New Internationalist

Climate control

Issue 397

Ocean and atmosphere are intricately linked. Dorrik Stow explores some of the facts, effects and challenges of global warming.

‘Polar Meltdown.’ ‘African Apocalypse.’ ‘Hurricane Hotspots.’ ‘Permafrost Destruction.’

By now we are well accustomed to such dramatic – and often exaggerated – headlines.

But what do we really know about climate change and its effects? Why is climate so inextricably bound up with the oceans? And why must ocean management feature so highly on the political agenda related to global warming?

The greenhouse

The mounting evidence is irrefutable and the scientific consensus is general – the Earth is warming rapidly under a cloak of greenhouse gases.

The world is now hotter than at any time in the last 800 years; 19 of the last 20 years are the warmest on record. Over the past 100 years, the average surface temperature has increased by about 0.7o Celsius, while the amount of carbon dioxide in the atmosphere has increased by over 25 per cent. Mean temperature is predicted to rise by a further 1.4-5.8oC over the next century.

Polar meltdown is firmly underway and appears to be gathering pace. Ice sheets over Greenland and Antarctica are both thinning and receding – almost 250 cubic kilometres of ice have been lost from Greenland alone in the past three years. The summer melt-back of sea ice has seen an even greater increase, culminating in the catastrophic collapse of the vast Larsen B Antarctic ice-shelf in March 2002. Five billion tonnes of ice shattered into tens of thousands of icebergs and drifted off into the Southern Ocean.

Average sea level rose by 10-20 centimetres through the 20th century. Our best predictions indicate a further rise of 25-100cm by 2100 as polar melting continues, and because seawater expands slightly as it warms. At the higher estimate, a large number of island nations, from the Pacific to the Caribbean, would vanish completely, together with several major coastal cities – New Orleans, Venice and Shanghai, for example. Huge tracts of low-lying coastline would be threatened or inundated, including 15 per cent of Bangladesh, leaving 15 million of the world’s poorest people homeless.

Not as well established is the direct link between global warming and extremes of weather. With warmer oceans, the frequency and scale of tropical cyclones will certainly increase, while storms in temperate latitudes also seem likely to intensify. Some parts of the world will become wetter, harsh climates more equable and growing seasons longer – but many others will be driven further into drought. Somalia, for example, is facing the worst drought in at least 40 years, with dire consequences for the lives of nearly two million people.

Although the absolute changes in temperature may seem small, even these can have dramatic effects on already fragile ecosystems. Some estimates place 25 per cent of land plants and animals under the threat of extinction from global warming.

Ocean regulator

In all of this, oceans play a critical role. Oceans and atmosphere are closely coupled as they respond to the constant influx of solar energy. Together they control the daily drama of Earth’s weather systems, as well as the longer-term changes in planetary climate.

Winds drive the currents that redistribute heat from the equator to the poles, just as they are engaged themselves in the atmospheric heat engine. Without such perpetual motion, the blistering heat delivered daily to the tropics would render them uninhabitable, while the rapid spread of ice caps and frozen lands from North and South would bring a harsher, crueller reality to the polar regions. A very different type of life would exist on planet Earth, most of it squeezed into a narrow band between these climatic extremes.

Thankfully, for our species at least, this is not the case. The oceans operate as a giant thermostat that regulates global temperature. The world’s climatic zones are determined by the ocean’s climatic zones – by the latent heat stored and transferred in the upper few hundred metres of our seas. The ocean’s energy storehouse also operates as a giant flywheel, both moderating change and prolonging it once it has begun.

Carbon sink

Oceans also provide one of the principal storehouses for carbon and carbon dioxide, and so act to regulate greenhouse gases in the atmosphere. They are like a gigantic sponge, holding 50 times more carbon dioxide than the atmosphere. They are thought to absorb 30-40 per cent of the carbon dioxide produced by human activity – the one ‘carbon sink’ we truly have.

This is a supremely important fact. Microscopic phytoplankton bloom in abundance at the ocean surface, living by sunlight and absorbing carbon dioxide. On death, they sink in countless billions, so releasing copious volumes of carbon dioxide to the ocean depths, much of it ultimately into sediment on the seafloor.

The sea surface also acts as a two-way control valve for gas transfer – opening and closing in response to gas concentration and ocean stirring. Carbon dioxide can be released back to the atmosphere just as it can be removed more permanently. The efficacy of these processes we do not yet fully understand – until we do, one of the crucial pieces of the climate jigsaw puzzle remains missing.

Cyclones

The largest, most violent storms the Earth’s heat engine can unleash are truly awesome and potentially devastating on land and at sea. Little wonder they are so revered, for the energy released by a single tropical cyclone in one day would be enough to power the entire industrial production of the United States for one year.

Tropical cyclones are born directly from overheating of the oceans – their increase worldwide is, therefore, a direct cause-effect of global warming. They only develop at tropical latitudes when sea surface temperatures exceed 27oC. As vast amounts of water evaporate from the overheated ocean surface, the hot moist air rises and then condenses as it cools to form billowing clouds. Air rushes inwards across the sea surface to fill the void, evaporating more water as it whirls past, encouraging more clouds to form. Heavy rainfall follows. Spiralling bands of thunderstorms begin to rotate threateningly around a calm clear eye of the storm.

The damage wreaked by this force of wind is extreme and much feared by vulnerable coastal communities. As the tropical cyclone tracks inland and becomes detached from the oceanic fuel supply, it slowly loses power. But its potential for destruction lingers on. Thunderstorms deluge the land with torrents of water, leading to landslides, mudflows and widespread flooding.

El niño

The complexity of the ocean-atmosphere system is well exemplified by the El Niño phenomenon – a natural climate oscillation in the equatorial Pacific Ocean whose effects are played out across the world. It can result in changes to global surface temperatures of up to 0.5oC in just one year. This is only a little less than the amount by which Earth’s average temperature has increased over the past century. The short-term effects are numerous and dramatic – with some of the most severe impacts being felt along the coast of South America.

The equatorial waters off the coast of Peru normally experience a strong upwelling of cold, nutrient-rich waters as warm surface water is blown out across the Pacific by strong trade winds. This upwelling supports very high plankton productivity, which is harvested by billions of anchovy and other fish, thereby supporting large seabird colonies as well as a thriving local fishing industry. Towards Christmastime each year, as the winds die down, the upwelling decreases and water warms up. Periodically, this effect is much more pronounced – warm waters spread right across the Pacific from Indonesia, reaching the coast of South America and completely suppressing the upwelling. The plankton factory switches down, fish die in their millions, seabirds and marine mammals starve, and the whole fishing industry is brought to its knees.

The coupling of El Niño with climate is very marked. Under normal conditions, warm moist air rises over the west equatorial Pacific, creating thunderstorms and heavy rainfall over Indonesia and northern Australia; whereas cold, dry air sinks in the east, leading to drought in South America. As El Niño intensifies, so conditions are reversed – heavy rains along the Peru-Chile coast result in floods and mudslides, while severe drought and wildfires occur in Australia and Indonesia. Further afield, droughts worsen in the Sahel, the Indian monsoon weakens and may even fail completely. Mid-latitude storms are pushed further north, bringing drought to the central United States.

In 1982-83 the strongest El Niño in living memory played havoc with global weather patterns and led to the near collapse of the Peruvian fishing industry. Such extreme events will become more common as ocean surface temperatures increase still further.

Climate challenge

The message could not be written more clearly. Global warming and its strong anthropogenic (human) drive are no longer in doubt. The cause and effects are global; the responsibility to act is therefore global. Urgent action is imperative if we are to slow the changes and mitigate the worst impacts on the poorest sector of Earth’s population. Tending the welfare of our common world ocean is not just an optional extra. It is central to effective action on climate change and poses serious questions that challenge the very fabric of our global society.

In my view, it is time that environmental concern replaced economic factors as the prime driver in society; that long-term planning replaced short-term politics; and that global responsibility replaced national self-interest.

Dorrik Stow is Professor of Ocean and Earth Science at the National Oceanography Centre, Southampton, England. He is also author of Encyclopedia of the Oceans (Oxford University Press, 2004) and Oceans: An Illustrated Reference (University of Chicago Press, 2005).

THE WASHING MACHINE

The remarkable properties of the humble water molecule have made life on Earth possible.

ELEMENTS

Every water molecule is made up of two atoms of hydrogen (the most abundant element in the universe) and one of oxygen. It is supersolvent, absorbing gases from the atmosphere and leaching salts from the continents.

Though it is not known for certain, the oceans probably formed as gases escaping from the Earth’s crust and mantle (‘outgassing’) condensed into water. Initially the water was fresh, if acidic. Today it contains over five trillion metric tons of dissolved salts and nearly 100 different elements, including some five billion kilos of gold. If the water evaporated completely, the residue would be enough to cover the entire planet with a layer of salts 45 metres thick.

The evidence suggests that ocean chemistry has changed little over the past 200-300 million years. Exploring why this is so – despite continued outgassing from mid-ocean ridges and the millions of tons of dissolved chemicals delivered by rivers every year – reveals an underlying chemical turmoil.

Carbon dioxide is particularly soluble in water, forming a weak acid. Human pollution adds around 22 billion extra tons of carbon to the atmosphere every year; great quantities of it are delivered to the ocean dissolved in rainfall.

A series of chemical reactions provides a buffering system that maintains seawater as a mildly alkaline solution, enabling animals to construct shells and skeletons. Because carbon dioxide is more soluble at low temperatures and high pressures, deep water is slightly more acidic and dissolves skeletal materials as they drift to the seafloor.

Ocean life is very sensitive to the amount of dissolved carbon dioxide in the water, where there is 60 times the quantity of the gas found in the air. During the process of photosynthesis, all marine plants and many bacteria take in carbon dioxide and release oxygen. During respiration, the reverse is true.

The sheer volume of organic activity absorbs a huge quantity of material. Other seawater elements are absorbed by clay particles as they fall through the water; by mid-ocean ridges; by manganese nodules; by deposits on shorelines. Eventually, after being uplifted as rocks, elements are returned to the ocean once more by weathering.

Different elements have different ‘residence time’ in the ocean, ranging from tens of millions of years for chloride and sodium to a few hundred years for manganese, aluminium and iron. Ocean water itself has a residence time of about 3,500 years.

Far from being a well-mixed bathtub, the oceans are arranged in a hierarchy of ever-changing layers, each with its own physical or chemical properties. Part of this layer-cake structure is as old as the ocean itself, part is constantly stirred and shaken.

The most important – and difficult to study – is the microlayer at the surface, only a few millimetres thick, where 70 per cent of Earth’s solar energy is absorbed and most of the water vapour, carbon dioxide and oxygen are exchanged. Enormous volumes of particulate matter and pollutants pass through it.

Water also has the capacity to store large amounts of heat. It can absorb or release it while changing relatively little in temperature. The marked difference in the input of solar energy between the equator and the poles is the main driver for atmospheric winds and ocean currents, and these in turn tend to equalize the heat imbalance. For example, the Gulf Stream transports 550 trillion calories of energy northwards every second – enough to run about a million domestic refrigerators for a year.

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