1.2: The Atmospheric Blanket and Its Warming Effect

The Earth’s atmosphere is an extremely thin shell compared with the size of our planet. The primary gases in the atmosphere by volume are nitrogen (78.1%), oxygen (20.9%), and argon (0.9%). These figures don’t include water vapor, which varies significantly with location and altitude but averages about 0.4% of the atmosphere globally. Other naturally occurring gases include carbon dioxide (designated by chemists as CO₂), ozone, and methane, which all occur in trace amounts. Although CO₂, methane, and ozone occur naturally, human activities are increasing their concentrations.

This blanket of atmosphere sustains life in many fundamental ways. First, it is vital to the cycle of plant and animal life. Plants grow by taking carbon dioxide from the atmosphere. In the process of photosynthesis, they use energy from the sun to synthesize carbon dioxide with water and release oxygen to the atmosphere. Plants incorporate the carbon into sugars that store energy and into structural materials such as cellulose, while they release the oxygen back into the atmosphere. When animals, including human beings, consume plant material (or other animals), they digest it: that is, they take in oxygen, which reacts with the food to release its stored energy. In the process, animals and humans convert some of the carbon back into carbon dioxide and re-exhale it into the atmosphere.

Water vapor is a crucial part of the atmospheric composition. Water vapor is produced primarily from evaporation from the oceans, surface soils, and subsurface aquifers and then spreads around the planet. It is the water vapor in the atmosphere that forms clouds and rain, thus creating rivers, lakes, glaciers, and ski slopes.

Figure 1.2.1 Atmosphere: a thin, fragile shell. The various ways the atmosphere sustains life on the planet. Image by V. Ramanathan. Icons designed by Freepik from Flaticon.com.

Most important for our purposes, the atmosphere plays a crucial role in determining the temperature of our planet, as we will see in our discussion of the greenhouse effect. Water vapor, carbon dioxide, and other greenhouse gases warm our planet. Without these greenhouse gases, our planet would be about as cold as Mars—far too cold to support liquid water and life. At the other extreme, without clouds, ice, and snow to reflect sunlight, the planet would be so hot that it would be unlivable.

Thus, the composition of the atmosphere keeps the Earth’s temperature at just the right level for water to be present in the planet in all three phases: gaseous water vapor, liquid water, and solid ice and snow crystals. The presence of all three forms of the water molecule is essential for the survival of Homo sapiens and most other species on Earth. The atmosphere thus protects life.

The natural greenhouse effect

Figure 1.2.2 The greenhouse effect. Adapted from NASA.

Our planet’s fundamental energy source is incoming radiation from the sun, which we will refer to as incoming solar energy. Not all of this solar energy is absorbed by the planet. About 29% of it is reflected back into space by the atmosphere, the land surface, and the sea surface. The percentage of solar radiation reflected back into space is called the albedo. The primary climate variables responsible for the Earth’s 29% albedo are clouds, snow cover, ice sheets, sea ice, glaciers, and oxygen and nitrogen in the atmosphere. In general, whiter substances (clouds, ice, and snow) reflect more solar radiation. The scattering of sunlight by oxygen and nitrogen gives the sky its blue color.

After 29% of the incoming solar radiation is reflected back to space, the Earth absorbs the remaining 71%, which heats the land, ocean surface, and atmosphere. In response, the surface and the atmosphere radiate (i.e., give off) this heat by emitting infrared radiation. This infrared radiation is commonly referred to as heat energy because the infrared radiation emitted by any substance depends on its temperature. The higher an object’s temperature, the more heat energy it emits.

However, not all of the emitted heat energy can escape to space. The greenhouse gases in the intervening atmosphere absorb (trap) some of this heat energy. As a result, the heat energy leaving the planet is reduced by the intervening atmosphere. It is this trapping of heat energy that otherwise would have escaped to space through the atmosphere that is referred to as the greenhouse effect.

