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Atmosphere, Climate, Cryosphere



Earth's Energy Budget Over the Poles

The Earth as a system is driven by the Sun's energy; and as the Sun's rays hit the Earth, a certain percentage of the energy is reflected back into space. The amount of radiation reflected by a surface is referred to as its albedo. Albedo can range from a value of 0 (no reflection) to a value of 1 (100% reflection). It is important to know how much of the Sun's energy at the poles is reflected, absorbed, or radiated back to into space. Many scientists are researching this question, and for a very good reason. What if, for example, more energy reached the Earth than was reflected or radiated back into space? The Earth as a system has an energy budget that includes all gains of incoming energy and all losses of outgoing energy. When the flow of incoming solar energy is balanced by an equal flow of heat to space, the Earth is in radiative equilibrium. Global temperature is relatively stable when the sum of the gains is approximately equal to the sum of the losses. Anything that increases or decreases the amount of incoming or outgoing energy disturbs Earth's radiative equilibrium; global temperatures rise or fall in response.

The amount of reflected energy changes with surface changes, especially at the poles or where snow and ice have traditionally dominated. The average albedo for the Earth is .31, meaning that the Earth reflects nearly a third of the incoming radiation back into space. Forests, oceans, cities, deserts all have different albedos. Forests have albedos of between .08 and .15; deserts have an albedo of about .30.

Bright snow and ice reflect sunlight back into space; the albedo of snow and ice is between .6 to .9. But what happens when Earth's temperature warms? Snow and ice melt, less energy is reflected and more is absorbed by the ground and water, warming the Earth and causing more snow and ice to melt. This cycle is known as the ice-albedo feedback. According to the Intergovernmental Panel on Climate Change (IPCC) Climate Change 2007 Synthesis Report, satellite data since 1978 indicate that arctic sea ice is shrinking at a rate of ~3% per decade.

Scientists are also interested in the role that clouds play in this scenario. With more snow and ice melting, more water vapor is in the air and available to create clouds. The question then becomes, what role do clouds play in reflecting energy?

Clouds reflect more light back to space than does a cloudless blue sky. A cloud's albedo depends on several factors, including the cloud's height, its size, and the number and size of droplets inside the cloud. Clouds range in color from bright white to dark grey, because of the water droplets scattering light; More drops as well as bigger drops have a larger surface area and reflect more light. A large cumulonimbus cloud casts a dark shadow because light does not go through it easily. From space this same cloud would look bright white because it has a high albedo. A cirrus cloud on the other hand is nearly transparent but seems more grey from the space because its albedo is lower.

Concerns about albedo apply to other areas in addition to clouds, snow and ice. Any Earth surface plays a role in this system. For example, if land vegetation is cleared (e.g., deforestation or agricultural burning), the bare surface reflects more sunlight back to space, creating a cooling effect. But, deforestation and biomass burning release carbon dioxide into the atmosphere. The vegetation that normally would have absorbed the carbon dioxide is gone. And while deforestation/reforestation may take place on annual to decadal time scales, the lifetime of carbon dioxide in the atmosphere is 50 to more than 100 years. This means that the cooling coming from solar reflectance and the warming coming from increases in the greenhouse effect take place on much different time scales. This leads initially to cooling followed later by a warming trend (source: NASA fact sheet: Earth's Energy Balance .)

Your Earth system science analysis of the role of albedo in the Earth-Sun relationship will help in understanding present and future climate issues. This is especially important in light of the present and projected loss of summer Arctic sea ice.



An Earth Observatory article suggests that Earth's albedo has been steadily declining since 2000. Between 2000 and 2004, the CERES instrument measured a decrease of in albedo of .0027 - or 0.9 watt of energy per square meter retained in the Earth system. The research team report being unsure as to what caused the decline in albedo. Because of your interest in the Earth's energy balance, your team has been asked to investigate this decline. Your supervisor is first interested in what has caused the decline and your recommendations as to how to mitigate this effect. The second interest is to what extent does polar snow and ice albedo play a role in climate change.


Date: 5/19/2009

Scenario Images:

Sun-Earth Connections
Credit: NASA Goddard Space Flight Center

In the image: the sun's rays are reflected back into space due to the albedo effect. Watch a short video in which Dr. Robert Cahalan, head of the NASA Goddard Climate and Radiation Branch, discusses Sun-Earth energy relationships.

Earth's Energy Budget
Image Courtesy: National Science Digital Library
Over a long term average, the Earth and its atmosphere must radiate as much energy out to space as it receives from the sun. In fact, the same type of balance exists between the Earth and its atmosphere! The question is, how does albedo fit into this equation? Click here for larger image.

