The Earth’s average surface temperature has increased by almost 1C in the past century, and the sea level has risen by about 8 inches.
There were about twice as many flood events in Southeast Asia in the first decade of the 21st Century than there were in the 1960s.
Greenhouse gases include CO2, CH4, N2O, and H2O.
The oceans would be frozen year-round if there were no greenhouse gases.
N2O emissions are caused by synthetic fertilizer use.
Ice-core samples indicate that the atmospheric concentration of CO2 is higher than at any time in the past 400,000 years.
During the past century, a few regions have cooled.
Most models predict that if we continue burning fossil fuels at the same rate, the concentration of CO2 in the atmosphere will triple.
A tree can absorb one ton of CO2 over its lifetime.
Alternative energy sources such as wind, solar, and geothermal have little effect on greenhouse gas emissions.
These graphic organizers are provided to help students locate and analyze information from the articles. Student understanding will be enhanced when they explore and evaluate the information themselves, with input from the teacher if students are struggling. Encourage students to use their own words and avoid copying entire sentences from the articles. The use of bullets helps them do this. If you use these reading strategies to evaluate student performance, you may want to develop a grading rubric such as the one below.
Complete; details provided; demonstrates deep understanding.
Complete; few details provided; demonstrates some understanding.
Incomplete; few details provided; some misconceptions evident.
Very incomplete; no details provided; many misconceptions evident.
So incomplete that no judgment can be made about student understanding
Links to Common Core State Standards for writing: Ask students to revise one of the articles in this issue to explain the information to a person who has not taken chemistry. Students should provide evidence from the article or other references to support their position.
Vocabulary that is reinforced in this issue:
Structural formulas. (You may want to have model kits available to help students visualize the structures.)
To help students engage with the text, ask students what questions they still have about the articles. The article about climate change, in particular, may spark questions and even debate among students.
Directions: As you read the article, complete the graphic organizer below to outline the evidence for climate change.
Beginning Ideas: Questions you have about climate change
Predictions: What do the models predict for the future?
Possible solutions: Include possible drawbacks.
How have your ideas about climate change changed?
(teacher information) More on the greenhouse effect and global warming In order to understand climate change, global warming or the greenhouse effect, students must understand the normal flow of energy through the Earth’s atmosphere. The energy that is radiated to the Earth comes, of course, from the sun—an annual average of 240 watts of solar power per square meter. We know that in order to remain in a stable state, the Earth must also, then, redistribute that energy. Some of it is distributed from equator to poles, since the equator receives more direct radiation from the sun. Some of the energy is used in processes like photosynthesis and evaporation. The rest is re-radiated into the atmosphere and then back into space. As a result, the incoming and outgoing energy at the Earth’s surface is balanced. And under those conditions the average temperature of the Earth remains relatively stable.
About 29 percent of the incoming solar radiation is reflected back into space and has no role to play in the Earth system. Of the other 71 percent that does enter the atmosphere, 23 percent is absorbed by water vapor, aerosols and ozone and 48 percent passes through the atmosphere and is absorbed by the Earth’s surface. Some of that energy is used to evaporate water as part of the natural hydrologic cycle and some of it drives convection currents. However, some of the incoming energy that is absorbed by the surface is reflected back through the atmosphere as infrared energy (heat) and into space. In general, the Earth maintains an energy balance in order to maintain a stable temperature.
Note that there is a shift in the specific kind of electromagnetic energy from incoming to outgoing from the Earth’s surface. The electromagnetic energy flowing into the Earth’s atmosphere ranges from ultraviolet to the visible spectrum and infrared range. Energy coming from the sun to the Earth is in the shorter wave length range—visible (0.4 to 0.7 μm) and ultraviolet ranges. We know this intuitively because the sun lights our daytime hours and because in recent years there is increasing evidence that UV radiation causes skin cancer. The reflected or outgoing energy, on the other hand, is in the thermal IR range. See also http://www.acs.org/content/acs/en/climatescience/energybalance.html.
