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The Oceans, the Atmosphere and Climate Change:

What should we know about?

Miroslav Gačić

National Institute of Oceanography and Experimental Geophysics (OGS)Trieste, Italy

EURopean network of excellence for OCean Ecosystems ANalysis (contract 511368)

Sixth EU Framework Programme for Research and Technological Development.

EUR-OCEANSNetwork of Excellence

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Contents

Heat in the oceans and atmosphere 3

Water exchange between the oceans and the atmosphere 6

Carbon dioxide and the greenhouse effect 7Natural mechanisms of CO

2 capture 8

Factors increasing the greenhouse effect 8Carbon reservoirs 9

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

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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 vol-

ume 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

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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. wa-

ter 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 neces-

sarily 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 dif-

ferences 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

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.

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In this way warm surface waters are forced to flow poleward, while the cold wa-

ters 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 keep-

ing 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).

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 atmos-

phere 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 atmos-

phere is more significant in the poleward heat transportation than the ocean.

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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. Evapora-

tion mainly balances precipitation. This does however, vary from one area to an-

other. 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

» This image from the Gulf Stream depicts the com-plex 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 intermedi-ate 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 Wisconsin-Madison’s MODIS direct broadcast receiving station. The MODIS (the Moderat-ing resolution Imaging Spectroradiometer) sensor flies aboard NASA’s Terra spacecraft, launched in December 1999.

Credit: courtesy Liam Gumley, MODIS Atmosphere Team, University of Wiscon-sin-Madison Cooperative Institute for Meteorological Satellite Studies.

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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 rap-

idly 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

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 organ-

isms extract carbon dioxide (inorganic carbon) and transform it into organic car-

bon 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 transmis-

sion 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-

» 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 riv-ers are visible close to the coast showing that fresh-water 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).

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ated back to the atmosphere. As the Earth is much colder than the sun, this back-

radiation 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, green-

house 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 tropi-

cal and equatorial waters tend to release the CO2 back to the atmosphere (outgas-

sing). 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 lay-

ers. 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 proc-

esses 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.

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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 situ-

ated 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.

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’.

» 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.

(http://interactive2.usgs.gov/learningweb/teachers/glo-balchange.htm)

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The increased concentration of carbon dioxide in the oceans reduces the con-

centration 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?

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 car-

bon 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

concentra-

tion 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-

» 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 bio-mass. On land, brown color shows areas of little vegeta-tion, while blue-green rep-resents 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.

Credit: Provided by the SeaWiFS Project, NASA/God-dard Space Flight Center, and ORBIMAGE

(http://visibleearth.nasa.gov/view_rec.php?id=1078)

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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 cli-

mate than land plants.

How do the oceans influence the climate?

Many people feel that as far as the climate is concerned it is more pleas-

ant 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 mid-

latitudes, 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 contin-

uation, 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 de-

crease 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 in-

crease in sea surface temperature would not only reduce the solubility of the CO2

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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 opposi-

tion to the two previously mentioned effects. Nevertheless warming the surface

ocean will increase its stratification and diminish the exchanges between the nu-

trient-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 sur-

face 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 melt-

ing 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-

» 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 main-land 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.

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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 thermo-

haline 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 sink-

ing 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 conse-

quences 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 indus-

trial revolution, emissions of CO2

and other greenhouse gases to the atmosphere

were balanced by their absorption. Greenhouse gas concentrations and temper-

ature 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 pre-

served 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

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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. Accord-

ing 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 ave-

rage 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

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 tem-

perature 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 con-

tinue 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

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shrinks, they are losing their habitat and access to

food. Tundra and permafrost are also thawing rap-

idly 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 butter-

flies were found to have shifted to the south4.

Scientists have also noticed that as a con-

sequence of increasing temperatures spring flow-

ering in Europe has occurred progressively earlier

since the 1960s, while fall events such as leaf col-

our 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 long-

term 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.na-

turescalendar.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)

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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 thou-

sands 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 19th century sea level was

almost constant, rising at a minimum rate of 0.1

to 0.2 mm/yr. Variations in sea-level since the last

ice age until the 19th century are natural phenomena and have happened slowly.

