1.4: Lab 4 - Earth's Energy Budget

Learning Objectives

• Define electromagnetic radiation and describe the electromagnetic spectrum.
• Analyze radiation data.
• Explain Earth’s energy budget.
• Differentiate among radiation, absorption, reflection, scattering, refraction, and transmission.
• Interpret radiation data collected through remote sensing techniques.

Introduction

In the previous lab, we studied how Earth’s annual revolution around the Sun influences the angle of incoming solar rays and the length of day at different latitudes. In this lab, we will look at how those angles play a role in insolation patterns at Earth’s surface and the amount of energy received on Earth during a given time period and at a given latitude. Insolation is the amount of incoming solar radiation. Radiation is both the emission and transmission of energy in the form of electromagnetic rays. Insolation is the energy emitted by the Sun, and transmitted through space, that reaches Earth.

Part A. Electromagnetic Radiation

Before we investigate Earth’s energy budget, let’s think about radiation—energy transmitted through space. Have you ever wondered how wifi, microwaves, or radios work? Each of these devices make use of electromagnetic waves. Electromagnetic radiation is made up of vibrating waves of electrical and magnetic fields that can travel through empty space, air, and solid objects. This radiation differs in terms of energy, wavelength, and frequency. Wavelength is the distance between the individual waves measured from crest to crest (Figure 4.1). Frequency is the number of waves which pass a point in space each second.

Figure 4.1: Electromagnetic Wave. Figure by OpenStax College is licensed under CC BY 4.0

Exercise \(\PageIndex{1}\)

1. Refer to Figure 4.2 (below). Which electromagnetic wave example represents a shorter wavelength?

Figure 4.2: Two Electromagnetic Waves. Figure by Waverly Ray is licensed under CC BY-NC-SA 4.0

In this part of the lab, we will focus on the forms of energy emitted by the Sun. There are several forms of electromagnetic radiation in the Universe (Figure 4.3). All objects with temperatures above absolute zero (-273.15 degrees Celsius; -459.67 degrees Fahrenheit) radiate energy proportional to its temperature. The Sun emits radiation at all wavelengths but 99% of the energy received at the Earth is in the infrared, visible light, or ultraviolet parts of the electromagnetic spectrum. Of this radiation received by our planet, 48% is infrared radiation, 44% is visible light radiation, and 7% is ultraviolet radiation. Note that infrared radiation is sometimes called longwave radiation or thermal radiation, and ultraviolet radiation is sometimes called shortwave radiation or UV radiation.

Figure 4.3: The Electromagnetic Spectrum. Figure by Waverly Ray is licensed under CC BY-NC-SA 4.0

Exercise \(\PageIndex{2}\)

1. Refer to Figure 4.3.

1. Does infrared, visible, or ultraviolet radiation have the longest wavelength?

2. Is wifi considered longwave or shortwave radiation?

3. Are X-rays considered longwave or shortwave radiation?

4. Which radiation has a longer wavelength, wifi radiation or microwave radiation?

Check It Out! Tour of the Electromagnetic Spectrum

Watch this video, Tour of the EMS 01 – Introduction, for a tour of the electromagnetic spectrum from NASA. (Video length is 5:03).

Part B. Net Radiation

Net radiation is the balance between incoming and outgoing energy at the top of the atmosphere. Some of the sunlight that reaches Earth is reflected back to space by bright surfaces like clouds or ice. The rest is absorbed by the atmosphere, oceans, and land. This absorbed light is converted to heat, which is emitted back to space by the surface of the earth and the atmosphere. Net radiation is the total amount of absorbed sunlight and heat energy that does not escape from the top of the Earth's atmosphere back into space. Specifically, net radiation is the sum total of shortwave and longwave electromagnetic energy, at wavelengths ranging from 0.3 to 100 micrometers, that remains in the Earth system. Net radiation is the energy that is available to influence the climate. On a global scale, net radiation must be zero or else the planet's overall temperature must rise or fall.^[59]

Most simply, the net radiation equation is as follows:

Net Radiation = Incoming Radiation - Outgoing Radiation

Incoming radiation is insolation; outgoing radiation is both reflected shortwave radiation and longwave radiation that is emitted from the Earth’s surface. It is important to point out that net radiation is calculated at the top of the atmosphere. If net radiation was calculated at the surface of the Earth, then we would include incoming longwave radiation that is absorbed and re-emitted by greenhouse gases in the atmosphere (the greenhouse effect, which is explained in greater detail in the climate change lab).

