Notes on global climates and oceans
We will skip the chapter on water resources for the time being and return to it later; for this week we will focus on global climate. The traditional format of the global climate chapter, followed in Christopherson as in most physical geography textbooks, is to develop a global climate classification scheme based on annual and seasonal patterns of temperature and precipitation. We can illustrate the pattern for any particular location using a climograph, which is a plot depicting monthly average temperature and precipitation values over the course of the year. These patterns are usually very distinctive and diagnostic of the climatic zone. For example, equatorial wet climates have very little variation in temperature or precipitation, and are both wet and quite warm throughout the year. Conversely, tropical wet-dry climates, or the even more extreme case, monsoon climates, tend to have only modest temperature variations but very pronounced wet and dry seasons.
It is critical to be aware of the factors that control the global distribution of these climate types. In fact it is almost impossible to memorize the Koppen regional climate map, but it is not very difficult to place most of the major climatic types within the context of a general understanding of atmospheric pressure and wind systems, their seasonal shifts, the relationship between pressure systems and global precipitation patterns, and the influence of ocean currents. Thus we can explain the progression of climates in Africa from the equatorial region to the Sahara desert and beyond to the Mediterranean, and south toward the tip of South Africa. Or we can observe the progression of climate types as we move up the west coast of the U.S. from Baja California to Seattle and even farther north into Canada and Alaska; and we can see similarities with the climates along the west coasts of South America, Europe, and to some extent Australia. Of course there are some differences induced by orographic barriers, which may create rain shadows further inland in some land masses but are absent on other land masses.
Once we are aware of the controlling factors, know how to read climographs, and can understand the Koppen system, we can then start to synthesize everything we have been talking about over the last several weeks and put together a conceptual picture of the entire global climate system. It is at this scale that we can also start to think about the effects of large-scale phenomena like El Nino, which is discussed in this chapter and is a prominent topic among those featured in this week's assignment. The book discusses El Nino in fairly brief form; you will find a much more extensive body of information, including current news stories, on the World Wide Web.
Following the discussion of global climate classification, the textbook devotes the remainder of chapter 10 to a discussion of global climate change. This is presented primarily in the context of global warming under the influence of increased levels of greenhouse gases that have been and are being injected into the atmosphere as a result of human activities. However it is important to retain some perspective on past climates: that is to say, that the climate is not changing from some fixed, stable position, but has in fact varied over all time scales. This does not invalidate the global warming scenarios being projected by atmospheric scientists, but it does place those scenarios within the context of "natural" or non-anthropogenic changes. You can find a brief discussion of paleoclimatology, or the reconstruction of past climates based on historical, geologic, or astronomical evidence, on pages 513-518 in the textbook. It is worth speculating on the scale of the anticipated changes coming in the next century or two as they compare with the changes depicted in some of the figures in this part of chapter 17. There are some predictions suggesting that we will see global average temperatures higher than at any time in the last 100,000 years.
I have also assigned part of chapter 16; since we are talking about ocean-atmosphere interactions, I thought it would be appropriate for you to have some basic familiarity with the ocean basins and the factors affecting temperature distributions and other physical and chemical parameters. Within the assigned section of the chapter you will also find information on tides and waves, and a brief discussion of sea-level changes. I probably will not lecture on most of this material in any formal way, but I am open to discussion of aspects that may attract your interest.
Chapter 10