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Miroslav Gačić
The Oceans, the Atmosphere
and Climate Change:
What should we know about
EUR-OCEANS
Network of Excellence
The Oceans, the Atmosphere
and Climate Change:
What should we know about?
Miroslav Gačić
National Institute of Oceanography and Experimental Geophysics (OGS)
Trieste, Italy
EUR-OCEANS
Network of Excellence
EURopean network of excellence for OCean Ecosystems ANalysis (contract 511368)
Sixth EU Framework Programme for Research and Technological Development.
1
Contents
Heat in the oceans and atmosphere
3
Water exchange between the oceans and the atmosphere
6
Carbon dioxide and the greenhouse effect
7
8
8
9
Natural mechanisms of CO2 capture
Factors increasing the greenhouse effect
Carbon reservoirs
2
How do marine plants “pump” the carbon?
10
How do the oceans influence climate?
11
How do climatic variations can affect the oceans?
12
Can we already see the consequences of climate
change in the oceans and in the atmosphere?
13
What will happen in the Mediterranean?
17
What can we expect in the near future?
19
What can we do to make a difference to climate
change?
21
Acknowledgements
24
Increasingly we are hearing, reading or discussing “climate” and “weather”,
and quite often the two terms are confused. What is the exact meaning of the
two terms? The weather describes whatever is happening outdoors in a given
place and time. It thus represents the instantaneous state of the atmosphere
characterized in terms of air temperature, humidity, wind, cloud cover and rainfall
with quantities mostly measured by meteorological instruments. We say that the
weather is windy, rainy, sunny, warm, cold etc. Climate represents average weather
conditions, regular weather sequences (like winter, spring, summer, and fall), and
extreme weather events (like tornadoes and floods). Thus, on radio or TV we hear
day-to-day weather forecast and not the climate prediction. In order to determine
the climate of a region we have to average weather observations typically for 30
years, but other periods may be used as well. The difference between weather
and climate is best described by saying: “Climate is what you expect, weather is
what you get”1. Climate has been considered to be changing slowly until recently
when scientists realised that changes are occuring faster than previously thought.
We have to keep in mind that instrumental meteorological measurements have
been carried out only in the last 200 years and reconstruction of climate beyond
that period has to be done using indirect information such as tree rings, ice cores
and sediments.
Heat in the oceans and atmosphere
The oceans and atmosphere are two fluids in close contact. The presence
of these fluids is very important for the Earth’s climate as they move and transport
heat and fresh-water. Without these fluids the Earth would have a very different
climate. Seawater is about 800 times denser than air and thus they do not mix at
all. Air is also in contact with the land, and the presence of land or ocean below
the atmosphere determines both the climate and the weather.
The oceans are made of water. Hence, they represent an enormous heat
reservoir that stores thousand times more heat than the air, i.e. in the same volume of water we can store a thousand times more heat than in the air. Water
also gains and releases heat very slowly. That’s why sea temperature differences
between summer and winter are not as large as those in the atmosphere. It heats
the atmosphere during the winter and cools it during the summer thanks to the air
- seawater heat exchanges. Heat is a form of energy which is in this case emitted
3
by the Sun (solar radiation is the term used for its energy). Despite the Earth being
very far from the sun (about 150 million kilometers) and only be able to intercept
a small fraction of its radiation, it represents the main source of energy for our
planet and climate. Only about half of the total energy coming from the sun that
reaches the Earth is stored in the oceans; the rest is immediately reflected back
to space or stored in the atmosphere (this heat storage is three times less than in
the ocean). Since, the Earth’s climate does not change much on average, we can
consider that our system is in an energy balance. The heating of the oceans should
thus be equal to the heat lost, otherwise we would have a continuous rise or fall
in seawater temperature. The most important heat loss process from the oceans
(more than a half of the total heat loss) is due to evaporation. Evaporation is the
process by which water absorbs heat and passes from liquid to vapor state, i.e. water is transferred from the ocean to the atmosphere. Water uses a large amount of
heat to evaporate, causing the surrounding area to cool, as a result seawater cools
down through evaporation. The process of evaporation produces vapour made of
pure water leaving behind the salt. This salt remains in the seawater leading to a
localised increase in salinity. The rate of evaporation depends on the humidity of
the air, the wind speed and the temperature of the seawater.
Although if we consider the entire Earth, the heat gained by solar radiation
and the heat lost are balanced, in some areas these two processes are not necessarily in net equilibrium. In polar regions the loss is much greater than the gain,
while near the equator the heating prevails over the loss. Why don’t we then have
a continuous heating and temperature increase in equatorial areas and continuous
cooling around the poles? This is because ocean currents flow and winds blow
poleward, so heat is carried from the equator toward the poles while the cold
water and air from poles move towards the equator keeping the temperature differences between the poles and the equator constant. In fact, cooling at the poles
and heating at the equator generates the ocean currents and winds that then
exchange heat.
