We employed the CCSM4 GCM (see Methods) to test whether glacial inception is simulated for the Stage 19 equivalent of the present day at 777 ka (Table 1), noting that the two largest forcings–insolation and CO 2 –are very close analogs to those proposed in the EAH. The negative greenhouse forcing in MIS19, relative to pre-industrial “PI” conditions at year 1850, causes a much colder climate. The mean-annual global temperature falls by 1.27 K, while the 5–6 K cooling in the high Arctic is the most pronounced anywhere (Fig. 2a). Polar temperatures drop the most over sea ice and along adjacent Arctic land, including Alaska and Baffin Island. High-latitude cooling during summer is especially important for promoting glaciations21, and the MIS19 simulation produces substantial and widespread summertime temperature decreases of over 3 K across much of the Arctic, especially over northeastern Siberia, northwestern North America, and the Canadian archipelago (Fig. 2b). Accompanying this terrestrial cooling is a comparable temperature decrease over the North Atlantic and Nordic Seas, where a pronounced expansion of sea ice occurs (Fig. 2c). The cooling in the North Atlantic versus the North Pacific is presumably enhanced by the response of the Atlantic Meridional Overturning Circulation (AMOC), which weakens by 2 Sv (25.5 Sv to 23.5 Sv) in MIS19.

Figure 2 Changes in surface temperature (K) and sea ice coverage (%) between MIS19 and PI. (a) Mean annual temperature, (b) Boreal summer temperature, June-August, (c) Boreal summer sea ice. Full size image

These regional summertime cooling maxima of over 5 K associated with expansions of terrestrial snow cover and marine ice cover are both expressions and drivers of incipient glaciation. One way to identify year-round snow cover is to apply the 5% grid-box snow concentration threshold (see Methods) to the PI and MIS19 simulations (Fig. 3). Based on this definition, much more permanent snow cover emerges in MIS19, especially in Siberia, Alaska, and the Canadian Northwest Territories. Overall, snow cover persisting throughout the year encompasses 9.92 million km2 in MIS19, compared with 5.99 million km2 in PI, representing a 66% increase (92% expansion excluding Greenland). Topography exerts a strong influence on where permanent snow cover forms (Fig. 3c), particularly in the PI simulation, as highly elevated regions are the preferred sites in Siberia and Alaska (Brooks Range and coastal Alaska Range). Elevation plays a less obvious role in the MIS19 run, which is cold enough to support a year-round snow pack even at lower altitudes. We assume that these regions of year-round snow cover would eventually become land ice if the model included glacial processes.

Figure 3 Regions with a year-round snow pack in (a) PI and (b) MIS19, based on August snow cover of at least 5% in a gridbox. (c) Model topography, meters. (d) Area of year-round snow cover in the Northern Hemisphere in final years of PI and MIS19 simulations. Full size image

Based on our standard definition of glacial inception (see Methods), the MIS19 simulation reaches this state, because every year at the end of the MIS19 run has more extensive year-round snow cover than PI (Fig. 3d). Conversely, the PI simulation is not cold enough to constitute a state of glacial inception relative to CCSM4’s transient 20th-century simulation (Table 1), because several years in the 20th-century run exhibit more permanent snow cover than the year with the least permanent snow cover in PI (not shown).

Locations of incipient glaciation can also be identified as grid boxes that reach the model-constrained snow depth limit of 1 m snow water equivalent (see Methods). The areal extent of such regions doubles in MIS19 versus PI (including Greenland) and encompasses most of Baffin Island and Novaya Zemlya and all of Svalbard, Franz Josef Land, Severnaya Zemlya, and the New Siberian Islands (Fig. 4a,b). Many of these locations are regarded as initial nucleation sites for marine ice sheets that developed over the Barents and Kara Seas22. Moreover, the snow depth limit is reached inland on Taimyr Peninsula and in a few isolated regions in far northeastern Siberia and northwestern North America. Over the entire Arctic (60°–90°N), the area reaching maximum snow depth increases during the MIS19 simulation (Fig. 4c), suggesting an expansion of glaciation with time. By contrast, the corresponding area in PI is fairly stable and far smaller than in MIS19, evident by the clear separation in their time series.