Table 4.1 provides monthly and annual average insolation data in watts per meters squared (W/m²) for three locations. This represents incoming radiation. A watt is measurement of power, or the amount of energy that something generates or uses over time. An incandescent light bulb uses anywhere from 40 to 100 watts. A microwave uses about 1000 watts. If you could capture and re-use, for just one hour, all the solar energy arriving over a single square meter at the top of the atmosphere directly facing the Sun—an area no wider than an adult’s outstretched arm span—you would have enough to run a refrigerator all day.^[60]

Table 4.1: Monthly and Annual Average Insolation (W/m2) for Three Locations

Location Approximate Latitude	March	June	September	December	Annual
Deadhorse, Alaska 70°N	133	488	181	0	198
Beijing, China 40°N	323	483	351	160	330
Kamala, Uganda 0°	439	388	429	413	420

Exercise \(\PageIndex{3}\)

3. Plot the monthly average data presented in Table 4.1 above on the blank graph provided in Figure 4.4. Use a different colored pencil for each location (blue for Deadhorse, green for Beijing, and red for Kampala) and connect the points with a line.

Figure 4.4: Blank Graph to Plot Insolation. Figure by Waverly Ray is licensed under CC BY-NC-SA 4.0

4. Refer to your completed Figure 4.4.

1. Describe the insolation variation for Deadhorse.

2. Use Your Critical Thinking Skills: What explains the insolation variation for Deadhorse?

3. Describe the insolation variation for Beijing.

4. Use Your Critical Thinking Skills: What explains the insolation variation for Beijing?

5. Describe the insolation variation for Kampala.

6. Use Your Critical Thinking Skills: What explains the insolation variation for Kampala?

5. Refer to Table 4.1 and your completed Figure 4.4.

1. Which location has the highest monthly average insolation? In which month does this occur?

1. Apply What You Learned: Why does this location receive the highest monthly average insolation?

In the net radiation equation, outgoing radiation includes reflected shortwave radiation and terrestrial radiation, which is in the form of longwave radiation. Terrestrial radiation is the outgoing longwave radiation from Earth. Table 4.2 provides all of the data needed to calculate net radiation at the top of the atmosphere for Deadhorse, Alaska.

Table 4.2: Monthly Average Radiation Data for Deadhorse, Alaska (~70°N) Data in W/m2

Month	Incoming Radiation	Outgoing Shortwave Radiation	Outgoing Longwave Radiation	Net Radiation
March	133	84	147	-98
June	488	332	204
September	181	84	147
December	0	0	154

In order to calculate net radiation for Deadhorse in March, subtract outgoing longwave radiation and outgoing shortwave radiation from incoming radiation:

133 - 84 - 147 = -98

Negative net radiation indicates an energy deficit. This means that more energy is lost to space than is received from insolation.

Exercise \(\PageIndex{4}\)

6. Following the example for March provided above, complete the net radiation column in Table 4.2.
7. Apply What You Learned: Why does Deadhorse have no incoming radiation in December?

8. Use Your Critical Thinking Skills: Why does Deadhorse have an energy deficit in June even though insolation is relatively high during this month?

Table 4.3 provides all of the data needed to calculate net radiation at the top of the atmosphere for Beijing, China.

Table 4.3: Monthly Average Radiation Data for Beijing, China (~40°N) Data in W/m2

Month	Incoming Radiation	Outgoing Shortwave Radiation	Outgoing Longwave Radiation	Net Radiation
March	323	106	223
June	483	161	246
September	351	106	223
December	160	55	210

Exercise \(\PageIndex{5}\)

9. Complete the net radiation column in Table 4.3.
10. How does the net radiation in Beijing change throughout the year?

Table 4.4 provides all of the data needed to calculate net radiation at the top of the atmosphere for Kampala, Uganda.

Table 4.4: Monthly Average Radiation Data for Kampala, Uganda (~0°) Data in W/m2

Month	Incoming Radiation	Outgoing Shortwave Radiation	Outgoing Longwave Radiation	Net Radiation
March	439	199	207
June	388	155	228
September	429	199	207
December	413	170	244

Exercise \(\PageIndex{6}\)

11. Complete the net radiation column in Table 4.4.
12. Apply What You Learned: Why does Kampala have relatively high net radiation throughout the year?

