|Figure 9.Trend of carbon dioxide concentration at two stations in the northern hemisphere, Mauna Loa, Hawaii, and the southern hemisphere at Cape Grim, showing the exponential increase in at 0.5% per year.9|
Global Environmental Problems
There is no upwind anymore. Everyone lives downwind from someone else. My students have analyzed sulfur pollution, lead, and soot at the peak of some of the highest mountains in the Swiss Alps and in the most remote regions of Norway. Polychlorinated biphenyls (PCBs) have entered the food chain of birds and mammals in the Arctic, Antarctica, and Midway Island in the middle of the ocean. Even though there are no pollution sources at those locations, and no human populations within 2000 miles, the chemicals are somewhat volatile and they travel long distances before being "cold-trapped" in these sensitive ecological areas.
The atmosphere is a relatively small compartment in the earth's conveyor belt of environmental pollutants. It contains a mass of kg of air that is dwarfed by the ocean ( kg), but the ocean is an average of 3800 meters deep. Furthermore, residence times of pollutants in the atmosphere are short compared to those in the ocean. It is relatively easy to pollute the atmosphere. It responds quickly to global emissions, but it also cleanses itself more quickly when emissions are abated.
Greenhouse gases are increasing in the atmosphere. Carbon dioxide () from fossil fuel emissions is increasing at 0.4-0.5% per year (Figure 9).9 A little more than half of the anthropogenic greenhouse gas effect is due to carbon dioxide (Table 1).3 Methane () concentrations in the atmosphere, another potent greenhouse gas, are increasing at ~0.7% per year, mainly due to flooded agriculture (rice production) and animal husbandry required to feed an expanding population (Figure 10).9 But there is some good news: the rate of increase in methane concentrations is beginning to decline, perhaps because leaks at natural gas pipelines have been controlled. Nitrous oxide () is the result of increasing fossil fuel emissions, slash burning of forests, and nitrogen fertilizer applications worldwide. As ammonia-nitrogen is oxidized in the soil and nitrate fertilizer is denitrified, nitrous oxide represents a chemical of intermediate oxidation state, which is volatilized to the atmosphere. Figure 11 shows that is increasing rapidly in the atmosphere and, even though it is only ~5% of the anthropogenic greenhouse gas effect, it will be a difficult gas to control.9 Fertilizer applications of ammonia, ammonium nitrate, and ammonium sulfate are likely to increase worldwide.
|Figure 10. Trend of methane concentrations at two stations in the northern hemisphere, Mauna Loa, Hawaii, and the southern hemisphere at Cape Grim, showing the exponential increase of methane at 0.9% per year (but concentrations appear to be leveling off). 9||Figure 11.Trend of nitrous oxide concentrations at two stations in the northern and the southern hemisphere, showing the exponential increase of at 0.25% per year.|
The increase in greenhouse gases in the atmosphere has led to concern regarding the potential for global warming and climate change. The effect is analogous to rolling up the windows of your car on a bright summer day. Short wave solar energy can pass the windows striking the interior surfaces of the car. But long-wave radiation cannot escape back through the windows, and the temperature rises. Greenhouse gases also absorb long-wave radiation, and they are increasing in the atmosphere (Table I), but it is not certain whether climate has been affected.
The vast majority (~98%) of earth's greenhouse effect is natural, caused by water vapor and carbon dioxide. Without these gases, earth would be so cold as to be uninhabitable, 33°C (59°F) cooler than it is presently. The annual global mean surface temperature of earth is 15°C (59°F), and the global temperature in the absence of natural greenhouse gases would be a frigid -18°C (0°F). The "greenhouse effect" is natural and it is beneficial to life on Earth.11
|Figure 12. Changes in contributions (in watts/m2) to global warming of various gases and processes between 1765 and 1990. 12|
Concern arises because anthropogenic greenhouse gases are increasing including carbon dioxide, methane, chlorofluorocarbons, nitrous oxide, and ozone (, , CFCs, , and ; Table 1). Ozone in the lower atmosphere, troposphere, is increasing despite the loss of the ozone layer in the upper atmosphere or stratosphere. These greenhouse gases have the potential to change climate if left unabated. Molecules with more than two atoms (like the windows of the car) absorb long-wave radiation, and when they do, the earth's surface warms. The relative effectiveness of each of the gases is not equal. Table 1 shows that (CFC-11) and (CFC-12) are 12,000 and 15,000 times more effective, respectively, at absorbing long-wave radiation compared to carbon dioxide on a molar basis.10
Calculated changes in the Earth's radiation budget as a result of increasing greenhouse gases are shown in Figure 12. Carbon dioxide has increased 27% from 280 ppm in 1765 to 355 ppm in 1990. A total change of 2.5 watts/ is attributed to anthropogenic greenhouse gases to date. To put the number into perspective, a standard Christmas tree light bulb puts out ~4 watts of energy, so if there were one additional light bulb of this kind shining continually on every square meter of the earth's surface, it would result in a climate forcing of 4 watts/m2. This climate forcing is small relative to the total input of energy from the sun (the solar irradiance or solar constant) of 1372 watts/. 11 The solar constant is not constant¾it varies with sunspot activities and other factors (± 0.1 %), and it may have increased a little in recent times (Figure 12).12 Also, humans have put more sulfur dioxide into the atmosphere (from coal combustion) which results in the formation of sulfate aerosols and brighter clouds (albedo increases) that cause "global cooling" (Figure 12).12 This cooling effect is somewhat less than the calculated warming effect due to greenhouse gases. However, there is considerable uncertainty in all of these effects, and they are estimated by use of global dynamic models, General Circulation Models or GCMs.