In polar areas the sea surface is cooled and the water partly freezes to form
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sea ice. Similar to the evaporation process of seawater, during sea ice formation salt
is extracted leading to ice made of pure water and to a localised increase in salinity.
Due to the strong cooling and the salinity increase, surface waters become heavier
than the waters below which makes them sink into the depths. This sinking results
in a surface water deficit that is compensated by water coming from warmer areas.
In this way warm surface waters are forced to flow poleward, while the cold waters flow toward the equator in the bottom layers of the ocean. This circulation is
called thermohaline circulation since it is due to temperature (thermo) and salinity
(haline) differences. However, the wind contributes significantly in generating a
surface water flow from equator toward polar areas. Thus, the general circulation
of the oceans is due to both winds and salinity-temperature differences and is
called Meridional Overturning Circulation. This ocean motion contributes to keeping the difference in temperature between polar and equatorial areas constant.
In the Atlantic ocean an important component of this circulation cell is the Gulf
Stream, one of the strongest ocean current in the world, which flows northward
and brings heat to polar areas. The equivalent current in the Pacific is the Kuroshio.
In the southern hemisphere there is a similar situation where, for example, the
Eastern Australian Current carries heat from the equator towards the south pole
(remember the cartoon “Finding Nemo” where the turtles were using this current
to travel southward, there was some artistic license in the film however as these
strong currents are not really like rivers as shown in the movie).
5
Similarly in the atmosphere, north-south circulation is established so that
the heat transport from equatorial to polar regions is really the sum of the atmosphere and ocean heat transport. The ocean carries poleward more heat than the
atmosphere only in tropical regions. At the rest of the Earth’s surface the atmosphere is more significant in the poleward heat transportation than the ocean.
» This image from the Gulf
Stream depicts the complex interaction of the sea
with the atmosphere. The
false colours in the image
represent heat radiation
from a combination of the
sea surface and overlying
moist atmosphere. The red
pixels show the warmer
areas, greens are intermediate values, and blues are
relatively low values. Heat
is being released into the
overlying atmosphere
from the ocean, raising
the humidity. Notice that
the Gulf Stream is a rather
irregular flow with a series
of meanders. The image
was produced from data
collected on May 2, 2001
and processed by the
University of WisconsinMadison’s MODIS direct
broadcast receiving station.
The MODIS (the Moderating resolution Imaging
Spectroradiometer) sensor
flies aboard NASA’s Terra
spacecraft, launched in
December 1999.
Credit: courtesy Liam
Gumley, MODIS Atmosphere
Team, University of Wisconsin-Madison Cooperative
Institute for Meteorological
Satellite Studies.
6
Water exchange between the oceans and the
atmosphere
The oceans and the atmosphere exchange not only heat but also water, in
the form of precipitation moving from the atmosphere to the ocean. The oceans
contain 97.5% of the Earth’s water, freshwater on land (rivers, lakes etc) represents
2.4%, and the atmosphere holds less than 0.001%.
The oceans lose water by evaporation, releasing vapor. Water continuously
evaporates from the oceans and the land, to the atmosphere at an average rate
of about 3 mm per day or one meter of water per year, if uniformly distributed
over the whole Earth. Obviously, as we can see, sea level does not drop due to
evaporation and this is because it is balanced by rainfall and river runoff. Thus, on
average the ocean loses as much water through evaporation as it gains. Evaporation mainly balances precipitation. This does however, vary from one area to another. At latitudes where land deserts occur the water loss is much higher than the
gain, while in subtropical areas rainfall prevails over evaporation. The influence of
freshwater input from rivers is small, constituting 1/10 of the rainfall and is mainly
concentrated in relatively small regions of the world’s oceans, usually in the coastal
zone. Precipitation plays an important role since it brings more than 30 times the
total atmospheric water content2 to Earth every year. This indicates that water rapidly circulates between the Earth’s surface, the oceans and the atmosphere . This
causes resultant effects on local salinity (and therefore density) of seawater and
large air-sea exchanges of heat and fresh-water.
Carbon dioxide and the greenhouse effect
» Satellite images of the
Northern Adriatic for 22nd
October (left) and 6th
December 2002 (right). In
the second image, the one
from the 6th December,
signs of spreading of highly
turbid riverine waters from
a series of north Italian rivers are visible close to the
coast showing that freshwater coming from rivers
remains confined in coastal
areas. On the other hand,
the image from the 22nd
October does not reveal
the presence of the coastal
turbid water since the
period preceding the date
the image was taken, was
relatively dry. White areas
are the snow and you can
see snow cover increase
over the Alps between the
end of October and the
first half of December.
(http://visibleearth.nasa.gov).
The air we breathe is mainly made of oxygen and nitrogen which represent
almost 99% of the mixture of gases that make the atmosphere. One of the gases
exchanged between the atmosphere and the ocean is carbon dioxide (CO2). We’ll
be talking more about it since despite being present in the atmosphere at low
concentrations (only 0.04%), CO2 is one of the most important greenhouse gases
and it is also very important to both land and marine plants growth. These organisms extract carbon dioxide (inorganic carbon) and transform it into organic carbon though a process discussed in more detail later on.
