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Introduction to Global Warming and Climate Change

Definition of Global Warming and Focus of this Class

Global warming refers to an increase in the global average surface temperature of the Planet Earth. Unfortunately, there is quite a bit of confusion about whether "Global Warming" means any increase in the average temperature of the Earth or specifically only temperature increases that are due to human activities. We can measure changes in the global average temperature of the Earth's surface with some accuracy (and we find that the average temperature has increased between 1900 and today), but we cannot determine very well how much of the temperature change is caused by human activities and how much is part of a natural fluctuation not related to human activity. We need to be clear on the definition of global warming whenever discussing this issue. For example, if someone were to ask me if I "believe" in global warming, I would say yes the measured global average surface temperature has increased over the last 100 years so in that sense I agree. However, if someone were to ask me if I "believe" that the increase in global average surface temperature is mostly caused by human activities, then the best answer I can give is that I am unsure that the measured temperature changes are mostly the result of human activity. In this class, global warming is taken to mean any increase in the average surface temperature of the Earth, regardless of the reason why the temperature is increasing. Using this definition, there has been observed (measured) warming of the global average surface temperature on Earth since 1900.

Many human activities may contribute to global warming or other climate changes, which include anthropogenic emissions of greenhouse gases (enhancing the natural atmospheric greenhouse effect), anthropological changes in land use (for example, deforestation or city-building), and many other human activities. In general, we should probably include any climate response to the anthropogenic forcing in this discussion. For example, if the average global surface temperature goes up, this will shift weather patterns causing some areas to warm, some areas to cool, some areas to become wetter, and some areas to become drier. No one expects that all areas will be affected in the same way by anthropogenic perturbations. Perhaps it would have been better to use a term such as "anthropogenically-induced climate changes" rather than global warming, which seems to imply that everywhere will get warmer by the same amount. In general, many human activities have the potential to impact climate in more ways than simply increasing the global average surface temperature. We do not have time to address all the possible impacts of human activity on climate. In this class, we will focus our attention on one specific aspect of possible human-caused climate change: What is the impact of human emissions of greenhouse gases on climate and climate change?

Fundamental Questions to Consider

The message to the general public concerning the impacts of human-emitted greenhouse gases is often oversimplified concerning both the potential problem and its potential solution. Yes, there is a significant greenhouse effect on Earth with the result that the Earth's surface temperature is much warmer than it would otherwise be due to the presence of greenhouse gases. Yes, human activity has been responsible for increasing the concentration of several greenhouse gases in the atmosphere. Yes, an increase in greenhouse gases would be expected to result in some warming of the Earth. However, these facts do not prove that recently observed global warming (and other recently observed changes in climate) have been mostly caused by human emissions of greenhouse gases.

Question #1. How much of the recent increase in global average temperature is due to increases in greenhouse gases?

There is widespread disagreement about how much of the recently observed climate changes are due to increases in greenhouse gases and how much of the observed climate changes may be due to natural variations in climate. The climate system is very complex and there is much that we do not understand about it. One thing that we do know is that significant, natural climate changes have occurred throughout Earth's history, i.e., without human influence. It is wrong to think that climate would be constant without human influence. It is very difficult, if not impossible, to separate recent climate change into two components: one due to human emissions of greenhouse gases and one due to natural climate change. In other words, it is difficult to determine the contribution to climate change due to greenhouse gas emissions given that we are observing climate changes in a system that has always been changing, including periods when greenhouse gases were steady.

Question #2. How much will climate change in the 21st century?

Obviously, if we are unable to answer question #1 with much certainty, there is even greater uncertainty in trying to predict how the combination of increased greenhouse gases and natural climate changes will act to change future climate. In spite of the great uncertainty, this is an important question. Knowing future climate changes would allow us to prepare for the changes and possibly mitigate the changes that might be caused by greenhouse gas emissions.

Question #3. Is warming "bad"?

