Table of Contents:
Climate and American People
Earth's Climate: A Dynamic System
Why Does Our Climate Change
Can We Change the Climate?
The Greenhouse Effect
Why are Greenhouse Gas Amounts Increasing?
Aerosols: Sunscreen for the Planet?
How has Climate Changed in the Last Century?
How Do We Predict Climate Change?
What do Climate Models Tell Us about Our Future?
Where do We Go from Here?
* * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Climate and American People
Climate has always had a profound effect on life in America. The
first people arrived in America between 15,000 and 30,000 years
ago. During that time much of North America was covered by two
great ice sheets that were nearly two miles thick in places. One
ice sheet followed the coastal mountains from Alaska to Washington
State, and another extended from the eastern slope of the Rocky
Mountains to the Atlantic Ocean and from the Arctic Ocean to Ohio.
Because so much water was piled up on land in ice sheets, the
sea level was about 350 feet lower at the peak of the last ice
age about 20,000 years ago. The lowered sea exposed a wide plain
between Siberia and Alaska, creating a land bridge across the
Bering Sea. Genetic, linguistic, and fossil evidence suggest that
the first humans in America came from northeast Asia, and it is
likely that the ice age climate made it possible for these people
to walk across the land bridge between Siberia and Alaska. After
crossing this plain, these hardy people made their way south between
the great ice sheets and spread across America.
Caption: Ice sheets and shifted coastlines at the time of the
last glacial maximum opened an access route from Asia to America.
The current continental outline is given for reference. [Graphic
by D.L. Hartmann and Kay Dewar.]
We know that some of these early Americans were big game hunters.
Their camps are marked by distinctive fluted spear points which
they used to hunt mastodons--extinct relatives of the modern elephant.
They also shared the land with saber-toothed cats, woolly rhinoceroses
and giant ground sloths. These and a variety of other species
all became extinct about 10,000 years ago. Some researchers argue
that efficient human hunters caused these extinctions, but others
believe that environmental change was the key factor.
The extinctions coincide with a time of enormous change in the
global climate. Some 14,000 years ago the great North American
ice sheets began to melt rapidly, and by 7,000 years ago they
were gone. This end to the ice age caused dramatic changes in
North America. As the ice melted and the climate warmed, the once-wet
region between the Cascade Range and Rocky Mountains became the
relatively dry landscape that we know today as the Great Basin.
Features like Utah's Great Salt Lake turned into shadows of their
former selves. Fifteen thousand years ago this body of water was
1200 feet deeper and covered an area the size of Lake Michigan.
Such changes had a marked impact on ice age plants and animals.
Cold-loving spruce trees, for example, withdrew their range northward
by about a thousand miles, giving way to grassland and broadleaf
trees. Mastodons and other large mammals that preferred cold climates
may not have been able to adapt to the warmer, drier conditions.
As their favorite game animal disappeared, the earliest Americans
would also have been forced to adapt.
The effect of climate on human settlement of America continued
into medieval times. The first Europeans to set foot on America
were Vikings who settled Greenland under the leadership of Eric
the Red in about 1000AD. His son, Leif Erikson, led an expedition
to colonize America that probably settled in Newfoundland. The
colony in Greenland was abandoned in about 1400AD when cooler
temperatures associated with the Little Ice Age made farming there
too difficult. Further to the south, climate changes also affected
the civilizations set up by the earlier Asian immigrants to America.
The Anasazi people of the Four Corners region of the American
Southwest provide an interesting example. They had an economy
centered around corn farming, and built large dwellings in river
valleys and along the ridges between canyons. The most famous
of these are the cliff dwellings and pueblos of the Mesa Verde
region near the junction of Colorado, Utah, Arizona, and New Mexico.
Beginning about 1150AD the Four Corners region experienced a series
of profound droughts, and by 1300AD the Anasazi had abandoned
Although we have more advanced technology than the Anasazi, modern
Americans are also affected by variations in our climate. Between
1934 and 1937 parts of Texas, Oklahoma, Colorado, New Mexico,
and Kansas became known as the Dust Bowl when severe drought afflicted
the area. Clouds of dust rolled across the vast area affected
by the drought, and many people were forced to move away to find
new sources of livelihood.
