The Greenhouse Effect (9.3) Increases in the Greenhouse Gases (9.4) Global Climate Change (9.5)
The Greenhouse Effect
- The greenhouse effect results in the surface temperature necessary for life on Earth to exist
The ultimate source of almost all energy on Earth is the Sun. In the most basic sense, the Sun emits solar radiation that strikes Earth. As the planet warms, it emits radiation back toward the atmosphere. However, the types of energy radiated from the Sun and Earth are different. As ultraviolet radiation travels toward the Earth, about one-third is reflected back into space. Although some UV is absorbed by ozone in the stratosphere, the remaining UV radiation, as well as visible light, easily passes through the atmosphere. Once it has passed through the atmosphere, this solar radiation strikes clouds and the surface of the Earth. Some of this radiation is reflected from the surface back into space. The remaining radiation is absorbed by clouds and the surface of the Earth, which become warmer and begin to emit lower-energy infrared radiation back toward the atmosphere. Unlike UV and visible light, infrared radiation does not easily pass through the atmosphere. It is absorbed by gases, which causes these gases to become warm. The warmed gases emit infrared radiation out into space and back toward the surface of Earth. The infrared radiation that is emitted toward Earth causes Earth's surface to become even warmer. This absorption of infrared radiation by atmospheric gases and reradiation of the energy back toward Earth is the greenhouse effect.
- The principal greenhouse gases are carbon dioxide, methane, water vapor, nitrous oxide, and chlorofluorocarbons (CFCs)
- While water vapor is a greenhouse gas, it doesn't contribute significantly to global climate change because it has a short residence time in the atmosphere
Certain gases in the atmosphere can absorb infrared radiation emitted by the surface of the planet and radiate much of it back toward the surface. The two most common gases in the atmosphere, N2 and O2, compose 99% percent of the atmosphere. Because these two gases do not absorb infrared radiation, they are not greenhouse gases (GHG) and do not contribute to the warming of the Earth. This means that GHGs make up a very small fraction of the atmosphere. The most common GHG is water vapor (H2O). Water vapor absorbs more infrared radiation from Earth than any other compound, although a molecule of water vapor does not persist nearly as long as other GHGs. Other important GHGs include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and tropospheric ozone (O3). All of these gases have been a part of the atmosphere for millions of years, and have kept Earth warm enough to be habitable.
Be careful when discussing ozone as a GHG. We have seen that the effects of ozone on Earth are diverse. Ozone in the stratosphere is beneficial because it filters out harmful UV radiation. In contrast, ozone in the lower troposphere acts as a GHG and can cause increased warming of Earth. It also is an air pollutant in the lower troposphere because it can cause damage to plants and human respiratory systems.
There is another type of GHG, chlorofluorocarbons (CFCs), which does not exist naturally. It occurs in the atmosphere exclusively due to production of CFCs by humans, and as we discussed in the last modules, these CFCs have contributed to the thinning of the ozone in the stratosphere while acting as a GHG in the troposphere.
Without these GHGs, the average temperature on Earth would be approximately -18 degrees Celsius (0F) instead of its current average temperature of 14C (57F). They are necessary to support life on Earth. The concern comes when there is an increase of these gases--as has occurred due to human activities--can cause the planet to warm more than usual.
- Carbon dioxide, which has a global warming potential (GWP) of 1, is used as a reference point for the comparison of different greenhouse gases and their impacts on global climate change. Chlorofluorocarbons (CFCs) have the highest GWP, followed by nitrous oxide, then methane
The contribution of each gas to global warming depends in part on its global warming potential (GWP). The GWP of a gas estimates how much a molecule of any compound can contribute to global warming over a period of 100 years relative to a molecule of CO2. In calculating this potential, scientists consider the amount of infrared energy that a given gas can absorb and how long a molecule of the gas can persist in the atmosphere. Water vapor has a lower potential compared with carbon dioxide. The remaining GHGs have much higher values, wither because they absorb more infrared radiation than a molecule of CO2 or because they persist longer in the atmosphere. Compared with CO2 with a GWP of 1, the GWP is 25 times higher for methane (CH4), nearly 300 times higher for nitrous oxide (N2O), and up to 13,000 times higher for CFCs.
