are 25-yr average temperatures, the solid line is a running three-point mean (from Wu et al., 2012).
As you probably pretty quickly picked out of Figure 2, the temperatures at the end of the Current Warm period (CWP), are not the highest of the entire reconstruction. In fact, there are indications that there were 25-yr periods during nearly all of the previously identified warm periods in which the reconstructed temperature exceeded the recent average.
Here is how the authors described the situation:
Based on our SST record, the MWP, usually thought to be a historical analogue for CWP, is characterized by large fluctuations from 25.1 to 26.8°C with a mean of 25.7°C, about 0.7°C lower than RWP (26.5°C) and STWP (26.3°C). On the centennial timescale, the mean SST of the 20th century (26.1°C) did not exceed the range of natural variability of the past 2700 years, such as RWP and STWP. The climate records from East China [Ge et al., 2004], north Icelandic shelf [Patterson et al., 2010] and Greenland [Kobashi et al., 2011] also comprise centennial-scale warm periods during the first millennia AD, comparable to or even warmer than mean 20th century conditions. However, we acknowledge that if the SST in the SOT keeps increasing at the current rate (ca. 0.3°C per decade based on mean SST in 2000 and 2010; http://www.nsof.noaa.gov), the 21st century warmth will be indeed unprecedented in the context of the past 2700 years.
(Our guess is that the reviewers probably encouraged (i.e., made) them put in the last sentence which seems a bit out of place in the general tone and analytical technique of the article. Perhaps this kind of strong-arming will show up in a new release of Climategate emails years from now.)
Look at Figure 2 as long as you may, and you are not going to be able to find a hockeystick. Instead what you see is a progression of natural variability as the East China Sea region fluctuated between multi-centennial scale cool and warm periods. The current warm period is by no means unique or unusual. Instead it appears rather matter-of-course.
Further, Figure 2 gives you an indication of just how meaningless the IPCC statement about the paleo-temperature history of the Northern Hemisphere really is. True, even in this proxy reconstruction, the “temperatures during the second half of the 20th century were very likely higher than during any other 50-year period in the last 500 years” and even “likely the highest in at least the past 1,300 years.” However, for the major portion of the millennium previous to that, the average proxy temperature was not much different from that observed during the latter half of the 20th century and in fact was probably higher on at least several occasions. Which means that what the IPCC is attempting to get you to infer—that human GHG emissions have pushed the temperature up to unprecedented levels—falls flat when a slightly longer historical time period is brought into the picture.
Apparently, the IPCC did not want that information prominently displayed.
We notice, for example, that the IPCC did not include a statement in the Summary for Policymakers like:
Average Northern Hemisphere temperatures during the second half of the 20th century were likely not higher than many 50-yr periods during the 1500-yr period from 500 BC to 1000 AD.
We are not saying the new results from Wu et al. are grounds for such a conclusion over such a large spatial scale, but one thing is for certain, they certainly don’t run counter to it.
Despite being an active area of research, there is still probably more that we don’t know about the spatial and temporal temperature variations over the past few thousand years than we do know. But what we do know seems to indicate periods in the not-too-distant past that were not-too-different than the present—at least over some portions of the planet. And the more that we look, the more places that we seem to find where this is the case.
Wu, W., et al., 2012. Sea surface temperature variability in southern Okinawa Trough during last 2700 years. Geophysical Research Letters, in press.