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Climate Change
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What is causing it? How do we know it is happening?
A blanket around the Earth
Figure 1: A layer of greenhouse gases – primarily water vapor, and including much smaller amounts of carbon dioxide,
methane and nitrous oxide – acts as a thermal blanket for the Earth, absorbing heat and warming the surface to a lifesupporting average of 59 degrees Fahrenheit (15 degrees Celsius).
Most climate scientists agree the main cause of the current global warming trend is human
expansion of the "greenhouse effect"1 — warming that results when the atmosphere traps heat
radiating from Earth toward space.
Certain gases in the atmosphere block heat from escaping. Long-lived gases that remain semipermanently in the atmosphere and do not respond physically or chemically to changes in
temperature are described as "forcing" climate change. Gases, such as water vapor, which
respond physically or chemically to changes in temperature are seen as "feedbacks."
We live in a greenhouse
Life on Earth depends on energy coming from the sun. About half the light reaching Earth's
atmosphere passes through the air and clouds to the surface, where it is absorbed and then
radiated upward in the form of infrared heat. About 90 percent of this heat is then absorbed by the
greenhouse gases and radiated back toward the surface, which is warmed to a life-supporting
average of 59 degrees Fahrenheit (15 degrees Celsius).
On Earth, human activities are changing the natural greenhouse. Over the last century the
burning of fossil fuels like coal and oil has increased the concentration of atmospheric
carbon dioxide (CO2). This happens because the coal or oil burning process combines carbon
with oxygen in the air to make CO2. To a lesser extent, the clearing of land for agriculture,
industry, and other human activities have increased concentrations of greenhouse gases.
The consequences of changing the natural atmospheric greenhouse are difficult to predict, but
certain effects seem likely:

On average, Earth will become warmer. Some regions may welcome warmer temperatures,
but others may not.

Warmer conditions will probably lead to more evaporation and precipitation overall, but
individual regions will vary, some becoming wetter and others dryer.

A stronger greenhouse effect will warm the oceans and partially melt glaciers and other ice,
increasing sea level. Ocean water also will expand if it warms, contributing to sea level rise.

Meanwhile, some crops and other plants may respond favorably to increased atmospheric
CO2, growing more vigorously and using water more efficiently. At the same time, higher
temperatures and shifting climate patterns may change the areas where crops grow best and
affect the makeup of natural plant communities.
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Climate change: How do we know?
Figure 2: This graph, based on the comparison of atmospheric samples contained in ice cores and more recent direct
measurements, provides evidence that atmospheric CO 2 has increased since the Industrial Revolution.
The Earth's climate has changed throughout history. Just in the last 650,000 years there have
been seven cycles of glacial advance and retreat, with the abrupt end of the last ice age about
7,000 years ago marking the beginning of the modern climate era — and of human civilization.
Most of these climate changes are attributed to very small variations in Earth’s orbit that change
the amount of solar energy our planet receives.
“Scientific evidence for warming of the climate
system is unequivocal.”
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- Intergovernmental Panel on Climate Change
The current warming trend is of particular significance because most of it is very likely human-induced and
proceeding at a rate that is unprecedented in the past 1,300 years.1
Earth-orbiting satellites and other technological advances have enabled scientists to see the big picture,
collecting many different types of information about our planet and its climate on a global scale. This body
of data, collected over many years, reveals the signals of a changing climate.
The heat-trapping nature of carbon dioxide and other gases was demonstrated in the mid-19th
century.2 Their ability to affect the transfer of infrared energy through the atmosphere is the scientific basis
of many instruments flown by NASA. There is no question that increased levels of greenhouse gases must
cause the Earth to warm in response.
Ice cores drawn from Greenland, Antarctica, and tropical mountain glaciers show that the Earth’s climate
responds to changes in greenhouse gas levels. They also show that in the past, large changes in climate
have happened very quickly, geologically-speaking: in tens of years, not in millions or even thousands.3
The evidence for rapid climate change is compelling:
Sea level rise
Global sea level rose about 17
centimeters (6.7 inches) in the last
century. The rate in the last decade,
however, is nearly double that of the
last century.4
Figure 3: Republic of Maldives: Vulnerable to sea level rise. Credit: Chumash Maxim/Shutterstock.com
Global temperature rise
All three major global surface temperature
reconstructions show that Earth has warmed
since 1880.5 Most of this warming has occurred
since the 1970s, with the 20 warmest years
having occurred since 1981 and with all 10 of
the warmest years occurring in the past 12
years.6 Even though the 2000s witnessed a
solar output decline resulting in an unusually
deep solar minimum in 2007-2009, surface
temperatures continue to increase.7
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Warming oceans
The oceans have absorbed much of this
increased heat, with the top 700 meters (about
2,300 feet) of ocean showing warming of 0.302
degrees Fahrenheit since 1969.8
Shrinking ice sheets
The Greenland and Antarctic ice sheets
have decreased in mass. Data from NASA's
Gravity Recovery and Climate Experiment
show Greenland lost 150 to 250 cubic
kilometers (36 to 60 cubic miles) of ice per
year between 2002 and 2006, while
Antarctica lost about 152 cubic kilometers
(36 cubic miles) of ice between 2002 and
2005.
Figure 4: Flowing meltwater from the Greenland ice sheet
Declining Arctic sea ice
Both the extent and thickness of Arctic
sea ice has declined rapidly over the
last several decades.9
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Visualization of the 2007 Arctic sea ice minimum
Glacial retreat
Glaciers are retreating almost everywhere
around the world — including in the Alps,
Himalayas, Andes, Rockies, Alaska and
Africa.10
The disappearing snowcap of Mount Kilimanjaro, from space.
Extreme events
The number of record high temperature
events in the United States has been
increasing, while the number of record
low temperature events has been
decreasing, since 1950. The U.S. has
also witnessed increasing numbers of
intense rainfall events.11
Ocean acidification
Since the beginning of the Industrial
Revolution, the acidity of surface ocean
waters has increased by about 30
percent.12,13This increase is the result of
humans emitting more carbon dioxide into
the atmosphere and hence more being
absorbed into the oceans. The amount of
carbon dioxide absorbed by the upper layer
of the oceans is increasing by about 2
billion tons per year.14,15
Decreased snow cover
Satellite observations reveal that the
amount of spring snow cover in the
Northern Hemisphere has decreased over
the past five decades and that the snow is
melting earlier.16
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Name: ______________________________________________________ Date: _____________________ Period: _____
Climate Change
Directions: Read about Climate Change in the articles provided and answer the questions below.
1. What is the greenhouse effect?
2. How do greenhouse gases act like a “thermal blanket?”
3. How have humans contributed to the increase in greenhouse gases and climate change?
4. Climate change and global temperature fluctuations have occurred naturally over time. How do we know that humans
are having an effect on climate change? Use the graph in Figure 2 to support your answer.
5. What evidence have scientists used to prove that global warming trends are man-made? Name at least 3 examples or
types of evidence.
6. Imagine you live in the city pictured in Figure 3. The sea level has already risen 6 inches, now imagine the sea level
continues to rise 12 more inches. How would this affect your life? What changes would occur? How would you solve
these problems?
7. How is the warming of the ocean going to affect wildlife? Be specific.
8. How is the melting of the ice caps and ice sheets going to affect wildlife? Be specific.
9. What is ocean acidification?
10. What can we do now to slow the rate of global warming and climate change? Hint: Think about our current energy
resources and options for the future.