What is the greenhouse effect? Sunlight that travels through space and
reaches the Earth is termed solar radiation. Radiation is the emission and transmission of energy through space or through a material medium. The wavelength of
the emitted radiation from a body depends on the temperature of that body. The
sun has an extremely high temperature (more than 6 000°C). It emits visible-light
and ultraviolet rays which are electromagnetic rays of a short wavelength. When
the rays reach the Earth’s surface (land or ocean) they are absorbed and/or radi-
7
ated back to the atmosphere. As the Earth is much colder than the sun, this backradiation is characterized by much longer wavelengths, essentially infrared rays,
the same kind that are released by domestic heaters. The atmosphere acts as a
transparent medium for short wavelength rays, such as visible-light (and therefore
we can “see” during the day-light). This is not true for longer wavelengths such as
infrared ones, and so part of this backradiated energy is absorbed by greenhouse
gases. Again, when a body absorbs energy, it has to radiate it back. Hence, greenhouse gases radiate this absorbed infrared energy back to the Earth increasing the
amount of energy received by the planet’s surface and therefore increasing the
overall heating of the planet.
Apart from carbon dioxide, other greenhouse gases include water vapor,
methane, ozone, nitrous oxide and some others that contribute less than 1% to
the atmosphere composition. All these gases occur naturally in the atmosphere
and they keep the Earth warmer, as without them all the radiated heat would
escape into space. In fact, without greenhouse gases the average Earth surface
temperature would be -18°C and not +15°C as it actually is. In the absence of the
greenhouse effect, the Earth would not be warm enough for humans and most
other living creatures. However, if the greenhouse effect intensifies, it could make
the Earth warmer than usual.
Natural mechanisms of CO2 capture
Carbon dioxide is soluble in seawater and as all gases is more soluble in
colder environments; the lower the temperature the stronger the solubility. Thus,
polar areas are very efficient at absorbing atmospheric carbon dioxide while tropi8
cal and equatorial waters tend to release the CO2 back to the atmosphere (outgassing). In polar areas where cooling leads to the sinking of surface water there is a
continuous vertical transfer of dissolved CO2, and carbon in general, to deeper layers. In addition, the surface CO2 is captured by marine plants for their growth and
used for the production of organic matter. This organic matter will subsequently
sink to deeper layers as plants die or are eaten by animals. There are thus two processes that “pump” the carbon from the surface to deeper layers decreasing the CO2
concentration in the surface layer. The transfer of carbon by marine plants is called
the biological pump while the physical pump is the transfer of carbon from surface
layers through the sinking of cooled water. The functioning of the biological pump
will be described in the next chapter.
Factors increasing the greenhouse effect
The greenhouse effect is a natural phenomenon since the greenhouse gases
occur naturally in the Earth’s atmosphere. However, since the industrial revolution
mankind has amplified this effect through increased use of fossil fuels, like coal
and oil, responsible of greater emissions of carbon dioxide. Deforestation leads to
a further reduction in carbon dioxide net consumption by plants through reduced
photosynthesis thereby further increasing its concentration in the atmosphere.
Deforestation is especially intense in Brazil, around the Amazon River where is situated the largest rainforest in the world. Between May 2000 and August 2005, Brazil
lost more than 132 000 km2 of forest – an area larger than Greece – and since 1970
– over 600 000 km2 have been destroyed. This area is three times larger than the
area of the Adriatic Sea.
» Brazilian rain forest in
1975 and in 1986 as seen
from the satellite. Forest
vegetation is red and the
roads and cleared areas
with houses and farms are
in blue. In 1975 (left) small
cleared areas are seen only
along the roads. By 1986
many secondary roads had
been built (right). Areas
where the forest has been
cut down for lumber or
crops extend out.
Carbon reservoirs
There are three places where carbon is stored, in the form of CO2 or organic
matter, the atmosphere, oceans, and land biosphere. The atmosphere contains the
least quantity of CO2 while approximately 93% is found in the oceans.. During the
preindustrial period the CO2 released from the ocean in tropical areas balanced
the uptake in the polar zones. However, the CO2 cycle has been much disturbed
due to anthropogenic emissions of CO2 into the atmosphere. Presently, the ocean
absorbs more carbon dioxide from the atmosphere than it releases.
The oceans can absorb about million tonnes CO2 per hour and so help to
slow the rate of global warming by taking up some of the excess CO2 produced by
burning fossil fuels. However, the increased concentration of CO2 in the oceans has
the effect of making seawater more acidic, a process known as ‘ocean acidification’.
(http://interactive2.usgs.gov/
learningweb/teachers/globalchange.htm)
9
The increased concentration of carbon dioxide in the oceans reduces the concentration of dissolved calcium carbonate that is necessary for small animals that
make their shells. Increasing ocean acidification will endanger these small animals
and in turn influence the entire food chain and marine ecosystem.
How do marine plants “pump” the carbon?
