Over the past 50 years, the average temperature in the U.S. has risen 2°F. Global temperatures have been higher than the 20th century average every year since the late 1970s. And the Earth will only get hotter throughout the 21st century.
While the Earth’s climate has undergone many changes over time, human activities over the past 150 years have greatly increased the levels of certain gases in the atmosphere, dramatically affecting the climate and chemistry of the planet.
Because these “greenhouse gases” have the overall effect of trapping additional heat from the sun, these changes are often referred to as “global warming.” However, this term doesn’t fully capture the nature and extent of impacts—such as the alterations in pattern, timing and amount of precipitation—so scientists often use the more general term “climate change.”
Greenhouse gases trap heat. The primary greenhouse gases are carbon dioxide, methane, nitrogen dioxide, and water vapor.
The term “greenhouse gas” refers to any gas that is transparent to incoming energy from the sun, but absorbs the slightly altered energy reflected back from the Earth. The sun’s energy reaches us as a mixture of ultraviolet, visible light and heat energy. In bouncing off the surface of the Earth, the energy is shifted and more bounces off as visible light and heat.
Greenhouse gases allow light and ultraviolet energy to pass through, but trap heat energy and send it back toward the Earth’s surface. Without any greenhouse gases, much of the sun’s energy would simply bounce off the Earth’s surface back into space, and the planet would be uninhabitable, like the surface of Mars or the Moon. On the other hand, there can also be too much of a good thing: Venus, with an atmosphere composed of 95% carbon dioxide, has a surface temperature of 460 degrees.
Carbon dioxide: the most important of the greenhouse gases.
Carbon dioxide (CO2) is a fundamental building block of photosynthesis: using the energy of the sun, plants build carbohydrate molecules out of CO2 and water, capturing the sun’s energy in storable chemical form. When that stored energy is used—by the plant itself, by an animal that eats it, or by decomposition or burning—the CO2 is returned to the atmosphere.
Countless plants have lived and died over the past millions of years. In some cases, these plants were buried before they had time to decompose, trapping their stored energy deep underground in deposits that eventually turned into peat, coal, oil and natural gas—collectively known as fossil fuels. Over the past 150 years, people increasingly unearthed these fossil fuels and burned them as sources of energy for transportation, heat, electricity and to power industrial processes.
The rapid release of millions of years’ worth of stored plant energy has helped drive incredible advances in technology and convenience. But it has also sent vast quantities of carbon dioxide into the atmosphere.Some of this CO2 is taken up by living plants, and some dissolves in ocean waters. However, natural uptake cannot keep up with emissions, which are increasing at an accelerating rate, causing CO2 to accumulate in the atmosphere.
Methane: even low levels of global warming could release vast quantities of this greenhouse gas.
Methane, or natural gas (CH4), is produced whenever organic matter decomposes in the absence of oxygen, such as in landfills, in the sediments swamps and rice paddies, in manure lagoons and in the digestive tracts of livestock animals. When methane is released directly into the atmosphere, it has more than twenty times the heat-trapping power of carbon dioxide. (This has sparked an interest in capturing methane from those sorts of operations that produce it as a byproduct—landfills, livestock operations and mining operations—and burning it to produce energy.)
Methane is also of concern because even a low level of additional warming of the planet could trigger the release of vast quantities from the ocean sediments and the tundra. Currently, large quantities of methane are contained at the bottom of the ocean, trapped by high pressures and cold temperatures. Warming of the ocean could allow this methane to escape. Unusually high methane concentrations have already been detected off the coast of Siberia. Even more worrying is the potential for melting of the permafrost in the tundra regions. Vast amounts of long-frozen organic matter would turn into swampy areas and start to decompose, releasing enough methane to trigger runaway climate change.
Other Greenhouse Gases
Nitrous oxide, which is predominately released following the application of nitrogen fertilizer, has 300 times the heat-trapping power of carbon dioxide. Thus, while it is released in much smaller quantities than CO2 or methane, it is a potent greenhouse gas. Another class of compounds, called halocarbons, is also on the rise. These chemicals are refrigerants and industrial chemicals that have been introduced as replacements for ozone-depleting chemicals like chlorofluorocarbons. Like nitrous oxide, the importance of these is measured not in quantity released, but in their heat trapping power: most have hundreds to thousands of times the greenhouse gas potential of CO2, and one of them, sulfur hexafluoride, has 23,900 as much—the most ever measured.
Water vapor is perhaps the most important greenhouse gas in the atmosphere, but its impacts are complicated. While it traps heat near the surface of the Earth, cloud cover also reflects incoming sunlight. Pollutant byproducts of combustion also behave in complicated fashion. Aerosols of sulfate from coal burning scatter light, but particles of soot trap heat.
Climate change is the disruption of our climate due to increased heat-trapping gases in the atmosphere. Also known as global warming, climate change is heating up the Earth’s air and oceans, melting glaciers and polar ice and increasing the severity of heat waves, droughts and strong storms.
The main reason why our planet is getting warmer is because of an increase in greenhouse gases. We need some greenhouse gases to trap heat and keep the planet warm, but too much of them traps more heat than we need and leads to a hotter Earth and a change in our climate.
A hotter Earth affects our lives in every way, from our food supply to the survival of animals such as penguins and polar bears.
Beyond Cutting Emissions
