1.3.1. GREENHOUSE GASES

Recently, scientific studies conducted indicate that both natural or anthropogenic factors are the primary cause of global warming. Greenhouse gases are also important in understanding earth’s climatic history. According to these studies, the greenhouse effect, the warming of the climate as a result of heat trapped by atmospheric gases, plays a significant role in regulating earth’s temperature (Fig. 1.1).

Over the last 600 million years, concentrations of greenhouse gases have varied from 5000 parts per million (ppm) to less than 200 ppm owing primarily to the effects of geologic processes and biologic interventions. Studies also have shown that there is a direct correlation between carbon dioxide (CO ₂) gas and global warming. Several historic examples of rapid change in greenhouse gas concentrations indicate a strong correlation with global warming during various geologic events, such as the end of the Varangian glaciation.

According to the Intergovernmental Panel on Climate Change (IPCC) in 2007, the atmospheric concentration of CO ₂ in 2005 was 379 ppm ³, compared with preindustrial levels of 280 ppm ³. These measurements have been substantiated by verifying the dynamic equilibrium of vast amounts of CO ₂ gas held in the world’s oceans, which move into and return from the atmosphere (Figs. 1.2 and 1.3).

Figure 1.1Greenhouse effect. Courtesy of United Nations Environmental Program/GRID-Arendal.

Figure 1.2Temperature and CO 2 concentration in the atmosphere over the past 400,000 years (from the Vostok Ice Core). Courtesy of United Nations Environmental Program/GRID-Arendal.

1.3.2. SOLAR VARIATION

Variations in sunspots and solar flare activity, which significantly affect the earth’s temperature, also have been observed and studied for several centuries. As we know, the sun is the ultimate source of essentially all heat in the climate system. The energy output of the sun, which is converted to heat at the earth’s surface, is the most significant factor controlling the earth’s climate. Ever since the Big Bang, the sun, which is a nuclear fusion reactor furnace, has been burning by converting hydrogen into helium and is getting brighter by outputting higher amounts of energy. In its earlier days, the earth went through several extreme cold and hot periods when liquid water at its surface was completely frozen and liquefied several times, a phenomenon referred to as the faint young sun paradox.

Recent climatology studies have determined that the sun undergoes 11-year cyclic modulations. The 11-year sunspot cycle, however, has so far not been established as having a definitive effect on the global climate. Solar intensity variations, though, did have influence in triggering the global warming effect recorded from 1900 to 1950.

Figure 1.3Global atmospheric concentration of CO 2. Courtesy of United Nations Environmental Program/GRID-Arendal.

1.3.3. ORBITAL VARIATIONS

Orbital variation patterns of the earth’s movement around the sun result in solar energy absorption variability because small variations in the earth’s orbit lead to much more considerable changes in the distribution and abundance of sunlight reaching the earth’s surface. Such orbital variations are a consequence of basic physics owing to the mutual interactions of the earth, its moon, and the other planets. These variations are considered the driving factors underlying the glacial and interglacial cycles of the last ice age. Some of the most notable climatic variations observed, such as the repeated advance and retreat of the desert, have been the result of these orbital variations.

1.3.4. VOLCANISM

Large volcanic activities that occur several times per century also have had a significant effect on climate, causing cooling for periods of a few years. For instance, the 1991 eruption of the Mount Pinatubo in the Philippines affected the global climate substantially. The huge eruptions that have taken place a few times every hundred million years can be verified based on the magmatic variations in rocks and have reshaped the climate for millions of years. It has been speculated that the dust emitted into the atmosphere from large volcanic eruptions in the past has been responsible for cooling owing to the fact that the dust particles have partially blocked transmission of the sun’s rays to the earth’s surface. However, recent studies of and measurements taken from volcanic eruptions indicate that most of the dust thrown in the atmosphere returns to the earth’s surface within 6 months.

Volcanoes also contribute to the extended atmospheric pollutants because over millennia of geologic time periods, they release carbon dioxide from the earth’s interior, counteracting the uptake by sedimentary rocks and other geologic carbon sinks. However, CO ₂ contribution resulting from volcanic eruptions is considered relatively insignificant compared with current anthropogenic emissions. Recent estimates indicate that anthropogenic activities generate more than 130 times the amount of carbon dioxide emitted by volcanoes.

1.3.5. GLACIATION

Glaciers are one of the most sensitive indicators of climate change. They advance substantially during climate cooling, as in ice ages, and retreat during climate warming on moderate time scales, a climatic cycle that has been repeating through the ages (Fig. 1.4). Glaciers are dynamic in nature; they grow in winter and collapse in summer, contributing to natural climatic variability. These are generally referred to as externally forced changes. However, in the last couple of centuries, glaciers have been unable to regenerate enough mass during the winter to make up for ice lost during summer months. The most significant climate processes that have taken place in the past several million years have been the glacial cycles that result from planetary gravitational forces and cause the formation of ice sheets.

