Planetary Watch: A New Ice Age IS Coming … but Don’t Hold Your Breathby Bill Chameides | November 17th, 2008
posted by Erica Rowell (Editor)
Permalink | 10 comments
In our current climate, ice ages are more common than not, but the next ice age is a long way away. Global warming, however, is with us now.
Skeptics have been arguing that we should forget about global warming — a new ice age is imminent. Maybe, some say, it’s already started. In fact, a new study does predict the coming of an ice age, one promising to be more permanent than others. Is it imminent? Depends on how you characterize 10,000 years.
It may surprise you to know that in our current climate, ice ages are more the norm than not. Over the past three million years, covering the end of the Pliocene and the present Pleistocene epoch, the Earth’s climate has oscillated between cold times (called ice ages or glaciations) and warmer times, interglaciations. In the recent past (the last one million years or so) the ice ages have lasted for about 100,000 years, and the warmer periods tens of thousands of years. The last ice age ended about 12,000 years ago. The questions most relevant to us are: when will the next ice age occur and should we be concerned about a global cold wave or the current global warming? The answers lie in the mechanism behind the climate swings.
The oscillations between ice ages and warm periods can be qualitatively explained by the Milankovitch theory (for more details see here). The theory’s basic tenet is that the ice age–interglacial swings are triggered by changes in the Earth’s orbit about the sun (eccentricity), rotational changes of the Earth on its axis (precession), and changes in the tilt of the axis (obliquity, which is what causes the seasons).
The orbital changes affect how much sunlight reaches the Earth at different latitudes. These changes in solar radiation are then amplified by feedbacks involving carbon dioxide and other greenhouse gases, the ice albedo, and the large temperature swings inferred between ice ages and interglacials.
One of the major puzzles in the Milankovitch theory is the so-called Mid-Pleistocene transition. Before about one million years ago, the glacial periods lasted about 40,000 years (which corresponds to the frequency of obliquity changes). Then the glaciations transitioned to a 100,000-year cycle (which corresponds to the frequency of changes in eccentricity).
Why this transition? Scientists continue to discuss the cause. Now Tom Crowley of the University of Edinburgh (previously at Duke University) and William Hyde of the University of Toronto have added a new wrinkle to the debate in a paper just published in Nature. Using a simplified, coupled climate-ice sheet model, they conclude that the shift in the ice age cycling kicked off a slow transition to a new climate regime, one that will be characterized by a permanent ice sheet in the northern mid-latitudes. They argue that this transition is being driven by snow-ice albedo effects.
A permanent ice sheet in the mid-latitudes of the North Hemisphere sounds like bad news. But panic is a little premature. Tom Crowley states that “our model predicts a rapid transition [to an ice age] beginning in the 10,000-100,000 years. But the timing of this transition is surely model dependent — it could easily be a quarter of million years or so — still short from the context of geology but almost infinite from the viewpoint of society. Our results in no way can be interpreted as justification for continued use of fossil fuels, as that problem is near term and very significant.”filed under: climate change, faculty, global warming, Planetary Watch, science
and: albedo, glaciation, ice age, interglacial, Milankovitch theory