By Robert Hazen, George Mason University
Global warming stands out as a starkly ominous feature of the landscape of environmental issues. The concept is that increased concentrations of carbon dioxide, and a few other atmospheric gases as well, causes a greenhouse effect, thereby increasing global temperature. It has a simple, intuitive logic, but it also holds tremendous dangers for human society.
Greenhouse Effect
The greenhouse effect is the tendency of some atmospheric gases to transmit visible light and UV wavelengths, just like a pane of glass. But when that radiation heats up the surface of the Earth, and the heat energy, the infrared radiation, comes back up from the surface of the Earth, it’s trapped by those gases, just like a greenhouse. If one has ever been in a greenhouse in the wintertime, they would know how warm it can be, even if the outside temperatures are quite cold, because the glass traps that infrared heat radiation.
The greenhouse effect is absolutely essential for survival of life on Earth. Without that trapped heat, Earth would become a frozen ball, so we have to have a greenhouse effect to some extent, but too much global warming could cause great changes. For example, we could have extensive melting of polar icecaps. We could have a substantial rise in the ocean levels. We could have shifts in global climate, so that areas that are now temperate would become tropical, and areas that are now quite cold would become much warmer and more moderate in climate.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Effects of Greenhouse Gases
We cannot eliminate greenhouse gases. It’s just a matter of a few percent change in the total amount of these gases that we’re looking at when we talk about the greenhouse effect. It’s a subtle effect, and therefore, a difficult effect to get an exact handle on.
Some prophets are foretelling eco-devastation unparalleled in Earth history. They talk about impending catastrophes of violent weather, of coastal flooding, of destroyed agricultural systems. There are others that fervently believe in a self-correcting planetary mechanism that’s going to buffer whatever global insults we throw.
To their profound frustration, scientists just don’t know the real answer at this point. They’re desperately trying to find out, so we have to look at global warming research; it’s advancing on many different fronts right now. The basic data are the ongoing measurements of global temperature, year by year, month by month, and also atmospheric composition, as it’s changing year by year.
Temperature Measurements
Let’s talk about the temperature measurements first. Historically, most temperature measurements on Earth have been made in urban areas, in cities. These are regions that have experienced significant warming over the past century, simply because of the increased area of pavement, of blacktop.
They are ‘heat islands’, and so as the city gets larger, they become more heat sinks. Therefore, we can’t really trust temperature measurements made in, for example, local airports, over the period of the last 50 years.
A System of Satellites
There are many detailed, annual measurements of temperature in rural areas, in the oceans, or on mountaintops. They provide a much better picture, and they do suggest a small, but statistically significant, global warming of about half a degree in the last half century or so; maybe half a degree in 50 years. Even more important, now, is a global network of microwave-sensing satellites. These are able to measure atmospheric temperature at high levels much more accurately, and they also—though the measurements are quite difficult—show a slight warming trend.
Infrared-sensing satellites that monitor the temperature of the ocean are going to be of very great importance for documentation in the coming years. Water has a very high heat capacity, and much of the heat energy at the surface of the Earth is stored in the ocean. Even a slight increase in ocean water temperature indicates a change in global temperature. We have an Earth-observing system of satellites now that’s going to provide the most accurate data of this sort, and ongoing data collection is absolutely essential in this whole effort.
Atmospheric Carbon Dioxide
The other half of the global warming cause-and-effect equation is carbon dioxide. Carbon dioxide is only one of several greenhouse gases, and by far it’s not the most important one in terms of total absorption of that infrared radiation. In fact, CO2 only accounts for about 10 percent of the greenhouse effect. Water vapor and methane are much more effective, molecule for molecule. But carbon dioxide appears to be the one gas subject to very wide variations in atmospheric composition; it’s the one gas that can increase in concentration significantly.
We have decades of detailed measurements of atmospheric carbon dioxide that have been made, for example, at Mauna Kea, at a high mountaintop observatory in Hawaii, and other places as well. These data provide unambiguous evidence that CO2 concentrations have increased dramatically during the last century, by as much as 15 percent in the last 40 years alone—presumably due to the increased burning of fossil fuels, because that’s where most of the CO2 is being generated, and being pushed into the atmosphere.
Common Questions about the Greenhouse Effect and Global Warming
The greenhouse effect is the tendency of some atmospheric gases to transmit visible light and UV wavelengths, just like a pane of glass. But when that radiation heats up the surface of the Earth, and the heat energy, the infrared radiation, comes back up from the surface of the Earth, it’s trapped by those gases, just like a greenhouse.
Too much global warming can cause great changes on Earth. We could have extensive melting of polar icecaps; a substantial rise in the ocean levels; and shifts in global climate, so that areas that are now temperate would become tropical, and areas that are now quite cold would become much warmer and more moderate in climate.
Infrared-sensing satellites that monitor the temperature of the ocean are going to be of very great importance for documentation in the coming years. Water has a very high heat capacity, and much of the heat energy at the surface of the Earth is stored in the ocean. Even a slight increase in ocean water temperature indicates a change in global temperature.
