Since Hansen’s article in September, we’re starting to hear the phrase “warmest in 12000 years”- google “warmest 12000 years” and you’ll see recent coverage. This immediately raises the question of the Holocene Optimum – a period from about 8000 to 5200 BP in which there is undisputed evidence of significant NH warmth. The Team has taken a preditable position on the Holocene Optimum: that it’s a regional and restricted event. realclimate:
The [Holocene Optimum] is a somewhat outdated term used to refer to a sub-interval of the Holocene period from 5000-7000 years ago during which it was once thought that the earth was warmer than today. We now know that conditions at this time were probably warmer than today, but only in summer and only in the extratropics of the Northern Hemisphere. This summer warming appears to have been due to astronomical factors that favoured warmer Northern summers, but colder Northern winters and colder tropics, than today (see Hewitt and Mitchell, 1998; Ganopolski et al, 1998). The best available evidence from recent peer-reviewed studies suggests that annual, global mean warmth was probably similar to pre-20th century warmth, but less than late 20th century warmth, at this time (see Kitoh and Murakami, 2002).
realclimate links in turn to a NOAA website which thanks “Dr. Keith R. Briffa, Dr. Phil D. Jones, Dr. Michael E. Mann, and Dr. Henry N. Pollack” for their contributions, and which takes a similar position to realclimate as follows:
In summary, the mid-Holocene, roughly 6,000 years ago, was generally warmer than today, but only in summer and only in the northern hemisphere.
The question that I’m wondering about: do we know that mid-Holocene warmth was only in summer and only in the northern hemisphere, and, if so, how do we know it? I’m not going to consider high-latitude NH evidence, as that does not appear to be in issue. At some point I might re-visit the question of whether we know that it was only in summer, but for now, I’m going to focus on the topics and the SH. A recent survey of NH northern sites from 0-180W (Kaufman et al 2004) – and results would be similar for 0-180E – stated:
The spatio-temporal pattern of peak Holocene warmth (Holocene thermal maximum, HTM) is traced over 140 sites across the Western Hemisphere of the Arctic (0–180W; north of ~60N). Paleoclimate inferences based on a wide variety of proxy indicators provide clear evidence for warmer-than-present conditions at 120 of these sites. At the 16 terrestrial sites where quantitative estimates have been obtained, local HTM temperatures (primarily summer estimates) were on average 1.6 +- 0.8 deg C higher than present (approximate average of the 20th century), but the warming was time-transgressive across the western Arctic. As the precession-driven summer insolation anomaly peaked 12–10 ka (thousands of calendar years ago), warming was concentrated in northwest North America, while cool conditions lingered in the northeast. Alaska and northwest Canada experienced the HTM between ca 11 and 9 ka, about 4000 yr prior to the HTM in northeast Canada
The proxy evidence for the Holocene is different than millenium proxies – ring width chronologies decline dramatically in relevance, while ocean sediment dO18 and Mg/Ca, ice core dO18 and stalagmite dO18 all increase dramatically in importance. On a million-year scale, there is a quite decent and interesting similarity between the pattern of ocean sediment Mg/Ca and O18 and Antarctic ice core dO18 and dD values. So one does get a sense that the data rises above cherry-picked red noise.Here’s some of the sites that I want to look at: Stott’s ocean sediment sites in the West Pacific Warm Pool; tropical ice cores (Thompson and others); southern ocean sediments; Antarctic ice cores. There is some surprisingly strong evidence of high-latitude warming in the Holocene Optimum – evidence which seems to be just ignored by the Team and by NOAA. In the face of this evidence, Lorenz et al 2006 (and predecessor article Kim et al QSR 2004) have proposed a newish theory that is gaining sway in Team-world – that there was Holocene Optimum warming in the extratropics, but there was tropical cooling in the Holocene Optimum. So I’ll probably start with some southern extratropic sites – oncluded some of the Lorenz et al concessions and then get to the tropic sites.
I’ve made a new category “Holocene Optimum” to index the posts. Past this, there is the interesting question of the Holocene interglacial as compared to other interglacials during what has been generally a very cold past million years (the Pleistocene).