The net heating imbalance between the equator and poles drives an atmospheric and oceanic circulation that climate scientists describe as a “heat engine”. (In our everyday experience, we associate the word engine with automobiles, but to a scientist, an engine is any device or system that converts energy into motion). The climate is an engine that uses heat energy to keep the atmosphere and ocean moving. Evaporation, convection, rainfall, winds, and ocean currents are all part of the Earth’s heat engine.^[62]

Let’s explore net radiation on animated world maps provided by NASA’s Earth Observatory. The animation provides net radiation in W/m² from July 2006 through July 2020.

Step 1

Go to the net radiation webpage provided by NASA’s Earth Observatory.

Step 2

Read the text beneath the map so that you understand the data shown and what the colors on the maps signify.

Step 3

Click the play button on the bottom left of the map.

Exercise \(\PageIndex{7}\)

13. Describe how net radiation fluctuates throughout the year.

14. Apply What You Learned: Why does net radiation fluctuate throughout the year?

Part C. Earth’s Energy Budget

Solar radiation reaches the surface of the Earth in the form of shortwave radiation and is considered incoming radiation. Radiation or energy that leaves the Earth in the form of longwave radiation is considered outgoing radiation, which is also known as terrestrial radiation. In the previous part of the lab, we took a simplified look at incoming and outgoing radiation. There are several important vocabulary terms related to Earth’s energy budget that will advance your understanding of net radiation:

➢ Emission is the process of energy being released or emitted by an object.

➢ Absorption refers to an object’s ability to absorb or take in electromagnetic waves.

➢ Reflection refers to electromagnetic waves “bouncing back” after hitting an object.

• The percentage of insolation reflected back out to space is called albedo. Earth’s average albedo is 31, this means that 31% of insolation is reflected back out to space in a given year.

➢ Energy is also distributed through scattering, which occurs when electromagnetic waves encounter gas molecules and particulates in the atmosphere and change direction.

➢ Refraction occurs when electromagnetic radiation hits an object and then changes both wavelength and direction.

➢ Energy that passes through a substance or a medium is called transmission. The more transparent the medium, the more transmission occurs. This is why energy passes through and is absorbed through water more easily than through land.

Figure 4.5 shows the total amount of energy coming into and leaving Earth in a given year, also known as an energy budget. Similar to an economic budget, energy budgets help us better understand how much energy Earth receives from the Sun (income), how that energy is distributed around the planet (transferred among checking and savings accounts), and how much of that energy is lost in a given year (expenses). Earth is always practicing a balancing act; therefore, it makes sense that the amount of energy received from the Sun is approximately how much energy is lost.

The energy budget accounts for energy in the form of units or a percent. Let’s say the Earth receives 100 units of energy from the Sun each year; Figure 4.5 shows how those 100 units are distributed throughout the planet in a given year.

Figure 4.5: Earth’s Energy Budget. Yellow numbers account for incoming shortwave radiation and red numbers account for outgoing longwave radiation. Figure by Jeremy Patrich is licensed under CC BY-NC-SA 4.0

Exercise \(\PageIndex{8}\)

15. What percent of energy received from the Sun each year is absorbed by clouds?

1. What percent of energy received from the Sun each year is reflected by clouds?

2. How much energy is absorbed by the surface of the Earth? Hint: your answer will be a percentage.

3. Of the total energy absorbed by the Earth’s surface (land and ocean) as incoming shortwave radiation, how much of it is emitted back out as longwave radiation?

4. What role do clouds play in the energy budget? Your response should be at least one sentence in length.

5. Use Your Critical Thinking Skills: When analyzing insolation data, why is it important to pay attention to whether or not the data was collected by satellites at the top of the atmosphere or by ground stations at the surface of the Earth? Your response should be at least one sentence in length.

Part D. Remote Sensing and Earth’s Energy Budget

Let’s go “behind the scenes” to learn how scientists calculate Earth’s energy budget. Part of the research comes from data collected by remote sensing. The term remote sensing is commonly used to describe the science—and art—of identifying, observing, and measuring an object without coming into direct contact with it. This process involves the detection and measurement of radiation of different wavelengths reflected or emitted from distant objects or materials, by which they may be identified and categorized by class/type, substance, and spatial distribution. Remote sensing is used in numerous fields, including geography, land surveying and most Earth Science disciplines; it also has military, intelligence, commercial, economic, planning, and humanitarian applications.⁸

