<pubnumber>236K97001</pubnumber>
<title>Reports to the Nation on Our Changing Planet: Our Changing Climate</title>
<pages>28</pages>
<pubyear>1997</pubyear>
<provider>NEPIS</provider>
<access>online</access>
<operator>BO</operator>
<scandate>03/02/98</scandate>
<origin>hardcopy</origin>
<type>single page tiff</type>
<keyword>climate ice atmosphere earth greenhouse warming change atmospheric global surface co2 years nation aerosols reports human solar radiation infrared gases</keyword>
Fall 1997 No.
On Our Changing Planet
H V
image:
Table of Contents
Climate and American People........"......
Earth's Climate: A Dynamic System,.........:.......................
Why Does Earth's Climate Change?... ,.......,„......,....,..
Can We Change the Climate?............. ..........................
The Greenhouse Effect.........,................................:......
Why Are Greenhouse Gas Amounts Increasing? ..............
Aerosols: Sunscreen for the Planet?..... .....v..v..-..'...,.:;..,....IB
How Has Climate Changed in the Past Century?......,.,... ..lEi
Can We Predict Climate Change? ......;.................:..,^...,:..:.,E
What Do Climate Models Tell Us About Our Future? .........
'Where Do We Go From Here? ,:........ ......;...:..'......„....
Cover. Peruvian coast and Andes Mountains
looking south. NASA Space Shuttle.
A publication of the University Corporation for Atmospheric Research pursuant to
National Oceanic and Atmospheric Administration Award No. NA576P0576. ,
I
Reports to the Nation • Fall1997
image:
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 'making.
Reports to the Nation - Fal|1997
image:
/'(tt'orahle temperatures and
abundant water near the
mrjace of Earth support a rich
ditvKity <>/ lift'. Patterns of
lemlK'ratnre and rainfall bare
shifted aifiiu'fkwilly over time
in ms/«tf/«' to natural forces.
and lime changes in climate
hare had important effects on
/wnpli' ami the natural world
In ichich ire lire.
V— -—y ! -' j J>7^
"
^pT-x-u^
\jf A * TI_
Reports to the Nation • fa//W97 ' •
image:
Reports to th« Mation • Fa(H997
image:
.. !',,«>"'! , •' >'.:'.'. ''.,• -.-'••
'i'Wif J« ij*'; &>?"-'•• . '
liJi,***'!-:1 >sj ." ,"•-•' • :
,iBI*J«. >?•
' '' ••••
T^lfcf' '
^siii* ;•
.-•.
tiKfe'.;*!
•;>. ^
Reports to the Nation • Fall 1997
image:
Climate
LIMATE AMD 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 F?3 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
V
Ohio Because so much water was piled up on land in ice sheets, the
sea level was about 3Sfeet lower at the peak of the last ice age about
_ » v
20,000 years ago than it is today The lowered sea exposed a wide
plain between SiberS and Alaska, creating a land bridge across the
"* *^^jr *
Bering -Sea Genetic, linguistic, and fossil evidence suggests that the
/ «°
first humanj^l America came from northeast Asia, and it is likely that
*" fr ^" %_tl& ajSgnif
the fee 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 could have made their way south past the great
ice sheets and spread across America
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
t
argue that efficient human hunters caused these extinctions, but
others believe that environmental change was the key factor F$]
fl e"p, 6 r'J s,.;t o, t h e M s t i o tv .,-, -fa(( .1897..
image:
Arctic
Ocean
*%-,;
m-
Pacific^
Oce'an
global dimateT?
, <•- ' •:••* i.$f iff1
the great North
" [ 'vVl^T^
to melt rapidly, ana
-. ''f'.^V «** 3——-™
were gone. Tins end to the
V? | ^ n '",,11 ii,™ .IIH!|I|||,I ilHijUI'jH'iiaililiiim
dramatic changes in
melted and the climate' waitneH, the once-weL
: :"* ^?^vWswiawBi
region between the Cascade Range and .Rod
Mountains fcOj became the relatively dry land-
scape that we know today as the Great Basin."
Features like Utah's Great Salt Lake shrank to
shadows of their former selves. Fifteen thou-
sand years ago this body of water was 1,200 feet
deeper and covered an area the size of Lake
Midiigan.
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 cli-
mates may not have been able to adapt to the
warmer, drier conditions. As their favorite game
animal disappeared, :the earliest Americans..
