Beneficent Greenhouse

Since the retreat of the last ice age, some 12,000 years ago, we have enjoyed the benevolence of a virtual greenhouse. A greenhouse or hothouse is a glass-enclosed structure devoted to the cultivation of flowers and plants—usually out of season—that's the key, out of season, because greenhouses are warm places, all year long. The glass of a greenhouse permits passage of the visible, shortwave radiation coming from the sun, but absorbs the reradiated longer infrared radiation, which warms the otherwise cool space. The incoming light (heat/energy) falls on the objects inside, warming them and raising their temperature. Our natural greenhouse, or greenhouse effect, is a highly salutary condition found only around planet Earth. Venus is a hothouse, a very hothouse, and Mars is a veritable icehouse.

Radiation, light, from the sun hurtling through space penetrates a blanket of gases before striking the earth. For some 300 miles from ground zero, the earth is sheathed in an envelope of gases, an actual protective cocoon, its atmosphere. This atmosphere consists of the radioactively active gases carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and water vapor, the so-called greenhouse gases, and is transparent to the incoming visible, shortwave solar radiation. For a comprehensive understanding of the greenhouse effect, the earth's energy budget must be known. This budget, like any budget, will have its credit and debit sides—energy in, energy out. The energy, heat, coming in from our closest star, the sun, heats the earth such that each square meter receives an average of 342 watts per square meter (W/m2) of solar radiation, but 30% is immediately reflected back as the longer, less energetic infrared radiation. The remaining 235 W/m2 is absorbed by the gaseous atmospheric "blanket"; 168 W/m2 warms the earth's land and ocean. Much of the solar radiation is absorbed by both the organic (living) and inorganic (nonliving) materials, resulting in a transformation into thermal energy that increases the temperature of the absorbing surfaces as well as the air in contact with these surfaces. For a stable climate, a balance is required between incoming and outgoing radiation. Consequently the earth must reradiate an average of 235 W/m2 back into space. Figure 5.1 describes this energy balance, with incoming energy on the right, and outgoing on the left.

Although the greenhouse gases (GHGs) are transparent to the incoming visible light, they are opaque to the reflected infrared. In effect, the greenhouse is actually a retention of heat energy by the earth's atmosphere, but the GHGs rereflect a portion of this retained heat earthward, while another portion of the infrared escapes back into space. This is and has been the natural order of the earth's balanced heat budget, and has worked in our favor for thousands of years until recently, when the balance began tilting in the direction of more heat being received than being reflected back into space.

The unusual buildup of carbon dioxide, as well as the other non-carbon dioxide GHGs in the atmosphere traps more of the infrared that would normally escape into space, rereflecting it back to earth. This additional heat, this

Figure 5.1. The earth's annual and global mean energy balance (follow the arrows).

CO2 forcing has been occurring over the past 200 years, but much more so over the past 100 years, and the earth's average temperature has risen along with it.

Dr. James Hansen, Director of NASA ' s Goddard Institute of Space Sciences, and his team have calculated that "these human-made gases were (are) heating the earth's surface at a rate of almost two watts per square meter. A miniature Christmas tree bulb delivers about one watt. So it is as if we humans had placed two of these tiny bulbs over every square meter of the earth' s surface, glowing 24/7" [3]. Was it—is it—possible that such minuscule amounts of heat could tweak—or more to the point, destabilize—the awesome forces of nature? Indeed, they could. We now know that small forces maintained over time can induce climate change. Again, Hansen informs us that "the earth is now absorbing nearly 1 watt per square meter more energy from the sun than it is emitting to space, portending further warming even if GHG levels were immediately stabilized " [4] .

The greenhouse has been our benefactor these past 12,000 years, maintaining a nicely balanced energy budget—equal amounts of heat coming in and going out. If CO2 were absent, the energy balance would tilt to the cold side and our earth's temperature would plunge some 70°F (35°C) below our current worldwide average of 14-15°C (58-59°F), which would bring us to a noninhab-itable -18°C (-0.4°F), well below the freezing point of water. On the other hand, if the blanket of GHGs swaddling us grew so thick, so deep, preventing all infrared from passing into space, our planet would be another Venus, another red-hot planet, far too hot for life as we have come to enjoy it. We have been fortunate. The quartet of GHGs have been nature's thermostat, automatically set for a splendid balmy temperature—on average. It is difficult to conceive that we are prepared to shut it down. The numbers suggest that we are.

Prior to 1850, the concentration of CO2 in the atmosphere was approximately 280 ppm. Currently it is 380 ppm—an increase of 35% in 156 years. To edge this a bit more starkly, in 1958 it was 315 ppm, an absolute increase of 65 ppm, or 17% in 48 years. Carbon dioxide is increasing, continuously increasing; that's the rub. But this should not come as a surprise. Who knew? A number of people knew. The sequence of events in which the shorter, visible-light radiation is absorbed by the earth and reradiated as the longer, less energetic infrared, trapped by the naturally occurring GHGs, was referred to as a "greenhouse effect" in 1827 by Jean-Baptiste Joseph Fourier, a French mathematician. But it was from a series of ingenious experiments conducted by John Tyndall, an Irish physicist, that water vapor was found to have a great absorptive capacity for solar radiation. Assigning a value of 1 to dried air, he calculated the absorption of ambient air containing water vapor and carbon dioxide as 15. Deducting the effect of CO2 , he showed the absorption of water vapor to be 13 times greater than air. In his published work Tyndall noted that [5]

If as the above experiments indicate, the chief influence be exercised by the aqueous vapor every variation of this constituent must produce a change of climate. Similar remarks would apply to the carbonic acid diffused through the air . . . a slight change in fact may have produced all the mutations of climate which the researches of geologists reveal . . . and they establish the existence of enormous differences among gases and vapors as to their action upon radiant heat. "

While not referring to it by name, Tyndall had revealed the existence of a greenhouse 146 years ago. Humankind however, can be forgiven for taking little notice of this esoteric discovery. Not so with the Swedish Chemist Svante August Arrhenius. By 1896, Arrhenius recognized that carbon dioxide allows solar radiation to pass unimpeded through the atmosphere but captures a portion of energy upon its re-radiation from earth [6] - Without benefit of a computer he calculated that doubling CO2 would produce a 6°F (3.3°C) increase in mean global temperature.

