Central and north amerioa

Some of the most notable past civilizations that have been directly influenced by climate are the Maya of Central America and the

Anasazi of the American Southwest. The Maya (also referred to as Mayans) were Central-American Indians. They were one of the greatest civilizations of the Western Hemisphere. Advanced for their time, they were well known for their extensive practice of agriculture, construction of enormous stone buildings and pyramid temples, artistic smithing of gold and copper, and development of a mathematics and hieroglyphic writing system. At its most advanced stage, Mayan civilization consisted of more than 40 cities, each populated with up to 50,000 people.

Today, all that is left of this impressive, sophisticated civilization are empty ruins deep within the wild jungles of southern Mexico, Guatemala, Belize, El Salvador, and northern Honduras. The civilization that thrived for 3,500 years (2000 b.c.e. to 1500 c.E.) abruptly disappeared. Just as Mayan society was reaching its height in 250 to 830 c.E., huge cities were suddenly abandoned, and the inhabitants seemed to vanish overnight. For years, historians pondered the reason for the sudden collapse of one of the greatest civilizations of all time. Then climatologists began to study sediment cores extracted from area lakes, and their findings offered clues to the mysterious disappearance of the Mayan civilization. Gill Richardson, an archaeologist who has spent considerable time studying the Maya, says that lack of water was a major factor in the collapse of the society. His most convincing evidence centers on sediment cores from several of Yucatan lakes acquired by David A. Hodell, Jason H. Curtis, and Mark Brenner from the University of Florida. Their measurements of the ancient deposits indicate that the driest interval of the last 7,000 years was between 800 and 1000 c.E. This coincides with the collapse of classic Maya civilization.

The Maya occupied the area known today as the Yucatán Peninsula, which has a seasonal desert climate and relies on annual rainfall for freshwater. Availability of abundant precipitation was important to the Maya because much of the area is underlain by limestone, a structure called karst topography. Karst topography erodes easily, so as rainwater collects in streams, it wears away the limestone beneath the ground and carves out caves, channels, and sinkholes. Although there were areas of abundant groundwater as a result, surface streams were not plentiful, which meant that rainwater for these people was very important.

Climate change played a role in the destruction of once-thriving Mayan civilizations. (Nature's Images)

Archaeologists have determined that because of this, the Maya developed extensive reservoir systems to capture, store, and distribute water. They were also very adept and efficient at agricultural practices and were able to make the desert productive. The more successful they were, the faster their population grew, until it reached a maximum of 200 people per 0.6 square mile (1 km2), one of the highest population densities of any ancient civilization.

Climatologists have retrieved sediment cores from both lakes and the ocean. What they found and were able to cross-verify from other sources put the Mayans' mysterious disappearance into perspective. According to experts at NOAA, the ratios of the oxygen isotopes 18O and 16O were analyzed in sediments taken from lakes in the Yucatán. These ratios were used to identify wet and dry periods in the climate. The oxygen isotopes were measured by the calcium carbonate (CaCO3) found in the shells of the aquatic organisms deposited within the sediment cores. Drier climates are signified by higher amounts of the oxygen-18 isotope. Therefore, when the 18O to 16O ratio is higher, it tells climatolo-gists the climate at the time was much drier and the area was probably experiencing a drought. Conversely, when the 18O to 16O ratio is lower, it signifies wetter conditions.

Climatologists discovered that shifts in atmospheric patterns caused a severe drought. Over an extended period, the Maya suffered several drought cycles, but from 800 to 1000, an extremely severe drought occurred in the region that devastated their ability to cultivate agriculture and maintain an adequate supply of water. During this time, the Mayan civilization collapsed. According to a 2003 article in National Geographic News, some archaeologists have estimated that the Maya were especially vulnerable to the climate because up to 95 percent of the population depended directly on lakes, ponds, and rivers for their supply of water, which contained only a year-and-a-half supply.

This conclusion was supported with evidence collected from ocean core sediments retrieved off the coast of Venezuela. Trace deposits of titanium, which are an indicator of weathering and erosion, were analyzed. High amounts indicate heavy weathering and erosion, which would indicate a wetter climate. What scientists found, however, were low amounts of titanium, indicating a drier climate. By dating the sediments, they were able to determine that not only was the area extremely dry, but there were several even more severe minidrought cycles within the longer period of drought. Dating techniques placed these minidrought cycles at 810, 860, and 910, which led scientists to believe that rapid climate change caused the rapid decline of Mayan civilization.

Also supporting this conclusion are data received from ice cores obtained in Greenland. During the same period, the ice cores had higher levels of ammonium, which indicates that the entire Northern Hemisphere experienced markedly drier climatic conditions during this period. According to NOAA scientists, the Mayan civilization faced a drought that lasted more than a century, and when their groundwater sources were depleted, their society collapsed.

Other cultures of Central America also met similar fates. The Teo-tihuacan, who lived north of present-day Mexico City, had an advanced civilization that fell around 800 due to deforestation of the surrounding o

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These two time lines illustrate that the collapse of two prominent civilizations—the Mayan and Akkadian—were related to severe climatic changes, including the abrupt onset of drought. (Source: Hodell, et al, 1995, Nature)

area and an increasingly arid climate. The Cacaxtla and Xochicalco, who lived near present-day Cuernavaca, also collapsed, and their settlements were abandoned around 800 because drought destroyed their food and water supplies. Climatologists believe this general trend occurred regionwide due to a general shift in climate toward aridity.

The American Southwest has been greatly affected by climate change and has seen the rise and fall of great civilizations. The Anasazi lived in the region that today is referred to as the four corners area (New Mexico, Arizona, Utah, and Colorado). Like the Maya, they were a very advanced civilization for their time. They had highly advanced astronomical calendars and tools. The Anasazi built vast networks of roads that connected major settlements and traded with tribes in Central and South America. They had extensive irrigation networks and knew how to farm efficiently.

In Chaco Canyon, New Mexico, and elsewhere, they built magnificent stone houses of sun-dried clay brick, some of which had 500 rooms and housed several thousand people at a time. The Anasazi thrived for more than 1,000 years. Then suddenly, between 1275 and 1300, all expansion stopped, and the Anasazi mysteriously disappeared.

Archaeologists have determined that the community was dependent on water gathered from ephemeral washes. They collected the water and diverted it through canals to where they needed it in extensive dam and irrigation systems. Then the region was struck with repeated droughts. Between 1125 and 1180, very little rain reached the dry, arid area. Another drought struck from 1270 to 1274, and then in 1275, yet another cycle of drought hit, this time lasting 14 years. Climatologists and archaeologists believe the culture could not maintain a stable supply of water. Because they had heavily deforested the area, extensive erosion took place, which cut arroyos (deep channels) into the landscape and lowered the groundwater table and hence water availability. Repeated episodes of drought were harsh enough to eventually cause the civilization to collapse. Today, the only remnants left of these magnificent settlements are a maze of stone ruins, a reminder of what climate change can do.

The Hohokam (their tribal name means "the people who vanished") were the ancestors of the present-day Pima Indians. Also living in the

Ruins found in Chaco Canyon left by the Anasazi are all that remain today of their great civilization that housed thousands of people. Long droughts contributed to the failure of their society 1,000 years ago. (National Park Service)

American Southwest, they were dwellers of the Salt-Gila River Basin in Arizona. Their civilization began around 300 b.c.e. and lasted until around 1450 c.e. To deal with the arid climate in which they lived, they built extensive networks of canals and irrigation systems. Then, beginning around 1080 and lasting for the next few hundred years, the area was repeatedly flooded, interspersed with long periods of drought. Their water supplies became contaminated with salt and dried out, spelling their eventual destruction.

There are also other signs of megadroughts throughout North America (900-1400 c.e.), such as massive sand dunes that are currently grassed over and stable. Much of the evidence found by climatologists has been derived from tree ring proxy records.

There have also been more recent examples of the repercussions of climate change. In the 1930s, the United States endured an extremely destructive environmental event during what is referred to as the dust bowl era. Because of improper farming practices, the topsoil was left damaged, dried out, and vulnerable to erosion. Unfortunately, the climate was in a drought phase at the time, and great amounts of silt, soil, and sand were blown hundreds and thousands of miles away, leaving the area infertile and uninhabitable. Farmers chose to abandon their farms and seek refuge elsewhere, such as California. It represented the largest migration in U.S. history.

Today, with ever-increasing population and settlement of communities in the Southwest, experts caution against rapid growth because of increased aridity predicted with an increase in global warming. Paleoclimatologists also warn that global climate is moving toward conditions that have not been seen for at least 10,000 years. To support this opinion, they note that the upward trend in CO2 and temperature in the last half century points directly to a strong human contribution because of heavy fossil fuel use, deforestation, and poor farming practices. Some scientists believe that climate itself may not be the sole reason for a society to collapse but that it could act as one of a multitude of reasons. For instance, climate could cause a shortage of food and water, which could then destroy a country's security, generate social unrest, lead to war, and cause a civilization to fail. There are many combinations of possibilities, but scientists agree that climate is a principal factor in determining a society's success. They stress that climate must be looked at as multidimensional, influenced by both geophysical and social components.

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