J Land Exposed by Lowered Sea Level
^fe>4^sSS»ijfi!S •••• • • A
Possible Human Migration Route
'Ice sfieets^anolowerecl'sealevei'a't1 tii'etii'ne' 'oftBe las? glacial maximum' opened an ':
access route from A|ia to 'America. The current .continent?! outline is given, for.
'.'•• reference. , ' ; ,,, '' _-'.'• • ..
' 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 r'
: Vikings who settled Greenland .under the lead-:;.
' ership of Eric the Red in about iOOOAD. His son,;; :
Leif Eriksdn, led an expedition to colonize .;
- America, that probably settled, in Ivfewfound- ,
land. The colony in Greenland was abandoned
.in about 1400AD when'cooler temperatures •"•
associate^ with the Little Ice Age made farming
Roports to the Nation • Fall 1997
image:
<,
there too difficult. Farther to the south, climate •
changes also affected the civilizations setup by
the earlier Asian immigrants to America The
<
Anasazi people pf the Four Corners region of
the southwestern United States' 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 experi-
enced a series of profound droughts, [g and by
1306AD the Anasazi had abandoned this area,
i
Although we have more advanced technol-
ogy than the Anasazi, modern residents of the
United States are also affected by variations in
our climate. Between 1934 and 1937 parts of
Texas, Oklahoma, Colorado, New Mexico, and
TCansas 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,1 rainy one m less
than an hour Farmers know that in one year the
amount and timing of rainfall can be nearly ideal
Canyon de Crjelly Anasazi rum, Arizona
i - *
f •, -. •. , ,
\ T.
^ Reports to tine Nation • Fall 1997
image:
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, gg
In many cases, variations in weather are
random; like the lucky and unlucky spells 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 persist month
after 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,- animals, aiid
people that inhabit them. Because the ocean
has a large capacity to store and release heat,
" it gives the climate system a long memory that
can result in variations lasting from seasons to
centuries. The number of hurricanes in the
Atlantic, for example, is known, to vary from
'year to year in synchrony with subtle shifts in
the sea surface temperature.arid seasonal wind
patterns. Similarly, long-term effects can result.
from changes in the biology and chemistry of
the climate system. For example; life in the sea
controls the, flux of carbon from atmospheric
Topeka, Kansas,
Climate Record
June, July, August
Average Temperature ;
and Rainfall
1890-1990
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 asso-'
mated, with the Dust Bowl.
period,of the 1930s.
Reports to the Nation • FalH997
image:
carbon dioxide into ocean sediments
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
Randorn variations in the weather frorn year to
year usually balance each other m these aver-
t-*1
ages and do not affect the mean climate
But sometimes abnormal temperature or rain-
fall persists for a few years or even a decade We
can think of these slow shifts in weather as
climate fluctuations One important source of
climate fluctuations is the El JSfino-Southern
-*t
Oscillation of the tropical Pacific The ocean
and atmosphere are closely linked in this region
and together produce important climate fluc-
tuations from one year to the next that have a
significant impact on the seasonal rainfall there
and in regions far removed from the tropical
Pacific. Events ranging from droughts in Austra-
lia to flooding in some parts of 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 1930s in the United States
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 m the
climate We can't give a simple explanation for
the warm, dry years that produced the Dust
.* s,,
Bowl event of the 1930s, but it is probably an
example of a natural fluctuation of the qlimate
system The effects of this fluctuation were
worsened by the agricultural practices in use in
the region at that time, and improved soil
conservation techniques were adopted after
the Dust Bowl experience g|
Climate varies not only from year to year and
decade to decade but also on time scales of
centuries Or longer Great continental ice sheets
Save 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
f
workings of the donate 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 Earth's Climate Change?
In 1924 the Serbian mathematician Milutin
Milankovitch offered a theory for what causes
the advances and retreats of ice sheets He
** « '*
hypothesized that the critical factor in determin-
ing ice sheet growth is the amount of sunshine
reaching high latitudes of the Northern Hemi-
sphere in the summer We call the energy
provided by sunshine the insolation
s
Milankovitch predicted that ice sheets would
grow when the insolation reaching the high
Reports to the Nation - Fain997
image:
1 4\;
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.
