Unit 1 Reading: Climate Forecasting and Adaptation through the Ages

by Cindy Shellito, University of Northern Colorado

In the 21st century, it is difficult to avoid news about climate change. Often, news of climate change is reported in the same breath as news about weather: "Greenland ice caps melt and midwestern crops destroyed by hail storm!" Climate is the term used to describe the weather conditions that prevail over a long period of time. Most people can describe the climate of their home town without thinking too much about it: it is warm at certain times of the year, cool in others. Sometimes it is wet, sometimes dry. Some places are always dry or wet. But over the course of a lifetime, we might see changes in what we expect from year to year. Ask someone who experienced the dust bowl in the 1930s. They will tell you that the climate of the American Midwest was different back then. Take a look at the Arctic today. The presence of sinkholes left by melting permafrost, and retreat of glaciers are an indicator that something is different now than it was only a couple of decades ago. Climate is never a constant. But it has such an impact on our lives. Today it not only dictates whether we wish to install air conditioning in our homes, but determines whether crops grow and thrive or shrivel and die. We depend on climate for food. And this is not a new phenomenon. It has always been so for us and for our ancestors.

Climate in a particular region can vary over a short or long period of time. In your lifetime, you might have noticed that some summers are wetter than others, or that some winters are much colder than others. These shorter-term changes in climate, which appear as anomalies—or differences from the average—in our records, are generally referred to as climate variability. When we see a long-term trend in climate, or a change in how frequently we experience anomalies associated with climate variability, we refer to it as climate change.

To better understand the full impact of climate on 21st-century life, we need to take a step back in time. This article surveys the impacts of climate on three past human cultures: the Incas, the Mayans, and the Vikings.

Begin your survey of climate impacts in:

• the snow-capped peaks of the Peruvian and Bolivian Andes
  
• the steamy jungles of Central America
  
• the rocky shores of coastal Greenland

Peruvian and Bolivian Andes: Forecasting Short-Term Climate Changes

In the high Andes, it is not unusual to find farms and villages at or above 12,000 feet elevation. Despite the fact that this region is within 15 degrees south of the Equator, the high mountains are a harsh, windswept place. There are few trees, and the natural vegetation consists mostly of scrub. Yet, if you were to drive one of the winding, gravel roads through the mountains, you would find (if you could look away from the precipitous cliff running alongside the road) many of the steep hillsides blanketed by a patchwork of crops. These are slopes that are so steep, they would be difficult to walk on, let alone drive a tractor. Corn, quinoa, and potatoes are the staples. Peru is known for having more varieties of potatoes than any other region of the world. Farmers working the land here today follow the same methods that have been in place for centuries, possibly since the time of the Incas, over 800 years ago. The Aymara- and Quechua-speaking people of these high peaks and plains have long depended on their crops for food and trade. Their survival depends on the success of their crops. As they understand it, they are at the whim of their gods. Not enough rain, or too much rain, in any given year could mean a failure of their crops, and the end of their way of life. To try to anticipate the whims of their gods, these people, long ago, developed their own means of forecasting the weather.

In mid to late June, often for many consecutive days, farmers observed the constellation Pleiades.

Around this time of year, the constellation first appears in the eastern sky, in the pre-dawn hours. The farmers would take note of several aspects of the Pleiades that relate to the overall brightness of this cluster of stars, the date the cluster first appears on the eastern horizon, the apparent size of the cluster, and the position of the brightest star in the Pleiades. Using these observations, the high mountain villagers would forecast the timing and quantity of precipitation for the upcoming year. This would also allow them to estimate the size of the potato harvest for the following fall (remember: fall occurs in March through June in the Southern Hemisphere).

A clear view of the Pleiades near the winter solstice (June 21) would lead to a forecast of sufficient rainfall and a large harvest the following autumn. If the villagers had an obscured view with less clear skies, they would forecast poor rainfall and delay planting of their potatoes. Potatoes are usually planted in October or November. During low rainfall years, farmers would delay this until December, when they were much more likely to receive rainfall. Potatoes have shallow roots and require a lot of water soon after planting in order to maximize tuber formation.

It is possible that this forecasting technique dates back several centuries to the Incas. The Incas worshipped the Pleiades, and there are notes about such forecasts soon after their conquest by the Spanish in the 16th century.

So, how well does this ancient forecasting work? Dr. Benjamin Orlove, from the University of California at Davis, and colleagues set out to verify the Incas' technique. They looked at satellite records of Pacific high cloud data archived by the International Satellite Cloud Climatology Project. They used these data as a substitute for the record of Pleiades visibility (in other words, lots of high clouds in the eastern Pacific means low or no Pleiades visibility), and compared the cloud data each year with crop yields in the Puno district of the high Andes (see map) over a period of eight years, from 1983 to 1991.