Now, let’s see how this trapping effect warms the surface. The greenhouse gases in the atmosphere trap heat energy and reradiate some of it back to the surface. The surface absorbs this reradiated heat energy, causing it to warm some more. The Earth will continue to warm until it reaches a temperature at which the net incoming solar energy equals the heat energy emitted to space by the warmer surface and the atmosphere.

Thus, the surface temperature of a planet is primarily determined by two factors: the net amount of incoming solar energy it receives, and the heat-trapping properties of any greenhouse gases in its atmosphere. Increasing the concentration of greenhouse gases shifts the balance between incoming solar energy and outgoing heat energy, requiring the planet to become warmer and emit more heat energy to restore equilibrium.

“Blanket” is a better metaphor

Figure 1.2.3 Blanket metaphor. Photograph by Matthew Henry on Unsplash.

The trapping of heat by the Earth’s atmosphere is typically referred to as the “greenhouse effect.” This metaphor compares the heat-trapping gases in the atmosphere to the glass panes of a greenhouse, which allow solar radiation to enter but slow down outgoing infrared heat radiation. Although this name has become standard, it’s not the best metaphor for understanding the effects of climate pollutants. In fact, the main reason it’s warmer inside a real greenhouse is not because it traps radiated heat energy, but because its walls and roof keep warm air from escaping and colder outside air from entering.

A more scientifically accurate metaphor for the warming effect of the atmosphere is the blanket effect. On a cold night, a blanket (the atmosphere) warms us by trapping some of the heat energy radiated by the body (the planet’s surface) and thus prevents some of it from escaping to the rest of the room (space). However, following well-established tradition, we will retain the terms greenhouse effect and greenhouse gases throughout this book.

What are the natural greenhouse gases?

So, what are the gases responsible for this natural heat-trapping effect? Most of the Earth’s atmosphere is made up of gases, primarily nitrogen and oxygen, that do not trap heat energy and do not contribute to the greenhouse effect. The term greenhouse gases refers to the small fraction of gases that do have the ability to trap infrared heat energy.

The dominant greenhouse gas in the Earth’s atmosphere is water vapor. Next is carbon dioxide. Other naturally occurring greenhouse gases in the atmosphere include methane, ozone, and nitrous oxide. Concentrations of these gases are extremely small when compared with oxygen and nitrogen, but they play a crucial role in regulating climate and climate change. They have a much larger role in determining the Earth’s climate than their tiny concentrations would suggest.

As we noted earlier, water exists in the atmosphere not only in the form of gaseous water vapor, but also in the form of clouds (liquid water droplets and ice crystals). Clouds also provide a large greenhouse effect, almost comparable to that of CO₂. However, clouds also reflect solar energy. The reflective effect of clouds is about twice as large as their greenhouse effect. Thus clouds, in spite of trapping significant amounts of heat, have a large net cooling effect on the planet.

Experimental validation of the atmospheric greenhouse effect

How do we know the greenhouse effect is real? One way is to look at the energy absorbed and emitted by the Earth. Satellites routinely measure the incoming solar energy and the outgoing heat energy from the planet. Independently, the heat energy emitted by the surface has been estimated using observed surface temperatures on land and sea.

Figure 1.2.4 Comparative statistics of the incoming solar energy and surface temperatures of Venus, Earth, and Mars. Adapted from NASA.

A note about units: scientists measure energy in units called joules. To describe incoming and outgoing energy for the Earth, scientists use watts. A watt is a unit describing the rate at which energy is emitted or absorbed; 1 watt is equal to a rate of 1 joule per second. To give a familiar example, a 60-watt light bulb, when lit, emits 60 joules of heat and light energy per second. Scientists measure the rate of incoming solar energy and emitted heat energy for a planet in terms of the energy rate per unit of its surface area. This is expressed as watts per square meter of the planet’s surface, denoted in short form as W/m² (where the slash means “per”).

Globally, measurements show that the Earth’s surface emits heat energy at 390 W/m². However, satellite measurements during the 1980s showed that the heat energy escaping to space through the atmosphere was only 260 W/m². Thus, the atmosphere traps about one-third of the surface-emitted heat energy. Clouds decrease the energy that escapes by an additional 25 W/m²; thus, the net heat energy escaping to space (with clouds) is 235 W/m².

