Lesson Objectives

• Describe some ways that climate change has been an important part of Earth history.
• Discuss what factors can cause climate to change and which of these can be exacerbated by human activities.
• Discuss the consequences of rising greenhouse gas levels in the atmosphere, the impacts that are already being measured, and the impacts that are likely to occur in the future.

Vocabulary

• El Niño
• global warming
• La Niña
• Milankovitch cycles
• slash-and-burn agriculture
• sunspot

Introduction

For the past two centuries, climate has been relatively stable. People placed their farms and cities in locations that were in a favorable climate without thinking that the climate could change. But climate has changed throughout Earth history, and a stable climate is not the norm. In recent years, Earth’s climate has begun to change again. Most of this change is warming because of human activities that release greenhouse gases into the atmosphere. The effects of warming are already being seen and will become more extreme as temperature rise.

Climate Change in Earth History

Climate has changed throughout Earth history. Much of the time Earth’s climate was hotter and more humid than it is today, but climate has also been colder, as when glaciers covered much more of the planet. The most recent ice ages were in the Pleistocene Epoch, between 1.8 million and 10,000 years ago (Figure below). Glaciers advanced and retreated in cycles, known as glacial and interglacial periods. With so much of the world’s water bound into the ice, sea level was about 125 meters (395 feet) lower than it is today. Many scientists think that we are now in a warm, interglacial period that has lasted about 10,000 years.

For the past 1500 years, climate has been relatively mild and stable when compared with much of Earth’s history. Why has climate stability been beneficial for human civilization? Stability has allowed the expansion of agriculture and the development of towns and cities.

Fairly small temperature changes can have major effects on global climate. The average global temperature during glacial periods was only about 5.5°C (10°F) less than Earth’s current average temperature. Temperatures during the interglacial periods were about 1.1°C (2.0°F) higher than today (Figure below).

Since the end of the Pleistocene, the global average temperature has risen about 4°C (7°F). Glaciers are retreating and sea level is rising. While climate is getting steadily warmer, there have been a few more extreme warm and cool times in the last 10,000 years. Changes in climate have had effects on human civilization.

• The Medieval Warm Period from 900 to 1300 A.D. allowed Vikings to colonize Greenland and Great Britain to grow wine grapes.
• The Little Ice Age, from the 14th to 19th centuries, the Vikings were forced out of Greenland and humans had to plant crops further south.

Short-Term Climate Changes

Short-term changes in climate are common (Figure below). The largest and most important of these is the oscillation between El Niño and La Niña conditions. This cycle is called the ENSO (El Niño southern oscillation). The ENSO drives changes in climate that are felt around the world about every two to seven years.

In a normal year, the trade winds blow across the Pacific Ocean near the equator from east to west (toward Asia). A low pressure cell rises above the western equatorial Pacific. Warm water in the western Pacific Ocean and raises sea levels by one-half meter. Along the western coast of South America, the Peru Current carries cold water northward, and then westward along the equator with the trade winds. Upwelling brings cold, nutrient-rich waters from the deep sea.

In an El Niño year, when water temperature reaches around 28°C (82°F), the trade winds weaken or reverse direction and blow east (toward South America) (Figure below). Warm water is dragged back across the Pacific Ocean and piles up off the west coast of South America. With warm, low-density water at the surface, upwelling stops. Without upwelling, nutrients are scarce and plankton populations decline. Since plankton form the base of the food web, fish cannot find food, and fish numbers decrease as well. All the animals that eat fish, including birds and humans, are affected by the decline in fish.

By altering atmospheric and oceanic circulation, El Niño events change global climate patterns.

• Some regions receive more than average rainfall, including the west coast of North and South America, the southern United States, and Western Europe.
• Drought occurs in other parts of South America, the western Pacific, southern and northern Africa, and southern Europe.

An El Niño cycle lasts one to two years. Often normal circulation patterns resume. Sometimes circulation patterns bounce back quickly and extremely (Figure below). This is a La Niña.

In a La Niña year, as in a normal year, trade winds moves from east to west and warm water piles up in the western Pacific Ocean. Ocean temperatures along coastal South America are colder than normal (instead of warmer, as in El Niño). Cold water reaches farther into the western Pacific than normal.

An online guide to El Niño and La Niña events from the University of Illinois is found here: http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/eln/home.rxml.

Other important oscillations are smaller and have a local, rather than global, effect. The North Atlantic Oscillation mostly alters climate in Europe. The Mediterranean also goes through cycles, varying between being dry at some times, and warm and wet at others.

