Page 105 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 3 earth’s Modern atmosphere 69
elements to form these materials. Both nitrogen and oxy- gen reserves in the atmosphere are so extensive that, at present, they far exceed human capabilities to disrupt or deplete them.
The gas argon, constituting less than 1% of the ho- mosphere, is completely inert (an unreactive “noble” gas) and unusable in life processes. All the argon present in the modern atmosphere comes from slow accumulation over millions of years. Because industry has found uses for inert argon (in lightbulbs, welding, and some lasers), it is extracted or “mined” from the atmosphere, along with nitrogen and oxygen, for commercial, medical, and in- dustrial uses.
Of the variable atmospheric gases in the homo- sphere, we examine carbon dioxide in the next section, and we discuss ozone later in this chapter. Water vapour is in Chapter 7; methane is discussed in Chapter 11. The homosphere also contains variable amounts of particu- lates, solids and liquid droplets that enter the air from natural and human sources. These particles, also known as aerosols, range in size from the relatively large liquid water droplets, salt, and pollen visible with the naked eye to relatively small, even microscopic, dust and soot. These particles affect the Earth’s energy balance (see Chapter 4), as well as human health (discussed later in the chapter).
Carbon Dioxide Carbon dioxide (CO2) is a natural by- product of life processes, a variable gas that is increas- ing rapidly. Although its present percentage in the atmo- sphere is small, CO2 is important to global temperatures.
The study of past atmospheres trapped in samples of glacial ice reveals that the present levels of atmospheric CO2 are higher than at any time in the past 800000 years. Over the past 200 years, and especially since the 1950s, the CO2 percentage increased as a result of human activities, principally the burning of fossil fuels and deforestation.
This increase in CO2 appears to be accelerating (see graphs for atmospheric CO2 concentrations in Chapter 11). From 1990 to 1999, CO2 emissions rose at an average of 1.1% per year; compare this to the average emissions in- crease since 2000 of 3.1% per year—a 2- to 3-ppm-per- year increase. Overall, atmospheric CO2 increased 16% from 1992 to 2012. During May 2013, CO2 levels reached 400 ppm. Today CO2 far exceeds the natural range of 180
to 300 ppm over the last 800000 years. A distinct cli- matic threshold is approaching at 450 ppm, forecasted for sometime in the decade of the 2020s. Beyond this tipping point, the warming associated with CO2 increases is ex- pected to bring irreversible ice-sheet and species losses. Chapters 4, 5, and 11 discuss the role of carbon dioxide as an important greenhouse gas and the implications of CO2 increases for climate change.
Atmospheric Temperature Criterion
By the criterion of temperature, the atmospheric profile can be divided into four distinct zones—thermosphere, mesosphere, stratosphere, and troposphere (labeled in Figure 3.1). We begin with the zone that is highest in altitude.
Thermosphere The thermosphere (“heat sphere”) roughly corresponds to the heterosphere (from 80 km out to 480 km). The upper limit of the thermosphere is the thermopause (the suffix -pause means “to change”). Dur- ing periods of a less active Sun, with fewer sunspots and eruptions from the solar surface, the thermopause may lower in altitude from the average 480 km to only 250 km. During periods of a more active Sun, the outer atmo- sphere swells to an altitude of 550 km where it can create frictional drag on satellites in low orbit.
The temperature profile in Figure 3.3a (yellow curve) shows that temperatures rise sharply in the ther- mosphere, to 1200°C and higher. Despite such high tem- peratures, however, the thermosphere is not “hot” in the way you might expect. Temperature and heat are differ- ent concepts. The intense solar radiation in this portion of the atmosphere excites individual molecules (principally nitrogen and oxygen) to high levels of vibration. This kinetic energy, the energy of motion, is the vibrational energy that we measure as temperature. (Temperature is a measure of the average kinetic energy of individual mol- ecules in matter, and is the focus of Chapter 5.)
In contrast, heat is created when kinetic energy is transferred between molecules, and thus between bodies or substances. (By definition, heat is the flow of kinetic energy from one body to another resulting from a tem- perature difference between them, and is discussed in Chapter 4.) Heat is therefore dependent on the density or mass of a substance; where little density or mass exists,
Georeport 3.3 Human Sources of Atmospheric Carbon Dioxide
Carbon dioxide occurs naturally in the atmosphere, as part of the earth’s carbon cycle (a focus of Chapters 11 and 19). However,
the recent acceleration in atmospheric CO2 concentrations is from human sources, primarily fossil fuel combustion for en- ergy and transportation. The main sources of U.S. CO2 emissions are electric power plants (40%) and transportation (31%); of the sources that produce electricity, coal burning produces more CO2 than oil or natural gas (see www.epa.gov/climatechange/ghgemissions/). China is now the leading emitter of CO2, responsible for 28% of all global CO2 emissions in 2011. However, the United States still leads in per capita (per person) CO2 emissions. The use of coal to meet energy demands is increasing worldwide, especially in China and india. Some estimates predict that coal will be supplying 50% of the world’s energy by 2035, an increase in 20% from 2012. Since CO2 is linked to ris- ing global temperatures, scientists think that such an increase could have irreversible impacts on climate.