Figure 4.6 reveals the variety of remote sensing platforms used today—offering a multi-scale, multi-resolution view of our planet. Remote sensing instruments are of two primary types: active and passive. Active sensors provide their own source of energy to illuminate the objects they observe. An active sensor emits radiation in the direction of the target to be investigated. The sensor then detects and measures the radiation that is reflected or backscattered from the target. Passive sensors, on the other hand, detect natural energy (radiation) that is emitted or reflected by the object or scene being observed. Reflected sunlight is the most common source of radiation measured by passive sensors.⁸

Figure 4.6: Remote Sensing Platforms. Figure and text by NASA’s Goddard Space Flight Center are in the public domain

Exercise \(\PageIndex{9}\)

16. Use Your Critical Thinking Skills: Why do you think there are so many applications, or uses, of remotely sensed data? Explain your response in at least one sentence.

17. Apply What You Learned: Let’s say you are looking out onto the Earth’s surface from a window seat on an airplane. Would your eyes be considered active or passive remote sensors? Explain your response in at least one sentence.

One of NASA’s remote sensing instruments used to understand Earth’s net radiation is CERES, which stands for Clouds and the Earth’s Radiant Energy System. Here are some interesting things to know about CERES:

➢ First launched in 1997, there are a total of five CERES instruments currently orbiting Earth (two on the Terra satellite, two on the Aqua satellite, and one on the Suomi National Polar-orbiting Partnership satellite).

➢ It weighs 54 kilograms (119 pounds).

➢ It’s accuracy is between 0.3 and 1%.

CERES program scientist explains that “CERES represents a serious attempt to create a true climate data record—that is, a time series that is precise enough and maintained for enough time that long-term trends in climate can be measured...There are some critically important science questions that cannot be answered without a very precise measurement made over multi-decadal time periods. In order to properly address these questions, we need to maintain the measurements over multiple generations of instruments—for the science”.^[65]

The animated net radiation false-color maps that you explored earlier in the lab were created using data from CERES. They show the net radiation (in Watts per square meter) that was contained in the Earth system for the given time period. Regions of positive net radiation have an energy surplus, and areas of negative net radiation have an energy deficit. The maps illustrate the fundamental imbalance between net radiation surpluses at the equator, where sunlight is direct year-round, and net radiation deficits at high latitudes, where direct sunlight is seasonal. This imbalance is the fundamental force that drives atmospheric and oceanic circulation patterns.^[66]

The CERES instrument also measures changes to Earth’s albedo. Figure 4.7 shows how the reflectivity of Earth—the amount of sunlight reflected back into space—changed between March 1, 2000, and December 31, 2011. This global picture of reflectivity (also called albedo) appears to be a muddle, with different areas reflecting more or less sunlight over the 12-year record. Shades of blue mark areas that reflected more sunlight over time (increasing albedo), and orange areas denote less reflection (lower albedo). Taken across the planet, no significant global trend appears. Global albedo rose and fell in different years, but did not necessarily head in either direction for long. Some regional patterns emerge, however.^[67]

Figure 4.7: Changes to Earth’s Albedo between March 1, 2000 and December 31, 2011. Figure by NASA Earth Observatory is in the public domain.

Exercise \(\PageIndex{10}\)

18. Refer to Figure 4.7.

​​​​​​​​​​​​​​

1. In two to three sentences, describe how Earth’s albedo changed. In your response, be sure to identify specific places on the planet that experienced increasing or decreasing reflectivity.

2. Use Your Critical Thinking Skills: List the factors that might explain why there are regional differences in albedo.

CERES and other satellite instruments provide data on anthropogenic (human-caused) effects on cloud cover. One example of this comes from a study of ship tracks (Figure 4.8). As ships cruise across the ocean, they emit a large number of small airborne particles—aerosols—into the lower atmosphere. Under the right conditions, these exhaust particles cause long, thin cloud patterns referred to as “ship tracks”. For decades, scientists have theorized that these cloud changes might alter climate by affecting the amount of sunlight reaching Earth’s surface. A 2020 study measured exactly how ship emissions affect clouds at a regional scale. University of Washington scientist Michael Diamond and colleagues examined more than a decade of cloud patterns over a busy shipping lane in the southeast Atlantic that connects Europe to southern Africa and Asia.^[69]

Figure 4.8: Ship Tracks In the North Atlantic Ocean on January 16, 2018. Figure by NASA Earth Observatory is in the public domain

