Introduction to Global Warming and Climate Change

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. You need to be clear on this point 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.

Many human activities may contribute to global warming or other climate changes, such as anthropogenic emissions of greenhouse gases (enhancing the natural atmospheric greenhouse effect) or anthropological changes in land use (for example, deforestation or city-building). We usually 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.

It is also 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. They are two separate environmental issues that should be discussed independently.

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 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.

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 the increase in greenhouse gases do significantly alter the natural global system, then we should expect that most climate zones will be affected worldwide. In addition, some issues like changes in global sea level, 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. 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. Note that 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 diagram is shown below.

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. UPPER FIGURE (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. 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.

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.

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 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 say don't worry at all, who cares 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. In this course, we will discuss what we know for certain about how human activities are affecting climate and what we do not understand about the potential influence of human activities. This will then help each of you to individually decide what, if anything, should be done.