Earth's Climate: A Dynamic System
Weather changes both rapidly and slowly. The passage of a thunderstorm
can change a bright sunny day into a dark, windy, rainy one in
less than an hour. Yet farmers know that in one year the amount
and timing of rainfall can be nearly ideal for growing crops,
while the next year might bring drought or floods. In some years
no hurricanes reach the Atlantic Coast, while in other years coastal
states are battered by one storm after another.
In many cases, these variations in weather are random; like the
lucky and unlucky days of a gambler they occur without any apparent
cause. The atmosphere, in isolation, has only short-term memory,
and so acting alone it cannot produce random variations that last
longer than about a month. But the climate is determined by the
workings of the climate system, which is composed of the atmosphere,
oceans, ice sheets, land, and the plants and animals that inhabit
them. Because the ocean has a large capacity to store and release
heat, it gives the climate system a long memory which can result
in variations lasting decades or longer. The number of hurricanes
in the Atlantic, for example, is known to vary from decade to
decade in synchrony with subtle shifts in the sea surface temperature.
Similarly, long-term effects can result from changes in the biology
of the climate system as well as its chemistry. Once perturbed,
for example, the carbon dioxide content of the atmosphere takes
more than a century to return to normal.
If weather varies over long intervals and climate does too, how
do we distinguish one from the other? One simple way to think
of it is that climate is what we expect; weather is what we get.
To describe climate, researchers look at the average weather over
a number of years in a particular region during a particular season.
Variations in the weather from year to year usually cancel each
other out and therefore the region's climate stays relatively
But sometimes changes in temperature or precipitation continue
for a few years or even a decade. We can think of these shifts
in weather as climate fluctuations. One example of an important
climate fluctuation is that associated with the El Nino-Southern
Oscillation of the tropical Pacific. The ocean and atmosphere
are closely linked in this region and together produce important
climate fluctuations on intervals of two to five years that have
a significant impact on the seasonal weather in regions far removed
from the tropical Pacific. Weather phenomena ranging from droughts
in Australia to flooding in the U.S. result from the intimate
slow dance of the atmosphere with the ocean.
Another example of a climate fluctuation is the Dust Bowl of the
1930's in America. While it had a very serious influence on the
lives of many people, it lasted only a few years and did not represent
a long-term change in the climate. We can't give a simple explanation
for the series of warm, dry years that produced the Dust Bowl
event of the 1930's, but it is probably an example of a natural
fluctuation of the climate system. The effects of this fluctuation
were worsened by the agricultural practices in use in the region
at that time, and improved conservation techniques were adopted
after the Dust Bowl experience.
Caption: The time series of summertime temperature and rainfall
at Topeka, Kansas gives a useful illustration of natural year-to-year
variations in local climate, as well as the major climate fluctuation
associated with the Dust Bowl period of the 1930s.
In addition to its fluctuations from year to year or decade to
decade the climate also varies on time scales of centuries or
longer. Great continental ice sheets have appeared and disappeared
again and again over the last several million years. What caused
these long-term variations? Scientists believe they stem from
something other than the internal workings of the climate system.
Just as a baseball player's home run statistics might change when
the fences are moved closer to home plate, the weather statistics
can change as a result of shifts in the planet's external conditions.
Why Does Our Climate Change?
In 1930 the Serbian mathematician Milutin Milankovitch offered
a theory for what caused the advances and retreats of ice sheets.
He hypothesized that the critical factor in determining ice sheet
growth was the amount of sunshine reaching high latitudes of the
Northern Hemisphere in the summer. We call the energy provided
by sunshine the insolation. Milankovitch predicted that ice sheets
would grow when the insolation reaching the high latitude continents
was less than normal during summer, since this would allow snow
cover to last through the melting season and gradually accumulate
over the centuries.
Caption: The oxygen isotope record in ocean sediments allows us
to estimate the mass of water contained in continental ice sheets
in the past. The global ice volume has varied dramatically from
ice age conditions to interglacial conditions more like today's
many times over the past 3 million years. This plot shows the
variation of global ice volume over the last 500,000 years. Figure
prepared by D.L. Hartmann from data supplied by Raymo, M. E.,
W. F. Ruddiman, N. J. Shackleton and D. W. Oppo , 1990: Evolution
of Atlantic-Pacific d 13C gradients over the last 2.5 m. y. Earth
Planet Sci. Lett., 97, 353-368.
He showed that changes in insolation result from subtle variations
in Earth's orbit. The planet's tilt as it revolves around the
sun, for example, varies with a period of about 41,000 years.