Increases in the Greenhouse Gases
While human activity appears to have little effect on the amount of water vapor in the atmosphere, it has caused substantial increases in the amount of other GHGs. Among these, carbon dioxide remains the greatest contributor to the greenhouse effect because its concentration is so much higher than any of the others. As a result, scientists and policy makers focus their efforts on ways to reduce carbon dioxide in the atmosphere.
Increasing the concentration of any historically present greenhouse gas should cause more infrared radiation to be absorbed in the atmosphere, which will radiate more energy back toward the surface of the planet and cause the planet to warm. There are both natural and anthropogenic (human) sources of GHGs. Natural sources include volcanic eruptions, decomposition, digestion, denitrification, evaporation and evapotranspiration. The most significant anthropogenic sources are burning fossil fuels, agricultural practices, deforestation, landfills, and industrial production of new greenhouse chemicals like CFCs.
Carbon dioxide concentrations have been increasing for the past 7 decades. In 1988 the United Nations and the World Meteorological Organization created the Intergovernmental Panel on Climate Change (IPCC), a group of more than 3,000 scientists from around the world working together to assess climate change. Their mission is to understand the details of the global warming system, the effects of climate change on biodiversity and energy fluxes in ecosystems, and the economic and social effects of climate change. This effort has produced an excellent understanding of how greenhouse gases and temperatures are linked.
Through the work of the IPCC, we now understand that CO2 is an important greenhouse gas that can contribute to global warming. There is a clear trend of rising CO2 concentrations across the years. This increase over time is correlated to increased human emissions of carbon from the combustion of fossil fuels and net destruction of vegetation.
- Global climate change, caused by excess greenhouse gases in the atmosphere, can lead to a variety of environmental problems including rising sea levels resulting from melting ice sheets and ocean water expansion, and disease vectors spreading from the tropics toward the poles. These problems can lead to changes in population dynamics and population movements in response.
While human activity appears to have little effect on the amount of water vapor in the atmosphere, it has caused substantial increases in the amount of other GHGs. Among these, carbon dioxide remains the greatest contributor to the greenhouse effect because its concentration is so much higher than any of the others. As a result, scientists and policy makers focus their efforts on ways to reduce carbon dioxide in the atmosphere.
Increasing the concentration of any historically present greenhouse gas should cause more infrared radiation to be absorbed in the atmosphere, which will radiate more energy back toward the surface of the planet and cause the planet to warm. There are both natural and anthropogenic (human) sources of GHGs. Natural sources include volcanic eruptions, decomposition, digestion, denitrification, evaporation and evapotranspiration. The most significant anthropogenic sources are burning fossil fuels, agricultural practices, deforestation, landfills, and industrial production of new greenhouse chemicals like CFCs.
Carbon dioxide concentrations have been increasing for the past 7 decades. In 1988 the United Nations and the World Meteorological Organization created the Intergovernmental Panel on Climate Change (IPCC), a group of more than 3,000 scientists from around the world working together to assess climate change. Their mission is to understand the details of the global warming system, the effects of climate change on biodiversity and energy fluxes in ecosystems, and the economic and social effects of climate change. This effort has produced an excellent understanding of how greenhouse gases and temperatures are linked.
Through the work of the IPCC, we now understand that CO2 is an important greenhouse gas that can contribute to global warming. There is a clear trend of rising CO2 concentrations across the years. This increase over time is correlated to increased human emissions of carbon from the combustion of fossil fuels and net destruction of vegetation.
Before we can determine if global temperature increases are a recent phenomenon and if these increases are unusual, we must establish how the temperatures of Earth have changed in the past. Since about 1880, there have been enough direct measurements of land and ocean temperatures that NASA Goddard Institute for Space Studies has been able to generate a graph of global temperature change over time. Comprising thousands of measurements from around the world, the graph shows global temperatures have increased 1.1 degree celsius (2F) from 1880 through 2017. In fact of the 18 warmest years since 1880, 17 of them have occurred between 2000 and 2017. The data collected by NASA clearly demonstrate that the globe has been slowly warming during the past 120 years. However, it is possible that such changes in temperature are simply a natural phenomenon. If we want to know whether these changes are typical, we must examine a much longer span of time.