1.3.6. OCEAN VARIABILITY

Climate changes also result from the interaction between the atmosphere and oceans. Many climatic fluctuations are a result of heat accumulation and storage in the oceans that cause water currents to move between different heat reservoirs. The movement process of oceanic water, thermoline circulation, plays a key role in redistributing and balancing heat and, consequently, climatic conditions throughout the globe.

Figure 1.4Trend in global average surface temperature. Courtesy of United Nations Environmental Program/GRID-Arendal.

Figure 1.5Great ocean conveyer belt. Courtesy of United Nations Environmental Program/GRID-Arendal.

1.5.1. THE USE OF SEQUESTERED SOLAR ENERGY AND ANTHROPOGENIC CAUSES OF ATMOSPHERIC POLLUTION

Anthropogenic factors are acts by humankind that affect the environment and influence climate. Various theories of human-induced climate change have been debated for many years. The biggest factor of present concern is the increase in CO ₂ levels owing to emissions from combustion. Other concerns are particulate matter in the atmosphere that exerts a cooling effect. Other factors, such as land use, animal husbandry, agriculture, and deforestation, also affect climate.

1.5.2. FOSSIL FUELS

Carbon dioxide variations over the last 400,000 years have shown a rise since the industrial revolution. Beginning in the 1850s and accelerating ever since, the human consumption of fossil fuels has elevated CO ₂ levels from a concentration of 280 ppm to more than 380 ppm today. These increases have been projected to reach more than 560 ppm before the end of the twenty-first century. It is known that CO ₂ levels are substantially higher now than at any other time in the last 800,000 years. Along with rising levels of atmospheric pollutants, it is anticipated that there will be an increase in global temperature by 1.4–5.6°F between 1990 and 2100 (Fig. 1.6).

1.5.3. MILLENNIAL PERSPECTIVE OF CARBON DIOXIDE VARIATIONS

Organic materials such as plants (mainly formed from hydrocarbons and water molecules) under certain conditions, such as natural disasters, fires, volcanic activity, tectonic plate displacement, and extreme geoclimatic changes, lose significant amounts of their water content and are left with solely carbon and mineral materials. It is because of the sun’s energy that organic life, in accordance with certain chemical processes, is transformed into various forms of hydrocarbon organic structures.

Figure 1.6Projected changes in CO 2 and global temperature: summary of assumptions in the IPCC 1992 alternative scenarios. Courtesy of United Nations Environmental Program/GRID-Arendal.

Through the passage of millennia, these remains of organic carbonized material are manifested in the form of solid coal or liquid crude oils, referred to as fossil fuels. Fossil fuels, therefore, are considered to be sequestered forms of concentrated solar energy.

One of the most common sources of energy known to humans is the chemical combination of carbon molecules with oxygen. At a certain kindling point (elevated temperature), one atom of carbon (C) combines with two atoms of oxygen (O ₂), giving rise to a carbon dioxide molecule (C + O ₂ = CO ₂). During this chemical combination, a certain amount of heat energy is released. We normally refer to this as a burning process.

Carbon dioxide, in its normal state, is a gas that is heavier than air. Under certain atmospheric pressures and temperatures, it liquefies. This liquid form of CO ₂ is used in common fire extinguishers. Under normal temperature conditions, CO ₂, when present in the air, displaces oxygen and ceases the spread of fire by preventing oxygen present in the air from reacting or combining with other forms of hydrocarbon-based material. This is referred to as oxygen starvation.

When heated, CO ₂ molecules, owing to thermal agitation, distance themselves from each other, rendering the gas lighter than air. Large quantities of CO ₂ result from burning fossil fuels. When heated by the sun’s energy, CO ₂ rises to higher elevations and surrounds the planet in a blanket of gas referred to as the inversion layer (Fig. 1.7).

When solar rays impinge on the earth, they are repelled from its surface and the north and south polar ice caps. The reflection of solar energy back into the earth’s outer stratosphere moderates global temperature to levels that promote the existence of various life forms. Minor elevations in the earth’s climatic conditions, in turn, affect the organic life and the reproductive cycles of all species.

As discussed earlier, global climatic temperature is moderated not only by the reflection of solar rays but also by the heat-absorption capability of the earth’s oceans. Two-thirds of the earth’s surface is covered by oceans, which absorb significant amounts of solar energy. Owing to earth’s relative rotational tilt angle to the sun (23.5 degrees), the oceanic water at the equator and poles absorbs unequal amounts of solar energy, giving rise to a water temperature gradient differential that creates convective water circulation within the oceans. The movement or displacement of lighter warm waters from the equator to the north and cold water from the north pole to the south is referred to as the gulf stream or the belt current and regulates continental climatic conditions.