New paper finds climate models unable to simulate the Holocene Climate Optimum & subsequent cooling

A paper under discussion for Climate of the Past finds climate models are unable to simulate the mid-Holocene Climate Optimum from ~4-6 thousand years ago when global temperatures were naturally 2-3C higher than the present. The models are also unable to simulate the gradual cooling from the Holocene Climate Optimum to the Little Ice Age and pre-industrial temperatures.

According to the authors,

"The model does not capture the mid-Holocene "thermal maximum" and gradual cooling to pre-industrial global temperature found in the data."

If climate models are unable to simulate the Holocene Climate Optimum, Roman Warm Period, Dark Ages Cold Period, Medieval Warm Period, and little Little Ice Age over the past 4,000 years, how can they possibly be relied upon to simulate the Current Warm Period or to distinguish natural variability from anthropogenic?

Clim. Past Discuss., 10, 2925-2978, 2014 www.clim-past-discuss.net/10/2925/2014/ doi:10.5194/cpd-10-2925-2014 Global climate simulations at 3000 year intervals for the last 21 000 years with the GENMOM coupled atmosphere–ocean model

J. R. Alder and S. W. Hostetler
US Geological Survey, College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, United States
Abstract. We apply GENMOM, a coupled atmosphere–ocean climate model, to simulate eight equilibrium "time-segments" at 3000 yr intervals for the past 21 000 years forced by changes in Earth-Sun geometry, atmospheric greenhouse gases (GHGs), continental ice sheets and sea level. Simulated global cooling during the Last Glacial Maximum (LGM) is 3.8 °C and the rate of post-glacial warming is in overall agreement with recently published temperature reconstructions. The greatest rate of warming occurs between 15 and 12 ka (2.4 °C over land, 0.7 °C over oceans and 1.4 °C globally) in response to changes in radiative forcing from the diminished extent of the Northern Hemisphere (NH) ice sheets and increases in GHGs and NH summer insolation. The modeled LGM and 6 ka temperature and precipitation climatologies are generally consistent with proxy reconstructions, the PMIP2 and PMIP3 simulations, and other paleoclimate data-model analyses. The model does not capture the mid-Holocene "thermal maximum" and gradual cooling to pre-industrial global temperature found in the data. Simulated monsoonal precipitation in North Africa peaks between 12 and 9 ka at values ~ 50% greater than those of the PI, and Indian monsoonal precipitation peaks at 12 and 9 ka at values ~ 45% greater than the PI. GENMOM captures the reconstructed LGM extent of NH and Southern Hemisphere (SH) sea ice. The simulated present-day Antarctica Circumpolar Current (ACC) is ~ 48% weaker than observed (62 vs. 119 Sv). The simulated present-day Atlantic Meridional Overturning Circulation (AMOC) of 19.3 ± 1.4 Sv on the Bermuda Rise (33° N) is comparable with the observed value of 17.4 Sv. AMOC at 33° N is reduced by ~ 15% during the LGM, and the largest post-glacial increase (~ 11%) occurs, unforced, during the 15 ka time slice.