Global Albedo
Map Courtesy NASA
The colors in this image depict the albedo over the Earth's land surfaces, ranging from 0.0 to 0.4. Areas colored red show the brightest, most reflective regions; yellows and greens are intermediate values; and blues and violets show relatively dark surfaces. No data was available in white areas; and no albedo data are provided over the oceans. This image was produced using data composited over a 16-day period, from April 7-22, 2002.



Applets Depicting Energy and Atmospheric Principles (Cycle A)
These applets were developed by Steve Ackerman and Tom Whittaker at the University of Wisconsin. They can be used to visualize energy and atmospheric principles to include albedo.


Arctic Climate Impact Assessment (Cycle A)
An international project of the Arctic Council and the International Arctic Science Committee (IASC), to evaluate and synthesize knowledge on climate variability, climate change, and increased ultraviolet radiation and their consequences. The results of the assessment were released at the ACIA International Scientific Symposium held in Reykjavik, Iceland in November 2004. This site contains information about changes in albedo and the impact on terrestrial ecosystems, snow cover and sea ice. This document is rather large. It may be best to download the index in order to find the appropriate topics.


Climate Feedback and Processes (Cycle A)
From the American Museum of Natural History's online Climate Change Course. Climate Feedback and Processes information details feedback, "a response to a change that influences the change itself."


Factors Affecting Surface Ultraviolet Radiation Levels In the Arctic (Cycle A)
Atmospheric ozone levels, solar zenith angle, clouds, aerosols, and altitude are all major factors affecting UV radiation levels reaching the surface of the Earth. In the Arctic, snow and ice cover add further complexity to the estimation of UV radiation exposure.


Retrieval of Surface Albedo From Space (Cycle A)
A technical article related to remotely sensing the Earth's absorbed, reflected and emitted radiation: "Location, topography, and the exchange of radiant energy between the sun and Earth ultimately determine weather. Absorbed, reflected, and emitted radiation are the driving forces causing temperature fluxes, ocean currents, wind, and evaporation which in turn becomes precipitation. And since climate is the study of weather at timescales ranging from a few weeks to decades and beyond, an understanding of the Earth's radiation budget is critical to climate research."


Scientist Studies Earth's Reflectivity in the Arctic (Cycle A)
This article shares how a young scientist began working for NASA and became interested in the role of snow, ice and clouds within the Earth's energy budget.


Cool Roofing and Its Impact on Albedo (Cycle B)
A technical article detailing the albedo of various roofing materials.


Cooling Urban Cities (Cycle B)
New Scientist reports on simulations at the UCAR suggesting that painting roofs white could cool cities.


Earth's reflectivity a great unknown in gauging climate change impacts (Cycle B)
" Earth's climate is being changed substantially by a buildup of atmospheric greenhouse gases, but a group of leading climate scientists contends the overall impact is not understood as well as it should be because data are too scarce on how much energy the planet reflects into space."


Global Cooling: Effect of Urban Albedo on Global Temperature (Cycle B)
This research paper estimates a potential U.S. savings of over $1 billion per year in energy costs by increasing roof reflectivity to 60%.


Ice-Albedo Feedback (Cycle B)
"As polar ice starts to melt... The ice is darker in color than the snow. Melt ponds and cracks form in the ice, which are also darker in color. And where the ice has melted, dark ocean water is exposed. These changes decrease the albedo. With a lower albedo more solar energy is absorbed and less is reflected. This causes more ice to melt, which in turn lowers the albedo, causes more energy to be absorbed and more warming to occur." See also this page on ice_albedo feedback from Windows to the Universe.


Planting Trees and Increasing Heat Island Albedo to Mitigate Heat Island Effects (Cycle B)
This dissertation by Ronnen M. Levinson, entitled Near-Ground Cooling Efficacies of Trees and High-Albedo Surfaces addresses two measures to combat urban area's heat island effect. The measures consist of planting trees and increasing the solar reflectivity of ground surfaces.


Chill out: in a world filled with worries over global warming, one teen searches for ways to help... (Cycle C)
Read what 14-year old Ryan did to investigate albedo: "It is becoming increasingly obvious that our climate is warming. I had seen news stories about global warming and Al Gore's film An Inconvenient Truth. I decided to do some research to try and better understand what is happening to our Earth. I had heard much discussion (and debate) about how people need to reduce their consumption of petroleum fuels and start driving electric cars. I had also heard a lot of disagreement about whether it is really practical to expect people to drastically change their daily behavior, and about whether people are ready or willing to abandon their gasoline-powered cars. I wanted to find out if there was anything that people might be able to do to reduce climate change without having to drastically change their lives."


