EC4. Changes in the Global System

The Americas

Dr. Steven Schneider, climatologist with the National Center for Atmospheric Research, has been very vocal in alerting the public to the impending dangers of global climate change. In an interview for the TV documentary, “Race to Save the Planet,” he shares his sense of urgency….

“Fifteen thousand years ago, the Ice Age began to break up. There were ice sheets literally a mile high over New York City, across through the Great Lakes, over most of northeastern Canada and parts of Europe. And over the next 10,000 years, the ice disappeared. Sea levels rose 100 meters — 300 feet or so. Forests moved thousands of kilometers. Literally, the ecological face of the planet was revamped. 

“It took 10,000 years to warm the planet up only 5 degrees Celsius, about 9 degrees Fahrenheit. [See explanation of temperature scales.] And we’re talking now about change on that magnitude in a century. So we could be modifying the climate — and the whole Earth’s environmental system — at a rate something between 10 and 100 times faster than nature. And that’s guaranteed to have nasty ecological surprises.”

I. The Ice Ages: A Natural Feedback Model

map of Ice sheet over North America, 18,000 years ago.
Ice sheet over North America, 18,000 years ago.

18,000 years ago, over half of North America was covered by a vast sheet of ice stretching from northern Canada to Long Island and across the country to the Pacific Ocean. The climate was very different during the ice age, with an average temperature 5°C lower than today. In some areas, the ice was as much as two miles thick. 

On Mount Washington in New Hampshire, the highest peak in the northeast at 6,280 feet, scratches on the rock point to evidence that a large ice sheet that was at least one mile thick, passed over the top. But by 6,000 years ago, all of the ice had melted away to its current levels in the western mountains, Alaska and Greenland. What happened?

There are a number of mechanisms that control why large ice sheets form on the Earth periodically. One reason, discovered by Milutin Milankovitch in the 1920’s, and since bolstered by new discoveries, is the Earth’s orbit. He proposed that the cycles of ice ages were the result of changes in the relationship between the Earth and Sun.

illustration of precession
Precession: Earth’s axis wobbles on a
19,000–26,000 year cycle.

The Earth’s orbit isn’t absolutely precise; various elements change over time. For example, the Earth’s axis is tilted 23° and is part of the reason we have seasons.

However, this tilt “wobbles” or in a sense, spins like a top, over a 26,000-year cycle; this is called Precession. In 13,000 years, the longest day of the year won’t be in June but in December. How might that affect winter? 

Also, the shape of Earth’s orbit changes over time as well. It becomes less circular and more oval in a pattern over many thousands of years. Over time, as this happens, the Earth’s distance from the Sun becomes even greater.

Eccentriciy - shape of Earh's orbit
Eccentricity: Earth’s orbit, though very nearly circular, has an elliptical shape that changes with a 100,000– 400,000 year cycle.

Right now the distance from the Earth to the Sun doesn’t vary that much, but if the orbit became more elliptical the difference between summer and winter temperatures would become much more significant. 

diagram Obliquity: The tilt of the Earth’s axis,
Obliquity: The tilt of the Earth’s axis, now 23.5°, changes in a 41,000 year cycle. Changes in Earth’s orbit adapted from

Finally, the Earth’s axial tilt changes over a 40,000-year period, creating less tilt and less dramatic seasons.

When the axis was tilted to a greater degree, winters would get colder and ice sheets would expand. If it were tilted less, then the seasons would be less extreme and ice sheets would melt. Taken together all of these changes to the Earth’s orbit can significantly affect the overall climate of the Earth. 

Ice sheet growth is also affected by atmospheric composition. If the carbon dioxide were to increase, then the atmosphere would warm. One natural feedback mechanism between the ice, the atmosphere and the Sun is called ice albedo feedback. 

As glaciers grow, their brilliant white color reflects away the majority of the sunlight that hits their surface keeping the ground colder. This causes them to grow larger and cover more area. As they grow they remove vegetation from the ecosystem. This increases the amount of CO2 in the atmosphere, which warms the atmosphere and melts the ice sheet. As the ice retreats, more trees once again grow and the carbon dioxide is buried in the reservoir. As a counterbalance, the amount of oxygen in the atmosphere increases. This creates a greater likelihood that forest fires will occur. In turn, this releases more carbon dioxide into the atmosphere, which warms the atmosphere and melts the ice sheets. 

The carbon dioxide found in the oceans is also released as the atmosphere warms and is taken in by the ocean as the atmosphere cools. Over time, coupled with the astronomical theory and ice albedo feedback, glaciers have come and gone during Earth’s history in a natural feedback model regulated by factors in nature. 

At present, glaciers around the world are melting. In the Antarctic, large chunks of ice have broken off from the ice caps that have been as large as the state of Delaware. While this is an enormous piece of ice, it is only a small piece of a vast ice sheet that covers much of Antarctica. The question remains, is it a natural feedback mechanism or are we having a negative effect on our atmosphere causing glaciers to melt catastrophically?