Regardless of the reason for the recently observed warming, there are questions about the overall impacts of a warming Earth on its inhabitants, including humans. The Earth has gone through countless warming and cooling periods throughout history and for the most part life has flourished. Some have actually argued that continued warming would result in an overall benefit for humans, while others say continued warming will be catastrophic for humans and many other species. It seems somewhat arbitrary to conclude that warming is "bad" and cooling is "good" considering the Earth's climate history.

Question #4. What, if anything, can we do about it?

This is where a possible solution is often oversimplified. The popular message seems to be that if we reduce or eliminate greenhouse gas emissions, then the climate will stop warming. This is far from certain. If the recent warming is mostly natural, with only a small influence from greenhouse gas emissions, then the warming may continue regardless of greenhouse gas emissions. In addition, if current climate model predictions are correct, warming will continue due to greenhouse gases that have already been released into the atmosphere, since the models indicate that there is a time lag between when greenhouse gases increase and the warming of the surface. Thus, actions that only modestly reduce greenhouse gas emissions will have little impact on slowing global warming according to the climate model predictions.

All four of these questions are complex and intensely debated. None of them can be currently answered using a scientific method that can produce provable answers and predictions.

In spite of the large uncertainty, it is important to consider the possible magnitude of human-caused climate change. There is almost no doubt that human activity, including the emissions of greenhouse gases, has and will continue to have an influence on climate and climate change. But how much? If human-caused climate changes are small compared to natural cycles of climate change, then we might conclude that the benefits of fossil fuel usage outweigh the slight climate changes due to higher levels of greenhouse gases. However, if human emissions of greenhouse gases were to lead to harmful climate changes for humans and other life that are outside the range of past natural climate changes, then we might conclude that we should do whatever it takes to reduce greenhouse gas emissions, even if this requires us to give up many of the benefits of fossil fuel use.

This issue, whether or not human activity is causing significant global warmining and what, if anything, we should do about it, is hotly debated and will continue to be for the foreseeable future. Some of you may find yourself in a position either in government or industry where you will have to make a decision that takes into account how human activities may affect the environment in which we live (both short-term and long-term changes). Even if all you do is vote for a particular issue or candidate based on the their stance about global warming, I think it is important that you can all make informed decisions. Too often, we are only told part of the story, especially by those who would like to sway opinions over to their point of view. If you don't know better, their arguments can be quite convincing. You need to understand this issue well enough to be able to make up your own mind about how to act.

As we go along, I will try to point out some common misconceptions about global warming. One that I would like to mention at the outset is that global warming and the ozone hole are not the same thing. Global warming due to greenhouse gas emissions and stratospheric ozone depletion due to CFCs are two separate environmental issues that can be discussed independently.

Brief Restatement

Global warming and climate change are real and are currently happening on Earth. But the role of humans in that change is uncertain and debatable. The connection between climate changes and human greenhouse gas emissions is uncertain because the Earth's clmate system has changed naturally throughout time and there is much that we do not understand about these natural changes. Today's climate is a mixture of both human-caused and natural climate change and we cannot separate the two effects with certainty. We are unable to definitively determine the impacts of human activity on recently observed climate changes against the long history of natural climate variability on Earth.

What is Climate Change?

We will define climate change as any significant shift in the atmospheric component of the climate system, which occurs at the spatial scale of a regionally-defined climate zone. We will take climate change to include both changes due to natural causes and changes due to human activities. Keep in mind that many people define climate change as a change in climate caused by human activity. Again when discussing this issue we need to be clear about what we mean by climate change.

We need to clearly define the spatial scales that will be used to determine climate change. Many times we are presented with data that shows only a change in the global average surface temperature. Although this may be relevant for something like global changes in sea level, a change in global average surface temperature is not enough to tell us about how life may be affected in smaller climate zones where organisms spend the majority of their lives. To understand the possible impacts of climate change, we need to look at smaller regions. An example of a climate classification system for the Earth is the Koppen map of world climates (linked from Wikipedia). We could say that climate change would mean a change in the distribution of the defined climate zones in this classification system. But even this would sometimes be too restrictive. For example, note that in the Koppen system the deserts of the southwestern United States and Mexico are classed in the same category as the Sahara Desert (warm, desert climate). However, we know that the climates are different. The Sahara is in general both hotter and drier than the deserts in northwest Mexico and the southwestern United States. If the deserts of the southwestern United States became like the Sahara that would be a huge climate change locally, but this would not change the Koppen class.