Today the angle is about 23.5 degrees, but it ranges between 22
and 24.5 degrees. The amount by which Earth's orbit deviates from
a perfect circle also varies, with periods around 100,000 and
400,000 years. And the month of Earth's closest approach to the
sun--currently January--varies on a 23,000-year cycle. The effects
of all these orbital variations are largest in middle and high
latitudes, where ice is more likely to form.
Over the last several decades Milankovitch's theory has received
a large boost. Modern techniques have allowed scientists to calculate
how much land ice once existed, based on information contained
in layered ocean sediments. For the last several million years,
the ice sheets have varied with the same regularity as Earth's
orbit. Summertime insolation at high latitudes drops at roughly
the same times that global ice volume peaks, in accordance with
the Milankovitch theory. In particular, the period of rapid ice
sheet melting about 10,000 years ago occurred at a time when greater
summertime insolation came to the high latitude continents of
the Northern Hemisphere.
Yet while external shifts of insolation appear to be the pacemaker
of ice ages, the nature and magnitude of the resulting climate
changes are still determined by processes that take place within
Earth's climate system. In order for the climate to swing from
ice age to warmer conditions, the climate system must amplify
the response to Earth's orbital changes. One way that this amplification
takes place is via a process known as ice-albedo feedback. "Albedo"
is, in short, a measure of Earth's reflectivity. Snow and ice
bounce the sun's rays back into space far more effectively than
unfrozen ocean or ice-free land. When temperatures are cold enough
for snow cover to last through a summer season, the planet absorbs
much less of the energy available in sunshine than it would without
a covering of snow. Thus, as the ice expands, less solar heat
is absorbed, which tends to cool the climate further and leads
to further expansion of the ice cover. This ice-albedo feedback
process can make the climate more sensitive, so that more temperature
change results from influences like shifting insolation.
An important clue to understanding how the climate can get cold
enough to sustain summertime snow comes from measurements of carbon
dioxide (CO2). CO2 is
a greenhouse gas that tends to warm the climate. Scientists can
determine how much CO2 existed in ancient
air because some of that air is trapped in bubbles inside cores
of ice from the Greenland and Antarctic ice sheets. These cores
show that the atmosphere contained 40% less CO2
when the ice reached its maximum extent 20,000 years ago than
it did just prior to the Industrial Revolution. Calculations show
that the reduced CO2 may account for nearly
half of the approximately 10_F global cooling during this glacial
The knowledge that variations in the chemical composition of the
atmosphere are important for explaining the ice ages has caused
scientists to broaden their view of the climate system to include
not only the physical processes that constrain energy and moisture,
but also the chemical and biological processes that control atmospheric
composition and land surface characteristics. Over the longer
periods of time that are required for major glacial cycles, the
atmospheric CO2 content is closely tied to
the amount of CO2 in the ocean. The amount
of CO2 in the ocean is dependent on marine
organisms that use CO2, sunlight, and nutrients
in the process of photosynthesis. Lowered atmospheric CO2
may have resulted from increased productivity of these marine
organisms during the ice age.
Caption: Estimates of past carbon dioxide concentrations derived
from ice cores drilled at Vostok, Antarctica and Siple Station,
Greenland are combined with the modern instrumental record from
Mauna Loa Observatory to show the relationship between atmospheric
CO2 changes associated with ice ages and the
modern increase in CO2 associated with human
activities. Natural control of atmospheric CO2
ended at the time of the Industrial Revolution, when humans began
burning fossil carbon fuels, manufacturing cement, and removing
forests at an increasing rate. [Prepared by D.L Hartmann from
public data sources. Data references are Barnola, J.M., D. Raynaud,
C. Lorius, and Y.S. Korotkevich, 1994. Historical CO2
record from Vostok ice core. p.7-10 in Trends '93: A Compendium
of Data on Global Change.; Neftel, A. H. Friedli E. Moor, H. Loetscher,
H. Oeschger, U. Siegenthaler, and B. Stauffer. 1994. Historical
CO2 recor from the Siple Station ice core.
pp. 11-14; and Keeling, C.D, and T.P. Whorf, 1994. Atmospheric
CO2 records from sites in the SIO air sampling
network. pp. 16-26. in Trends '93: A Compendium of Data on Global
Some things cause climate to change over periods shorter than
glacial cycles. Climate change could, for example, be produced
by variations in the energy output of the sun. Observations taken
over the last few decades indicate that output is about 0.1% greater
when the number of dark spots on the sun is at its maximum--roughly
every 11 years--than when it is at a minimum. This change in energy
output is too small to cause important climate variations, but
the sun's output may vary more on longer time scales. Some evidence
suggests that weakened solar energy output may have helped produce
the Little Ice Age of 1350-1800AD. During this period temperatures
were a few degrees colder than now in middle latitudes. But while
mountain glaciers expanded, major ice sheets did not form.