» This image shows the
amount of marine plants
present in the oceans, and
the amount of vegetation
on land. Purple and blue
represent low quantity of
marine plants, while green,
yellow, and red indicate
progressively higher biomass. On land, brown color
shows areas of little vegetation, while blue-green represents dense vegetation.
Ocean areas poor in marine
plants are extension of the
desert on land. Viceversa
parts of the ocean rich in
phytoplankton represents
an extension of the dense
vegetation areas on land.
10
Credit: Provided by the
SeaWiFS Project, NASA/Goddard Space Flight Center, and
ORBIMAGE
(http://visibleearth.nasa.gov/
view_rec.php?id=1078)
Marine algae, including microscopic phytoplankton and sea-grasses are
equivalent to trees, shrubs and herbaceous plants on the land. They use sunlight
as energy, take up inorganic nutrients in the sea, transform CO2 into organic carbon for grow and release oxygen, through a process known as photosynthesis.
Eventually phytoplankton dies and sinks contributing to the vertical transport
of carbon towards deeper layers of the ocean and its accumulation at depth. At
the same time in the surface layer the reduction of the dissolved CO2 concentration due to photosynthesis increases the CO2 flux into the ocean. The process of
carbon capture and vertical flux is thus called the biological pump. Although the
total oceanic phytoplankton biomass is only about 5% of that of the terrestrial
plants, marine phytoplankton is responsible for about half of the global photosyn-
thesis. Compared to terrestrial plants, phytoplankton is very small but transforms
large amounts of carbon, because it is eaten by zooplankton about as quickly as it
grows. The entire global phytoplankton biomass is eaten every two to six days, in
contrast to land plants that might live several months or even hundreds of years.
This rapid change of phytoplankton biomass, along with the fact that it is limited
to the upper 100 m layer of the ocean (where there is enough sunlight to sustain
photosynthesis) makes phytoplankton much more responsive to changes in climate than land plants.
How do the oceans influence the climate?
Many people feel that as far as the climate is concerned it is more pleasant to live near the coast than far from it. Those living close to a coastal area will
know that if the wind blows from the land, the air is very cold and dry in winter
or hot and dry in summer. In California for example, in a season when the wind
blows from the surrounding desert the air is dry and hot. This creates favorable
conditions for spreading of forest fires over a wide area. On the other hand, at midlatitudes, the wind coming from the sea is humid and warm during the winter.
Summers in coastal areas are not as hot and winters are milder compared
to inland areas. This is due to the fact that the sea gains and releases heat slower
than the land, its temperature varies little and therefore it heats the atmosphere in
winter and cools it in summer. So to what extent is the sea important in determi­
ning the climate? This can be demonstrated by comparing the west coast of Great
Britain and the east coast of Canada both located at the same latitude. The two
areas should have the same climate if there were no interactions with the ocean.
The west coast of France and the UK, however, have a much milder climate than
Canada. This is due both to the transport of heat by the Gulf Stream and its continuation, the North Atlantic Drift, and by the westerly winds. In the range of latitudes
between 40°N and 60° N the winds transport as much as 80% of the total quantity
of heat that is transferred meridionally.
The oceans control concentration of CO2; thus an eventual increase or decrease of its absorption would change the CO2 concentrations in the atmosphere,
changing in turn the intensity of the greenhouse effect. As mentioned before the
solubility of CO2 depends on the sea surface water temperature. Therefore, an increase in sea surface temperature would not only reduce the solubility of the CO2
11
» Satellite image of the
huge forest fire in California
from October 22nd, 2007.
You can see the smoke
over large areas of the
eastern Pacific associated
with a series of forest fires,
outlined in red. The wind
is blowing from the mainland desert areas (called
Santa Anna wind) carrying
dry and warm air as can
be seen from the smoke
extending seawards.
Credit: NASA/MODIS Rapid
Response.
12
but also weaken the physical “pump” as the vertical sinking would decrease. In
contrast, higher temperatures should reinforce plant productivity, strengthen the
biological “pump” and cause stronger absorption of atmospheric CO2 in opposition to the two previously mentioned effects. Nevertheless warming the surface
ocean will increase its stratification and diminish the exchanges between the nutrient-rich deep waters and the nutrient-poor upper layers. The net result should
be a decrease in the biological productivity of surface waters.
How climatic variations can affect the ocean?
Heating of the Earth includes heating of the oceans too, primarily at the surface layer. Water volume increases with temperature. Thus, as seawater is heated it
will expand causing a rise in sea level. Sea level can also increase due to the melting of glaciers and of polar ice caps, however, the on-going sea level rise is mainly
due to the thermal expansion of the ocean.
In the North Atlantic polar region, the heating of the planet could stop sur-
face water from sinking (as seawater needs to be colder and denser than the water
below for this process to occur) and thus could block the north-south thermohaline component of the Meridional Overturning Circulation. This would have a
major effect on climate as less equatorial heat would be exported by the oceans
poleward. However, as shown above, the role of the atmosphere in the meridional
transport of heat must also be taken into account before coming to conclusions
about the net effect of global warming on changes in oceanic circulation.
