Orbital changes to solar radiation inside the Arctic circle alone are insufficient to end all ice ages. A new paper by Ralf Ellis proposes an elegant solution to the mystery of how ice ages terminate. Life itself could provide the ultimate thermostat control.
Variations in insolation at high latitudes occur in tune with the 23Ky precession cycle, but their strength is strongly modulated by the 41Ky obliquity cycle and the 100Ky eccentricity cycle. Glaciations used to follow the 41Ky obliquity cycle, but for the last 800Ky the maximum summer insolation in the arctic has not been enough to fully terminate an ice age. Ice sheets in the Northern hemisphere have grown so large that the ice-albedo feedback suppresses solar melting alone. Something else is now needed to break the ice albedo feedback effect. The basic proposal is as follows.
As ice sheets expand and ocean temperatures fall so CO2 levels will eventually fall dangerously below 200ppm, threatening the survival of plants based on C3 photosynthesis. Boreal forests and temperate grasslands will begin to die back exposing soil to weathering. Desertification ensues and strong winds transport dust storms over the ice sheets. With very little annual snowfall during a glacial maximum, this dust layer will build up and thereby reduce net ice albedo. The next ‘Grand Summer’ precession cycle at maximum eccentricity will now be able to rapidly melt back the ice sheets, because finally a lower albedo ensures that more heat is absorbed by the ice sheets. Does the data actually support this hypothesis ?
The last glacial cycle is particularly interesting because the Great Summer maxima are suppressed by low values of orbital eccentricity. This results in the classic sawtooth shape showing how two insolation maxima had very little effect on ice volume growth. However, when CO2 levels fall below ~220 ppm the deposition of dust increases dramatically. The onset of the next summer maximum triggered the current interglacial. We are very lucky that human civilization developed during an era of low orbital eccentricity since the current summer minimum insolation in the arctic is very weak. Otherwise we would already be heading for the next ice age. The most similar interglacial occurred 400,000 years ago and lasted ~30,000 years (see figure 2). That too shows a large dust peak prior to rapid warming.
I think Ellis’s proposal is attractive and it is likely to be the correct explanation for the last glacial termination, as no other explanation so far makes sense to me. However the evidence is less strong for the previous glacial cycle. What is clear is that whenever CO2 levels fall below 200ppm there is a steep increase in dust deposition. The lack of moisture and low CO2 levels kills off boreal forests and soil turns to dust. The previous glacial cycle shows a long period of dust but without a final peak. However, the final Great Summer insolation is very strong, reaching an average daily insolation at the pole of 600W/m2. Note however that a similarly large excursion during the last ice age had no effect because CO2 levels were still high without any dust deposition on the ice sheets.
Figure 2 above shows the last 800k years and demonstrates why it is eccentricity that drives the long-term glacial cycle. What is also evident is that there are always peaks of dust deposition whenever CO2 levels fall below 200ppm. However, the data implies that it is only when eccentricity is small that the dust-albedo effect becomes critical. Otherwise the boost to a Great Summer Insolation caused by large eccentricity is usually sufficient to terminate a glaciation. Note that the glacial ‘cycle’ from -630Ky to -480Ky essentially consisted of three mini-cycles, each broken by an enhanced Great Summer Insolation, because eccentricity remained continuously above current values.
The IPCC AR5 report is closer to a ‘CO2 controls everything’ scenario, because they see the feedback of CO2 forcing feedback as the ‘extra’ mechanism for ending ice ages. However cooler oceans absorb more CO2 and rises in CO2 follow those first in temperature. There clearly must be some enhanced CO2 greenhouse feedback, but rises in humidity(H2O) with warming are probably far more important. Taken together rising CO2 and H2O levels boost the regrowth of boreal forests thereby ending dust storms. Life rapidly recovers following a deep shock. Gaia lives on.
In conclusion Ralf Ellis has made a very interesting proposal which I believe could well be the correct explanation for the rapid recovery from the last glacial minimum (LGM), and also that of 400,000 years ago. During the LGM CO2 levels reached dangerously low levels of ~180 ppm causing arid desertification as temperate trees and savannah died off. The resulting dust storms then deposited huge amounts of dust on the ice sheets increasing its albedo. The consequent Great Summer Insolation, coinciding with maximum eccentricity, finally melted back the ice sheets through reduced albedo, perhaps aided by increasing CO2 and H2O. However, the evidence is far less strong for previous glacial cycles. A fall in dust induced ice-albedo is less likely to be the primary cause of interglacial warming. This is because the orbital eccentricity was much larger and was sufficient to melt back ice sheets for the Great summer maxima.