Phytoplankton productivity and climate change

Posted by on March 11, 2012
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12th March 2012.  It’s gratifying to see that the oceans are slowly beginning to attract the attention they should when people consider climate change and other aspects of the global environmental crisis.  The oceans play a major role in determining our weather, the composition of our atmosphere, and the rate of climate change.  They also play a major role in driving primary production – the creation of organic matter through photosynthesis.  How they play all these roles is now changing.

Last week in Nature, Paul Falkowski provided a highly readable review of the role of phytoplankton in the functioning of marine food webs, carbon cycling, carbon sequestration, and regulation of the chemistry of our atmosphere.  The phytoplankton are composed principally of two main groups of organisms, the photosynthetic cyanobacteria (that used to be called the blue-green algae), and a number of different, mostly single-celled true algae.  They occupy the upper 100 meters of ocean waters, where there is sufficient light to permit their photosynthesis, and together their photosynthesis incorporates some 45 to 50 billion tonnes of inorganic carbon into their tissues each year.  Land plants, which are generally larger and more conspicuously present, manage only slightly better and incorporate about 52 billion tonnes of carbon per year.  To say that phytoplankton are important ecologically is stating the obvious.

Yet the well-being of phytoplankton is strongly dependent on ocean physics and chemistry, and climate change is changing these.  Living phytoplankton must remain in the top 100 meters where there is sufficient light.  If they sink below this level they die.  More to the point, all organisms living in the top 100 meters tend to sink below this level when they die, so that there is normally a constant rain of organic material down to deeper waters.  Phytoplankton use inorganic nutrients in their photosynthesis.  The organic compounds formed are incorporated first into the phytoplankton themselves, then into organisms which consume them and on up the food chain.  And as organisms drift downwards, they are effectively removing nutrients from the shallow waters of the ocean.  Nutrients get returned to the upper layers primarily in places where there are strong upwellings due to bottom topography, particular current patterns and so on, but much of the open ocean is nutrient-poor in its upper layers.  Falkowski and others have determined that about 15% of the organic matter created by phytoplankton each year is lost to the deep ocean, while the remaining 85% is recycled among organisms living in the lighted waters.

Above 4oC, water gets less dense as it is warmed (below this temperature it becomes less dense as it cools, thus ensuring that ice floats and Canadian lakes do not freeze from the bottom up).  A layer of warmer water will float on top of a layer of cooler water and mixing between the two is reduced if the temperature difference is greater.  In the ocean, this situation is complicated somewhat by salinity, as water of lower salinity is less dense than water of higher salinity, and the right combination of salinity and temperature differences can make two layers of quite different temperature mix easily.

Climate change begins with CO­­2 (and other greenhouse gases) being emitted by human activities, changing the composition of the atmosphere.  The increased concentration of greenhouse gases warms the planet, including the surface of the ocean.  This warming at the surface enhances temperature differentials in the ocean making the surface waters less likely to mix with deeper waters below.  While the warmer surface waters might be expected to encourage more rapid metabolism of organisms, and therefore more rapid photosynthesis by phytoplankton, it is now clear that the reduced mixing is the more important process.  The surface waters become further depleted of nutrients, and phytoplankton activity slows.  Back in 2006, Michael Behrenfeld of Oregon State University, with several colleagues including Falkowski, reported in Nature that global phytoplankton primary production declined measurably during the period 1999 to 2005, when sea surface temperature was warming.  A more recent modeling study (Hofmann et al, 2011), published in Environmental Research Letters, largely supports this trend and extends it through the 21st century.  The warming that is taking place is going to slow the production of organic matter through oceanic photosynthesis, and, by extension, reduce the ocean’s capacity to produce food.

Given that we gain about 16% of our animal protein from oceanic fisheries, and given that these fisheries are already yielding less than they did in the mid-1980s because of our ineffective management of our rates of extraction, this temperature-driven reduction in productivity is not good news.  The modeling study estimates the decline as about 40% between 1900 and 2100.  And, just to make bad news even worse, the reduced photosynthetic activity by phytoplankton will reduce the rate at which oceanic processes remove CO2 from the atmosphere, leading to – you guessed it – a more rapidly warming world.

I’ll continue this tale in my next post, which will consider ocean acidification.  In the meantime, how is this for an alarming quote?  It is taken from a paper presented by the Australian, Ian T. Dunlop, at the Club of Rome conference “The Future of Energy and the Interconnected Challenges of the 21st Century”, held in Basel, Switzerland, 17-18 October 2011.  Dunlop is an energy engineer, whose career was spent in the coal and oil industries; he expressed these views prior to the Durban climate meeting.  (Sorry to end on a downer.)

“Historic emissions [of CO2] have probably already locked in a temperature increase of around 20C relative to pre-industrial conditions, sufficient in due course to melt large parts of the Greenland and West Antarctic ice sheets, leading to a 6-7 metre sea level rise once a thermal equilibrium point is reached. Even if current climate policies are fully implemented, the likely temperature increase will probably be in excess of 40C, leading in due course to a 70 metre sea level increase. Empirical evidence suggests that climate modelling has probably badly underestimated both the speed and the extent of climate change.

“The implication is that the official target of limiting temperature increase to below 20C is inadequate if dangerous climate change is to be avoided, a more realistic target being below 1.50C. This requires the reduction of atmospheric carbon concentrations from 391ppm CO2 currently to below 350ppm CO2. Even to achieve the current 20C official target requires global emissions to peak immediately and decline rapidly.

“Despite two decades of negotiation, virtually nothing has been done to constrain emissions and there is no sign of this changing in the short term if current attitudes prevail. Adaptation to a 40C temperature rise is talked about glibly in policy circles as a realistic strategy if required. Such an outcome would be catastrophic, with a possible reduction in global population to less than 1 billion people from the current 7 billion. If the global community is serious about addressing climate change, prudent risk management dictates that action must be elevated to an emergency footing, outside the arena of adversarial politics, as time for a graduated incremental response has run out. The cost of taking action now if the science turns out to be wrong is vastly outweighed by the catastrophic cost of inaction if the science is right.”

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