Global Warming is defined as an increase in the average temperature on Earth (Miller 2002). There are many causes of global warming, ranging from natural causes to human induced causes. The greenhouse effect is actually a natural cause of global warming, defined as "a natural situation in which heat is retained in the Earth's atmosphere by carbon dioxide, methane, water vapor, and other gases (Miller 2002)." The greenhouse effect is actually a vital component to maintaining a relatively constant surface temperature on Earth and keeping temperatures in "a range suitable for life as we know it (Miller 2002)." Human activity, however, has led to an increase in the amount of greenhouse gasses in the atmosphere. The students will look at the greenhouse effect, greenhouse gases, long vs. short term climate changes and finally they'll look at human activity. I believe that understanding the human activity aspect of global warming is highly important for the students to learn about because it's the one thing that we have control over, but I also see value in understanding the non-man-made causes of global warming so that we can distinguish the difference between the two types of causes. Human activity is the most prominently talked about cause of our current global warming trend, but because there is such great variety as to the causes of global warming over time, the subject of human activity and global warming is a topic of great debate and controversy and the reason being that there is no definitive piece of evidence linking human activity to the cause of global warming and that global warming is actually taking place. People have the most difficult time with the idea of global warming because they don't quite understand what global warming is and they're not experiencing immediate effects. Al Gore described it this way, he said that if you put a frog in a beaker of water on a hot plate and slowly turn up the heat, the frog adjusts to the slow temperature change and does not realize that it's getting too hot and will sit in the hot water until it boils to death unless someone rescues the frog. This same phenomenon is analogous to humans here on Earth. Global Warming is slowly turning up the burner of the hot plate, so humans are not seeing any drastic and lasting effects, so why should they make any changes? This is where the underlying debate question will become important for the students, should we do anything about global warming even if there is only a small amount of evidence. They will look at what global warming is and some of the causes of it, and they will analyze some of the arguments that Scientists are making for and against it and will look at some of the evidence and decide, is there enough information to make a decision that humans need to make changes in their daily habits.
The first cause of global warming is the greenhouse effect, which is "the buildup of heat beneath substances such as glass, water vapor, and carbon dioxide that allow incoming short-wavelength solar radiation to pass through, but block the return of long-wavelength solar radiation [...] (Abbott 2004)." The greenhouse effect is actually quite important because it actually keeps the temperature on earth relatively constant and is what sustains life on the planet, that is, if the greenhouse gases remain in the proper proportions. The greenhouse gases, specifically carbon dioxide, methane, nitrous oxide, ozone, water vapor, and chlorofluorocarbons, prevent the reradiated infrared wavelengths from escaping out of the earth's atmosphere, thus causing the earth to heat up. The more greenhouse gases in the atmosphere, the more heat that stays inside of the earth's atmosphere that won't escape back into space, which ultimately causes the earth to get hotter. Carbon Dioxide is often blamed as the culprit of global warming, but it's heat absorbing potential is a lot smaller than the other gases mentioned above. For instance, methane has a heat absorbing ability of 21 times more than that of Carbon Dioxide and ozone's heat absorbing ability is 2,000 times greater than Carbon Dioxide, so then why is Carbon Dioxide often blamed for global warming? First of all there's a lot more CO 2 in the atmosphere than the other greenhouse gases. Since there is such a high amount of CO 2 in the atmosphere therefore causing increased temperatures, this has led to more water vapor in the atmosphere, which is also a green house gas. Secondly, sixty percent of the earth's warming due to the greenhouse effect is caused by humans releasing Carbon Dioxide into the atmosphere. CO2 is naturally cycled in the environment, through the process of photosynthesis, plants take in CO 2 which aids in the process of tissue production, when they die the unused CO2 is returned to the atmosphere. Humans have significantly changed this cycle by burning wood and burning fossil fuels. This has caused a dramatic increase in the amount of CO 2 in the atmosphere because when wood and fossil fuels are burned, it causes chemical reactions to take place that end up releasing excess CO2 into the atmosphere. Fossil fuels were formed when organic-rich decaying plant materials were buried beneath sediments and the pressure of the ever increasing buildup of sediments above, caused the material to transform into the coal, oil, and natural gas that we use to power and heat our homes, and run our cars, etc. According to Abbott author of,
the CO2 concentration in 1800 was 280 parts per million (ppm), but by 2000 the amount had increased by greater than 30 percent to 370 ppm, and 70 percent of that increase came after the year 1950. So while there are other gases that have the ability to trap more of the reradiated infrared wavelengths of light, Carbon Dioxide is the gas that we have significantly increased over the last several decades.
