|The Greenhouse Gases|
Our study of climate change begins with a look at the greenhouse gases and the greenhouse effect. When first asked, students unsurprisingly respond that the greenhouse effect is a "bad thing," exhibiting their imperfectly developed understanding of this natural phenomenon. I must often confront this and other types of misconceptions in my science classroom, thanks mainly to the disinformation that students have been exposed to through television media.
The greenhouse effect refers to the ability of Earth's atmosphere to retain additional heat energy because of the presence of various greenhouse gases, such as water vapor and carbon dioxide. The greenhouse effect is a wholly natural phenomenon; without it, Earth's average temperature would be 33°C lower than it is now, and the planet would be completely frozen. No greenhouse effect means no liquid water, which means no plants, which means no life.
So, this natural greenhouse effect is truly a good thing for our planet. But what happens when the greenhouse effect is altered, by adding additional greenhouse gases to Earth's atmosphere? Any change in the composition of Earth's atmosphere will alter its chemistry and add imbalance to Earth's energy budget. (Earth's energy budget is a balance of energy: solar energy from the Sun flows into Earth's atmosphere, is converted to infrared heat energy by the atmosphere, clouds, and surface of the Earth, which eventually flows back into space.) The Earth system will respond to changes in atmospheric chemistry through a variety of feedback mechanisms, but ultimately the physics dictates that increased greenhouse gases create a warmer planet.
There are four major, naturally-occurring greenhouse gases in Earth's atmosphere: water vapor, carbon dioxide, methane, and nitrous oxide. As mentioned above, these gases help create a favorable temperature on our planet. However, since the beginning of the Industrial Revolution back in the 1700's, humans have been adding more of these gases plus a host of new, synthetic gases to the atmospheric mix. The concentrations of almost all of these gases have been rising steadily for the past 200+ years.
Why are these invisible, colorless, odorless gases called "greenhouse gases?" What do they actually do in the atmosphere? This is an abstract concept for students to grasp, but it is imperative that they understand how these greenhouse gases behave to appreciate their role in maintaining an atmospheric heat balance. One excellent tool is the Greenhouse Effect simulation, part of the PhET collection developed by the University of Colorado. In this interactive simulation, students can see what happens when both solar energy from the Sun and infrared heat energy from the Earth interact with various gaseous molecules present in Earth's atmosphere. Molecules such as nitrogen and oxygen—the two most abundant molecules in Earth's atmosphere—allow both solar and infrared energy to pass through the atmosphere uninterrupted. In contrast, greenhouse gas molecules such as carbon dioxide and methane allow the solar energy to pass into the Earth's atmosphere freely, but disrupt the return flow of infrared energy back into space. Because of the greenhouse gases, the heat energy leaving planet Earth takes longer to return to space, thus warming the planet.
What data do we have to support and validate the phenomenon of the greenhouse effect? For the past 50 years, NOAA scientists have been collecting air samples all around the globe and measuring the concentrations of different greenhouse gases in our atmosphere. The concentrations are steadily increasing as humans add more of these gases into the atmosphere through the burning of fossil fuels and the release of gases through various industrial and agricultural processes. NOAA developed a simplified measure of the combined effects of these gases, the Annual Greenhouse Gas Index (AGGI). The AGGI provides mathematical values for each of the greenhouse gases that indicate their individual contribution to the greenhouse effect and the warming of our planet. My students graph the AGGI data each year and draw conclusions about the patterns and trends in the data. SPOILER ALERT: The data support greenhouse theory.
The approach to which I introduce students to global climate change is entirely evidence based. For any scientist to make claims or propose hypotheses about the natural universe (including planet Earth) requires that she or he follow the evidence trail. An overwhelming body of evidence, including laboratory testing, supports scientists' explanations about the greenhouse effect, the changes to the greenhouse balance, and its impacts on global climate. To ignore both the evidence and the rigorous science behind it presents a moral and ethical dilemma that I believe is irresponsible to the inhabitants of this planet—our only home in the universe. My greatest hope is that my students develop an appreciation for science so that they can make logical and informed decisions based on data and evidence, not hype and hot air.
Classroom resources for global climate change studies: