I. Identifying Links Between Climate Change and Tornadoes?
The outbreak of recent killer weather events including US tornadoes hitting Joplin, Missouri and Tuscaloosa, Alabama has everyone asking whether there is a link between tornadoes and human-induced climate change. In this writer’s experience when US TV or radio weathermen are asked about the cause of recent strong tornadoes, they most always ignore climate change as a potential cause and point to a cyclical ocean circulation event known as La Niña as the cause of recent tornadoes if they comment on causation at all.
Rarely is human-induced climate change mentioned as a cause or contributing factor in the recent outbreak of sever tornadoes although questions about causation are becoming more frequent on TV and newspapers in this writer’s experience.
This post argues that ethics requires acknowledging the links between tornadoes and climate change despite scientific uncertainties about increased frequency and intensity of tornadoes in a warming world. However, because there are also scientific reasons to doubt that tornado propagation and intensity will increase in a warming world, as we shall see, care is necessary about how we should discuss these risks.
As we shall see there are certain aspects of atmospheric conditions necessary to produce violent tornadoes that climate change is enhancing while there are other atmospheric conditions necessary to form tornadoes about which scientists are uncertain exactly how a warming world will affect them. To figure out whether climate change will cause more intense and frequent tornadoes requires asking lots of smaller questions about the atmospheric conditions necessary to produce tornadoes and to determine how climate change will affect each of these various atmospheric conditions that combine to propagate tornadoes.
Before discussing tornadoes, it is important to note that it is scientifically uncontroversial to conclude that climate change is causing more violent weather particularly in the form of: (a) more damaging thunder storms, (b) the kind of devastating flooding we have seen this year in Australia, Pakistan, Brazil, Columbia, Venezuela, along the Mississippi and the Tennessee valleys, and (c) more severe droughts such as those experienced this year in China, Brazil, and Texas. Similarly more intense hurricanes have been linked to climate change although it is still uncertain whether global warming will increase hurricane frequency. (Emanuel, 2005)
Most climate scientists agree that future weather will be characterized by far more chaotic weather causing greater damage to human life, health and ecological systems and so tornadoes are not the only intense weather events that could be enhanced by climate change and that will likely cause increased damage and suffering. .
It also can be said that in one way climate change is already changing all global weather including tornadoes. This is so because climate change has already caused changes to the global climate system such as raising ocean temperatures and increasing the amount of water in the atmosphere. Increased ocean temperatures and the water content of air have an effect on the amount and timing of precipitation that is being experienced in any one location. And so a strong claim can be made that climate change is now at least partially responsible for all global weather although the part played by climate change could be small for any individual climate event relative to other causes such as normal ocean circulation patterns. Yet, no tornado or hurricane experienced recently would likely be the same without some contribution from climate change. That is no tornado would appear at the same place, the same time, with the same wind speed without changes to the climate system that have been caused by human impacts on climate And so every tornado is very likely affected somewhat by climate change. That is although strong tornadoes have occurred before recent human-induced climate change, no recent tornado is likely to have happened in the same way at the same place in the absence of global warming.
This is not to say, however, that the intensity and frequency of tornadoes will surely increase in the years ahead.. Yet, although it is not clear that climate change will be responsible for more tornado caused damages, other kinds of storm damages are virtually certain to increase.
This post, however, looks at links between tornado intensity and frequency and climate change and what ethics requires when discussing these links. That is, this post does not examine other links between climate change and damaging weather.
II. The Scientific Links
A. The Earth Is Warming Because of Human Activities
We know with high levels of confidence that the Earth is warming and that most of the warming is attributable to human activities despite the natural variability of thee Earth’s climate. Earlier this month, the United States Academy of Science issued its most recent report on the science of climate change that once again concluded that human-induced climate change was a very serious threat to humans and ecological systems around the world and that the Earth is getting warmer as predicted by mainstream climate change science. . This Report was entitled “America’s Climate Choices 2011” (US Academy, 2011) Among other conclusions, this report found:
Climate change is occurring, is very likely caused primarily by the emission of greenhouse gases from human activities, and poses significant risks for a range of human and natural systems. Emissions continue to increase, which will result in further change and greater risks. In the judgment of this report’s authoring committee, the environmental, economic, and humanitarian risks posed by climate change indicate a pressing need for substantial action to limit the magnitude of climate change and to prepare for adapting to its impacts. (US Academy, 2011)
That the Earth is warming is based upon many robust lines of independent evidence including temperature measurements of many different types of land, ocean, and atmosphere, diminishing sea ice, retreating glaciers, the timing of the formation and melting of river ice, the timing and aerial extent of snow cover, and the migration of plants and animals among other things.
There are also multiple robust lines of evidence of human causation of this undeniable warming that the Earth is experiencing that include multiple attribution studies, fingerprint studies based upon how the Earth would warm if greenhouse gases are causing the warming compared to how it would warm is climate change is caused by the sun, volcanic activity or ocean cycles, carbon isotope analyses that have demonstrated that carbon compounds appearing the atmosphere can be traced to fossil fuel burning, and model results that achieve the best fit between prediction and observation when both natural climate forcing and greenhouse gas climate forcing are combined. (Forcing means changes in heat energy from a baseline year.)
B. There Will Be More Intense Droughts, Floods, and Destructive Storms As the Earth Warms.
As the Earth warms, more water continues to be transported into the atmosphere under the forces of evapotranspiration. The global water cycle is believed to be at least 4% wetter from recent warming. As a rule of thumb, every 1 degree F in temperature will result in 4% more water in the air.
Basic physics predicts that increased warming and atmospheric water content will lead to increased droughts, floods, and intense storms. More droughts will be experienced despite more water in the atmosphere because the atmosphere will become more turbulent creating a propensity to discharge the water in stronger storms. Also as the Earth warms it will also more quickly deplete soil moisture in some areas therefore further intensifying drought conditions. And so, some places will get wetter and some drier and some places may cycle between drought and floods. The precipitation increases due to more water in the atmosphere under some conditions will include both more snow and rain. As a result, recent heavy snows are predicted by climate change theory despite somewhat counter intuitive notions that snow should decrease in a warming world. Of course, as temperatures continue to arise eventually snow will decrease in some areas.
C. Atmospheric Winds and Global Temperatures Will Be Affected and Weather Will Become More Violent As Oceans Warm.
Winds are caused by uneven heat causing differences in high and low pressures and pressure differences are caused by differences in temperature. Cold air is denser and exerts more pressure than worm air. Therefore increased warming changes global wind patterns. As one observer sums this up in regard to ocean temperature:
Heat exchange between the ocean and atmosphere drive atmospheric circulation over the entire planet and modifies temperatures and atmospheric wind patterns. Ocean surface currents play an important role by redistributing some of the heat the ocean absorbs and affect local weather patterns around the world. Understanding this complex relationship between surface ocean currents and weather is important to understanding global climate patterns and the changes that they are undergoing due to global warming. . .. The ocean-atmosphere system is a delicate balance between incoming and outgoing energy. If this balance is upset, even slightly, global climate can undergo a series of complicated changes. (Missouri, 2011)
Both an increase in water vapor and a rise in temperature will boost a metric that meteorologists use to forecast severe thunderstorms, known as Convective Available Potential Energy, or “CAPE.” A higher CAPE value indicates that there is more potential energy in the atmosphere to fuel thunderstorm development, should some trigger come along and set them off.
D. Weather Is Becoming More Chaotic As Predicted.
Not only is more chaotic weather predicted by climate change, it is actually happening. There is strong scientific evidence that “indeed the weather has become more extreme, as expected, and that it is extremely likely that humans are a contributing cause.” (Romm, 2011)
We also know that oceans are warming due to human activities and that warmer oceans both lead to higher levels of moisture in the atmosphere and more energy in the climate system. (Romm, 2011) Romm, quoting Dr. Kevin Trenberth, Senior Scientists with the Climate Analysis Section of the National Center for Atmospheric Research describes the evidence of how recent temperatures rise in the oceans has affected storms.
The SSTs (Sea Surface Temperatures) in the Gulf have been running perhaps 2 deg F above pre 1970 values. Warm waters also extend across the tropical Atlantic north of the equator in the region favored for hurricanes, and hence the recent NOAA forecast for an above average hurricane season (although the La Nina is fading and will likely be over by August, so there may be more competition from the Pacific).
Of those 2 def F, 1 can be assigned to human influence. With 1F increase in SST there is 4% increase in water holding capacity over the oceans and hence in this case the plentiful supply of moisture means there is likely to have been 8% increase in moisture flowing in the southerlies into the warm sector, thereby acting as fuel for the thunderstorms, and thus increasing the likelihood they would become super cells, with the attendant risk of tornadoes. And of course heavy rains. In spring the westerly jet stream aloft and southerlies at the surface create a wind shear environment that is favorable for tornadoes as the wind shear can be turned into rotation. This part of the situation is largely in the realm of weather. The climate part is the warmth and moistness of the air flowing out of the Gulf and the resulting very unstable atmosphere. So while a big part of that is natural variability, a substantial part was anthropogenic global warming.
These changes are making the weather more violent . In fact a believed contributor to the recent severe weather is an unusually warm Gulf of Mexico, where sea surface temperatures are running 1 to 2.5 degrees Celsius above average. (Freedman, 2011) The Gulf is the main moisture source for storm systems as they move east from the Rockies, and the additional moisture is helping to fuel thunderstorm development.
E. Tornadoes are Caused By A Combination of A Certain Type of Atmospheric Instability and Wind Shear
Tornadoes are caused when certain atmospheric temperature conditions exist along with certain wind conditions.
Tornadoes form in unusually violent thunderstorms when there is sufficient (1) instability and (2) wind shear present in the lower atmosphere. Instability refers to unusually warm and humid conditions in the lower atmosphere, and possibly cooler than usual conditions in the upper atmosphere. Wind shear in this case refers to the wind direction changing, and the wind speed increasing, with height. An example would be a southerly wind of 15 mph at the surface, changing to a southwesterly or westerly wind of 50 mph at 5,000 feet altitude.
This kind of wind shear and instability usually exists only ahead of a cold front and low pressure system. The intense spinning of a tornado is partly the result of the updrafts and downdrafts in the thunderstorm (caused by the unstable air) interacting with the wind shear, causing a tilting of the wind shear to form an upright tornado vortex. Helping the process along, cyclonically flowing air around the cyclone, already slowly spinning in a counter-clockwise direction (in the Northern Hemisphere), converges inward toward the thunderstorm, causing it to spin faster. This is the same process that causes an ice skater to spin faster when she pulls her arms in toward her body.
(Weather Questions 2011)
And so tornadoes form when warm moist air collides with cold air that is moving at a different direction and speed under certain conditions. For this reason, the two main ingredients for tornado formation most often quoted are atmospheric instability and wind shear.
F. There Is A Likely Connection Between El Niño and La Niña and Tornadoes.
El Niño and La Niña are extreme phases of a naturally occurring ocean climate cycles referred to as El Niño/Southern Oscillation. Both terms refer to large-scale changes in sea-surface temperature across the eastern tropical Pacific. (Weather Almanac, 2011) Usually, sea-surface readings off South America’s west coast range from the 60s to 70s°F, while they exceed 80°F in the “warm pool” located in the central and western Pacific. Weather Almanac, 2011) This warm pool expands to cover the tropics during El Niño, but during La Niña, the easterly trade winds strengthen and cold upwelling along the equator and the west coast of South America intensifies. (Weather Almanac, 2011) Sea-surface temperatures along the equator can fall as low as 7°F below normal during La Niña. Both La Niña and El Niño impact global weather patterns. (Weather Almanac, 2011)
The scientific community has identified at least a weak correlation between El Niño and La Niña ocean circulation patterns and the frequency of tornadoes. As the US National Weather Service has recently concluded:
Since a strong jet stream is an important ingredient for severe weather, the position of the jet stream helps to determine the regions more likely to experience tornadoes. Contrasting El Niño and La Niña winters, the jet stream over the United States is considerably different. During El Niño the jet stream is oriented from west to east across the southern portion of the United States. Thus, this region becomes more susceptible to severe weather outbreaks during La Niña. During La Niña the jet stream and severe weather is likely to be farther north.
(Weather Almanac, 2011)
G. Evidence Exists That Climate Change Is Affecting the Intensity and Frequency of El Niño and La Niña Events.
Although the jury is still out over how climate change is affecting El Niño and La Niña events there is increasing evidence that climate change is already affecting the frequency and intensity of these ocean circulation cycles. According to Kevin Trenberth, a senior climate scientist with the National Centre for Atmospheric Research (NCAR):
There have been changes in the El Niño-La Niña cycle since the 1970s. It’s a complex cycle but the associated droughts, flooding and other manifestations have been stronger over the last 30 to 40 years……It would be surprising if there wasn’t an effect [between climate change and the strength of El Niño-La Niña cycles] .
(Leahy, 2011, quoting Trenberth)
Since climate change has fundamentally altered the global climate system, trapping more heat and about four percent more water vapor in the atmosphere, it is reasonable to conclude it is affecting the has affected the El Niño and La Niña cycle. For these reasons climate scientists believe a warming world may be increasing the intensity and frequency of these ocean cycles. If La Nina’s are increasing from climate change, it is reasonable to conclude tornado propagation will be affected.
III. Not All Atmospheric Conditions Required For Tornado Generation Will Necessarily Increase In A Warming World.
In order for tornadoes to form, several factors have to combine in just the right way. (Freedman, 2011) These ingredients include a warm and humid atmosphere, strong jet stream winds, and atmospheric wind shear (winds that vary with speed and/or direction with height), as well as a mechanism to ignite this volatile mixture of ingredients – such as a cold front. (Freedman, 2011). Neither trend data on tornado propagation nor our understanding of future atmospheric conditions as the Earth continues to warm allow strong predictions on the number and intensity of future tornadoes in a warming world. Some of the reasons for this uncertainty is as follows:
A. There is No Trend Data Supporting Greater Tornado Production Over A Long Enough Time To Draw Conclusions About Future Tornado.
There is no clear indication that severe thunderstorms and tornadoes have become more common due to climate change, in part because of major limitations in relying on the historical record of severe weather reports. (Freedman, 2011) While the number of tornadoes recorded in the U.S. has just about doubled during the past 50 years, the number of strong tornadoes (EF2 and above) has actually been decreasing. (Freedman, 2011) It may be the case that more tornadoes are being noticed today, given a network of trained storm spotters and a national Doppler radar network, both of which didn’t exist as recently as the early 1980s. (Freedman, 2011)
The Joplin tornado is the deadliest since modern record keeping began in 1950 and is ranked 8th among the deadliest tornadoes in U.S. history. (NOAA, 2011) Preliminary estimate is that there have been approximately 1,314 tornadoes so far this year. (NOAA, 2011). The previous yearly record number of tornadoes was set in 2004 with 1,817. (NOAA, 2011). The overall yearly average number of tornadoes for the past decade is 1,274. (NOAA, 2011) The preliminary estimated number of tornado fatalities so far this year is 512. (NOAA, 2011). There were 365 tornado fatalities before the Joplin event. There were 132 fatalities from the Joplin tornado. (NOAA, 2011) An additional 18 fatalities were reported for a tornado outbreak on May 24, 2011 (NOAA, 2011). 2011 is preliminarily ranked 7th among the deadliest tornado years in U.S. history. (NOAA, 2011)
And so trend data is inconclusive in regard to how climate change may be affecting tornado propagation. There appear to be more tornadoes recently but not necessarily stronger or more deadly tornadoes. Most likely it will be decades before sufficient trend data will emerge that allow scientists to what extent a warming world is affecting tornadoes in frequency and intensity. A warming world could increase frequency but not intensity or increase intensity but not frequency or have little affect on tornado propagation.
B. The Relative Strength Of Cold Fronts Could Be Reduced in A Warming World.
Because cold fronts colliding with warm moist air masses are optimal for tornado production, it is not clear what happens to tornado production as differences between artic and tropic temperatures decrease in a warming world. A warming world could actually reduce differences betweem Earth’s cold spots compared to warmest parts of our planet. Because tornadoes are propagated when warm moist air collides with cold air, future frequency of tornadoes are in question despite a warming world.
C. As the Planet Heats Up, Wind Shear May Not Increase.
Some modeling studies indicate that a warmer world may also have less wind shear, which is necessary in order to transform ordinary thundershowers into organized squall lines and supercells, capable of dropping large hailstones, producing damaging straight line winds, and spawning tornadoes. (Freedman, 2011) And so, if a warming world produces less wind sheer, tornado intensity and frequency may actually decrease as the world experiences other increased weather caused damages. It will take years if not decades before models can predict wind sheer. As one commentator has noted:
It will take years or even decades to build the scientific models that would allow us to actually attribute individual weather events to manmade global warming. This will be even tougher with tornadoes, where the historical records are less well understood than other extreme weather events. (Walsh, 2011)
And so it is possible that average frequency and intensity of tornadoes decrease in warming world. Only time will tell.
