Page 366 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

330 part II The Water, Weather, and Climate Systems
CO2 ranging between 100 ppm and 300 ppm over that time period, while never changing 30 ppm upward or down- ward in any span of less than 1000 years. Yet in May 2013, atmospheric CO2 reached 400 ppm after rising over 30 ppm in only the last 13 years (having hit 370 ppm in May 2000).
Carbon dioxide has a residence time of 50 to 200 years in the atmosphere; however, rates of uptake vary for different removal processes. For example, the uptake of atmospheric CO2 into long-term carbon sinks such as marine sediments can take tens of thousands of years. As mentioned earlier, CO2 emissions come from a number of sources: the combustion of fossil fuels; biomass burning (such as the burning of solid waste for fuel); removal of forests; industrial agriculture; and cement production. (Cement is used to make concrete, which is used glob- ally for construction activities, accounting for about 5% of total CO2 emissions). Fossil-fuel burning accounts for over 70% of the total. Overall, from 1990 to 1999, CO2 emissions rose an average of 1.1% per year; from 2000 to 2010 that rate increased to 2.7% per year.
Methane After CO2, methane is the second most prevalent greenhouse gas produced by human activities. Today, at- mospheric methane concentrations are increasing at a rate even faster than carbon dioxide. Reconstructions of the past 800000 years show that methane levels never topped 750 parts per billion (ppb) until relatively modern times, yet in Figure 11.22a we see present levels at 1890 ppb.
Methane has a residence time of about 12 years in the atmosphere, much shorter than that of CO2. How- ever, methane is more efficient at trapping longwave ra- diation. Over a 100-year timescale, methane is 25 times more effective at trapping atmospheric heat than CO2, making its global warming potential higher. On a shorter timescale of 20 years, methane is 72 times more effective than CO2. After about a decade, methane oxidizes to CO2 in the atmosphere.
The largest sources of atmospheric methane are an- thropogenic, accounting for about two-thirds of the total. Of the anthropogenic methane released, about 20% is from livestock (from waste and from bacterial activity in the animals’ intestinal tracts); about 20% is from the mining of coal, oil, and natural gas, including shale gas extraction (discussed in Chapter 1, Geosystems Now); about 12% is from anaerobic (“without oxygen”) pro- cesses in flooded fields, associated with rice cultivation;
and about 8% is from the burning of vegetation in fires. Natural sources include methane released from wetlands (associated with natural anaerobic processes, some of which occur in areas of melting permafrost, as described in Geosystems Now) and bacterial action inside the di- gestive systems of termite populations. Finally, scientists suggest that methane is released from permafrost areas and along continental shelves in the Arctic as methane hydrates thaw, potentially a significant source. Focus Study 11.1 discusses methane hydrates (also called gas hydrates), a controversial potential energy source that could have serious consequences for global climates if large amounts of methane are extracted.
nitrous Oxide The third most important greenhouse gas produced by human activity is nitrous oxide (N2O), which increased 19% in atmospheric concentration since 1750 and is now higher than at any time in the past 10000 years (Figure 11.22b). Nitrous oxide has a lifetime in the atmosphere of about 120 years—giving it a high global warming potential.
Although it is produced naturally as part of Earth’s nitrogen cycle (discussed in Chapter 19), human activi- ties, primarily the use of fertilizer in agriculture, but also wastewater management, fossil-fuel burning, and some industrial practices, also release N2O to the atmosphere. Scientists attribute the recent rise in atmospheric con- centrations mainly to emissions associated with agricul- tural activities.
Halogenated Gases Containing fluorine, chlorine, or bromine, halogenated gases are produced only by human activities. These gases have high global warm- ing potential; even small quantities can accelerate greenhouse warming. Of this group, fluorinated gases, sometimes called F-gases, comprise a large portion. The most important of these are chlorofluorocarbons (CFCs), especially CFC-12 and CFC-11, and hydrochlorofluorocar- bons (HCFCs), especially HCFC-22. Although CFC-12 and CFC-11 account for a small portion of the rising green- house gas accumulations since 1979, their concentrations have decreased in recent years owing to regulations in the Montreal Protocol (Figure 11.22c; also see discus- sion of stratospheric ozone in Focus Study 3.1). However, hydrofluorocarbons (HFCs), fluorinated gases that are used as substitutes for CFCs and other ozone-depleting
Georeport 11.2 China Leads the World in Overall CO2 Emissions
Over the past several years, China, with 19.5% of global population, took the lead in overall carbon dioxide emissions (29%). With 4.5% of world population, the United States was second (16%), and the european Union (7% of the population)
was third (11%). On a per capita basis in 2012, China produced 7.1 tonnes per person, on par with the european Union
(at 7.5 tonnes per person). With 0.5% of global population Canada produced 1.6% of overall carbon dioxide emissions, at a rate of 16.0 tonnes of CO2 emissions per person. in the United States 16.4 tonnes of CO2 emissions per person were produced, and among the world’s major industrialized countries, australia had the highest per capita CO2 output at 18.8 tonnes per person in 2012. [emissions Database for global atmospheric research (eDgar), edgar.jrc.ec.europa.eu/news_docs/pbl-2013-trends-in-global-co2- emissions-2013-report-1148.pdf.]