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The effects of global climate change on our oceans. An essay by Courtney.


The Effects of Global Warming on Our Oceans - Ocean Warming


A Short Article by Courtney


 

INTRODUCTION

We are all aware of the issue of global warming - see my article on global warming - and global climate change. Of course, whether each of us chooses to attribute it to natural cycles or human activity, is a personal matter. Human activity is clearly only one source of atmospheric CO2, but the burning of such huge quantities of fossil fuels may well be the activity that tips the global balance. The evidence is highly convincing that we must bear at least some of the responsibility. Regardless of its causes, few would argue that the mean surface temperature is rising and most are aware of the potential consequences to the land environment. What far fewer people understand is the significance of this temperature rise to our oceans and the life that dwells in it as well as the effects on our climate. I call it Ocean Warming.

At the surface of the oceans a number of processes take place. Heat energy moves between the two mediums - so global warming will cause ocean warming. Gas exchange at the surface allows oxygen to diffuse from the sea to the atmosphere and CO2 to be absorbed into the oceans. Colder water is capable of containing greater amounts of dissolved gas than warm water; so as ocean temperatures rise the oceans' ability to store dissolved CO2 reduces causing it to be released back into the air. Evaporation of water vapour is an important part of the earth's water cycle and this, combined with heat exchange, drives our weather patterns; the more energy (heat) that is available in surface waters, the more powerful the weather generated - this is the process behind the formation of tropical storms (hurricanes and cyclones). Of course, water vapour in the atmosphere also has a greenhousde effect. The effect of wind on the ocean is to generate and drive waves and currents. So this interface between air and water is a very active boundary and has a huge effect on many global processes. I would suggest that an increase in the heat energy in the seas would contribute to greater extremes of weather and climate.

Ocean surface temperatures globally have risen on average 0.5ºC and ocean waters in many tropical regions have risen by 1ºC over the past century. This is 30 times the amount of heat energy that has been added to the atmosphere, a significant amount even though the ocean has a lot more mass than the atmosphere.

Recent reports by The World Wildlife Fund and The Marine Conservation Biology Institute in Seattle have suggested that rising global warming is having a far greater impact on ocean ecosystems than was previously thought. It is clear from these reports that wide-ranging changes are occurring in marine life as a direct result of rising water temperatures - ocean warming. This is as true for tropical waters as it is for the seas at or surrounding the poles. Moreover, the evidence shows that these events are happening far sooner than previously predicted. The report, Turning Up the Heat: How Global Warming Threatens Life in the Sea, is based on a comprehensive review of the latest scientific literature, including work that was yet to be published, but that had been subject to scientific scrutiny. The key issue is that global water temperatures are rising in line with other global warming events. Ocean warming is following global warming very closely.

Dr. Elliot Norse, President of the Marine Conservation Biology Institute, stated that, "Warmer temperatures are raising the biological cost of living for marine species. This is true in polar seas, where climate changes have been most pronounced, as well as on tropical coral reefs, which are suffering unprecedented devastation due to heat stress." The Institute says the findings clearly demonstrate the urgent need to reduce carbon dioxide emissions and other greenhouse gases.

 

EFFECTS ON PHYTOPLANKTON

The ocean's food chain is based on the growth of billions of microscopic phytoplankton. Disturbing news has emerged from the analysis of data from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) instrument on the OrbView-2 spacecraft, launched in 1997. The data show that ocean warming is reducing the numbers of these plants, endangering ocean fisheries and marine life.

Phytoplankton growth slows as water temperature rises because, as the temperature of the upper ocean increases it becomes less dense than the cold water beneath it. This results in stratification of ocean waters creating a barrier between the surface layer and the nutrients below, cutting off the phytoplankton's food supply. The reduction in phytoplankton reduces the food available to fish and other organisms.

Phytoplankton are responsible for more photosynthesis (and, therefore, the consumption of atmospheric carbon and production of oxygen) than all the plants on land combined. Therefore, an alarming result of reduced phytoplankton populations is that less carbon dioxide is taken up by the ocean, which accelerates global warming, warms the oceans further and causing a vicious cycle.

 

EFFECTS ON HIGHER ANIMALS

Effects of a warming climate are appearing throughout the marine food web, from the plankton to polar bears, walruses, seals, sea lions, penguins, seabirds, coral reefs and the fish on which millions of humans depend for food and income.

As an example, Pacific salmon may no longer be able to find suitable habitat in the Pacific Ocean. These fish are extremely sensitive to temperature in that their metabolism increases with rising water temperature. This means that as the seas become warmer they require larger amounts of food to survive. Therefore, the salmon must either move into deeper water or migrate into the cooler Bering Sea further from the rivers where they spawn. It is unlikely that they can adapt to this longer migration and it should, therefore, come as no surprise that their population crashed following a 5ºC rise in ocean temperatures in the region. Salmon are crucial sources of food and income in Alaska and British Columbia.

Reef fish and shallow water invertebrates such as crabs and sea anemones in Californian waters provide compelling evidence that fish and other species are moving their habitat toward the poles in response to warming – seeking colder water.

Scientists have observed decreased reproduction and an increased death rate in seabirds, coinciding with warmer water. At the end of the last century populations off the Californian coast of Cassin's auklets declined by 50% and Sooty Shearwaters by a staggering 90%. In both cases the cause seems to be declining plankton numbers. In Alaska a similar decline in shearwaters was clearly due to starvation, as the abundance of their crustacean prey was dramatically reduced in the unusually warm waters.

 

CORAL

Coral reefs are also at extreme risk – see my article on coral. Coral polyps are only able to survive temperatures a few degrees above the normal range and only for a limited time. If this upper temperature limit is exceeded, the polyps expel the zooxanthellae that provide them with food and give many coral species their colour; a process known as coral bleaching. If temperatures return to normal soon enough, coral can recover, but if they remain too high for too long, bleached corals invariably die. High water temperatures in 1997 and 1998 triggered unprecedented bleaching in all major tropical waters: the Indian and Pacific Oceans, the Caribbean, Mediterranean and Red Seas and the Persian Gulf. Large quantities of coral are bleaching and dying, most extremely in areas of the Indian Ocean where some 90% of the coral has already been lost, mainly to this process.

Coral reefs lock up large quantities of carbon, helping the oceans to regulate the amounts of CO2 in the atmosphere. Raised levels of CO2 dissolved in the water from the atmosphere cause ocean acidification; this reduces the availability of calcium that the corals require to build reefs, so less carbon can be stored. The destruction of the reefs will cause the release of stored carbon into the sea, which is then diffused back into the atmosphere. More greenhouse effect, more global warming.

 

POLAR REGIONS

It is likely that the polar regions will experience even greater temperature rises and, therefore, more extreme effects than tropical waters. Sea ice is diminishing in both the Arctic and Antarctic, depriving birds and marine mammals of their hunting and breeding grounds. The edges and undersides of sea ice are home to the algae that form the base of the polar food web. As the ice shrinks these algae become less plentiful and so does the food available higher up the food web. The result is a serious lack of food for numerous bird and mammal species.

Over the past 40 years the average temperature in the Arctic region has risen by around 2ºC, at least double the global average. The extent of Arctic sea ice has been reducing, on average, by over 70,000 km2 each year (see Figure 1 below). It is a very real possibility that the Arctic Ocean could see ice-free summers by the middle of this century, probably much sooner.

 

Arctic Ice

Figure 1: Graph showing the extent of Arctic Ice.

 

Research in Canada has revealed significantly decreased weight in adult polar bears and a decline in the birth rate since the early 1980s. The suspected cause is global warming causing the earlier breakup of sea ice each spring. The bears use the ice floes as a platform from which to hunt seals. During the summer months, bears are confined to the land and survive on their fat reserves. Effectively, the entire population must fast for at least four months after the ice has broken up; pregnant females must fast for up to eight. As their fat reserves are now significantly lower, they are less well placed to survive their fast.

Alaska's Bering Sea has experienced many ecological changes in the first ten years of the 21st century: sharp declines in Northern sea lion numbers (Figure 2) led the species to be considered threatened in the 1990s. They are now on the endangered species list. Northern fur seals and bird communities appear to be similarly threatened.

 

Northern Sea Lion

Figure 2: Northern Sea Lion.

