FEATURE ARTICLE
The Source of Europe's Mild Climate
The notion that the Gulf Stream is responsible for keeping Europe anomalously warm turns out to be a myth
Richard Seager
If you grow up in England, as I did, a few items of unquestioned
wisdom are passed down to you from the preceding generation. Along
with stories of a plucky island race with a glorious past and the
benefits of drinking unbelievable quantities of milky tea, you will
be told that England is blessed with its pleasant climate courtesy
of the Gulf Stream, that huge current of warm water that flows
northeast across the Atlantic from its source in the Gulf of Mexico.
That the Gulf Stream is responsible for Europe's mild winters is
widely known and accepted, but, as I will show, it is nothing more
than the earth-science equivalent of an urban legend.
This is not to say that there is no climatological mystery to be
explained. The countries of northern Europe do indeed have curiously
mild climates, a phenomenon I didn't really appreciate until I moved
from Liverpool to New York. I arrived in the Big Apple just before a
late-summer heat wave, at a time when the temperature soared to
around 35 degrees Celsius. I had never endured such blistering
temperatures. And just a few months later I was awestruck by the
sensation of my nostrils freezing when I went outside. Nothing like
that happens in England, where the average January is 15 to 20
degrees warmer than what prevails at the same latitude in eastern
North America. So what keeps my former home so balmy in the winter?
And why do so many people credit the Gulf Stream?
Like many other myths, this one rests on a strand of truth. The Gulf
Stream carries with it considerable heat when it flows out from the
Gulf of Mexico and then north along the East Coast before departing
U.S. waters at Cape Hatteras and heading northeast toward Europe.
All along the way, it warms the overlying atmosphere. In the seas
between Norway and Newfoundland, the current has lost so much of its
heat, and the water has become so salty (through evaporation), that
it is dense enough to sink. The return flow occurs at the bottom of
the North Atlantic, also along the eastern flank of North America.
This overturning is frequently referred to as the North Atlantic
thermohaline circulation, or simply the "Atlantic
conveyor." It is part of the global pattern of ocean
circulation, which is driven by winds and the exchange of heat and
water vapor at the sea surface.
The Gulf Stream indeed contributes to Europe's warmth, but it is
wrong to conflate the climate difference across the North Atlantic
with the northward flow of warm water in the Gulf Stream. This
erroneous logic leads to such statements as (from The
Times of London): "The British Isles lie on the same
latitude as Labrador on the East Coast of Canada, and are protected
from a similarly icy climate by the Atlantic conveyor belt."
Such claims are absolutely wrong.
The statements scientists make about Atlantic thermohaline
circulation typically read more like this one from my Columbia
University colleague, Wallace S. Broecker:
One of the major elements of today's ocean system is a
conveyor-like circulation that delivers an enormous amount of
tropical heat to the northern Atlantic. During winter, this heat is
released to the overlying eastward air masses, thereby greatly
ameliorating winter temperatures in northern Europe.
This assertion has the benefit of being both correct and misleading.
Because it does not specify what European climate is ameliorated
relative to (the climate of eastern North America?), it leaves
unchallenged the incorrect version expounded in the popular
media—thus contributing to the erroneous beliefs of millions.
The idea that the Gulf Stream is responsible for Europe's mild
winters seems to have originated with Matthew Fontaine Maury, an
American naval officer who in 1855 published The Physical
Geography of the Sea, which is often considered the first
textbook of physical oceanography. The book was a huge success, went
through many printings and was translated into three languages. The
role of the Gulf Stream in shaping climate is a recurring theme in
Maury's book. For example, he stated:
One of the benign offices of the Gulf Stream is to convey
heat from the Gulf of Mexico, where otherwise it would be excessive,
and to disperse it in regions beyond the Atlantic for the
amelioration of the climates of the British Isles and of all Western
Europe.
According to Maury, if this transport of heat did not take place,
"… the soft climates of both France and England would be
as that of Labrador, severe in the extreme, and ice bound."
Despite the differences in language and style, the modern statements
clearly owe their provenance to this 1855 treatise.
Maury thought that God set the ocean up to work this way apparently
as part of His design to keep Europe warm (for unspecified reasons).