Global climate change is something that has occurred since the beginning of the earth, so it's important for students to understand that not only is global warming and climate change something that humans can impact, but there are also natural phenomena that can cause the earth to warm and/or cool. Abbott tells us that the Earth's climate is dependent upon a balance between the heat that comes into the Earth's atmosphere and the heat that leaves the Earth's atmosphere. The Earth is divided into three temperature zones which are described as frigid, temperate, and torrid and these zones are all dictated by latitude. Looking at the Earth's climate history, the climate has gone through many irregular changes throughout time. There have been times when the Frigid Zone's temperature became dominant over most of the Earth's surface, typically known as an Ice Age, while there have been other points in history where warmer temperatures from the Torrid Zone dominated the Earth. Three Hundred Sixty Million years ago, was one such period of time when the Earth's climate was in a cold state, known as the Late Paleozoic Ice Age, which lasted for 100 million years. The reason for this Ice Age was due to changes in the sizes, shapes, and orientations of the oceans and the continents. There had to be a huge land mass at one of the poles, which would gather the snow to create massive ice sheets. Ocean water circulation was also an important factor. Water at the equator always receives more solar energy than the water at the poles, if there were no continents, then the water would flow in an east-west pathway because of the earth's spin. "The geologic record shows that Ice Ages are favored when oceanic circulation is more longitudinal (north-south) than latitudinal (east-west) (Abbott 2004)." North-South continental alignment causes ocean water to circulate longitudinally and propels the warm equatorial water to each of the poles, and because warm water evaporates much better than cool water, this allows clouds to form over the poles and dumps massive amounts of snow on to the land masses at the poles and causes the formation of glaciers. The Late Paleozoic Ice Age most likely ended because the Southern Portion of Pangaea broke apart causing the circulation of the ocean waters to change, the warm waters stayed near the equator, while the cold water circulated at the poles, thus preventing the needed evaporation and cloud formation at the poles to create the snow necessary to maintain the glaciers. This was an example of the opposite of global warming, and that global climate change can take place without the humans. There have also been times in the geologic time scale that indicate a warming period of the Earth. One such event occurred during the Late Paleocene Torrid Age, which was about 65-55 million years ago. During this warming event there came to be less of a difference between the temperatures of the waters at the equators and the poles. When the temperatures in these areas become less diverse, meaning without extremely cold water, there is no longer a sinking action of the colder more dense water in the polar regions which causes the temperature at the surface and in the depths to be more uniform, which means that ocean circulation would be slow-moving. With slow-moving surface waters, also come changes in the atmospheric temperatures, worldwide there was little temperature difference which caused more peaceful weather, meaning that the world-climate was without extremes. The reason for the torrid-dominated temperatures of the world was because of a several factors, the first being that at the equator there was mainly oceans, which meant that the ocean was absorbing more of the sun's energy. The second reason was that warmer oceans caused more snow and ice to melt and exposed more land, while snow and ice reflect sun rays, the land absorbs it. The third factor was probably that in the North Atlantic Ocean lava was escaping at fault lines as the ocean was opening up in this area, which caused a release of profuse amounts of gas, which would have increased the amounts of greenhouse gases in the atmosphere, thus causing a warming effect throughout the entire earth. The final reason for the warming was because the warm waters became extremely dense, which is opposite of the current state of the ocean, where the cold waters are more dense. The change in density happened because of the higher evaporation rate of the ocean waters, leaving more salt in the warm waters, which increased the water's density. Warm water was able to sink to the bottom of the ocean, and this caused the methane hydrates at the bottom of the ocean floor to melt and get released. Methane has a heat trapping ability of 21 times more than Carbon Dioxide, so releasing the Methane from the ocean floor caused a drastic increase in the greenhouse effect thus changing the temperature on earth and life on earth. Today on our current ocean floors there is still large amounts of these methane hydrates. Climate change is nothing new to the earth. It has undergone many changes over the course of its history and happens naturally without the help of humans, however, humans can and do play a significant role. Climate change usually takes place in 10s of 1000's of years, the climate change that we are currently seeing is happening at a much faster rate.
There are also shorter term climate changes that take place such as El Nino. El Nino is a phenomenon that occurs because of the weakening of the trade winds. Normally there is a low pressure system that remains over Indonesia, but when the trade winds weaken, it causes the low to move over into the Pacific Basin, causing the warm waters to move toward South and Central America. When this happens, the currents get reversed and the winds from the west blow the surface water to the east, causing the Americas to receive a large amount of warm ocean water that easily evaporates. When water evaporates more freely, more clouds are produced and the atmospheric pressure is lowered, causing the warm moist air to flow in an easterly direction towards the Americas and heavier rains in areas that are not used to getting rain. Not only do the coastal areas of the Americas receive heavier rains but because the jet stream flows differently in an El Nino, the southeastern United States is also hit with more rain. The number of hurricanes is reduced because the normally moist areas of the West Pacific Ocean, Indian Ocean, and Africa dry up due to the weakening of the trade winds, the winds enter the Central Pacific Ocean from both sides, and the warm water and moist air spread into Eastern Pacific causing a weaker North Atlantic Hurricane season. The weaker trade winds are unable to transport the cyclones to the west. The jet stream also flows in between the Atlantic and Caribbean breaking apart major storm systems.