Increasing the sea surface temperature will increase the temperature and
density differences between the surface and deep reservoirs of the oceans. This
results in an increased stratification of the ocean a reduced nutrient inputs from
below and thus in an expected decrease in the world ocean primary production.
One important consequence of a weakening or complete blocking of the
vertical water sinking would be reduction or a complete cessation of ventilation of
the deep ocean layers. In the bottom waters of oceans and seas, bacteria consume
oxygen in the process of decomposition (transformation of dead organisms into
inorganic nutrients). This oxygen is brought down by the vertical mixing and sinking of the surface oxygen-rich waters. The ocean areas where sinking occurs are
like open windows in our houses. Life is impossible without oxygen, thus if that
process stops, all dissolved oxygen will be consumed in the upper layers and deep
water creatures will not be able to survive.
Can we already see the consequences of climate
change in the oceans and in the atmosphere?
Increased concentrations of greenhouse, and climate change or its consequences are already evident in various ways. Firstly, it has been noted from direct
measurements that concentrations of CO2 have increased by more than 30% since
the beginning of the industrial revolution. For thousands of years prior to the industrial revolution, emissions of CO2 and other greenhouse gases to the atmosphere
were balanced by their absorption. Greenhouse gas concentrations and temperature were then fairly stable and this stability has allowed human civilization to
develop within a consistent climate. Thanks to the analysis of tiny air bubbles preserved in Antarctic ice for millenia, scientists have been able to determine that
there is currently more CO2 in the atmosphere than at any time during the last
850 000 years. The main concern today is that changes in climate due to global
13
warming as a result of the increased greenhouse gas concentrations will happen so
quickly that living organisms (including humans) might not have time to adapt.
Globally averaged Earth temperature shows a clear increasing trend. According to the National Atmospheric and Space Agency (NASA) the year 2005 was the
hottest on record (updated until 2005)3. The average global surface temperature
of 14.8°C was the highest recorded since measurements began in 1880. January,
April, September, and October of that year were the hottest months on the whole
record, while March, June, and November were the second warmest ever. In fact,
the six hottest years since 1880 have all occurred between 1998 and 2005. After
2005, 1998 was the warmest year, with an average global temperature of 14.7°C.
During the last century, temperatures have risen by 0.8°C, of which 0.6°C occurred
during the last three decades, a rate not seen over the last millennium. The average temperature of 14.0°C in the 1970s rose to 14.3°C in the 1980s. In the 1990s it
reached 14.4°C. This trend seems to be continuing and the autumn of 2006 and
14
winter of 2007 were the warmest in Europe in the last 500 years!
The temperature increase is not uniform over the entire planet. In particular
the northernmost region, the Arctic, has been experiencing the strongest temperature variations. Increased temperatures have caused the summer sea ice cover
in the Arctic to be 15–20% smaller than it was 30 years ago. If such trends continue there could be serious consequences to living organisms. It is thought that
polar bears are not likely to survive to the end of the century because, as the ice
shrinks, they are losing their habitat and access to
food. Tundra and permafrost are also thawing rapidly across the Arctic, threatening the survival of
many land species. In Europe, as a consequence of
global warming, about 40 butterfly species have
shifted northward by about 200 km in 27 years,
consistent with temperature increases. No butterflies were found to have shifted to the south4.
Scientists have also noticed that as a consequence of increasing temperatures spring flowering in Europe has occurred progressively earlier
since the 1960s, while fall events such as leaf colour changes and falling have been delayed. Seasonal cycles of various terrestrial
plants have been observed continuously in the last fifty years and it was noticed
that the snowdrop for example responds to 1°C warmer temperature in February
by flowering about 8 days earlier. In fact, in Europe the snowdrop flowers on ave­
rage 15 days earlier now than fifty years ago. Since 1990 this process has shown
signs of acceleration, with the snowdrop flowering 13 days earlier than the longterm average. Similarly earlier flowering has been noticed in a number of other
terrestrial plants. In this last extremely warm winter snowdrops started to flower
two months earlier than usual5. (Further information on the response of terrestrial
plants to recent temperature rise can be found at the web page: http://www.naturescalendar.org.uk6).
Glaciers are large rivers of compact snow and ice that move slowly down
valleys in mountains and polar caps7. They can be found in high mountains at all
latitudes. Although glaciers represent a relatively small quantity of the total water
on the Earth (less than 5%), they are good indicators of climate change, and this
is why climatologists have observed their changes very carefully. Glaciers have
been melting very fast over the last century especially those at low latitudes which
have shrunk by more than 70% on average. In Europe, Alpine glaciers have melted
intensely and lost about 50% of their volume in the last 150 years. Similar losses
have been noted in Russian, South American and New Zealand’s glaciers. All these
changes can be associated with the rise in the Earth’s temperature.
