Global Warming(Note: Hyperlinked images coming soon!)
Earth's surface is warmed by a group of atmospheric gases known as greenhouse gases, most of which--carbon dioxide, methane, water vapor, nitrous oxide, and ozone--have both natural and human-made sources. Chlorofluorocarbons (CFCs), however, are produced solely by humans, and are so damaging to the ozone layer that an international agreement was made to ban their production (see Merritts et al., Environmental Geology, Chapter 9). CFC emissions have been significantly reduced, but CFC molecules may remain in the atmosphere well beyond the year 2050.
Since the Industrial Revolution, combustion of fossil fuels for electricity and transportation has led to great increases in atmospheric C02 concentrations (Figure 1). Every year 7 billion to 9 billion metric tons of C02 enter the atmosphere as a result of human activities. At the same time, deforestation is destroying one of the greatest sinks for atmospheric carbon-trees. Today C02 concentrations are 25 percent higher than in preindustrial days.
Methane concentrations have increased by 100 percent primarily because of expanding rice cultivation and cattle ranching. Rice cultivation requires swampy conditions, which release methane, while cows and other ruminants release methane in intestinal gas. Ozone is accumulating in the troposphere because the extreme heat generated in automobile engines splits oxygen molecules into highly reactive atoms (see Merritts et al., Environmental Geology, Chapter 9). Nitrogen oxides result from the oxidation of fertilizers applied to farmlands and from industrial processes such as nylon production.
While not nearly as abundant as carbon dioxide, these other gases are much more effective heat absorbers and cause greater temperature changes than the same volume of C02. Molecule for molecule, methane is 25 times as efficient and CFCs are 20,000 times as efficient as C02 at trapping heat. Although emissions of these other gases are low compared with CO2 emissions, together they almost equal the warming effect of total C02 emissions (Figure 2).
In the last few decades, scientists have become increasingly aware that the human-induced increases in greenhouse gases have the potential to cause a significant warming of the planet. Greenhouse gases have residence times in the atmosphere ranging from 50 to 200 years, so even if human-induced emissions were stopped today, they would linger in the atmosphere for several generations. Long-term warming could have devastating consequences. First, it could change the distribution and amount of precipitation on the continents. Eventually, the polar ice caps could melt and cause a rise in sea level.
Depending on the climate model used to compute global temperature changes, a doubling of C02 may warm Earth's surface 2° to 5°C on average. Small shifts in average global temperature can cause significant environmental changes and social upheaval, as occurred during the Little Ice Age. Furthermore, this projected temperature increase may be an underestimate. If all the fossil fuels on Earth were used in the near future, atmospheric C02 could reach eight times the modern values and cause a 6° to 10°C rise in global temperature, making Earth as warm as it was during the Cretaceous, one of the warmest periods in the history of Earth. Finally, if greenhouse gases such as methane, nitrous oxide, and CFCs were allowed to increase, their concentrations would yield yet more warming.
Climate models exhibit varying degrees of sophistication and may differ in their mathematical treatment of certain Earth processes. These disparities result from gaps in our understanding of the physics underlying atmospheric processes. For instance, clouds may cause both a cooling and a heating effect on Earth. While clouds reflect some incoming solar radiation back to space, effectively cooling the planet, they also trap outgoing infrared radiation, keeping the surface warm. Suppose that Earth were blanketed with a thin, high-altitude cloud bank. The clouds could reflect enough incoming solar radiation to cool the planet's surface by 20°C or more. However, they could also trap enough infrared radiation to warm the surface by as much as 27°C. The difficulty of predicting the extent of cloud cover and the complex nature of the impact of clouds on temperature leads to uncertainty.
Consequences of Global Warming
The consequences of an increase in global temperature are somewhat conjectural. Warmer oceans will release more moisture into the atmosphere and enhance the hydrologic cycle. Lands at high latitudes, such as Canada and Scandinavia, are expected to receive increased precipitation throughout the year. Lands at mid-latitudes, such as the United States and central Europe, would get increased precipitation only during the winter. In some areas, however, higher evaporation rates probably will also increase the frequency of summer droughts, causing problems for farmers; climate models indicate that parts of North America and Eurasia may be particularly hard hit (Figure 3).
Parts of the Antarctic ice sheet and mountain glaciers could melt, leading to higher sea levels and flooding of low-lying coastal areas. Island countries in the Pacific, Bangladesh, and the Netherlands, among other nations, are justifiably concerned given the predicted increase in sea level of between 5 and 40 cm by the year 2050. Furthermore, some models suggest a total rise of between 5 and 35 m if all fossil fuels are burned and methane, ozone, and nitrous oxides continue to increase at their present rates. In the United States, the Atlantic and Gulf Coast states have cause for worry. The Environmental Protection Agency has estimated that Louisiana could lose up to 5000 square miles of wetlands and coasts if sea level were to rise 1 m. The higher sea-surface temperatures associated with global warming may lead to a greater frequency of hurricanes and tropical storms. Again, coastal regions will bear the brunt.