Some energy, then, passes through the gases in the atmosphere twice, incoming and outgoing. As noted above, the energy that the Earth reflects back into the atmosphere is in the infrared region of the energy spectrum. The gases that make up the bulk of the atmosphere, oxygen and nitrogen, do not absorb that reflected infrared thermal energy. We say that they are transparent to infrared thermal radiation. But other gases present in the atmosphere in lesser concentrations—carbon dioxide (CO2), methane (CH4), water vapor, nitrous oxide (N2O) and other trace gases—absorb some of the out-going reflected energy—about 5–6 per cent of it—trapping it in the atmosphere, and thus preventing it from returning to space. These gas molecules, in turn, radiate energy out in all directions and in so doing increase the energy of nearby molecules. The net result is an increase in the kinetic energy of the molecules in the atmosphere and a consequent increase in the ambient temperature of the atmosphere. This is the greenhouse effect.
Remember that this process has gone on for centuries. It is a natural process. The heat trapped in the atmosphere by gases we now call greenhouse gases—mainly carbon dioxide, methane, water vapor and nitrous oxide referenced above—is a natural part of the Earth’s energy budget. In fact, without these heat-absorbing molecules the Earth would be an icy planet devoid of life. Remember also that these gases, especially carbon dioxide and water, are cycled in and out of the atmosphere naturally over shorter time periods and over centuries. Water vapor is cycled back to the Earth’s surface in liquid form by the hydrologic cycle. Carbon dioxide, produced and sent into the atmosphere by naturally-occurring oxidation, primarily as products of respiration and combustion, is cycled out of the atmosphere again by photosynthesis. Thus, carbon dioxide concentrations are at a maximum, at least in the northern hemisphere, in May when plants begin their growing cycle and reach a minimum in November at the end of the growing cycle. And there have been broader cyclic changes in average atmospheric CO2 concentration historically, as the graph below shows.
(http://www.ncdc.noaa.gov/paleo/icecore/antarctica/vostok/vostok_co2.html) The data from which this graph was created is the result of measuring CO2 concentrations in ice cores taken from Antarctic ice sheets. Air trapped in the ice is retrieved and various gas concentrations measured. The article describes this method of determining ancient temperatures. See “More on measuring past climate conditions” below.
But on average the concentration of CO2 has remained relatively stable over centuries, thanks primarily to the photosynthesis-oxidation cycle. However, in recent decades the CO2 story has changed, as the graph below indicates.
According to NOAA:
The carbon dioxide data (red curve), measured as the mole fraction in dry air, on Mauna Loa constitute the longest record of direct measurements of CO2 in the atmosphere. They were started by C. David Keeling of the Scripps Institution of Oceanography in March of 1958 at a facility of the National Oceanic and Atmospheric Administration [Keeling, 1976]. NOAA started its own CO2 measurements in May of 1974, and they have run in parallel with those made by Scripps since then [Thoning, 1989]. The black curve represents the seasonally corrected data. Data are reported as a dry mole fraction defined as the number of molecules of carbon dioxide divided by the number of molecules of dry air multiplied by one million (ppm).
So, although the total concentration of heat-absorbing gases in the atmosphere has been relatively stable, on average, for centuries, recently the average CO2 concentrations have been steadily increasing. Most scientists attribute the increase to increased human activity, for example, in the form of burning fossil fuels which produces CO2. The reasoning goes that humans are pumping carbon dioxide into the atmosphere at a rate too fast for the Earth’s natural systems to cycle it out. And at the same time the rate of deforestation has been increasing, thus removing the trees that consume so much carbon dioxide for photosynthesis.
The net effect is that heat-absorbing gases are being added to the Earth’s atmosphere as a result of human activity and that is happening faster than the Earth can remove them. Heat reflected from the Earth is, therefore, trapped in the atmosphere due to more molecules absorbing more heat, which contributes to the greenhouse effect, raising the temperature of the atmosphere and the Earth’s surface above natural levels, causing global warming.