It's important that the students understand that there are many causes to global climate change, and that global climate change can and does happen without humans, however it is important for students to understand that while global climate change does occur naturally it happens at a rate of 10s of 1000s of years. When we compare that time scale to the climate change that is currently taking place, it's evident that our climate change is happening at a much quicker rate, and it's due mainly to the fact that humans are releasing Carbon Dioxide into the atmosphere in excess quantities. So I want students to be able to compare the difference between the warming during the Late Paleocene period and the warming that is occurring now.
A hurricane is a large cyclonic storm that occurs in warm tropical waters with sustained winds of greater than 74mph, the reason why 74mph makes a hurricane a hurricane, is that at this speed the winds are no longer able to reach the center of the storm, thus creating the hurricane's eye (Abbott 2004). A hurricane is often referred to as a heat engine. They work by the process of convection, which "is a process of heat transfer where hot material at depth rises upward due to its lower density while cooler material above sinks because of its higher density (Abbott 2004)." Basically a hurricane is able to take the heat energy from the ocean and change it to winds and waves. The energy of a hurricane has the ability to generate winds greater than 150mph, push seawater onshore to heights of 20ft or more and produce heavy rains leading to floods far from the coastal shores.
In order for a hurricane to form several criteria must be met, first within the top 200ft of the ocean's surface, the water temperature must be at least 80EF. The second criterion is the air above the water must be warm, humid, and unstable. Finally there should be weak upper level winds blowing in the same direction of the developing storm. A hurricane begins as a tropical disturbance, where a low pressure zone draws in groups of "poorly organized" thunderstorms (Abbott 2004). From that point a tropical depression begins to form "as surface winds strengthen and flow more efficiently around and into the growing storm [. . .] (Abbott 2004)." In the Northern Hemisphere the storm rotates in a counterclockwise direction around the core. At this central core the surface winds converge and this core "acts like a chimney" pushing warm, moist air up toward the stratosphere. As the air rises it begins to cool and reaches the dew point temperature, causing condensation of the water vapor and a large amount of latent heat to be released. This heat release warms the air within the core, thus "causing stronger updrafts," increasing the rate of upward movement of the warm, moist air from below (Abbott 2004). The tropical depression grows in strength as the converging winds gain speed as they continually spiral up into the core and it becomes a tropical storm when the converging winds sustain speeds between 39 and 74mph. The tropical storm develops into a hurricane when the surface winds remain above 74mph. A hurricane's strength is dependent upon "the speed that the surface winds can flow into the central core, race up its sides, and easily flow out and away in the upper atmosphere. The path that the hurricane follows is due to the trade winds and Coriolis Effect. Trade winds push the hurricanes toward the west. While the Coriolis Effect causes hurricanes to curve to the right, in the Northern Hemisphere. Abbot explains that "[i]f the Earth's surface were as smooth as a billiard ball and solar energy were received equally over its surface, then the atmosphere would be rather still[,]" however this is not the case, Earth's surface is not smooth and further more it does not receive equal solar radiation (Abbott 2004). The earth rotates rapidly, causing warm and cold air masses to move around the earth's surface. At different latitudes the velocity of the rotation of the earth's surface varies, at the equator it moves a 1,037mph and at the poles it moves at 0mph (Abbott 2004). These changes in velocity at different latitudes cause bodies to follow curved paths as they move across the latitudes. This is known as the Coriolis Effect, so hurricanes located in the Northern Hemisphere will always follow a path that curves to the right, while hurricanes in the Southern Hemisphere will always follow a path that curves to the left.
Connecting Global Warming to Hurricanes
There is disagreement in the scientific community over whether the increase in the number of more intense hurricanes is a result of global warming. However, naturally since warm water is a key and necessary ingredient to forming a hurricane, common sense would tell us that even warmer water would yield stronger hurricanes. The problem is that the warmer water may not necessarily be the result of global warming, and even if it is, Scientists still are not sure about the idea that an increase in the number of more intense hurricane is truly an indicator of global warming. The students will be looking at both sides to this argument, and because the students must research the reason, I need to have a strong understanding of what both sides are to this argument. An article in the American Meteorological Society journal, entitled,
Hurricanes and Global Warming
, looks at the 2004 hurricane season to help to bring light to the idea of the disagreement.
