Soundings: Atmospheric turns on the horizon
Polaris has not always shown the way to the north. Earth wobbles; throughout the last 20-plus millennia, its axis has pointed to a variety of different pole stars (the next Sky Watch, this Saturday night at 7 p.m., will focus on pole stars past and future).
Other parameters of Earth’s orbit also change: our distance from the sun in winter or summer (we’re now closest in early January, farthest in early July); how sharply we tilt toward the sun in summer, away in winter.
Capping centuries of hypotheses, a mathematician in the 1920s correlated these orbital influences on sunlight with the comings and goings of ice ages over the past couple-of-million years.
Problem: The orbital changes don’t change insolation (sunlight) nearly enough to cause ice ages. The ” Milankovitch cycles” must trigger some other factor(s) that has or have a stronger effect on climate.
For some millions of years, Earth’s polar regions have been capped with ice – on Greenland in the north, on Antarctica in the south. The ice fell as snow; miles thick, it compressed, and metamorphosed to ice – ice which retains the layered structure of the snow from which it formed.
The ice also retains traces of the atmosphere through which the snow fell. Like tree rings, these layers can be counted backwards. Analyzing the gases they’ve trapped, we can determine the composition of the atmosphere in times past.
When light hits a wall, most of its energy is absorbed. The wall then radiates that energy back out again, as heat – infrared radiation (IR; shade a sun-lit wall with your body and feel the IR radiating from it).
From earliest times, practitioners of ancient chemistries – potters and glass-makers – have exploited the fact that the electrons in atoms selectively suck up certain wavelengths – colors – of electromagnetic radiation. Just as a rock band would sound different if you took out one of the instruments, light looks different when you subtract one or more of its colors.
Just which wavelengths an electron absorbs depends on its environment – the type of atom in which it resides, the types of atoms nearby, and so on. In atoms of iron, the wavelengths not absorbed result in earthy ochres and umbers; copper yields blues and greens; uranium, yellows and oranges (uranium was used to color glass and glazes for millennia before it was isolated as a metal).
Except for pollutants like nitrogen dioxide, which colors smog reddish brown, most of the gases in the atmosphere are transparent to both visible light and IR. But a number of trace gases – carbon dioxide and methane, for instance – are opaque to IR. They allow light to reach the Earth from above, but absorb the IR Earth would radiate back out to space. Re-radiating this absorbed energy in all directions, they warm the Earth – they are “greenhouse gases.”
Polar ice reveals that when the Earth was cold, there were less greenhouse gases in the air – less carbon dioxide and methane. When it was warm, there were more greenhouse gases – enough to explain the coolings and warmings of glacial and interglacial epochs.
But another problem arises: Atmospheric greenhouse gases concentrations rise and fall after the climate has already begun to change – there’s a lag. Greenhouse gases warm the atmosphere (and their absence cools it) – big time but they are not what the Milankovitch orbital changes first trigger.
The complexity – and mystery – of Earth’s climate system alerts us to the “inevitable surprises” (a term found in the scientific literature) to be expected when we tweak the atmosphere … most obviously, by pumping in huge quantities of greenhouse gases.
Alan Stahler trained as a biologist and is an amateur astronomer. He teaches private enrichment classes for students of all ages. His science programs can be heard at noon on alternate Tuesdays on KVMR-FM (89.5).
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