The ABC News video explores the relationship of El Niño to global warming. El Niño is named as the cause of strange weather across the United States in the winter of 2007 in this video (5g): http://www.youtube.com/watch?v=5uk9nwtAOio (3:33).

Causes of Long-term Climate Change

Many processes can cause climate to change. These include changes:

• in the amount of energy the Sun produces over years.
• in the positions of the continents over millions of years.
• in the tilt of Earth’s axis and orbit over thousands of years.
• that are sudden and dramatic because of random catastrophic events, such as a large asteroid impact.
• in greenhouse gases in the atmosphere, caused naturally or by human activities.

Solar Variation

The amount of energy the Sun radiates is variable. Sunspots are magnetic storms on the Sun’s surface that increase and decrease over an 11-year cycle (Figure below). When the number of sunspots is high, solar radiation is also relatively high. But the entire variation in solar radiation is tiny relative to the total amount of solar radiation that there is and there is no known 11-year cycle in climate variability. The Little Ice Age corresponded to a time when there were no sunspots on the Sun.

Plate Tectonics

Plate tectonic movements can alter climate. Over millions of years as seas open and close, ocean currents may distribute heat differently. For example, when all the continents are joined into one supercontinent (such as Pangaea), nearly all locations experience a continental climate. When the continents separate, heat is more evenly distributed.

Plate tectonic movements may help start an ice age. When continents are located near the poles, ice can accumulate, which may increase albedo and lower global temperature. Low enough temperatures may start a global ice age.

Plate motions trigger volcanic eruptions, which release dust and CO₂ into the atmosphere. Ordinary eruptions, even large ones, have only a short-term effect on weather (Figure below). Massive eruptions of the fluid lavas that create lava plateaus release much more gas and dust, and can change climate for many years. This type of eruption is exceedingly rare; none has occurred since humans have lived on Earth.

Milankovitch Cycles

The most extreme climate of recent Earth history was the Pleistocene. Scientists attribute a series of ice ages to variation in the Earth’s position relative to the Sun, known as Milankovitch cycles.

The Earth goes through regular variations in its position relative to the Sun:

1. The shape of the Earth’s orbit changes slightly as it goes around the Sun. The orbit varies from more circular to more elliptical in a cycle lasting between 90,000 and 100,000 years. When the orbit is more elliptical, there is a greater difference in solar radiation between winter and summer.

2. The planet wobbles on its axis of rotation. At one extreme of this 27,000 year cycle, the Northern Hemisphere points toward the Sun when the Earth is closest to the Sun. Summers are much warmer and winters are much colder than now. At the opposite extreme, the Northern Hemisphere points toward the Sun when it is farthest from the Sun. This results in chilly summers and warmer winters.

3. The planet’s tilt on its axis varies between 22.1° and 24.5°. Seasons are caused by the tilt of Earth’s axis of rotation, which is at a 23.5° angle now. When the tilt angle is smaller, summers and winters differ less in temperature. This cycle lasts 41,000 years.

When these three variations are charted out, a climate pattern of about 100,000 years emerges. Ice ages correspond closely with Milankovitch cycles. Since glaciers can form only over land, ice ages only occur when landmasses cover the polar regions. Therefore, Milankovitch cycles are also connected to plate tectonics.

Changes in Atmospheric Greenhouse Gas Levels

Since greenhouse gases trap the heat that radiates off the planet’s surfaces what would happen to global temperatures if atmospheric greenhouse gas levels decreased? What if greenhouse gases increased? A decrease in greenhouse gas levels decreases global temperature and an increase raises air temperature.

Greenhouse gas levels have varied throughout Earth history. For example, CO₂ has been present at concentrations less than 200 parts per million (ppm) and more than 5,000 ppm. But for at least 650,000 years, CO₂ has never risen above 300 ppm, during either glacial or interglacial periods (Figure below).

Natural processes add and remove CO₂ from the atmosphere

• Processes that add CO₂
  • volcanic eruptions
  • decay or burning of organic matter.
• Processes that remove CO₂
  • absorption by plant and animal tissue.

When plants are turned into fossil fuels the CO₂ in their tissue is stored with them. So CO₂ is removed from the atmosphere. What does this do to Earth’s average temperature?

What happens to atmospheric CO₂ when the fossil fuels are burned? What happens to global temperatures?

Fossil fuel use has skyrocketed in the past few decades more people want more cars and industrial products. This has released CO₂ into the atmosphere.