Investigating the Climate System: Energy (Cycle C)
This Penn State University problem-based learning module (PBL)* is a model to use in setting up your own lessons. It begins with a scenario in which students play the role of researchers.

* Note: while labeled as PBL, the lesson is still very structured and could be more appropriately named: guided discovery.


Sample Investigations:


Energy Budget Model (Cycle A)
This activity takes a bit of exploring to learn the ropes, but it is well worth the effort. This lab explores climate at Mono Lake using an energy-budget model. Energy is input from the sun and is stored in the atmosphere and the Earth's surface. The fraction of energy budgeted to the surface is what controls the surface temperature, which you will compute. But does the model work correctly? You can use the model to estimate temperature for any month for modern climate and then see how well the model can estimate ice-age climate, when temperatures were about 7°C colder than today.
Difficulty: beginner


Radiation Budget Lesson: Exploring Albedo (Cycle A)
In this grades 5-8 activity, students answer the question of how albedo impacts surface temperature.

Difficulty: beginner


Understanding Albedo (Cycle A)
This impressive site has a full range of activities concerning Arctic climate. Activities are available for K-12, to include an albedo activity for grades 9-12.
Difficulty: beginner


Earth's Albedo and Global Warming (Cycle B)
This interactive activity adapted from NASA and the U.S. Geological Survey illustrates the concept of albedo: the measure of how much solar radiation is reflected from Earth's surface. The balance between the amount of solar radiation reflected and absorbed by Earth's surface plays an important role in regulating global temperature.
Difficulty: beginner


Earth's Energy Budget Activity (Cycle B)
Dynamic balance in a bottle activity from Dr. Art's guide to planet Earth.
Difficulty: beginner


Using NASA NEO and ImageJ to Explort the Role of Snow Cover in Shaping Climate (Cycle B)
Investigate satellite images displaying land surface temperature, snow cover, and reflected shortwave radiation data from the NASA Earth Observation (NEO) website.
Difficulty: beginner


Geographic Effects on Precipitation, Water Vapor and Temperature (9-12) (Cycle C)
Students improve their analytical and basic science skills by collecting, comparing, and analyzing atmospheric conditions between two geographic locations using a spreadsheet and data sets.
Difficulty: beginner


Variables Affecting Earth's Albedo (Cycle C)
In this activity from MY NASA DATA, students:

  • collect data on Earth's albedo from two surface types at the same latitude,
  • graph the differences in the data for the two locations over a period of two years,
  • use the data to calculate how much difference there is in Earth's albedo between the two locations, and
  • suggest reasons for the differences.

Difficulty: beginner




  • Science
    National Science Education Standards - Science Content Standards The science content standards outline what students should know, understand, and be able to do in the natural sciences over the course of K-12 education.
      The understandings and abilities associated with the following concepts and processes need to be developed throughout a student's educational experiences:
      • Systems, order, and organization
      • Evidence, models, and explanation
      • Constancy, change, and measurement
      • Science as Inquiry (Std A)
        • Abilities necessary to do scientific inquiry
        • Understanding about scientific inquiry
      • Physical Science (Std B)
        • Interactions of energy and matter
      • Earth and Space Science (Std D)
        • Energy in the earth system
      • Science and Technology (Std E)
        • Understanding about science and technology
      • Science in Personal and Social Perspectives (Std F)
        • Environmental quality
        • Natural and human-induced hazards
        • Science and technology in local, national, and global challenges
  • Geography
    Geography for Life: National Geography Standards, 1994
      Geography studies the relationships between people, places, and environments by mapping information about them into a spatial context. The geographically informed person knows and understands:
      • How to use maps and other geographic representations, tools and technologies to acquire, process, and report information from a spatial perspective
      The identities and lives of individuals and people are rooted in particular places and in those human constructs called regions. The geographically informed person knows and understands:
      • The physical and human characteristics of places
      • That people create regions to interpret Earth’s complexity
      Physical processes shape Earth’s surface and interact with plant and animal life to create, sustain, and modify ecosystems. The geographically informed person knows and understands:
      • The physical processes that shape the patterns of Earth’s surface
      • The characteristics and spatial distribution of ecosystems on Earth’s surface
      People are central to geography in that human activities help shape Earth’s surface, human settlements and structures are part of Earth’s surface, and humans compete for control of Earth’s surface. The geographically informed person knows and understands:
      • The characteristics, distribution, and migration of human populations on Earth’s surface
      • The processes, patterns, and functions of human settlement
      The physical environment is modified by human activities, largely as a consequence of the ways in which human societies value and use Earth’s natural resources, and human activities are also influenced by Earth’s physical features and processes. The geographically informed person knows and understands:
      • How human actions modify the physical environment
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