If you’ve wondered where to look for signs that Earth is entering a geological epoch of our own making, the Anthropocene, what’s a good place to start?

I’d suggest the growing body of research concluding that what was once seen as an inevitable descent into the next ice age has been put off for a very long time by the building blanket of greenhouse gases generated by humanity’s burst of fossil fuel combustion.

A new addition to that literature — “Determining the natural length of the current interglacial” – is being published today in the online edition of Nature Geoscience.

The research, led by Chronis Tzedakis of University College, London, examined similarities between the current warm interval between ice ages and a particular point, around 780,000 years ago, during a past warm period known as Marine Isotope Stage 19. Using a variety of methods, the authors conclude that the onset of a new ice age would likely begin about 1,500 years from now, if the concentration of carbon dioxide was back below the levels produced since the Industrial Revolution.

I first explored when the next ice age would begin, and whether humans had forestalled that transition, in a Science Times article in 2003. James Hansen of NASA already had concluded at that time that the heat-trapping property of humanity’s gigaton-scale emissions of carbon dioxide was swamping the slight flux in incoming solar energy from periodic changes in Earth’s orientation relative to the Sun. ”We have taken over control of the mechanisms that determine the climate change,” he said.

In a news release one of the co-authors of the new study, James E.T. Channell of the University of Florida, echoes that point, saying: “The problem is that now we have added to the total amount of CO2 cycling through the system by burning fossil fuels. The cooling forces can’t keep up.” (Click here to read two news releases summarizing the work; a Popular Science post has more.)

I circulated the paper, under the journal’s embargo rules, to a variety of researchers focused on this question, including Hansen. Here’s the roundup of reactions:

Richard Alley, a longtime analyst of ice and climate at Pennsylvania State University:

Interesting. David Archer and [Andrey] Ganopolski, back in 2004 [“A movable trigger: Fossil fuel CO2 and the onset of the next Glaciation”], used modeling to show that human CO2 is heading off ice ages for perhaps a long time if we keep burning. This has been confirmed by others. In case you missed it, Shaffer, Gary, 2009 (“Long time management of fossil fuel resources to limit global warming and avoid ice age onsets,” Geophys. Res. Lett., 36, L03704, doi: 10.1029/2008GL036294), suggested that we if we saved the remaining fossil fuels, we could head off the next few ice ages by burning on the appropriate orbital schedule (although I have heard people suggest that we could manufacture much more potent greenhouse gases to do the same job). Cochelin et al used a model of intermediate complexity to show that the orbital variations over the next 100,000 years are weak enough that even a little human CO2 remaining in the atmosphere is enough to keep the earth out of an ice age (“Simulation of long-term future climate changes with the green McGill paleoclimate model: The next glacial inception”). So, overall, the idea that our CO2 is having a large impact on the climate that will last a long time, and exceed any natural trend to start a new ice age, is rather well established in the scientific literature (also see the references 2-3 and 5-7 in the new paper). The argument in the new paper is interesting, attempting to use the paleoclimatic record to quantify the threshold for glacial inception. The approach is clever and plausible, but I suggest a little caution on the exact quantification. First off, the orbital parameters are not exactly the same then as now; similarities surely exist, but there are differences. Also, the authors use the onset of a bipolar seesaw as the marker of glaciation. This may be true. However, there is enough remaining uncertainty about the controls on the “conveyor” that I would like a little more process understanding. We have fairly high confidence that we observe the history of Heinrich events (huge discharges of ice-rafted debris from the Laurentide ice sheet through Hudson Bay that are roughly coincident with large southern warming, southward shift of the intertropical convergence zone, extensive sea ice in the north Atlantic, reduced monsoonal rainfall in at least some parts of Asia, and other changes), and also cold phases of the Dansgaard/Oeschger oscillations that lack Heinrich layers and are characterized by muted versions of the other climate anomalies I just mentioned. We have a pretty good idea that the Heinrich events, with the most prominent bipolar seesaw behavior, are linked to ice-sheet behavior, but we’re less confident about the non-Heinrich cold phases of the D/O oscillations (the cold phases do have more ice-rafted debris in these non-Heinrich cold-phases than in warm phases, but is that an ice-dynamical signal, a survival-of-icebergs signal, or something else?). So, it would be nice to have stronger documentation that the bipolar seesaw behavior in the past is indeed Heinrich and not non-Heinrich D/O. Furthermore, I am among many people who have studied the Heinrich layers and tried to explain them, but we haven’t quite nailed it all down. I still believe that the Heinrich layers are primarily ice-dynamical (big ice doing interesting things and driving bipolar see-saw), but some uncertainties remain. In the modern world, for example, the big ice shelves—Ross and Filchner-Ronne—produce almost no ice-rafted debris, because the ice shelf is melting beneath, and keeps the ice attached to Antarctica until all the rocks have melted out before making icebergs. Warming would likely increase ice-rafted debris from the Ross and Filchner-Ronne. Thus, I’d be a little happier if we had a stronger process understanding of the Heinrich events, the D/O oscillations, and their dependence on the size of ice sheets. The authors are probably right, but I’m not positive that it rises to the pound-on-the-table level. So, I tend to believe the new results, they confirm what has been shown by several other studies—human-caused CO2 has a large enough effect lasting long enough that it will greatly affect the natural ice-age cycling—but I’ll listen to Peter, Andre and the others with great interest, and I don’t think this is the last word on exactly what CO2 level is needed for exactly what orbital configuration for ice-age initiation. –Richard