Global Climate Change
Scientists can estimate global temperatures and greenhouse gas concentrations for over 500,000 years. Since no one was measuring temperatures thousands of years ago, we must use indirect measurements such as chemical analysis of air bubbles formed in ice long ago. In cold areas such as Antarctica and at the top of the Himalayas, the snowfall each year eventually compresses to become ice. Similar to marine sediments, the youngest ice is near the surface and the oldest ice is much deeper. During the process of compression, the ice captures small air bubbles. These bubble contain tiny samples of the atmosphere that existed at the time the ice was formed. Scientists drill deep into the ice and extract long tubes called ice cores. Samples of ice cores span up to 500,000 years of ice formation. Scientists determine the age of different layers in the ice core and then melt the ice from a piece associated with a particular time period. When the piece of ice melts, air bubbles are released and scientists measure the concentration of GHGs in the air when the bubbles were trapped in the ancient ice.
Oxygen atoms in melted ice cores can also be used to determine temperatures from the distant past. Oxygen atoms occur in two different isotopes; light oxygen with 8 protons and 8 neutrons or heavy oxygen with 8 protons and 10 neutrons. Ice formed during a period of warmer temperatures contains a higher percentage of heavy oxygen than ice formed during colder temperatures. By examining changes in the percentage of heavy oxygen from different layers of the ice core, we can indirectly estimate temperatures from hundreds of thousands of years ago.
In the graph below, you can see the pattern of atmospheric carbon dioxide. Even though there are natural rises and falls in levels of carbon dioxide, for most of the time shown, the atmosphere never contained more than 300 ppm. The rise of carbon dioxide over the past 50 years is unprecedented.
- The Earth has undergone climate change throughout geologic time, with major shifts in global temperatures causing periods of warming and cooling as recorded with CO2 data and ice cores.
Scientists can estimate global temperatures and greenhouse gas concentrations for over 500,000 years. Since no one was measuring temperatures thousands of years ago, we must use indirect measurements such as chemical analysis of air bubbles formed in ice long ago. In cold areas such as Antarctica and at the top of the Himalayas, the snowfall each year eventually compresses to become ice. Similar to marine sediments, the youngest ice is near the surface and the oldest ice is much deeper. During the process of compression, the ice captures small air bubbles. These bubble contain tiny samples of the atmosphere that existed at the time the ice was formed. Scientists drill deep into the ice and extract long tubes called ice cores. Samples of ice cores span up to 500,000 years of ice formation. Scientists determine the age of different layers in the ice core and then melt the ice from a piece associated with a particular time period. When the piece of ice melts, air bubbles are released and scientists measure the concentration of GHGs in the air when the bubbles were trapped in the ancient ice.
Oxygen atoms in melted ice cores can also be used to determine temperatures from the distant past. Oxygen atoms occur in two different isotopes; light oxygen with 8 protons and 8 neutrons or heavy oxygen with 8 protons and 10 neutrons. Ice formed during a period of warmer temperatures contains a higher percentage of heavy oxygen than ice formed during colder temperatures. By examining changes in the percentage of heavy oxygen from different layers of the ice core, we can indirectly estimate temperatures from hundreds of thousands of years ago.
In the graph below, you can see the pattern of atmospheric carbon dioxide. Even though there are natural rises and falls in levels of carbon dioxide, for most of the time shown, the atmosphere never contained more than 300 ppm. The rise of carbon dioxide over the past 50 years is unprecedented.
The graph above charts historic temperatures and carbon dioxide concentrations. We see that temperatures have changed drastically over the past. Most of these rapid shifts occurred during the onset of an ice age or during the transition from an ice age to a period of warming temperatures. Because these changes occurred before human could have had an effect, scientists suspect the changes were caused by small, regular shifts in the orbit of the Earth. The path of the orbit, the amount of tilt of Earth's axis and the position relative to the Sun all change regularly over hundreds of thousands of years.