While global warming will most certainly be detrimental to some countries, those in cold climates may benefit. Russia and Canada, for example, stand to gain from a longer growing season, increasing perhaps up to 20 days. In addition, milder winters would reduce expenditures on heating oil, natural gas, and electricity.
Global Warming: Real or Imagined?What evidence do we have that global warming is occurring? Do two summers in a row with hotter than usual temperatures indicate a warming trend? What about locations that appear to be experiencing a cooling of summer temperatures while other locations are heating up? Partly because of such contradictory observations and partly because disagreements exist among scientists and between climate models, some argue that global warming is not a problem and thus requires no solution. The danger in doing nothing, however, is that we risk losing an opportunity to slow or halt global warming before significant, and perhaps catastrophic, climatic changes occur. While Earth scientists may not agree about the future extent of global warming, enough experimental evidence exists for most to believe that it is now occurring. Measurements conducted over the last century show that Earth has warmed roughly 1°C in that time. Superimposed on this long-term warming trend are short periods of slight cooling, lasting a few years each and attributable to volcanic eruptions and changes in the intensity of solar radiation. With the benefit of more than a century of climatic data, we now know that mean global temperature is on the rise despite brief coolings. Is the increase in temperature over the last century related to increased greenhouse gas concentrations from human activities, or has it resulted from some natural cause? Because short-term records are insufficient as a basis for conclusions about climate change, a better approach to answering this question is to combine historical data with much longer records of climatic change.
Data from tree rings and oxygen isotopes in ice cores suggest that the last 50 years have been warmer than at any time in the last few hundred to few thousand years. The coincidence of this warming trend with years of high greenhouse gas emissions does not conclusively prove a link, but it does provide suggestive evidence. Furthermore, ice-core data have shown that greenhouse gas concentrations and global temperature have varied together in the past 150,000 years, and there is no reason to think this relationship has changed in modern times. The composition of bubbles in ice cores has shown that the concentration of the atmospheric carbon dioxide fluctuated between about 180 and 280 ppm over the last 160,000 years up until the last two centuries (Figure 4). These fluctuations occurred gradually, over tens of thousands of years. In the last two centuries, however, concentrations of C02 have risen to between 350 and 355 ppm, a rate of increase unprecedented in recent Earth history. Whether ecosystems can tolerate this rapid change remains to be seen.
Reducing Global Warming What, if anything, is to be done? Greenhouse gas emissions are largely the byproducts of the activities of affluent societies dependent on fossil fuels for most of their industrial and transportation needs. However, greenhouse gas production is not entirely a function of industrialization. France and Germany are both industrialized nations, yet they consume less energy per capita and produce far less C02 than does the United States (Figure 5). In France, widespread use of nuclear power allows relatively high consumption of energy while limiting emissions of greenhouse gases. In both France and Germany, the high cost of oil and other fossil fuels (between $3 and $5 per gallon of gasoline) limits energy consumption. Furthermore, distances between population centers in Europe tend to be much less than those in the United States, so less fuel is consumed in transportation. Bicycles are a common sight in European cities, and are even more common in China (Figure 6), particularly in the megacities of Beijing and Shanghai.
Many scientists and policymakers worry that economic development will replace all these bicycles with cars, but why shouldn't the rest of the world enjoy the same standard of living that Americans take for granted? Perhaps the most practical solution is for Americans voluntarily to increase their reliance on alternative energies such as solar and wind power; to improve the efficiency of appliances and motor vehicles dependent on fossil fuels, and to use more mass transit and modes of transportation such as walking and biking. These changes would enable us to make our supplies of fossil fuels last much longer; make us healthier from the added exercise, reduce concentrations of air pollutants such as ozone and nitrogen oxides which lead to ill health, and help curb global warming. The following table lists the energy savings and reductions in carbon emissions if individuals take some simple steps.
| Ways to Save Energy and Reduce CO2 Emissions | ||
| Action | Energy savings | Reduction in carbon emissions |
| Improving insulation of hot water heater | 300 kWh/yr | 55 kg (120 lbs) |
| Substituting 18-W compact fluorescent light for 75-W incandescent bulb (8 hrs/day) | 170 kWh/yr | 32 kg (70 lbs) |
| Carpooling 5 people instead of driving solo | 3790 liters/yr (1000 gallons/yr) | 2270 kg (5000 lbs) |
| Driving 10,000 miles in a car that gets 30 mpg (instead of 20 mpg) | 630 liters (167 gallons/yr) | 400 kg (880 lbs) |
| 40 mpg | 950 liters/yr (250 gallons/yr) | 600 kg (1320 lbs) |
| 50 mpg | 1140 liters/yr (300 gallons/yr) | 120 kg (265 lbs) |
| Regular car maintenance | 190 liters (50 gallons/yr) | 120 kg (265 lbs) |
| Planting trees to block the Sun | 500-1500 kWh/yr | 455 kg/yr (1000 lbs) |
| Source: Modified from F. Lyman et al., The Greenhouse Trap: What We're Doing to the Atmosphere and How We Can Slow Global Warming (Boston: Beacon Press), 1990, 132-133. | ||