(7) Air Pollution

Pollution (Land, Air and Water Pollution)

Pollution7

What Is the Scientific Consensus about Future Global Temperature Changes?

 To project the effects of increasing greenhouse gases on average global temperatures, scientists develop complex mathematical models to simulate interactions among the earth’s sunlight, clouds, landmasses, oceans, ocean currents, concentrations of greenhouse gases and pollutants, and positive (runaway) and negative (corrective) feedback loops within the climate system. Then they run these elaborate and continually improving climate models on supercomputers and use the results to project future changes in the earth’s average atmospheric temperature. Such models provide scenarios or projections of what is very likely (90–99% level of confidence) or likely (66–89% level of confidence) to happen to the average temperature of the lower atmosphere. How well the results correspond to the real world depends on the validity of the assumptions and variablesbuilt into the models and on the accuracy of the data used.

In 1990, 1995, 2001, and 2007, the Intergovernmental Panel on Climate Change (IPCC) published reports on how global temperatures have changed in the past and made forecasts of how they are likely to change during this century. According to the 2007 report, based on analyzing past climate data and using 19 climate models, it is very likely (a 90–99% probability) that human activities, led by burning fossil fuels, have been the main cause of the observed atmospheric warming during the past 50 years.

The 2007 report and recent runs of 19 different climate models suggest that it is very likely that the earth’s mean surface temperature will increase by 2–4.5 Co (3.6–8.1 Fo) between 2005 and 2100, with about 3 Co (5.4 Fo) the most likely rise, unless the world makes drastic cuts in greenhouse gas emissions from power plants, factories, and cars that burn fossil fuels. This is a major increase in such a short period. The lower temperature in this range is likely only if global greenhouse gas emissions fall 50–85% by 2050-an unlikely assumption.

There is overwhelming consensus among the world’s climate scientists that global warming is occurring at a rapid rate, that human activities are the major factor in this temperature increase since 1950, and that human activities will play an even greater role in the warming projected to take place during this century (Concept 15-3). Energy use accounts for about two-thirds of the CO2 emitted byhuman activities, followed by agriculture (14%) and deforestation and other forms of land use (10%). In 2007, the leaders of the American Association for the Advancement of Science (AAAS)-the world’s largest general science body-said that Global warming is not a theory; it is a fact based on a growing torrent of information . . . . The pace of change and the evidence of harm have increased markedly over the last five years. The time to control greenhousegas emissions is now. . . . Delaying action to address climate change will increase the environmental and social consequences as well as the costs.

The global warming hypothesis is based on very reliable science. The question now is, what do we do about it?

Critical Thinking

If projected temperature increases 15-C take place, list three ways in which this will affect your lifestyle.

Can the Oceans Save Us?

Scientists have identified a number of natural and human-influenced factors that might amplify or dampen projected changes in the average temperature of the atmosphere (Figure 15-C). The oceans, for example, help moderate the earth’s average surface temperature by removing about 30% of the excess CO2 pumped into the lower atmosphere by human activities. The oceans also absorb heat from the lower atmosphere and use currents to slowly transfer some of it to the deep ocean, where it is removed from the climate system for unknown periods.

We do not know whether the oceans can continue to absorb more CO2. But the solubility of CO2 in ocean water decreases with increasing temperature. Thus, if the oceans heat up, some of their dissolved CO2 could be released into the lower atmosphere-like CO2 is bubbling out of a warm carbonated soft drink. This could amplify global warming.

Scientific measurements show that the upper portion of the ocean warmed by 0.32–0.67 C° (0.6–1.2 F°) during the last century-an astounding increase considering the huge volume of water involved. According to a 2007 study of the vast Southern Ocean around Antarctica, led by researcher Corinne Le Quere, the ability of the oceans to absorb more CO2 from the atmosphere is weakening.

In 2005, the U.K. Royal Society reported that higher levels of CO2 in the ocean have increased the acidity of the ocean surface by 30% from preindustrial times and could reach very harmful levels by 2150. This happens because much of the CO2 absorbed by the ocean reacts with water to produce carbonic acid (H2CO3)-the same weak acid found in carbonated drinks. The scientists involved in this study warn that this may reduce the ability of the oceans to remove CO2 from the lower atmosphere and thus could accelerate global warming.

This increase in seawater acidity also threatens coral reefs and alters seawater life by impairing the ability of certain shellfish (including certain plankton and tiny snails) to produce shells, which like coral reefs, are made of calcium carbonate (CaCO3). You can see this effect by dropping a piece of chalk (made of calcium carbonate) in a glass of vinegar (a weak acid) and watching it rapidly dissolve.