Volcanic eruptions can affect the climate over the short term
by sending large amounts of sulfur dioxide (SO2)
gas into the stratosphere, about ten miles above Earth's surface.
In the stratosphere the SO2 gas is converted
into tiny sulfuric acid droplets that remain there for a year
or more. These droplets reflect sunlight and reduce the solar
heating of the planet. The eruption of Mt. Pinatubo in June of
1991 cooled the climate by a few tenths of a degree for about
a year. But such effects fade as the volcanic particles slowly
fall out of the stratosphere. Only a succession of major volcanic
eruptions could cause a longer-lasting change in climate.
Can We Change the Climate?
At the end of the last ice age, there were perhaps a million people
in North America, or about one for every 7 square miles. Today,
excluding Alaska and Hawaii, there are about 80 people for every
square mile of land area in the United States. To sustain this
population growth and raise our standard of living, we employ
natural resources and technologies that were unknown to our ice
age forebears. These technologies also allow us to see the invisible
impact of our population on the broader terrain of the planet's
In 1896 the Nobel-Prize-winning Swedish chemist Svante Arrhenius
predicted that humans would warm the global climate by increasing
the carbon dioxide content of the atmosphere. And in fact, measurements
show that the levels of this gas have increased by about 30% since
the late 1700's. That time coincides with the beginning of the
Industrial Revolution when the use of coal as an energy source
began to increase rapidly. Burning coal releases CO2
to the atmosphere. Other fossil carbon fuels, like petroleum and
natural gas, also release CO2 when they are
burned. Such fuels are used in electrical generation plants, automobiles,
home heating, and in a variety of other ways. Carbon dioxide also
escapes to the atmosphere during the process of cement manufacture
and as a result of deforestation.
The yearly rise in CO2 has increased in recent
times, and continued growth of both population and per capita
energy use will force atmospheric CO2 to even
higher levels. In addition, the levels of other greenhouse gases
in the atmosphere have increased during the industrial age, in
most cases as a direct result of human activities. These include
halocarbons, methane (CH4), nitrous oxide
(N2O), and tropospheric ozone (O3).
Is it possible that, because of our numbers and our greater use
of resources and technology, modern humans are directly influencing
the global climate of Earth just as Arrhenius predicted?
Caption: Atmospheric carbon dioxide has increased from a value
of about 275 parts per million before the Industrial Revolution
to about 360 parts per million in 1996, and the rate of increase
has speeded up over this span of time. The amount of CO2
in the atmosphere has been measured with instruments since 1957.
CO2 concentrations farther into the past can
be estimated from CO2 amounts trapped in bubbles
in ice cores from Greenland and Antarctica. Atmospheric CO2
began to rise rapidly in about 1700 at the beginning of the Industrial
Revolution. It is certain that the predominant cause of this increase
is burning of fossil carbon fuels such as coal, oil and natural
gas. Prepared by D.L Hartmann from public data sources. Data references
are Neftel, A. H. Friedli E. Moor, H. Loetscher, H. Oeschger,
U. Siegenthaer, and B. Stauffer. 1994. Historical CO2
record from the Siple Station ice core. pp. 11-14; and Keeling,
C.D, and T.P. Whorf, 1994. Atmospheric CO2
records from sites in the SIO air sampling network. pp. 16-26.
in Trends '93: A Compendium of Data on Global Change.
The Greenhouse Effect
Carbon dioxide gas constitutes a tiny fraction of the atmosphere.
Only about one air molecule in four thousand is CO2.
Yet in spite of the fact that so little of it is around, CO2
can have a big effect on the climate. To understand why we need
to understand the greenhouse effect of the atmosphere. Earth's
atmosphere lets in the rays of the sun which warm the surface.
The planet, in turn, keeps cool by emitting heat back into space
in the form of infrared radiation--the same radiation that warms
us when we sit near a campfire or stove. But while the atmosphere
is fairly transparent to sunshine, it is almost opaque to infrared
radiation. Much like a garden greenhouse, it traps the heat inside.
