Changes in Extreme Weather Events?

Introduction

One of the points often brought up in arguments as to why we must reduce greenhouse gas (GHG) emissions is that global warming due to GHG increases will cause an increase in extreme weather events, such as stronger hurricanes, more tornadoes, more severe droughts, and more severe flooding. The simple line of reasoning is that if the temperature increases, there could potentially be more heavy rain events, since warmer air can contain more water vapor (remember the the relationship between saturation vapor pressure and temperature). One can also make the argument that if the temperature increases, there could be more severe droughts, since where it is not raining, the rate of evaporation is faster at higher temperature, which could dry these regions more. However, the occurence of floods and droughts depends on other variables beside temperature, such as weather patterns, so the prediction of this change is more complicated than the simple argument. For hurricanes, it is simply argued that warmer ocean water will lead to more and stronger hurricanes because we know that hurricanes need warm water to form. Remember that we covered this issue in module one. Even though there has been global warming and measurable warmer ocean surface temperature, the oberservational evidence does not show in increase in either the number or intensity of hurricanes. For tornadoes, warmer surface air can contain more water vapor and may be more unstable. But again, as discussed in module 2, the warming so far has not lead to more or stronger tornadoes in recent years.

So while simple arguments can be made as to why global warming may result in more extreme weather events, up until this point in time, even though there has been measured global warming, the observational evidence does not necessarily show that extreme weather events have increased in either number or severity. But this has not prevented some in the media and even some textbooks from presenting this as a fact ... that it is already happening and is sure to get worse in the future with continued global warming. Have you ever seen a documentary on severe weather? Those shows always end by claiming these events are only going to get worse as the Earth warms due to GHG emissions.

Almost anytime there is a particularly severe and damaging weather event, there seem to be no shortage of people who claim it was either entirely due to global warming or was made worse by global warming. Somehow they forget that severe weather events have always happened and are part of Earth's climate history. Those who repeatedly make this claim are practicing a sort of self-fulfilling prophesy. Everytime there is an extreme weather event, they say it is "evidence" of the impact of GHG emissions.

This is an important debate to have, as possible increases in severe weather are often given as a strong reason why we need to take immediate action to reduce GHG emissions. This is not to say that there are not other good reasons to reduce emissions or that it is not possible for global warming to lead to more or stronger extreme weather events in the future. The point is that the available evidence does not clearly support that it has happened yet and it is not certain to happen in the future.

Below is another angle regarding the uncertainty of this claim, which was part of a reading page that we did not cover this semester.

Long Term Trends in Worldwide Extreme Weather Events?

There have been many recent news stories about extreme weather events and their effects on people both within the United States and around the world. A recurring question is whether or not these extreme weather events are becoming more frequent and severe in recent years and if so what might be the causing it. This question is briefly discussed in this section, however, a definitive answer is not given as there continues to be much scientific debate on this issue. I encourage all of you to keep an open mind on this issue, especially when considering the arguments of anyone who claims to know for sure.

One question to address is how well can we determine if the number of extreme weather events is actually increasing or even changing significantly with time? In order to answer that we would need to have some idea about what is "normal" or "expected." By definition extreme events are rare. We also know from history that extreme weather events have always happened and that they are not evenly distributed in time, e.g., some past decades had more extreme events than others. Are these past fluctuations in extreme weather events randomly distributed or are there some underlying patterns in time? We know that there are natural cycles of climate oscillations that operate on a range of different time scales. For example, El Nino/La Nina oscillations happen on timescales of a few years; Hurricanes in the north Atlantic Ocean oscillate over multi-decadal timescales, which is something we covered earlier; Ice Age cycles have been occurring over timescales of about 100,000 years. All of these natural climate oscillations likely influence weather patterns and hence influence the liklihood of extreme weather events. We certainly do not understand all of these natural climate cycles that occur on Earth and it is likely that they interact with each other. In addition, given that some of these natural climate cycles have only recently been recognized, there are probably other cycles that we do not even know about. Our incomplete understanding of these cycles combined with the fact that good observations of extreme weather events have only been available for recent decades, makes it difficult for us to determine if there really are more extreme weather events today than there "should be." One must also keep in mind that there are more people and more weather observations than ever. Unnoticed extreme events in the past (perhaps because there were no people living where the event took place) can make it appear as if there are increasing numbers of extreme events in recent years, even when there may not be a real change in the number of extreme weather events.

Let's go though a hypothetical example that illustrates some of the potential problems with having only a limited understanding of the underlying statistics on extreme weather events. Assume that there were more extreme weather events in the first decade of the 2000s compared to the decades of the 1980s and 1990s. In this example the actual trend is increasing over this time period, so one may jump to the conclusion that this will continue with higher numbers of extreme events as we move forward in time. However, this is based on only 30 years of knowledge. The recent increase may all be part of a much longer period cycle ... perhaps the occurrence of extreme weather events peaks every 50 years with less frequent extreme weather events between the peaks. One point is that we do not have enough understanding of the climate system and natural climate variability to answer questions similar to the hypothetical question presented here. In other words, we do not have enough information to determine whether or not there has actually been any significant change in the number or intensity of extreme weather events when put into the context of the entire history of extreme weather events that have happened on Earth. It is important to realize that our lives are short compared to some of the natural cycles of climate variability. We do not experience the full range of weather and climate extremes over a lifetime. We tend to think that our notion of climate is the way things "should be" or way things "have always been." Therefore, we can easily misinterpret changes from our perspective.

Whenever a disaster occurs, people wonder what caused it to happen. Prior to the large increase in greenhouse gases due to human activity (pre 1950), we can definitely say that extreme weather events happened naturally, i.e., they were not influenced much by human activity. Today, many people are quick to attribute extreme weather events to increased greenhouse gases. However, it is difficult to show that there has been a statistically significant change in extreme weather events in recent decades due to our lack of knowledge concerning the historical occurences (natural variability) of these events. In other words, how can we be sure that a particular extreme event would not have happened (or that it would not have been as severe) if humans did not add greenhouse gases to the atmosphere? Does this mean that increased greenhouse gases have not had a significant effect on extreme weather events? No! All we can say now is that based on our current knowledge of past climate cycles and extreme weather events, we are unable to determine if recent severe weather events are outside the range of what would occur naturally.

The idea that extreme weather events are more common and severe today and that these events will only get worse in the future because of human-added greenhouse gases is not wild speculation though. One simple line of reasoning is this ... adding greenhouse gases should cause the surface temperature of the Earth to increase, which would cause the rate of evaporation to increase. Thus, in places where it does not rain, droughts may be more severe, and since warmer air is able to hold more water vapor, in places where it does rain flooding may be more severe. These conclusions, while plausible, are far from certain. There have also been several recent studies using climate models which indicate that extreme weather events become more common and severe after greenhouse gases have been added to the atmosphere. These modeling studies do support the possibility that human-added greenhouse gases may significantly influence extreme weather events, however they cannot be used a scientific proof, since climate models are not reality.

Here is a local example about possible attribution of severe weather and climate events. We know that there have periodic severe droughts in the western United States. Evidence for this abounds, which includes tree-ring observations done right here at the University of Arizona. Based on historical evidence we can expect more severe droughts in the western United States. In other words periodic extreme droughts are normal for this region. The most severe of these droughts can be separated by hundreds of years. Thus, it is very possible that there are many people that have never experienced a severe drought, nor have their recent ancestors such as parents and grandparents. Now suppose an extreme drought does occur. People who have never experienced a drought (or do not know climate history) think this is not normal and there must be a reason this change is happening. This is the point where someone could claim that the drought was caused by humans, possibly due to greenhouse gas emissions and their subsequent influence on climate. I hope you can see why this would be difficult to prove. Of course it would also be difficult to prove that severe weather events today are not at all influenced by the higher levels of greenhouse gases in the atmosphere. There are those who say that a good reason to reduce our emissions of greenhouse gases is because increased greenhouse gases will either cause droughts in the western United States or make drought more severe in spite of the fact that it is difficult to scientifically prove that reducing greenhouse gas emissions will prevent or minimize future droughts in the western United States. Perhaps a more prudent course of action would be to prepare for severe droughts in the western United States regardless of whether or not we can prove that they will be influenced by higher levels of greenhouse gases because we know from the past that they will happen again.

Two Perspectives on Recent Trends in Extreme Weather Events

Perspective One

The Intergovernmental Panel on Climate Change (IPCC) has released a set of reports, which comprise the Fifth Assessment Report (AR5) on climate change. Material from the Summary for Policymakers will be referenced in this section. The IPCC has modified some of its statements about recent changes in extreme weather events in this latest report.

In the Fourth Assessment Report (AR4) published in 2007, the following claim was made "it is likely to very likely that events such as heat waves, floods, droughts, and tropical storms have become more severe since 1960." Based on my personal research and some of the arguments presented above, I think these claims are difficult to prove given our rather poor understanding of historical climate and severe weather events and our poor understanding of the mechanisms driving past natural climate changes and fluctuations in extreme weather events. Given that these extreme weather events occur so infrequently, we would need many more years of good observations before we understand the natural variability of these events. And we need to understand the natural variation in order to determine if there has been some significant change since 1960. Here we are just considering the question as to whether or not there are provable changes in the frequency and severity of extreme weather events, without speculating on the cause of the (possible) changes. Of course, the IPCC reports go further in concluding that recent climate changes are dominated by greenhouse gas emissions from human activities.

In Table SPM.1 in the 2013 Summary to Policy Makers, the following information about the possible changes in severe weather since the year 1950 is presented. First, "medium confidence" that heat waves have increased worldwide and "likely" that heat waves have increased in Europe, Asia, and Australia. Second, "low confidence" that the intensity and duration of drought has increased. Third, "low confidence" of increases in tropical storm activity globally, but "virtually certain" in the North Atlantic since 1970. (Instructor's Note. It is interesting to me that 1970 was near the beginning of the less active part of the Atlantic multi-decadal cycle in hurricanes, which means that the observed increase in north Atlantic hurricanes from 1970 to today could be largely explained by the multi-decadal cycle.) The latest AR5 has backed away from many of the claims of increased severe weather events in recent times that were made in earlier reports, due to criticism that the claims could not be backed up with the available evidence. The previous conclusion based on the 2007 report, either implicitly or explicity stated, was that extreme weather events were becoming more severe as a result of warmer surface temperatures. Unfortunately, this connection has been accepted by the popular media and many in the public believe that there is a proven connection between higher surface temperature and extreme weather events. It is commonly presented as fact in some textbooks and TV "science" shows that extreme weather events are becoming more severe and that if the surface temperature were to increase further, we can expect even more severe weather events. You should understand that this connection is far from scientifically established and for the most part cannot be supported by available data. In this discussion we avoided the question of why surface temperature increased between 1900 and today and are just considering if there is observational evidence that proves extreme weather events are becoming more severe.

Beside the International IPCC report, there are reports by US government agencies that claim there are climate change impacts that are already easily observable (e.g., see this EPA page on climate change indicators). In many of these reports, the reader is lead to believe, either explicitly or implicitly, that the climate changes mentioned are largely the result of global warming caused by human additions of greenhouse gases.

Skeptical Reaction to Perspective One

First, it can be quite difficult to prove that climate has changed significantly given our incomplete knowledge of past climates and our incomplete understanding of how the climate system operates. Then on top of that we would need to prove that the climate changes were significantly influenced by higher levels of greenhouse gases. Most of the climate changes discussed in the EPA reports would be considered negative or bad effects of climate changes and end up being discussed in context of endangered species, forest resources, water availability, disaster planning, and the like. Often these possible bad effects of climate change are given as reasons to guide policy on greenhouse gas emissions in spite of the lack of scientific proof connecting them with increases in greenhouse gases.

Does this rule out a connection between higher surface temperature and extreme events? The answer is no. However, what we can say is that if there is a connnection, the connection so far has been too weak for us to distinguish statistically significant changes in extreme events from natural variations in these events based on available observations. Many of the studies that conclude a warmer surface temperature on Earth will lead to more frequent and intense severe weather events are based on computer models of climate. While these models are good tools that can help us to better understand some of the processes that regulate climate, their projections are not scientific proof for how climate is going to change. Their results cannot be verified with empirical evidence. Scientific proof requires that repeatable experiments can be performed to verify predictions. If you are interested in this discussion, a good debate among scientists is contained in The uncertainty in climate modeling from the Bulletin of Atomic Scientists. This is not required reading and students will not be tested on the material discussed in the article.

Perspective Two

On the other hand, there are many climate scientists who do not fully agree with the reports published by government agencies. It is important to realize that there is nowhere near a scientific consensus that fully agrees with the conclusions and predictions of future climate change made within the IPCC report. A comprehensive report, which challenges some of the claims made within the IPCC reports, has been published by the Nongovernmental International Panel on Climate Change (NIPCC), Climate Change Reconsidered II: Physical Science and was last updated in 2013. The full report is over 1200 pages and was written by a team of over 50 international climate scientists. There is also a shorter "Summary for Policymakers" similar to that produced by the IPCC. Quoting the first two paragraphs from the NIPCC Summary for Policymakers:

"The [NIPCC] is an international panel of scientists and and scholars who came together to understand the causes and consequences of climate change. NIPCC has no formal attachment to or sponsorship from any government or governmental agency. It is wholly independent of political pressures and influences and therefore is not predisposed to produce politically motivated conclusions or policy recommendations.
NIPCC seeks to objectively analyze and interpret data and facts without conforming to any specific agenda. This organizational structure and purpose stand in contrast to those of the United Nationsântergovernmental Panel on Climate Change (IPCC), which is government-sponsored, politically motivated, and predisposed to believing that climate change is a problem in need of a U.N. solution."

Chapter 7 in the NIPCC Report, Observations: Extreme Weather specifically addresses the debate about possible increases in severe weather due to global warming. Historical observations of exteme weather events to look for trends related past and current temperature changes. If you are interested in this question, I encourage you to look over the chapter and consider the evidence presented. Below is a list of the key findings quoted from chapter 7.

"Air temperature variability decreases as mean air temperature rises, on all time scales.

Therefore the claim that global warming will lead to more extremes of climate and weather, including of temperature itself, seems theoretically unsound; the claim is also unsupported by empirical evidence.

Although specific regions have experienced significant changes in the intensity or number of extreme events over the twentieth century, for the globe as a whole no relationship exists between such events and global warming over the past 100 years.

Observations from across the planet demonstrate droughts have not become more extreme or erratic in response to global warming. In most cases, the worst droughts in recorded meteorological history were much milder than droughts that occurred periodically during much colder times.

There is little or no evidence that precipitation will become more variable and intense in a warming world; indeed, some observations show just the opposite.

There has been no significant increase in either the frequency or intensity of stormy weather in the modern era.

Despite the supposedly unprecedented warming of the twentieth century, there has been no increase in the intensity or frequency of tropical cyclones globally or in any of the specific ocean basins."

Another piece that presents a slighltly different perspective on the possible connection between global warming, changes in exteme weather impacts, and the impacts on human life is linked below. This piece is from the Global Warming Policy Foundation (GWPF), which is a think tank. The stated purpose of the GWPF (taken from their web page) is "to help restore balance and trust in the climate debate that is frequently distorted by prejudice and exaggeration." This group is often critical of the information and reports published by the Intergovernmental Panel on Climate Change (IPCC). You should keep that in mind when reading the essay below. Also keep in mind that this article was written to respond to some of the claims made in the 2007 IPCC AR4 report on climate change. The article, A Primer on the Global Death Toll from Extreme Weather Events - Context and Long Term (1900-2008), claims that even if the IPCC is correct and there have been and will continue to be more extreme weather events due to global warming from GreenHouse Gas (GHG) emissions, the subsequent toll on human life and property has been and will continue to decrease mainly due to technological advances that were only possible due to the use of fossil fuels. The point made in the article is that even if there have been more extreme weather events in recent years due to global warming from GHG emisssion, many fewer people die from these events simply due to our advanced technology, which was made possible by using fossil fuels. In other words is the overall impact of GHG emissions on severe weather events that more or fewer people die from these events?