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The Link between Global Warming and Extreme Weather

Wed ,22/08/2012

A large body of scientific evidence, going back to the middle of the 19th century, links the concentration of atmospheric carbon dioxide,  the temperature of the Earth, and the Earth’s climate. Those who study the Earth and its ecosystems have found ample evidence that the climate is changing. The USDA recently acknowledge that fact by shifting the plant hardiness zones for gardeners northward, acknowledging that frosts occur later in the fall and the last freeze in spring occurs earlier. However many people still doubt climate change and point to weather events as evidence.

Theory: Climate scientists would like to clearly establish the link between climate change and extreme weather events, but that is difficult because of the natural variability of the weather.  The link between global warming, heat waves and droughts would seem unquestionable, but it is difficult to prove. Global warming has increased the energy and moisture in the atmosphere, making conditions for severe storms and floods more likely.  In the last century, the Earth’s average temperature has increased by about 0.8°C, increasing the amount of water the air can hold by about 7%.  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.  (1) However, it has been very difficult to prove, and certainly even more difficult to convince skeptics that that might be the case.

Climate Models: Another approach to linking extreme weather events to global warming has been through the use of climate models. The models take into account the factors that influence climate and weather, and are often used by meteorologists for “future casting” the weather for 10 day forecasts, which is about as long as normal weather patterns last. However, the models may also be used to examine the effect of global warming on the weather events. The models are used to compare the prediction for a weather event assuming that there is no global warming with a prediction of the weather event that includes global warming. In many cases, it can be shown that the weather and rainfall will be more extreme under the global warming conditions. The results are often challenged by climate Skeptics, who claim that the models do not accurately represent the data, or that the models are “falling apart”. The models were developed to fit a century’s worth of the weather and climate data, and there is little evidence to support the Skeptics claims. However climate scientists would like to show a definite link between global warming and weather events to silence those criticisms.

Statistical Evidence: A recent NOAA report, edited by Petersen, et al. (2) , examined 6 extreme weather events that occurred in 2011 and found that there was a link between climate change and the extreme weather event. One of the most interesting reports (3) ,  found that the 2011 heat wave and drought in Texas were 20 times more likely to happen than they would have been in the 1950’s. How did they arrive at that conclusion? A recent paper by Hansen et al.  (4), shows that extreme temperatures are much more likely to occur worldwide than in the 1950’s, and over 10 times as likely to occur as in 1980. As Hansen puts it, the extreme temperatures “which covered much less than 1% of Earth in 1950, now typically covers about 10% of the land area. It follows that we can state, with a high degree of confidence, that extreme anomalies such as those in Texas and Oklahoma in 2011 and Moscow in 2010 were a consequence of global warming because their likelihood in the absence of global warming was exceedingly small.”

Those two papers are important as they have been able to establish a quantitative link between the probabilities of weather events and global warming. More importantly, the link does not depend on theory or on climate models, and relies only on a straight forward statistical analysis of the data. The method depends on computing the normal distribution of the Earth’s temperature anomalies for each decade and then comparing how the distribution of extreme weather events change with time.

Normal distributions:  Before examining how the method works for weather events, it might be useful to examine how it works with something more familiar, like the height of American men. How could we show whether the number of extremely tall men was increasing as time went by?  This could be done by taking a representative sample of men and examining a graph of the normal distribution. We could find the average, μ , and then repeat the process every 10 years to see how the average changed with time. An increase in the average height might indicate that there would be more extremely tall men, but that is not the full story.

Another piece of information that needs to be considered is the variance, or how widely the height of men vary about the mean. The variance is usually measured by the standard deviation , σ, which can be easily calculated from the measurements done to compute the mean. A  graph of the normal distribution  is shown at the right.  “Normal” means that the data has been divided by the total number of men in sample, so that the area under the entire curve represents 100%. That feature is very useful for comparing heights, and it also allows us to associate an area under the curve with  probabilities.

The average height, μ on the graph, is 5’10”, and the standard deviation, σ, is 3 inches. About 95% of the sample falls within 2 standard deviations of the mean, which also says that the probability is 95% that a man selected at random would fall between 5’4″ and 6’4″. Those over 2σ  from the mean, or 6’4″, make up about 2% of the sample and are considered very tall. Finally, those over 3σ  from the mean , over 6’7″, are considered extremely tall and make up only 0.15 %. Michael Jordan and a host of other National Basketball Association players fall into that 3σ category.

How would it be possible to tell whether the incidence of extremely tall men is increasing? One way would be to take height data collected every 10 years, plot the normal distribution, and see how the area of the graphs out past 3σ change. We could not only tell whether there were more extremely tall men, but we could calculate how the probability of finding an extremely tall man changed, just by comparing areas on the graph.

Weather events. Enough data and computing power is now available to calculate normal distributions of temperature data every 10 years for many decades. Having the normal distribution of the temperature data by decade can be used to find whether the probability of extreme temperatures is increasing or decreasing. The Earth’s temperature was fairly stable from about 1950 to 1980, making it a convenient standard for comparing changes. Rather than using temperatures, the graph uses temperature anomalies, which measure how far a temperature reading was above or below average. 

The procedure is similar to the one described for examining the height of men. Hansen, et al. used the Earth’s temperature data to graph normal distributions of the Earth’s temperature anomalies by decade, from 1950 to the present. They found that the distribution of temperature anomalies approximate a normal distribution. 

The results of their work for the summer months show that beginning in about 1970, the mean begins to move to the right toward higher temperatures. It can also be seen that the variance of the data increased and shifted to the right, showing that the probability of extreme temperatures increase greatly from 1950 to 2011.  It can be seen that the number of extreme temperatures, those out past 3 ( meaning 3σ), almost nonexistent in the 1950s, have grown significantly larger in each decade after 1980. A similar graph, using  σ for the last 30 year period (not shown), found the probability of temperatures past 3 sigma is 10 times as great as for the 198o2 to 2010 years.

It should also be noted that the left side of the graph flattens, but that the probability of extremely cool temperatures is not zero. Though  hot temperatures became more probable, that there was still a significant likelihood of cooler temperatures.

Climate Skeptics often argue that an extremely cold weather event disproves global warming. The normal distributions by decade for the winter months is given at the right.  The graph shows the average winter temperatures have increased significantly during the last 30 years and the variance in the temperature has become greater as time progressed. However, the left side of the graph shows there is still a significant probability of extremely cold weather even though global warming is occurring. This means that the skeptics argument is baseless. It is also sometimes argued that extreme snowfalls disprove global warming, but that is also a baseless argument. Extremely cold air can hold little moisture, and it is warmer air, slightly below freezing, that produces the greatest amount of snow. The Inuit know that a warm spell brings a much greater chance of snow.

So there we have it. Climate physics predicts that global warming should cause higher incidences of extreme weather. Climate models find that global warming makes increased rainfall and storms more probable. A straightforward statistical analysis of temperature data not only shows that extreme temperatures are more likely, but has allow climate scientists to calculate how global warming affects the probability of extreme temperatures. A definite link between global warming and extreme weather has been established by the research.

 (1) http://jcmooreonline.com/2011/03/22/the-case-of-global-warming-and-extreme-weather/ 

(2) http://www1.ncdc.noaa.gov/pub/data/cmb/bams-sotc/2011-peterson-et-al.pdf 

(3) http://usnews.nbcnews.com/_news/2012/07/10/12665235-2011-texas-drought-was-20-times-more-likely-due-to-warming-study-says?

4) http://www.pnas.org/content/early/2012/07/30/1205276109.full.pdf+html

(C) 2012 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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