⏳ Estimated Reading Time: 9 - 11 minutes

Identify greenhouse gases and the sources and potency of these gases.

The Greenhouse Effect

This image illustrates a winter scene with a greenhouse amidst snow. The sun shines in the clear sky, casting rays into the greenhouse where various green plants thrive. A cheerful snowman with stick arms stands outside, completing this picturesque setting. Ideal for screen readers, this description conveys the warmth and growth within the greenhouse contrasted against the snowy landscape. — **The ozone layer protects Earth from the most harmful rays of the sun's radiation.**

The image to the right is a greenhouse. If you have ever been in a real greenhouse, you know that the temperature inside is typically much warmer than the temperature outside.

These structures are made of clear glass or acrylic or plastic. High-energy incoming solar radiation can easily pass into the greenhouse but, once inside, the sunlight bounces off of the plants and surfaces in the greenhouse, loses a lot of its energy, and cannot easily get back out.

This same principle is also why your car is very hot inside during the summers and can even be warm inside on a cold winter’s day.

This phenomenon is known as the greenhouse effect. Earth has a similar system because it acts like a greenhouse. Instead of a clear covering, though, the earth has an atmosphere composed of different gases such as nitrogen (78%), oxygen (21%), and a very small — but very important! — contingent of other gases. Some of the minor gases play a very important role in creating the greenhouse effect for Earth and are called greenhouse gases (GHGs). Greenhouse gases include water vapor, carbon dioxide, methane, nitrous oxide, sulfur hexafluoride, ozone, chlorofluorocarbons (CFCs), and a few others.

Earth’s greenhouse effect is a function of incoming solar radiation that strikes surfaces then re-radiates back into the atmosphere. Solar radiation enters Earth’s atmosphere, most is absorbed by the Earth’s surface, and some bounces off Earth’s surface and tries to leave. Historically, our atmosphere keeps some of this radiation from trying to escape in the Earth’s atmosphere and only lets some escape. Greenhouse gases facilitate this process.

the natural greenhouse effect. The Sun emits solar radiation toward Earth, with some heat escaping into space and the rest being re-radiated by the atmosphere. The image visually represents this process, emphasizing the interaction between solar energy, Earth, and its atmospheric layers. — **The greenhouse effect naturally warms the Earth.**

We want the greenhouse effect to occur. If it did not, our climate would be too cold for life. The problem we now face is that human activities such as burning fossil fuels (coal, oil, and natural gas), agriculture, and land clearing are increasing the concentrations of GHGs.

When greenhouse gases increase in concentration in the atmosphere, they trap more infrared radiation, increasing the ambient temperature of Earth. However, the greenhouse effect can increase too much, making our planet too hot. Move the slider to see how increasing the greenhouse effect can warm the planet:

Although the concept of the greenhouse effect is indisputable, the notion of climate change being caused by human activities is rooted in controversy as we will see in another lesson.

Essential Knowledge

The greenhouse effect results in the surface temperature necessary for life on Earth to exist.

The Greenhouse Gases

Human-produced GHGs are of great concern to climate scientists and much effort is going into monitoring, studying, and modeling of climate change in response to those GHGs.

The Global Warming Potential (GWP) was developed to allow comparisons of the global warming impacts of different gases. Specifically, it is a measure of how much energy the emissions of 1 ton of a gas will absorb over a given time period, relative to the emissions of 1 ton of carbon dioxide (CO₂). Carbon dioxide, which has a global warming potential (GWP) of 1, is used as a reference point for the comparison of different greenhouse gases and their impacts on global climate change. The larger the GWP, the more that gas warms the Earth compared to CO₂ over a 100-year time period. The table below shows the GWP of the main GHGs:

**Global Warming Potential (GWP) of the Main Greenhouse Gases (GHG)**
Greenhouse Gas	Pre-Industrial Concentrations	2021 concentrations	Human Source(s)	Global Warming Potential (GWP)
Carbon dioxide (CO₂)	278 ppm	414.7 ppm	fossil fuel combustion, land use changes, cement production	1
Methane (CH₄)	700 ppb	1895.7 ppb	fossil fuels, rice paddies, waste dumps, livestock	25
Nitrous Oxide (N₂O)	270 ppb	334.3 ppb	fertilizer, industrial processes, fossil fuel combustion	298
Hydrofluorocarbons (HFC-23)	0	data not available	liquid coolants	14,800
Perfluorocarbons (CF₄)	0	data not available	refrigerant, electronic industry and aluminum industry	6,500
Sulfur hexafluoride (SF₆)	0	0.39 ppt	insulator in electronics and magnesium industry	22,800

Note that, compared to carbon dioxide, the other GHGs in the table have much higher global warming potentials and, thus, a greater impact to the greenhouse effect over time.

Explore the tabs below to learn more about each greenhouse gas. Are there any that surprise you?

CO₂
CH₄
H₂O Vapor
NO_x
CFCs/HFCs
O₃

Carbon Dioxide (CO₂)

What is it?

Carbon dioxide (CO₂) is an odorless, colorless naturally occurring gas that plays a significant role in the Earth's atmosphere. Carbon dioxide has become the subject of much debate and controlling the anthropogenic production of this gas has come to the forefront of corporate policy and politics.

What are the sources?

