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Ocean processes

and greenhouse warming
David Tranter

Summary


Earth is a fine-tuned ecosystem, warmed by the sun, the global greenhouse and the ocean. The alternating advance and retreat of polar ice and snow over the past 600,000 years (ice ages) were initiated by orbitally induced variations in the angle of the summer sun above the polar horizon (orbital effect). So much fresh water was returned to the ocean each inter-glacial phase by the melting of continental ice that, combined with thermal expansion of the water column, sea level rose by as much as 120m.

The orbital effect was reinforced by greenhouse warming. This, in turn, was further reinforced by ocean processes that limited how much aerial carbon dioxide (CO2) could dissolve in the ocean and be stored in the ocean interior; and how much dissolved CO2 could be removed from the ocean by photosynthesis or returned to the air at upwellings.

Man-made fossil fuel emissions have now increased the atmospheric concentration of CO2 from the ice age maximum (~280 ppm) to ~390 ppm and the ocean has become more acid. As fossil fuel emissions continue to increase, greenhouse warming will raise sea levels high enough to inundate low-lying coastal places such as coral atolls, deltas and those densely populated foreshores where many of the largest cities and airports in the world have been built. That scenario is a matter of choice, not necessity, and it can be limited by governmental intervention.

1. Introduction


Science is driven by curiosity, observation, experiment and pattern recognition. Experiments in 1859 by Irish scientist John Tyndall, Professor of Natural Philosophy at the Royal Institution in London, showed that CO2 is a powerful greenhouse gas. It dissolves less readily in warm water than cold, a fact you can confirm yourself by observing how much gas is released from a bottle of fizzy drink that’s been sitting in the sun compared with one that’s been kept in the refrigerator.

But there is only one earth, so despite the prevailing habit of using its atmosphere as a rubbish dump for fossil fuel wastes, it would be logistically impossible and socially irresponsible to experiment with it. The recent erosion of the ozone layer that protects life on Earth from harmful ultraviolet radiation has confirmed that small traces of chemical can have a big effect, most of those that did the damage coming from small cooling tubes at the back of man-made refrigerators.

A useful scientific tool is pattern recognition, where correlations of observations lead us to the ultimate identification of the mechanism involved. That’s how Darwin and Wallace arrived at the theory of evolution by natural selection; how Wallace and Wegener came up with the theory of continental drift; how Milankovich conceived the theory that recent ice age cycles were due to variability in Earth’s orbit around the sun [1]; each theory with a global dimension and each now verified by further observation. So too it is with the current theory that fossil fuel emissions warm the earth.

2. Air-Sea Interaction


The ocean is a giant solar-heated cauldron, whose ingredients are temperature, salinity, currents, winds, sea-ice and living organisms. It is now absorbing more than 80% of the heat that Earth receives from the sun [2], the sea-air interface acting as a busy transit station for heat, moisture and CO2; and as the source of clouds, cyclones, and tornados. The oceanic equivalent of atmospheric pressure is density, a blend of temperature and salinity that, in concert with the wind, drives the surface circulation of the sea.

An example of a transient air-sea circulation pattern is the El Nino Southern Oscillation (ENSO). Cool, high-latitude Pacific air, converging from each hemisphere towards the equator, is deflected westward in the wake of the earth’s rotation to form the trade winds that drive equatorial surface water towards Australasia. Contained by this barrier, that wedge of warm surface water becomes the heat engine that generates what we in Australia regard as normal weather.

When the trades lose their clout, that vast body of warm surface water surges back towards the Americas leaving Australia in drought - a fairly regular 3-4 year cycle that is sometimes confused with climate change. It is not; it is a normal pan-Pacific weather pattern. Climate is weather averaged over time, the period adopted by climatologists being ~30 years. It follows that weather at any particular point in time might have little to do with climate; it could well be little more than “noise” or “static” rather than a genuine climate signal.

3. Evolution of “Air”


The fact that air contains so little CO2 (less than 0.1%) might appear to be at odds with the fact that it is a powerful greenhouse warming agent; however such apparent contradictions are fairly common in nature: For example, the nutrient resources that sustain the luxuriance of a coral reef or tropical rainforest are so scarce in the immediate environment they are barely detectable; their recycling rate is so rapid, or a mere 0.05% of alcohol in our blood stream can so severely impact our decision processes.

Primeval air had little oxygen and a lot of CO2 (of volcanic origin), whereas today’s air has a lot of oxygen (21%) and only enough CO2 to warm the earth. This apparent “anomaly” is due to the evolution of life on a lifeless planet. The primaeval forms of life were bacteria that used chemical energy for their metabolism, generating methane (natural gas) as a by-product, some of which is now imprisoned within icy matrices on the sea floor at depths greater than 300m. In today’s oxygen-rich world, such anaerobic bacteria are confined to smelly refuges, superseded by green photosynthetic bacteria that draw their energy from sunlight. Extracting hydrogen and dissolved carbon dioxide from the surrounding water, they produce carbohydrates, yielding oxygen (O2) as a by-product. Over the ages, this oxygen accumulated in the air in pace with the decline of CO2, shaping and fine-tuning the composition of the air.

Descendants of those primaeval green bacteria are the phytoplankton, a community of unicellular algae that inhabit the sunlit layer of the ocean, collectively withdrawing as much CO2 from the sea each year as land vegetation withdraws from the air. Another favoured habitat is polar sea-ice at the interface between snow and ice, fed by a rich supply of nutrients from the underlying sea and continually sunlit for a few months of the year. In that brief period, as much CO2 is withdrawn from the ocean [3] as the Amazon rain forest removes from the air in a year.

There are hundreds of different kinds of phytoplankton. They are dust-sized particles of spectacular beauty, some radially symmetrical like minute snowflakes, some encased in shells of glass, chitin or calcium carbonate. The ocean floor is littered with their remains, which are ultimately recycled in the earth’s interior, surfacing later on as limestone or chalk, a cycle thought to regulate the global climate on a geological time scale. What those species have in common are structures to keep them up in the sunlight, their nemesis the stratification of the water column that isolates them from their sub-surface nutrient supply, their saviour winter mixing.

4. Sea-ice


The axis of Earth’s rotation is so inclined to the plane of its orbit around the sun that the poles receive less heat than the equator and become covered in sea-ice every winter. An alien intelligence studying Earth from afar in fast forward mode might think Earth was breathing as its polar sea-ice frontier advances and retreats with the seasons and the ice-age cycles - an image that is more than metaphor.

Sea-ice sheds brine as it forms and the underlying cold, saline, aerated seawater, enriched with carbon dioxide of atmospheric origin, sinks under its own weight into the abyss as a gigantic sub-surface waterfall. Without such ventilation, bottom-living animals would suffocate and rot for want of oxygen, generating clouds of poisonous, buoyant, hydrogen sulphide gas that are thought to have wiped out life on Earth at least once in the distant past.

This bottom water weaves its tenuous way across the sea floor, ultimately surfacing a millennium later at upwelling sites, such as those off the west coasts of continents and along the equator. Under the climate regimes that prevailed over the past 600,000 years (Figure 1), when the CO2 concentration of the air was always less than 280ppm , those (low latitude) upwellings would have released their load of super-saturated CO2 into the air like bubbles from a newly opened soda water bottle

Tranter1 trends

Figure 1: Warming-cooling cycles (the “Ice Ages”) in the last 600,000 years. - Le Page 2007

 

5. Continental Ice


Whereas today’s sea-ice advances and retreats with the seasons, continental ice has been accumulating, layer by layer for at least a million years, each layer containing bubbles of air from the atmosphere of the day [RD1]. This historic archive of temperature and CO2 (Figure 1), which has been accessed by coring through the permanent Antarctic ice sheet, covers the past 600,000 years (six ice age cycles). Compiled by Michael Le Page [1] from the 2004 work of Royer et al, with a shorter, independent record of sea level, those data confirm the orbital cycle that Milankovich had predicted from astronomical calculations. Among its most interesting features are the following:

  • Temperature and CO2 cycled up and down with a period of about 100,000 years.
  • Those periods were fairly regular and their amplitude limited.
  • Temperature and carbon dioxide were closely correlated, temperature leading with CO2 close behind.
  • Reversals of temperature and carbon dioxide trends were swifter after each minimum than each maximum.


Those phenomena raise the following interesting questions:

  • How could so weak a signal as the orbital effect have generated such severe global climate consequences, its warming influence (as calculated from astronomical observations) being only 10 Watts/m2, which is about the same as a 100 Watt globe in a closed room 3m square? [4]
  • Where did all the CO2 go that was lost from the atmosphere each glaciation (when most high latitude vegetation would have been covered in snow and ice and unable to withdraw much CO2 from the atmosphere for want of light)?
  • Why did temperature lead and CO2 lag?


The answer to the first question is that the orbital effect was reinforced by greenhouse warming. Although less at any one point on the earth than the orbital effect (which is limited to polar ice in the polar summer), CO2 induced warming (~1.7 W/m2) influences the entire cloud-free surface of the earth every day of the year, land and sea alike. Answers to the other questions are to be found in the working principles of the ocean:

  • Most of the CO2 that was lost during the glacial phase of each ice age went directly into the ocean when the sea was cold and CO2 solubility high [5].That’s why the ocean now contains about 90% of all the free CO2 on earth, making it a major global climate force.
  • Because CO2 is less soluble in warm water than cold, global warming releases dissolved CO2 into the air, amplifying pre-existing greenhouse warming. This draws even more CO2 from the ocean (and so on) until a new balance is reached. That’s why CO2 is considered to be the main mechanism reinforcing global swings in temperature.
  • The most likely reason why the ice age warming trend was faster than the subsequent cooling trend is because the flow of CO2 across the air-sea interface is more effective in colder seas (when the water column is well mixed) than in warmer seas (when surface waters become isolated from the interior of the ocean by a sharp temperature gradient known as the thermocline.
  • Current acidification of the ocean is most likely due to the current retention of CO2 by upwelled water, constrained as it now is by the stronger CO2 gradient that has developed across the air-sea interface since that upwelled water left the surface ~ 1000 years earlier.

 

6. Ocean Amplifiers of Greenhouse Warming


In the centre of the conceptual warming spiral illustrated in Figure 2 is what I have called a stable temperature cycle - for example the seasons of the year or the annual advance and retreat of Antarctic sea-ice.

Tranter2 amplifiers

Figure 2: Global Warming Amplifiers: Oceanic and Social Feedback Loops. - Tranter, 2009

 

These are cycles, rather than trends, and by definition reversible. By contrast, if you follow the spiral clockwise you will notice the following series of gains in global temperature, labelled G1-G5, which amplify the previous greenhouse warming trend:

  • Sea-ice albedo (G1). Imagine how much hotter it would be standing in your bare feet in summer on black bitumen compared with white pavement. This phenomenon (albedo) has a strong influence on global temperature because sea-ice is a mirror, which reflects the warmth of incident sunlight back to space. When sea-ice melts each summer, that part of the ocean absorbs heat instead of cooling and, as more sea-ice melts, warming gains momentum like a locomotive gaining speed. Should the sea-ice frontier recede each successive winter, albedo warming would become a long term trend until it was reined in by orbital cooling.
  • Sea-ice habitat (G2): The winter extent of Antarctic sea-ice, which is 50% greater than the whole Antarctic mainland, is a powerful seasonal bio-sequestration agent by virtue of the CO2 uptake of the algae that live within its interstices. When sea-ice melts, that algal biomass empties into the underlying sea and sinks rapidly to the deep sea floor. Should the sea-ice frontier retreat year by year due to global warming (as predicted by IPCC2), this valuable cooling agent would weaken; and the organic carbon that would otherwise end up on the sea floor would accumulate in the air as CO2, reinforcing pre-existing greenhouse warming.
  • Temperature Induced stratification (G3). Most of the global ocean is located in low latitudes, where the thermocline isolates surface phytoplankton from their sub-surface nutrient source, limiting their capacity to transform dissolved CO2 into particulate matter. The immediate result is a reduced flow of CO2 from air to sea, reinforcing greenhouse warming, ultimately strengthening the thermocline and extending its areal expanse.
  • CO2 solubility (G4). As the earth warms up, so too does the ocean, which then accepts less CO2 from the air, reinforcing pre-existing greenhouse warming, an effect calculated to account for ~ 20% of the warming that brings the glacial phase of each ice age cycle to a close [2].
  • Methane hydrate release (G5). Methane hydrate exists in abundance in frozen cages on the sea floor at depths greater than 300m. Should bottom temperatures warm, methane molecules are likely to escape into the water column and eventually the air above. Since methane is an even more powerful greenhouse gas than CO2, the consequence of its release would be to dramatically reinforce pre-existing global warming.

 

7. Man-made Amplifiers of Greenhouse Warming


All of the above amplifiers are elements of natural climate change - a card that humanity has been dealt by nature. That constraint does not apply to man-made warming, driven as it is by “social” rather than natural feedbacks, such as those shown on the left in Figure 2. They are matters of choice not necessity and, since neither the greenhouse layer nor any of its reinforcing agents discriminate between natural and man-made molecules of CO2, the consequence is the same, namely further greenhouse warming. The difference is that each molecule added to an atmosphere that already contains more CO2 than at any previous time during the ice ages is a molecule that takes Earth into uncharted territory whose main symptom is fever.

The Intergovernmental Panel on Climate Change (IPCC) has identified the likely impacts of man-made global warming [2]. However changes are now happening at the high end of IPCC projections, due to the combined influence of the oceanic and social feedbacks illustrated in Figure 2. Although such non-linear effects are difficult to model, they are nonetheless real, present and significant. Amplified by increasing fossil fuel emissions, they could easily carry the global climate towards some rather nasty tipping points much sooner than most of us would wish.

Figure 3 illustrates the warming consequence of doubling the CO2 concentration of the air (the current trend). It shows that that independent reinforcing feedbacks, such as those identified in Figure 2, have a cumulative effect whose ultimate consequence is irreversibility. Figure 4 shows that CO2 stayed below 280 ppm throughout the past millennium until the Industrial Revolution, when it rose rapidly towards 400 ppm, a trend that may have begun as far back as 8000 years ago (Figure 5) when natural forests were cleared and burnt for agriculture [5].

Tranter3 overheating

Figure 3: The Cumulative Global Warming Effect of Multiple Feedback Gains. - Paltridge, 2009

Tranter4 CO2

Figure 4: Exponential rise of Atmospheric CO2 since 1800 A.D. – Internet

Tranter5 anomaly

Figure 5: The CO2 “anomaly” (deviation from the norm) over the past 8000 years. - Ruddiman 2005

 

8. Sea Level Rise


Only 20,000 years ago, at the peak of the last glaciation, when ice sheets 1000m thick covered Canada and Northern Europe, global sea levels were 120m lower than they are today (Figure 1) and the straits separating the Australian mainland from New Guinea and Tasmania were dry land. Sea level is now rising at about 3mm per year, half of which is due to thermal expansion caused by fossil fuel emissions [6]. During the previous inter-glacial era (125,000 years ago), when polar regions were significantly warmer than now for an extended period, sea levels rose a further 4-6m [2]. Should that happen again in the current inter-glacial era, when man-made warming is adding its weight to natural warming, the consequences could well be dire. Continental ice-sheets on Greenland and the Antarctic mainland, which contain most of the world’s fresh water, would start to melt and discharge into the global ocean elevating the sea level. The social impact of rising seas would be greater than in earlier inter-glacials because large cities and airports have since developed along the sea shore and they would be swamped by rising seas, as well as densely populated deltas and low lying islands, such as Boigu and Saibai in Torres Strait, and all the coral atolls in the world. The resulting influx of refugees to Australia from Bangladesh alone would dwarf the current influx, which already concerns so many Australians.

9. Climate Action


There is always more to learn, but the fundamental processes and consequences are clear-cut. Now is the time for action and that’s the job of society, not climate science. "We live", as the Chinese are wont to say, “in interesting times”. Perhaps future historians will recall the (paraphrased) words of Charles Dickens a century and a half ago:

“It was the best of times, it was the worst of times, it was the age of wisdom, it was the age of foolishness, it was the epoch of belief, it was the epoch of incredulity, it was the season of light, it was the season of darkness, it was the spring of hope, it was the winter of despair, we had nothing before us, we had everything before us.”

Sources


1. Le Page, M. 2007 New Scientist May 19, 34-42, After Royer, D.L. et al, CO2 as a primary driver of Phanerozoic climate, GSA Today, 14, 3, 4-10, (2004).

2. Intergovernmental Panel on Climate Change (IPCC) 2007, Climate change 2007. The physical science basis, 21pp.

3. Lizotte 2001. American Zoologist 41 (1), 57-73.   Back to text

4. Godfrey 2008 (Retired CSIRO Physicist). Personal communication.   Back to text

5. Ruddiman 2005. Plows, Plagues and Petroleum, Princeton University Press, N.J. USA   Back to text

6. Church 2009 (CSIRO Physicist). Personal communication.   Back to text

7. Paltridge 2009. (Retired CSIRO Climatologist). The Climate Caper, Connor Court, Vic. Aust.

[RD1] The continental ice has also been increasing and decreasing with the ice-ages.  Only in the coldest places (Greenland, Antarctica) is the accumulation continuous, balanced by a flow of ice to the coast and the creation of glaciers.