A new analysis of the dramatic cycles of ice ages and warm intervals over the past million years, published in Nature, concludes that the climatic swings are the gyrations of a system poised to settle into a quasi-permanent colder state — with expanded ice sheets at both poles. [UPDATE 11/13: Authors and critics debate the findings.]

In essence, says one of the two authors, Thomas J. Crowley of the University of Edinburgh, the ice age cycles over the past million years are a super-slow-motion variant of the dramatic jostlings recorded by a seismograph in an earthquake before the ground settles into a new quiet state. He and William T. Hyde of the University of Toronto used climate models and other techniques to assess the chances that the world is witnessing the final stages of a 50-million-year transition from a planet with a persistent warm climate and scant polar ice to one with greatly expanded ice sheets at both poles.

Their findings have stirred a lot of skepticism in the community of specialists examining ancient records of past climate changes and how they might relate to variations in Earth’s orbit and orientation toward the Sun and other factors. I’ll be adding some of their reactions overnight (I’m on the road).

The Nature paper (abstract and citation below) goes on to propose that humans, as long as they have a technologically powerful society, would be likely to avert such a slide into a long big chill by adding greenhouse gases to the atmosphere. That doesn’t obviate the need to curb such emissions and the prospect of dangerous climate warming in the short run, Dr. Crowley said. But it is more evidence that like it or not, the future of conditions on Earth is likely to be a function of human actions, whether chosen or not.

The idea that human actions can dominate the climatic influence of things as grand as shifts in a planet’s orbit is hard to grasp, but quite a few climate specialist say it’s pretty clear this is the case. In 2003, I wrote an article exploring when scientists think we’ll slide into the next ice age (the conventional variety). James Hansen of NASA echoed Dr. Crowley, saying that as long as we’re technologically able, we’ll be able to keep the big ice at bay. Strange, wonderful stuff, climate science.

The paper citation details and abstract are below (it’s not online except for subscribers):

Nature Vol 456| 13 November 2008 doi:10.1038/nature07365

LETTERS

Transient nature of late Pleistocene climate variability

Thomas J. Crowley & William T. Hyde

Climate in the early Pleistocene1 varied with a period of 41 kyr and

was related to variations in Earth’s obliquity. About 900 kyr ago,

variability increased and oscillated primarily at a period of ,100

kyr, suggesting that the link was then with the eccentricity of

Earth’s orbit. This transition has often2–5 been attributed to a nonlinear

response to small changes in external boundary conditions.

Here we propose that increasing variablility within the past million

yearsmay indicate that the climate system was approaching a second

climate bifurcation point, after which it would transition again to a

new stable state characterized by permanent mid-latitude Northern

Hemisphere glaciation. From this perspective the pastmillion years

can be viewed as a transient interval in the evolution of Earth’s

climate. We support our hypothesis using a coupled energybalance/

ice-sheet model, which furthermore predicts that the future

transition would involve a large expansion of the Eurasian ice sheet.

The process responsible for the abrupt change seems to be the albedo

discontinuity at the snow–ice edge. The best-fit model run, which

explains almost 60%of the variance in global ice volume6 during the

past 400 kyr, predicts a rapid transition in the geologically near

future to the proposed glacial state. Should it be attained, this state

would be more ‘symmetric’ than the present climate, with comparable

areas of ice/sea-ice cover in each hemisphere, and would

represent the culmination of 50 million years of evolution from

bipolar nonglacial climates to bipolar glacial climates.