Want to stir up some controversy? Just make some definitive statement about global warming, either good or bad. Chances are high that someone will strongly disagree. The scientific data demonstrates pretty conclusively that Earth’s surface temperature has risen by 1°F over the last 40 years.
While widespread consensus exists regarding the rise in temperature, major disagreements occur when discussing the cause of this warming. Is it a result of changing astronomical conditions like fluctuating galactic cosmic rays or a variability in the distance from the sun? What effect does variability in atmospheric and ocean circulation play? What about aerosols and particulates in the atmosphere? Are greenhouse gases, particularly those generated by humans (like carbon dioxide and methane), the cause of the temperature rise? Have humans made any contribution to the warming? The short answer is yes! All of these factors affect the global temperature.
Political positions seem to play an inordinate role in the public discussion of global warming. Rather than enter that minefield, I want to address the importance of global warming in a very different context—how Earth’s atmosphere plays a critical and fundamental role in the planet’s capacity to host a teeming and diverse array of life.
Putting Things in Perspective
The worst-case warming scenarios for the next few centuries typically have temperature increases around 10°F. Clearly, such an increase would have a major impact on the planet, but consider this number from a different perspective. What would Earth’s average global temperature be without any greenhouse gas–induced warming? It would be a bone-chilling 0°F! That’s an arctic winter kind of cold, a temperature well below Earth’s current 60°F. What makes the difference? It is due to an abundance of greenhouse gases in Earth’s atmosphere.
The Basic Principles of Greenhouse Heating
Earth’s surface receives a known quantity of energy from the sun—240 W/m2. Because the surface is in equilibrium, Earth also radiates this much energy back into space. One can use the Stefan-Boltzmann law to calculate the temperature that corresponds to this amount of energy radiation. For those who like equations, S = σT4, where S is power per unit area, σ is the Stefan-Boltzmann constant, and T is the absolute temperature. Plugging in S = 240 W/m2, σ = 5.67 x 10-8 W/m2 / T4, and solving gives T = 255K. Converting to more familiar units yields the temperature of 0°F mentioned above. However, adding an atmosphere changes the picture.
Most of the radiation Earth receives from the sun arrives in the form of visible light, and Earth is surrounded by a largely transparent atmosphere. The gases in the atmosphere allow the light to penetrate to the surface, where it is absorbed and heats up the surface. The surface then radiates its energy back toward space as heat. However, this heat radiation may or may not pass through the atmosphere depending on the gases present. The two gases that dominate Earth’s atmosphere, nitrogen (78 percent) and oxygen (21 percent), allow the heat radiation to pass unaffected. Other natural gases (water vapor, carbon dioxide, methane, nitrous oxide, and ozone) and man-made gases (chlorofluorocarbons and hydrofluorocarbons) have molecular structures that will absorb the heat radiation coming from Earth’s surface and heat up the atmosphere. Just like the surface, the atmosphere will eventually reradiate this energy. Unlike the surface, some will radiate to space and some will radiate toward the surface, resulting in an increased surface temperature. Getting all the details exactly right requires fairly complicated calculations, but the net result is a life-friendly surface temperature of 60°F.
The larger the amount of greenhouse gases in the atmosphere, the more warming happens on the planet’s surface. For the last 500 million years, when large-bodied animals roamed the Earth, oxygen and nitrogen dominated the gas budget of the atmosphere. Interestingly, the greenhouse gases that make Earth so habitable compose just a tiny part of the atmosphere—less than a fraction of a percent. This fact stands out when compared to the composition of the atmospheres of Mars and Venus. Each planet’s atmosphere has minuscule amounts of oxygen and is comprised of about 3 percent nitrogen. Carbon dioxide dominates both atmospheres with a percentage higher than 95! Additionally, the atmosphere on Venus is 90 percent more dense than Earth’s. Consequently, Earth’s sister planet has a surface temperature around 880°F—hot enough that paper would spontaneously combust if Venus had any oxygen. The global surface temperature of -67°F on Mars results from an atmospheric density less than 1 percent of Earth’s and a farther distance from the sun.
The largest contributor to Earth’s temperate climate, water vapor, receives very little press in the current discussions about global warming. In all fairness, however, humans have little control over the amount of water vapor and a much larger influence over carbon dioxide, methane, and the particulate matter in the atmosphere.
The topic of global warming, or global climate change, will continue to generate lots of heated discussion, but one fact remains: without substantial heating of Earth’s surface by greenhouse gases, no life would exist here on Earth.