Link to the Greenhouse Effect and Climate Disruption Student Learning Guide

1. Introduction: Carbon cycle review

In the previous tutorial, you learned about the Carbon Cycle. Take the click-on quiz below to review.

2. Through the combustion of fossil fuels, humans have disrupted the carbon cycle

Over the past 200 years, humans have changed the Earth’s natural carbon cycle.  As you’ve learned, there are natural flows that take carbon dioxide out of the atmosphere (photosynthesis – letter “A” in the carbon cycle diagram above) and release carbon dioxide back into the atmosphere (plant and animal respiration – letters “B” and “D”, and decomposition – letter “G”).  But, through burning fossil fuels (letter “J”), humans have added enough carbon dioxide to the Earth’s system to change the balance. The result is more carbon dioxide remaining in the atmosphere.

2a. Humans are adding enough  CO₂ to the atmosphere to make a difference

How much carbon dioxide? Combustion of fossil fuels from CO₂ adds about 3% to the natural flow of CO₂ into the atmosphere.  While 3% might not sound like a lot, that CO₂ has been accumulating for the last 200 years.

Think of all the cars and factories and power plants all over the world. Most of these are powered by fossil fuels. As a result, most of these are emitting carbon dioxide. To emit means to produce and release something into the environment. What gets emitted is called an emission. Combustion emits carbon dioxide (and water vapor) into the air.

Here are a few examples of activities that result in carbon dioxide emissions.

• Electrical power generation: When you open up your refrigerator, it might not feel like you’re burning anything. But the electricity that powers your refrigerator (and every other electrical appliance in your house) was probably generated in a power plant that made the electricity by burning coal or natural gas. That’s how 60% of the electricity in the United States is generated.
  Note that there are ways to make electricity that don’t result in carbon dioxide emissions. These include solar power, wind power, and hydroelectric power. We’ll briefly discuss these at the end of this tutorial.
• Transportation: Unless it’s an electric vehicle, the energy that powers cars, buses, trucks, and trains is a liquid fuel made from oil (petroleum). That includes the gasoline or diesel that we put in our cars. It also includes the jet fuel that powers airplanes. Even if you’re driving an electric car, the electricity itself might have been generated through the combustion of fossil fuels.
• Home heating and cooking: If you cook with gas, you’re burning what’s called natural gas. Many home heating systems work by burning natural gas. And, as with electrical vehicles, even if you cook with an electric stove, the electricity might come from a fossil fuel-powered generating station.

 

You can see how much carbon dioxide humans have emitted into the air in the graph on the right. Note the increase between 1800, when the Industrial Revolution began, and now.

In 2020, global carbon dioxide emissions were 39 billion tonnes. A metric tonne is 2205 pounds: a bit more than the weight of a car. That’s a lot of carbon dioxide emitted into the air, every year.

 

2b. Emissions of CO₂ are increasing the amount of CO₂ in the atmosphere

Given all of the carbon dioxide we’ve been emitting into the atmosphere, it’s no surprise that the amount of CO₂ in the air is going up.

Since 1958, scientists have been directly measuring the amount of carbon dioxide in the atmosphere. The gas is sampled at various sites around the globe. The most famous of these sites is the Mauna Loa Astronomical Observatory in Hawaii, which is where atmospheric carbon dioxide was first sampled. The scientist who began this effort was named Charles Keeling, and the line showing atmospheric carbon dioxide change is often called the Keeling Curve. 

Carbon dioxide makes up only a small fraction of our atmosphere: much less than one percent. By contrast, nitrogen gas makes up about 78% of the molecules in the air, and oxygen makes up about 21%.  Because there’s so little carbon dioxide, its level is measured in parts per million (ppm). For example, in 1960, the carbon dioxide level was 320 ppm. That means that for every million molecules of gas making up the atmosphere, there were 320 molecules of carbon dioxide. In percentage terms, 320 ppm is 0.032%.

Study the graph above. Over the past 60 years, the fraction of carbon dioxide in the atmosphere has steadily increased. In 2013, the level of carbon dioxide passed 400 ppm. In 2021, the level reached 419 ppm. That’s a 30.9% increase in 60 years.

If you look closely at any year on the Keeling curve, you’ll note that there’s a seasonal rise and fall. The rise and fall are related to the carbon cycle, and two features of the Earth.

1. The Earth has more land mass north of the equator than south of the equator.
2. Enormous forests blanket much of the northern part of the Earth (think of the forests of Canada and Russia).