A rise in sea-level has also been noted8. Historically, Earth’s climate has
shifted regularly back and forth between temperatures like those we experience
» Rate of change of the
Greenland ice cover. Notice
that the ice cover has
reduced a lot, the rate of
change reaching in some
coastal areas values of 60
cm per year.
(http://en.wikipedia.org/wiki/
Image:Rate_of_change_in_
Ice_Sheet_Height.jpg)
15
today and temperatures cold enough for large
sheets of ice to cover much of North America
and Europe. The difference between average
global temperatures today and during those
ice ages is actually only about 5°C, but these
swings happened slowly, over hundreds of thousands of years. The last ice age occurred about
18 000 years ago and since its peak, sea level has
risen around 100-120 m, most of this rise having
occurred 6 000 years ago. From 3 000 years ago
to the beginning of the 19 th century sea level was
almost constant, rising at a minimum rate of 0.1
16
» Scientists have been
very carefully monitoring
the formation of icebergs
in order to follow closely
changes in the ice cover
on earth. The icebergs are
enormous pieces of ice
that after detaching from
the mainland, are moved
by ocean currents and
survive for years before
melting completely. This
satellite image shows one
of the biggest iceberg that
has been observed. The
iceberg has collided into
a neighbouring ice sheet.
This collision caused the
ice sheet to break up into
smaller parts. The iceberg
has been blocking shipping lanes and the feeding
grounds of 3 000 penguins,
for over 4 years.
Credit: NASA/Goddard
Space Flight Center Scientific
Visualization Studio
to 0.2 mm/yr. Variations in sea-level since the last
ice age until the 19 century are natural phenomena and have happened slowly.
th
Since 1900 however, sea levels have risen at a rate of 1 to 2 mm/yr. Since 1992
altimetry from TOPEX/Poseidon and JASON satellites (they measure the height of
the sea surface) indicates a rate of rise of about 3 mm/yr fully explained by the sea
water volume expansion due to global warming. So, there has been an acceleration of the rise in sea level since 1900. In some areas, for example Venice, the sea
level rose at an even higher rate. There were however, two processes contributing.
First, the global sea level rise and second, the subsidence i.e. sinking of the land
» Piazza San Marco (main
Venice square) during
a high-tide event locally named “acqua alta”.
Venice has been known to
flood for many years, but
recently the frequency of
such events and extension
of the city area flooded
have increased largely due
to sea level rise.
Source: http://www.venezia.
net/blog-eventi/wp-content/
uploads/2007/10/acquaalta.
jpg
due to water pumping for industrial purposes from beneath the city that took
place in the first half of the 20th century. Sea level rise is hence not uniform across
the entire planet, but depends on ocean circulation patterns. Thus, in some areas
a decrease in sea level has been observed.
Long-term atmospheric and meteorological observations are essential to
understand possible changes due to global warming. As an example, a longterm observing network has been set up in the Atlantic to record any changes
in the north-south Meridional Overturning Circulation, which depends partly on
the intensity of the polar water sinking and sea ice melting. Therefore, long-term
changes in the Meridional Overturning Circulation could be a consequence of the
global warming.
What will happen in the Mediterranean?
From the thermohaline circulation pattern perspective the Mediterranean
behaves like a small ocean. It is cooled in its northern part (Gulf of Lion, Adriatic
and Aegean Sea) where vertical water sinking takes place in winter. This water then
spreads over the entire bottom layer of the Mediterranean ventilating the deepest
17
part. The Mediterranean is connected to the Atlantic Ocean via the very narrow
and shallow (400 m) Gibraltar Strait. In general, but especially in its eastern part,
evaporation is much stronger than precipitation, thus the salt content in Mediterranean waters is higher than that in the Atlantic (1 m3 of the Mediterranean water
has in average 38 kg of salt while the Atlantic water has only 35 kg). Less salty water
from the Atlantic enters the Mediterranean from the surface layers of the Gibraltar
Strait, flows eastward and becomes more and more salty due to evaporation. On
the other hand, through the bottom currents, salty and dense Mediterranean wa18
ter exits into the Atlantic.
As it is much smaller and shallower than an ocean, the Mediterranean is more
responsive to changes in climate. Indeed, the circulation in the Mediterranean was
completely different only 6-9 000 years ago. At that time, the bottom layer was anoxic i.e. not ventilated at all. This was due to the fact that much more freshwater was
discharged into the basin than today, and the cooled surface water was not dense
enough to reach the depths. Scientists have studied bottom sediments in certain
places of the Mediterranean which are very rich in dead organisms. These organisms could not survive but were not eaten by bacteria either, which is a sign that
there was a lack of oxygen. These types of sediments are called sapropels9.
There are evidences, from oceanographic measurements, of warming and
increasing salinity of the Mediterranean waters since the 1990s which has been
attributed to global warming10. Scientists have predicted that this increase in both
temperature and salinity will lead to a weakening of the thermohaline circulation.
Temperature increase will certainly have an influence on the Mediterranean
ecosystem enabling survival of organisms that previously only lived in tropical seas.