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 accelera-

tion 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

» 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 ship-ping lanes and the feeding grounds of 3 000 penguins, for over 4 years.

Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio

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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 long-

term 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

» Piazza San Marco (main Venice square) during a high-tide event lo-cally 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

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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 Mediter-

ranean 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 wa-

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 an-

oxic 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 organ-

isms 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 Gi-

braltar 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

19

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 infor-

mation 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 analy-

sis, 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 follow-

ing climate evolution due to global warming:

•Bythesecondhalfofthe21st century, wintertime precipitation in the north-ern mid to high latitudes and Antarctica will rise

•Bythesametime,Australasia,CentralAmericaandsouthernAfricaarelikelyto see decreases in winter precipitation

•Inthetropics,itisthoughtsomelandareaswillseemorerainfallandotherswill see less

•ItisthoughttheWestAntarcticicesheetisunlikelytocollapsethiscentury.If it does break up, sea level rises would be enormous

•Global average temperatures are predicted to rise by between 1.4°C and5.8°C by 2100

20

•Maximumandminimumtemperaturesareexpectedtorise•Morehotdaysoverlandareasandfewercolddaysandfrost•Moreintenseprecipitationevents•Sealevelswillprobablyriseby18to59cm

These predictions are based on numerical models. We can see that the un-

certainty 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 chan-

ges 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 stand-

point that recent global warming is due to humans has faced opposing opin-

ions. 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

» 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 sea-level wind set-up. Espe-cially 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

21

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 inter-

ested 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 air-

conditioning 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

22

waste paper at school or in the office and print only when necessary - think about

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 in-

creased 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 (EUR-OCEANS 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 Phy-sique 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 Ocea-nografia e di Geofisica Sperimentale – OGS, Trieste, Italy) have further improved various parts of the manu-script. Excellent work in English proof reading was done by EUR-OCEANS volunteers Jessica Heard, Paul Mat-thews, Claire Enright as well as by Nicola Murray from the National Marine Aquarium, Plymouth.

23There 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 Technol-ogy Teacher, September 2001 by the International Technology Education Asso-ciation.

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

10 Somot, S. et al., 2006: Transient climate change scenario simulation of the Medi-terranean Sea for the twenty-first century using a high-resolution ocean circula-tion model. Climate Dynamics, 27.

11 http://en.wikipedia.org/wiki/Intergovernmental_Panel_on_Climate_Change

12 http://www.ncpa.org/pub/st/st279/st279.pdf

24

EUR-OCEANS complementary material and resources available for free download!

Educational pack: http://www.eur-oceans.info/EN/education/cards.php

•Teachingguide:‘Embracingenvironmentalissuesinyourcurriculum’

•Educationalfactsheets(impactofglobalchangeontheocean,responsible

fisheries, penguins, etc,.)

•Workshops(Deepwaterformation;Planktonoftheword)

•Poster:‘Thegreatoceanconveyorbelt’

Films: http://www.eur-oceans.info/EN/medias/films.php

•Polarstern:Intheclimaticdepths(English,FrenchandGerman)

•GulfStream:thefallofamyth?(English,French)

•Oceanacidification(English,French,Spanish,German)

•Shouldwemanipulatetheoceans?(English,French,Spanish,German)

•Thestoryofsardines(English,French,Spanish,German)

•CalanusinSpitzberg(English,Spanish,German)

•Thefishermenreserve(Spanish,English)

•TheInternationalPolarYear(English)

•Observatories:keepingawatchonourchangingoceans(English)

Animations: http://www.eur-oceans.info/EN/medias/animation.php

•TheEarth:thermalmachine

•OceanconveyorbeltandGulfStream

•Upwellingformation

•Thebiologicalpump

•Thephysicalpump

•DeepoceanicandgeologicalCO2 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!