Climate Patterns

Climatologists have determined that climate change is not only constant but that it also occurs at multiple time scales. It occurs at geologic time scales of millions of years, and nested within that at scales of hundreds of thousands of years, and within that at tens of thousands of years to thousands, hundreds, and even decades and annual scales. Long-term changes are seen in changes in the Earth's tilt, axis, and precession. Cycles are seen in ice ages. Shorter-term changes are seen in patterns of phenomena, such as El Niño. Historical climate change will be looked at in more detail in chapter 2.

Some scientists have even suggested that climate has a "memory" and that by understanding past conditions and behaviors it is possible to

Bryce National Park, Utah. This area was once covered by seas, mountains, deserts, and coastal plains. The geologic formations within this area testify to the various climates that existed when each layer was deposited. (Nature's Images)

predict and plan for future conditions. According to scientists at NOAA, "Understanding 'climate surprises' of the past is critical if we are to avoid being surprised by abrupt climatic change now." For example, abrupt climate change has had drastic consequences for ancient civilizations that can serve as warnings for society today. According to Dr. Harvey Weiss, a professor of archaeology at Yale University, as reported by CNN in April 2005, climate change was a fact of life for earlier civilizations. Egyptian pharaohs and medieval Vikings both had to deal with violent changes in weather patterns, which sometimes prompted mass migrations.

According to Dr. Weiss, "Those episodes proved to be the single most important stimulus for the major transformations in human history." As information is discovered about prehistoric climates, archaeologists are looking for connections between climate change and human development. They have linked the collapse of early Bronze Age civilizations in Greece and India to abrupt climate changes about 4,200 years ago. Drought has been cited as a factor in the collapse of the Anasazi civilization of the American Southwest during the 13 th century.

Dr. Weiss says results were not always negative, however—it was drought that drove farmers in ancient Mesopotamia to build irrigation channels, which enabled farmers to grow enough food so that for the first time everyone did not need to farm. Instead, it allowed some inhabitants to pursue other paths, such as architects, politicians, and artists. According to Weiss, "The historical lesson is that those societies had no knowledge of what was happening to them and certainly no historic knowledge of what could happen to them, where we have both." With today's technology, scientists are able to improve predictions for the future based on knowledge learned from incidents in the past.

Because different aspects of the environment respond in different ways, physical characteristics of areas play a significant role in determining climate. As an example, if a highly vegetated area were to have repeated years of lower-than-normal rainfall, the soil would dry out, lakes would shrink, and water sources would diminish. In response, the vegetation would die off. Over time, with less vegetation, less biomass would enter the soil, causing the soil to lose fertility. There would be less evapotranspiration, which would also change the characteristics of the area to become more arid. Evapotranspiration is the transfer of moisture from the Earth to the atmosphere through evaporation of water and transpiration from plants. Dry conditions would contribute to even further evaporation and shrinkage of lakes, cause grasslands to dry up, and cause desert areas to begin expanding. This is a process called "desertification" that is becoming common in the American Southwest as global warming continues, and it has caused enormous problems for the peoples of Africa. It has destroyed water supplies, drinking water, and farming and has subsequently contributed to the spread of disease and starvation. Desertlike conditions can then lead to less rainfall in subsequent years. When this works as a cycle, scientists view this as a self-perpetuating process. This happens because different parts of the environment respond over different time intervals and in different ways, sometimes feeding off of each other and either enhancing or minimizing the responses.

Some scientists in England have noticed that there is a correlation of summer rainfall amounts to the wintertime air pressure pattern of the North Atlantic Ocean; the summer climatic pattern seems to "remember" what the previous winter pattern was. Because of this correlation, farmers have been able to predict which years will be better for growing wheat, because they have been able to predict which years will have wetter or drier summers.

Being able to look at characteristics of climate in certain locations and to predict how climate will act in the future is a valuable tool. One thing that climatologists agree on is that the Earth's climate is always changing. Its variability changes on multiple timescales—short-, medium-, and long-term intervals. No matter what the scale of these changes, they have a significant effect on humans. Because of this, it is important that scientists study past climatic variability in order to better understand future climate change and the potential impacts it may have on society.

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