About half of the solar energy that reaches Earth passes through
the atmosphere and is absorbed at the surface. About 90% of the
infrared radiation emitted by the surface is absorbed by the atmosphere
before it can escape to space. In addition, greenhouse gases like
CO2 as well as clouds can re-emit this radiation,
sending it back toward the ground. The fact is, Earth's surface
receives almost twice as much energy from infrared radiation coming
down from the atmosphere as it receives from sunshine. If all
greenhouse gases were removed from the atmosphere, the average
surface temperature of Earth would drop from its current value
of 59F (15C) to about 0F (-18C). Without the atmosphere's greenhouse
effect, Earth would be a frozen and probably lifeless planet.
Caption: The atmosphere allows solar radiation to enter the climate
system relatively easily, but absorbs the infrared radiation emitted
by the Earth's surface. Although about half of the energy coming
from the sun is absorbed at the surface of the Earth, almost twice
as much heating of the surface is provided by downward infrared
emission from the atmosphere. This "greenhouse effect"
causes the surface of Earth to be much warmer than it would be
without the atmosphere. This diagram shows the flow of solar (yellow)
and infrared (red) radiative energy through the climate system
in Watts per square meter of surface area. On average, 168 Watts
of solar radiation energy reach each square meter of the surface
area, but the heating of the surface from the downward infrared
radiation emitted by the atmosphere is almost twice that, 324
Watts per square meter. Prepared by D.L. Hartmann and Kay M. Dewar
from data supplied in Kiehl, J . T. and K. E. Trenberth, 1996:
Earth's annual global mean energy budget. Bull. Amer. Meteor.
Soc., 77, submitted.
It is the distinctive molecular structures of the greenhouse gases
that allow them to absorb and re-emit infrared radiation in this
way. Although the atmosphere is about 78% nitrogen and 21% oxygen,
these gases have a simple structure consisting of two identical
atoms. As a result, they have a relatively minor effect on the
transmission of solar and infrared radiation through the atmosphere.
But the three-atom molecules of water vapor, carbon dioxide, ozone
and a host of other gases can efficiently absorb and emit infrared
energy by storing and releasing it in molecular vibration and
rotation. Though some of these gases constitute only a tiny fraction
of the atmosphere, they can nevertheless make significant contributions
to the greenhouse effect.
The molecule that makes the largest contribution is water vapor,
which is relatively abundant in the atmosphere. The amount of
water vapor in the air is determined by the balance between evaporation
from the surface and precipitation as rain or snow. An average
water molecule stays in the atmosphere only a few days between
evaporation from the surface and falling out of the atmosphere
as precipitation, and the water vapor content of the atmosphere
adjusts quickly to changes in surface temperature. There is little
that humanity can do to directly control global atmospheric water
vapor amounts. Because atmospheric water vapor tends to increase
with increasing temperature, however, it can amplify climate changes
produced by other means--a process called water vapor feedback.
Why are Greenhouse Gas Amounts Increasing?
Carbon dioxide has a much longer lifetime in the atmosphere than
water vapor. If CO2 is suddenly added to the
atmosphere, it takes between 50 and 200 years for the amount of
atmospheric CO2 to establish a new balance,
compared to several weeks required for water vapor. That's because
CO2 is cycled between the atmosphere and the
ocean or land surface by chemical and biological processes. Plants,
for example, use it to produce energy in a process known as photosynthesis.
Through millions of years of Earth's history, trillions of tons
of CO2 were taken out of the atmosphere by
plants and buried in sediments that eventually became coal, oil
or natural gas deposits. In the last two centuries these deposits
have been employed at an increasing rate as an economical energy
source, and today humanity releases about 5.5 billion tons of
carbon to the atmosphere every year through fossil fuel burning
and cement manufacture. Approximately another 1.5 billion tons
per year are released through land use changes such as deforestation.
These releases result in an increase of atmospheric CO2
of about one-half percent per year.
Other naturally occurring greenhouse gases such as methane and
nitrous oxide have also been increasing, and entirely man-made
greenhouse gases such as halocarbons have been introduced into
the atmosphere. Many of these gases are increasing more rapidly
than carbon dioxide. The amount of methane, or natural gas, in
the atmosphere has doubled since the Industrial Revolution. Although
its sources are many, the increase is believed to come mainly
from rice paddies, domestic animals, and leakage from coal, petroleum
and natural gas mining. Halocarbons are a family of industrial
gases that are manufactured for use in refrigeration units, as
cleaning solvents, and in the production of insulating foams.