During the Northern summer, the trees in the northern forests are doing photosynthesis. That pulls carbon out of the air through carbon fixation, pulling the Earth’s carbon dioxide level down. In the winter, those trees can’t do very much photosynthesis (because of the short days). These plants keep themselves alive by doing cellular respiration. That increases the Earth’s carbon dioxide level.

We know the level of carbon dioxide in the air since 1958 through direct measurements of the air. But the record of carbon dioxide levels goes back hundreds of thousands of years. That’s because of air bubbles trapped in glaciers. The air in these bubbles can be hundreds of thousands of years old. Measuring the amount of carbon dioxide in those air bubbles tells us how much carbon dioxide was in the air before 1958.

Based on these samples, it’s been determined that the amount of carbon dioxide in the atmosphere at the start of the Industrial Revolution was 280 ppm. That means that the combustion of fossil fuels has increased the Earth’s carbon dioxide level by just under 50% during the past 200 years.

In the next section, we’ll see the effect that this increase in carbon dioxide levels has had on the Earth’s systems for regulating climate. But first, consolidate your understanding of the terms and concepts above with a set of flashcards.

3. Altering the Carbon Cycle: Flashcards

4. Understanding the Greenhouse Effect

Among the planets in the solar system, the Earth has a uniquely moderate temperature. The Earth has some hot places and some cold places, but if you averaged out all the temperatures around the globe, you’d find that the Earth’s average temperature is 59 °F (15 °C).

The Earth has several systems that control average temperature. One of the most important of these is called the Greenhouse effect.

Why is it called the greenhouse effect?  A greenhouse is a special structure for growing plants. The roof and walls are made of glass or plastic, which let light from the sun enter. The windows also trap heat, which usually warms the greenhouse to a comfortable, plant-friendly temperature, even in the winter.

The greenhouse effect involves the same process of letting light through and trapping heat, but on a planetary scale. Here’s how it works.

4a. CO₂ and Methane are Greenhouse Gases

In the diagram to your right, “1” represents the sun. Number 2 represents the sun’s energy. When the sun’s energy strikes the Earth (“3”), two things happen.

First, some sunlight is reflected out into space. Light reflected from the Earth is represented by arrow # 4.

Second, some of the sun’s energy is converted into heat energy, which is represented by arrow # 5. If you’ve ever walked on hot sand on a beach or hot pavement on the street, then you’ve experienced how the sun’s energy can heat the Earth’s surface.

Heat energy is also called infrared radiation. You can’t see infrared radiation (except with special goggles). But you can feel it. That infrared radiation warms the atmosphere and heads back out into space. In the diagram, the atmosphere is represented by “8,” and heat from the Earth is represented by “5.”

Our atmosphere is mostly made out of nitrogen and oxygen gas, which make up 78% and 21% of the gases in the air. Nitrogen and oxygen let infrared radiation pass right through the atmosphere. However, a few other gases are greenhouse gases. These are represented by “9” in the diagram. Note that in reality, they don’t form a layer, but are mixed throughout the atmosphere.

The most important greenhouse gases are carbon dioxide (CO₂) and methane (CH₄). We’ve talked a lot about carbon dioxide in the context of the carbon cycle. Methane is produced naturally by bacteria that live in the guts of animals like cows, sheep, and people. Every time a cow or sheep (or person) farts or belches, methane is released into the air. Methane is also naturally produced by bacteria in swamps or lakes. In addition, many human activities also produce methane. These activities include coal mining, wastewater treatment, landfill emissions, and industrial processes.

4b. Greenhouse Gases Trap the Earth’s Heat

When infrared radiation from the Earth hits greenhouse gas molecules, those molecules absorb the energy and then re-emit that energy. Some of that energy goes out into space (arrow number 6). Some of the energy gets sent back down into the Earth’s atmosphere (arrow number 7).

Arrow number 7 is what the greenhouse effect is all about. The greenhouse effect traps some of the Earth’s heat and sends it back to the Earth’s surface. It’s estimated that without the greenhouse effect, the Earth’s average temperature would be -15 °C (5 °F). The Earth’s surface would be frozen solid.

So, the greenhouse effect is a good thing. It’s responsible for the mild temperatures that support life on Earth, and which make civilization possible. We’ll see below, however, that human activities — especially human-caused emissions of carbon dioxide and methane — have altered the natural greenhouse effect in a way that is disrupting our climate. We’ve taken the natural greenhouse effect, and created an intensified greenhouse effect.

