Key Concepts

After completing this chapter, you will be able to:

1. Outline the physical basis of Earth’s greenhouse effect.
2. Explain the term greenhouse gas (GHG).
3. Describe how the various GHGs vary in their effectiveness and influence on the greenhouse effect.
4. Identify which GHGs have been increasing in concentration in the atmosphere, and give the reasons for those changes.
5. Explain the climatic consequences of an intensification of the greenhouse effect, and describe economic and ecological effects.
6. Discuss strategies for reducing the intensity of the human influence on the greenhouse effect.

Introduction

In this chapter we examine how Earth’s naturally occurring greenhouse effect keeps the surface of the planet relatively warm. We also describe how certain atmospheric constituents influence this phenomenon. These constituents are known as greenhouse gases (GHGs) and they work by slowing the rate by which Earth is able to cool itself of absorbed solar radiation. It is well documented that the concentrations of some of the GHGs, particularly carbon dioxide, are increasing because of emissions associated with various human activities. The effect of these increased emissions on the climate can have devastating consequences for the human economy and natural ecosystems.

The Greenhouse Effect

Earth’s greenhouse effect is a well-understood physical phenomenon, and it is critical in maintaining the average surface temperature of the planet at about 15°C (59°F). Without this influence, the surface temperature would average about -18°C (-0.4°F) or 33° cooler than it actually is. This would be frostier than organisms could tolerate over the long term, because at -18°C (-0.4°F) water is in a solid state. Liquid water is crucial to the proper functioning of organisms and ecosystems. At Earth’s actual average temperature of 15°C (59°F), water is unfrozen for much or all of the year (depending on location). This means that enzymes can function and physiology can proceed efficiently, as can the many important ecological processes that involve liquid water.

To understand the nature of Earth’s greenhouse effect, it is necessary to comprehend the planet’s energy budget. As we examined in Chapter 3, an energy budget is a physical analysis that deals with the following:

1. All of the energy coming into a system
2. All of the energy going out
3. Any difference that might be internally transformed or stored

Solar electromagnetic radiation is the major input of energy to Earth. On average, this energy arrives at a rate of about 8.4 J/cm²•min. Much of the incoming solar radiation penetrates the atmosphere and is absorbed by the surface of the planet. However, the surface temperature does not increase excessively because Earth dissipates the absorbed solar energy by emitting long-wave infrared radiation. The surface temperature is determined by the equilibrium rates at which (1) solar energy is absorbed by the surface, and (2) the absorbed energy is re-radiated in a longer-wavelength form (see Figure 3.10 for a diagram of the greenhouse effect).

If the atmosphere were transparent to the long-wave infrared radiated by the surface, then that energy would travel unobstructed to outer space. However, this is not the case because greenhouse gases (GHGs) are present in the atmosphere. GHGs efficiently absorb infrared radiation, and become heated as a consequence. They then dissipate some of this thermal energy through yet another re-radiation. (This re-radiated energy has a longer wavelength than the electromagnetic energy that was originally absorbed. This is necessary to satisfy the second law of thermodynamics.) The re-radiated energy of the GHGs is emitted in all directions, including back toward the surface. The net effect of the various energy transformations and re-radiations involving atmospheric GHGs is a reduction in the rate of cooling of Earth’s surface. Thus, the equilibrium temperature of the planet’s surface is warmer than it would be if the GHGs were not present in the atmosphere.

The process just described is known as the greenhouse effect because its physical mechanism is similar to the warming of a glass-encased space by solar radiation. The encasing glass of a literal greenhouse is transparent to incoming solar radiation. The solar energy is absorbed by, and therefore heats, internal surfaces of the greenhouse, such as plants, soil, and other materials. These warmed objects then dissipate their absorbed energy by re-radiating longer-wave infrared energy. However, much of the infrared is absorbed by the glass and humid atmosphere of the greenhouse, which are somewhat opaque to those wavelengths of electromagnetic radiation. That absorption of some re-radiated infrared slows the rate of cooling of the greenhouse, causing it to heat up rapidly on sunny days. (In addition, a greenhouse is an enclosed space, so it traps heat because its warmed interior air cannot be dissipated by convection higher into the atmosphere, with cooler air drawn in below.)

Greenhouse Gases

Water vapor (H₂O) is the most important of the greenhouse gases of Earth’s atmosphere, accounting for about 36% of the overall greenhouse effect, followed by carbon dioxide (CO₂; about 20%). Lesser roles are played by trace concentrations of methane (CH₄), nitrous oxide (N₂O), ozone (O₃), carbon tetrachloride (CCl₄), and chlorofluorocarbons (CFCs).

These latter compounds are, however, much stronger absorbers of infrared energy than is CO₂ (on a per molecule basis, they are more efficient GHGs). A molecule of CH₄ is about 28 times more effective than one of CO₂ at absorbing infrared radiation, while N₂O is 265 times more effective (these are known as greenhouse warming potentials, with CO₂ assigned a value of 1.0; Table 21.1).