They found that crop yields were well correlated with Pleiades visibility. In other words, the ancient forecasting technique worked quite well!

Understanding that climate variability had an impact on their survival, and astute observations of their environment gave the Andean people an enormous advantage.

Be prepared to discuss the following questions in class:

• List as many conditions as you can think of that might affect the appearance of a star or group of stars on the horizon. (There may be more than were discussed in this paper.)
  
• Based on the article and your understanding of food production, list as many factors as you can that might affect a crop yield.
  
• Orlove examined 8 years of cloud data and crop yields in the high Andes, and found a very good correlation between them. Do you consider this to be sufficient evidence that the Inca forecasting method works? Why or why not?
  
• The Incan forecasting method is based on the idea that there are clouds in some years and not in others. List all the factors you can think of that might affect whether clouds can form off the west coast of Peru. What do you think are the most important factors? (Remember, this is very near the Equator.)
  

Central America: a Story of Long-Term Adaptation and Downsizing

The Mayan civilization flourished in Central America from A.D. 250 to 900. They lived in a region that today is occupied by the Yucatan in Mexico, Belize, Honduras, and Guatemala. Their civilization reached a cultural peak about A.D. 800, and today, millions of tourists flock to their pyramids each year to marvel at their ingenuity. The collapse of this productive, successful civilization from A.D. 800 to 1000 has long been shrouded in mystery, although climate change has been thought to play an important role.

The rainforests of central America provided a gold mine of food and resources for these people. Ample rainfall throughout the year fed the growth of a verdant, lush canopy of incredible diversity. There is little the Mayans would have needed beyond their forest home, if anything. The Mayans had a rather sophisticated system of agriculture, with raised fields and terraces. It is thought that some of these agricultural systems contributed to deforestation as their cities grew and the demand for food increased. The Mayan culture flourished for centuries under this system. Under ample rainfall, this likely would have continued. But the health of the forest and large crops are very sensitive to minute changes in rainfall.

In the early 1990s, a group of scientists from the University of Florida examined lake sediments from Lakes Punta Laguna and Chichancanab on the Yucatan Peninsula of southern Mexico. Lake sediments can provide an excellent record of climate change and may also provide some indicator regarding the events that led to the demise of the Mayans. Over the decades, as sediments settle out of the water column to the lake floor, they preserve a record of the chemistry of the lake, as well as a record of evolution of the life in the lake. Punta Laguna and Chichancanab are even better than most lakes for such studies because of their high sedimentation rate. Punta Laguna also has no streams or rivers branching away from the lake. It loses most of its water via evaporation. The water column is rarely disturbed, and sediments are deposited in smooth layers.

Within the sediments, researchers examined a number of climate proxies, or physical characteristics preserved in the geologic record that can be used to infer past climate. In the sediments of both lakes, researchers examined the chemistry of shells of microscopic fauna that were buried in the sediments after they died. Specifically, they looked at the ratio of heavy oxygen (¹⁸O) to light oxygen (¹⁶O), as this ratio can provide an indicator of relative evaporation and precipitation over the ages. At Lake Punta Laguna, they also examined shells and pieces of wood buried in the sediments.

What the researchers found at both of these lakes are records, going back several thousand years, of multiple wet and dry episodes, each lasting, quite possibly, for decades. It is hypothesized that these wet and dry episodes may have had an influence on Mayan history and cultural evolution.

What the proxies show is that for the 1000 years or so prior to about A.D. 280, the region was rather wet over all. Around 280 A.D. things began to dry out, but at the same time, Mayan cultural evolution accelerated. For the next 500 to 600 years, there was a gradual drying, punctuated by more severe droughts. Between A.D. 800 to 900, the Classic Mayan civilization collapsed. This time period also corresponds to what is estimated to be the most severe drought in the past 8000 years. Archeological evidence points to the movement of people to the northern Yucatan at this time. It has been suggested that these people were dependent on precipitation for water. With reduced precipitation and a drop in the water table, the only way they could adapt was to move somewhere with more water.

Considering that the climate of the Yucatan Peninsula has returned to its previously moist conditions since the collapse of the Mayan civilization, it is likely that the Mayans were victims of long-term climate variability.

Be prepared to discuss the following in class:

• Unlike the Incas, we have no evidence that the Mayans were able to make weather or climate forecasts. Had they been able to make a long-term forecast, do you think it would have made much of a difference in the survival of their civilization?
  
• Much of our understanding of the climate history of the Mayan region comes from data collected in lake sediments. Can you think of other things that might serve as indicators of past climates?
  
