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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 4 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 23 24 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