The Earth’s Temperature: Certainly, the average temperature of the Earth has varied greatly over the last million years, from about 2°C (36°F) during the ice ages to about 15°C (59°F) during the warmer interglacial periods. We are now in an interglacial periodic and the Earth’s average temperature for the last century averages 13.9°C (57°F). Much of the research on the Earth’s temperature has been an attempt to understand the coming and going of the ice ages. We now know that the Earth’s temperature is correlated with the Milankovitch cycles , which affect how much sunlight the Earth receives, but that is not the whole story. That greenhouse gases play a role in warming the Earth was shown by Joseph Fourier in the 1820s. Using the differential equations he developed for heat transfer, Fourier calculated that the Earth, considering its size and its distance from the Sun, should be considerably colder than it actually is. He proposed the Earth must be kept warmer by its atmosphere, which acts much as the glass in a greenhouse. The actual amount of warming that could be attributed to the greenhouse effect was later found from the Stephen Boltzmann law, developed in the early 1900s. If the Earth had no atmosphere, its average temperature would be 33°C lower, at -19.0°C (-2.2°F). Without greenhouse gases, the Earth would be a frozen block of ice.
Greenhouse Gases: Heat energy leaves the Earth as infrared radiation, which makes up a part of the spectrum that is absorbed by many molecules as they vibrate. As infrared radiation leaves the Earth, it is absorbed then reemitted in all directions, some of it going back toward the Earth where it further warms the Earth. In the 1850’s, John Tyndall’s infrared research found that nitrogen and oxygen, the major components of the atmosphere, do not absorb infrared radiation. He discovered that the molecules responsible for the greenhouse effect were water vapor and carbon dioxide. Water varies from a trace up to about 4% depending on the humidity; carbon dioxide’s concentration was about 0.0028% in Tyndall’s time. In spite of their low concentration, CO2 and H2O both absorb strongly in the infrared region of the spectrum. Also, radiation leaving the Earth must traverse several kilometers of atmosphere, greatly increasing the probability of the radiation being absorbed and readmitted. Carbon dioxide plays a large role for its concentration, as it absorbs strongly in regions of the infrared spectrum where water does not.
Recent research by Kiehl and Tenebreth on the Earth’s energy budget identified five naturally occurring gases that contribute to the greenhouse effect. The gases, along with their contribution in both clear sky and cloudy conditions, are listed in the table.The infrared spectra of the major greenhouse gases can be found at http://chemlinks.beloit.edu/Warming/pdf/greenIR.pdf .
Each of the greenhouse gases has several absorption bands, and there are some regions of the spectrum where the bands overlap, as noted in the table. Once clouds form, the liquid droplets absorbed broadly across most of the infrared region, so cloud formation reduces the contributions of the other gases. Overall, clouds and H2O account for about 75% of the greenhouse effect and carbon dioxide and the other greenhouse gases for about 25%. Some of the coldest nights on Earth are when the humidity is low and the night is still and clear, as the contribution of H20 is reduced far below the 60% given in the table.
The average residence time of a water molecule in the atmosphere is only about nine days. Because precipitation removes water from the air in such a short time, the concentration of water in the air varies from a trace in cold arid region up to about 4% in warm humid regions. The average residence time in the atmosphere of CH4 is 12 years, while the residence times of NO2 and CO2 are more than a century. Gases with long half-lives reside in the atmosphere long enough to become evenly distributed throughout the atmosphere. Ozone (O3), which has a residence time of a few months, is constantly beingformed in the atmosphere from photochemical processes, many of which are initiated by methane and hydrocarbons.
The Limit of Humidity: The pressure of the atmosphere is made up of contributions from all the molecules in the atmosphere and the share that each gas contributes is called its partial pressure. The amount of water in the air can be measured by its partial pressure. There is a limit on the amount of water the air can hold as the humidity becomes 100% when the partial pressure equals the saturated vapor pressure, and the air can hold no more water.
The Saturated Vapor Pressure of Water
The saturated vapor pressure depends only on the temperature and is listed in the table at the right.That limit of water in an air mass can be reached by water evaporating from the surface until the partial pressure reaches the saturated vapor pressure given in the table. Alternatively, the limit can be reached when a mass of air is cooled until its saturated vapor pressure is lowered to the air’s partial pressure. Any further decrease in temperature will cause air to be oversaturated and cloud formation and precipitation is likely to occur. For example, at the equator, where the temperature averages 26°, water will evaporate until it reaches the saturated vapor pressure of 25.2 mmHg. However, over the Arctic Ocean where the temperature averages 1°C, the air is saturated at 4.9 mmHg. Not surprisingly, the air can hold almost 5.1 times as much water at the equator. Or, on a clear night, when the temperature drops until the saturated vapor pressure is less than the air’s partial pressure, dew will form. The weatherman usually reports the temperature when that will happen as the “dew point”.