He showed that changes of insolation result
from subtle variations in Earth's orbit. Today
the Earth's axis of rotation is tilted about ,23.5
degrees relative to the plane of the Earth's orbit
about the sun, and this tilt gives us pronounced'
seasons in middle and high latitudes. This tilt
angle varies between 22 and 24.5 degrees with
a period of about 41,000 years. The amount by.
Which Earth's * orbit deviates 'from; a perfect,
circle also varies, with periods around 100,000
arid 400,000 {years. And the day of the year
•when Earth is closest to' the sun—-cuiTently-
: January 3rd^-varies on a 23,000-year cycle.
The effects, pf all these; orbital variations on
insolation are largest in middle arid high lati-
tudes, where colder temperatures^ make the,
.'development of large ice sheets possible. ||]
Over the last several decades Milarikpvitch's;
theory has received, a large boost. Modern
techniques enable .scientists'to estimate,past
Oxygen Isotope—Global Ice Volume Past 500,000 Years
Tlw oxyflen isotope record in ocean sediments can be used to estimate the mass of
water contained in continental ice sheets in the past, Many times over the past 3
million years the global ice volume has varied dramatically from ice age conditions
to ihterglacial conditions more like.today's. This plotshqyvs the variatibh.of global
' ice volume over the last 500,000 years, plotted upside dbwrrso-thaf peaks indicate
-Warm intervals and lo.w p'oints indicate ice ages.
Reports to the Nation • Fall1997
image:
amounts of land ice, based on information
contained in layered ocean sediments For the
last several million years, the ice sheets have
varied with the same rhythm as Earth's orbit In
> s
agreement with the Milankovitch theory, global
ice volume peaked at about the same times that
summertime insolation at high latitudes dipped
The period of rapid ice sheet melting about
10,000 years ago occurred at a time when
greater summertime insolation was reaching
the high-latitude continents of the Northern
Hemisphere
While external shifts of insolation appear to
be a 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 condi-
tions, the climate system must amplify the 4
response to Earth's orbital chafiges One way"
climate change can be amplified is via a process
known as ice-albedo feedback "Albedo" is a
measure of how much insolation Earth reflects
back to space Snow and ice bounce the sun's
rays back into space far more effectively than
unfrozen ocean or ice-free land g 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
v
energy is absorbed, which tends to cool the
High Northern Hemisphere Summer Jnsolation Glaciers Melt
**low Northern Hemisphere Summer Insolation Glaciers Grow and Expand
•- l *- .
/lilankovitch's Orbital Parameter Theory
^jlankovitch theorized that'subtle variations in Earth s orbit can add up to ~
grjrjie shifts in climate Alarms tilt in Earth saxtsof rotatioifcombmed with an ^
Kentrfc orBit an3 alummenime Sate for Earth s closesf approach to (he sun t
[topjcauses more sjinsriineto'reacrTriigfi latitudes of the^NortBern Hemisphere
nfrftr'f he resultls"*a warm'cflipafe Th'e opposite extreme in which T
of Eartffs VMS is smaller, its orbit is lessjeecentric. arfl it is closer to the "r
gj^afestsaafiwwfaawte&SWNsssSas&S^sWssBrf^siiii ^MMBHKO f «-affi
^urjngtne RgrtnenTHemisphere s winter (bpttomj causes snow cover to
ist thxpusfrthe meTtingseaso*n The accumulafranoTsnow over many seisons
'"""
climate further and leads to further expansion
of the ice cover This ice-albedo feedback
< " £ >
process can make the climate more sensitive to
> ( -*
outside influences like shifting msolatipn
f <r
An important clue to understanding how the
"- %
climate can get cold enough to sustain summer-
time snow comes from measurements of car-
>• »
bon dioxide (CO2) gas, whose presence in the
atmosphere tends to warm the climate, as
explained on p 14 Scientists can determine
how much CO2 existed in ancient air because
some^of that air is trapped in bubbles inside
~< -1 ^t v '
cores of ice from the Greenland and Antarctic
ice sheets These cores show that the atmo-
-> fc
sphere contained 40% less CO. when the ice
-*- T- V ^
reached its maximum extent 20,000 years ago
^
than it did just prior to the Industrial Revolution
^ 3
in the 18th century Estimates suggest that the
*• ^
reduced CO2 may account for nearly half of the
approximately 10°F global cooling during this
t f *
glacial maximum
t " <- * '
The discovery that variations in the chemical
i *•
composition of the atmosphere are important
Reports to the Mation • Fa/U997
image:
for explaining the ice age's 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 char-
acteristics. Over the longer time spans required
for major glacial cycles, the atmospheric CO2V
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 photo-
synthesis. Lowered atmospheric CO2 may haye
Past and Present Atmospheric C02 Concentration
Estimates of past carbon dioxide concentrations derived from ice cores drilled at
Vostok In Antarctica and Siple Station in Greenland are combined with the modern
instrumental record from Mauna Loa Observatory to create a continuous record that
shows both natural changes associated with ice ages and the modern increase in C02
associated wilh human activities. Natural control of atmospheric C02 ended at the time
o( the Industrial Revolution, when humans began burning fossil carbon fuels,
manufacturing cement, and removing forests at an increasing rate. >
resulted from increased productivity of these
marine organisms during the ice-age. :
Some "things cause climate to change over
periods shorter than glacial cycles. Climate,
change could, for example, be produced by
variations Jn 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
maximumr--roughly every 11 years—than when
it is at a minimum. This change in energy output
is too small to cause important climate variar,
tions, 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 1350-1850AD.
During this period cold spells were more com-
mon and temperatures were a few degrees
colder than npw in middle latitudes. But;while
mountain glaciers [o].expanded in :some re-
gions, major ice.sheets did notform. ,
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
Reports to the Nation • Fa/11697
image:
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 predecessors
Human activities are leading to a buildup of
certain trace gases in Earth's atmosphere Mea-
surements show that the level of carbon dioxide
has increased by about 30% since the late 1700s,
That time coincides with the beginning of the
Industrial Revolution when the use of coal as an
energy so~urce began to increase rapidly. Burn-
ing 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 the
destruction of forests.
Atmospheric CO2 has been increasing more
rapidly in recent times, and continued growth
of both population and per capita energy use
will make it rise even faster in the 21st century
In addition, the levels of other trace gases in the
atmosphere have increased during the indus-
trial age, in most cases as a direct result of
"* * s
human activities These include hialocarbons,
methane (CH4), nitrous oxide(N2O), andtropos-
phenc ozone (O3) In 1896, the Nobel Prize-
winning Swedish chemist Svante Arrhemus
predicted that the buildup of CO2 m the atmos-
phere would warmthe global climate How can
^such a small change in atmospheric composi-
tion have such a big effect on climate? [g]
Atmospheric Carbon Dioxide Since Premdustnal Times
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 It js certamlhat the
predominant cause of this increase is burning of fossil carbon fuels sucji as coal oil
and natural gas The amount of G02 in the atmosphere has been measured with
instruments since 1957 C02 concentrations prior to 1957 are estimated from C0a
amounts trapped irr bubbles in ice cores from Greenland and"Antarctica
Reports to the Nation • Fail 1997
image:
reenhouse effect, Earth tuouldbe
a\.
The Greenhouse Effect
Carbon dioxide gas constitutes a tiny fraction of
the atmosphere. Only about one air molecule in
three thousand is CO2. Yet despite their small
numbers, CO2 molecules can have a big effect on
the climate. To understand why they, are'-.'so-
important, we need to know about the green-
house effect of the atmosphere. Earth's atmos-
phere lets in rays of sunshine and, they warm the
surface. The planet keeps cool by emitting heat
back into space in the form of infrared radia-
tion—die 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 hea.t inside. [$>]
The Greenhouse Effect
The atmosphere allows solar radiation to enterthe climate system relatively easily, but
absorbs the infrared radiation emitted by the Earth's surface. Although about half of
the energy corning from the sun is absorbed at the surface of the Earth, almost twice
as much surface heating is provided by downward infrared emission from the
atmosphere as from sunshine. This "greenhouse effect" causes the
surface of Earth to be much warmer than it would be without the •^
atmosphere. The graphic on this page shows the flow of solar rff-f
(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 metarof the surface
area, but the heating of the surface from , .
the downward Infrared radiation .....'
emitted by the atmosphere Is
almosttwfce that,324watts
per square meter. - • > . -
About half-of the .solar energy that reaches,;' ,
Earth passes through the atmosphere and is; j
absorbed at the surface. In contrast, about 90%".
of the infrared radiation emitted by,the .surface;
is absorbed by'the atmosphere 'before'If can' ;
... escape to,space. In addition, greenhouse gases
like CO2 as well as clouds can re-emit this radia-'
tion, sending it back toward the ground. The fact ,.
as, .Earth's surface receives almost twice as much (
.yF^-arj^--'intrared
'
T». through
atmosphere
40 W
Reports to the Nation • Fall1997
image:
energy from infrared radiation coining down
j-, * t
from the atmosphere as it receives from sun-
shine IfJ all greenhouse gases were removed
from the atmosphere, the average surface tem-
perature of Earth would drop from its current
value of 597 (15*C) to about 0*F (-18°C) With-
.jib. *
out the atmosphere's greenhojfeLeffect. Earth
would be a frozen and nearl«iiLeless?Qknet.
Reflected solar radiation 107 W
Thermals 24 WC
Latent heat
o| condensation
78 W ,
"^'*"*"**Solar
radiation
reflected
by surface
SOW
• Evaporation,
transpiration
78 W
*~ * Incoming
' solar
radiation
342 W
o"Ju ifOBOJurKa,!! ri ^
Solar
radiation
absorbed by
atmosphere
67 W
Solar radiation
reflected by
clouds, aerosols
and atmosphere
77 W
Solar- radiation
absorbed by surface
168 W
••'Mj'iSsi^"^^8'3***
image:
Deoali National Park, Alaska,
It is the distinctive molecular structures of the
greenhouse gases that make them strong ab-
sorbers and emitters of infrared radiation. About
99% of air molecules are nitrogen'and oxygen,
which have a simple structure consisting of two
identical-.atoms. Because of this simple struc-
ture, they have a relatively minor effect on the
transmission of solar and infrared radiation.
through the atmosphere. Molecules with three :
or more atoms like water vapor, 'carbon diox-
ide, ozone, and a host of other trace gases can
efficiently absorb and emit infrared energy by
storing and releasing it in molecular vibration-
and rotation. Though sbme of these gases
constitute only a tiny fraction of the atmo-
sphere, they can;nevertheless make significant
contributions to the!greenhouse effect.
The molecule that makes the, largest contri-
bution is water vapor, which is a relatively
abundant greenhouse gas, An average water
molecule stays in the atmosphere Only a few
days from the time it, evaporates from the
surface to the time it falls out, of the atmosphere
as precipitation, so the water vapor content |^|
of the atmosphere adjusts quickly to changes in
surface temperature. Humanity can do little to
directly control the global amount of atmos-
pheric water vapor. Because atmospheric wa-
ter .vapor tends to increase: with increasing
temperature, however, it can amplify climate.
changes produced by other means—a process
called water vapor feedback.
Reports to the Nation • Fall1997
image:
Why Are Greenhouse
Gas Amounts Increasing?
Carbon dioxide has a much longer lifetime in the
f V T
atmosphere than water vapor. If CO2 is suddenly
added to the atmosphere, it takes 100 to 200 years
•i •->
for the amount of atmospheric CO2 to establish a
new balance, compared to several weeks for
water vapor. That's because the carbon in CO2 is
cycled between the atmosphere and the ocean or
land surface by slow chemical and biological
processes. Plants, for example, use CO2 to pro-
duce energy m a process known as photosynthe-
sis. Through millions of years of Earth's history,
trillions of tons of carbon 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 humans have
used these deposits at an increasing rate as an
economical energy source. In a simila^ way,
cement manufacture releases carbon atoms bur-
ied in carbonate rocks. Today humanity releases
about 5.5 billion tons of carbon to the atmos-
phere eyery year through fossil fuel burning ands
cement manufacture Approximately another I 5
billion tons per year are released through land
use changes such as deforestation. These,re-
leases 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 green-
house gases such as halocarbons have been
introduced into the atmosphere. Many of these
gases are increasing more rapidly than carbon
dioxide. The (amount pf 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
,» " f
fronrrice paddies, domestic animals; and leak-
-l -|s
age from coal, petroleum, and natural gas
mining. Halocarbons are a family of industrial
gases that are manufactured for use in refrigera-
tion units, as cleaning solvents, and in the
production of insulating foams, g They were
first manufactured in the 1940s, and because
they do not readily react with other chemicals
Methane
047W/m2
Nitrous Oxide
014W/m2
Halocarbons
0 24 W/m2
Carbon Dioxide
1 56 W/m =
Climate Forcing by Greenhouse Gas Increases Since the Industrial Revolution.