 

Fish such as herring and capelin, that provide food for young seals and sea lions, have been declining markedly in recent years. Species that previously lived in lower latitudes have started to appear much further north in Alaska including Pacific white-sided dolphins, albacore and yellow-fin tuna; these species all compete with young seals and sea lions for food leading to reduced growth rates and higher mortality.

In the Antarctic things seem a little different. The average temperature rise has been half that in the Arctic; about 1ºC. While this would appear to be less significant, it must be considered that there are large areas of the region that have seen no significant warming yet, due largely to the reflectivity of the ice (the albedo effect). However, as the ice retreats here, this effect will weaken and temperatures will start to rise more quickly.

 

OCEANIC CURRENTS

The World's oceanic currents play a fundamental role in regulating the Earth's climate and circulating nutrients and oxygen around the globe. The oceans distribute 25 to 50% of the energy received from the sun, and the oceanic “conveyor belt” is a major source of heat to the North Atlantic .

In tropical waters, the sun warms the surface water causing evaporation and the water becomes saltier. The warm, salty water is carried northward along the East Coast of the United States by the Gulf Stream and then across the Atlantic towards Europe. As this massive volume of water becomes colder and denser it plunges downward to the ocean depths. As cold, salty water sinks in the North Atlantic, it pulls more warm, salty, tropical waters northward to replace it.  This huge plunge of water drives the ocean's conveyor belt, sending deep currents travelling along the ocean bottom to surface elsewhere around the world. Eventually these currents resurface to be warmed again and the cycle starts again.  

The whole cycle takes over one thousand years to complete, unlike global wind and air circulation patterns, which take place over days or weeks. They may be slow, but these currents move massive amounts of energy around the globe. It is called the thermohaline circulation from the Greek words "thermos" (heat) and "halos" (salt).

Thermohaline Circulation
Figure 3: The Earth's Thermohaline circulation


The release of the heat from the warm waters of the Gulf Stream moderates Northern Europe's winter temperatures. It also generally distributes heat more evenly around the planet, moderating extremes between cold northern latitudes and hot equatorial regions.

It is highly likely that global warming could effect thermohaline circulation. As global temperatures rise, the north-flowing surface currents would cool less quickly. Global warming is also likely to increase rainfall in the northern region, causing more fresh water to fall or drain into the North Atlantic, reducing salinity. The melting of polar ice caps and the Greenland ice sheet will further freshen the surface ocean waters.

So, as the surface waters become less salty and remain warmer during their journey north, they remain more buoyant and less likely to sink; so the thermohaline circulation would slow and, at a certain point, completely stop. Thermohaline circulation is not easily restarted. Models indicate that it could take about 500 years to recover from a 30% reduction of the thermohaline circulation. How long it would take to restart after a complete shutdown is hard even to guess.

There is some evidence that something of this sort is going on now. We have data showing that the North Atlantic has been growing steadily less saline over the last 40 years. Measurements of the deep ocean currents moving south from the Nordic seas show a reduction of 20% since 1950. These deep waters have been growing less saline for almost half a century.

The likelihood of a shutdown as a result of global warming is difficult to assess. Estimates suggest it would require a global temperature increase of 4 to 5ºC to trigger such an event - probably more than is currently expected within the next century. The more likely scenario may be simply a reduction in the thermohaline circulation, though the magnitude of that reduction is, again, hard to predict.

The effects of a shutdown, if it were to occur, are likely to be most acute in Europe. As we have seen, Europe's unusual warmth (for its latitude) depends on the warm surface currents that bring heat up from the equator. There have been widely circulated predictions that failure of the thermohaline circulation would cause average temperatures in Europe to drop by up to 11ºC. In this case Iceland would be glaciated down to sea level and Northern Europe would be something like Siberia. These predictions probably overstate the case. Though global warming could shut down the thermohaline circulation, it would not shut down the North Atlantic gyre (I don't intend to get into this here). The forces driving gyres, including winds and expansion of water at the equator, would continue, although their pattern and strength could alter. Therefore, a shutdown of the thermohaline circulation should not completely halt the flow of the Gulf Stream, which is what the predictions above presume. However, the Gulf Stream's influence on European climate is only about half as much as that of the thermohaline circulation, and without the influence of the thermohaline circulation the Gulf Stream might shift further southward, lessening its influence on Europe.

