Global warming is defined as climate change that causes an increase in the temperature of Earth's lower atmosphere. Anthropogenic global warming is the observed increase (from recorded measurements using modern instruments) in the average temperature of the near-surface atmosphere, land, and ocean environments of Earth during the past 50 years as a result of human activities. A growing volume of evidence suggests that we are now in a period of global warming resulting from burning vast amounts of fossil fuels. Does this mean we are experiencing human-induced global warming? Many scientists now believe that human processes, as well as natural ones, are contributing significantly to global warming.
The Greenhouse Effect
For the most part, the temperature of Earth is determined by three factors: the amount of sunlight Earth receives, the amount of sunlight Earth reflects (and, therefore, does not absorb), and atmospheric retention of re-radiated heat. Earth's energy balance today is slightly out of equilibrium, with about 1 W/m2 more energy coming from the Sun that is lost to space. The energy we are talking about, W/m2, is energy per unit time (joules/sec) per unit area (m2). The units W/m2 is power per unit area, but we speak of it as solar energy. The units W/m2 are widely used in global warming and climate change research. Earth receives energy from the Sun in the form of electromagnetic radiation. Radiation from the Sun is relatively short wave and mostly visible, whereas Earth radiates relatively long-wave infra-red radiation. The hotter an object, whether it is the Sun, Earth, a rock, or a lake, the more electromagnetic energy it emits. The Sun, with a surface temperature of 5800°C (10,472°F), radiates much more energy per unit area than does Earth, which has an average surface temperature of 15°C (59°F). Absorbed solar energy warms Earths atmosphere and surface, which then re-radiate the energy back into space as infra-red radiation . Water vapour and several other atmospheric gases including carbon dioxide methane and chlorofluorocarbons (CFCs), human-made chemicals used in air conditioners and refrigerators tend to trap heat. That is, they absorb some of the energy radiating CH4, CO2, from Earth's surface and are thereby warmed. As a result, the lower atmosphere of Earth is much warmer than it would be if all of its radiation escaped into space without this intermediate absorption and warming. This effect is somewhat analogous to the trapping of heat by a greenhouse and is therefore referred to as the greenhouse effect. It is important to understand that the greenhouse effect is a natural phenomenon that has been occurring for millions of years on Earth, as well as on other planets in our solar system. Without heat trapped in the atmosphere, Earth would be much colder than it is now, and all surface water would be frozen. Most of the natural greenhouse warming is due to water vapour and small particles of water in the atmosphere. However, potential global warming due to human activity is related to carbon dioxide, methane, nitrogen oxides, and chlorofluorocarbons. In recent years, the atmospheric concentrations of these gases and others have been increasing because of human activities. These gases tend to absorb infrared radiation from Earth, and it has been hypothesized that Earth is warming because of the increases in the amounts of these so-called greenhouse gases. Table 16.1 shows the rate of increase of these atmospheric gases due to human-induced emissions and their relative contribution to the anthropogenic, or human caused, component of the greenhouse effect. Notice that carbon dioxide produces 60 percent of the relative contribution.
Measurements of carbon dioxide trapped in air bubbles of the Antarctic ice sheet suggest that, during most of the past 160,000 years, the atmospheric concentration of carbon dioxide has varied from a little less than 200 ppm to about 300 ppm.5 The highest levels are recorded during major interglacial periods that occurred approximately 125,000 years ago and at the present. Major interglacials occurred about four times during the past 400,000 years about every 100,000 years. During each of these, the concentration of CO2 in the atmosphere was similar to that of the most recent interglacial event about 125,000 years ago.7 At the beginning of the Industrial Revolution, the atmospheric concentration of carbon dioxide was approximately 280 ppm. Since 1860, fossil fuel burning has contributed to the exponential growth of the concentration of carbon dioxide in the atmosphere. Data for before the mid-twentieth century are from measurements made from air bubbles trapped in glacial ice. The concentration of carbon dioxide in the atmosphere today exceeds 380 ppm, and it is predicted to reach at least 450 ppm more than 1.5 times the pre-industrial level by the year 2050. Changes in atmospheric concentration of carbon dioxide at Mauna Loa. The annual cycles with a lower concentration of during the summer are due to the summer growing season in the Northern Hemisphere, when plants extract more carbon dioxide from the atmosphere by photosynthesis.