Extensive loss of these forms of plankton would disrupt food webs, killing seals, whales, and fish and perhaps disrupting human food supplies from the ocean. This would also decrease the ability of such plankton to slow global warming by removing CO2 from the atmosphere.

Bottom line: Changes in the temperature and acidity of the oceans as a result of human activities are likely to accelerate global warming. There Is Uncertainty about the Effects of Cloud Cover on Global Warming A major unknown in global climate models is the effect that changes in the global distribution of clouds might have on the temperature of the atmosphere. Warmer temperatures increase evaporation of surface water and create more clouds. Depending on their content and reflectivity, these additional clouds could have two effects. An increase in thick and continuous clouds at low altitudes could decrease surface warming by reflecting more sunlight back into space. But an increase in

thin and discontinuous cirrus clouds at high altitudes can warm the lower atmosphere and increase surface warming. We need more research to understand which of these effects might predominate globally and in various parts of the world.

In addition, infrared satellite images indicate that the wispy condensation trails (contrails) left behind by jet planes might have a greater impact on atmospheric temperatures than scientists once thought. NASA scientists found that jet contrails expand and turn into large cirrus clouds that tend to release heat into the upper troposphere. If these preliminary results are confirmed, emissions from jet planes could be responsible for as much as half of the warming of the lower atmosphere in the northern hemisphere.

Outdoor Air Pollution Can

Temporarily Slow Global Warming Aerosols (microscopic droplets and solid particles) of various air pollutants are released or formed in the troposphere by volcanic eruptions (Core Case Study) and human activities. They can either warm or cool the air and hinder or enhance cloud formation depending on factors such as their size and reflectivity.

Most aerosols, such as light-colored sulfate particles produced by fossil fuel combustion, tend to reflect incoming sunlight and cool the lower atmosphere and thus temporarily slow global warming. On the other hand, a 2004 study by Mark Jacobson of Stanford University indicated that tiny particles of soot or black carbon aerosols-produced mainly from incomplete combustion in coal burning, diesel engines, and open fires-may be the second biggest contributor to global warming after the greenhouse gas CO2.

Climate scientists do not expect aerosol and soot pollutants to counteract or enhance projected global warming very much in the next 50 years for two reasons. First, aerosols and soot fall back to the earth or are washed out of the lower atmosphere within weeks or months, whereas CO2 remains in the lower atmosphere for about 120 years. Second, aerosol and soot inputs into the lower atmosphere are being reduced because of their harmful impacts on plants and human health-especially in developed countries. Some scientists have suggested using balloons, large jet planes, or giant cannons to inject sulfate particles into the stratosphere as a possible way to slow global warming by reflecting some of the incoming sunlight into space and cooling the troposphere. The effect might be similar to the estimated 0.5 C° (0.9 F°) cooling effect that lasted about 15 months from the 1991 volcanic eruption of Mt. Pinatubo (Core Case Study). Huge amounts of SO2 would have to be injected into the stratosphere about every two years at an average cost of about $1 billion a year-nearly 100 times cheaper than the estimated cost of cutting CO2 emissions.

Other scientists reject this idea as being too risky because of our limited knowledge about possible unknown effects. Such a scheme could increase ozone depletion by boosting levels of ozone-destroying chlorine in the stratosphere. This short-term technological fix could also destroy much of the life in the oceans and their ability to remove CO2 by allowing CO2 levels in the lower atmosphere to continue to rise and increase the acidity of the oceans. As the oceans become more acidic, they absorb less CO2, which can accelerate global warming. Plants Can Remove More CO2 from the Atmosphere but the Effect Is Temporary

Some studies suggest that larger amounts of CO2 in the lower atmosphere could increase the rate of photosynthesis in some areas with adequate water and soil nutrients.

This would remove more CO2 from the lower atmosphere and help slow global warming.

However, recent studies indicate that this effect would be temporary for two reasons. First, the increase in photosynthesis would slow as the plants reach maturity and take up less CO2. Second, carbon stored by the plants would return to the lower atmosphere as CO2 when the plants die and decompose or burn. According to the 2007 IPCC report, plants now absorb more CO2 than they release, but by 2050 will likely release more CO2 than they take up.

Reducing the clear-cutting of rain forests in the Amazon and other tropical areas will preserve trees and other vegetation that remove some of the CO2 we add to the atmosphere. However, even if we halted all clear-cutting, climate models forecast that global warming will convert much of the Amazon’s wet forests into dry savannah during this century. This would reduce CO2 uptake by tropical forests, further accelerate global warming, and greatly decrease tropical biodiversity.

THINKING ABOUT

Tinkering with the Stratosphere

Explain why you agree or disagree with the proposal to inject large quantities of sulfate particles into the stratosphere every two years to help cool the troposphere

 

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