CO2 Controls the Temperature: One of the great mysteries confronting science in the 1800’s was the cause of the ice ages. The role that greenhouse gases had in keeping the Earth warm provided a clue for Arrhenius, who thought that changes in their concentration might be the cause of the coming and going of the ice ages. He set out to find the climate sensitivity, the temperature change expected if the concentration is doubled, for the individual greenhouse gases. Arrhenius understood that the concentration of water vapor in the air was limited by its saturated vapor pressure, which is dependent on the temperature. How then, could an increase in H2O increase the temperature when it was itself limited by the temperature? Carbon dioxide has no such limitation, so Arrhenius turned his attention to finding the climate’s sensitivity to carbon dioxide. Though Arrhenius’s model was simple and the calculations were laborious, he found that doubling the carbon concentration would increase the temperature of the earth by about 5°C. However, the increase in temperature would allow a greater concentration of water vapor in the air which would amplify the warming. Thus, the concentration of CO2 acts as a regulator of water vapor, and ultimately determines the planet’s long-term equilibrium temperature. Recent work using better data and models have found that the climate sensitivity to carbon dioxide is in the range of 3 to 4°C, and carbon dioxide has been proposed as the “control knob” for the Earth’s temperature. Still, water vapor and clouds contribute the most to greenhouse warming, and their contribution is considered to be a positive feedback to the increasing carbon dioxide concentration.
No one in Arrhenius’s day could imagine how the atmosphere’s carbon dioxide concentration could possibly double, and some of Arrhenius’ contemporaries proposed setting some poor quality coal seams on fire to ward off another Ice Age. That proved not to be unnecessary as in 1900 Arvid Hgbom, a volcanologist, calculated that industrial sources were adding CO2 to the atmosphere at roughly the same rate as volcanoes. No one thought much of it as, at that rate, it would take centuries for the amount of CO2 to increase significantly. However, no one imagined that we would burn fossil fuels at today’s rate, putting 30 billion tons of CO2 into the air each year. The amount of carbon dioxide in the air has increased by 40% since Arrhenius’s day, and the temperature of the Earth has increased by about 0.85°C, well in line with Arrhenius’s predictions.
Alternate Theories: There are a number of alternate theories as to why the Earth is warming, and most of the recent ones center around water and clouds, as that is still an active area of research. The most easily dismissed one is that water vapor is responsible for global warming rather than carbon dioxide. Arrhenius showed that was false over 100 years ago, yet some Skeptics are still saying it. Another theory, credited to Svensmark, is that cosmic rays from the stars produce charged particles that promote cloud formation. There is little evidence that the cosmic rays reaching Earth have increased and there are plenty of particulates in the air to seed clouds besides the charged particles. Another theory is Iris Effect which has been promoted by Richard Lindzen, mostly in op-ed pieces that are not peer-reviewed. His theory is that the earth’s sensitivity to greenhouse gases is low as the increasing surface temperature at the equator will cause the rising columns of moist air to rain out more moisture, leaving less to form high ice clouds, known to be a positive forcing. Aside from the fact that it seemed a little unreasonable to claim that more moisture in the air will lead to fewer high clouds, other climate scientists have found significant errors in Lindzen’s published works.
A recent paper in Remote Sensing by Roy Spencer attributes global warming to cloud formation and it was claimed to “blow a gaping hole in global warming theory.” Its main theory was that clouds were driving global warming, rather than being a feedback mechanism. The paper was quickly refuted by climate scientists by pointing out that Spencer’s model of the Earth’s atmosphere was terribly inadequate. There is also evidence that Spencer’s paper gained publication by gaming the peer review system. Another theory comes from Roger Pielke Jr., who claims that hurricanes and tornadoes are becoming less frequent and destructive, based on an economic analysis of storm damage. Global warming is likely to increase the probability of severe storms, so his work has been used to discount global warming. However, his theories stand in sharp contrast to the number of events and the amounts paid out in storm damage by Munich Re (the fourth and fifth graphs) , a large secondary insurance company, that analyzed the issue without the benefit of some of Pielke’s assumptions.
The final theory, which would be laughable if it weren’t repeated by many Skeptics to discredit climate science, is that climate scientists have created the CO2 global warming theory purely for their own economic benefit. The greenhouse gas theory was developed, and the main points understood by the end of the 19th century, long before any of today’s climate scientists were even born. Fourier, Tyndall, and Arrhenius established that H2O and CO2 were main factors in warming the earth, with changes in CO2 concentrations being the primary driving force and H2O being a feedback to changes in the CO2 levels. Research since then has confirmed their findings, and their theories have been borne out by the global warming we have experienced since their day. It is hard to believe that any credible scientist would reject such well-established theories.
Note: Much of the historical data in this article came from http://www.aip.org/history/climate/co2.htm
(C) 2013 J.C. Moore