But first, take the quiz below to make sure that you understand the diagram and information above.

4c. Understanding the Natural Greenhouse Effect

5. Combustion of CO₂, the release of methane, and other changes are creating an Intensified Greenhouse Effect

Let’s look at the carbon cycle and the greenhouse effect side by side. That will enable us to see how human changes to the carbon cycle have changed the greenhouse effect. Those changes are making the greenhouse effect more intense. That is changing the Earth’s climate system.

Here’s a short list of some of the human-caused changes that are intensifying the greenhouse effect.

5a. Increasing the density of the Earth’s Greenhouse Gas Layer

Look at the left side of the carbon cycle diagram. Since 1800, humans have been burning fossil fuels (“J”) to power the generators and other machines  (“6”) that have become the basis of our civilization. The combustion of fossil fuels has increased the amount of carbon dioxide in the atmosphere.

Now, look at the greenhouse effect diagram. Carbon dioxide emissions have increased the density of the greenhouse gas layer (number 9) in the Earth’s atmosphere (8). That’s increased the amount of heat reflected back to the Earth by greenhouse gases (the arrow at 7). It’s as if you were cold at night, and put on a thicker blanket. The blanket would trap more heat. By releasing more carbon dioxide from the combustion of fossil fuels, we’ve thrown a thicker blanket over the Earth, trapping more heat. That’s the intensified, greenhouse effect. That’s increasing temperatures.

Emissions of methane have added to this effect. While methane is much less abundant than carbon dioxide, it’s about 80 times more powerful than CO₂ as a greenhouse gas. Since the Industrial Revolution, methane levels have soared, increasing about nine times from their pre-industrial levels.

Here are some activities that have increased atmospheric methane.

1. Mining coal, oil, and natural gas cause methane to leak into the atmosphere.
2. More livestock. As the human population has increased and eating patterns have changed, more cattle and sheep are being raised than ever before. The Earth has over a billion cattle. That number has been constant since about the 1970s, but it’s much more than the Earth had in 1960 (about 750 million ^(source)), or in 1900 (about 460 million ^(source)). That’s a lot of additional methane released into the air from animal farts and belches. On top of that, ranches are often on land that used to be forested. That means that the carbon that was locked up in the trees is now in the atmosphere, and the remaining plants are doing much less photosynthesis.
3. More landfills and more wastewater treatment.

5b. Deforestation reduces carbon fixation

Deforestation involves cutting down the trees in a forest. The purpose of deforestation can be to harvest wood (logging) or to clear the land for other uses (usually for farming and ranching). Deforestation removes or reduces the size of the plant carbon reservoir at “2,” and reduces the amount of carbon fixation from the atmosphere into plants (at “A”). Because carbon dioxide isn’t being pulled out of the atmosphere, deforestation has the effect of increasing the density of the Earth’s greenhouse gas layer.

5c. Changes to the Earth’s surface that reflect less light and create more heat.

Cities have a lot of concrete and pavement which absorb light and convert it into heat. That makes each city a kind of “heat island:” cities are a few degrees hotter than the surrounding countryside. That means that more of the sun’s light becomes heat, and that adds to the greenhouse effect.  In terms of the diagram, that would make the arrow at “5” thicker.

Melting ice (which we’ll discuss more below) causes less light to be reflected from the Earth into space. When ice is replaced by seawater or land, that land converts the light into heat, which warms the atmosphere. In the greenhouse effect diagram, that makes the arrow at “4” smaller, and the arrows at “5,” “6,” and “7” bigger. We’ll discuss this more below.

5d. Intensifying the Greenhouse Effect: Checking Understanding

Answer the questions below.

6. Consequences of Disrupting the Carbon Cycle and Intensifying the Greenhouse Effect

We’ve seen above how human changes to the carbon cycle (and other changes to the planet) are intensifying the greenhouse effect.

• As humans emit more carbon dioxide and methane into the atmosphere, the layer of greenhouse gases at “9” is becoming denser. That’s increasing the arrow at “7,” trapping more heat in the Earth’s atmosphere.
• The arrow at “4” is getting smaller as ice melts and cities spread. That’s increasing the arrows at “5,” “6,” and “7,” also trapping more heat within the atmosphere.

Here are some of the consequences.