But holding such religious beliefs did not stop Maury from also
providing a scientific explanation for the Gulf Stream. His idea was
that it was the oceanic equivalent of what in the atmosphere is
known as a Hadley cell, a convection cell wherein warm air
flows upward and poleward, and cold material flows downward and
equatorward. In the ocean, heated surface waters take a
northeastward route, in Maury's view, because of the need to
conserve angular momentum as they move north and, hence, closer to
the axis of the Earth's rotation. Maury did not recognize that winds
drive ocean currents. And it was not until a century later that a
valid explanation of the Gulf Stream emerged: In the jargon of
oceanographers, it is a westward-intensified boundary current within
a subtropical gyre (a large circular current system) driven
by the trade winds, which blow from east to west in the tropics, and
mid-latitude westerlies, which move in the opposite direction.
Questioning the Myth
After completing my Ph.D. at Columbia University in New York City, I
took a temporary postdoctoral position at the University of
Washington in Seattle, where I should have immediately realized that
something was wrong with the Gulf Stream-European climate story.
Seattle and British Columbia, just to the north, I discovered, have
a winter climate with which I was very familiar—mild and damp,
quite unlike the very cold conditions that prevail on the Asian side
of the Pacific Ocean. This contrast exists despite the fact that the
circulation of currents in the Pacific Ocean is very different from
the situation in the Atlantic.
The analogue of the Gulf Stream in the Pacific Ocean is the
Kuroshio Current, which flows north along the coast of
Asia until it shoots off into the interior of the Pacific Ocean east
of Japan. From there, it heads due east (unlike the Gulf Stream,
which heads northeast) toward Oregon and California. As such, there
is almost no heat carried northward into the Pacific Ocean at the
latitudes of Washington and British Columbia. Hence oceanic heat
transport cannot be creating the vast difference in winter climate
between the Pacific Northwest and similar latitudes in eastern
Asia—say, chilly Vladivostok.
Strangely, experiencing a Seattle winter firsthand was not enough to
make me question the myth. However, in Seattle I did become good
friends with David S. Battisti, a professor of atmospheric sciences
at the University of Washington. Battisti is one of those great
scientists who, with relish and an air of mischief, loves to
question conventional wisdom. Over the years he and I have enjoyed
many a long evening indulging our shared passions for Italian
cooking and wine while talking about climate research. During one of
those conversations, sometime in 2000 as I recall, he brought up
that he wanted to test the Gulf Stream-European climate idea. It was
perfect timing, because just then I had been conducting a series of
experiments with a numerical climate model, ones designed to examine
the role the ocean plays in determining the global and regional
features of the Earth's climate. So Battisti and I went to work.
First we had to consider the range of possibilities. If oceanic heat
transport does not create the differences in regional climate across
the North Atlantic (or North Pacific), what does? An obvious
alternative explanation is that standard of high school geography
education: Because the heat capacity of water is so much greater
than that of rock or soil, the ocean warms more slowly in summer
than does land. For the same reason, it cools more slowly in winter.
That effect alone means that the seasonal cycle of sea-surface
temperature is considerably less than that of land surfaces at the
same latitude, which is why summers near the sea are cooler and
winters are warmer than at equivalent sites located inland.
The effect of differing heat capacities is augmented by the fact
that the Sun's heat is stored within a larger mass in the ocean than
on land. The heat reservoir is bigger because, as the Sun's rays are
absorbed in the upper several meters of the ocean, the wind mixes
that water downward so that, in the end, solar energy heats several
tens of meters of water. On land, the absorbed heat of the Sun can
only diffuse downward and does not reach deeper than a meter or two
during a season. The greater density of soil and rock (which ranges
up to three times that of water) cannot make up for this difference
in volume of material that the Sun heats and for the difference in
heat capacity of water compared with soil or rock.
Because sea-surface temperatures vary less through the seasonal
cycle than do land-surface temperatures, any place where the wind
blows from off the ocean will have relatively mild winters and cool
summers. Both the British Isles and the Pacific Northwest enjoy such
"maritime" climates. Central Asia, the northern Great
Plains and Canadian Prairies are classic examples of
"continental" climates, which do not benefit from this
moderating effect and thus experience bitterly cold winters and
blazingly hot summers. The northeastern United States and eastern
Canada fall somewhere in between. But because they are under the
influence of prevailing winds that blow from west to east, their
climate is considerably more continental than maritime.
