New Paper: “Reconciling divergent trends and millennial variations in Holocene temperatures”

Guest post by David Middleton

This appears to be a fairly high resolution regional Holocene climate reconstruction of North America and Europe…

Reconciling divergent trends and millennial variations in Holocene temperatures

Jeremiah Marsicek, Bryan N. Shuman, Patrick J. Bartlein, Sarah L. Shafer & Simon Brewer

Nature volume 554, pages 92–96 (01 February 2018)
doi:10.1038/nature25464

Abstract

Cooling during most of the past two millennia has been widely recognized1,2 and has been inferred to be the dominant global temperature trend of the past 11,700 years (the Holocene epoch)3. However, long-term cooling has been difficult to reconcile with global forcing4, and climate models consistently simulate long-term warming4. The divergence between simulations and reconstructions emerges primarily for northern mid-latitudes, for which pronounced cooling has been inferred from marine and coastal records using multiple approaches3. Here we show that temperatures reconstructed from sub-fossil pollen from 642 sites across North America and Europe closely match simulations, and that long-term warming, not cooling, defined the Holocene until around 2,000 years ago. The reconstructions indicate that evidence of long-term cooling was limited to North Atlantic records. Early Holocene temperatures on the continents were more than two degrees Celsius below those of the past two millennia, consistent with the simulated effects of remnant ice sheets in the climate model Community Climate System Model 3 (CCSM3)5. CCSM3 simulates increases in ‘growing degree days’—a measure of the accumulated warmth above five degrees Celsius per year—of more than 300 kelvin days over the Holocene, consistent with inferences from the pollen data. It also simulates a decrease in mean summer temperatures of more than two degrees Celsius, which correlates with reconstructed marine trends and highlights the potential importance of the different subseasonal sensitivities of the records. Despite the differing trends, pollen- and marine-based reconstructions are correlated at millennial-to-centennial scales, probably in response to ice-sheet and meltwater dynamics, and to stochastic dynamics similar to the temperature variations produced by CCSM3. Although our results depend on a single source of palaeoclimatic data (pollen) and a single climate-model simulation, they reinforce the notion that climate models can adequately simulate climates for periods other than the present-day. They also demonstrate that amplified warming in recent decades increased temperatures above the mean of any century during the past 11,000 years.

Nature

I might just be willing to spend $20 to buy this paper… It’s behind the paywall, of course.

Naturally, the  University of Wyoming press release is alarmingly misleading:

University of Wyoming researchers led a climate study that determined recent temperatures across Europe and North America appear to have few, if any, precedent in the past 11,000 years.

What the paper says:

Although our results depend on a single source of palaeoclimatic data (pollen) and a single climate-model simulation, they reinforce the notion that climate models can adequately simulate climates for periods other than the present-day. They also demonstrate that amplified warming in recent decades increased temperatures above the mean of any century during the past 11,000 years.

“Amplified warming in recent decades” relative to centennial means doesn’t make the current climate unprecedented.

Furthermore, climate models can’t adequately simulate the present-day climate.

And they also appear to be using Marcott et al., 2013 as a benchmark.

Figure 1

Otherwise it looks like an interesting paper.  If I have time, I might dig into the extended data and supplemental information.

Feature image from NOAA.

via Watts Up With That?

http://ift.tt/2nwnuj5

February 1, 2018 at 07:48AM

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