There is no evidence that the concentration of water vapor in the atmosphere has increased recently. However, concentrations of CO₂ and other GHGs have increased markedly during the past several centuries because of emissions associated with human activities (Table 21.1). Prior to 1750, the atmospheric concentration of CO₂ was about 280 ppm, whereas in 2014 it had reached 399 ppm, which is a 43% increase. Other GHGs have also increased during this period. The increases have been especially rapid since the middle of the twentieth century, coinciding with enormous increases in population, industrialization, and deforestation. As of 2019, CO₂ levels have reached 409.85 ppm (Global Carbon Budget 2020).

Because the various GHGs influence the greenhouse effect, their increasing concentrations can intensify that process. Such an environmental change is viewed as an anthropogenic intensification of Earth’s naturally occurring greenhouse effect. Overall, the increased concentration of CO₂ is estimated to account for about 57% of this possible enhancement of the greenhouse effect, while CH₄ is responsible for 15%, tropospheric O₃ for 12%, halocarbons for 8%, and N₂O for 5% (Table 21.1).

Table 21.1. Increases and Characteristics of Greenhouse Gases. Source: Data from Blasing, 2014.

Atmospheric Carbon Dioxide

Concentrations of CO₂ in the atmosphere have been increasing steadily for at least the past century. The data record supporting this change is excellent and demonstrates one of the most convincing examples of long-term changes of any aspect of environmental chemistry. For example, atmospheric CO₂ has been monitored continuously since 1958 at a remote observatory located on Mauna Loa, a mountain on the island of Hawaii (Figure 21.1). Data are also shown for Alert, a high-Arctic station located at the northern tip of Ellesmere Island, Nunavut. The data from both places clearly show steadily increasing concentrations of CO₂ in the atmosphere during the past five decades.

Figure 21.1. Increases in Atmospheric CO₂. These data are from measurements made on an equatorial station on Mauna Loa, Hawaii, and in the High Arctic in northern Ellesmere Island, Canada. Each datum represents an annual average. Note that prior to 1750, the concentration of CO₂ in the atmosphere was about 280 ppm (see text). Source: Data from Keeling et al. (2015).

A seasonal cycle of CO₂ concentration is illustrated in Figure 21.2, again using data from Mauna Loa and Alert. The annual periodicity is caused by high rates of CO₂ uptake by vegetation of the Northern Hemisphere during the growing season. This seasonal CO₂ fixation occurs at rates that are high enough to depress its overall concentration in the global atmosphere. The effects are larger in the Arctic than at the Equator, although both regions have the same annual average concentration of CO₂.

Figure 21.2. Seasonal Changes in Atmospheric CO₂. These data are based on measurements made at Mauna Loa, Hawaii, and Alert, Ellesmere Island, Canada. Source: Data from Keeling et al. (2008, 2015).

A significant body of research has demonstrated that increased concentrations of atmospheric CO₂ are due to emissions associated with various human activities. The two most important sources of anthropogenic emissions are examined in more detail in the following sections:

• The combustion of fossil fuels, during which the carbon content of the fuel is oxidized to CO₂, which is emitted to the atmosphere (Image 21.1)
• Deforestation, an ecological conversion in which mature forests that store large amounts of organic carbon are converted into ecosystems that contain much less, with the difference being made up by a released of CO₂ to the atmosphere

Image 21.1. A Packed Parking Lot. The combustion of fossil fuels for transportation and commercial energy is the leading anthropogenic source of emissions of carbon dioxide to the atmosphere. Source: B. Freedman.

CO₂ from Fossil Fuels

Fossil fuels are the most important source of energy in industrialized countries, followed by hydroelectricity, nuclear power, and relatively minor sources such as wood, solar, and wind energies (Chapter 18). The rates of utilization of coal, petroleum, natural gas, and oil sand have increased enormously during the past century, mostly to satisfy surging energy demands for industry, transportation, and space heating. The manufacturing of cement also results in large emissions of CO₂ to the atmosphere.

In total, since about the beginning of the Industrial Revolution in 1750, about 365-billion tonnes of CO₂-C (carbon in the form of CO₂) have been released to the atmosphere from the consumption of fossil fuels and the production of cement (Boden et al., 2013). Half of these fossil-fuel CO₂ emissions have occurred since the mid-1980s.

Between 1860 and 1869, during the middle part of the Industrial Revolution, the combustion of fossil fuels, mainly coal, resulted in the global emission of about 422-million tonnes of CO₂ per year (Boden et al., 2013). By the year 2012, global emissions from fossil-fuel combustion had increased by a factor of 80, to 35.4 billion tonnes per year (Boden et al. 2013). Globally we currently emit around 36 billion tonnes per year, although there was a reduction in 2020 (34 billion tonnes) that was attributed to the COVID-19 pandemic (Global Carbon Project, 2020; Figure 21.3). The majority of global CO₂ emissions by fuel type comes from coal, then oil, then gas (Figure 21.4). Table 21.2 provides a comparison of global CO₂ emissions to the U.S. by fuel type.

Figure 21.3 Annual Global CO₂ emissions 1800-2019. CO₂ emissions depicted here are from the burning of fossil fuels for energy and cement production. Land use is not included. Source: OurWorldinData.org with data from Global Carbon Project, supplemental data; Carbon Dioxide Information Analysis Center, licensed CC BY 4.0.