Global Temperature Change
The Pleistocene ice ages began approximately 2 million years ago, and, since then, there have been numerous changes in Earth's mean annual temperature. Figure 16.13 shows the changes of approximately the past million years on several time scales. The first scale shows the entire million years, during which there have been major climatic changes involving swings of several degrees Celsius in mean temperature. Low temperatures have coincided with major glacial events that have greatly altered the landscape; high temperatures are associated with interglacial conditions. Interglacial and glacial events become increasingly prominent in the scales, showing changes over 150,000 and 18,000 years. The last major interglacial warm period, even warmer than today, was the Eemian. During the Eemian, the sea level was a few meters higher than it is today. The cold period that occurred about 11,500 years ago is known as the Younger Dryas, followed by rapid warming to the Holocene maximum, which preceded the Little Ice Age. A scale of 1000 years shows several warming and cooling trends that have affected people. For example, a major warming trend from approximately A.D. 1100 1300 allowed the Vikings to colonize Iceland, Greenland, and northern North America. When glaciers made a minor advance around A.D. 1400, during a cold period known as the Little Ice Age, the Viking settlements in North America and parts of Greenland were abandoned. In approximately 1750, an apparent warming trend began that lasted until approximately the 1940s, when temperatures cooled slightly. Over the last 140 years, more changes are apparent, and the 1940s event is clearer. It is evident from the record that, in the last 100 years, global mean annual temperature increased by approximately 0.8°C (1.4°F). Most of the increase has been since the 1970s, and the 1990s and first 8 years of the twenty-first century had the warmest temperatures since global temperatures have been monitored. This period may become known as the early twenty-first century increase in global temperature.
Why Does Climate Change?
The question that begs to be answered is: Why does climate change?. There are cycles of change lasting 100,000 years, separated by shorter cycles of 20,000 to 40,000 years in duration. These cycles were first identified by Milutin Milankovitch in the 1920s as a hypothesis to explain climate change. Milankovitch realized that the spinning Earth is like a wobbling top, unable to keep a constant position in relationship to the Sun; this instability partially determines the amount of sunlight reaching and warming Earth. He discovered that variability in Earths orbit around the Sun follows a 100,000-year cycle that is correlated with the major glacial and interglacial periods. Earths orbit varies from a nearly circular ellipse to a more elongated ellipse. Over the 100,000-year cycle of Earth, when the orbit is most elliptical, solar radiation reaching Earth is greater than during a more circular orbit. Cycles of approximately 40,000 and 20,000 years are the result of changes in the tilt and wobble of Earth's axis, respectively. Milankovitch cycles reproduce most of the long-term cycles observed in the climate, and they do have a significant effect on climate. However, the cycles are not sufficient to produce the observed largescale global climatic changes. Therefore, these cycles, along with other processes, must be invoked to explain global climatic change. Thus, the Milankovitch cycles that force (push) the climate in one direction or another can be looked at as natural processes (forcing) that, when linked to other processes (forcing), produce climatic change. We now will consider this forcing concept in more detail. Climate forcing is defined as an imposed change of Earth's energy balance. The units for the forcing are W/m2 and can be positive if a particular forcing increases global mean temperature or negative if temperature is decreased. For example, if the energy from the Sun increases, then Earth will warm (this is positive climate forcing). If CO2 were to decrease, causing Earth to cool, that is an example of negative climate forcing. Climate sensitivity refers to the response of climate to a specific climate forcing after a new equilibrium has been established, and the time required for the response to a forcing to occur is the climate response time. A significant implication of climate forcing is that, if you maintain small climate forcing for a long enough time, then large climate change can occur.The climate forcing that produced the last ice age 22,000 years ago (last glacial maximum). Notice that 1 W/m2 produces a temperature change of about 0.75°C. Total positive forcing is about 1.6 W/m2, most of which is due to greenhouse gas forcings (CO2, CH4, N2O). They have increased dramatically in the past 100 years. We now believe that our climate system may be inherently unstable and capable of changing quickly from one state to another in as short a time as a few decades. However, very short or abrupt climate change is unlikely. Part of what may drive the climate system and its potential to change is the ocean conveyor belt, a global-scale circulation of ocean waters, characterized by strong northward movement of 12° to 13°C (53° to 55°F) near-surface waters in the Atlantic Ocean that are cooled to 2° to 4°C (35° to 39°F) when they arrive near Greenland. As the water cools, it becomes saltier; the salinity increases the waters density and causes it to sink to the bottom. The current then flows southward around Africa, adjoining the global pattern of ocean currents. The flow in this conveyor belt current is huge, equal to about 100 Amazon Rivers. The amount of warm water and heat released to the atmosphere, along with the stronger effect of relatively warm winter air moving east and northeast across the Atlantic Ocean, is sufficient to keep northern Europe 5° to 10°C (8.5° to 17°F) warmer than it would be otherwise. If the conveyor belt were to shut down, it would have an effect on the climate of Europe. However, the effect would not be catastrophic to England and France in terms of producing extreme cold and icebound conditions.