Burning tropical rainforests, to clear land for agriculture, a practice called slash-and-burn agriculture, also increases atmospheric CO₂. By cutting down trees, they can no longer remove CO₂ from the atmosphere. Burning the trees releases all the CO₂ stored in the trees into the atmosphere.

There is now nearly 40% more CO₂ in the atmosphere than there was 200 years ago, before the Industrial Revolution. About 65% of that increase has occurred since the first CO₂ measurements were made on Mauna Loa Volcano, Hawaii, in 1958 (Figure below).

CO₂ is the most important greenhouse gas that human activities affect because it is so abundant. But other greenhouse gases are increasing as well. A few are:

• Methane: released from raising livestock, rice production, and the incomplete burning of rainforest plants.
• Chlorofluorocarbons (CFCs): human-made chemicals that were invented and used widely in the 20th century.
• Tropospheric ozone: from vehicle exhaust, it has more than doubled since 1976.

Global Warming

With more greenhouse gases trapping heat, average annual global temperatures are rising. This is known as global warming.

Global warming – How Humans are Affecting our Planet from NASA, discusses the basics of global warming science (4c): http://www.youtube.com/watch?v=VXvGPbHXxtc (7:58).

Temperatures are Increasing

While temperatures have risen since the end of the Pleistocene, 10,000 years ago, this rate of increase has been more rapid in the past century, and has risen even faster since 1990. The nine warmest years on record have all occurred since 1998, and the 10 of the 11 warmest years have occurred since 2001 (through 2012) (Figure below). The 2000s were the warmest decade yet.

Annual variations aside, the average global temperature increased about 0.8°C (1.5°F) between 1880 and 2010, according to the Goddard Institute for Space Studies, NOAA. This number doesn’t seem very large. Why is it important? http://www.giss.nasa.gov/research/news/20100121/

The United States has long been the largest emitter of greenhouse gases, with about 20% of total emissions in 2004 (Figure below). As a result of China’s rapid economic growth, its emissions surpassed those of the United States in 2008. However, it’s also important to keep in mind that the United States has only about one-fifth the population of China. What’s the significance of this? The average United States citizen produces far more greenhouse gases than the average Chinese person.

An animation of CO₂ released by different fossil fuels is seen here: CO₂ release by different fossil fuels at http://www.nature.nps.gov/GEOLOGY/usgsnps/oilgas/CO2BTU_3.MPG

If nothing is done to decrease the rate of CO₂ emissions, by 2030, CO₂ emissions are projected to be 63% greater than they were in 2002.

A number of videos on the National Geographic site deal with global warming. Go to National Geographic Videos, Environment Videos, Global Warming, http://video.nationalgeographic.com/video/player/environment/.

• A no-nonsense look at global warming and what we can do about it is found in “A Way Forward: Facing Climate Change.”
• “Antarctic Ice” describes the changes that are already happening to Antarctica and what the consequences of future melting will be.
• “Glacier Melt” looks at melting in a large alpine glacier and the effects of glacier loss to Europe.
• In “Greenhouse Gases” researchers look at the effects of additional greenhouse gases on future forests.
• Researchers look for changes in the range of a mountain-top dwelling mammal, the pika.
• Polar bears, in their specialized habitat in the Arctic, are among the species already affected by warming temperatures.

KQED: Climate Watch: California at the Tipping Point

Warming temperatures are bringing changes to much of the planet, including California. Sea level is rising, snow pack is changing and the ecology of the state is responding to these changes. Learn more at: http://science.kqed.org/quest/video/climate-watch-california-at-the-tipping-point/.

Future Warming

The amount CO₂ levels will rise in the next decades is unknown. What will this number depend on in the developed nations? What will it depend on in the developing nations? In the developed nations it will depend on technological advances or lifestyle changes that decrease emissions. In the developing nations, it will depend on how much their lifestyles improve and how these improvements are made.

Computer models are used to predict the effects of greenhouse gas increases on climate for the planet as a whole and also for specific regions. If nothing is done to control greenhouse gas emissions and they continue to increase at current rates, the surface temperature of the Earth can be expected to increase between 0.5°C and 2.0°C (0.9°F and 3.6°F) by 2050 and between 2° and 4.5°C (3.5° and 8°F) by 2100, with CO₂ levels over 800 parts per million (ppm). On the other hand, if severe limits on CO₂ emissions begin soon, temperatures could rise less than 1.1°C (2°F) by 2100.

This video explores the tools NASA scientists use to determine how the climate is changing (6d): http://www.youtube.com/watch?v=JRayIgKublg (4:00).