Figure 4 [Top]: Fraction of years during the final 50 years of the simulations when snow depth reaches the model-constrained 1 m SWE limit in (a) PI and (b) MIS19. [Bottom]: Time series from MIS19 (black) and PI (gray) simulations showing the fraction of surface area poleward of 60°N that reaches this limit during August. The 50-year running mean is shown in red for MIS19 and blue for PI. Full size image

Particularly interesting for this study is the emergence of maximum snow depth in MIS19 over most of the Canadian Archipelago and Baffin Island within the first few years of the simulation. This region has long been supposed to be the main area of glacial inception for the Laurentide ice sheet23, and there is clear evidence that glaciers expanded on Baffin Island during the Little Ice Age24,25. Our PI simulation corresponding to the end of the Little Ice Age shows most of the island with permanent snow cover, based on the 0.05 concentration, and occasional years when the maximum snow depth threshold is reached (Figs 3,4). This area and the expanded region of maximum snow depth along the Greenland coast in the MIS19 run are likely sources for ice-rafted debris deposited in MIS 193. Although CCSM4 does not represent glacial processes, the model’s pronounced cooling and sea ice expansion in the North Atlantic and Labrador Sea would have created a favorably chilled marine environment for the production and preservation of calving icebergs, whose deposits associated with MIS19 glacial inception have been identified in sediments at North Atlantic Ocean Drilling Program Site 9834 south of Iceland, around the region of extreme temperature decreases (Fig. 2b).

Large areas of permanent snow cover blanket northeastern Eurasia, but no regions reach the snow depth limit east of the Taimyr Peninsula except near the Chuhotka Peninsula in easternmost Siberia. Glaciated landscapes in northeastern Russia are known to be limited to mountainous regions because of pervasive aridity and strong continentality with relatively warm summers26. There is evidence of a possible MIS5d local glacier advance in the Chuhotka Peninsula27 and Verkhoyansk Mountains of Siberia28 but not in other parts of northeast Siberia.

Although a colder climate in MIS19 is the proximate cause of permanent snow cover emerging in these three regions, the CCSM4 simulation reveals important contributions from circulation changes that appear to play a major role in shaping these patterns. Both increased accumulation of snowfall and reduced ablation act to enhance snow depth, but most studies have found that a decreased ablation effect is more important for generating permanent snow cover29,30,31.

For glacial inception in MIS19, both processes contribute but their relative importance differs by region, as is the case for ice sheets generally32. Over Baffin Island and the Archipelago, reduced ablation is promoted by the development of a summertime high-pressure anomaly centered over the broader Greenland area that extends across much of the Arctic Ocean and into Siberia (Fig. 5a). This anomalous anticyclone is fostered by the strong surface cooling of the ocean surrounding Greenland (Fig. 2b), which in turn is caused by the combination of a weakened AMOC and expanded sea ice (Fig. 2c). This ice expansion results in a pronounced spread of sub-freezing surface conditions during summer in the North Atlantic-Labrador Sea region, approximately delineated by where sea ice cover expands by at least 15%. A consequence of the anomalous high pressure around Greenland is a change in the lower-atmospheric wind field (Fig. 5b). This results in enhanced flow over the much-colder ocean surrounding Greenland and onto Baffin Island, where the most pronounced emergence of maximum snow thickness occurs (Fig. 4). The circulation change over the Arctic Ocean also favors less ablation over Siberia, due to the weaker but consistently onshore flow coming from the Arctic Ocean.

Figure 5 Changes between MIS19 and PI simulations. (a) Sea level pressure (hPa) and (b) lower atmosphere (surface-750 hPa) wind velocity (m s−1) in summer; (c) snowfall (cm) and (d) lower atmosphere (surface-750 hPa) wind velocity (m s−1) annually. Full size image

Over northwestern North America, by contrast, the emergence of a perennial snowpack is fostered by atmospheric circulation changes that favor increased accumulation of snowfall through enhanced transport of onshore, upslope flow into the Rocky Mountains and Alaska (Fig. 5c,d). The large increase of annual snowfall over this region matches well with the appearance of permanent snow cover (Fig. 3). A similar type of circulation change also favors the increase of snowfall over the mountains of Norway, where a few grid cells become permanently snow covered in MIS19. However, the accumulation effect is not responsible for the development of permanent snow cover over Baffin Island, which receives slightly less snowfall in MIS19.