II. Venus and Mars: A Positive Feedback Model

What happens when there is no controlling mechanism to keep a planet’s climate system in dynamic equilibrium? The planets Mars and Venus are instructive examples. Venus is the second planet from the Sun. It is nearly the same size as Earth, has an orbit that’s just a little faster and has a dynamic weather system similar to ours. However, what lies below is quite different.


When scientists first started to study Venus, they imagined a planet like Earth but with more water since it had thick clouds. Maybe it even had life! However, when NASA and the Soviet Union began to send probes to the planet, an entirely different picture emerged from the data.

The atmosphere of Venus is 90 times heavier than Earth’s (you would be crushed) and has a composition of 96% carbon dioxide, 3% nitrogen and 1% assorted gases like carbon monoxide, which is a gas that comes out of the exhaust pipe of your car. It has volcanoes and a cloud cover that goes nearly to the surface. Venus does have water, but it has evaporated into the upper atmosphere where it mixes with the sulfur spewed from volcanoes and forms deadly acids such as Sulfuric Acid. The surface temperature of Venus is more than 400°C, hot enough to melt lead. If you could land on the surface, your spacecraft would be eaten away by the acid rain and would quickly melt if you didn’t takeoff quickly. You would not survive very long. 

Venus is an example of a “runaway greenhouse effect.” Because of the volcanoes carbon dioxide is added to the atmosphere. This is turn causes clouds to form which traps the heat, preventing any moisture from making it to the surface of the planet. Venus is a planet that might have supported life if it had reached dynamic equilibrium before conditions became catastrophic. 


Mars on the other hand, is further from the Sun than the Earth and has an average surface temperature of -60°C, far too cold for hospitable conditions. The reason for this is that Mars lost most of its atmosphere billions of years ago. Its current atmospheric pressure is less than a hundredth that of Earth and there are only tantalizing glimpses of water that may have been on the surface eons ago. It can’t retain any heat. 

While Mars is also in dynamic equilibrium, it is the opposite of Venus. We find that Venus is too hot and Mars is too cold. Earth is the only planet that is just right. This has come to be known as the Goldilocks Effect after the fairy tale, Goldilocks and the Three Bears. What about the Earth? Is the Earth still “just right” or are we heating up?

III. Is the System Heating Up?

If the input of energy from the Sun exceeds the output of reflected light and heat energy from Earth, then the global system may begin to warm. We haven’t been able to put Earth in a giant terrarium to monitor the total energy input to our system and the output of energy to space. However, scientists have been very inventive in using a variety of atmospheric tests, core samples from polar ice, soil and ocean samples, as well as historical records of climate to support the idea that increasing levels of carbon dioxide in the atmosphere are triggering global warming.

Since the beginning of the industrial revolution, enormous amounts of fossil fuels have been burned, releasing large quantities of carbon dioxide into the atmosphere. 


IV. Energy and Matter In and Out of Ecosystems

Analyzing the flow of energy and matter in an ecosystem can provide clues for understanding the global impact of human activities. For example, in a forest ecosystem, plants use the Sun’s energy, water, minerals and nutrients from dead organisms in the soil to grow and produce more plant matter. Animals consume plants and/or other animals, thereby producing new body tissue. Heat and waste products such as carbon dioxide and droppings are also produced. 

Each ecosystem has an input of matter. This can be in the form of gases, minerals and nutrients from the atmosphere, from the weathering of rocks and from other ecosystems. For example, the monarch butterfly migrates south to Mexico over winter in a different ecosystem. Air and water may enter your terrarium ecosystem. Energy also enters the system as light, heat, and chemical energy from living and dead organisms. What forms of energy enter your terrarium?

Each ecosystem has a flow of matter and energy through it, with much of the matter being recycled through the ecosystem food web again and again. What organisms in your terrarium are producing substances that eventually get used by other organisms? The reuse of nutrients by organisms in a system is another example of a negative feedback response that promotes equilibrium in the system.

Each ecosystem has an output of matter and energy. This may take the form of gases and heat energy to the atmosphere or of nutrients and organic matter to other ecosystems, often in the water runoff in streams. 

Question 4.1. 
What matter and energy is being lost from your terrarium?

This diagram shows the flow of energy and matter through an ecosystem and may assist you in identifying the input and output of your terrarium system.

The flow of energy and matter in an ecosystem

V. Why Are We Worried About Level of CO2?

The simplified diagram on the previous page reveals many interrelated processes that might be affected by a change such as the atmospheric increase of CO2.The structure of the carbon dioxide molecule enables it to absorb more heat energy than the dominant gases in our atmosphere, nitrogen and oxygen. Methane and chloroflouro-hydrocarbons (CFC’s) absorb even more energy than CO2. Although these gases are found in very small amounts in our air, we call them the “greenhouse gases” because of their capacity to capture heat and warm the atmosphere. 

In 1750 the atmospheric concentration of CO2 was 280 parts per million (ppm). In 1991, the concentration had risen to 355 ppm and many scientists predict CO2levels will double within the next century if the present rate of emissions continues. Because our atmosphere and oceans are intertwined as part of one gigantic water cycle, climate changes in one region may trigger changes half a world away. The following are just a few of the questions scientists are investigating in the quest to understand the potential effects of global changes in the Earth’s atmosphere. 