Peter Huybers, a Harvard University researcher focused on ice sheets and influences on the climate system (Huybers made an earlier appearance on Dot Earth):

I don’t have a whole lot to add beyond Richard’s discussion, but would like to make a few points about how well we can predict the future from the analogies available to us. [Marine isotope] Sub-stage 19c [the period that Tzadekis et al focus on] does seem one of the better analogies with our current interglacial, but the inference that our current interglacial would end within 1,500 years can be questioned on several accounts. First, as Richard noted, the orbital parameters are close but not exactly the same. Second, sub-stage 19c lies near the middle-Pleistocene, a time when the climate system appears to have been most clearly transitioning from smaller amplitude, shorter period, and more symmetric glacial cycles, to the larger, longer, and more saw-toothed glacial cycles of the late Pleistocene. Does stage 19c have the same background climate state as our present interglacial, or are the factors that gave rise to different glaciations in the early Pleistocene also important? Finally, there has been a long, on-going discussion of whether glacial cycles are primarily deterministic or stochastic. There is good evidence and physical grounds for CO2 and orbital variations influencing glacial cycles, but that by no means shows that they completely control the glacial system. For instance, it could be that differences in vegetation and the associated albedo vary stochastically from one epoch to another. Similar speculation could be made regarding specific states of ocean and atmospheric circulation. There is also the issue of how nonlinear the system is, and whether slight differences in earlier conditions could lead the present interglacial to differ substantially from 19c. Having only a small number of glacial/inter-glacial cycles to work with makes it difficult to test whether a plausible analogy is actually predictive.

James Hansen, the director of NASA’s Goddard Institute for Space Studies:

What would have been (absent humans) is only of academic interest. The two principal mechanisms by which the orbital effects on the regional/seasonal distribution of insolation instigate climate change are melting/growth of ice sheets (thus albedo change –> temperature change) and the slow feedback effect of GHGs responding to temperature change, thus increasing the greenhouse effect. But the huge human-made GHG changes cause ice to be melting all over the planet. So the growth of Northern Hemisphere ice sheets to start the next ice age can’t happen — on the contrary, as you can see, the tundra, Greenland and sea ice are melting and shrinking in area. So the increasing albedo mechanism needed to move the planet into the next ice age can’t happen — unless humans go extinct. Of course I know this is already well known by you.

André Berger, a remarkable Belgian climate scientist (whom I first met on the sea ice near the North Pole):

The future of our interglacial is indeed an old story. I give you here down some of our papers devoted to this problem. A long interglacial with CO2 equal or larger than 280 ppmv is what we claim since 1996. The paper in Ambio (1997) shows in Fig 2 the entrance into glaciation starting now if CO2 is 210 ppmv, but the threshold in our model for entering into glaciation has been indicated many times to be 240 ppmv (see paper by Loutre and Berger cited by Tzedakis et al., and the papers Berger and Loutre in Science 2002, Berger et al. in Surveys in Geophysics 2003). A more recent paper by Ganopolsky as a co-author confirmed such a finding. In the paper Crucifix et al., 2005, sensitivity to the entrance in glaciation is discussed in the framework of the Ruddiman hypothesis showing that low CO2 levels are necessary to enter into glaciation now. What is “new” and stressed in the paper by Tzedakis et al. is the discussion about the length of the interglacials. Ruddiman has also recently published the intercomparison of many interglacials in defense of his hypothesis claiming that we have avoided entering into glaciation already thousands of years ago. About the strategy used by Tzedakis et al. I remind you of the paper by Kukla et al and the special issue of Quaternary Research in 1972, concluding that based on the comparison with the other interglacials we will enter soon in glaciation (see the introduction of our papers to have a summary of that story). Berger A. , Loutre M.F., 1996. Modelling the climate response to astronomical and CO2 forcings. C.R.Acad.Sci.Paris, t.323, série IIa, 1-16 Berger A. , Loutre M.F., 1997. Paleoclimate sensitivity to CO2 and insolation. Ambio, 26(1), 32-37 Loutre M.F., Berger A., 2000. Future climatic changes: Are we entering an exceptionally long interglacial? Climate Change, 46 (1-2), 61-90 – [pdf] Berger A., Loutre M.F., 2002. An exceptionally long interglacial ahead? Science 297. 1287-1288 – [pdf] Berger A., Loutre M.F., Crucifix M., 2003. The earth’s climate in the next hundred thousand years. Surveys of Geophysics, 24, 117-138 Crucifix M., M.F. Loutre, and A. Berger, 2005. Commentary on ‘The Anthropogenic Greenhouse Era began thousands of years ago’. Climatic Change, 69, 419-426

Here are two relevant Dot Earth posts:

“Will the Next Ice Age Be a Very Long One?”

“More on Whether a Big Chill Is Nigh”

And here’s a suitable parting shot from my 2004 exploration of Greenland ice-sheet trends: