The Earth is getting warmer. NASA has compiled the Earth's annual mean temperatures since 1880 by using ships logs, weather station, and satellite data. Their data shows that the trend in the Earth's temperature has been mostly upward and that the Earth is now about 1.3 °F warmer than it was a century ago. Scientists point to an increase in greenhouse gases from fossil fuel use as the major cause, but skeptics claim the warming observed is natural. The Sun, clouds, particulates, volcanoes, and greenhouse gases all affect the Earth's temperature. The question is "What part does each play in global warming and what part does man play?"
Energy balance: The Earth has a solar energy balance. Of all the Sun's energy coming to Earth, 30% is reflected immediately back into space by the surface, particles, and clouds. About 20% is absorbed into the atmosphere where it runs the weather cycle.The remaining 50% heats the land and oceans. All the absorbed heat is eventually radiated back into space as infrared radiation. It's a balanced energy budget with 100% of the incoming energy eventually going back into space. Anything that reflects sunlight into space, such as an increase in particulate matter in the air, would cause the Earth to cool. Anything that increases the amount of sunlight received or delays the energy's trip back to space such as greenhouse gases do, would cause the Earth to warm. What factors determine the energy balance?
The Sun: Solar radiation from the Sun has varied very little over the last few million years. The Sun has cycles where its output varies slightly such as Sunspots which cause the amount of solar radiation to vary in approximately 11-year cycles. Increased sunspot activity causes the Earth to warm. However, the effect of Sunspots is so small that they do not show up above the other small variations in NASA's temperature record. Long term variations in the Sun's intensity does not seem responsible for the current warming. Satellite measurements of solar radiation show that the solar radiation reaching Earth has declined slightly over the last 30 years - yet the Earth still warmed.
Milankovitch Cycles: Small wobbles in the Earth's orbit such as variations in the eccentricity, axial tilt, and precession of the Earth's orbit affects the amount of solar radiation the Earth receives. Both scientists and skeptics seem to agree that these cycles are responsible for the Earth's change from warmer to cooler periods and for ice ages that occur in roughly 100,000-year cycles. In the part of the cycle where the Earth receives more solar radiation, the oceans slowly warm and release CO2. The CO2 further amplifies the warming by the greenhouse effect. As the Earth moves into the part of the cycle where it receives less solar radiation, the oceans slowly cool, the CO2 dissolves back into the oceans and another ice age starts. The patterns of wobble in the Earth's orbit are predictable and the model predicts that a minor cooling trend, which began some 6,000 years ago, will continue for the next 23,000 years. The current warming trend is too rapid and in the wrong direction to be a part of the Milankovitch Cycles.
Clouds: Clouds have a dual role. They cool the Earth in the daytime by reflecting solar radiation back into space but they also warm the Earth by their greenhouse effect. First frosts are more likely to occur on cold clear nights. The average cloud cover over the entire Earth remains relatively constant from year to year. Clouds could be considered to contribute to the current warming trend only if the average cloud cover could be shown to be increasing over the years.
One unusual theory tries to do just that. It claims that the number of cosmic rays from the stars that strike the Earth is increasing. That would lead to more clouds since cosmic rays produce charged particles in the atmosphere that seed clouds. It's an interesting theory, but there is no data showing that the number of cosmic rays are actually increasing. Also, there are plenty of particulates in the air to seed clouds and any effect from cosmic rays would be small in comparison.
Greenhouse gases: If the Earth had no atmosphere, its average temperature would be much colder. Most of the Sun's energy comes to Earth as light in the visible and ultraviolet region of the spectrum but it leaves as heat energy in the infrared region. The atmosphere's clouds, water vapor, and CO2 traps some of the infrared radiation headed into space and directs it back to Earth, helping to warm the Earth. This is called the greenhouse effect, as it is similar to the effect that keeps greenhouses warm on cold nights. Recent research shows that about 75% of the greenhouse effect is due to water vapor and clouds and the rest is due mostly to CO2.
Water vapor is removed from the air whenever it rains so the average amount of water vapor in the air remains somewhat constant from year to year. However, the amount of CO2 in the air has been observed to be increasing. Measurements of the concentration of CO2 in the air have shown that it has increased from 280 ppm to 385 ppm in the last century. The physicist G. N. Plass determined the effect of the increase of CO2 on the Earth's temperature. In 1956, he calculated that doubling the concentration of CO2 in the air would cause a 3 to 4 °C increase in the Earth's temperature. A number of more recent studies have confirmed his results but the most important thing is that the rise in temperature he predicted 50 years ago in consistent with the increase in temperature we have observed. Certainly, the increasing amount of CO2 is a main factor global warming. But, is the increase in CO2 natural or man-made?
Volcanoes: Although impressive, the amount of heat released by volcanoes has a minuscule effect on the Earth's temperature compared to that of the particulates and greenhouse gases they release. Particulates reflect sunlight back into space and cause the Earth to cool while the greenhouse gases released cause the Earth to warm. Particulates from large eruptions, such as Mt. Pinaturbo, cause a very quick decline in the Earth's temperature. However, the particulates settle out or are removed by rain in a few years after the eruption stops. The CO2 released by the eruption causes greenhouse warming for a long time as it persists in the air for many decades. Volcanoes, however, play a small role in the current global warming compared to man. Man's activities currently emit about seven times the particulates and 150 times the CO2 as all the world's volcanoes together.
Fossil fuels: Since the 1800′s, scientists have been concerned with whether our use of fossil fuels might affect the temperature of the Earth. Burning carbon fuels releases large amounts of CO2, and there was speculation about whether an increase of CO2 in the air might actually change the energy balance of the Earth. Recent research has shown that CO2 plays a very important role in global warming and one scientist even labeled CO2 the "control knob" for the temperature of the Earth. The amount of CO2 man releases into the air is no longer minuscule as our emissions amount to 30 billion tons of CO2 each year. The concentration of CO2 in the air has increased from about 280 ppm to 385 ppm and the Earth's temperature has increased by 1.3 °F. Clearly, CO2 is the main factor in global warming and our use of fossil fuels is the main cause of the increase in atmospheric CO2.
It's not natural: Climate scientists agree. In 2004, a survey of 928 climate papers published in Science found that none of the authors argued for a natural explanation of global warming. So there we have it. Global warming is not caused by volcanoes, clouds, sunspots, changes in solar output, or cosmic rays from the stars - and it is not part of the natural cycles of nature. As much as we may dislike the idea, the major cause is man.
Dr. J.C. Moore is a physical chemist whose interests are spectroscopy,computational chemistry, professional ethics, and science education. He taught chemistry, physics, and general science at the college level for 38 years. Since retirement, he has established http://JCMooreonline.com, a website that examines current events from a science and research perspective.