The team measured the cloud properties inside the shipping corridor using satellite data and compared them to what they estimated the values would be without shipping activity. The scientists found that the shipping activity increased the number of cloud droplets over the shipping lane. They further showed that those clouds prevented about 2 Watts of solar energy from reaching each square meter of ocean surface along the shipping lane. The team’s results agreed with previous computer modeling studies that predicted a sizeable cooling effect due to shipping. Diamond explained that the aerosols from the ships create “seeds” in the atmosphere that water vapor can latch onto and condense into small cloud droplets. These smaller droplets make clouds brighter so that they reflect more sunlight, which creates a local cooling effect at the planet’s surface.¹⁵

That change is small on a regional scale, but it could be enough to affect global temperatures if the same phenomenon occurs worldwide, according to study co-author Hannah Director of the University of Washington. When the team scaled their findings across the planet, they found that changes in low-lying clouds from all industrial pollution sources could block about 1 Watt of energy per square meter globally. For context, greenhouse gases from industrial activities have trapped roughly 3 Watts per square meter so far.¹⁵

Exercise \(\PageIndex{11}\)

19. Apply What You Learned: In your own words, explain how the aerosols in ship tracks create a local cooling effect. Be sure to indicate which of the atmospheric effects (absorption, reflection, scattering, or refraction) is responsible for the cooling.

20. Scientists create knowledge using three main types of research: lab experiments, field experiments, and computer modeling.

• Lab experiments are conducted in a controlled setting.
• Field experiments are conducted in a natural, real-world setting.
• Computer models use mathematical equations based on the laws of physics to understand the natural world; data collected from lab experiments and/or field experiments are often used in computer models.

What types of research contributed to the analysis of ship tracks? Explain your response in one to two sentences.

21. Use Your Critical Thinking Skills: Do you think it is valid to extrapolate the findings from a study done on one part of the planet (e.g., ship lanes in the southeast Atlantic) to create a global analysis based on computer modeling (e.g., ship lanes in other parts of the world)? Explain why or why not in two to three sentences. Tip: extrapolate means to “extend the application of (a method or conclusion, especially one based on statistics) to an unknown situation by assuming that existing trends will continue or similar methods will be applicable”.

Part E. Wrap-Up

Consult with your geography lab instructor to find out which of the following wrap-up questions you should complete. Attach additional pages to answer the questions as needed.

Exercise \(\PageIndex{12}\)

22. What is the most important idea that you learned in this lab? In two to three sentences, explain the concept and why it is important to know.

23. What was the most challenging part of this lab? In two to three sentences, explain why it was challenging. If nothing challenged you in the lab, write about what you think challenged your classmates.

24. What is one question that you have about what you learned in this lab? Write your question along with one to two sentences explaining why you think your question is important to ask.

25. Review the learning objectives on page 1 of this lab. How would you rate your understanding or ability for each learning objective? Write one sentence that addresses each learning objective.

26. Sketch a concept map that includes the key ideas from this lab. Include at least five of the terms shown in bold-faced type.

27. Create an advertisement to educate your peers on the important information that you learned in this lab. Include at least three key terms in your advertisement. The advertisement should be about half a page in size (about 4 inches by 6 inches).

28. One way to think of physical geography is that it is the study of the relationships among variables that impact the Earth's surface. Select two variables discussed in this lab and describe how they are related.

29. How does what you learned in this lab relate to your everyday life? In two to three sentences, explain a concept that you learned in this lab and how it relates to your day-to-day actions.

30. How does what you learned in this lab relate to current events?
    1. Write the title, source, and date of a news item that relates to this lab.
    2. In two to three sentences, discuss how the news item relates to what you have learned in this lab.
    3. In one to two sentences, discuss whether or not the news item accurately represents the science that you learned. Tip: consider whether or not the news item includes the complexity of the topic.

31. Search O*NET OnLine to find an occupation that is relevant to the topics presented in today's lab. Your lab instructor may provide you with possible keywords to type in the Occupation Quick Search field on the O*NET website.
    1. What is the name of occupation that you found?
    2. Write two to three sentences that summarize the most important information that you learned about this occupation.
    3. What is one question that you would want to ask a person with this occupation?

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^[59] Text by NASA Earth Observatory is in the public domain

^[60] Text by NASA Earth Observatory is in the public domain

^[62] Text by NASA

^[65] Quote from NASA is in the public domain

^[66] Text by NASA Earth Observatory is in the public domain

^[67] Text by NASA Earth Observatory is in the public domain

^[69] Text by NASA Earth Observatory is in the public domain