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Posts Tagged ‘greenhouse gases’

Global Warming: The Role of Water Vapor

Sun ,20/01/2013

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 .

GasesEach 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.

VP1

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

The Case of Global Warming and Extreme Weather

Tue ,22/03/2011

“We know who the culprit is, we just can’t prove it – yet.”  Detective Dick Tracy

The Case. That statement could have been uttered by climate scientists. They know global warming is the cause of the more extreme weather events we are experiencing, but it is hard to prove it.  Global warming has increased the energy and moisture in the atmosphere, and that combination makes conditions ripe for severe storms and floods. Certainly, hurricanes occur and intensify over low-pressure areas fed by moisture and warmer oceans. To be accurate, however, climate scientists could only say, “When weather events occur, global warming is likely to make them more extreme.”  However, the case against global warming is growing stronger. A number of recent research papers have shown global warming is the cause of extreme weather events, and the business community, particularly insurers, are beginning to take notice. (1)

The Link. The vapor pressure of water is one of the most important factors in determining weather. Water will evaporate from the surface until the air above it reaches its saturated vapor pressure. The saturated vapor pressure depends only on the temperature, which makes temperature the determining factor controlling the amount of moisture that the air can hold. (2) If a mass of air saturated with moisture moves to higher altitudes or encounters a cold front and is cooled, the air becomes supersaturated, which leads to precipitation. Over the last century, the Earth’s average temperature has increased by about 0.8°C, which translates into an increase in the saturated vapor pressure of water of about 7%. When precipitation occurs, on the average, 7% more moisture is available. It is a reasonable conclusion that when it rains, it will rain more and when it snows, it will snow more. So strangely enough, global warming could actually lead to greater snowfall. However, it has been very difficult to prove, and certainly even more difficult to convince skeptics that that might be the case.

Floods. Two recent research papers have established a link between global warming, increased rainfall, and flooding. A recent paper in Nature (3) reported that the observed increase in rainfall in the Northern Hemisphere in the past 50 years and climate change are linked. The researchers analyzed the rainfall data in areas prone to flood and found that the rainfall has increased due to the warmer temperatures of the Earth. Their results “were based on a comparison of observed and multi-model simulated changes in extreme precipitation over the latter half of the twentieth century analyzed with an optimal fingerprinting technique.” They also found that the models seem to underestimate the observed increase in heavy precipitation with warming temperatures. Extreme precipitation in the future may be even more severe than now predicted.

The second paper, also published in Nature (4), has linked the increasing floods in England and Wales and global warming. The researchers generated several thousand climate model simulations of the autumn 2000 weather by using actual conditions and also by assuming conditions as they would have been had no greenhouse gas emissions or global warming occurred. They concluded that “the precise magnitude of the anthropogenic contribution remains uncertain, but in nine out of ten cases our model results indicate that twentieth-century anthropogenic greenhouse gas emissions increased the risk of floods occurring in England and Wales in autumn 2000 by more than 20%, and in two out of three cases by more than 90%.

Rivers in the Sky. Normally, when an air mass saturated with water moves ashore and is forced upward, it  cools and precipitation falls until the clouds are no longer over-saturated. However, that is not the case for “rivers in the sky, weather patterns that carry a stream of air saturated with water into coastal regions  continually for days. These  “rivers in the sky”,  cause flooding rains in coastal and inland mountains causing untold costs in property damage and human lives. One such river produced more than 40 inches of rainfall in the mountains of southern California in only four days in early January 2005. It caused widespread flooding and massive mudslides such as the one in La Conchita, California, which took 10 lives. The researchers say these “rivers in the sky” will become more common as global temperatures rise since warmer air means that the atmosphere can hold more water vapor. That is, unless global warming changes the weather patterns that produce them. (5)

Droughts. The link between global warming and droughts has not been yet established by research. Areas that receive little moisture from the oceans would not benefit from the fact that the air can hold more water. Though higher temperatures mean that more water evaporates into the air, it also means that the air can hold more moisture before becoming saturated. Areas that normally experience droughts are much more likely to have less rainfall in the future. This past year has seen droughts in Russia, China, and South America that have limited the production of grain and increased the chances that some species may become extinct. The heat waves in Europe in 2003 and 2010, that caused widespread crop failure and wildfires, may have been the worst in 500 years. Certainly, more frequent and extensive droughts may occur in a warming world. (6)

The Smart Money. The widespread damage caused by weather events related to global warming has caught the attention of the business community, particularly those who pay out insurance claims or invest large sums of money. Insurance companies ranked 2010 among worst years ever for climate disasters. Climate change is the culprit in many of the catastrophic natural disasters in 2010, according to insurance company Munich Re, one of the largest global insurance carriers. It added that trends are pointing to more frequent and riskier events in the future. (7) Recently, a group of International investors, responsible for more than $15 trillion in assets, issued a global warming warning. (8) They called for the world’s nations, particularly the United States, to move decisively to combat climate change or face the possibility of economic disruptions even worse than the global recession of the last two years. They also pointed out that “The economic opportunities are enormous for nations with the foresight to seize them while the risks of inaction are potentially catastrophic.”

(1) http://www.smartplanet.com/business/blog/intelligent-energy/studies-prove-link-between-human-activity-and-extreme-weather-events/4835/

(2)  http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/relhum.html#c3

(3) http://www.nature.com/nature/journal/v470/n7334/full/nature09763.html

(4) http://www.nature.com/nature/journal/v470/n7334/full/nature09762.html

(5)  http://www.noaanews.noaa.gov/stories2005/s2529.htm

(6) http://www.sciencedaily.com/releases/2011/03/110318091141.htm

(7) http://www.scientificamerican.com/article.cfm?id=insurance-ranks-2010-worst-for-climate-disasters&page=2

(8)  http://www.latimes.com/business/la-fi-climate-financiers-20101117,0,6204171.story

(C) 2011 J.C. Moore

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