We can use another, more whole-system approach to validate the greenhouse effect: comparing planet Earth with its neighbors, Venus and Mars. On one hand, the average surface temperature of Earth is 15°C. The Venusian surface, on the other hand, is searing hot at 462°C—well above the melting point of lead. Why is this the case? The first obvious suggestion would be that Venus is hot because it is so close to the sun. Indeed, Venus is close to the sun, and its incoming solar energy is 659 W/m², compared with 341 W/m² for Earth.

But there is a second factor to consider: Venus is completely cloud covered and as a result reflects as much as 75% of its incoming solar energy (that is, the albedo of Venus is 75%). Taking this into account, we find that Venus actually absorbs solar energy of 165 W/m², slightly less than the amount of solar energy that Earth absorbs (242 W/m²). On that basis, we would expect Venus to be cooler than the Earth.

The only remaining explanation for Venus’s searing hot surface temperature is the greenhouse effect of the CO₂ in its atmosphere. It turns out that the concentration of CO₂ on Venus is about 200,000 times more than that on Earth, creating a superstrong CO₂ greenhouse effect, which maintains Venus’s hot temperature.

Mars, on the other hand, is much farther from the sun and receives less than half the solar energy that Earth receives. Mars is nearly cloudfree (except for some dust clouds), and its albedo is only 18%. The net effect is that the solar energy that Mars absorbs (125 W/m²) is only half of that absorbed by Earth. This is the primary reason for the frigid average temperature on Mars (−55°C). Mars’s atmosphere is mostly CO₂, and the amount of CO₂ on Mars is actually about 15 times larger than that on Earth, but the stronger greenhouse effect is not enough to compensate for the lower incoming solar energy.

Earth in the Goldilocks zone

The above exercise illustrates an important message about the optimal climate on Earth. The surface temperature is determined by a delicate balance between the amount of incoming solar energy, the reflected solar energy, and the greenhouse gases in the atmosphere.

As we saw earlier, water vapor has the strongest warming effect of the naturally occurring greenhouse gases. At the same time, clouds made up of condensed water vapor have a net cooling effect. If water vapor plays such a significant role in our climate, why do discussions of climate change mostly focus on emissions of carbon dioxide? Where does the water vapor greenhouse effect fit in this picture?

While carbon dioxide is emitted by geological processes (and more recently, human activities), the concentration of water vapor is primarily governed by surface and atmospheric temperatures. The warmer the atmosphere, the higher the concentration of water vapor, assuming there is an abundant source (such as oceans or water “cooked out” from minerals deep in the Earth’s interior).

Figure 1.2.5 Goldilocks principle. Reproduced from NOAA.

Because the concentration of water vapor depends on temperature, climate scientists refer to it as a climate feedback that amplifies warming, rather than a direct cause of warming. If there were no carbon dioxide in the Earth’s atmosphere, temperatures would fall to the point that most of the water vapor would condense or crystallize out of the Earth’s atmosphere as well. Without carbon dioxide, there would be very little water vapor greenhouse effect and the Earth would be much cooler, if not frozen.

Thus, the Earth seems to have just the right amount of incoming solar radiation, clouds, and CO₂ to maintain an equitable climate. Among the three planets, Earth is the only one whose temperature is not too hot, not too cold, but “just right” for Goldilocks’s porridge—and for life.

CO₂ increased by human activities

While the natural greenhouse effect is vital for maintaining life on Earth, humans have added an enormous amount of carbon dioxide to the thin shell of the atmosphere since the dawn of the Industrial Revolution. As of 2017, we have dumped 2,200,000,000,000 (2.2 trillion) tons of carbon dioxide into the atmosphere over the past 240 years. About 45% of that carbon dioxide still remains in the air today. That leaves a blanket of human-generated carbon dioxide in our thin atmospheric shell whose sheer weight is astounding—990 billion tons. That’s equivalent to the weight of about 490 billion cars circling the planet all the time.