Laying out his calculations in the Quarterly Journal of the Royal Meteorological Society for 1938, George S. Callender, a British engineer, who believed that the world was warming, estimated, again without benefit of a computer, a 2°F (1.1°C) increase in air temperature for a doubling of carbon dioxide, and identifying the burning of fossil fuels as the culprit [7]. But concern was elsewhere. Two decades later Gilbert N. Plass of Johns Hopkins University took up the question and categorically established CO- as the major greenhouse gas. Listen to him [8]: "If the total CO2 amount is reduced 50 percent or less of its present value, then a permanent period of glaciation results until the total CO2 amount again increases." He also contended that "The burning of fossil fuel . . . had greatly disturbed the CO2 balance. If all this additional CO2 remains in the atmosphere there will be 30 percent more CO2 in the atmosphere at the end of the 20th century than at the beginning. Man' s activities are increasing the average temperature by 1.1°C per century" [8] - Plass was way ahead of the pack. But there was no urgency. Plass fingered both fossil fuel and those who were burning it. Unfortunately there was no critical mess prompting him to act on it—until 1988, when James E. Hansen sounded the tocsin. Here was a call to arms, and it came at a propitious moment. It was hot. Weeks of drought had parched the American grain belt, and the north and southeast were in the throes of an extended hot spell. Hansen, speaking with a force and authority rarely encountered in an established scientist, not only stated that the world was getting warmer, but that "he was 99 percent certain that the accumulation of greenhouse gases was responsible for the warming trend." Hansen had the heat working for him. The media embraced his account, and the threat of an overheated greenhouse raced around the world. Global warming moved from academic journals to the fast lanes of the electronic media, and "greenhouse" took on a new meaning—and there was a splendid curve—a trendline to back up the caution that atmospheric temperature was rising along with CO2 on the greenhouse.

In 1958, Charles D. Keeling and Robert B. Bacastow of Scripps Institute of Oceanography, together with the National Oceanic and Atmospheric Administration (NO A A), established a CO- monitoring station on the summit of

Mauna Loa Observatory Monthly Average Carbon Dioxide

Mauna Loa Observatory Monthly Average Carbon Dioxide

1950 1960 1970 1980 1990 2000 2010

1950 1960 1970 1980 1990 2000 2010

Climate Monitoring £ Diagnostics Laboratory National Oceanic A Atmospheric Administration

Climate Monitoring £ Diagnostics Laboratory National Oceanic A Atmospheric Administration

Figure 5.2. Monthly average global carbon dioxide levels, 1979-2005. (Figure courtesy of Climate Monitoring & Diagnostics Laboratory, National Oceanic & Atmospheric Administration.)

Mauna Loa, a dormant volcano on the island of Hawaii, well isolated from industrial areas and local air pollution. From 1955 on, Keelings ' group collected air samples and analyzed them for carbon dioxide. Figure 5.2 depicts the rendering of monthly mean CO2 levels and clearly shows the steady, upward trend. In fact, each of the curve's data points is the residual of thousands of analyses. This sawtoothed projection reflects the fact of the warmth of spring and summer with plants and trees absorbing CO2 and bursting into bloom as CO2 is removed from the atmosphere during their photosynthetic cycle. In fall and winter, respiration occurs; leaves fall, flowers wither and die; fruit disappears, as all green things exude CO2. Each leg of the graph presents a half-year's reading. The descending leg represents the photosynthetic absorption and sequestering of CO2, while the ascending leg represents respiratory activity.

At the outset (in 1958), readings of 315 ppm were obtained. Twenty years later they had arrived at 330 ppm, and by 2004, at 380. Obviously, and unfortunately, an increasing rate of increase at work. Atmospheric CO2 is increasing faster than ever, which parallels the increased use of fossil fuels around the world, especially in China, India, and the United States. The Mauna Loa curve has been corroborated and verified by an additional station in Antarctica. The manometers have been running continuously for 47 years. The data are solid. Along with these worrisome, steadily increasing CO2 levels, the IPCC has documented a 0.6°C rise in temperature at the surface of the earth. And

Dr. Hansen recently remarked that "It became clear very quickly that his [Keeling ' s] measured CO2 increase was proportional to fossil fuel emissions and that humans were the source of the change. He altered our perspective about the degree to which the earth can absorb the human assault." Dr. Keeling will be missed. He died at age 77, on June 20, 2005. His work goes on [9].

Clearly, CO2 has been closely monitored, but what of the other non-CO2 GHGs that interrupt the passage of infrared? What is their contribution, and what of their levels? Are they rising along with CO2? If they are, it would mean that they, too, would be adding to radiative forcing, and any factor that alters the radiation from the sun can affect climate. A change in the net radiative energy available is referred to as a "forcing." Positive forcings warm the earth; negative, cool it. Let us then consider each of the half-dozen gases that collectively appear to be enhancing the greenhouse.

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