6a. Rising temperatures and Climate Disruption

Trapping more heat in the atmosphere is causing the Earth to warm up. Scientists have accurate records of the Earth’s temperature going back to 1880. During that time, the Earth’s average temperature has risen about 2 °F (slightly more than 1 °C). The rate of temperature increase has doubled in the last 50 years^(source). The hottest seven years on record have all occurred since 2015 ^(source).

Climate is the long-term weather conditions in an area. The increase in average planetary temperatures is referred to as

• Global warming
• Climate change
• Climate disruption

A 1 °C increase in average temperature might not seem like a big deal. But that 1 °C of warming has already caused serious consequences around the globe. If humanity can limit the total amount of warming to 1.5 °C, the consequences will continue to be serious, but not catastrophic. A 2 °C (3.6 °F) increase is widely thought to be the level that could generate a  climate catastrophe that would affect billions of people.

The consequences will vary around the globe. Not all of them are negative.

In some areas that were formerly too cold for farming, agriculture might become possible (see How Russia Wins the Climate Crisis).

In other parts of the globe, the effects will be harmful These include

• More droughts and heat waves
• Stronger and more frequent hurricanes
• Increased wildfires

Temperature changes are altering the habitats that wild animals and plants depend on. This is adding to an ongoing extinction crisis. If you want to learn more about ecosystem disruption (caused by climate change and other human activities), you can do so at this tutorial in our AP Biology Curriculum. 

6b. Rising Sea Levels

As the atmosphere has warmed up, the oceans have warmed up too. All liquids (including water) expand when they are warmed up. As a result, the warming seas are rising.

According to Climate.gov, the website of the U.S. National Oceanic and Atmospheric Administration (NOAA), “sea level has risen 8-9 inches (21 – 24 cm) since 1880.” A new record was set in 2020, and the rate of increase is accelerating.

What’s the problem with sea-level rise? Most people in the world live close to the coastlines. As sea levels rise, coastal cities and towns will face increased flooding and damage from storms.

How much will the sea level continue to rise in the future? It depends on us. If humans continue to emit high levels of carbon dioxide and methane, the rise could be from 0.6 to 1.0 meters by 2100.  That level of sea-level rise will have significant consequences for many coastal towns and cities.

In some predictions, sea-level rise could be much worse. If the large glaciers in Greenland and Antarctica melted, sea levels around the world could rise by many meters. According to the United States Geological Survey, this would flood “every coastal city on the planet.”

With lower emissions, the level of sea-level rise could be less.

6c. Ocean Acidification

Over the past two hundred years, a lot of the carbon dioxide that humans have emitted into the atmosphere has been absorbed by the oceans. When carbon dioxide dissolves in seawater, it becomes a weak acid called carbonic acid. As carbonic acid has built up in the oceans, the oceans have become more acidic. Since the Industrial Revolution began, the pH of the oceans has fallen by 0.1. That’s a 30% increase in acidity ^{(source: NOAA)}.

This is a challenge for many ocean species, especially those with shells. Around the planet, the dual threat of warming temperatures and ocean acidification is endangering coral reefs, which are the most diverse ecosystem in the oceans.

The death of the coral that makes up coral reefs is called coral bleaching. You can learn more about coral bleaching on Wikipedia, or from this article from the Woods Hole Oceanographic Institution.

6d. Positive feedback loops are causing more changes

Human-caused emissions from fossil fuel combustion have caused the atmosphere to warm up. This warming can unleash positive feedback loops, causing other changes to the Earth’s systems which can wind up making the problem much worse.

A positive feedback loop is a system that feeds back on itself in a way that increases the output of the system. In the diagram above, increased temperature causes sea ice to melt (step 1). The darker sea water absorbs more sunlight, causing more warming (2). That warming feeds back into the system and causes more melting (3). Now we’re back to 1, but there’s less ice, which will lead to more melting, which leads to less ice.

Positive feedback loops can lead things to change very quickly. When a positive feedback loop has a harmful consequence, it’s called a vicious cycle. Unfortunately, when it comes to climate change, there could be many such vicious cycles awaiting us.

Another feedback loop involves permafrost and methane. Places like Russia, Canada, and Alaska have huge areas covered with permafrost. Permafrost is ground that remains frozen throughout the year. A lot of permafrost has been frozen since the end of the last Ice Age, about 10,000 years ago. As the temperature rises, the permafrost melts. As the permafrost melts, bacteria in the soil become active and release carbon dioxide and methane into the air. As more carbon dioxide and methane are released, temperatures rise even faster, melting more permafrost, releasing more methane and carbon dioxide, and causing more global warming.