IV. The Ethical Obligation To Discuss Tornado/Climate Change Links Despite Scientific Uncertainty.
And so, as we have seen, there are scientifically sound reasons to conclude that as the world heats in response to human activities tornadoes will increase in intensity and/or frequency. That is there are both scientifically sound theoretical basis for linking increased warming with more frequent and intense tornadoes and empirical evidence supporting the theory. Yet there are some theoretical reasons for being doubtful that tornado activity will increase in frequency and intensity in a warming world and insufficient trend data to draw conclusions at this time. Yet, it is very likely that storm damage will increase in a warming world because of human activities but it is not clear that this increase in damages will be caused by increased tornado activity.
Because of the complexity of the climate system and the need to predict atmospheric conditions at specific locations to be able to predict future tornado activity, it is unlikely that strong proof about the causal connection will emerge in the near future. Compelling proof would require a much better understanding of how the timing and magnitude of local atmospheric conditions will change than our current modeling capability allows or decades of experience with tornado formation to be able to establish credible trend data.
And so, in summary, when it comes to tornadoes and climate change there is reason to believe that tornado caused destruction will increase due to human induced climate change and also reason for doubt.
Many commenting on the connection between climate change and destructive tornado equate the lack of proof with the lack of any scientific evidence. In so doing they are implicitly claiming only absolute proof counties as evidence. Yet, as we have seen, it would be untruthful to conclude there is no scientific basis for connecting climate change to more damaging tornadoes. Climate change will clearly enhance certain atmospheric conditions that should lead to more intense and frequent tornadoes while possibly diminishing others. Evidence of a connection exists despite lack of conclusive proof.
We also know, that any tornado that greatly harms people in a specific place and at a specific time would not likely have happened at that place and time without climate change because climate change has already changed aspects of the climate system so as to influence where and when tornadoes will form and with what intensity. That is although killer tornadoes may have formed in May of 2011 somewhere in the United States without climate change, the tornadoes experienced in Joplin and Tuscaloosa would not likely have formed at the same exact time and place in the absence of climate change because climate change has already transformed the initial conditions which trigger tornadoes. When and where a tornado is generated is dependent upon initial conditions and climate change has changed initial conditions around the world.
As Bill McKibben has stated there is now no place on Earth that is now unaffected by climate change and human activities. (McKibben, 1989). As a result all tornadoes have been affected somewhat by global climate change although tornado frequency and intensity need not have increased.
In a warming world it is possible that for some time periods tornado activity may increase while decreasing in other periods. Yet is not truthful to claim there is no connection between climate change and tornadoes because all weather events are being affected by climate change and some of the atmospheric conditions needed to propagate tornadoes have been enhanced by climate change . Yet it is not proven that tornadoes will in general increase in intensity and frequency.
It would appear that many weatherman and the media claim that there is no connection between climate change and tornadoes because of the absence of proof of increases in tornado frequency and intensity from climate change. Yet it is simply not true that there is no scientific basis for being concerned that tornado damage may increase as human activities warm the planet. And so it is untruthful to say that there is no evidence of a connection although truthful to claim there is no proof of increased intensity and frequency of tornadoes.. That is, it is truthful to claim that there is no absolute proof that climate change is causing more intense and frequent tornadoes. Yet it is not true to claim that there is no evidence of scientific links between climate change and tornado damage.
Given this, a strong case can be made that when talking about the connection between climate change and tornadoes there is a duty to warn people of the possible connection even in the absence of absolute proof once it is established scientifically that behavior causing climate change is increasing the risk of harm from tornadoes. This is particularly true in cases when waiting for the proof will make it too late to avoid the harm from risky behavior.
To fully understand this it is helpful to understand why climate change is essentially an ethical problem. Climate change is an ethical problem because: (a) Some people in some parts of the world are putting others at risk, (b) The harms to those at risk could be catastrophic, and (c) Most of the victims of climate change can do little to avoid harm, they must rely on a sense of justice will motivate those who are putting others at risk to reduce their climate changing causing behavior.
For this reason, since we now know that it is scientifically plausible that tornado frequency and intensity will increase as the world warms and climate change is already affecting timing, location, and intensity of tornadoes that will form, it is not ethically acceptable to assert there is no link because such a claim implies that there is no scientifically valid basis for concern or risk. To understand why this it is ethically problematic to deny evidence. it is necessary to review the ethics of dangerous behavior.
From a proposition that a problem like global warming creates a particular threat or risk, one cannot, however, deduce whether that threat is acceptable without first deciding on certain criteria for acceptability. The criteria of acceptability must be understood as an ethical rather than a scientific question. For instance, although science may conclude that a certain increased exposure to solar radiation may increase the risk of skin cancer by one new cancer in every hundred people, science cannot say whether this additional risk is acceptable because science describes facts and cannot generate prescriptive guidance by itself. The scientific understanding of the nature of the threat, of course, is not irrelevant to the ethical question of whether the risk is ethically acceptable, but science alone cannot tell society what it should do about various threats. In environmental controversies such as global warming where there is legitimate concern, important ethical questions arise when scientific uncertainty prevents unambiguous predictions of human health and environmental consequences. This is so because decision-makers or those engaged in risky behavior cannot duck ethical questions such as how conservative “should” scientific assumptions be in the face of uncertainty or who “should” bear the burden of proof about harm. To ignore these questions is to decide to expose human health and the environment to real risk before changing the risky behavior, that is, a decision to not act on a serious environmental threat has serious potential consequences. Science alone cannot tell us what assumptions or concerns should be considered in making a judgment about what to do about potentially dangerous behavior. This is an ethical question. And so from the standpoint of ethics, potential risks are relevant to what should be done.
For this reason, environmental decisions in the face of scientific uncertainty must be understood to raise a mixture of ethical and scientific questions. Yet, the scientific skeptics on global warming or those denying connections between tornadoes and climate change often speak as if it is irrational to talk about duties to reduce greenhouse gases until science is capable of proving with high levels of certainty what actual damages will be.
That only proven facts should count about dangerous behavior can be shown to be ethically problematic by looking at how societies often deal with other kinds of unsafe behavior. For instance, many societies make dangerous behavior criminal such as dangerous driving, irresponsible use of hazardous substances, and negligently setting fire to a forest. Many social norms about dangerous behavior can be found in various cultures that recognize that burdens of proof and quantity of proof should shift depending upon what is at stake, who has been harmed, whether society can wait until the uncertainties are resolved, whether those harmed by the decision have consented to be put at risk. In other words, when the burden of proof should shift to those proposing to do something dangerous or how much proof should satisfy the burden of proof are ethical questions that need to take into consideration many different factors. Because these are ethical questions, they cannot be answered by an algorithm or a “value-neutral” scientific calculation. That is, there is no escaping asking the question what is the right thing to do given the uncertainty about links between tornadoes and climate change.
Because scientists are expected to produce scientific knowledge that can be applied to public policy questions, they must be able to describe threats that are not fully proven. From the standpoint of public policy, therefore, scientists should not deny that climate change creates risks of increased damage from tornadoes. A claim that there is no link between climate change and tornadoes is misleading. If someone is concerned about whether to adopt policies reducing the threat of climate change they need to know whether climate change creates risks of damage from tornadoes even if there are open questions about what happens to tornado frequency and intensity in a warming world.
In other words, when science is applied to public policy where there is reasonable basis that some human activity is dangerous, science has an important role in communicating any scientifically plausible dangerous risks-not just proven facts.
As long as anyone is asking the question of whether there is a link between climate change and tornado damage because they want to know whether there is reason to limit greenhouse gas emissions, it is therefore ethically problematic to say there is no link
However, it is also ethically required to acknowledge that increased tornado damage and frequency are not yet proven. When talking about these risks it is important to acknowledge that there is also scientific basis for doubt about increased tornado and frequency in a warming world. However, if this said, it is also ethically important to acknowledge that increased damage from other kinds of storms is virtually certain as the planet warms. Furthermore, it is ethically important to acknowledge that tornadoes will appear in places that they would not likely occur in the absence of global warming even if tornado frequency and intensity decrease because a changing climate is already affecting tornado propagation.
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Donald A. Brown
Associate Professor, Environmental Ethics, Science, and Law,
Penn State Univesity