Quantifying Climate Forcings and Feedbacks over the Last Millennium in the CMIP5–PMIP3 Models*

Abstract

The role of radiative forcings and climate feedbacks on global cooling over the last millennium is quantified in the CMIP5–PMIP3 transient climate model simulations. Changes in the global energy budget over the last millennium are decomposed into contributions from radiative forcings and climate feedbacks through the use of the approximate partial radiative perturbation method and radiative kernels. Global cooling occurs circa 1200–1850 CE in the multimodel ensemble mean with pronounced minima corresponding with volcanically active periods that are outside the range of natural variability. Analysis of the global energy budget during the last millennium indicates that Little Ice Age (LIA; 1600–1850 CE) cooling is largely driven by volcanic forcing (comprising an average of 65% of the total forcing among models), while contributions due to changes in land use (13%), greenhouse gas concentrations (12%), and insolation (10%) are substantially lower. The combination of these forcings directly contributes to 47% of the global cooling during the LIA, while the remainder of the cooling arises from the sum of the climate feedbacks. The dominant positive feedback is the water vapor feedback, which contributes 29% of the global cooling. Additional positive feedbacks include the surface albedo feedback (which contributes 7% of the global cooling and arises owing to high-latitude sea ice expansion and increased snow cover) and the lapse rate feedback (which contributes an additional 7% of the global cooling and arises owing to greater cooling near the surface than aloft in the middle and high latitudes).

The PMIP4 contribution to CMIP6 – Part 3: The last millennium, scientific objective, and experimental design for the PMIP4 <i>past1000</i> simulations

“…Whereas model simulations generally reproduce the summer cooling, as well as aspects of regional and delayed responses to volcanic eruptions (Zanchettin et al, 2012Atwood et al, 2016), there are discrepancies between model results and the observed climate evolution, in particular regarding the amplitude of the response to volcanic eruptions (e.g. Brohan et al, 2012;Evans et al, 2013;Wilson et al, 2016;Anchukaitis et al, 2010).…”

Freshening of the Labrador Sea as a trigger for Little Ice Age development

“…Precisely dated records of ice-cap growth from Arctic Canada and Iceland (Miller et al, 2012) showed that LIA summer cooling and ice growth, potentially linked to volcanic forcing, began abruptly between AD 1275 and 1300, followed by a substantial intensification at AD 1430-1455. Moreover, a recent study about the role of radiative forcings and climate feedbacks on global cooling over the last millennium also concluded that the volcanic forcing is the factor that contributed the most (Atwood et al, 2016).…”

Comparing proxy and model estimates of hydroclimate variability and change over the Common Era

“…The former, namely stratospheric aerosols from strong volcanic eruptions, is the most important of these two forcings at the global scale during the preindustrial interval of the CE. Although the impacts persist for only a few years, volcanic forcing from multiple events constitutes the most important contributor to the global centennial-scale cooling observed from 850-1800 CE based on comparisons of single-forcing experiments or through the use of all-forcing simulations in which the contributions from individual forcings are isolated offline (Atwood et al, 2016). For this reason, volcanic eruptions are particularly important for gauging the forced hydroclimatic response in lastmillennium simulations.…”