The World in 2050_ Four Forces Shaping Civilization's Northern Future - Laurence C. Smith [12]
The basic physics of this was worked out in the 1890s by the Swedish chemist Svante Arrhenius.36 Like glass, greenhouse gases are transparent to short-wavelength sunlight, allowing it to pass unimpeded through the atmosphere to warm the Earth’s surface (unless blocked by a cloud). But they are opaque to the (invisible) long-wavelength infrared radiation returned from the warmed Earth back to space, instead absorbing it and thus becoming infrared radiators themselves.
Arrhenius was trying to solve the puzzle of ice ages, so was initially interested in global cooling, not warming, but his calculations worked easily well in either direction. He later wondered if humans, by adding carbon dioxide to the air through fossil-fuel burning, could also influence the planet’s climate. He ran the numbers and found that they certainly could, and substantially, too, if the gas’s concentration was raised high enough. His initial estimate of +5°C warming for a doubling of atmospheric CO2, calculated by hand, was remarkably close to the ones generated by far more sophisticated computer models running today. But Arrhenius didn’t think much of this at the time, because he couldn’t imagine humans ever releasing that much carbon dioxide. For humans to double the atmosphere’s CO2, he reasoned, would take at least three thousand years .37
Apparently, the physics of greenhouse gas warming is a lot easier to comprehend than the pace of human industrialization. We’ve already raised the concentration of CO2 in the atmosphere nearly 40%, up from ~280 parts per million by volume (ppmv) in preindustrial times to ~387 ppmv as of 2009. Two-thirds of that rise has been carefully documented since 1958, when the first continuous air sample measurement program was begun by Charles Keeling at Hawaii’s Mauna Loa Observatory as part of the International Geophysical Year. Atmospheric measurements of two other powerful greenhouse gases also released by human activity, methane and nitrous oxide levels, have followed a similar rising pattern. Depending on the choices we make about carbon emissions, CO2 projections for century’s end range anywhere from 450 to 1,550 ppmv, corresponding to a +0.6 to +4.0°C increase in average global temperature on top of the +0.7°C increase already experienced in the twentieth century.38 Many policy pragmatists now feel a +2°C increase is all but assured, after the 2009 Copenhagen Climate Conference failed to produce anything resembling a binding international agreement to curb carbon emissions.
These numbers may sound small but they’re not. At the height of the last ice age, when Chicago was buried under a mile-deep sheet of ice, global temperatures averaged just 5°C (9°F) cooler than today. From historical weather-station data the global average temperature is already +0.8°C warmer than in Arrhenius’ time, with most of that rise since the 1970s. An increase of that magnitude is already much larger than the difference between any one year and the next. As expected, this warming trend varies strongly with geography, with even some local cooling in some places (the details and reasons for this are known and discussed further in Chapter 5). But the global average is trending upward, along with the steady measured growth of greenhouse gas concentrations in the atmosphere.
Not only are average temperatures rising, the way they are rising is consistent with the greenhouse effect but inconsistent with other natural cycles and processes also known to influence climate. Temperatures are warming more by night than by day; more in winter than in summer; more over oceans than over land; more at high latitudes than in the tropics; and in the troposphere