Figure 21.4. Global CO₂ Emissions by fuel 1750-2019. Source: OurWorldinData.org from Global Carbon Project, supplemental data, licensed CC BY 4.0.

Table 21.2. Global and U.S. Emissions of Carbon Dioxide by fuel type. Data modified from: OurWorldinData.org from Global Carbon Project, supplemental data.

Fuel type	Global (billion tonnes)	U.S. (billion tonnes)
Coal	14.36	1.09
Oil	12.36	2.34
Gas	7.62	1.71
Cement	1.56	0.0412
Flaring	0.429	0.0717
Other industry	0.115	0.0275

The global emissions of CO₂ are equivalent to about 4.73 tonnes CO₂/person/year. Of course, per-capita use of fossil fuels differs greatly among countries, depending on their kind and degree of industrialization, types of energy sources, climate, and other factors (Figure 21.5). The greatest emissions are in wealthy, energy-intensive countries, such as Australia, the U.S., China, and most of Western Europe. The smallest emissions are in the poorest, least-developed countries, where there is relatively little use of fossil fuels because of the expense to purchase them.

Figure 20.5. Per Capita Emissions CO₂ Emissions by Selected Countries from 1800 to 2017. This figure depicts carbon dioxide emissions from the burning of fossil fuels for energy and cement production. Land use change is not included in this figure. Source: OurWorldinData.org from Global Carbon Project, supplemental data; Gapminder, and UN is licensed under CC-BY 4.0.

Future emissions of CO₂ from fossil-fuel combustion are predicted to be much larger than those occurring today, mainly because of the anticipated industrialization of poorer countries as they develop economically. One prediction suggests that global emissions by the middle of the twenty-first century could be up to 55 billion tonnes of CO₂ per year, about double the current releases.

CO₂ from Clearing Forest

Mature forest stores large amounts of organic carbon in vegetation and the dead organic matter of soil. All other kinds of ecosystems, including younger forests that are regenerating from a disturbance, store much less organic carbon than occurs in older forests. This observation suggests that whenever an area of mature forest is disturbed by timber harvesting, or is cleared to provide land for agricultural or urbanized use, much less organic carbon will be stored on the land.

If a harvested stand is allowed to regenerate to another mature forest, then the depletion of stored carbon will be a medium-term phenomenon. However, if forest is converted into an anthropogenic land-use, such as for agriculture or urbanization, there is a permanent loss of carbon stored on the land. In either case, the difference in the average quantity of organic carbon stored in the ecosystem is balanced by an emission of CO₂ to the atmosphere. The CO₂ release mostly occurs by decomposition of the forest biomass or by burning. To a lesser degree, and for similar reasons, a carbon loss also occurs when natural grassland is converted into cultivated agriculture.

It is well known that humans have caused enormous reductions in the area of mature forest in most regions of the world (Chapters 11, 12, and 14). These changes began slowly, initially perhaps with the domestication of fire and its widespread use to improve the habitat of hunted animals. Deforestation proceeded more rapidly when it was discovered that fertile agricultural land could be developed by removing the natural cover of forest or grassland. (The harvested trees were also valuable commodities.) Deforestation has proceeded especially quickly during the past several centuries because of population growth, agricultural expansion, and industrialization.

Prior to any substantial clearing of Earth’s natural forests, the global terrestrial vegetation stored an estimated 900-billion tonnes of organic carbon. About 90% of that carbon was stored in forest, of which half was in tropical forest. Now, only about 400-billion tonnes of carbon are stored in terrestrial biomass (Pan et al. 2013). Moreover, the stocks of global biomass are diminishing further as more-and-more natural ecosystems are converted into agricultural and urban ones that store much less carbon.

During the 143-year period from 1870 to 2013, changes in land-use (mostly conversions of forest into agricultural land) resulted in the emission of about 145-billion tonnes of CO₂-C. This quantity is about 45% of the emissions due to fossil fuel combustion during the same period (320-billion tonnes of CO₂). In 2013, the combustion of fossil fuels emitted about 9.9-billion tonnes of CO₂-C into the atmosphere, while deforestation accounted for another 0.9-billion tonnes.

As was previously noted, forest and grassland ecosystems store large amounts of carbon in the biomass of their vegetation and soil. When these “high-carbon” ecosystems are converted into agricultural or urban ones, there is a large emission of their organic carbon to the atmosphere (mostly as CO₂ from decomposition and fires).

The disturbance of forests by harvesting timber also results in a large emission of CO₂, because mature stands support much more biomass than younger ones (old-growth forest stores the most). However, the carbon emission scenario is complicated by what is done with the harvested timber. For example, if the tree biomass is burned as a fuel, the release of CO₂ to the atmosphere occurs rapidly. On the other hand, if the harvested wood is used to manufacture lumber, furniture, or violins, all of which are “enduring” products with an extended lifespan, the release of CO₂ to the atmosphere occurs slowly. It must also be remembered that much of the initial release of CO₂ may eventually be offset by regeneration of the harvested forest (unless this is prevented, as happens when deforestation occurs to develop agricultural or urban land-use).