Although scientific uncertainties exist, there is sufficient evidence to state that
(1) there is a discernible human influence on global climate;
(2) warming is now occurring; and
(3) the mean surface temperature of Earth will likely increase by between 1.5° to 4.5°C (2.6° to 7.8°F) during the twenty-first century. The human-induced component of global warming results from increased emissions of gases that tend to trap heat in the atmosphere. There is good reason to argue that increases in carbon dioxide and other greenhouse gases are related to an increase in the mean global temperature of Earth. Over the past few hundred thousand years, there has been a strong correlation between the concentration of atmospheric and global temperature. When has been high, temperature has also been high, and, conversely, low concentrations of CO2 have been correlated with a low global temperature. However, to better understand global warming, we need to consider major forcing variables that influence global warming, including solar emission, volcanic eruption, and anthropogenic input.
Solar Forcing
Since the Sun is responsible for heating Earth, solar variation should be evaluated as a possible cause of climate change. When we examine the history of climate during the past 1000 years, the variability of solar energy plays a role. Examination of the solar record reveals that the Medieval Warm Period (A.D. 1000 1300) corresponds with a time of increased solar radiation, comparable to that which we see today. Evaluation of the record also suggests that minimum solar activity occurred during the fourteenth century, coincident with the beginning of the Little Ice Age. Therefore, it appears that variability of the input of solar energy to Earth can partially explain climatic variability during the past 1000 years. However, the effect is relatively small, only 0.25 percent; that is, the difference between the solar forcing from the Medieval Warm Period to the Little Ice Age is only a fraction of 1 percent. Brightening of the Sun is unlikely to have had a significant effect on global warming since the beginning of the Industrial Revolution.
Volcanic Forcing
Upon eruption, volcanoes can hurl vast amounts of particulate matter, known as aerosols, high (15 25 km) into the atmosphere. The aerosol particles are transported by strong winds around Earth. They reflect a significant amount of sunlight and produce a net cooling that may offset much of the global warming expected from the anthropogenic greenhouse effect. For example, increased atmospheric aerosols over the United States (from air pollution) are probably responsible for mean temperatures roughly 1°C (1.7°F) cooler than they would be otherwise. Aerosol particle cooling may, thus, help explain the disparity between model simulations of global warming and actual recorded temperatures that are lower than those predicted by models. Volcanic eruptions add uncertainty in predicting global temperatures. For instance, what was the cooling effect of the 1991 Mt. Pinatubo eruption in the Philippines? Tremendous explosions sent volcanic ash to elevations of 30 km (19 mi) into the stratosphere, and, as with similar past events, the aerosol cloud of ash and sulfur dioxide remained in the atmosphere, circling Earth, for several years. The particles of ash and sulfur dioxide scattered incoming solar radiation, resulting in a climatic forcing of about 3 W/m2, cooling Earth about 2.3°C in 1991 and 1992. Calculations suggest that aerosol additions to the atmosphere from the Mt. Pinatubo eruption counterbalanced the warming effects of greenhouse gas additions
through 1992. However, by 1994, most aerosols from the eruption had fallen out of the atmosphere, and global temperatures returned to previous higher levels. Volcanic forcing from pulses of volcanic eruptions is believed to have significantly contributed to the cooling of the Little Ice Age.
Anthropogenic Forcing
Evidence of anthropogenic climate forcing, resulting in a warmer world, is based, in part, on the following:
- Recent warming over last few decades of 0.2°C per decade cannot be explained by natural variability of the climate over recent geologic history.
- Industrial age forcing of 1.6 W/m2 is mostly due to emissions of carbon dioxide that, with other greenhouse gases, have greatly increased in concentration in the past few decades.
- Climate models suggest that natural forcings in the past 100 years cannot be responsible for what we know to be a nearly 1°C rise in global land temperature.
When natural and anthropogenic forcing are combined, the observed changes can be explained. The present warming greatly exceeds the natural variability (climate forcing) and closely agrees with the response predicted from models of greenhouse gas forcing. Human processes are also causing a slight cooling. Reflection from air pollution particles (aerosols) has reduced incoming solar energy by as much as 10 percent. This is termed global dimming. Negative forcing from aerosols in the industrial age is 1.4 W/m2 and may be offsetting up to 50 percent of the expected warming due to greenhouse gases
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