Experimenting with Earth [Archive.org URL]

The planet absorbs only about 70 percent of the incoming solar energy and, in turn, emits infrared energy into space to offset the solar heating.

Atmospheric gases, such as water vapor and carbon dioxide, absorb infrared energy emitted by the planet’s surface that would otherwise escape into space. These gases also emit infrared energy into space, but because the surface of the planet is, on average, much warmer than the atmosphere, the eventual result is a net trapping of infrared energy within the atmosphere. (Atmospheric gases absorb some incoming solar radiation as well, but this has only a negligible impact.) This reduction of the outgoing infrared energy by atmospheric gases is what we call the greenhouse effect.

Water vapor, carbon dioxide, and clouds are the major contributors to the natural atmospheric greenhouse effect, with water vapor the dominant greenhouse gas. (Some of the major gaseous constituents of the atmosphere, such as nitrogen and oxygen, do not contribute to the greenhouse effect.) […] For a long time, the prevailing view with respect to anthropogenic (human-generated) sources was that carbon dioxide was the only one of concern. The importance of the dozens of other greenhouse gases (including CFCs, methane, and ozone) was not recognized until the mid-1970s […] The CFCs and other anthropogenic greenhouse gases besides carbon dioxide currently contribute about 40 percent of the total anthropogenic greenhouse effect.

Enter James Watt, who ushered in the industrial era with his invention of the modern steam engine in 1784. Nobel Prize-winning chemist Paul Crutzen has argued that this development jolted Earth into a new geological era he calls the Anthropocene, with human beings profoundly influencing the environment. Greenhouse gases begin to accumulate in the atmosphere, and, as a consequence, more infrared energy is trapped and the amount of infrared leaving the planet to balance the solar heating is reduced. (However, because scientists lack suitable measurements from space, this reduction has not been confirmed.) Earth warms until the excess infrared energy is finally radiated into space to reach a new equilibrium that is warmer than the preindustrial climate. In sum, the warming of the planet in response to a buildup of greenhouse gases is indisputable; it’s based on fundamental and well-tested laws of thermodynamics and physics.

The important practical issue concerns the magnitude of the warming. How great is it? […] Although we still have a long way to go in developing the models, they have improved a great deal in the past decade.

Feedback effects are one of the greatest imponderables in these models. The ultimate source of water vapor in the atmosphere is evaporation from the oceans. More moisture will evaporate from a warmer ocean. Basic water vapor thermodynamics dictates that the amount of moisture the atmosphere can hold increases exponentially with temperature. (This explains why winters outside the Tropics tend to be dry and summers humid—the colder winter air simply can’t hold as much moisture as the warm summer air.) As a result, the greenhouse warming of the atmosphere increases the amount of water vapor, which, in turn, can further amplify the warming. A conceptually simple model incorporating these deductions has been built, and it, along with many of its variants, suggests that the increase in greenhouse gases since the 1850s should have committed the planet to a warming of about 1ºC. (Without the positive water vapor feedback, the estimated warming would be smaller by about 30 to 50 percent.)

However, because of the strong links among the atmosphere, the cryosphere (ice and snow), the oceans, and the land, the predicted warming is not uniform, but varies significantly with latitude, longitude, altitude, and season.

To sort out these feedbacks, we must turn to more sophisticated climate models than the one with which we began. The most advanced […] suggest that the committed warming should have been between 1.5ºC and 2ºC.

It’s important to recognize that the extent of global warming is not fully and immediately reflected in Earth’s surface temperatures…Basically, the oceans sequester the additional heat, delaying the full impact of greenhouse warming. Only much later will the heat stored in the ocean depths warm the oceans and the atmosphere. “Later” could mean anything from a few decades to a few centuries. Thus, the realized warming will always be smaller than the committed warming. Our best understanding of climate feedbacks and the rate at which heat is stored in the oceans suggests that the realized warming during the 20th century should have been about 50 percent to 75 percent of the committed warming.

To see how well all of these deductions and model predictions square with reality, we can compare them with real-world observations.

1. Taking into account the thermal inertia of the oceans, the realized warming we should have observed from 1850 to 2000 is about 0.5ºC to 1.5ºC. (These figures refer to the global average of surface temperatures, over land and sea.) The 0.5ºC is based on the simple model’s result of 1ºC committed warming and a 50 percent value for the realized warming. The 1.5ºC warming is obtained by using the upper range of value for the more sophisticated model’s estimate and the upper range of 75 percent for the realized warming.In fact, global surface temperature records reveal a warming trend of about 0.6ºC (give or take 0.15º) between 1850 and 2000. That’s certainly within the range predicted by the models, though it’s less than half of what the more sophisticated model predicts. Some or most of this discrepancy can be accounted for by the cooling effect of sulfate particles of anthropogenic origin (which we’ll describe later). In addition, natural causes contributed to the observed climate changes during this period.