In summary, both our model simulations and proxy evidence suggest currently suitable conditions for incipient glaciation, in the absence of anthropogenic carbon emissions, due to the same favorable orbital and greenhouse forcing that triggered the cessation of interglacial warmth at the end of MIS19. Present-day orbital forcing is virtually the same as in MIS19, and contemporary GHG forcing would be virtually equivalent to MIS19’s if the Holocene climate had followed the expected late-interglacial GHG decline33. In that case, our present-day natural climate should be approximately the same as MIS19’s, including glacial inception. If the upward GHG trends during the late Holocene were caused by early agricultural carbon emissions, then ancient farming was apparently sufficient to avert a contemporary glacial inception.

Support for this possibility comes from our supplemental “Natural PI” CCSM4 model experiment, PI_NAT, which used contemporary orbital parameters and estimated natural GHG concentrations (Table 1; see Methods). Its expanded permanent snow cover (Fig. 6) is virtually identical to the MIS19 run (Table 2, Fig. 3b), indicating that the similarities in greenhouse forcing between MIS19 and “natural” present-day dominate over the slight orbital forcing differences.

Figure 6 Regions with a year-round snow pack in PI_NAT, based on August snow cover of at least 5% in a gridbox. Full size image

Table 2 Comparison of Arctic (60–90°N) insolation forcing annually (W m−2), GHG radiative forcing (W m−2), permanent snow cover area (×106 km2), and Arctic (60–90°N) temperature change (K) relative to PI among CCSM4 simulations. Full size table

Although the PI_NAT results are based on estimated levels of greenhouse gases in the absence of all anthropogenic carbon emissions, these estimated concentrations can be partially constrained. Removing the CO 2 contribution from industrialization lowers the contemporary concentration to 285 ppm, which therefore serves as an absolute upper limit for a natural present-day. This CO 2 value was preceded, however, by rising concentrations since 8000 years ago, when the concentration had fallen to 260 ppm. Because CO 2 levels in all recent, comparable interglaciations identified by ref.34. (MIS 5, 7, 9, 11, 17, and 19) declined during their final eight millennia (up to their insolation minimum), an expected present-day CO 2 concentration is no higher than 260 ppm. In fact, CO 2 values have fallen by a minimum of 5 ppm (MIS 7 and MIS 11) and a maximum of 16 ppm (MIS 9 and MIS 17) in their final 8000 years. This spread implies an expected contemporary concentration of CO 2 as low as 244 ppm and sets another plausible upper limit of 255 ppm, bracketing a range that encompasses the 245 ppm concentration used in PI_NAT as our best estimate of a natural present-day value (see Methods).

That a fairly small CO 2 decrease, relative to the CO 2 rise from industrialization, can leverage a large climate change is supported by findings of enhanced cold-climate sensitivity. Stronger positive feedbacks between temperature and albedo (in this case illustrated by a pronounced expansion of sea ice area and permanent snow cover) occur in relatively cold climates, such as those with greenhouse gas concentrations only slightly below those of PI35, implying that only a modest external forcing perturbation is then required to push the climate system into glacial inception. Further evidence for this enhanced cold climate sensitivity is found in comparisons of external forcing changes and climatic responses in the four CCSM4 simulations summarized in Table 2. PI_NAT has only modestly reduced GHG forcing relative to PI (−1.02 W/m2), but there is a large increase in permanent snow cover and a large decrease in Arctic surface temperature35. MIS 19 has only a small net negative Arctic insolation forcing (−0.75 W/m2) and a slightly smaller reduction in GHG forcing relative to PI (−0.92 W/m2), yet the response is an equally large increase in permanent snow cover and nearly as large a decrease in Arctic temperature as in PI_NAT (Table 2).

In contrast, the glacial inception simulation of ref.36 for 115 ka (the end of MIS 5e) employs a much larger annual negative insolation forcing (−4.3 W/m2) in the Arctic (60–90°N) with no change in GHG forcing relative to PI and produces a large increase in permanent snow cover (although not quite as large as PI_NAT or MIS 19), along with a small decrease in Arctic annual temperature. The large orbitally forced decrease in summer temperature is sufficient to initiate the increase in permanent snow cover, even with GHG forcing equivalent to PI (the annual Arctic temperature decrease is smaller because the winter temperatures are higher). These experiments thus demonstrate “bookends” of forcing required for glacial inception: either strong orbital forcing with no change in greenhouse gases (MIS5e) or strong greenhouse forcing with little or no orbital influence (MIS19, PI_NAT) relative to present-day. Glacial inception occurs in all three cases, but cold climate sensitivity via a strengthened positive temperature/albedo feedback is most enhanced for PI_NAT and MIS 19, whose GHG forcing is slightly reduced relative to PI.