Science of Climate Change

Chemistry of the Greenhouse Effect

The two gases contributing most significantly to the natural greenhouse warming of the earth are water vapor and carbon dioxide.  Methane, nitrous oxide, ozone and sulfur hexafluoride are also greenhouse gases but make a smaller contribution to the greenhouse effect because their concentrations are so low. 

Since the beginning of the Industrial Revolution, human activities have caused an increase in several greenhouse gases, most notably carbon dioxide, a trend most scientists believe is causing anthropogenic greenhouse warming. Over the past two centuries the concentration of carbon dioxide in our atmosphere has increased about 30 percent, from a pre-industrial level of about 270 parts per million to a current level of 380 parts per million.  Carbon dioxide concentrations in the atmosphere are already higher today than at any time in the past 150,000 years. And if the consumption of fossil fuels such as coal and oil continues into the next century at projected rates, the carbon dioxide concentrations in the atmosphere would reach the 600-700 parts per million range by 2100. 

Other greenhouse gas emissions have been rising as well. Methane concentrations in the atmosphere have doubled since pre-industrial times. Other greenhouse chemicals, such as chlorofluorocarbons, perfluorocarbons, and hydrofluorocarbons, are synthetic and have only appeared in the atmosphere since the Industrial Revolution. 

Each compound has a distinct capacity for greenhouse warming and a distinct chemical half-life — that is, the time a typical molecule spends in the atmosphere before reacting and forming a new compound. Many greenhouse substances, including methane and the halogen-containing compounds, contribute many times more pound-for-pound to the greenhouse effect than carbon dioxide.  However, the sheer volume of carbon dioxide in the atmosphere compared to these other trace gases means that carbon dioxide is still by far the largest contributor to anthropogenic greenhouse warming. Additionally, while some greenhouse gases have a half-life of several decades, the half-life of carbon dioxide is on the order of a century. Most of the carbon dioxide we release today will still linger in the atmosphere in 2075 and even 2100. 

The climate system is extremely complex, and many forces other than the greenhouse effect contribute to the swings in our climate patterns. However, evidence is building that human influence is changing the climate of this planet. Many of the world's leading scientists argue that the warming experienced in the 20th century is at least partially anthropogenic in origin. In addition, the Intergovernmental Panel on Climate Change has concluded, "The balance of evidence suggests that there is a discernible human influence on global climate."

Physics of the Greenhouse Effect

The greenhouse effect is absolutely vital to allowing life, as we know it to survive on earth.  Without the greenhouse effect, Earth would be a cold planet, with a mean surface temperature well below freezing. The greenhouse effect insulates earth, resulting in the mild temperatures at the earth's surface that have allowed life to flourish. 

Using a very simple model, we can predict the mean surface temperature of the earth in the absence of a greenhouse effect. We know that about 340 W/m2 of solar power per unit surface area insolates our planet. About 30 percent of this energy is reflected, leaving an average of 240 watts to be absorbed by each square meter of surface area on earth. 

All objects with a temperature above absolute zero emit radiation — and the earth is no exception. According to physics, the power emitted by a black body (which for our purposes we will assume the earth to be) is sT4, where T is the surface temperature of the earth and s the Stefan-Boltzmann constant. 

If the earth and space are at radiative equilibrium, meaning there is no net gain or loss of heat by the earth, we can solve for the temperature of the earth as a function of the insolation and Stefan-Boltzmann constant.  Our model yields an average surface temperature of earth of 255 K, or about 0 degrees Fahrenheit.  Many parts of the earth would be even colder.  Imagine a world where much of the planet is covered by conditions we associate only with polar or subpolar regions - clearly this planet would be inhospitable to many forms of life on earth today. 

Fortunately, the mean surface temperature of our planet is a much more pleasant 288 K (58 degrees Fahrenheit), allowing for temperate conditions over most of the planet suitable for the forms of life we know today.  The missing piece of our model is the greenhouse effect - gases that warm our planet the approximately 60 degrees Fahrenheit and produce the climate we know today.  The two principal greenhouse gases in our atmosphere are carbon dioxide and water vapor.  Other greenhouse gases include methane and the chlorofluorocarbons.  These substances absorb heat in the infrared, the band of wavelengths at which the earth emits energy.  They then reradiate this energy, directing some of it back toward the earth's surface.  This is the extra source of heat that warms the earth beyond the frigid temperatures expected from our non-greenhouse model. 

Remaining Uncertainties

The rich complexities of the earth's climate mean we cannot be sure what changes will result from the increase in carbon dioxide concentrations.  More research and refinement of the global climate models (GCMs) are needed to reduce the range of error in predictions about future climate. 

The GCMs are computer programs that simulate the earth's climate, taking into account an extraordinary number of variables describing the physical and chemical properties of the atmosphere, oceans, and continents.  Over the past decade the quality of GCMs has improved dramatically as computers have become faster and more powerful.  However several weaknesses remain to be corrected in order to improve the GCMs' accuracy.  Much work remains to be done in accurately simulating the behavior of the earth's oceans in the GCMs.  Additionally, the physics of clouds are poorly understood and add another measure of uncertainty to the GCMs. 

With additional research fueled by the ever-growing power of computers, the resolution of models should improve in the next few years, allowing scientists to pinpoint a more accurate range of warming and sea level rise and perhaps allowing for better evaluation of regional effects of global warming.