Inside Global Warming
Miller, Roxanne Greitz, Science Scope
Byline: Roxanne Greitz Miller
Over the last 15 years, much attention has been given to global warming, and whether the increase in the Earth's temperature in recent decades threatens the survival of life on Earth. Release of the films The Day After Tomorrow (2004), where North America is rapidly plunged into a new ice age, and An Inconvenient Truth (2006), where Al Gore focuses on the future impacts of global warming, has raised much public and media attention on the subject. As such, it's important that science teachers understand the basics behind the scientific phenomenon, the controversy surrounding the topic, and how to discuss and explore global warming with their students.
What is global warming and what causes it?
Global warming is the observed overall statistical increase in the average temperature of the Earth's atmosphere and oceans over recent years. According to the U.S. National Academy of Sciences, the Earth's surface temperature has risen by about one degree Fahrenheit in the last century, with accelerated warming in the last two decades. While there are a number of causes for global warming, strong scientific evidence proposes that over the last 50 years the largest contributor to global warming has been human activities that have released certain heat-trapping gases into the atmosphere, increasing global mean temperature because of the greenhouse effect.
The greenhouse effect refers to the radiative effect by which the atmosphere warms a planet, i.e., the trapping of the Sun's rays within atmospheric gases and how that trapping of the rays-and their heat-moderates global temperature (Figure 1).
Without the greenhouse effect, our planet would not be able to sustain life-it would be too cold. The increased retention of solar radiation as a result of increased amounts of greenhouse gases in the atmosphere-called greenhouse warming-is proposed by scientists to be responsible for 50% of the global warming in the past 50 years.
The naturally occurring gases in our atmosphere that trap solar radiation-and therefore contribute to greenhouse warming-include water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide (NO2), and ozone (O3). While all of these gases are naturally occurring (water vapor and CO2 are primary gases emitted in volcanic eruptions, for example), significant levels of these gases are also produced via human activity. CO2 is a product of combustion (burning of solid waste, wood, and fossil fuels such as oil, gas, and coal). CH4 is emitted during the production and transport of fossil fuels and the decomposition of organic wastes. NO2 is emitted during agricultural and industrial activities, as well as during combustion of fossil fuels and solid waste.
In addition, other non-naturally occurring gases that greatly contribute to the atmosphere's ability to trap solar radiation include hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6), all of which are generated via industrial processes.
Each individual greenhouse gas differs in its ability to absorb heat in the atmosphere. Of the non-naturally occurring gases, HFCs and PFCs are the most heat-absorbent. Of the naturally occurring gases, methane traps over 21 times more heat per molecule than carbon dioxide and nitrous oxide absorbs 270 times more heat per molecule than carbon dioxide. Therefore, it is important when discussing emissions of gases into the atmosphere to remember that not all greenhouse gases are equal in contributing to greenhouse warming; both the quantity and heat-trapping potential of each gas must be considered.
How is the Earth's temperature different now than it has been in the past?
Reliable data on global temperature is available from approximately 1880 forward. As stated earlier, it is estimated that the Earth's global mean temperature has risen approximately 1[degrees]F in the last 100 years (see Figure 2). …