How do we know the weight of the human-made CO₂? From direct measurements initiated by Charles David Keeling of the Scripps Institution of Oceanography (UC San Diego). This wiggly curve (Figure 1.2.6) is called the Keeling Curve and shows the concentration of carbon dioxide in the atmosphere. When Keeling first started making measurements in 1958, the atmospheric carbon dioxide concentration was 313 parts per million (abbreviated as 313 ppm). That is, out of every million molecules in the atmosphere, 313 were carbon dioxide molecules in 1958.

Figure 1.2.6 The Keeling Curve shows the increase in CO2 from 1958 to 2017. Reproduced from the Scripps CO2 Program from the Scripps Institution of Oceanography.

Passing a major threshold

In the year 2016, we passed a major threshold—one that we should not be passing. Based on measurements of ancient air bubbles trapped in ice and other data, scientists estimate that the concentration of CO₂ before 1850 was 275 ppm. That concentration has since increased steadily, shooting past 300 ppm by 1950, 369 ppm by 2000, and 400 ppm by 2016. Carbon dioxide concentration was about 410 ppm in 2018, meaning that humans have now increased the overall concentration of carbon dioxide by nearly 50% since the preindustrial era. Crossing the threshold of 400 parts per million signifies that the planet could be transitioning into an era of major climate changes.

The increase is seen everywhere on the planet: the ocean surface, mountaintops, and deserts. Whether the data are collected in Hawaii, the Arctic, or the Antarctic, the findings are the same. Basically, the additional CO₂ has covered the planet like a blanket.

How did that happen? Air travels fast. Pollution from North America travels to Europe in days; pollution from Asia travels to North America in a week; and pollution from South America travels to the Antarctic in a few weeks. Air takes a few years to travel from the Arctic to the Antarctic. Travel times for pollution are much shorter than the lifetime of the CO₂ molecule in the air, which is 100 to 1,000 years. That’s why the CO₂ increase is found everywhere on the planet. Carbon dioxide is what scientists refer to as a well-mixed gas—one that remains in the atmosphere much longer than the time it takes to spread around the world.

What is the take-home message?

The atmosphere connects every part of the world with every other part in a matter of days or weeks. Therefore, we can only solve the climate change problem through global cooperation.

Greenhouse gases as pollutants

Why do we call carbon dioxide and other anthropogenic greenhouse gases “pollutants”? Pollute means “contaminate something with a harmful or toxic substance.” Carbon dioxide is a natural component of the atmosphere and a vital part of the respiration cycle that sustains life, so how can it be a pollutant? Although carbon dioxide and most other greenhouse gases exist naturally in the atmosphere, human emissions are increasing their concentrations, causing warming that will most definitely have harmful impacts, as we will see later in this chapter. The harmful impacts of these emissions make it appropriate to refer to greenhouse gases as “pollutants.”

What are the sources for the observed increase in CO₂?

Figure 1.2.7 Sources of greenhouse gas emissions. Reproduced from UNEP/GRID-Arendal.

Many human activities that address our basic needs, development, and well-being are sources of greenhouse gases. Most of the energy used by society since the Industrial Revolution has come from fossil fuels: coal, oil, and natural gas. Burning fossil fuels emits the largest amount of CO₂ by far, contributing an estimated 34 billion tons in 2016. Major anthropogenic sources of carbon dioxide include the following:

• Using fossil fuels to produce electricity: In 2016, 65% of electricity worldwide was generated by burning fossil fuels, including 38% from coal and 23% from natural gas. Coal emits roughly twice as much CO₂ per unit of electricity generated as natural gas, so burning coal to generate electricity is particularly concerning.
• Transportation: There are about 1 billion motor vehicles in use around the world, the vast majority of which use oil-based fuels. Aviation and commercial shipping are also major emitters of carbon dioxide.
• Residential and commercial buildings and activities: In addition to indirect emissions from electricity use, buildings can be a direct source of CO₂ emissions, primarily through heating. In developed countries, natural gas is frequently used for space heating, water heating, and cooking. The least affluent 3 billion, with limited access to fossil fuels, frequently burn wood or animal dung for heating and cooking, which also release CO₂.
• Industrial processes: A range of industrial processes, in particular cement and steel production, emit significant amounts of CO₂. Cement production alone is estimated to have been responsible for 2 billion tons of CO₂ emissions in 2016.
• Land use: Changes in land use, in particular burning forests to clear land for farming, grazing, or housing, also emit significant amounts of carbon dioxide. Over the decade 2007–2016, CO₂ emissions from land use averaged about 5 billion tons per year.

How much additional heat energy is trapped by the 990-billion-ton CO₂ blanket?

As of 2010, the heat-trapping effect of human-emitted carbon dioxide was about 860 terawatts (1 terawatt equals 1,000 billion watts—that’s a 1 followed by 12 zeros). This represents about 50 times our total global rate of energy consumption! To understand the enormity of 860 terawatts, let’s look at another statistic. The heat energy trapped by our human-made blanket is equivalent to burning 40 trillion 60-watt lightbulbs every second, every day, every month, every year. We are trapping an enormous amount of heat in the land, oceans, and atmosphere. Based on fundamental physics, the temperature of the planet and the atmosphere will be forced to increase until the extra 860 terawatts are radiated away into space. If we continue to increase the concentration of CO₂ in the atmosphere, even more heat will be trapped, forcing the planet to warm even further. This, in a nutshell, is the cause of global warming.

Is CO₂ the only important anthropogenic greenhouse gas?

Until 1975, we thought that CO₂ was the only source of anthropogenic warming. Then the greenhouse effect of chlorofluorocarbons (CFCs)—a group of artificially produced molecules used as refrigerants, solvents, and propellants—was discovered in 1975. Soon after, a host of other anthropogenic gases (more than 20) were added to the list of climate-warming gases. The most important of these, in terms of their warming impact, are methane, ozone, nitrous oxide, and another group of refrigerants known as hydrofluorocarbons (HFCs). The sources of these pollutants are the following:

• Methane is the main natural gas that we use for power generation, heating, and cooking. Natural gas leaks (called “fugitive emissions”) at production and processing facilities and through distribution pipes are a significant source of methane emissions. Another major source is methane produced by bacteria in the guts of cattle, sheep, and goats. Wet rice agriculture (rice paddy fields), wood burning, landfills, and sewage water treatment plants are among the other significant sources.
• CFCs (chlorofluorocarbons) and HFCs (hydrofluorocarbons) are artificially produced for refrigeration and air conditioning. CFCs have been phased out by international treaty since the late 1980s, but work to phase out HFCs is just beginning.
• Ozone is not directly emitted by human activities, but fossil-fuel power plants and automobile engines emit gases known as ozone precursors (methane, nitrogen oxides, and volatile organic compounds) that react with sunlight to produce ozone in the lower atmosphere.
• Nitrous oxide is released by bacteria in the soil. Nitrogen-based fertilizers used in agriculture increase the activity of soil bacteria and their nitrous oxide emissions. The IPCC estimates that as of 2010 (the IPCC data is available for only up to 2010), CO₂ has trapped 1.8 W/m2 of heat, which is 860 terawatts when integrated over the surface area of the whole planet. All of the anthropogenic non-CO₂ gases have added another 1.2 W/m2, bringing the total heat trapped by all anthropogenic greenhouse gases to 3 W/m2 (about 1,500 terawatts).

Is the climate responding to this added heat?

Undoubtedly, the climate is responding to this added heat, according to data that scientists have collected at the surface, in the atmosphere using aircraft and balloons, and from space using satellites. The entire atmosphere, most of the land surface, and the oceans to depths of as much as a kilometer have warmed to unprecedented levels compared with the temperatures of the last 100,000 years. In the following section, we will review the vast amount of past climate data that provide quantitative answers about the magnitude of the climate’s response.