Changes in the atmospheric concentration of CQ2, methane, nitrous oxide, 'and
halQcarbonsthat have occurred since the Industrial Revolution have altered theenergy
budget of Earth The difference is about 2 4 watts per square meter, or roughly 1 % of
the energy flow through the global climate system
Reports to trie Nation'- Fall 1997
image:
bv trapping infrared radiation.
vJ1- .*..., ,; -...:. ,. • .• „•,
Clouds reflect sunlit
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 halo-,
carbons. Production of some of the halocar-
bons that are important greenhouse gases has
been regulated by international agreements to
preserve Earth's protective ozone layer, so their
influence on climate should decline in the
future. Nearer to Earth's surface, in the tropos- -
phere, ozone amounts have been increasing
because of human activities. , Ozone at the
surface has harmful effects on the health of
plants, animals, and humans. - . . , :
Aerosols: Sunscreen for the Planet?
Although raising the levels of greenhouse gases.
in the atmosphere is our most important direct
influence on the global climate, human actions :
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 .are present even in rela-
tively dry air. Atmospheric aerosols have many
sources and are composed of many different
materials including sea salt, soil, smoke,^and
sulfuric acid. Although aerosols have many.
natural sources, 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 agricul-.'. •
tural land. Human production of sulfur gases;,
•accelerated rapidly in the 1950s. :: ; :
It appears that the cooling effe'ct of aerosols ,
has canceled out part of the warming that might:
•have been associated with recent greenhouse'
gas increases. • Aerosols can, reflect, solar radia-
tion or absorb.and emit infrared radiation, and ,
afe bften.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,- enough to. offset about
20% of the greenhouse gas warming effect., g
The aerosols produced by humans could also.
have a. significant, effect ori the, amount or ,
'properties ofelouds. Every cloud droplet or ice.
i particle has at its center an aerosol called a cloud
condensation nucleus, on which the water ya- .
por collected to form the cloud droplet. Aerosols.
; that attract water, such as those composed^bf salt
Or sulfuric: acid, are. particularly effective as. ;
•[: cloud condensation nuclei. The increased,num- •
ber of aerosols produced by, humans could
. cause the water in clouds, to be distributed into
more,, but smaller, cloud droplets. WitK their :,
water spread more diffusely;. the clouds would
reflect more solar radiation. The existence of
Rsports to the Nation • Fall1997
image:
such clouds would cause a cooling that might
offset part of the greenhouse gas warming, but
the size of this effect is very uncertain [2]
We must 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, aerosols released into the lower atmos-
phere remain there only a few weeks There- A
fore, human-produced aerosols are not distrib-
uted evenly over the globe, but tend to be
concentrated near the points where^ they^ are
released into the atmosphere Most of them
originate in industrialized countries of the North-
em Hemisphere, where fossil fuels are burned,
and in land areas where vegetation is burned
Because their effects are more localized, aero-
sols may cause regional shifts in climate Also,
because of their short lifetimes in the atmos-
phere, the effect of aerosols on today's climate
is determined by the amount of aerosol pro-'
duced during the previous couple of weeks In
contrast, the CO2 that we release into the
atmosphere today will affect the climate for
more than 100 years
For these reasons the greenhouse gas warm-
ing must eventually overwhelm any human-
induced aerosol cooling that may be taking
place 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 the proper 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 Past Century?
Measurements indicate that global mean surface
temperature has increased by about 1°F (0 5°C)
in the past century The warming has been
greatest over the continents between 40 and 70
degrees North latitude Over this same period of
time global sea level has risen between 4 and 10
inches (10-25 cm) Scientists do not yet know
with certainty what part of these changes is,
caused by human activities and what part would
have .occurred anyway Part of this warming
may be a rebound from the cooling of the little
IceAge during the 1350-1850 period, which was
probably unrelated to human activities But this
warming also happened during the period when
human activities were increasing CO2 and other
greenhouse gas amounts in the atmosphere.
Many scientists are convinced that human ac-
tivities have made a major contribution to the
warming of the past century, and that warming
caused by greenhouse gas increases willjbe a
continuing part of our future,
A rapid greenhouse warming of the climate
Reports to the Nation • Fafl1997
image:
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 what part of it can be attributed
to human actions. The record of global tempera-
ture obtained from thermometers around the
world extends back in time only a little over a
century. This record shows a steady increase up
until 1940, followed by a period of slight cool-
ing. Since the 1970s the temperatures have
gone up rapidly, and many of the warmest years .
in the global temperature record have occurred
in the past 15 years. It is not known with
certainty whether this recent warming trend will
continue, or whether it is caused by the increas-
ing 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 riot yet clear how
to separate this variability from changes that
have resulted from human activities. , : ; • ,
Can We Predict Climate Change?
