Holocene

Zeke Announces That The Holocene Climatic Optimum Has Been Cancelled

Forget the erasing of the inconvenient Medieval Warm Period, or the even warmer Roman Warm Period, the Klimate Konsensus are now coming for the entire Holocene, replacing it with a radically altered version which much better conforms to what their models say should have happened: a steady, linear rise in temperatures throughout after an initial steep rise at inception, followed by a glorious mann-made hockeystick at the end. Zeke loves it:

This means of course that we have broken through the glass ceiling of the Holocene with our demonic expellations of greenhouse gases and are now firmly within the new epoch of the Anthropocene. Not only that, we are probably well on our way to shattering the previous warm record of the interglacial preceding the Holocene, which will mean that we have indeed ‘broken’ the entire Quaternary Ice Age cycle.

Here is the abstract of the paper which has got Zeke so excited:

Proxy reconstructions from marine sediment cores indicate peak temperatures in the first half of the last and current interglacial periods (the thermal maxima of the Holocene epoch, 10,000 to 6,000 years ago, and the last interglacial period, 128,000 to 123,000 years ago) that arguably exceed modern warmth1,2,3. By contrast, climate models simulate monotonic warming throughout both periods4,5,6,7. This substantial model–data discrepancy undermines confidence in both proxy reconstructions and climate models, and inhibits a mechanistic understanding of recent climate change. Here we show that previous global reconstructions of temperature in the Holocene1,2,3 and the last interglacial period8 reflect the evolution of seasonal, rather than annual, temperatures and we develop a method of transforming them to mean annual temperatures. We further demonstrate that global mean annual sea surface temperatures have been steadily increasing since the start of the Holocene (about 12,000 years ago), first in response to retreating ice sheets (12 to 6.5 thousand years ago), and then as a result of rising greenhouse gas concentrations (0.25 ± 0.21 degrees Celsius over the past 6,500 years or so). However, mean annual temperatures during the last interglacial period were stable and warmer than estimates of temperatures during the Holocene, and we attribute this to the near-constant greenhouse gas levels and the reduced extent of ice sheets. We therefore argue that the climate of the Holocene differed from that of the last interglacial period in two ways: first, larger remnant glacial ice sheets acted to cool the early Holocene, and second, rising greenhouse gas levels in the late Holocene warmed the planet. Furthermore, our reconstructions demonstrate that the modern global temperature has exceeded annual levels over the past 12,000 years and probably approaches the warmth of the last interglacial period (128,000 to 115,000 years ago).

This looks like another classic case of ‘the models don’t fit the data, so let’s change the data’. In this case, they’ve refashioned the entire Holocene interglacial! That’s quite a historical/geological revision. The paper is pay to view but I imagine what they are arguing is that orbital forcings (which were at maximum during the Holocene Climatic Optimum) turn out to be a lot less important when determining average annual temperatures and are primarily a seasonal effect, with carbon dioxide dominating the mean annual temperature. The GHG-driven climate models must be right.

Update 29 Jan 2021

Nature have an explanatory article on this study now which provides further information. As I said, it was basically an exercise in making the data fit the models:

However, computational simulations of Holocene climate reveal only a long-term warming trend3Writing in Nature, Bova et al.4 report an analysis that effectively brings Holocene climate reconstructions in line with computational simulations.

The apparent temperature peak during the early Holocene (known as the Holocene thermal maximum) is a prominent feature in global syntheses of proxy-based climate reconstructions1,2 (Fig. 1). Its notable absence from computational modelling has been dubbed the Holocene temperature conundrum, and has puzzled climate scientists for years3

The authors calibrated their adjustment for seasonal bias using a period during the Eemian interglacial when orbital forcing exceeded those during the present interglacial:

Bova and colleagues’ new method identifies seasonal biases in SST records and enables the calculation of mean annual SST from seasonal SST. It takes advantage of the characteristics of the last interglacial period (128,000–115,000 years ago), which was marked by mild global temperatures, smaller ice sheets and higher sea levels than those of today7. This period is advantageous for the authors’ purposes in that the seasonal difference of incoming solar radiation (insolation) was greater than during the Holocene, whereas the effects of other factors that alter climate, such as greenhouse gases and ice, were weaker, making it easier to identify seasonal biases.

More specifically, the authors’ method involves identifying seasonal bias in the portion of an SST record that corresponds to the last interglacial, and in which there was a stronger correlation of SST with seasonal insolation than with mean annual insolation. The sensitivity of the SST record to seasonal insolation during this period is then calculated, and used as a benchmark to remove seasonal bias from the entire record, thereby allowing mean annual SST to be determined from that record. 

Bova et al. went on to create a synthesis of previously published SST records spanning the last interglacial and the Holocene periods. These records are based on two common proxies used for reconstructing SST: the chemical composition of the fossilized calcium carbonate shells of surface-dwelling unicellular marine organisms known as foraminifera; and organic biomarkers known as alkenones, which are synthesized by marine algae and settle into marine sediments. The authors found that the majority of the examined SST records are indeed seasonally biased.

After converting the seasonally biased SST records into mean annual SST records, Bova and colleagues infer that the climate has been warming since the early Holocene — that is, there is no evidence for a Holocene thermal maximum in mean annual temperatures (Fig. 1). They suggest instead that the Holocene thermal maximum is a seasonal feature driven by a peak in summer insolation in the Northern Hemisphere that occurred during the early Holocene.

Their explanation for the ‘new improved’ Holocene temperature reconstruction seems a bit lame to me. They assign the initial rise in temperatures during the early Holocene to the retreat of ice sheets, without bothering to say what caused the ice sheets to retreat and they blandly assert that the steady rise in Holocene temperatures during the last 6500 years has been due to increasing CO2.

This enabled Bova and colleagues to shed new light on the drivers of Holocene climate change. They find that the increase in global mean annual temperatures that occurred during the early Holocene (12,000–6,500 years ago) was a response to retreating ice sheets, whereas the continued increase in temperatures over the past 6,500 years is due to rising greenhouse-gas concentrations.

Only at the very end of the article does Nature inform us that the SST proxies used by the authors to correct for seasonal bias and to construct a new Holocene temperature profile were from an area of the globe spanning 40 degree north and south of the equator. It does seem a bit odd that they would exclude the high latitudes where seasonal insolation effects are so much greater in order to correct for significant seasonal biases . . . . . .

One limitation of the findings is that the new synthesis of proxy SST records is limited to the global region between 40° N and 40° S. Proxy records from higher latitudes were deliberately excluded because of the scarcity of such records for the last interglacial, and because of the proximity of those regions to ocean fronts, where ocean dynamics can affect SST. However, the inclusion of these regions might be needed in the future, given that processes at high latitudes have a substantial role in many climate feedback processes. Moreover, the new synthesis examines records based on only two SST proxies. Future work should include more records based on other temperature proxies. Nevertheless, by solving a conundrum that has puzzled climate scientists for years, Bova and colleagues’ study is a major step forward for the field.