Monday, July 30, 2007

Climate on ice

The Earth has been going through a series of Ice Ages in the last few million years. Their discovery and investigation is one of the great stories of modern science. The understanding that human civilization began recently (about 12,000 years ago) during an interglacial (when the ice temporarily retreated) connected the historical and near-historical past with the "deep" human past - before the invention of agriculture and writing - and with our earlier ancestors (genus Homo and genus Australopithecus), whose rise was probably forced by the cooling and drying Ice Age climate.*

Before the Ice Ages started, Earth climate was apparently hotter, more humid, and ice-free - at the upper end of the "Goldilocks regime." Since then, Earth has oscillated between the middle and the cold end of "Goldilocks regime," cooling down and drying out - punctuated by episodes such as our current epoch of middling climate.

Are the Ice Ages understood by modern science? Yes and no. The basic facts of the Ice Ages have been established since the 19th century. Traveling through northern Europe and the northern US and much of Canada, you notice rocky soil, large boulders strewn about, and sharply-etched lakes and rivers - all results of the slowly grinding ice sheets advancing southward and retreating northward. More of the Earth's water volume was tied up in ice during the Ice Ages, and the sea level was lower. There was more coast, and America and Asia were connected by a land link at what is now the Bering Strait. From Siberia, the ancestors of the native Americans crossed the bridge, which was closed off when the last Ice Age ended. The Eskimos (Inuit) continued to cross back and forth by water.

Why the Ice Ages are happening is still not completely clear. Their timing is best explained by the insights of Milankovic first published in the early 20th century. He noticed some striking facts about the shape of the Earth's orbit and the tilt of its axis relative to the plane of its orbit around the Sun. At present, the Earth's orbit is nearly circular, and its tilt from vertical about 23.5o. That tilt defines the relative latitude-dependence of incoming solar radiation (insolation) and the strength of seasonal climate variations. But the tilt is not constant; it's disturbed by the gravity of the Moon and the Sun. The Earth's orbital shape is affected by the gravitational pull of other planets, notably Jupiter, and, if it becomes more elliptical, keeps the Earth slightly farther from the Sun for significant parts of the year. Each of these changes is a complex of oscillations with different periods, notably 41,000 years for the oscillation of the Earth's tilt and about 100,000 years for the Jupiter-induced disturbances. (There is also a weak 26,000-year period from the precession of the equinoxes.) These are exactly the periodicities needed to explain the advance and retreat of the ice sheets. Modern versions of this theory are based on more detailed field work uncovering the history of soil layers and ocean sediments that reveal far more about the ice and its history than was available a century ago to Milankovic. Modern computer calculations of the Earth's orbit and orientation allow more exact reconstructions over long periods.

And modern theories also incorporate a crucial enabling factor missing from Milankovic's proposal, the reduction or cessation of the mighty poleward ocean currents that move excess equatorial heat away from the tropics.** The present positions of the continents (sitting atop the South Pole and nearly landlocking the North Pole) reduce what would otherwise be a stronger poleward oceanic heat flow that would even out the Earth's temperatures more completely. Without these currents, the polar regions would be significantly colder than they already are. In fact, the Earth's climate would be cooler overall, but would also exhibit an even more extreme variation by latitude - another example of an important climate rule of thumb mentioned earlier. The era of ice ages features continental positions and ocean current reductions favorable to polar glaciation. During ice advance, these currents weaken further; during ice retreat, they become stronger. Recent work has led to an expansion of the list of secondary factors to include the effect of cloud and plant variability, in addition to ocean current and insolation changes.

The formation of ice sheets on land is not a simple matter of the polar regions becoming colder. Indeed, colder air has a much lower water saturation vapor pressure and tends to be drier than warmer air. Forming ice on the surface is not the same as the ocean turning into an ice cube. It requires enough warmth to evaporate liquid water, which is then subject to rapid cooling with altitude and latitude, leading to condensation as clouds and then precipitation as snow. And the growth and shrinkage of surface ice sheets formed from snow is not as obvious as it sounds. Cooling air in the polar regions is often correlated with shrinking ice sheets. Colder years still have warm summers, when some of the ice melts. The colder winter air is drier and produces less snow. The net effect, integrated over a year, is glacier shrinkage. Strangely, ice sheet growth is often correlated with somewhat warmer years, for just the opposite reasons. These facts are another reminder that climate is an on-going process, and "things" like ice sheets are really snapshots of processes. Indeed, ice sheets are best viewed, not as signs of cold, but as relics of a rapid and wet cooling episode at some time in the past. They also remind us of the independent importance of water and hydrology, alongside temperature, as a climate variable in its own right.