   About half of the observed warming occurred between 1900 and 1940. After that period, the global mean temperature went into a cooling trend until the mid-1960s, followed by a larger warming trend that has continued to the present day. Warming induced by greenhouse gases cannot, by itself, explain these swings. Natural variations in the energy output of the Sun, cooling due to the scattering of solar energy back to space by sulfate aerosols from volcanic eruptions (in addition to sulfates from anthropogenic emissions), and nonlinear climate dynamics account for some of the warming trend until the 1940s and the cooling trend from the 1940s to mid-1960s. But none of these other factors can account for the large warming trend of the latter part of the 20th century. At least a half-dozen global climate model studies show, to a high degree of statistical certainty, that greenhouse gases are the dominant contributor to this warming.

2. Because some of the excess infrared energy is stored in the oceans, the amount of heat stored in the oceans should be increasing with time.Records of ocean temperatures down to a depth of about 3,000 meters stretch back to the 1950s. A recent study demonstrates that the heat content of all the world’s oceans has increased steadily during the past 50 years. The rate of increase matches very closely, with a high degree of statistical significance, the model-simulated increase attributable to greenhouse gases.
3. The water vapor content of the lower atmosphere should be increasing with time.The vertical distribution of water vapor is gauged by humidity-measuring instruments flown on balloons, but the measurements are few and far between. Nevertheless, available records do show that the amount of water vapor in the lower atmosphere (up to five kilometers from Earth’s surface) has increased during the past 50 years.
4. The warming should be greater at higher latitudes because the brighter ice and snow reflect more radiation.This is also happening. Alaska, for example, is experiencing a significant warming, along with ecosystem changes.
5. The snow cover on land, especially glaciers, and the sea ice cover should be retreating toward the poles.It’s been shown that arctic sea ice has thinned by about 45 percent over the past 30 years, and landlocked glaciers such as those in the Himalayas are in retreat in most parts of the globe. If the warming continues, it’s estimated that the Asian glacier field, the third largest collection of ice on the planet after the Antarctic and Greenland icecaps, will vanish during this century.

In short, there’s compelling evidence to conclude that the observed warming over the past 50 years is attributable largely to anthropogenic increases in greenhouse gases.

Jack Hollander’s main reason, in his essay elsewhere in this issue, for skepticism about the role of anthropogenic greenhouse gases in global warming is the 20-year record (1979–98) of satellite-based estimates of atmospheric temperature change […] Hollander relies on an analysis by John Christy and his colleagues in 2000. An analysis by Frank Wentz and several colleagues in 2002, using exactly the same data employed in the Christy study, reveals a warming of the atmosphere in closer agreement with greenhouse models.

Uncertainties surround any attempt to predict climate changes […] In other words, clouds have a cooling effect about five times larger than the warming effect from a doubling of carbon dioxide. The great unknown, however, is what will happen in the future, after all possible feedbacks are considered, if cloudiness increases or decreases. It’s possible that cloudiness could lessen future warming—and possible, too, that it could increase warming.

Another uncertainty has been introduced by human activities, such as the release of sulfur dioxide from coal combustion, that have altered the sulfur cycle. Anthropogenic emissions of sulfur dioxide, which converts into sulfate particles in the atmosphere, exceed those from natural sources, such as volcanic emissions, by more than a factor of two. Sulfate particles exert a cooling effect in two ways: directly, by scattering incoming sunlight back into space, and indirectly, by nucleating more cloud drops and increasing the brightness of clouds. These direct and indirect effects may have counteracted as much as 30 to 75 percent of the greenhouse warming the planet might otherwise have experienced.

The key issue with respect to clouds and aerosols is the extent to which the solar radiation reflected by the planet is out of equilbrium with its pre-Anthropocene values. Accurate radiation-budget measurements from space were begun only in the 1980s, and we need to continue them to document this major human impact.

The greenhouse gases we’re now adding to the atmosphere have very long lifetimes—on the order of centuries. (Aerosols, in contrast, survive only for weeks.) Put in simple terms, about 10 to 30 percent of the carbon dioxide we release from our cars today will still be circling the globe 100 years from now.

Our observational records and models are far from perfect, and it may take decades to make them sufficiently conclusive to convince everyone. By then the deed will have been done. As we continue to ignore or debate the issue, the question we must ask ourselves grows ever more urgent: How much risk do we want to take before slowing down the experiment human beings are performing on the planet?