In North America, extensive forest clearing began when the continent was colonized by Europeans and continued until the 1920s. Since then, however, large areas of marginally economical agricultural land have been returned to forest. Overall, the net emission of CO₂ by changes in forest area has recently been close to zero—that is, agricultural land is regenerating back to forest about as quickly as forest elsewhere in North America is being converted into agricultural and urban land-uses. The European situation is similar, and forest biomass (and carbon storage) there has also increased since the 1920s.

However, in relatively poor, less-developed, tropical countries of Africa, Asia, and Latin America, forests are being cleared rapidly. This is being done mostly to develop agricultural land to provide livelihoods and grow food for increasing numbers of people, and also to provide agriculture commodities for export. This is a serious problem not only because of the large emissions of CO₂, but also because of the consequences for biodiversity (Image 21.2).

Image 21.2. The conversion of carbon-dense ecosystems, such as forest, into agricultural and urban ecosystems that store much less carbon is an important source of CO₂ emissions. This site on Sumatra has had its tree cover felled and the woody debris burned. The land will be planted with a variety of crops. Deforestation is proceeding rapidly in this region of Indonesia, and in most tropical countries. Source: B. Freedman.

Overall, in modern times, most CO₂ emissions associated with deforestation have been occurring in less-developed tropical countries. In contrast, most CO₂ emissions from the combustion of fossil fuels have been occurring in relatively wealthy, industrialized, higher-latitude countries, of which the U.S. is a leading example.

Global Carbon Geochemistry

Key anthropogenic influences on the global carbon budget are summarized in Figure 21.5, which shows the major compartments in which carbon is stored as well as transfers between them.

Figure 21.5. The Carbon Cycle. This figure shows the movement of carbon between land, atmosphere and ocean. The yellow numbers depict natural fluxes, the red numbers indicate human contributions in gigatons of carbon per year, and the white numbers indicate stored carbon. Diagram adapted from U.S. DOE, Biological and Environmental Research Information System.

The concentration of CO₂ in the atmosphere has increased from approximately 277 ppm in 1750 to 409.85 ppm in 2019 (Global Carbon Budget 2020). Before humans began to modify the character of Earth’s ecosystems, especially by extensive deforestation, the global emission and fixation of atmospheric CO₂ were approximately in balance. In other words, on a global basis, the gross primary production (GPP) was about equal to ecosystem respiration (ER), and biologically fixed carbon was not changing over time. However, land use change including deforestation and fires are resulting in emissions of CO₂, amounting to about 1.8 gigatons/y of CO₂-C.

Ultimately, the oceans are the most important sink for CO₂ emitted through human activities. The oceans have a net absorption of about 3.1 gigatons/y of CO₂-C from the atmosphere. However, this is much less than the anthropogenic emissions of about 8.6 gigatons/y of CO₂-C, and so the amount of CO₂ stored in the atmosphere is increasing. The oceans have an enormous capacity for absorbing atmospheric CO₂, which is ultimately deposited as calcium carbonate (CaCO₃), a mineral that accumulates in sediment (mostly as the shells of mollusks, foraminifera, and other invertebrates). However, the rate of formation of CaCO₃ is affected by various factors, including the concentration of inorganic carbon in seawater as well as acidity. This concentration is determined by the rate at which CO₂ enters the oceans from the atmosphere, minus its biological uptake (mostly by phytoplankton during photosynthesis). Although anthropogenic CO₂ eventually ends up as CaCO₃ in oceanic sediment, there is a substantial time-lag in the response of oceanic sinks to increasing concentrations of CO₂ in the atmosphere. This lag allows atmospheric CO₂ concentrations to increase because of anthropogenic emissions.

Acidification of the ocean is an additional issue. In actual fact, the ocean is maintained as a non-acidic environment by carbon dynamics and a variety of other influences, with a typical pH between about 7.5 and 8.4 (Chester and Jickells, 2012). In this case, acidification would be represented oceanic water becoming less alkaline over time. The acidification is caused by atmospheric CO₂ dissolving into oceanic water, a process that forms carbonic acid (H₂CO₃), a weak acid, according to this equation:

CO₂ + H₂O ⇌ H₂CO₃

The carbonic acid may then dissociate to form bicarbonate (HCO₃^–) and carbonate (CO₃^2–), as follows:

H₂CO₃ ⇌ HCO₃– + H⁺ ⇌ CO₃–2 + H⁺

The rate at which CO₂ can dissolve into the ocean is in equilibrium with its atmospheric concentration. As a result, the rapid increases of atmospheric CO₂ (to 409.85 ppm in 2019) have resulted in more dissolving, more production of carbonic acid, and the beginning of acidification of the vast aquatic ecosystem. Studies have demonstrated that over the last decade ocean carbon dioxide levels have risen in response to increased atmospheric carbon dioxide, leading to an increase in acidity in ocean waters (EPA, 2021). Ocean acidification is a potentially serious problem, because many marine organisms can only live within a narrow range of tolerance of this aspect of water chemistry.