Burning fossil fuels, burning trees, and volcanic eruption are sources of carbon dioxide. Oceans and terrestrial ecosystems, especially forests, are sinks of carbon dioxide. Deforestation is especially problematic, because it releases CO₂ and destroys a carbon sink.

Is it increasing?

Carbon dioxide (CO₂) is the most prevalent greenhouse gas. In 2009, its concentration in the atmosphere was 39% higher than it was before the industrial revolution. Explore the graph below and see how carbon dioxide emissions have steadily increased since the Industrial Revolution. Check out the worldwide dip in CO₂ emissions in 2020 due to COVID-19!

Methane (CH₄)

What is it?

Methane (CH₄) is a greenhouse gas. It is 25 times more potent as a greenhouse gas than carbon dioxide and can last in the atmosphere for 25 years. It is the second-largest contributor to climate warming after carbon dioxide (CO₂).

What are the sources?

The primary natural sources of methane include wetlands, termites, volcanoes, seafloor vents, and methane hydrates in polar permafrost.

Human activities contribute approximately 70% of total annual methane emissions, with the major anthropogenic sources being rice cultivation, livestock farming, coal and natural gas burning, biomass combustion, and organic matter decomposition in landfills.

Is it increasing?

Methane in the atmosphere has more than doubled since the industrial revolution, primarily due to human activities.

Explore the graph below, and see how methane emissions are steadily increasing:

Water (H₂O) Vapor

What is it?

Water vapor (H₂O) is the most important GHG, since globally it is the most abundant of these gases, although it varies from 0-3% in a given location. It is less potent than carbon dioxide and lasts around 9 days in the atmosphere.

What are the sources?

Evaporation, transpiration, and sublimation contribute to the water vapor content in the atmosphere. Because water vapor has a short residual time in the atmosphere and it is not a human-produced greenhouse gas, scientists are not concerned about it as one of the gases involved in climate change. However, water vapor is a consequence of warming, amplifying the warming caused by other greenhouse gases through a positive feedback loop.

Is it increasing?

It has been increasing in the atmosphere since 1988. As humidity rises, it traps more heat, increasing the amount of water vapor in the atmosphere.

Nitrous Oxide (NO_x)

What is it?

NOx refers to a group of highly reactive gases, primarily nitric oxide (NO) and nitrogen dioxide (NO2), which are key components of the nitrogen oxides that contribute to air pollution. They are 300 times more potent than carbon dioxide and can last for 114 years in the atmosphere.

What are the sources?

Agriculture, oceans, lightning, biomass burning, burning of fossil fuels, and industry can all release nitrous oxide into the atmosphere.

Is it increasing?

The atmospheric concentration of N₂O was 270 parts per billion in 1750 and rose to 331 parts per billion in 2018, with the fastest rise occurring in the last five decades. Explore the graph below and see how NO_x emissions are steadily increasing. Check out the worldwide dip in NO_x emissions in 2020 due to COVID-19!

CFCs/HFCs

What are they?

CFCs and halocarbons are also greenhouse gases. They are banned now but are 1600 to 13,000 times more potent than carbon dioxide and can last 55 years in the atmosphere.

What are the sources?

Refrigerators, air conditioning, solvents, aerosol sprays, and fire retardants are all sources of CFCs/halocarbons.

Are they increasing?

The levels of certain CFCs have been increasing in the atmosphere, which is surprising since their production was banned under the Montreal Protocol 🕶️. This increase is attributed to their use in the production of other chemicals, including HFCs.

The production of HFCs is predicted to further increase over the next few years, potentially leading to increased emissions.

Explore the graph below and see how CFC emissions have decreased from a maximum in 1988. Notice how natural emissions of these substances have remained constant since the 1960s:

Ozone (O₃)

What is it?

Tropospheric ozone (O₃) is a significant greenhouse gas. It is a secondary air pollutant, created when NO_x and VOCs react with sunlight.

What are the sources?

Sunlight on pollutants, emissions from industrial facilities, electric utilities, vehicle exhaust, gasoline vapors, burning solvents, burning forests, and agricultural wastes can all produce tropospheric ozone.

Is it increasing?

Ground-level ozone has been increasing, particularly since the industrial revolution. It has increased 36% since 1750. This increase is tied to NOx gases and VOCs, which are byproducts of combustion. With more heat and sunlight, especially in the summer months, more ozone is formed, leading to higher pollution levels.

Explore the graph below and see how tropospheric ozone emissions have increased since the 1990s. When you play the time-lapse, notice how the maximum concentration on the y-axis increases:

Essential Knowledge

The principal greenhouse gases are carbon dioxide, methane, water vapor, nitrous oxide, and chlorofluorocarbons (CFCs).

While water vapor is a greenhouse gas, it doesn't contribute significantly to global climate change because it has a short residence time in the atmosphere.

Carbon dioxide, which has a global warming potential (GWP) of 1, is used as a reference point for the comparison of different greenhouse gases and their impacts on global climate change. Chlorofluorocarbons (CFCs) have the highest GWP, followed by nitrous oxide, then methane.

AP Exam Tip

You should know the relative GWP of the greenhouse gases.

You should understand WHY we don't worry about water vapor as a greenhouse gas as much as we worry about the other greenhouse gases even though it is more potent than CO₂.

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