Whatever the temperature increase, it will not be uniform around the globe. A rise of 2.8°C (5°F) would result in 0.6° to 1.2°C (1° to 2°F) at the equator, but up to 6.7°C (12°F) at the poles. So far, global warming has affected the North Pole more than the South Pole, but temperatures are still increasing at Antarctica (Figure below).

The following images show changes in the earth and organisms as a result of global warming: Figure below, Figure below, and Figure below.

The timing of events for species is changing. Mating and migrations take place earlier in the spring months. Species that can are moving their ranges uphill. Some regions that were already marginal for agriculture are no longer farmable because they have become too warm or dry.

Modeled Climate Induced Glacier change in Glacier National Park 1850-2100: http://www.nrmsc.usgs.gov/research/glacier_model.htm

What are the two major effects being seen in this animation? Glaciers are melting and vegetation zones are moving uphill. If fossil fuel use exploded in the 1950s, why do these changes begin early in the animation? Does this mean that the climate change we are seeing is caused by natural processes and not by fossil fuel use?

Animations of temperature anomalies for 5- and 10-year periods: http://data.giss.nasa.gov/gistemp/animations/

As greenhouse gases increase, changes will be more extreme. Oceans will become slightly more acidic, making it more difficult for creatures with carbonate shells to grow, and that includes coral reefs. A study monitoring ocean acidity in the Pacific Northwest found ocean acidity increasing ten times faster than expected and 10% to 20% of shellfish (mussels) being replaced by acid tolerant algae.

Plant and animal species seeking cooler temperatures will need to move poleward 100 to 150 km (60 to 90 miles) or upward 150 m (500 feet) for each 1.0°C (8°F) rise in global temperature. There will be a tremendous loss of biodiversity because forest species can’t migrate that rapidly. Biologists have already documented the extinction of high-altitude species that have nowhere higher to go.

Decreased snowpacks, shrinking glaciers, and the earlier arrival of spring will all lessen the amount of water available in some regions of the world, including the western United States and much of Asia. Ice will continue to melt and sea level is predicted to rise 18 to 97 cm (7 to 38 inches) by 2100 (Figure below). An increase this large will gradually flood coastal regions where about one-third of the world’s population lives, forcing billions of people to move inland.

Weather will become more extreme with heat waves and droughts. Some modelers predict that the Midwestern United States will become too dry to support agriculture and that Canada will become the new breadbasket. In all, about 10% to 50% of current cropland worldwide may become unusable if CO₂ doubles.

Although scientists do not all agree, hurricanes are likely to become more severe and possibly more frequent. Tropical and subtropical insects will expand their ranges, resulting in the spread of tropical diseases such as malaria, encephalitis, yellow fever, and dengue fever.

You may notice that the numerical predictions above contain wide ranges. Sea level, for example, is expected to rise somewhere between 18 and 97 cm — quite a wide range. What is the reason for this uncertainty? It is partly because scientists cannot predict exactly how the Earth will respond to increased levels of greenhouses gases. How quickly greenhouse gases continue to build up in the atmosphere depends in part on the choices we make.

An important question people ask is this: Are the increases in global temperature natural? In other words, can natural variations in temperature account for the increase in temperature that we see? The answer is no. Changes in the Sun’s irradiance, El Niño and La Niña cycles, natural changes in greenhouse gas, and other atmospheric gases cannot account for the increase in temperature that has already happened in the past decades.

This video discusses how, by using the CERES satellite, scientists monitor energy in the atmosphere, including incoming solar energy and reflected and absorbed energy. Greenhouse warming that results from atmospheric greenhouse gasses is also monitored (4c): http://www.youtube.com/watch?v=JFfD6jn_OvA (4:31).

KQED: Going UP: Sea Level Rise in San Francisco Bay

Along with the rest of the world’s oceans, San Francisco Bay is rising. Changes are happening slowly in the coastal arena of the San Francisco Bay Area and even the most optimistic estimates about how high and how quickly this rise will occur indicate potentially huge problems for the region. Learn more at: http://science.kqed.org/quest/video/going-up-sea-level-rise-in-san-francisco-bay/.

Lesson Summary

• Climate has changed throughout Earth history. In general, when greenhouse gas levels are high, temperature is high.
• Greenhouse gases are now increasing because of human activities, especially fossil fuel use.
• We are already seeing the effects of these rising greenhouse gases in higher temperatures and changes to physical and biological systems.
• Society must choose to reduce greenhouse gas emissions or face more serious consequences.