Keep in mind that some talk about climate change only in terms of the changes in the average surface temperature of the Earth. You may have heard predictions such as "the global average temperature is expected to increase by 4°C over the next 100 years." The change in global average temperature is only part of the story. A change in the global average surface temperature does not mean all regions of the Earth will experience the same change in temperature. In some areas, the average temperature may change drastically, while in other areas, no changes may occur. Also keep in mind that temperature is only one component of climate as discussed above. There can also be changes in precipitation, winds, cloud cover, and other components of climate. There are many different climate zones around the Earth. It is the regional changes in a particular climate zone that are most important to the inhabitants of that area, not the change in global average temperature. The ultimate question is will the inhabitants of a particular ecosystem be able to adapt to the changes? This is why for this class we will define climate changes as occurring on small regional scales, like a moderately-sized ecosystem, as opposed to the entire planet Earth. We must track regional climate changes if we are going to understand how life will be affected by climate change.

While I believe it best to define climate changes at the regional scale, increases in greenhouse gases caused by human activity do alter the composition of the atmosphere globally because once those gases are released they spread quickly around the globe. Thus, if increases in greenhouse gases do significantly alter the natural global system, then we should expect that most climate zones will be affected worldwide. However, there is no reason to believe that future climate changes will necessarily be harmful or detrimental for all different climate zones. There are issues, such as changes in global sea level, that are tied to changes in the global average temperature. For these reasons, some prefer to define climate change as any long-term alteration in global weather patterns ... though ultimately changes in global weather patterns will result in climate changes within many individual climate zones.

It is not just changes in the average temperature for a region that determines climate change. Other examples of climatic components were listed in the previous lecture page and include things like precipitation, frequency of droughts, frequency of severe storms, etc. The distribution and frequency of extreme events is more important in determining what type of life can exist in a given ecosystem than the average conditions, since those are the conditions that will likely cause the most stress for life. All organisms do well when temperatures are near average, but can be stressed when the temperature is extremely high or extremely low. In general the distribution of extreme events can change even if the average conditions do not. You should also realize that even a relatively small change in the average conditions can correspond to relatively larger changes in the frequency of extreme events, since most climate variables fall into a Gaussian shaped distribution. This is important to understand because many people do not realize that a seemingly small change in say the average temperature in a climate zone can rather have a large impact on the life that lives there. An illustrative example is shown below. In the example, a small increase in the average temperature (from 50°F to 55°F), resulted in more than a 100% increase in the probablility that the temperature will be above 80°F. All plants and animals would have to endure this climate change in temperature. Suppose there is a particular plant that experiences severe heat stress when the temperature is above 80°F. With this climate change, there will be a large percentage increase in the number of days above 80°F, and this plant may not survive. This may affect animals that rely on this plant as well. The point is that a 5°F increase in the average temperature does not simply mean it gets 5°F warmer every day ... as the liklihood of extreme temperature can change more than the average.

An illustration of how a small change in the mean or average value of a meteorological variable can have a large effect on the expected number of extreme readings.

UPPER FIGURE (black curve, A): probability of different temperature readings when the mean temperature is 50°F. (blue curve, B): the same, when the mean temperature rises 5°F to 55°F. The shape or spread of the bell-shaped pattern by which expected values are distributed on either side of the mean is the same in both cases. Curve B is drawn by simply sliding curve A to the right by 5°F.