A Model of Contrasts
Battisti and I naturally wondered whether we could explain the
difference in winter conditions between Europe and eastern North
America as simply the difference between a maritime climate and a
more continental one. To find the answer, he and I used two climate
models, ones that normally serve for studies of natural climate
variability or for assessments of future climate change. As in all
such models, Earth's atmosphere is represented on a
three-dimensional grid (latitude, longitude and pressure level in
the vertical). For each grid point, the computer solves the relevant
equations for the winds, temperature, specific humidity, fluxes of
solar and terrestrial radiation and so forth while keeping track of
the precipitation and energy fluxes at Earth's surface. The packing
of the grid points was sufficiently dense so that we could
accurately capture the endless progression of storm systems, which
transport vast quantities of heat and moisture poleward. As with the
computer models used to forecast the weather (which are basically
the same as climate models), the computer code we used calculated
conditions forward in time until, for these experiments, a
statistical steady state was achieved. To get a representative
picture of overall climate, we averaged together many years of
simulated weather.
The joy of such numerical models is that you can make radical
changes to a virtual Earth's climate system with nothing more than a
click of the mouse. To assess the importance of the heat transported
by ocean currents such as the Gulf Stream, we compared the results
of two versions of these climate models. The first versions were the
standard ones, which compute sea-surface temperature after
accounting for the heat moved by ocean currents, the absorption of
the Sun's rays, and the exchange of heat between the ocean and the
atmosphere. In the second versions, the computer code accounted for
solar warming and the relevant surface heat exchanges but did not
allow the model ocean to transport heat horizontally.
What we found in these tests was that, south of northern Norway, the
difference in winter temperature across the North Atlantic was
always the same, whether or not we let the ocean move heat around.
This result would suggest that oceanic heat transport does not
matter at all to the difference between the winter climates of
western Europe and eastern North America! We concluded that the
temperature difference must, as we had speculated before, be caused
by other processes, most likely the seasonal absorption and release
of heat by the ocean and the moderating effect this process has on
maritime climates downwind.
Our revised view of things did not, however, mean that heat
transport in the ocean does not influence climate. The ocean indeed
absorbs more heat from the Sun near the equator than it loses back
to the atmosphere (primarily by evaporation). And oceanic currents
indeed move the excess heat poleward before releasing it to the
atmosphere in the middle latitudes. Consequently, removal of the
oceanic heat transport globally in our modeling exercise warmed the
equator and cooled everywhere else. The climates produced by the
models deprived of oceanic heat transport were colder in the
subpolar North Atlantic by as much as 8 degrees in some places. The
cooling over land areas was more modest, typically less than 3
degrees. These temperature changes, large as they are, are not
terribly dramatic compared with the much larger temperature contrast
across the North Atlantic Ocean.
Why doesn't the ocean exert a greater influence on North Atlantic
climate? According to scientists' best estimates, the ocean and
atmosphere move about an equal amount of heat in the deep tropics.
But at mid-latitudes, the atmosphere carries several times more
heat. Thus, if one considers the region north of, say, 35 degrees
North, the atmosphere is much more effective than the ocean in
warming winter climates. Also, the winter release of the heat
absorbed during the summer is several times greater than the amount
of heat that the ocean transports from low to high latitudes in a
year. Hence it is the combined effect of atmospheric heat transport
and seasonal heat storage and release that keep the winters outside
the tropics warmer than they otherwise would be—by several
tens of degrees.
Although these numbers are instructive, they are not directly
relevant to understanding the warming of Europe. For that, one needs
to consider some details of geography. The Gulf Stream and
associated current systems in the North Atlantic focus heat (and
lose it to the atmosphere) in two clearly defined areas. One is
immediately to the east of the United States, where the warm Gulf
Stream flows north after leaving the Gulf of Mexico and rounding the
tip of Florida. During winter, the prevailing winds blow frigid, dry
air off the North American continent and across the Gulf Stream.
Because of the large difference in moisture and temperature content
between air and sea, the heat lost from the ocean through
evaporation and direct heat transfer is immense—a few hundred
watts per square meter. Much of this heat is picked up by storms in
the atmosphere and carried over the eastern United States and
Canada, effectively mitigating what would otherwise be a cold
continental climate.
Where else does the Gulf Stream deposit its heat? After departing
the American coast, the Gulf Stream heads northeast and turns into
what is called the North Atlantic Drift and, farther downstream, the
Norwegian Current. After spawning many Atlantic storms, it loses
most of the remainder of its heat in the Nordic seas. There the heat
can effectively be moved eastward by the prevailing winds to warm
northwest Europe. Thus the transport of heat taking place in the
North Atlantic warms both sides of the ocean and
by roughly the same amount, a few degrees. This leaves the much
larger, 15-to-20-degree difference in winter temperatures to be
explained by other processes.