Climate Change

As was previously examined, Earth has a naturally occurring greenhouse effect, the physical mechanism of which is relatively simple and understood by scientists (Chapter 4). Moreover, the greenhouse effect helps to maintain the surface temperature within a range that is comfortable for organisms – averaging about 15°C, or 33° warmer than it would be with a non-greenhouse atmosphere. It is also well documented that the concentrations of CO₂ and other greenhouse gases are increasing in the atmosphere, which can then intensify the greenhouse effect, having significant climactic and ecological impacts.

Climate change refers to long-term variations of the weather that are experiences in a region. One of the most important indicators of climate change is the temperature of the surface atmosphere. Air temperature is measured routinely in many places throughout the world. These data can be used to calculate estimates of the average surface temperature of Earth and to detect changes over time. Recent analyses suggest that there has been a definite warming trend since the mid-nineteenth century. The average global surface temperature has increased by more than 0.8°C over the past 150 years (Figure 21.6). The warmest years since 1850 have all occurred since about 1990. This warming partly reflects the end of a 400-year period of climate cooling, known as the Little Ice Age, which lasted until the mid-1800s (Figure 21.7). However, there appears to have been a particular intensification of warming during the most recent several decades.

Figure 21.6. Recent Changes in Global Surface Temperature. The data are the global annual temperature anomaly (°C), calculated relative to the average for 1961-1990. A negative value means a year was relatively cool, while a positive number means it was warmer. Source: Data from Jones et al. (2013).

a)

b)

c)

Figure 21.7. Deviation of Global Average Surface Temperature from Present Conditions. Curve (a) shows estimated global temperatures over the last 500 million years. Curve (b) shows long-term trends since the end of the most recent ice age. Curve (b) shows the past two millennia. Note that a value of “zero” in (a) and (b) means that no temperature change (deviation) has occurred. Sources: NOAA, NOAA, NOAA.

Paleoclimactic studies of long-term changes have provided evidence of a link between concentrations of atmospheric CO₂ and temperature in the atmosphere. Especially valuable data come from a core of glacial ice taken in Antarctica, representing a record of 417-thousand years (Figure 21.8). Results of this important study suggest a strong correlation between CO₂ concentration and air temperature, implying a possible causal relationship.

Figure 21.8. Variations in Atmospheric CO₂ and Surface Temperature. These data were obtained by studying a 417,000-year glacial-core record from Vostok, Antarctica. The red data are the temperature deviation and the blue are CO₂ concentration. The two data sets are strongly correlated, with a coefficient of 0.82. Sources: Data from Petit et al. (1999) and Barnola et al. (2003).

Other valuable insights have been obtained by running sophisticated mathematical models of global climate processes on high-powered supercomputers. These “virtual experiments” examine the potential climatic responses to increases in atmospheric CO₂. The computer simulations are known as three-dimensional general circulation models (GCMs). The models simulate the complex movements of energy and mass in the global circulation of the atmosphere. They also examine the interactions of these processes with physical variables that are important aspects of climate, such as temperature and precipitation. Many simulation experiments have been run using various GCMs, and the results are variable. Nevertheless, a strong tendency that emerges from these virtual experiments is that global warming and associated climate changes are a likely consequence of the anthropogenic, well-documented increases of CO₂ and other GHGs in the atmosphere.

Many such simulation experiments have examined the scenario of a doubling of CO₂ concentration from its recent concentration of about 400 ppm. These experiments suggest that such a doubling would result in an increase of 1°C to 4°C in the average temperature of the surface atmosphere. The intensity of warming is predicted to be greatest in high-latitude regions, where the temperature increases might be two to three times greater than in the tropics.

Warming of the lower atmosphere will be one likely change that will be caused by an increased intensity of the greenhouse effect. However, there could also be important effects that occur indirectly, in response to changes in the distribution of heat in the atmosphere. The most important of the indirect changes would include large-scale shifts in the patterns of atmospheric circulation. Such shifts would likely result in changes in the amounts, spatial distribution, and seasonality of precipitation. Changes in precipitation regimes would influence soil moisture, which would greatly affect the distribution and productivity of vegetation, both natural and managed. These changes in precipitation regime would likely have much greater effects on agricultural and wild ecosystems than would any direct influence of a warmed atmosphere.

Ecological Effects

In terrestrial ecosystems, the effects of global warming and associated climatic changes would most notably be related to changes in the amounts and seasonal patterns of precipitation. Soil moisture is often a key environmental influence on the distribution and productivity of vegetation. For instance, a decrease in the amounts of precipitation or soil moisture in the North American prairies would likely cause the natural mixed-grass prairie to change into short-grass prairie, or even to semi-desert. Decreased soil moisture would also affect the kinds of crops that could be grown in many regions, as well as their productivity. That could make present agricultural systems more difficult or even impossible unless irrigation was practiced.

About 14 thousand years ago, the continental glaciers started to melt back, and they were about 80% gone by 8-10-thousand years ago. Vegetation in the regions of North America changed substantially during the warming climates that followed this deglaciation. One of the paleoecological tools that have been used to study the changes involves the examination of fossil pollen grains extracted from dated sections of cores of lake sediment (these studies are known as palynology). This kind of analysis has provided a record of vegetation changes extending as far back as early deglaciation.