They were first manufactured in the 1940's, and because they do
not easily react with other chemicals they can have a lifetime
in the atmosphere of more than 100 years. Halocarbons are also
responsible for the Antarctic Ozone Hole and a more general decline
in global stratospheric ozone, but this is a separate problem
from the greenhouse warming contributed by the halocarbons. Production
of some of the halocarbons that are most important for climate
have been regulated by international agreements to preserve Earth's
protective ozone layer, so their influence on climate will decline
in the future.
Climate Forcing Pie Chart
Caption: Changes in the atmospheric concentration of CO2,
methane, nitrous oxide and halocarbons that have occured since
the Industrial Revolution have altered the energy budget of Earth.
The difference is about 2.4 Watts per square meter, or roughly
1% of the energy flow through the climate system. [Prepared by
D.L. Hartmann from public data supplied in Houghton, J. T., L.
G. Meira Filho, B. A. Callander, N. Harris, A. Kattenberg and
K. Maskell, Eds., 1996: Climate Change 1995: The Science of Climate
Change. Intergovernmental Panel on Climate Change, Cambridge,
Aerosols: Sunscreen for the Planet?
Although our effect on the levels of greenhouse gases in the atmosphere
is the most important direct influence that we can have on the
global climate, humans also contribute to the aerosol content
of the atmosphere. Aerosols are tiny particles of liquid or solid
matter that are suspended in air. They are different from water
cloud droplets or ice particles in that they appear even in relatively
dry air. Atmospheric aerosols have many sources and are composed
of many different materials including sea salt, soil, smoke, and
sulfuric acid. Although there are many natural sources of aerosols,
it is estimated that aerosols resulting from human activities
are now almost as important for climate as naturally produced
ones. Most of the human-induced aerosols come from sulfur released
in fossil fuel burning and from burning vegetation to clear agricultural
land. Human production of sulfur gases accelerated rapidly in
It appears that the cooling effect of atmospheric aerosols has
canceled out part of the warming that might have been associated
with greenhouse gas increases. Aerosols can reflect solar radiation
or absorb and emit infrared radiation, and are often visible as
haze or smog. By reflecting sunlight, they cool the climate. The
human-induced increase in atmospheric aerosols since preindustrial
times is believed to have reduced the energy absorbed by the planet
by about half a Watt per square meter, which would offset about
20% of the greenhouse gas warming effect.
The aerosols produced by humans could also have a significant
effect on the numbers or properties of clouds. Every cloud droplet
or ice particle has at its center an aerosol, called a cloud condensation
nucleus, on which the water vapor collected to form the cloud
droplet. Aerosols that attract water, such as those composed of
salt or sulfuric acid, are particularly effective as cloud condensation
nuclei. The increased number of aerosols produced by humans could
cause the water in clouds to be distributed into more, but smaller,
cloud droplets. With their water spread more diffusely, the clouds
would reflect more solar radiation. The existence of such clouds
would cause a cooling that might offset part of the greenhouse
gas warming, but the size of this effect is very uncertain.
We must also keep in mind some very important differences between
the greenhouse warming and the aerosol cooling. While greenhouse
gases such as CO2 and halocarbons remain in
the atmosphere for about a century after being released, only
a few weeks transpire between when an aerosol is released into
the lower atmosphere and when it is washed out entirely. Therefore
human-produced aerosols are not distributed evenly over the globe,
but tend to be concentrated near the points where they are released
into the atmosphere. Aerosols that result from human actions originate
predominantly in industrialized countries of the Northern Hemisphere,
where fossil fuels are burned, and in land areas where vegetation
is burned. Because their effects are more localized, aerosols
may cause regional shifts in climate. Also, because of their short
lifetimes in the atmosphere, the effect of aerosols on today's
climate is determined by the release of aerosols that occurred
during the previous couple of weeks. In contrast, the CO2
that we release into the atmosphere today will affect the climate
for 50 to 100 years into the future.
For these reasons the greenhouse gas warming must eventually overwhelm
the human-induced aerosol cooling. Nonetheless it is important
to understand the effect of aerosols on the climate so that we
may better predict how changing greenhouse gas amounts will affect
the future climate and assign causes to temperature changes when
we observe them. Efforts are underway to reduce the release of
SO2 gas from coal-fired energy plants because
it causes acid rain and lung disease, and this may have the effect
of reducing aerosol amounts in some regions.