One subtle but important effect stems from a fundamental principle
in physics: the conservation of angular momentum. In meteorology,
this principle translates to a rule that atmospheric flow must
closely conserve the total angular momentum of a column of air. The
angular momentum of the air contains two components: one arising
from the rotation of the Earth (which meteorologists call the
"planetary component") and another from the curvature of
the fluid flow itself. The planetary component, which in the
Northern Hemisphere is directed counterclockwise, is at a maximum at
the pole and zero at the equator.
The conservation of angular momentum, it turns out, causes the
mountains of North America to contribute substantially to the
dramatic difference in temperatures across the Atlantic. To fathom
why, you must first understand that the troposphere (the
lower part of the atmosphere, where weather takes place) is bounded
at the top by the tropopause, a region of stability where
temperature increases with height and which acts somewhat like a
lid. Thus when air flows over a mountain range—say, the
Rockies—it gets compressed vertically and, as a consequence,
tends to spread out horizontally. When a spinning ice skater does as
much, by spreading his arms, the conservation of angular momentum
slows his spin. An atmospheric column going up a mountain
behaves in a similar way and swerves to the south to gain some
clockwise spin, which offsets part of the counterclockwise planetary
component of its spin.
On the far side of the Rockies, the reverse happens: The air begins
to stretch vertically and contract horizontally, becoming most
contracted in the horizontal when it reaches the Atlantic. And as
with an ice skater pulling in his arms, conservation of angular
momentum demands that the air gain counterclockwise spin. It does so
by swerving to its left. But having moved to the south after
crossing the mountains, it is now at a latitude where the planetary
component of its angular moment is less than it was originally. To
balance this reduction in angular momentum, the air acquires more
counterclockwise spin by curving back around to the north. This
first southward and then northward deflection creates a waviness in
the generally west-to-east flow of air across North America and far
downwind to the east.
Such waves are of massive scale. The southward flow takes place over
all of central and eastern North America, bringing Arctic air south
and dramatically cooling winters on the East Coast. The return
northward flow occurs over the eastern Atlantic Ocean and western
Europe, bringing mild subtropical air north and pleasantly warming
winters on the far side of ocean.
Topographically forced atmospheric waves contribute significantly to
the large difference in winter temperature across the Atlantic. When
Battisti and I removed mountains from our climate models, the
temperature difference was cut in half. Our conclusion was that the
large difference in winter temperature between western Europe and
eastern North America was caused about equally by the contrast
between the maritime climate on one side and the continental climate
on the other, and by the large-scale waviness set up by air flow
over the Rocky Mountains.
A Sea Change in Climate?
Evidence from ocean sediments suggests that at times during the last
Ice Age the North Atlantic thermohaline circulation was considerably
weaker than it is today, or perhaps it even shut down entirely. One
such event took place about 12,900 years ago, during the last
deglaciation, and is called the Younger Dryas (after a European
cold-dwelling flower that marks it in some terrestrial records). The
Younger Dryas began with a dramatic reversal in what was a general
warming trend, bringing near-glacial cold to the North Atlantic
region. This episode ended with an even more dramatic warming about
1,000 years later. In Greenland and western Europe, the beginning
and end of the Younger Dryas involved changes in winter temperature
as large as 20 degrees taking place in little more than a decade.
But the Younger Dryas was not a purely North Atlantic phenomenon:
Manifestations of it also appeared in the tropical and southern
Atlantic, in South America and in Asia.
For many years, the leading theory for what caused the Younger Dryas
was a release of water from glacial Lake Agassiz, a huge, ice-dammed
lake that was once situated near Lake Superior. This sudden outwash
of glacial meltwater flooded into the North Atlantic, it was said,
lowering the salinity and density of surface waters enough to
prevent them from sinking, thus switching off the conveyor. The
North Atlantic Drift then ceased flowing north, and, consequently,
the northward transport of heat in the ocean diminished. The North
Atlantic region was then plunged back into near-glacial conditions.
Or so the prevailing reasoning went.
Recently, however, evidence has emerged that the Younger Dryas began
long before the breach that allowed freshwater to flood the North
Atlantic. What is more, the temperature changes induced by a
shutdown in the conveyor are too small to explain what went on
during the Younger Dryas. Some climatologists appeal to a
large expansion in sea ice to explain the severe winter
cooling. I agree that something of this sort probably
happened, but it's not at all clear to me how stopping the Atlantic
conveyor could cause a sufficient redistribution of heat to bring on
this vast a change.