The research in North America and elsewhere suggests that plants responded to post-glacial warming in a species-specific manner. This occurred because of the different abilities of species to migrate to and colonize newly available habitats released by the melting of glacial ice. As a result, the species composition of early post-glacial plant communities was different from that occurring today under similar climatic regimes. We can expect the responses of natural vegetation to future climate changes to also be species-specific. This will result in the development of plant communities that are different from those that occur now. If climate change results in substantial modifications in the character of plant communities, there will also be adjustments in the species of animals, microbes, and other organisms that can be supported on the landscape. Challenges to native biodiversity will be an important consequence of climate change in North America and everywhere else in the world (Image 21.3).

Image 21.3. An ‘Island” of Trees. This is a small “island” of trees in the midst of tundra near Tuktoyaktuk in the Northwest Territories of Canada. These short individuals of white spruce (Picea glauca) are remnants of a more widespread population that established during a period of warmer climate more than about six centuries ago. If an anthropogenic intensification of Earth’s greenhouse effect were to result in a warming climate, as has been predicted, then these tree-islands may be focal points from which trees could colonize the tundra. If this kind of change occurs over a large area, there would be profound consequences for the biota and for human interests. Source: B. Freedman.

Climate change in tropical countries, which support much larger numbers of species than North America does, would have great ecological consequences. For example, most of northern and central South America is now characterized by a warm and humid, tropical climate. However, this region is thought to have been considerably drier during the past glacial period, which ended 10-14 thousand years ago. During that time, much of the tropical region was covered by an open-canopied savannah, while rainforest occurred only in isolated regions with relatively high rainfall, known as refugia. In terms of the landscape, the refugia of tropical forest occurred as “islands” within a more extensive matrix of savannah, which is an inhospitable habitat for species of moist forest. The restructuring of tropical ecosystems during the Pleistocene Ice Age, which was driven by climate changes of the time, must have had enormous impacts on the multitudes of rare species of the rainforest. It is likely that many of those species became extinct as a result of the habitat changes. In modern times, an anthropogenic intensification of the greenhouse effect will also cause substantial changes to occur in the character of tropical habitats over enormous areas, and similar ecological calamities would again result.

As was just noted, changes in climate would influence the ability of landscapes to support agricultural production. In the U.S., this would be particularly true of the great expanses of agricultural land in the Midwest. Much of this terrain is already marginal from a rainfall perspective, and is vulnerable to years of severe drought. Wheat, for example, is a vital crop that is grown extensively in areas that were originally short-grass prairie. In North America, as much as 40% of this 400-million hectare, semi-arid region has already been desertified to some degree as a consequence of ecological changes associated with agricultural practices. Sporadic crop-threatening droughts occur widely. If the land is irrigated, the limitations of sparse precipitation in this region can be alleviated. However, insufficient water is available for this purpose, and secondary problems, such as salinization, can be caused by irrigation. Clearly, any further losses of soil moisture in this important agricultural region would be extremely damaging to agricultural production and to food security.

The extent and severity of forest fires would also likely be affected by changes in the amount and distribution of precipitation and evapotranspiration, and to their secondary effects, such as soil moisture. We are already starting to see some of these effects in the frequency and intensity of fires. Since the 1970s, the wildfire season in the western U.S. has lengthened by about three days due to rising temperatures and snow melting earlier in the season, creating overall drier conditions. The amount of burned area in the U.S. has been increasing over the past two decades and it is estimated that in 2050 burned area in the western U.S. could increase 2-6 times from what it is in 2020. Furthermore, increased emissions result in higher temperatures, which result in drier, more fire-prone conditions and with more fires, comes more emissions, perpetuating a climate feedback loop (Harris et al. 2020).

In marine ecosystems, increases in water temperature would adversely affect some biota. Prolonged warming may cause corals to lose their symbiotic algae (known as zooxanthellae), sometimes resulting in death of the coral. This syndrome of damage, known as coral bleaching, can be induced by unusually high or low temperature, changes in salinity, and other stresses (Image 21.4). Coral reefs are the world’s most biodiverse marine ecosystems, and they are already threatened by many stressors associated with human activities, including coastal pollution, mining of the coral, and overly intensive fisheries.

Image 21.4. Coral Bleaching. Depicted here is a colony of soft coral called “bent sea rod” in Islamorada, Florida that is losing its symbiotic algae due to warmer waters. Source: “Bent Sea Rod Bleaching” by Kelsey Roberts, USGS, licensed by Public Domain.

Another predicted consequence of climate change is the accelerated melting and retreat of glaciers. And in fact, we are already seeing this phenomenon occurring. In Alaska, the Muir Glacier, like many Alaskan glaciers, has retreated and thinned dramatically since the 19th century (Image 21.5). This can have consequences for the flow of rivers that are substantially dependent on glacial meltwater, including the Copper River Region of Alaska. Rapid glacial retreat is also well documented in the Alps of Europe and on Mount Kilimanjaro in Kenya, the top of which may be ice-free by 2050. It is also affecting the world’s most massive glaciers, in Greenland and Antarctica.