Review Questions

1. Why is the climate currently warming?

2. Why does sea level rise and fall during interglacial and glacial periods?

3. How can the human history of Greenland be related to climate cycles?

4. If climate has been much warmer in Earth history, why do we need to worry about global warming now?

5. When the weather along coastal California is especially rainy with many winter storms, what is likely to be happening in the equatorial Pacific?

6. The Peruvian anchovy fishery collapsed in 1972. Using what you know about climate and food webs, can you devise an explanation for this event?

7. What two events must occur for there to be an ice age?

8. What human activities increase greenhouse gases in the atmosphere? Explain.

9. Why are CO₂ emissions projected to increase during the next few decades?

10. What role will the developed nations play in increasing CO₂ emissions in the next few decades?

11. Why do storms increase in frequency and intensity as global temperatures increase?

12. Earth is undergoing some important changes, some of which are known about because of and monitored by satellites. Describe the sort of global change that satellites can monitor.

13. What will happen if sea level rises by 60 cm (2 feet) by the end of this century? Which locations will be hardest hit?

14. What can be done to reduce greenhouse gas emissions?

Virtually all credible scientists agree that Earth is warming and human actions are largely to blame. The evidence comes from many areas of science: atmospheric chemistry, earth history, glaciology, ecology, astronomy (stars, the Sun), energy (fossil fuels), oceanography, remote sensing, agricultural science, and others. Because the media like to present a “balanced” story, media outlets often present the side of climate skeptics who do not believe that global warming is happening, or that if it is happening, that human actions are largely responsible.

From the following videos you can learn basic global warming science, the effects already being seen from changing climate, and learn a bit about risk assessment:

Global Warming 101 touches on all aspects of the global warming story in just a few minutes (1l – I&E Stand.): http://www.youtube.com/watch?v=-lubjnPA0b0 (1:28).

Observations made for the past decade by the TERRA satellite shows how Earth is changing because of warmer temperatures (1l – I&E Stand.): http://www.youtube.com/watch?v=h-VvMUseE_o (4:57).

The Most Terrifying Video You’ll Ever See evaluates the risks of choosing action or inaction on global warming (1l – I&E Stand.): http://www.youtube.com/watch?v=zORv8wwiadQ (9:34).

There are many other videos that look at the issue of climate change, some by those who deny that it is happening. Look at some videos created by the so-called climate skeptics and write down their arguments, then write down the scientific counter-arguments. Next check out this series: Climate Crock of the Week ‘(1l – I&E Stand.) http://www.youtube.com/watch?v=_KK8F5noCrA (2:02), which dismantles the arguments made by those who deny global warming science one by one.

California has gone its own way by passing legislation to reduce the state’s greenhouse gas emissions to 1990 levels. A strong proponent is Governor Arnold Schwarzenegger, who has broken with the Republican party in accepting that global warming is real and that something must be done to slow its effects. The following videos address California’s cap-and-trade policy and the legislation:

Governor Schwarzenegger discusses why California Chose Cap-and-Trade in regulating carbon emissions (1m – I&E Stand.): http://www.youtube.com/watch?v=fON7t5DPQbk (3:40).

Governor Arnold Schwarzenegger explains why the emissions standards adopted by California should be picked up by the rest of the country (1m – I&E Stand.): http://www.youtube.com/watch?v=VnZtT7Nj1rI (3:52).

Here Governor Schwarzenegger addresses the impact of global warming on fires, attacks the Bush administration on its policies on global warming and drilling for oil off the coast of California, and reviews recent U.S. history on alternative energy research (1m – I&E Stand.): http://www.youtube.com/watch?v=osBNMvp2Cws (5:24).

California water board official answers questions about California’s legislation on global warming (1m – I&E Stand.): http://www.youtube.com/watch?v=h-ZMsNdd-34 (4:03).

Further reading/supplemental links

Illustrating the concept of El Niño and La Niña: http://earthguide.ucsd.edu/enso/.

Points to Consider

• Nearly all climate scientists agree that human activities are causing the accelerated warming of the planet that we see today. Why do you think that the media is still talking about the controversy about this idea when scientists are almost entirely in agreement?
• If greenhouse gas emissions must be lowered to avoid some of the more serious consequences of global warming, why have humans not done something to lower these emissions instead of letting them increase?
• In what ways can progress be made in reducing greenhouse gas emissions? Think about this on a variety of scales: for individuals, local communities, nations, and the global community.