by Rasmus Benestad, with contributions from Caspar & Eric
In a recent article in Climatic Change, D.G. Martinson and W.C. Pitman III discuss a new hypothesis explaining how the climate could change abruptly between ice ages and inter-glacial (warm) periods. They argue that the changes in Earth’s orbit around the Sun in isolation is not sufficient to explain the estimated high rate of change, and that there must be an amplifying feedback process kicking in. The necessity for a feedback is not new, as the Swedish Nobel Prize winner (Chemistry), Svante Arrhenius, suggested already in 1896 that CO2 could act as an amplification mechanism. In addition, there is the albedo feedback, where the amount of solar radiation that is reflected back into space, scales with the area of the ice- and snow-cover. And are clouds as well as other aspects playing a role.
Martinson & Pitman III’s hypothesis states that the fresh water input works in concert with the Milankovitch cycle and the albedo feedback. They conclude that ‘major’ terminations can only follow from glacial build-up of sufficient magnitude to isolate the Arctic, inhibiting the inflow of fresh water to the point that salinity buildup in the surface layer from slow but continuous growth of sea-ice, causes overturn of the Arctic (through the effect on the atmospheric circulation and the ocean currents). The vertical overturning brings warmer water up from below, setting conditions that are more favourable for ice metling. Salinity plays a role too, but the hypothesis does not mention variations in the greenhouse gases (GHGs). A few questions: Did Martinson and Pitman III forget this last point? Or did the GHGs only represent a minor contribution? And, could not changes in GHGs explain much of the variability? On the other hand, it sounds plausible that changes in salinity and fresh water input may affect the sea-ice formation and the deep convection. However, so far, the hypothesis proposed by Martinson and Pitman III is merely a speculation, and we are waiting to see if the hypothesis can be tested through numerical model experiments (which would require higher resolution sea-ice and ocean models than used in todays global climate models). It would be interesting to carry out experiments to assess the significance of the fresh water only, GHGs, and the combined effect.
One reaction to the Martison and Pittman paper is: Where is the calculation of energy? Greenhouse gases only contribute a couple of W/m2, vs. the seasonal Milankovich forcing of >40. For this new idea to have merit, it had better have heat fluxes at least on par with the radiative forcing from CO2. Previous modeling studies find that GHG make up roughly 50% of the total LGM to present temperature response (see e.g. Broccoli & Manabe), the other part being albedo etc that respond to the seasonal cycle of irradiance. It is tricky to completely isolate the individual causes because changes in GHG may produce altered cloud and sea ice distribution. But roughly speaking, if you do an LGM run and only reduce sea level, put in the ice sheets, change the vegetation, add some dust (though that one is still rough), then you get about 50% the way you want to go. Change the GHG concentrations and you get close. This is more or less what Manabe and Stouffer showed 15 years ago. The question is do we need anything else, really, and does that ‘anything else’ pack sufficient punch.