The behavior of the climate system can be
simulated with computer, models, [pi] and the
.simulations can then be tested'against observa-
tions of current "and past climate. They/.can be
used to .study the response of the climate to
changing :amounts of greenhouse gases and
MkMHVMHMUMV>'MMMtf*4l!lS
Instrumental Temperature Record 1865-1995
The record of global mean surface air temperature from thermometer readings
Indicates a global warming over the past century, with many peaks and .troughs
suggesting the natural year-to-year variability of climate. " .
Reports to the Nation • Fal11997
image:
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
shortcomings *
Modeling the climate on a computer is diffi-
cult because processes with very large spatial
scales, such as the transport of energy from the
tropics to the poles by atmospheric motions,
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 in a single model that is
efficient enough to run on available computers
in a reasonable length of time? The standard
approach js to represent the globe with a grid"
of boxes about 100 miles on a side and then
predict the average properties in these boxes
v
using known laws of physics The effects of
processes that occur pn^ 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 m a more approximate fashion are
also central to the feedback effects that deter-
mine how much climate change will result from
human actions. For example, clouds have a
" i
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 p] in climate models Important feed-
— - # '
backs such as those involving surface ice and
atmospheric water vapor also involve pro-
cesses occurring on small scales that must be
treated with approximate formulas As com-
puter power and'understandlng both increase,
some of the uncertainty associated with feed-
back processes will decline and more accurate
* * t. ^ *"
climate forecasts will become available
V
What Do Climate Models
fell Us About Our 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 climate^ A
typical experiment of this nature is to extend the
"~ tv ^ *
20th century's increase in greenhouse gases into
the next century and see how the climate model
responds to this change Because of the approxi-
* "' •>- ;
mations in the models, however, the projected
warming over the next century is quite uncertain,
jangmg 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,
t * * ~ O 7
-Reports to the- Nation • Fa/M997
image:
especially during summer in North America and
Southern Europe. Warming may cause agricul-
tural zones in North America to move north-'
ward, 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 fluctua-
tions cause sea surface temperature in the tropi-
cal North Atlantic to increase, hurricane .activity
also seems to increase, but it is not certain that
a global surface temperature rise caused by
greenhouse gas increases would have a, proper-;
tional 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 organizes itself to adapt to climate
change. If the warming is as fast and as large as
some of the models suggest, then the effects on ;
people and our natural environment could be
quite serious. Agriculture and.water supplies
can take decades to adapt, and natural ecosys,-
tems take, centuries. Therefore, a more rapid:
change would pose more difficult problems.
Where Do We Go from Here?
When planning for the future, we often assume-
thatthe climate we have experienced in the past
will continue,, but this may not be: the ease. 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 ^in-
creasing as a result of human activities. The
well-understood physics of the greenhouse ef-
fect indicates that the changing composition of
the- atmosphere 'should ^warrh the surface cli-
niate of Earth, Current estimates of the expected
Images of Earth
Cllnwlolojlcal values for the December through February
saason of (from left to right): sea surface temperature,
rainfall, albedo, and outgoing Infrared emission. Values
range from low to high as the color goes from blue to red to
whit*. The heavy precipitation over South America during
this season is accompanied by the high albedos and low
Infrared emission associated with tall convective clouds.