• What factors can you think of that might affect rainfall in this region of Mexico?

Greenland: When the Going Gets Tough, Pack up and Flee

A bit off the beaten tourist path, Greenland today strikes many as a somewhat forbidding destination. It is a land of rocky coastlines and massive glaciers occupying most of the continent. It is a place where you could easily escape the summer heat. Average high temperatures in Greenland's capital city, Nuuk, max out around 50 degrees Fahrenheit in July. In winter, high temperatures average in the mid-20's. If you like the dark, you would enjoy December—cold, snow, and less than three hours of daylight. Despite the prohibitively cool environment, archeologists have found evidence of four different groups of people living in Greenland over the past 5000 years. Many of them lived in or around the present-day city of Nuuk.

About 4500 years ago, Inuit tribes from North America crossed over a frozen ocean into Greenland. (At least, that is what we imagine. We have not found any evidence of boats from that time.) First the Saqqaq people, and then the Dorset people, occupied the mostly frozen continent until about 2200 years ago. About 1000 years later, Norse explorers known as the Vikings arrived in Greenland. The Vikings had already settled in Iceland, so Greenland was just one more hop, skip, and step beyond that. Greenland must have been a sort of high-latitude "promised-land" at the time, because the Vikings did not have it to themselves. A couple of hundred years after they arrived, another Inuit group, the Thule people, arrived from North America. Both groups occupied Greenland until the Vikings abandoned their settlements about 600 years ago. The Thule people stayed on, however, and their descendants still live in Greenland today.

How did these different groups manage to make a living in such a harsh land? And why did the Vikings abandon their settlements and leave Greenland entirely to the Thule people? There is little evidence of interaction between the two groups, so it is generally not thought that there were any conflicts between them. Clues lie in the ice and in the lake sediments of Western Greenland.

Dr. William D'Andrea, from Brown University, and colleagues examined sediments deposited over the past 5600 years in two lakes in West Greenland. They wanted to create a long-term record of temperature in this region. In the mud samples they collected, they measured "alkenones," or fat deposits made by algae in the lakes (didn't imagine algae could get fat, did you?). The abundance of algae depends on the temperature of the water, which depends on the temperature of the air. So, D'Andrea and his team were able to reconstruct a record of air temperature based on these alkenones.

Chemical analyses of ice cores near the summit of Greenland, specifically, the ratio of heavy oxygen (¹⁸O) to light oxygen (¹⁶O), also provide a record of temperature changes over the past 5000 years. But the record of temperature inferred from mud samples provides a much clearer picture of what temperatures were like on the coast of Greenland, where people actually lived. (Only scientists are crazy enough to live on the summit of the Greenland ice sheet!)

These records show us that at the time the Vikings lived in Greenland, the climate was quite different than what you would experience today. When the Vikings first arrived in Greenland, it was warm enough for birch trees to grow in some of the fjords! The Vikings quickly cut these down to help build their houses, and their goats and sheep grazed on any remaining vegetation. Nevertheless, trees are not something one would associate with Greenland today. The Norse community did quite well for some time, with as many as 600 farms. They traded ivory, sheep, and seals with Iceland and their homeland in Norway for wood, iron, and food they could not grow.

There are no written records explaining why the Vikings left Greenland, but their departure has long been attributed to regional cooling. Both the ice core and the lake sediment records show that the timing of the cooling along the western coast coincides well with the abandonment of Viking settlements. Excavations at settlement sites show that prior to abandonment, there was a change in diet among the Vikings, from sheep, goats, and cattle to seals and fish. It is thought that the Norse, whose lifestyle depended on farming rather than hunting, were unable to adapt to harsher winters, and that they eventually abandoned their settlements. The Thule people, on the other hand, were hunters and fishers, and it seems, they were able to adapt much more easily to harsher conditions.

What was happening in Europe while the Viking outposts were succumbing to those colder, harsher winters? As it turns out, as climate was cooling in Greenland, it was warming in other parts of the North Atlantic! A record of temperature inferred from cave deposits in southwestern Ireland showed that at the same time Greenland was cooling, Ireland experienced warming. Likewise, when Greenland was warming, Ireland experienced cooling. This strongly suggests that there is likely some type of long-term climate oscillation in the North Atlantic, and locations on the opposite ends of this oscillation will experience opposite trends in climate. The Vikings, lulled into Greenland by a relatively long-term warm episode around 1000 A.D., were ultimately victims of these long-term oscillations in the climate of the North Atlantic.

Be prepared to discuss the following in class:

• What factors seem to be important in whether a culture can adapt to changes in climate?
  
• Over how long did climate changes in Greenland occur? Do you think the changes are something you would notice on a year-to-year basis?