How has Climate Changed in the Last Century?
Measurements suggest that global mean surface temperature has
increased by about 1_F in the last century. The warming has been
greatest over the continents between 40 and 70 degrees north latitude.
Over this same period of time measurements indicate that global
sea level has risen between 4 and 10 inches. Scientists do not
yet know with certainty what part of these changes is caused by
humanity and what part would have occurred without us. Part of
this warming may be a rebound from the cooling of the Little Ice
age during the 1350-1800 period, and the causes of the Little
Ice Age were probably unrelated to human activities. However the
period of this warming also coincides with the period when human
activities have increased CO2 and other greenhouse
gases in the atmosphere. Many scientists are convinced that human
activities have made a major contribution to the warming of the
last century, and that warming caused by greenhouse gas increases
will be a continuing part of our future.
A rapid greenhouse warming of the climate would cause serious
problems. Because such a warming, once initiated, would last for
a long time, scientists and civic planners are very interested
in knowing how much warming is occurring and whether that warming
can be attributed to human actions. The record of global temperature
obtained from thermometers around the world extends back in time
only a little over a century. This record shows a steady increase
up until the 1940's, followed by a period of slight cooling between
the 1940's and 1970's. Since the 1970's the temperatures have
gone up rapidly, and many of the warmest years in the global temperature
record have occurred in the last 15 years. It is not known with
certainty whether this recent warming trend will continue, nor
whether it is caused by the increasing trend of greenhouse gas
concentrations in the atmosphere. The natural random variability
of the climate system on decade-long time scales is fairly large,
and it is not yet easy to separate this variability from changes
that have resulted from human activities.
Temperature Anomaly Plot
Caption: The record of global mean surface air temperature from
thermometer readings indicates a global warming over the past
century, with many bumps and wiggles suggesting the natural year-to-year
variability of climate. Prepared by D.L Hartmann from data supplied
in Hansen, J., R. Ruedy, M. Sato and R. Reynolds, 1996: Global
surface air temperature in 1995: Return to pre-Pinatubo levels.
Geophys. Res. Lett., 23, 1665-1668. and in Wilson, H. and J. Hansen.
1994. Global and hemispheric temperature anomalies from instrumental
surface air temperature records. pp. 609-614 in Trends '93: A
Compendium of Data on Global Change. and including later additions
to the online data set.
How do We Predict Climate Change?
The behavior of the climate system can be simulated with computer
models, and the simulations can then be tested against observations
of the current and past climates. They can be used to study the
response of the climate to changing amounts of greenhouse gases
and aerosols, to changes in land surface conditions, and to other
natural or human-caused changes. But while such models capture
many of the key features of the present climate, they do have
Modeling the climate on a computer is difficult because processes
with very large spatial scales, such as the transport of energy
from the tropics to the poles by atmospheric motions ranging over
thousands of miles, are just as important as small-scale processes
like the collection of water molecules into raindrops. How do
we represent this wide range of spatial scales and also produce
a model that is efficient enough to run on available computers
in a reasonable length of time? The standard approach is to represent
the globe with a grid of boxes about 100 miles on a side and then
predict the average properties in these boxes using known laws
of physics. The effects of processes that occur on smaller scales
are represented with approximate formulas that relate them to
the averaged properties in the grid boxes. The problem with this
approach is that some of the small-scale processes that must be
treated in a more approximate fashion are also central to the
feedback effects that determine how much climate change will result
from human actions. For example, clouds have a huge influence
on the transmission of solar and infrared radiation through the
atmosphere, yet the processes that determine the properties of
clouds occur on scales that are much smaller than a climate model
grid box. A large part of the uncertainty in forecasts of future
climates derives from uncertainty about how to treat clouds in
climate models. Important feedbacks such as those involving surface
ice and atmospheric water vapor also involve processes occurring
on small scales and must be treated with approximate formulas.
As computer power and understanding both increase, some of the
uncertainty associated with feedback processes will decline and
more accurate climate forecasts will become available.
What do Climate Models Tell Us about the Future?
Once a climate model has been tested against current and past
observations, it is reasonable to ask what it can tell us about
future climates. A typical experiment of this nature is to extend
the past century's increase in greenhouse gases into the next
century and see how the climate model responds to this change.
