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 been warming recently), but cannot determine very well how much of the temperature change is caused by human activities. You need to be clear on this point whenever discussing this issue.

Many human activities may contribute to global warming, 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.

This issue, whether or not human activity is causing 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 issues, although both result from anthropogenic activities.

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 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.

It is not just changes in the average temperature for a region that determines climate change. Other examples of climatic components were listed in a 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. 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 shown on the bottom scale. The solid blue curve (with scale at left) is the difference in probability of the readings 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.

People often 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 we define climate changes as occurring on smaller regional scales as opposed to the entire planet Earth.

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. 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, human beings and other life will just have to adapt to the changes (life on Earth has survived large climate shifts in the past). In this course, we will discuss what we know about how human activities are affecting climate. This will then allow each of you to individually decide what should be done.