The modern theory of the Ice Ages (continental positions, poleward ocean currents, Milankovic's astronomical insights) is still not totally satisfactory. But it does fit the evidence well, better than anything else proposed so far, and Milankovic's ideas are as fundamental to modern geoscience as Wegener's theory of continental drift. Unlike continental drift, however, modern science has not directly witnessed episodes of dramatic ice advance and retreat. That leads to some degree of uncertainty.

It also leaves a hole temptingly exploited by the fanatics of "global warming" - proponents of the idea that minor atmospheric gases like carbon dioxide (CO2) and methane (CH4) are a major causative element in climate change. Such an idea collides head-on with the success of this modern picture of the Ice Ages that explains the Quaternary climate pretty well.†

More complete and refined measurements of ice advance and retreat and inferred temperature and atmospheric changes (from gas trapped in ice) provide no support for this obsession either. On the contrary: the temperature changes associated with Ice Age changes consistently precede changes in infrared-active gas concentrations, a fatal blow to the CO2 monomania. The pattern seems to be: temperature change > atmospheric chemistry change > ice change. The missing piece that can't be measured directly is probably the change in ocean currents, although changes in clouds and plants are also possible contributors. All three affect the ice coverage, but they also affect CO2 and CH4 concentrations in the atmosphere. These facts reinforce a conclusion arrived at by other lines of reasoning: these gases are mainly effects, not causes, in the Earth's climate. Changes in their concentration can have a weak, secondary causal effect on climate. The size of the resulting effect is limited to augmentation of temperature swings by no more than about 1 oC for roughly a doubling of the recent CO2 concentration (about 2-3 parts in 10,000). Shamefully, the IPCC's executive summary makes an exaggerated and unsupportable claim of 3 oC for this change, another striking case of bad science based on sloppy reasoning and misuse of evidence.

A simple but complete estimate given earlier was +0.3-0.4 oC for a doubling of CO2 concentration. Loosening the assumptions, but keeping all the crucial pieces of climate (evaporation, water vapor, clouds, convection, etc.), allows a larger estimated range of +0.3-0.8 oC. This estimate might be in serious error. But it is reassuringly both less than the rough upper limit of +1 oC, but not drastically smaller. Violation of the limit would mean the estimate is definitely wrong. Being too small (say, by a factor of 10 or more) would suggest something important missing from the estimate. And keep in mind that this estimate was atmosphere-only; it did not include the mitigating effects of ocean and plant absorption of CO2.

Attempts to invoke minor infrared-active gases as causes of Ice Age changes have flopped. These attempts to rewrite paleoclimate history with minor gases like CO2 and CH4 playing starring roles are reminiscent of the junk science of the "hockey stick." The claims of the "global warming" cult require not only a sense of certainty about climate models and immediate crisis that the science does not warrant; they also require overturning much of what has been learned in the last century and a half about climate and replacing real knowledge with cult "knowledge" - in this case, about the Ice Ages.

POSTSCRIPT: Recent precise measurements of ice core properties: Petit et al. (1999); Fischer et al. (1999); Delmotte et al. (2004).

Recent reviews on the Ice Ages, ice core science, and climate change: Masson-Delmotte et al. (2006); Peacock et al. (2006); Soon (2007).
* An interesting, if rather off-beat, version of this theory is the "aquatic ape" theory for the immediate ancestor of Australopithecus, living in a cooling and drying East Africa. The independent science writer Elaine Morgan has popularized and extended this idea. Like the Gaia concept, it has gotten entangled with a certain amount of touchy-feely New Age meshuggas. But the idea has a distinguished pedigree going at least as far back as the early Greek philosopher Anaximander and does explain many odd things about humans.

** The Gulf Stream, running between the Caribbean Sea and the northwest Atlantic, is the best known, but far from the only one.

† The Quaternary Epoch is the age of glaciation-deglaciation and also of hominids, modern humans and their immediate ancestors.

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