Because of the approximations in the models, however, the predicted
warming over the next century is quite uncertain, ranging from
a modest warming of 2_F(1_C) to a very substantial warming of
8_F(4.5_C). Models consistently predict that the warming would
be greater in high latitudes than in the tropics, and greater
over land than ocean. Many models predict larger increases in
evaporation than in precipitation over midlatitude land areas,
which would result in drier conditions in those regions, especially
during summer in North America and Southern Europe. Warming may
cause agricultural zones in North American to move northward,
which would benefit some communities and harm others. Changes
in the climate of specific small regions and changes in the activity
of tropical storms cannot yet be predicted with much confidence.
When natural climate fluctuations cause sea surface temperature
in the tropical Atlantic to increase, hurricane activity also
increases, but it is not certain that a global surface temperature
rise caused by greenhouse gas increases would have a proportional
effect on hurricane activity.
The effect of the warming on humanity depends on the magnitude
of the warming, the speed with which the warming occurs, and the
way society is organized to adapt to climate change. If the warming
is as fast and as large as some of the models predict, then the
effects on people and our natural environment could be very serious.
Agriculture and water supplies take decades to adapt, and natural
ecosystems take centuries. Therefore, rapid change would pose
more difficult problems.
Where do We Go from Here?
When planning for the future, most Americans assume that the climate
we have experienced in the past will continue, but this may not
be the case. Rain, snow, and temperature affect many aspects of
human life, including public health, agriculture and the way we
manage our water and energy resources. We know that the amounts
of some greenhouse gases in the atmosphere are increasing as a
result of human activities. The well-understood physics of the
greenhouse effect indicates that the changing composition of the
atmosphere should warm the surface climate of Earth. Current estimates
of the expected climate change over the next 50 years range from
a future climate modestly warmer than today to one warmer than
any that has occurred on Earth for more than a million years.
This range of uncertainty is uncomfortably large. Moreover, current
models cannot make accurate predictions of how temperature and
the availability of water might change in a particular state or
county, where measures to adapt to climate change would need to
Scientists are working hard to improve our understanding of the
climate system and our ability to predict its future course. This
work involves taking careful observations to monitor subtle changes
in the climate system, conducting intensive observational programs
to study the processes that determine how much climate change
to expect, and continuing to improve climate models and test them
against observations. We also need to better understand the complex
relationship between humans and climate. Because of the long lifetime
of greenhouse gases in the atmosphere and the slow but steady
response of the climate to them, it is very important to have
accurate forecasts of how the future climate will evolve in response
to both natural and human forces. The potential exists for very
significant shifts in climate. An accurate assessment of what
those shifts will bring will help us to either mitigate climate
change or adapt to its effects.
Given the current level of uncertainty and the complexity of the
climate system the future will certainly bring surprises, both
of the pleasant and the unpleasant variety. Information about
how the climate is changing, the assignment of causes to these
changes, and accurate prediction of future climates will be very
important for the public and policy makers. Efficient communication
of this information to all concerned will be an important part
of the process of deciding how to respond to the challenge of
our changing climate.
Meltzer, D. J., 1993: Search for the First Americans. Smithsonian Books, Washington, D.C., 176.
Hartmann, D. L., 1994: Global Physical Climatology. Academic Press, San Diego, 411.
Graedel, T. E. and P. J. Crutzen, 1995: Atmosphere, climate, and change. W.H. Freeman, 196pp.
Imbrie, J. and K. P. Imbrie, 1979: Ice Ages: Solving the Mystery.
Enslow Publishers, Short Hills, N. J., 224.
Boxed text and wavy text for top channel:
page 1, boxed text:
"We have now entered an era when actions by humanity may have as much
influence on Earth's climate as the natural processes that have driven climate
change in the past. Our future climate will be partly of our own
page 2, boxed text:
"Favorable temperatures and abundant water near the surface of Earth
support a rich diversity of life. Patterns of temperature and rainfall have
shifted significantly over time in response to natural forces, and these
changes in climate have had important effects on people and the natural
world we live in. "
page 3, wavy text in top channel:
"Life on Earth responds to the climate and also helps to
page 14, wavy text in top channel:
"Without the greenhouse effect, Earth would be a frozen planet."
pages 24-25, wavy text in top channel:
"Humanity, long affected by Earth's changing climate, now
plays a role in shaping it."