We’ve all become rather used to graphs looking like the one that I’ve drawn below (other than the horizontal blue line that I will explain later). This, together with the corresponding graph of CO2 levels, is almost certainly same as Gore’s graphic on page 66 of Inconvenient Truth. I’ll bet dollars-to-doughnuts that some variation of this will be featured in the AR4 Summary for Policy-Makers. The Hockey Stick became important because it was promoted not simply in TAR, but in the SPM, which was the only document available for several months. So here are some thoughts on this.
The figure below shows the deuterium isotopes from the EPICA ice core in Antarctica, published in a variety of articles including Nature 2004 here
Figure 1. EPICA deuterium values. Blue line at -400, more or less marking retreat of continental glaciation from southern Ontario (and elsewhere). Grey marks interglacial periods with values above -400.
The blue line is set at a value to try to isolate interglacials. Values of -400 were reached about 12,300 BP. I tried to pick a value at which glaciers would not be covering Toronto. Toronto was free of glaciers by 12000 BP, though not 13000 BP., though glaciers still covered Ottawa.
I realize that attaching value to the absence of glaciers in Toronto may be controversial and be contested by lovers of ice shelves, glaciers and continental ice sheets as a parochial point of view, but as a holder of real estate in Toronto, I confess to being biased towards the absence of glaciers from southern Ontario. I think that one could reasonably take an opposite point of view about the presence of glaciers in Ottawa. So depending on your point of view, you might move the blue line a few points up or down.
If you overlay ocean sediment Mg/Ca series of Medina-Elizalde, Lea et al (or other such series), you get a remarkable coherence between it and EPICA deuterium. While I’ve been critical of many paleoclimate series, I’m inclined to believe that this series contains valid and valuable information.
An obvious observation is that for the past million years, Toronto has mostly been under a glacier. Interglacials seem to be relatively short – with amazingly fast entries and exits. In a commentary on the original publication of EPICA data, James C White (Science 2004) said that our present climate is a “rarity if not an anomaly”:
It’s worth noting that in the past 430,000 years, the percentage of time that climate was as warm as it is today is quite small, about 5 to 10%, and before that time, it appears to never have been that warm. Our current climate appears to be a rarity, if not an anomaly. Again, we don’t know why … We have basked in relative warmth for the past 12,000 years, considerably longer than the 4000- to 6000-year lengths of the previous three interglacials as seen in the Vostok ice core (3).
If one examines the length of previous interglacials (using EPICA deuterium gt -400, the blue line, as a metric), one can summarize the interglacials in which Toronto was not glaciated as follows. The period of the graph is 740 kyr. So as a first cut, Toronto was glaciated for over 90% of the time and unglaciated only during the interglacials listed below:
|Start (kyr)||End (kyr)||Length(kyr)|
Now how does the Holocene compare to other interglacials (rare as they are)? Of the 4 other interglacials shown above, the Holocene is already longer than two of them and the Eemian was not a whole lot longer. Stage 11 was nearly twice the length of the Holocene to date, but it’s the only such interglacial.
Now the current explanation for glacial-interglacial transitions is Milankovitch theory expressed in 4AR as follows:
The Milankovitch theory proposed that ice ages were triggered by changes in 65°N summer insolation minima, enabling winter snowfall to persist all year trough and therefore accumulate to build northern hemisphere glacial ice sheets.
There is certainly evidence that changing insolation minima resulting from orbital precession, obliquity and eccentricity changes has something (or a lot) to do with the ice ages, but trying to match the observations to insolation changes has not been accomplished. Gavin Schmidt, for example, says:
“We have a trigger for a new glaciation — insolation goes down. But there are times when it does, and times when it doesn’t … It’s not clear how all these things fit together.”
I agree with Gavin about this. If one tries to actually implement Milankowitch insolation theory to yield observed glaciations, it’s hard to get everything to work together. One of the big problems is the “100,000 year problem” – the recurrence of interglacials seems to have an approximate 100,000 year period, but the orbital power is at 41,000 years (obliquity) and 23,000 years (precession). There are any number of interest efforts to extract the observed cycle from different non-linear combinations of the orbital phenomena, but, as far as I can tell, there is far from being any “consensus” on the recipe.
Despite the absence of any consensus on exactly how orbital variations translate into ice ages, IPCC 4AR says that it is very likely (TM-IPCC) that the earth would not enter another ice age for at least 30,000 years in a non-anthropogenic situation:
There is no evidence that the current warming will be mitigated by a natural cooling trend towards glacial conditions. It is very likely that the Earth would not naturally enter another ice age for at least 30,000 years. ,,,, For Stage 11, these conceptual models show that the deglaciation is triggered by the insolation maximum at ~427 kyr, but that the next insolation minimum is not sufficiently low to start another glaciation. The interglacial thus lasts an additional precessional cycle, yielding a total duration of 28 kyr.
Adding in the already elapsed portion of 14,000 years to the very likely 30,000 years mentioned above, IPCC therefore believe that it is very likely that the present interglacial (absent anthropogenic effect) would last at least 42,000 years – a length which is nearly twice the length of the longest previous interglacial. I guess that means that the Holocene is an unprecedented interglacial.
Whether or not one believes IPCC here, the present interglacial is already not a “small” interglacial. One could reasonably expect some impact of a not-small interglacial on things like Antarctic ice shelves, an issue that I’ll return to on another occasion.