Ups and downs of the “greenhouse” effect

Several gases have the property of absorbing radiation in the wavelength range emitted by the Earth because of its surface temperature, including methane as well as carbon dioxide, the usual culprit.  By doing so, they delay the escape of thermal energy through the atmosphere to outer space and give the Earth a higher surface temperature than it would have if they were not present.  Because methane oxidizes to CO2 more rapidly than the latter gas’s recycling time, a record of atmospheric carbon dioxide is the best guide to fluctuations in the “greenhouse” effect through the past glacial-interglacial cycles.  Bubbles in cores through the ice sheets of Greenland and Antarctica trapped air at the time when snow converted to ice within a few decades after it fell in polar regions.

The publication of data of all kinds from the ice-core drilled beneath the Vostok camp in Antarctica (see Earth Pages archive – Milankovic forcing flawed? July 2000) opened up 420 000 years worth of atmospheric composition shifts.  Daniel Sigman and Edward Boyle, of Princeton University and MIT, Massachusetts, review all the bio-geochemical factors that might have contributed to the CO2 time series for the last 4 major climate cycles (Sigman, D. M. & Boyle, E.A.  2000.  Glacial/interglacial variations in atmospheric carbon dioxide.  Nature, v. 407, p. 859-869).

While work continues to fully grasp this climate forcing function, Sigman and Boyle argue convincingly that the overwhelmingly dominant influence on it is the combined biological and physical carbon “pump” of the ocean around Antarctica.

News from the South

Increasingly, evidence of many kinds points to a dominant influence on climatic ups and down through the last 2.5 Ma by processes in the northern hemisphere.  Empirically, at least, the global-climate time series seems to show patterns that closely resemble Milankovic’s predictions of varying insolation at high northern latitudes.  For millennial-scale fluctuations, such as Heinrich events and the Dansgaard-Oeschger cycles in ocean and ice-sheet cores respectively, the focus is on changes in deep-water formation in the North Atlantic.  The South cannot be set aside, however, and there are two important issues that crop up in October’s publications.  One is the extent to which climatic events in the southern hemisphere tracked those in the North, and the other is the role of the southern oceans in the global carbon cycle that underpins the climate-related fluctuations in atmospheric CO2 concentrations.

Both the Greenlandic and Antarctic ice cores show synchronicity of CO2 trapped in air bubbles with the records of local air temperature and global land-ice volume, going back over 400 ka in Antarctica.  With more or less constant additions from volcanism, the ups and downs of the primary “greenhouse” gas have to be mediated by removal of carbon in one form or another from the ocean-atmosphere system through the agency of biological processes.  Just what process, where it is most active and the controls underlying it form a topic of continual discussion and research.  One possibility is variation in the biological productivity of the open oceans, coupled with removal of carbon from the ocean-atmosphere interface.

In terms of size and potential, the Southern Ocean is overwhelmingly the most likely candidate for a control.  It is today the largest repository of unused nutrients in surface waters (by comparison with its potential for supporting phytoplankton it is a “wet” desert), but also a major source of deep-water formation that could sequester carbon from the surface environment.  The late John Martin suggested that the main control over ocean productivity is soluble iron, currently at low concentrations far from land.  The first realistic experiment to verify this involved “seeding” a small area of the equatorial Pacific with iron sulphate in 1995.  Sure enough, that provoked a short-lived bloom of microscopic marine plants and local changes in dissolved CO2, but a boost in productivity at low latitudes is unlikely to lead to carbon removal from the surface part of the C-cycle.

Eighteen months ago, a multinational team of 35 ocean scientists conducted a similar experiment off Antarctica at 60°S (Boyd, P.W. et al. 2000.  A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization.  Nature,  407, 695-702.  See also: Chisholm, S.W. 2000.  Stirring times in the Southern Ocean. Nature,  407, 685-687).  Once again bio-productivity soared by three times, and an input of 9 t of ferrous sulphate into about 50 km2 of ocean triggered an estimated 600 to 3000 t of extra algal carbon production.  The “bloom” lasted for at least 6 weeks, being transformed into a swirling ribbon 150 km long.  But it did not seem to be absorbed into deep water, merely mixing at the surface.  In principle, iron dissolved from dust blown far from land during cold, dry episodes might have drawn down CO2 levels, but it is still uncertain.  Yet the dust records trapped in ice cores do show a pronounced negative correlation with both CO2 and climate proxies.

Millennial-scale climate shifts are best known from the area around the North Atlantic.  The most recent of these, and the most dramatic, was a sudden reversal from the warming trend out of the last glacial maximum around 13 ka ago, which lasted around 1800 years.  This is recorded in many ways everywhere around the North Atlantic, and takes the name Younger Dryas (YD) from the associated increase in sediment cores of pollen of the cold-resistant mountain avens (Dryas octopetala).  For some years there have been reports of a YD signal in climate records from the southern hemisphere, and some suggesting it was not felt there at all, the most detailed counter-evidence being the lack of the YD signal in Antarctic ice cores (ascribed by some to climatic inertia of the ice-bound continent).

The YD interrupted warming and wetting in the lead-up to the Holocene interglacial, so its signal ought to be easy to verify or rule out, simply because no later glacial advances have obliterated suitable investigation sites and many lakes at high altitudes and latitudes formed about that time.  The problem for southern-hemisphere work has been a lack of precise dates.  Southern Chile proves to be an excellent place to check, because lakes there go back further and contain evidence for many glacial advances and retreats (Bennett, K.D. et al. 2000.  The last glacial-Holocene transition in Southern Chile.  Science, 290, 325-328).  Moreover, the sediment cores provide sufficient high-precision dates to construct a believably detailed time scale.  Bennett and co-workers show that during the YD Chilean glaciers were retreating rather than advancing.  That seems to knock the idea of “teleconnections” spanning both hemispheres for this particularly dramatic event, although its signal extends to the north Pacific.  Like the mountain avens, however, disputing palaeoclimatologists are a hardy lot.  It could be that the site of Bennett and colleagues work was far from a boundary between pollen-shedding species that was sensitive to climate change, despite the excellence of their record (see also Rodbell, D.T. 2000.  The Younger Dryas: cold, cold everywhere?  Science, 290, 285-286).