Some prominent scientists proposed that the intense 2004 hurricane season and its considerable impacts, particularly in Florida, could be linked to global warming resulting from the emissions of greenhouse gases into the atmosphere. But the current state of climate science does not support so close a linkage.
The scientists agree that because a hurricane acts as a heat engine, which works by transferring energy from a warm region to an area of cooler temperatures. So a hurricane is able to take the heat energy from the warm waters and transfer it to the cooler air above it thus increasing the wind velocity and moisture content of the hurricane. In this sense global warming does play a role because it has increased the surface temperatures of the water. However Scientists are having difficulties buying this as an explanation because the current models are showing mixed results. Some models show that there is no theoretical foundation for evidence to support a change in the numbers of higher intensity hurricanes, while other models do show that if you increase the temperature of water that the intensity of the hurricane does increase. This inconsistency in the models is preventing Scientists from definitively saying that increased hurricane intensity indicates global warming. There are qualitative observations that suggest that tropical cyclones need "thermodynamical and dynamical" factors to fuel the intensity of hurricanes.
Since 1995 there has been an increase in the number of [. . .] major hurricanes (categories 3, 4, and 5) in the Atlantic. But the changes in the last decade in these [parameters] are not so large as to clearly indicate that anything is going on other than the multidecadal variability that has been well documented since at least 1900.
As I mentioned above, warmer waters may not necessarily be the result of global warming, and from the above quote from the
Hurricanes and Global Warming
article, there is some controversy as to whether warmer ocean waters and more intense hurricanes is due to something other than global warming. The ocean goes through periods of warm and cold temperatures that can last between 20 and 30 years, known as multidecadal oscillations. During these periods of time the ocean temperatures will remain at a warm or cool state for decades. If the ocean is in a warm state, then there will be an increase in the numbers of higher intensity hurricanes, and if the ocean is in a cool state, then there will be fewer hurricanes. The Atlantic Ocean, within the last few years has entered into a warm oscillation period, so until Scientists are able to collect more data, which will need to happen over the course of several decades, they can't say with certainty that the increase in hurricane intensity is not due to the multidecadal oscillations and that global warming is the definitive cause. What this means is that there are no trends yet to suggest that there is a significant increase in stronger hurricanes due to greenhouse gases and global warming. Recent history has helped in showing the weak trend, but it could also very well indicate the multidecadal oscillations. In the '70s, '80's, and early '90s there was relatively very little hurricane activity, while compared to the 40s, 50s, and 60s there was a lot more hurricane activity especially in Florida which was hit with 11 hurricanes from 1944-1950 (Pielke 2005). The other evidence that is out there is that in a global sense over a span of several recent decades there has not been an increase in the frequency of tropical cyclones.
In the grand scheme of the Earth's history, global warming has just begun and Scientists have just recently begun recording data on it, so the students will run across very little distinctive evidence indicating whether global warming has caused an increase in the numbers of more intense hurricanes, especially since the recorded measurements of the increase in Carbon Dioxide is no longer than a decadal oscillation, so determining if global warming is really the cause will only happen over a period of several more decades. Scientists have however developed theoretical predictions of future hurricane activity, based on simulated hurricane activity; one such study was done looking at a 1% increase in CO2 per year for the next 80 years. These simulated models compare the present day hurricane intensities to the future and the prediction is that the Earth will have more Category 5 hurricanes in 80 years (Knutson 2004). We can't however rule out another possibility which is that global warming could send the earth into a permanent state of El Nino conditions which would therefore contradict all arguments that increased numbers of more intense hurricanes is an indicator of global warming, because as it was already mentioned in the "Content Background Information" in the section on global warming, the El Nino phenomenon actually produces fewer hurricanes.
The trends in tropical cyclone intensity is a lot more complicated, there are a lot of things that need to be taken into consideration such as, maximum potential energy, average intensity, average storm lifetime, maximum storm lifetime, accumulated cyclone energy, etc. Because of these varying factors affecting hurricane strength and intensity, it's difficult for Scientists to say with certainty that hurricanes are getting stronger.
This research that the students will be conducting should lead them to truly question Al Gore's statement in
An Inconvenient Truth
, that evidence to global warming is that the numbers of higher intensity hurricanes has doubled over the past 30 years. Mr. Gore failed to talk about other factors that could potentially influence hurricane intensity, the most important one being the multidecadal oscillations of the oceans' temperatures. This evidence, I would hope would lead the students to look for other pieces of evidence to suggest that global warming really is occurring.