In its 2001 report, the U.N.'s International Panel on Climate Change (IPCC) described a number of very different scenarios for greenhouse gas emissions for the rest of the century. The magnitude of the emissions predicted at century's end vary dramatically: 5, 13.5, 20, and 29 Gt of carbon annually, the range spanning from 0.6 to 3.5 times the current value of about 8 Gt C/year. The carbon dioxide concentrations projected for 2100 for the IPCC scenarios range from 500 to more than 900 ppm, compared to today's 373- and the preindustrial 280-ppm levels.
Even with constant carbon dioxide emissions at current levels or a few percentage points lower, the carbon dioxide concentration in the atmosphere will continue to grow. Some policymakers have promoted the idea that, through international agreements or allocation schemes, the world should control future CQ2 emissions so that the atmospheric level of the gas never exceeds some specific concentration. Although there is no consensus on the most appropriate target, for our discussion we shall use 550 ppm. This value is twice the preindustrial value—in other words, a situation in which human actions have doubled the natural atmospheric carbon dioxide concentration.
One way in which global C02 emissions could rise and fall with time in order to eventually achieve the 550-ppm concentration target is shown by the curve in Figure 7-1 la. Figure 7-1 lb shows how the corresponding atmospheric C02 concentration would change with time for this emission scenario. The scenario was developed assuming that international agreement on C02 emissions can be achieved in the relatively near future. Consequently, it assumes modest growth in C02 releases until about 2060, at which point a decline would set in. The temperature increase—which tracks the C02 concentration curve closely—by 2100 would be just under 2°C (relative to that for the year 2000). The rise in sea levels would be reduced by about
FIGURE 7-11 Approximate (a) annual C02 emission rates and (b) resultant atmospheric CO, concentrations to meet a 550-ppm stabilization target.
12 11 10 9
one-third if we embark soon on the scenario to never exceed the 550-ppm concentration of carbon dioxide.
An alternative scenario to the one shown by the curves in Figures 7-1 la and 7 -11 b, in which effective C02 controls are not implemented until several decades later, would eventually require a sharper decline in emissions and would reach the 550-ppm limit sooner. Such alternative proposals allow more time to further develop replacement technologies, such as the solar energy techniques discussed in Chapter 8, before we begin to end our reliance on fossil fuels. Such scenarios require the world to generate more emissions-free power than today's total power consumption by about mid-century, a major challenge to achieve. By the end of the century, almost all power would have to be emissions-free. It is not possible to defer emission reductions indefinitely if the 550-ppm concentration target is to be achieved.
Green Chemistry: Polylactic Acid—The Production of Biodegradable Polymers from Renewable Resources; Reducing the Need for Petroleum and the Impact on the Environment
Our everyday lives are permeated by the chemicals in products such as pharmaceuticals, plastics, pesticides, personal hygiene products, cleaners, fibers, dyes, paints, clothes, building materials, computer chips, packaging, and food. The vast majority of these chemicals are ultimately made from oil, consuming approximately 2.7% of the production of this natural resource. The compounds that are isolated from oil and used to produce chemicals are known as chemical feedstocks. Approximately 60 billion kg of these feedstocks are employed to create 27 billion kg of polymers (many are loosely referred to as plastics) each year. Some of the more familiar polymers (as will be discussed in Chapter 16) that are produced from crude oil include polyethylene tereph-thalate (PET), which is used to make plastic beverage bottles and fibers for clothes; polyethylene, which is used to produce plastic grocery and trash bags; and polystyrene, which we discussed in the green chemistry section in Chapter 1. Trade names of polymers, such as Dacron, Teflon, Styrofoam, and Kevlar, represent polymers that are part of our everyday lexicon.
Approximately 2 billion kg of PET are produced each year. PET is one of the main targets for the recycling of plastics, yet less than one-quarter ot this total is recycled in the United States; the rest is landfilled or incinerated. Even when PET is recycled, it generally can't be reused as beverage bottles; it is downward recycled into polyester fiber products such as carpets, T-shirts, fleece jackets, sleeping bags, and car trunk linings, or into thermoformed sheet products such as laundry scoops, nonfood containers, and containers for fruits.
When we use oil to produce items that we dispose of or incinerate (including the use of oil as a fuel), we are consuming a resource that has taken nature millions of years to produce. Petroleum is a finite, nonrenewable resource. Although there are still considerable oil reserves, at the rate of our current use we will deplete the supply of cheap, readily accessible oil within the next 30 to 40 years. We must learn to use renewable resources such as biomass rather than petroleum to produce chemical feedstocks.
Scientists at NatureWorks LLC (formerly Cargill Dow LLC") have developed a method for producing a polymer called polyiactic acid (PLA) from renewable resources—such as corn (called maize in the UK and elsewhere) and sugar beets—for which they won a Presidential Green Chemistry Challenge Award in 2002. NatureWorks produces PLA at a plant in Blair, Nebraska. Ultimately, the goal is to utilize waste biomass as the source of this polymer. As in the steps shown in Figure 7-12, the corn is milled into starches, which are then reacted with water to yield glucose, which is then converted to lactic acid by natural fermentation. This naturally occurring compound is then converted to its dimer, followed by polymerization to PLA.
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If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.