  • Will increasing levels of carbon dioxide and greenhouse gases in the atmosphere stimulate further global warming?
  • Can we expect flooding of coastal areas due to melting ice caps and expansion of warm water?
  • What effect will an increase in carbon dioxide have on vegetation; will food crops grow slower or faster? 
  • How might a warming trend affect the diversity of species in each region?

VI. Is More Methane On the Way?

Buried just beneath the floor of the Arctic Ocean, and also beneath the Arctic tundra, are vast quantities of a substance that could cause the world’s temperature to rise much faster than the models are projecting. The substance, methane hydrate, is a combination of methane gas and frozen water. It looks like snow but is 90% methane, a potent greenhouse gas. Methane is 20 to 30 times more effective than CO2 at trapping heat in the Earth’s atmosphere. If global warming thaws the tundra and warms the oceans sufficiently, huge amounts of this gas could be released.

VII. What Might Happen If the Oceans Warm?

If projections are correct and there is a rise in the level of the oceans due to expansion of warmed water and melt from the ice caps, the sea will come up about one foot in the next 25 to 40 years. This would result in flooding of many coastal and island areas around the world, affecting not only human resources but natural coastal ecosystems as well.


EC4.1. Investigation:
Mapping Future Wetlands

Find out how a coastal area might be affected if ocean levels rise.

Find out how a coastal area might be affected if ocean levels rise.

What You Need

  • several pieces of clear acetate
  • fine tip pens for writing on acetate
  • topographic map and street map of a low-lying coastal area

Getting Started

  1. Choose a coastal area of the country that interests you, preferably one that includes urban as well as natural areas. 
  2. Locate a topographic map that shows contour lines for sea level and 5 foot increments of elevation. Locate a street map that shows urban landmarks. 
  3. Lay a clear sheet of plastic or acetate over the topographic map and trace the coastline with an indelible pen. Trace any reefs or tide pools shown on the map.
  4. Using another color, trace and label the five-foot contour line.
  5. Refer to the street map to determine the urban developments located in the zone at risk, and label major landmarks.
  6. Write a one-page report on the potential effects of a rise in sea level on urban and natural areas. Include your recommendations for how the community might respond to the threat of flooding.

Questions to Research

Have there been recent floods or storms that have damaged the area?

What was the natural ecosystem like before the urban development?

VIII. How Might Temperature Changes Affect Organisms

The plants and trees used by wildlife are adapted to specific climate conditions that have been relatively stable for thousands of years. Within the next fifty years, the average global temperature is predicted to rise by 3 degrees Celsius. Computer climate predictions show entire temperature and rainfall belts shifting several hundred miles by the middle of the next century. Those species of plants that are unable to quickly disperse to the new regions where the climate is suitable may become extinct over large regions.

Plant species can move—up to half a mile a year, as their seeds are blown by the wind or carried by animals. 

During the Ice Ages, the plant communities were very different than they are today. 

Many wild places are hemmed in by towns, cities and farms, so without natural corridors forests could end up being marooned in the wrong climate. Those species of plants and animals that require a mature forest in order to complete their life cycle, may become extinct if old forests die out before being replaced elsewhere.

map - Present potential vegetation (i.e. if there were no agriculture)
map- vegetation 18,000 radiocarbon years ago.
white space

IX. Negative Feedback Systems On a Global Scale

Scientists who are skeptical of the climate change models point to the possibility of negative feedback systems moderating the increase in greenhouse gases. They point out that any or all of the following responses could prevent or counteract increasing levels of CO2.

Diagram - negative feedback on a global scale

A: Exploding algae populations in the warmed oceans may use more CO2. In the diagram below, the large “A” arrow represents atmospheric CO2 being taken up by ocean plant life.

B: Land plants may photosynthesize more and absorb more CO2. This possibility is represented by “B” as the increased uptake of CO2 by land plants.

C: A warmer Earth may produce more cloud cover reflecting more heat back into space. This is shown by arrow “C” reflecting the Sun’s rays away from Earth.

X. Positive Feedback Systems On a Global Scale

It is also possible that the following positive feedback responses could accelerate the rate of global warming. A warmer climate and ocean might result in any or all of the following:

diagram - Positive feed back on a global scale might increase global warming
  • Warmer oceans might add more water vapor to the atmosphere. The “A” arrow in the diagram below indicates that more cloud cover could trap more heat at the Earth’s surface and accelerate climate warming.
  • Climate change might warm the permafrost represented by “B”, thereby releasing large amounts of methane gas into the atmosphere. Methane absorbs more of the Sun’s energy than does carbon dioxide, and could accelerate a warming trend. 
  • Warmer oceans might increase low cloud cover and thereby decrease the amount of heat reflected back into space represented by arrow “C”.
  • A warmer climate might stimulate urban populations to increase the use of air conditioning further increasing emissions of CO2 and CFCs shown by arrow “D”.

See Staying current for this chapter.