Reports to the Nation • Fall1997
image:
climate change over the next century 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 accu-
rate predictions of how temperature and the
.6
availability of water might change in a particular
y
state or county, where measures to adapt to
climate change would need to be taken
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, conduct-
ing intensive observational programs to study
the processes that determine how much cli-
mate change to expect, and continuing to
improve climate models and test them against
observations We also need to improve our
knowledge of the two-way relationship be-
tween humans and climate Because of the
long lifetime of greenhouse gases in the atmo-
sphere, decisions that are made during the next
decade or so could affect the quality of life for
generations to come
•* f f f
Given the current level of Uncertainty and the
complexity of the climate system, the future will
likely bring surprises, which could be of either
the pleasant or the unpleasant variety Informa-
tion about how the climate is changing, knowl-
edge of why observed changes occur, ^and
accurate prediction of future climates will be
very important for the public and policy mak-
Efficient communication of this mforma-
ers
tion to all concerned will be^an important part
of the process of deciding how to respond to
the challenge of our changing climate [g
Bibliography
Graecfel T E and P J Crutzen, 1995, Atmosphere
Climafe and Change WH Freeman 196, '
Hartmann D L 1994 Global Physical Climatology
Academic Press San Diego 411
Houghton J T etal 1996 Climate Change 1995 the
Science of Climate Change Cambridge University Press
Cambridge, 572
Imbne J andK P Imbrie 1979 IceAges Solving the
Mystery Enslow Publishers Short flills N J 224
Meltzer D J 1993 Search for the First Americans
SmithsoniahBooks Washington DC 176
Somerville R, C J , 1996 The Forgiving Air
Understanding Enviromental Change University of
California Press 216
Reports to t)ie Matiop - Faff1997
image:
Writers and Contributors
Writer. Dennis Hartmann, University of Washington
Contributors; Shawna Vogel, Science Writer
Lisa Farrow, National Oceanic and Atmospheric Administration
Our Changing Climate is the fourth in a series of publications on climate
and global change Intended for public education. The documents are a
collaborative effort of the UCAR Joint Office for Science Support and the
NOAA Office of Global Programs, for the purpose of raising the level of
public awareness of Issues dealing with global environmental change. The
reports are written by wen-recognized scientists and science writers on •,.
timely subiects and are guided by a scientific editorial board.
Editorial Board
Daniel L. Albtitton, National Oceanic and Atmospheric Administration
Francis Brelherton, University of Wisconsin
Michael Glantz, National Center for Atmospheric Research
J. Michael Hall, National Oceanic and Atmospheric Administration . ,
Mark Meier, University of Colorado.
Stephen Schneider, Stanford University -
John M. Wallace, University of Washington
Additional Manuscript Reviewers
Thomas Karl, National Oceanic and Atmospheric Administration
Jerry Melilfo, Office of Science and Technology Policy
Eric Miller, Falrview High School
Richard SomerviHe, Scripps Institution of Oceanography
Production Management and Distribution :
Paula Robinson, University Corporation for Atmospheric Research
Karyn Sawyer, University Corporation for Atmospheric Research
Design, Illustration and Production
Internetwork, Inc.. Del Mar, CA 92014. www.in-media.com
Payson R. Stevens, Leonard Slrota, Roger Parker, Eric Arterburn
Editing and Graphic Support .
Grace C. Gudmundson, Kay M. Dewar, Marc L. Michelsen,
University of Washington
Denise Cook-Clampert, Copy Editor
For additional copies contact Reports to the Nation, UCAR Joint Office for
Science Support, P.O. Box 3000. Boulder, CO 80307-3000 USA; phone
(303) 497-8666; fax (303) 497-8633; internet: rtn@joss.ucar.edu
Imago Credits
All computer graphic illustrations: InterNetwork, Inc.
Cover/Page 1: NASA/Space Shuttle; Pages 2-3: top row, left to right-
NASA Space Shuttle. U.S. Geological Survey, NOAA/NESDIS, EROS Data,
Center, middle row, Surface Temperatures, Jet Propulsion Laboratory,
©Payson R. Stevens,© Richard N. Carter, bottom row, © Payson R. Stevens,
U.S. Geological Survey, © Payson R. Stevens; Page 7: © Payson R. Stevens;
Page 8; Western History Collections, University of Oklahoma Libraries; Page 9:
NASA Space Shuttle; Page 10: top row, U. S, Geological Survey, bottom
© Bruce F. Molnia; Page 16: © Payson R. Stevens; Page 22: Marc L.
Michelsen, University of Washington; Page 24-IBC: © Payson R. Stevens;
9/97 610K
Reports to the Nation • Fall1997
image:
*-:" .-•'.'•.'• -t;'•',','.;.'.- .^ .'••; -...-"'.-.,'•.• v ".;. •••.,:n-,--^ .-.-. f-5 ,,
;*i
" •Bepo'rtsMoKtYe Natio-n • Fail'1997'
;;';V^'V'';'-:^>r,r^;r,'s-':.'.rr:''
image:
UCAR/JOSS
Office of
Global Programs
Printed on Recycled Paper.
image:
