ice of mountain glaciers, polar
regions, and the Arctic and Antarctic Seas.
The questions now are at what rate will
these changes occur and how will they be
regionally manifested?
During the summer of 2000, the authors
traveled, as part of a team of lecturers,
aboard a Russian nuclear icebreaker to the
North Pole. The lecturers were well
informed regarding recent scientific papers
that discussed climate change in the Arctic
but had no expectations that any of this
would be visible to casual observers like
ourselves. However, a paucity of ice greet-
ed us along our journeythere was open
water at the North Poleand upon the
ships return, these observations quickly
made their way into a global web of print
and electronic media.
This article begins by reviewing current
understandings of anthropogenic climate
change. Within this context, it describes
the special sensitivities of polar regions to
climate change and reviews recent evi-
dence for such change. We then discuss
our own experiences in the Arctic this past
summer and the public reaction to them.
How
Earths
Ice Is
Changing
W Our Changing Climate
E a rt h s history is of cl i m ate va ri ation or
cl i m ate ch a n ge on time scales ra n gi n g
f rom seasons to multiple millennia. How-
eve r, d i rect observations of cl i m ate are a
c o m p a rat ive ly recent phenomenon, a n d
the written re c o rd of cl i m ate observe rs is
o n ly a few centuries old. Further back in
t i m e, the diary of cl i m ate ch a n ge ex i s t s
p ri m a ri ly in the fo rm of proxies encoded
in lake and ocean sediments, ro ck s , fo s-
sils of plants and animals, t ree ri n g s , a n d
ice cores. Ranges of distri bution of ter-
re s t rial and marine orga n i s m s , e s t i m at e s
of their growth rates (as with tree ri n g s ) ,
and chemical analy s e s , s u ch as isotopic
ratios in fossils and ice, all indicat e
cl i m ate stat e.
It is widely agreed among climate scien-
tists that humans have the potential to
influence Earths climate at local and
global scales, and there is strong evidence
that this has already occurred. The true
issues of debate among scientists knowl-
edgeable on this topic concern the rate at
which anthropogenically influenced cli-
mate change is occurring, how it is now
being or will be regionally manifested, and
whether this change will have predomi-
nantly positive or negative effects on nat-
ural and human systems. The strongest
global component of human alteration of
the climate system is the alteration of
by James J. McCarthy
and Malcolm C. McKenna
The Yamal in a typical stretch
of intermittent and thin ice in
summer 2000, within a few
hundred miles of the North
Pole. Note the thin edges of the
ice and the absence of pressure
ridges. 10
E
NVIRONMENT
D
ECEMBER
2000
at m o s p h e ric concentrations of gre e n-
house gasesthe most notable of which
is carbon dioxide (CO
2
)arising from
fossil fuel combustion.
1
To d ay, no earth scientist disputes
w i d e s p read evidence that at times in the
p a s t , mu ch of Eart h s surface (perhaps all
of it) has been cove red by ice. The last
glacial event re a ched its maximum ex t e n t
about 18,000 ye a rs ago. At that time,
C a n a d a , the nort h e a s t e rn United Stat e s ,
S c a n d i n av i a , and nort h e rn Eurasia we re
c ove red with a glacial mass more than a
mile thick. When this concept of Eart h s
glacial history was fi rst introduced in the
mid-1800s by Louis A ga s s i z , a Swiss
n at u ra l i s t , the scientific community of
his day found this notion too radical to
a c c ept. Many of his prominent scientifi c
c o n t e m p o ra ries in Europe we re accus-
tomed to altern at ive ex p l a n ations for the
ge o l ogical fe at u res A gassiz used as ev i-
dence for his glacial theory. Notabl e
among these competing theories was the
w i d e ly perc e ived influence of the gre at
b i blical flood in shaping the fe at u res of
their familiar landscap e.
People now know that the condition
deduced by A gassiz was one phase of a
cy cle that has rep e ated about eve ry
150,000 ye a rs for the past million ye a rs .
Th e re are many complex and some poor-
ly understood positive and negat ive phy s-
i c a l , b i o l ogi c a l , and bioge o ch e m i c a l
fe e d b a ck s
2
in Eart h s cl i m ate system that
give rise to the uneven progre s s i o n
t owa rd a peak glacial condition fo l l owe d
by a more rapid re t re at to an interg l a c i a l
p e riod like that of the last few millennia.
3
Ice core data from A n t a rctica and
Greenland help to reveal relationships
between physical controls of climate and
the effects of atmospheric abundance of
greenhouse gases on climate. Some of
these analyses indicate a sensitive and
p rominent role for biologi c a l / b i oge o-
chemical processes that respond to
changes in the distribution of solar ener-
gy on Eart h s surfa c e, and in turn
enhance climate conditions conducive
for growth of glacial ice volume.
4
Without contemporary human influ-
ence on the climate system, geologic
history would suggest a future cooling
for Eart h s cl i m ate with an eve n t u a l
return to a glacial condition. However,
late in the 1800s, not long after Agas-
sizs ideas about ice ages became well
accepted, Svante Arrhenius, a chemist
f rom Swe d e n , b egan thinking ab o u t
h u m a n k i n d s increasing use of fo s s i l
fuels and calculated that this could have
a global effect on climate. It is a great
curiosity that his calculations and those
of others ear ly in the 1900s gave results
for global temperature increases that are
not wildly different from the estimations
derived from todays sophisticated sce-
narios for future climate.
5
As we contemplate the habitat of our
ancestors during the last ice age, it is
worth noting that the relatively warm
and stable climate of the last few millen-
nia has been unusually conducive to the
development of temperate-region agri-
culture and the increase in human popu-
lation that followed. The consequences
of the two unusual climate periods in the
last millennia are illustrat ive. Th e
M e d i eval Wa rm Pe riod (about 1000
A.D.) allowed agriculture to flourish on
the southern tip of Greenland. Converse-
ly, the Little Ice Age was an unusually
cold period , primarily in Europe, in the
late 1500s and early 1600s. The warm
period in southern Greenland seems to
h ave been a local phenomenon, a n d
average temperatures in the Northern
Hemisphere during this time were in fact
cooler than those for the 20th century.
6
The Little Ice Age condition was more
widespread, with cool conditions spread
u n eve n ly across the Nort h e rn Hemi-
sphere (Greenland, for example, was rel-
atively warm), which finally dissipated
in the late 1700s or early 1800s. Average
annual temperatures during the Little Ice
Age were about 1 degree C colder than
the Northern Hemisphere average for the
20th century. Records indicate that the
period of the Little Ice Age was one of
low solar activity.
7
An unusual condition
The Yamal leading a diesel icebreaker, the Dranitsyn, to the North Pole in 1994. This is
typical of the multiyear ice encountered for several hundred miles around the pole. Note the
block of 23 meter-thick ice being tipped up beside the ship and the crack being formed
ahead of the ship. V
OLUME
42 N
UMBER
10
E
NVIRONMENT
11
in Nort h e rn Hemisphere at m o s p h e ri c
circulation may have played a role in the
uneven distribution of cooling during
this time.
D u ring the last few ye a rs , v i rt u a l ly
eve ry month has brought a new scientifi c
p aper that adds weight to the scientifi c
consensus that globally ave raged cl i m at e
conditions are consistent with scenari o s
for a wa rming of Earth caused by
i n c reasing at m o s p h e ric concentrations of
greenhouse gases. The 20th century is
the wa rmest century of the millennium.
The 1990s we re the wa rmest decade in
the century, and the 1980s we re the sec-
ond wa rmest. Ten of the 15 wa rm e s t
ye a rs in the instrumental temperat u re
re c o rd occurred in the 1990s, and 1998
was the wa rmest year on re c o rd.
8
Ice and Snow
At this time, the largest masses of
glacial ice on Earth are on Antarctica
and Greenland (85 percent and 10 per-
cent of Earths freshwater, respectively).
These masses rise to elevations of more
than 3 kilometers. The oldest ice on
Antarctica is at least half a million years
old. As the massive ice cr own in central
A n t a rctica slow ly streams down and
away from the center of the continent, it
flows around mountains and re-fuses
into massive flat ice sheets that extend
into the sea. Two of these cover substan-
tial portions of the large Antarctic conti-
nent embayments known as the Ross and
Weddell Seas. When the perimeter of
these and many lesser so-called ice
shelves calve, they release massive tabu-
lar icebergs into the adjacent sea, some
of which can be as large as a small U.S.
state. Glaciers flowing down the steeper
p e rimeter of Greenland calve small
rugged icebergs. The proximity of ship-
ping lanes to Greenlands southern coast
is the reason these icebergs are a partic-
ular hazard to navigation, appreciated in