Image 21.5. Muir Glacier Retreat. These photos demonstrate how Muir Glacier in Alaska has retreated within the last 60 years. The top photo was taken in 1941 and the bottom photo was taken in 2004. Photographed by William O. Field on Aug. 13, 1941 (top) and Bruce F. Molnia on Aug. 31, 2004 (bottom). From the Glacier Photograph Collection. Boulder Colorado USA: National Snow and Ice Data Center/World Data Center for Glaciology.

An additional predicted effect of global warming is an increase in sea level. This change would be caused mostly by a thermal expansion of seawater, because as water warms, its volume increases. There would also be an influence on sea level from the melting of massive glaciers, particularly those in Antarctica and Greenland, which would release some of their enormous mass to the oceans. Even an increase of sea level of a meter or so would have massive implications for low-lying populated regions, such as the Netherlands in Europe and the Maldives and other archipelagos in the Indian and Pacific Oceans. These low coastal places would become much more vulnerable to the devastating effects of storm surges. There would also be risks for shallow-water marine ecosystems, such as coral reefs. Global sea level has risen about 8-9 inches (21-24 centimeters) since 1880, with about a third of that rise within the last 25 years (Lindsey, 2021). The rate of global sea level rise is also accelerating. In the U.S., the sea level is rising four times faster than the global average, specifically along parts of the Atlantic coast (Sallenger et al., 2012). The U.S. is particularly vulnerable to sea-level rise as almost 40% of the U.S. population lives in high-population density coastlines (Lindsey, 2021). Thus, sea-level rise could have devastating impacts on infrastructure and local economies.

It is also predicted that global warming might increase the frequency, and perhaps the severity, of events of severe weather. This means that hurricanes, tornadoes, and even El Niño events could become more frequent, and perhaps also more intense. In addition, greater water vapor in the atmosphere may lead to higher levels of flooding due to heavier rainfall and snowfall. Heat waves may be longer, more frequent, and more intense. These extremes of weather have well-known, devastating effects on economic and ecological systems.

Effects of CO₂ on Plants

Carbon dioxide is an important nutrient for plants. As a result, increased concentrations of CO₂ can stimulate the productivity of some plants, especially if moisture and nutrients are abundant.

Many laboratory experiments have shown that agricultural plants can be more productive when fertilized by CO₂. In fact, some commercial greenhouses increase the productivity of crops such as cucumber, tomato, and ornamental plants by fertilizing the air with CO₂ at concentrations of 600-2000 ppm.

Usually, however, the productivity of crops grown under field conditions is limited by an inadequate supply of nutrients other than CO₂, usually nitrogen, phosphorus, or potassium, and often the availability of water is also a constraint. Under these kinds of conditions, the responses of plants to CO₂ fertilization are small and short term, or non-existent.

Increased concentrations of CO₂ can also affect many plants by decreasing their rate of water loss by transpiration. Most water loss occurs through tiny pores, known as stomata, on the leaf surfaces. The size of the stomatal opening is controlled by specialized guard cells. Activity of the guard cells is influenced by CO₂, and stomata tend to close partially or entirely when its concentrations are high. Because the availability of moisture is an important factor affecting plant productivity in agricultural and forest ecosystems, decreased water losses from lessened transpiration could be a beneficial effect.

It appears that some benefits might be realized from CO₂ fertilization and decreased transpiration, especially in intensively managed agricultural systems. It is important to recognize, however, that these gains are likely to be minor. Moreover, the possible benefits would probably be overwhelmed by the negative consequences of anthropogenic climate change. The distribution and composition of natural and managed ecosystems could be greatly affected by effects on precipitation and other climatic factors, and that could result in enormous damage being caused to economic resources in agriculture, forestry, and fisheries, and also to natural biodiversity.

Reducing Carbon Dioxide

Because of the potential consequences of anthropogenic climate change, governments are considering actions to reduce the emissions of CO₂ and other GHGs in the atmosphere, or at least to slow their rates of increase. This goal could be achieved in two ways: (1) by reducing the emissions of GHGs, and (2) by increasing the rates at which they are removed from the atmosphere. The latter tactic is especially relevant to CO₂, the most abundant of the anthropogenic GHGs.

Ultimately, large decreases in the emissions of GHGs, particularly CO₂, must be the major tactic of any strategy to deal with an intensification of the greenhouse effect. However, it is extremely difficult to rapidly reduce emissions of CO₂ because they are associated with so many economically important activities. As we previously examined, the major CO₂-emitting activities include the use of fossil fuels in industry, transportation, and space heating; the manufacturing of cement; and ecological conversions, particularly of forest to agriculture. Thus there are concerns about the shorter-term economic consequences of actions necessary to rapidly reduce the emissions of CO₂ to the atmosphere. Some believe it is more prudent to reduce those emissions through more protracted actions.

Planting large numbers of trees is an option that would contribute to reducing the CO₂ concentration in the atmosphere. As trees and other plants grow, they fix CO₂ into the organic carbon of their accumulating biomass. Depending on the species and growing conditions, that biomass can eventually reach several tonnes of dry weight per large tree, about half of which is carbon.

Studies have shown that substantial carbon credits can be gained by planting large numbers of trees in urban and rural environments. The carbon credits are especially large if the tree-planting involves afforestation, or the creation of forest on disused agricultural land. (Afforestation converts non-forested land into a forest, while reforestation ensures that another forest regrows on a site from which timber was harvested.) Agroecosystems typically store small amounts of carbon in biomass, while forests store much more. The carbon-storage function would be optimized if mature or old-growth forest is established, and if that ecosystem were maintained in its high-carbon condition for as long as possible. (Harvesting of mature trees would detract from the carbon-storage function.) Moreover, the afforestation of extensive areas would achieve many additional, non-carbon benefits, such as the enhancement of biodiversity.

Although tree-planting and afforestation are attractive options toward reducing CO₂ in the atmosphere, these tactics cannot offset more than a portion of the CO₂ emitted by fossil-fuel combustion and deforestation. An enormous area of land would have to be afforested to achieve full offsets. For example, to fully offset the CO₂ emissions from one 200 MW coal-fired generating station (which would emit about 0.34-million tonnes of CO_2-C per year), the carbon-fixing services of about 500,000 ha of natural forest of would be required. If the forest productivity were increased by silvicultural management on a fertile site, as little as one-tenth of that area might be required, but that would still be a huge area (Freedman et al., 1992). Only a limited amount of land is available, in the U.S. or elsewhere, for afforestation to provide carbon offsets. The use of larger areas would withdraw too much land from other productive uses, especially agriculture.

In any event, dealing effectively with an anthropogenic climate change will require a comprehensive, integrated strategy, of which reduced emissions of GHGs must be the major component. Carbon offsets such as tree-planting will be a useful element, but they will not be sufficient.

The most important means of reducing CO₂ emissions would potentially involve the following:

• Aggressive conservation of energy through more efficient use, which would result in a decreased demand for fossil fuels
• Increased use of non-carbon energy (such as solar, wind, tidal, hydro, biomass, and nuclear) to displace many uses of fossil fuels
• Prevention of further conversions of forest into agricultural and other land-uses, to avoid CO₂ emissions that are associated with deforestation
• Afforestation, which would increase carbon stored in ecosystems

However, it must be recognized that the implementation of an effective strategy involving these actions would be politically and economically difficult. Industrialized nations depend heavily on fossil fuels, and changes in this reliance will have huge implications for economic systems, industrial capitalization, resource use, and citizens’ expectations of lifestyle. Similarly, deforestation in tropical countries is a primary means by which impoverished people gain access to opportunities and livelihoods, and harvested timber helps to earn the foreign exchange that is necessary to fund development activities.

The societal changes that would be necessary to effectively deal with an intensified greenhouse effect are revolutionary in their nature and magnitude. Designing the required economic and energy systems will be a tremendous challenge, and implementing them will require enlightened and forceful leadership. Unfortunately, there are no easy solutions to an environmental problem as potentially damaging as anthropogenic climate change. Moreover, it appears that it will be necessary and precautionary to implement effective actions as soon as possible, even before it is definitely known that many of the damages are occurring.

Conclusions

Earth’s natural greenhouse effect is caused by the activity of greenhouse gases in the atmosphere, and it helps make the planet habitable. The concentrations of key GHGs are increasing rapidly, particularly carbon dioxide, resulting in rising temperatures and contributing to many other climactic effects, such as changes in precipitation regimes and in the frequency of severe weather events. These changes have severe consequences for agroecosystems and the human economy in general, and also for natural ecosystems. At the international level, the Kyoto Protocol was the key first action being taken to reduce the emissions of GHGs and has helped lead to the current Paris Agreement. Many countries have ratified this treaty and are taking steps to reduce their emissions of GHGs. This Agreement may indicate a turning point in the global race against climate change.

Questions for Review

1. Describe Earth’s natural greenhouse effect and the factors that create it.
2. How may human influences be making the greenhouse effect more intense?
3. What is a greenhouse gas (GHG)? What are the most important GHGs in the atmosphere, and how are human actions affecting their concentrations?
4. What are the likely climatic and ecological consequences of an intensification of the greenhouse effect?

Questions for Discussion

1. How might the U.S. economy and the lifestyles of typical U.S. citizens be affected if serious actions are taken to reduce the emissions of greenhouse gases?
2. How do you think the U.S. government should handle reduction of greenhouse gases? Explain your answer.
3. Even with the implementation of the Paris Agreement, an expected increase in population and therefore urbanization will contribute to the steady rise of greenhouse gas emissions. Although the Paris Agreement calls for increasingly ambitious measures to combat climate change, do you think this will be effective considering the expected population growth? What would you suggest to combat the expected effects of increased urbanization?

Exploring Issues

1. Your state government has struck a committee of politicians and citizens to recommend actions to reduce the net emissions of greenhouse gases. As the principal science advisor to the committee, you have been asked to develop a list of practical options that should be undertaken. What actions would you recommend for implementation immediately, and which more gradually (that is, progressively during the next 10 years)? Justify each of your recommendations.
2. For one day, make a list of your activities that result in emissions of carbon dioxide or methane to the atmosphere. These should include direct emissions (for example, by breathing or driving a vehicle) and indirect ones (as when trees must be harvested to provide you with paper, or when organic garbage is disposed into a landfill). Estimate the percentage reduction in emissions that you think you could make without suffering an unacceptable degree of change in your lifestyle.

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