Scientists have noticed for example, the presence of some tropical fish never seen in
the area before. It has been noticed that about 60 tropical fish species have entered
through the Suez Channel and remained in the Mediterranean, while about 30 fish
species have migrated from the Atlantic. With continued increases in temperature
one could expect the occurrence and survival of more and more tropical creatures
in the Mediterranean, resulting in a change of the ecosystem equilibrium.
But how else might these creatures enter the Mediterranean except via Gibraltar or through the Suez Channel, transported by currents? They are brought in
and discharged with ballast water from tankers and other commercial ships that
enter in large number the Mediterranean. Ballast water is carried in unladen ships
to provide stability. It is taken on board at port before the voyage begins and tiny
stowaways, in the form of marine organisms, are taken on board with it. At the ships’
destination, the cargo is loaded and the ballast water, with its surviving stowaway
organisms, is pumped out. Some of these organisms then establish populations in
the new environment if the conditions are favourable. According to statistics last
year about 2 000 ships released ballast water in the Adriatic and if in average each
ship carry about 10 000 m3 of ballast water that means in only one year 10 000 000
m3 or 10 000 000 tonnes of imported water were discharged into the Adriatic.
What can we expect in the near future?
United Nations (UNESCO) and United Nations Environment Programme
(UNEP) consider climate change a significant issue. In 1988 the Intergovernmental
Panel on Climatic Change (IPCC)11 was established aiming to assess scientific information relevant to:
1. Human-induced climate change,
2. The impacts of human-induced climate change,
3. Options for adaptation and mitigation.
The IPCC continuously collects scientific information on climatic change
published in international journals, analyses them and on the basis of their analysis, prepares periodical reports to address the three issues. The Norwegian Nobel
Committee awarded the 2007 Nobel Peace Prize to IPCC for its efforts to build
up and disseminate greater knowledge about man-made climate change. Four
reports have been prepared so far and the last one was issued in 2007. These
reports summarize scientific results, suggest the possible progression of climate
change and propose solutions for mitigation. The last report predicts the following climate evolution due to global warming:
•By the second half of the 21st century, wintertime precipitation in the northern mid to high latitudes and Antarctica will rise
•By the same time, Australasia, Central America and southern Africa are likely
to see decreases in winter precipitation
•In the tropics, it is thought some land areas will see more rainfall and others
will see less
•It is thought the West Antarctic ice sheet is unlikely to collapse this century.
If it does break up, sea level rises would be enormous
•Global average temperatures are predicted to rise by between 1.4°C and
5.8°C by 2100
19
20
» Tropical storms are very
violent meteorological
events characterized by
strong winds and rainfall
that subsequently can
cause floodings in the
coastal areas due to a sealevel wind set-up. Especially destructive was the
tropical storm Katrina that
hit New Orleans causing
heavy damages and life
losses. Scientists have not
yet found clear indications
whether global warming
will cause an increase of
strength and frequency of
these storms.
Credit: NOAA
•Maximum and minimum temperatures are expected to rise
•More hot days over land areas and fewer cold days and frost
•More intense precipitation events
•Sea levels will probably rise by 18 to 59 cm
These predictions are based on numerical models. We can see that the uncertainty is rather large (the predicted temperature increase varies between 1.4
and 5.8°C, sea level rise between 18 and 59 cm, etc...). These uncertainties are due
to the fact that models are based on different types of scenarios for economy and
demography. Also, numerical models are still rather simplified and for instance the
water cycle and the role of aerosols is still poorly understood.
We also have to consider that there will be significant differences between
regions and thus the whole planet will not necessarily experience the same changes such as more intense precipitation events. However, an important element in
the last report was that for the first time it was clearly stated that unequivocally
the temperature increase is due to anthropogenic greenhouse gas emissions.
It is important to say that as any other scientific idea in history, the standpoint that recent global warming is due to humans has faced opposing opinions. According to Singer and Avery12, the Earth experiences 1 500-year natural
warming-cooling cycles. The current warming began in about 1850 and could be
part of one of those natural cycles. According to them the warming will continue
for possibly another 500 years. Their findings are drawn from physical evidence
of past climate cycles that have been documented from tree rings and ice cores,
stalagmites and dust plumes, prehistoric villages and collapsed cultures, fossilised
pollen and algae. In addition, to the two mentioned scientists, there is a list of
those opposing the prevailing interpretation of global warming. If you are interested in reading about their reasoning, see the web page: http://en.wikipedia.org/
wiki/List_of scientist_opposing_global_warming_consensus.
What can we do to make a difference to climate
change?
All of us can contribute to the reduction in the emission of greenhouse
gases, mainly by saving electricity or energy in general. Instead of immediately
getting in the car we can turn to public transportation and walking or cycling
wherever possible. We could also help by turning off lights and only using airconditioning when absolutely necessary or put on an extra layer and turning our
heating down a notch in winter.
Planting trees is also very important for reduction of greenhouse gases since
they use carbon dioxide from the atmosphere to grow; if you cannot do it yourself
you could help local conservation organizations with their tree planting schemes.