In any event, the still-tentative connections investigators have
made between thermohaline circulation and abrupt climate change
during glacial times have combined with the popular perception that
it is the Gulf Stream that keeps European climate mild to create a
doomsday scenario: Global warming might shut down the Gulf Stream,
which could "plunge western Europe into a mini ice age,"
making winters "as harsh as those in Newfoundland," or so
claims, for example, a recent article in New Scientist.
This general idea been rehashed in hundreds of sensational news stories.
The germ of truth on which such hype is based is that most
atmosphere-ocean models show a slowdown of thermohaline circulation
in simulations of the 21st century with the expected rise in
greenhouse gases. The conveyer slows because the surface waters of
the subpolar North Atlantic warm and because the increased transport
of water vapor from the subtropics to the subpolar regions (where it
falls as rain and snow) freshens the subpolar North Atlantic and
reduces the density of surface waters, which makes it harder for
them to sink. These processes could be augmented by the melting of
freshwater reserves (glaciers, permafrost and sea ice) around the
North Atlantic and Arctic.
But from what specialists have long known, I would expect that any
slowdown in thermohaline circulation would have a noticeable but not
catastrophic effect on climate. The temperature difference between
Europe and Labrador should remain. Temperatures will not drop to
ice-age levels, not even to the levels of the Little Ice Age, the
relatively cold period that Europe suffered a few centuries ago. The
North Atlantic will not freeze over, and English Channel ferries
will not have to plow their way through sea ice. A slowdown in
thermohaline circulation should bring on a cooling tendency of at
most a few degrees across the North Atlantic—one that would
most likely be overwhelmed by the warming caused by rising
concentrations of greenhouse gases. This moderating influence is
indeed what the climate models show for the 21st century and what
has been stated in reports of the Intergovernmental Panel on Climate
Change. Instead of creating catastrophe in the North Atlantic
region, a slowdown in thermohaline circulation would serve to
mitigate the expected anthropogenic warming!
The Longevity of a Legend
When Battisti and I had finished our study of the influence of the
Gulf Stream, we were left with a certain sense of deflation: Pretty
much everything we had found could have been concluded on the basis
of results that were already available. Ngar-Cheung Lau of the
National Atmospheric and Oceanic Administration's Geophysical Fluid
Dynamics Laboratory (GFDL) and Princeton University had published in
1979 an observational study in which he quantitatively demonstrated
the warming and cooling effects that large-scale waves in the
atmosphere had in Europe and eastern North America, respectively. In
the 1980s, atmosphere modelers such as Brian J. Hoskins and Paul J.
Valdes at the University of Reading in England and Isaac M. Held and
Sumant Nigam at GFDL had shown how such stationary waves, including
those forced by mountains, warm western Europe. In the late 1980s,
two other GFDL researchers, Syukuro Manabe and Ronald J. Stouffer,
had used a coupled ocean-atmosphere climate model to determine the
climate impacts of an imposed shutdown of the North Atlantic
thermohaline circulation. Their modeled climate cooled by a few
degrees on both sides of the Atlantic and left the much larger
difference in temperature across the ocean unchanged. Other
published model experiments went on to show the same thing. Further,
the distinction between maritime and continental climates had been a
standard of climatology for decades, even centuries. What is more,
by the late 1990s satellite data, and analyses of numerical models
into which those data had been assimilated as part of the
weather-forecasting process, had shown that in mid-latitudes the
poleward transport of heat by the atmosphere exceeds that by the
ocean several-fold.
All Battisti and I did was put these pieces of evidence together and
add in a few more illustrative numerical experiments. Why hadn't
anyone done that before? Why had these collective studies not
already led to the demise of claims in the media and scientific
papers alike that the Gulf Stream keeps Europe's climate just this
side of glaciation? It seems this particular myth has grown to such
a massive size that it exerts a great deal of pull on the minds of
otherwise discerning people.
This is not just an academic issue. The play that the doomsday
scenario has gotten in the media—even from seemingly reputable
outlets such as the British Broadcasting Corporation—could be
dismissed as attention-
grabbing sensationalism. But at root,
it is the ignorance of how regional climates are determined that
allows this misinformation to gain such traction. Maury should not
be faulted; he could hardly have known better. The blame lies with
modern-day climate scientists who either continue to promulgate the
Gulf Stream-climate myth or who decline to clarify the relative
roles of atmosphere and ocean in determining European climate. This
abdication of responsibility leaves decades of folk wisdom
unchallenged, still dominating the front pages, airwaves and
Internet, ensuring that a well-worn piece of climatological nonsense
will be passed down to yet another generation.