LOWER FIGURE: the corresponding changes in the probability of the temperature readings if the temperature distribution were to change from curve A to curve B. The solid blue curve (with scale at left) is the difference in probability of the of various temperatures as shown on the bottom scale. The greatest difference is for temperatures about 10°F above and below the mean value of 55°F. The dashed blue curve (scale at right) is the percentage change in probability. Readings of 60° to 70°, for example, are expected roughly twice as often as before. Notice that the largest percentage change in temperature are for those tempertures that happen on both extremes of the distribution (large percentage increase in extremely warm days and large percentage decrease in extremely cold days). Thus, a seemingly small increase in the average temperature, resulted in a large percentage increase in the number of days with extremely warm temperatures and a large percentage decrease in the number of days with extremely cold temperatures. There is a much smaller percentage change for temperatures near the average.

We know that all regions of the Earth have experienced climate changes throughout history and that until recently, all of these changes were due to natural as opposed to human causes. We can explain some of the reasons for past climate changes (for example, ice age cycles are probably related to slight variaitions in the Earth's orbit about the Sun), but we certainly cannot explain everything because the climate system is so complex. We also know that past climate changes have had a significant impact on the distribution evolution of the various forms of life on the planet. This works both ways though because life itself also helps to determine climate.

Uncertainty Makes it Difficult to Agree on How to Act

The big questions today are (1)how are human activities affecting climates? and (2)what, if anything, should we do about it? Because the climate system is so complex and by no means fully understood, we are forced to try to address the latter question before we can definitively answer the first. The basic issue with the first question is that we are unable to separate natural climate changes from human-caused climate changes and we should not expect a definitive answer to that question in the near future. However, if human activity is responsible for detrimental climate changes, the longer we wait to act the worse the situation is likely to become. There will be no way to quickly reverse climate changes. Another issue associated with the second question is that to be effective, action will require global scale cooperation. This because once greenhouse gases are emitted they spread around the globe. So just moving emission from one place to another will not have much effect.

This uncertainty makes it hard for people to come to an agreement. On one extreme some believe that humans should make drastic changes, such as eliminating the use of all fossil fuels, to minimize our influence on climate (either as a precautionary measure or based on a belief that all human meddling is bad), while on the other extreme some don't worry at all about climate change, arguing that humans have a right to exploit the Earth, and even if we are causing climate changes, human beings and other life will just have to adapt to the changes (life on Earth has survived large climate shifts in the past). I think most people fall somewhere between these two extremes, but there remains a huge spread in public opinion on this issue. The title of following report from the Pew Research Group, Polls show most Americans believe in climate change, but give it low priority, sums up the most common attitudes of the American public. According to the survey, while 61% of Americans believe there is solid evidence that the Earth is getting warmer, only 40% of Americans believe that warming has been primarily caused by human activity. As you may guess, there is a sharp divide among different political groups on this issue. According to the Pew article, "Nearly all Solid Liberals (96%) and just 21% of Steadfast Conservatives said the U.S. should do whatever it takes [to protect the environment]." A poll in 2015, which asked Americans to rank what they consider to be the top priorities for government in 2015, Public’s Policy Priorities Reflect Changing Conditions at Home and Abroad, found that action on global warming came in 22nd out of 23 priorities listed. Government action on global warming has consistently been considered low priority by the American public. Only 38% of Americans thought global warming was a top piority. A more recent poll in 2020, Increased support for prioritizing policies on the environment, climate change since 2011, indicates that the percentage of Americans who think global warming is a top priority rose from under 30% in 2011 to 52% in 2020. An International Poll in June 2013 shows that US citizens are less concerned about the threat from global climate change compared with all other countries included in the poll.

In this course, we will discuss what we know for certain based on observations and measurements, such as the increase in greenhouse gases in the atmosphere due to human activity and the recent changes in global average temperature. We will also discuss the uncertainty, such as our incomplete understanding of the climate system and the use of computer climate models to predict how increases in greenhouse gases will change climate in the future. After studying an unbiased presentation of this issue, each of you will be asked to individually decide what, if anything, should be done about the potential problem of climate changes caused by human emissions of greenhouse gases.

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