The more important insight from the graph above is the close correspondence between historic temperatures and carbon dioxide concentrations. But the graph does not tell us if increased carbon dioxide caused increased temperature or vice versa. Scientists believe that the relationship between fluctuating levels of carbon dioxide and the temperature is complex and that both factors play a role. As we know, the increase of carbon dioxide in the atmosphere causes a greater capacity for warming through the greenhouse effect. However, when the Earth experiences higher temperatures, the oceans warm and cannot contain as much carbon dioxide gas, and as a result, they release carbon dioxide into the atmosphere.
The more important insight from the graph above is the close correspondence between historic temperatures and carbon dioxide concentrations. But the graph does not tell us if increased carbon dioxide caused increased temperature or vice versa. Scientists believe that the relationship between fluctuating levels of carbon dioxide and the temperature is complex and that both factors play a role. As we know, the increase of carbon dioxide in the atmosphere causes a greater capacity for warming through the greenhouse effect. However, when the Earth experiences higher temperatures, the oceans warm and cannot contain as much carbon dioxide gas, and as a result, they release carbon dioxide into the atmosphere.
- Effects of climate change include rising temperatures, melting permafrost and sea ice, rising sea levels, and displacement of coastal populations.
- Climate change can affect soil through changes in temperature and rainfall, which can impact soil's viability and potentially increase erosion.
Warming temperatures are expected to have a wide range of impacts on the environment. Many of these effects are already happening, including melting of polar ice caps, glaciers, and permafrost and rising sea levels. Other effects are predicted to occur in the future, including an increased frequency of heat waves, fewer and less intense cold spells, altered precipitation patterns and storm intensity, and shifting ocean currents.
Not only has the size of the ice cap that surrounds the North Pole been reduced, the remaining ice is considerably thinner, making it more vulnerable to future melting. Over the next 70 years, the Arctic is predicted to warm by an additional 4 degrees Celsius. This will cause larges openings in the sea ice to continue to expand and the ecosystem of the Arctic region will be negatively affected. In addition to the polar ice cap, Greenland and Antarctica have also experienced melting. Such large amounts of melted ice have caused sea levels to rise. Global warming has also caused glaciers to melt. In many parts of the world, the melting of glaciers each spring provides a critical source of water for many communities. As summers become warmer, glaciers are melting faster than they can grow back in the winter, which leaves some without a reliable water supply.
- Global climate change response time in the Arctic is due to positive feedback loops involving melting sea ice and thawing tundra, and the subsequent release of greenhouse gases like methane.
As warmer temperatures cause ice caps and glaciers to melt, areas of permafrost are also melting. Permafrost is permanently frozen ground that exists in the cold regions of high altitudes and high latitudes, which include the tundra and boreal forest biomes. About 20% of land on Earth contains permafrost. In some places it can be as much as a mile thick. Melting of the permafrost causes overlying lakes to be smaller as the lake water drains deeper into the ground. Melting can also cause substantial problems with human-built structures that are anchored into the permafrost, including houses and oil pipelines. As the frozen ground melts, it can subside and slide away. Melting permafrost also means that the massive amounts of organic matter contained in the tundra will begin to decompose, releasing substantial amounts of methane, increasing the concentration of this potent GHG. This chain of events could produce a positive feedback in which the warming of the Earth melts the permafrost, releasing more methane that causes further global warming.
- One consequence of the loss of ice and snow in polar regions is the effect on species that depend on the ice for habitat and food.
Polar bears live in the Arctic and play a key role in the ecosystem by hunting for seals on the polar ice cap. The bears hunt for the holes in the ice and pounce on any seals that come up for air. The bears usually consume the seal blubber, leaving the carcass as a food source for other animals such as the Arctic fox. As the ice cap retreats far away from the land each summer, the bears can no longer reach the ice to hunt for seals. With less seal predation, there will be fewer seal carcasses for other animals like the Arctic fox to consume, so there is a cascade of effects when polar bears are affected by global warming.
- Marine ecosystems are affected by changes in sea level, some positively, such as in newly created habitats on now-flooded continental shelves, and some negatively, such as deeper communities that may no longer be in the photic zone of seawater.