Here’s a third positive feedback loop. The oceans store huge amounts of carbon dioxide. As the Earth warms, so does the water in the oceans. Warm water can hold less carbon dioxide than cold water. As the oceans warm, they release their carbon dioxide. That increases the greenhouse effect, which warms the oceans even more.

 

6e. Carbon Cycle and Greenhouse Effect Disruption Consequences: Checking Understanding

7. Solutions are possible, but humanity has to act fast

Humanity has been aware of the intensification of the greenhouse effect and its consequences for over 40 years. Yet despite numerous studies, conferences, and agreements by the highest levels of world leadership, emissions of greenhouse gases are rising at an accelerating rate.

According to the latest report from the United Nations Intergovernmental Panel on Climate Change,  40% of the world’s population lives in situations and places that are “highly vulnerable to climate change.” Climate change is already causing death and suffering around the world from heat waves, catastrophic forest fires, flooding, and severe storms. If we allow global average temperatures to rise beyond 1.5 °C  above their pre-industrial levels, the positive feedback loops described above might lead to irreversible and catastrophic change.

What actions could reverse our current direction?

ACTION # 1: Aggressive reduction of human-caused greenhouse gas emissions through switching from fossil fuel consumption to the use of renewable, carbon-free energy sources. Instead of combusting fossil fuels, we’d have to power our civilization through electricity generated by wind, solar energy, hydroelectric generators, and geothermal energy.

Action # 1 is far more important than anything else. But the next moves are important too.

ACTION # 2: Reforestation: Caring for current forests, and replanting forests that have been destroyed. New trees could soak up billions of tons of carbon dioxide.

ACTION # 3: Making agriculture less carbon-intensive.  Changing our eating patterns — eating less meat — and raising cattle in ways that reduce their methane output are important moves in reducing climate disruption. That’s because cattle are responsible for about 14.5% of the world’s greenhouse gas emissions. In addition, new methods of farming are being developed that keep more carbon in the soil.

ACTION # 4: Improving our buildings.  Buildings contribute about 33% of worldwide greenhouse gas emissions. The key is to improve insulation and overall energy efficiency.

ACTION # 5:  Reduce fossil fuel-powered transportation. In 2019, transportation accounted for 29% of US greenhouse gas emissions. Improving public transportation, moving closer to work, and biking instead of driving are all key steps toward a carbon-neutral future.

These five are just the beginning. For a more comprehensive approach, take a look at the Stabilization Wedges Approach from Princeton University’s Carbon Mitigation Initiative.

8. The Greenhouse Effect and Climate Disruption Cumulative Quiz

The quiz below will test you on everything above. If you don’t know the answer, take a hint, or guess. The goal is to know all the material by the end. You can do it!

9. Questions you (and everyone) should be able to answer about climate disruption

10. Learn More About Climate Disruption and Solutions

I’m hoping that the material above has given you a solid foundation in terms of understanding climate disruption. If you want to learn more, here are a few links to get you started.

1. United Nations Climate Action Page
2. NASA’s  Global Climate Change Page
3. The US Environmental Protection Agency’s Climate Change Page
4. The US National Oceanic and Atmospheric Administration Climate Change Page. 
5. Climate Resources from The US National Academy of Sciences 
6. The Climate Challenge, from the UK Climate Change Committee
7. Princeton’s Carbon Mitigation Initiative has developed an innovative Stabilization Wedges Approach to reducing carbon emissions. I found it so inspiring and hopeful that I wrote a song about it (see “50 Ways to Reduce Carbon” below.

11. Mr. W’s Climate Change Music Videos

Do you want to learn about climate change through music, rhyme, and animated diagrams? Try the videos below.

1. Greenhouse Effect: How the natural greenhouse effect works, and how humans have disrupted it
2. Keep Those Fossil Fuels in the Ground: The consequences of climate change.
3. Seven Ways to Reduce Carbon: A guide to carbon reduction, based on Princeton’s Carbon Mitigation Initiative Stabilization Wedges Approach, sung to Paul Simon’s “50 Ways to Leave Your Lover.”
4. Disrupt Climate Disruption: What we can do about climate change

12. What’s next?

1. Ecosystems (HS Level) Main Menu
2. Use the menus above to choose another tutorial.