No escape from global warming?

Palaeoclimatology is well-funded because it is believed to shed light on the likely consequences of anthropogenic warming caused by CO2 emissions, and perhaps even technical solutions that allow us to continue burning fossil fuels.  There is no doubt that throwing money at the range of associated phenomena and data has produced many astonishing findings and connections for the last 2.5 Ma.  There is now sufficient high-quality data to reviewing them in their proper context; that of the climatic aspect of the “human condition”.  That is the task that yet another multinational group of scientists set themselves at an International Biosphere-Geosphere Programme (IBGP) workshop at the Royal Swedish Academy of Science in November 1999 (Falkowski, P. and 16 others 2000.  The global carbon cycle: a test of our knowledge of earth as a system.  Science, 290, 291-296).

The workshop used two generalized outcomes of many years of work on Antarctic ice cores: the variation over more than 400 ka of CO2 in trapped air bubbles with temperature shifts; the frequencies and amplitudes of changes in atmospheric CO2.  They compare these with human effects over the last 200 years.  A great deal of discussion and qualification surrounds the workshop’s conclusions, but they are stark and simple.  Anthropogenic change falls way outside that induced by natural processes (whatever they are), and its period bears no relationship to those involved in short- to long term processes.  Despite the seeming attraction of technical fixes, such as boosting ocean productivity and the deep-water carbon sink (above), and intervention in terrestrial plant processes to increase CO2 sequestration from the atmosphere, both face the likelihood of weakening natural feedbacks due to the massive change that has taken place.  Indeed, the consequences of strategies of these kinds aimed at mitigating climate change cannot be known in advance.  This grim conclusion stems from the fact that no matter how well we get to know the climate system of the past, it is no longer what it was.  Even a complete halt to all anthropogenic emissions now cannot reverse the trend in the short to medium term.

The group suggests Earth’s entry into a new Epoch (the Anthropocene) of uncertainty, but brimming with growing knowledge.  To them, this seeming paradox must not be “used as an excuse to postpone prudent policy decisions based on the best information available at the time”.  They also highlight the disciplinary compartmentalization of research that hinders a “proper” understanding of the Earth system. I suppose what they are getting at is the continuing ethos of Descartes’ 400-year old reductionism in science, yet surely their call for a “systems approach” is merely dressing up reductionist empiricism in a more complicated guise; hurling yet more intricate maths at the problem.  That is indeed the goal of climate modelling and has been for well over a decade.  Perhaps the solution lies not in descriptive retrospection by scientists and in “policy”, but with society as a whole that now begins to confront the mismatch between several thousand years of divided human activity with the rest of the world.

Daniel Sigman and Edward Boyle, of Princeton University and MIT, USA, usefully review the whole issue of varying CO2 through the 420 ka Antarctic ice-core record, together with its environmental buffering (Sigman, D.M. and Boyle, E.A.  2000.  Glacial/interglacial variations in atmospheric carbon dioxide.  Nature,  407, 859-869).  Their article helps see the views of Falkowski et al. from a broad and detailed context, and links to News from the South (above), because Sigman and Boyle conclude that while the pacing of climate change tracks the combined effects of orbital processes on solar energy input at high northern latitudes the “greenhouse” effect  changes because of biological and physical processes in the Southern Ocean that surrounds Antarctica.

The nudge of noise

The emergence of a signal in the climate shifts through the last ice age and the Holocene with a roughly 1 000 to 1 500 year period (see Earth Pages Archive, A new regular pulse in recent climate, September 2000) finds no link with processes linked to Earth’s orbital behaviour.  It must be generated within the Earth system itself.  That being said, there is a lot of debate over what precisely is involved.  It’s safe to say that debate will continue.

However, another factor might well be involved; one that is as much to do with statistics as with phenomena with sufficient power to flip climate patterns.  Random noise is everywhere in nature.  If strong enough at a critical time, such stochastic noise might resonate with an otherwise weak, periodic phenomenon to give it sufficient push that it shows up in a climate change.  Let’s say that there is some weak pulsation that bears on climate – not really known with certainty, but having a 1 500 year period.  If resonance with noise was involved, we might expect to see 1 500, 3 000, 4 500 year periods in the climate record (1-, 2- and 3-cycle shifts), with the first more common than the last two – that is how the statistics should work.  The fact that short-term climate pulses (the stadial-interstadial events) cluster around 1 000 to 1 500 years might indicate that random noise is implicated.  However, only the last 120 000 years of climate data have sufficient precision for such statistical analyses, so it might be fortuitous.

The same nudge of randon climatic noise has also been called on to explain the jump from a roughly 41 000 year cyclicity to the present one of 100 000 years about 700 000 years ago.  The first correlates with the period of changes in the Earth’s axial tilt, and the second with changes in the eccentricity of its elliptical orbit. The effect of orbital variations on the energy received from the Sun is so very small that it cannot have much of an effect on climate by itself, but changes related to axial tilt are ten times bigger.  The change in behaviour seven ice ages ago is therefore hard to explain, without the nudge of noise.

Source:  Kerr, R.A.  2000.  Does a climate clock get a noisy boost.  Science, v. 290, p. 697-698.

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