The rise in global temperatures affects sea levels in two ways. First the water from melting glaciers and ice sheets on land adds to the total volume of ocean water. Second, as the water of the oceans becomes warmer, it expands. Sea levels have risen 9.5 inches since 1870. Scientist predict sea levels could rise an additional 5 to 21 inches by the end of the century. This could endanger coastal cities and low-lying island nations by making them more vulnerable to flooding, especially during storms, with more salt water intrusion into aquifers and increased soil erosion. Currently, 100 million people live within 3 feet of sea level. The actual impact on these areas of the world will depend on the steps taken to mitigate these effects. For example, some countries may be able to build up their shorelines with dikes to prevent inundation from rising sea levels. Countries possessing less wealth are not expected to be able to respond as effectively to coastal flooding.
- Oceanic currents, or the ocean conveyor belt, carry heat throughout the world. When these currents change, it can have a big impact on global climate, especially in coastal regions.
- Winds generated by atmospheric circulation help transport heat throughout the Earth. Climate change may change circulation patterns, as temperature changes may impact Hadley cells and the jet stream.
Global ocean currents may shift as a result of more fresh water being released from melting ice. If currents change, distribution of heat on the planet could be disrupted. Scientists are particularly concerned about the thermohaline circulation, a deep ocean circulation driven by water that comes out of the Gulf of Mexico and moves up to Greenland where it becomes colder and saltier and sinks to the ocean floor. The sinking water mixes with the deep waters of the ocean basin, resurfaces near the equator, and eventually makes its way back to the Gulf of Mexico. Increased melting could dilute the salty ocean water sufficiently to stop the water from sinking near Greenland and thereby shut off the thermohaline circulation. If this occurs, much of Europe would experience significantly colder temperatures.
Feedbacks can increase or decrease the impact of climate change
The global greenhouse system is made up of several interconnected subsystems with many potential positive and negative feedbacks. Positive feedback amplifies changes, often leading to unstable situations in which small fluctuations in inputs lead to large observed effects. For examples, global soils contain more than twice as much carbon as the amount currently in the atmosphere. Higher temperatures are expected to increase the biological activity of decomposers in these soils. Because this decomposition leads to the release of additional carbon dioxide from the soil into the atmosphere, the temperature change will be amplified even more. Another example of positive feedback occurs as a result of polar ice melt.
The global greenhouse system is made up of several interconnected subsystems with many potential positive and negative feedbacks. Positive feedback amplifies changes, often leading to unstable situations in which small fluctuations in inputs lead to large observed effects. For examples, global soils contain more than twice as much carbon as the amount currently in the atmosphere. Higher temperatures are expected to increase the biological activity of decomposers in these soils. Because this decomposition leads to the release of additional carbon dioxide from the soil into the atmosphere, the temperature change will be amplified even more. Another example of positive feedback occurs as a result of polar ice melt.
- Earth's polar regions are showing faster response times to global climate change because ice and snow in these regions reflect the most energy back out to space, leading to a positive feedback loop.
- As the Earth warms, this ice and snow melts, meaning less solar energy is radiated back into space and instead is absorbed by the Earth's surface. This is turn causes more warming of the polar regions.
Negative feedback dampens changes. One of the most important negative feedbacks occurs as plants respond to increases in atmospheric carbon. Because carbon dioxide is required for photosynthesis, an increase in carbon dioxide can stimulate plant growth, The growth of more plants will cause more carbon dioxide to be removed from the atmosphere. A second negative feedback example exists in the oceans. As carbon dioxide concentrations increase in the atmosphere, more carbon dioxide is absorbed by the oceans. When carbon dioxide dissolves in the water, much of it combines with water molecules to form carbonic acid (H2CO3). Since this is an equilibrium reaction, an increase in ocean carbon dioxide causes more carbon dioxide to be converted to carbonic acid, which lowers the pH of ocean water in a process known as ocean acidification. Ocean acidification is of particular concern for the wide variety of species that build shells and skeletons made of calcium carbonate including corals, mollusks, and crustaceans. As pH decreases, the calcium carbonate in these organisms can begin to dissolve.