|Figure 13. Annual deviations of the global mean (land and sea) temperature change over the past century relative to the average for 1951-1980. (The curve shows the results of a smoothing filter applied to the annual values.)|
We are currently in a relatively warm climate pattern (Figure 13).13 The global average surface temperature of the earth is ~0.5°C warmer than the earliest period of record, 1860. But this temperature change is still within the inter-annual variability of temperature (±0.7°C) due to large-scale circulation patterns (e.g., El Niño Southern Oscillation events) that we do not fully understand. Eight out of nine of the warmest years on record have occurred since 1980 (1980-1995) in a 135-year period. Last year, 1995, was the warmest year on record and 1990 and 1991 were close seconds. 1992 and 1993 were relatively cool because sulfur dioxide and ash particles were blown high into the atmosphere (>35,000 ft) by the eruption of Mt. Pinatubo volcano. All of these observations have been modeled with some success using General Circulation Models (GCMs).
Last December, representatives from 120 nations and the Intergovernmental Panel on Climate Change met to discuss the issues of global change. For the first time, they agreed that while many uncertainties remain, "the balance of evidence... suggests a discernible human influence in global climate."
The best estimate of warming is 1.0-3.5 °C (1.8-6.3 °F) with a most probable estimate of 2.0 °C (3.6 °F) by the middle of the 21st century (Intergovernmental Panel on Climate Change, IPCC, Second Assessment Report, 1996). If carbon dioxide and other greenhouse gases continue to increase, all models predict a warming trend in the 21st century. Precipitation would increase globally, but mid-continental areas, like Iowa, would probably become warmer and drier. Sea level has been rising already, about 3.9 ± 0.8 mm/year in 1993 and 1994.14 It would continue to rise by 15-95 cm by the year 2100, enough to cause salinity intrusion into the drinking water supply of coastal cities and inundation of coastal properties on barrier islands. One of the greatest economic costs of global warming is expected to be health related, including an increase in malaria and schistosomiasis and problems of extreme heat affecting the elderly. Although an average temperature increase of 3.6 °F may not seem like much warming, it could affect the extremes of weather considerably, resulting in greater hurricanes, droughts, floods, and costs to society. The biggest proponent of further controls on carbon dioxide emissions is the insurance industry, which has been monitoring the Climate Convention and the Commission on Sustainable Development closely. Insurers are worried that the escalating claims in recent years due to storms will leave them bankrupt.
|Figure 14. Scenarios of the atmospheric COªconcentration and the carbon reservoirs of terrestrial biosphere. Case A is the 1.5% increase per year, case B is the constant emission, and case C is the 5% decrease per year from year 1989. 15|
We have developed a model to predict atmospheric concentrations of carbon dioxide.15 Our global budget for anthropogenic carbon sources and sinks to the atmosphere in Table II shows that most of the carbon dioxide is transferred to the biosphere (uptake by terrestrial vegetation) and absorbed by the oceans, but the remainder accumulates in the atmosphere with a lifetime on the order of 125 years. If emissions continue as projected in the mid-case scenario by the Intergovernmental Panel on Climate Change at approximately 1.5% per year, carbon dioxide will increase to 550 parts per million by volume (ppmv) by the middle of the 21st century, and global warming will certainly occur (Figure 14). Assuming that we are able to control emissions at 1990 levels, as recommended by the Climate Convention that was signed at the Rio Earth Summit in 1992, COª concentrations will begin to level off at ~450 ppmv, and a more modest increase in global warming would be expected. The grim news is that it would take a substantial reduction in carbon dioxide emissions to limit concentrations to their present levels. A cutback of ~70%, a rollback of emissions to levels similar to 1950, would be required to maintain emissions at the current 360 ppmv.
As humans become ever more numerous and consumptive, our emissions and pollution begin to rival natural processes of nature's cycles. At first this occurs locally, then at regional scales, and finally globally. Global atmospheric pollution is the first of earth's reservoirs to be affected because it is the smallest. Table III shows that for methane releases and carbon monoxide (CO), anthropogenic emissions already surpass those of nature.16 It is also true for sulfur dioxide and nitrogen oxides. Human effects are pervasive. In 1750, when the global population was one billion, people had an average of 29 acres of habitable land per person, but today the number is about five acres. And in these five acres per person, we must feed ourselves, supply shelter and energy, and allow for the cohabitation of four million or more species of plants and animals. The world is still a big place and much accommodation is possible, but only with education and good government can we make progress towards providing for humans while not despoiling the environment.
Speech | Acknowledgements | References | About Jerald L. Schnoor
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