In general the use of recyclable products can futher help to save energy. Try not to
21
waste paper at school or in the office and print only when necessary - think about
22
the wood that was cut to produce the paper.
Some materials use enormous amounts of energy during production e.g.
aluminum cans, so avoiding products packed in them helps save energy.
Generally, try to avoid waste. Do not change your TV set, HiFi or cellular
phone just for the sake of it, since by increasing consumption you contribute to increased energy production and then the resultant emission of greenhouse gases.
Remember that for any product that you buy, there is a certain quantity of energy
consumed and greenhouse gases released in the air not only to produce it, but
later on to destroy it once used!
Acknowledgements:
The publishing of this book was possible thanks to the enthusiasm and support by Osana Bonilla-Findji (EUROCEANS Public Outreach Team Océanopolis, Brest, France) Stefano Angelini and Stefano Argentero from the
Genoa Aquarium. Critical reading and completing of the manuscript by Sabrina Speich (Laboratoire de Physique des Océans, Brest, France) and Paul Tréguer (European Institute for Marine Studies, Université de Bretagne
Occidentale, Brest, France) have helped me to eliminate number of inconsistencies and mistakes. Review by
Patricija Mozetič (Marine Biological Station, Piran, Slovenia) and Alessandro Crise (Istituto Nazionale di Oceanografia e di Geofisica Sperimentale – OGS, Trieste, Italy) have further improved various parts of the manuscript. Excellent work in English proof reading was done by EUR-OCEANS volunteers Jessica Heard, Paul Matthews, Claire Enright as well as by Nicola Murray from the National Marine Aquarium, Plymouth.
There is a wide variety of information available if you would like to learn more
about global warming and climate change. To get you started here are some of
the web pages or papers used in the preparation of this booklet:
1
http://www.wrh.noaa.gov/twc/
2
Fisher, D.: Water Works on the Blue Planet, Originally published in The Technology Teacher, September 2001 by the International Technology Education Association.
3
http://www.earth-policy.org/Indicators/index.htm
4
Gian-Reto, W. et al., 2002: Ecological responses to recent climate change. Nature,
Vol. 416.
5
Luterbacher, J. et al., 2007: Exceptional European warmth of autumn 2006 and
winter 2007: Historical context, the underlying dynamics, and its phenological
impacts. Geophysical Research Letters, Vol. 34.
6
http://www.naturescalendar.org.uk
7
http://en.wikipedia.org/wiki/Glacier
8
http://en.wikipedia.org/wiki/Sea_level_rise
9
Rohling E. J., 2002: The Dark Secret of the Mediterranean, a case history in
past environmental reconstruction. http://www.soc.soton.ac.uk/soes/staff/ejr/
DarkMed/dark-title.html
Somot, S. et al., 2006: Transient climate change scenario simulation of the Mediterranean Sea for the twenty-first century using a high-resolution ocean circulation model. Climate Dynamics, 27.
10 11
http://en.wikipedia.org/wiki/Intergovernmental_Panel_on_Climate_Change
12
http://www.ncpa.org/pub/st/st279/st279.pdf
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EUR-OCEANS complementary material and resources available for free
download!
Educational pack: http://www.eur-oceans.info/EN/education/cards.php
•Teaching guide : ‘Embracing environmental issues in your curriculum’
•Educational fact sheets (impact of global change on the ocean, responsible
fisheries, penguins, etc,.)
•Workshops (Deep water formation; Plankton of the word)
•Poster: ‘The great ocean conveyor belt’
Films: http://www.eur-oceans.info/EN/medias/films.php
•Polarstern: In the climatic depths (English, French and German)
•Gulf Stream: the fall of a myth? (English, French)
•Ocean acidification (English, French, Spanish, German)
•Should we manipulate the oceans? (English, French, Spanish, German)
•The story of sardines (English, French, Spanish, German)
•Calanus in Spitzberg (English, Spanish, German)
•The fishermen reserve (Spanish, English)
•The International Polar Year (English)
•Observatories: keeping a watch on our changing oceans (English)
Animations: http://www.eur-oceans.info/EN/medias/animation.php
•The Earth: thermal machine
•Ocean conveyor belt and Gulf Stream
•Upwelling formation
•The biological pump
•The physical pump
•Deep oceanic and geological CO2 storage
I bet you are asking yourself how many trees had to be cut down to print this booklet.
Don’t worry no trees had to be cut down to make it, we used recycled paper!
Educational booklet
English version, March 2008
Coordination: Osana Bonilla-Findji
Graphical editing and illustration: Pika Vončina, Zagreb, Croatia
Graphic design: Ana Baričević, Zagreb, Croatia
Publisher: EUR-OCEANS 2008
This publication is funded by the EUR-OCEANS Network of Excellence.
More information about EUR-OCEANS and its educational programme on:
www.eur-oceans.info
Contact: Osana Bonilla-Findji
Public Outreach Team
[email protected]
EUR-OCEANS
Network of Excellence