There are more optimistic predictions say that if the thermohaline circulation failed, then the cooling in Greenland would be around 8ºC, Europe would only cool about 4º, which would appear conveniently to counteract the global warming that triggered the shutdown in the first place. Different models give very different results. What is more certain is that weather patterns in northern Europe are likely to become much more extreme and we can expect to experience severely cold weather. Though it is hard to be sure what the likelihood of a shutdown of the thermohaline circulation is, it seems almost certain that some slowdown is happening and we are already starting to see harsher winters in the UK.

 

CLIMATE AND WEATHER

The effects of warming on thermohaline circulation, the faster evaporation of water from warmer ocean surfaces and the generation of larger tropical storms all point not just to warmer global weather, but more extreme global weather. It would be simple to assume that the Earth could warm evenly, moving climate bands towards the poles. But this is clearly not the case. Instead we can expect more frequent, more extreme events: colder winters, hotter drier summers, more destructive storms, longer droughts and greater flooding.

I mentioned the albedo effect earlier and this too will play an increasing role. Antarctica is currently covered in snow and ice. This white material effectively reflects on average about 80% of solar radiation back into space, reducing the local heating effect. Unless there is a significant rise in the temperature of the atmosphere, the albedo effect tends to protect ice-bound regions from thawing.

But an effect of global warming is to begin to melt ice. As it melts, darker rock and soil below is exposed, which reflects only about 10% of incoming radiation and so more readily absorbs solar radiation, inreasing local heating. This, of course, melts more ice and so the process accelerates (the ice-albedo positive feedback). The converse is also true in that areas of the Earth that become glaciated tend to be cooled further by the reflectivity of the ice. This effect could be expected to exagerate the cooling effect of the other mechanisms alreasdy described.

 

SUMMARY

So global warming is increasing the mean temperature of our seas, reducing the overall productivity of the oceans and could be the killer blow for many species who's populations are already under pressure from over-fishing and habitat loss. Marine life is a vital source of food and medicines, and provides livelihoods for millions of people around the globe through tourism and fishing.

Together, our oceans form the largest active carbon sink on Earth, absorbing more than a quarter of the carbon dioxide that humans release into the atmosphere. The seas may contain as much as 37,500 billion tons of CO2. Global warming and the associated oceanic warming reduce the oceans' capacity to contain carbon through a number of mechanisms. The effect is that less CO2 is absorbed into the seas and more is diffused back into the atmosphere adding further to the greenhouse effect.

The deep ocean currents play a massive role in the global distribution of heat energy. It is likely that this thermohaline circulation will be slowed (or in extremis, stopped) causing significant lowering of temperatures in some northern landmasses, including northern Europe.

Heat energy in surface waters at lower latitudes provides the fuel to start and feed hurricanes and cyclones - the tropical storms. The U.S. National Oceanic and Atmospheric Administration performed simulations to determine if there is a correlation between global warming and the frequency or strength of tropical storms. This work concluded that "the strongest hurricanes in the present climate may be upstaged by even more intense hurricanes over the next century as the earth's climate is warmed by increasing levels of greenhouse gases in the atmosphere". The more heat that is available, the greater the potential for the formation of larger, more destructive storms - events we are already seeing in North America (Katrina 2005, Ike & Norbert 2008), Central America (Hermine 2010), Western Pacific (Saomai 2006, Parma 2009) and northeastern Australia (Yasi 2011).

One of the direct effects of global warming is raise the mean temperatures of our oceans or, more specifically, increase the amount of heat energy stored in the oceans. This is having immediate and unprecedented effects on marine life, the oceans' capacity to store carbon and, through a number of processes, our climate and weather. Much more research is required into all these effects and processes, but we can be sure that the effects are already becoming apparent and changing our world at an alarming rate.

 

"Turning up the Heat: How Global Warming Threatens Life in the Sea", was written by Amy Mathews-Amos and Ewann A. Berntson, Marine Conservation Biology Institute, Seattle, USA, for the WWF.

 



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