The 30- or more year long debate about anthropogenic climate change resulting from the ‘greenhouse’ effect of carbon dioxide releases by fossil-fuel burning has grown sharper in recent years. Some specialists have cast doubt on climatologists’ ability to unravel human effects on the undoubted rise in mean surface temperature over the last 150 years from underlying fluctuations that stem from natural processes. Considering the number of forcing factors, both large and small and with different periodicities, such doubts are valid, even though they may well be overemphasised in order to support continued and rising use of petroleum and coal.
The climate record for the last millennium is known to have been one of considerable change, involving a mediaeval warm period during its first half and the so-called ‘Little Ice Age’ from around 1600 to the mid 19th century. Even within the recent warming trend there have been climatic ups and downs in the northern hemisphere, such as the mid-20th century warm period and several documented examples of cooling associated with major volcanism than punched aerosols into the stratosphere.
Thomas Crowley of Texas A&M University (Crowley, T.J., 2000. Causes of climate change over the past 1000 years. Science, v. 289, p 270-277) has attempted to isolate the known natural forcing functions and model their individual effects on mean surface temperatures in the northern hemisphere, to isolate meaningful signs of human effects from the various records temperature changes. Even charting the changes is no simple task, as most records are local to regional, rather than valid for the whole hemisphere. The most comprehensive climate data model uses proxy indicators from ice cores, tree-ring studies and coral time series, scaled to instrumental temperature records for the century from 1860-1965. Uncertainty increases backwards with time, to around + 0.3°C in year 1000 AD. This is somewhat greater than the fluctuations recorded in the temperature reconstructions, that Crowley and others have derived by a complex statistical method that fits a smoothed trend to the temperature fluctuations modelled from proxy data.
The approach used in Crowley’s analysis is to identify each likely, natural factor that influences energy balance in the northern hemisphere, and then to model the trends that they produced, and should continue to be producing. There are two factors that are significant in millennial to shorter timescales: influences of volcanic aerosols, as timed by ash layers in ice-sheet cores; and variations in solar output based on 10Be and 14C variations in ice cores and tree rings (solar radiation generates both at the top of the atmosphere). Possible anthropogenic forcing factors are numerous, including industrial aerosols and emitted gases other than the usual suspect, carbon dioxide. The end product is a temperature time series which removes the effects of all known forcing factors, except those connected with ‘greenhouse’ gases, from the northern-hemisphere temperature model. Until the early 19th century, this hovers close to zero variation, and then starts an upward rise to a value of about 0.75°C by now.
Crowley’s modelling adds significantly to the case for a detectable economic influence on climatic warming, by showing that known natural forcings cannot account for the rising trend since the start of the Industrial Revolution. It does not prove the case, and leaves several features in recent climatic change unexplained, notably the cooling in the late 19th century. Also, this approach does not exhaust all possibilities involved in climate shifts, such as linked fluctuations in energy movement by ocean-atmosphere circulation that work to make some regions experience cooling while others warm. Such processes might respond to variations in solar-energy input by a kind of complex resonance, which remains to be looked at.
See also 20 percent more oil in the ground
Methane hydrate – more evidence for the ‘greenhouse’ time bomb
Where ocean water is more than 400 metres deep and bottom temperatures fall below 1 to 2°C methane and water can freeze to form crystals of methane hydrate. These efficiently absorb more methane in a gas-solid solution, known as a clathrate. Being lighter than seawater, methane clathrates do not carpet the ocean floor, but occupy pore spaces in sediments. Under the anaerobic conditions of marine sediments, bacteria break down buried organic matter to release methane. Build up of the gas in clathrates forms distinct reflecting horizons seen on many seismic sections of marine basins. Estimates suggest that methane clathrates contain around the same amount of buried carbon as all fossil fuels lumped together. Since methane is a powerful ‘greenhouse’ gas when released into the atmosphere, breakdown of the clathrates is a potential mechanism for global warming.
In 1995, evidence began to emerge that 55 million years ago, at the Palaeocene-Eocene boundary, a pulse of global warming probably stemmed from catastrophic release of methane from ocean-floor clathrates. The signs lay in the proportions of 12C to 13C in organic matter within marine sediments of that age. Since organisms selectively take up the light isotope of carbon, when organic matter becomes buried, seawater becomes enriched in 13C. Buried carbon, both in organic molecules and in marine carbonates, takes on the isotopic signature of seawater at the time. This means that the ups and downs of carbon burial leave an imprint in the carbon-isotope record of marine sediments. When warming of deep-ocean water or a pressure release caused by a lowering of sea-level releases biogenic methane from clathrates, its high 12C content quickly appears in the carbon in seawater as a whole, by oxidation to CO2 and solution. This reduces the proportion of heavy to light carbon in both buried organic matter and marine carbonates, forming a downward ‘spike’ in the carbon-isotope record. Other processes can produce the same effect, such as increased release of volcanic CO2, which is also isotopically light, or a collapse of the marine biosphere. So 12C ‘spikes’ need to be matched with other evidence.
Co-workers from Britain and Denmark have just reported in detail on just such an excursion that took place about 183 Ma ago, during a period of massive burial of carbon in the Jurassic Period (Hesselbo, S.P. et al., 2000. Massive dissociation of gas hydrate during a Jurassic ocean anoxic event. Nature, v. 406, p. 392-395). The Early Toarcian was a period where circulation in the deep ocean stopped, to give anoxic conditions, ideal for burial of dead organisms. While that lasted, important hydrocarbon-rich source rocks for petroleum reserves were laid down, and the carbon isotopes in sea water became unusually ‘heavy’. Several stratigraphic sections show evidence for a 12C ‘spike’ in the middle of this period of general 13C enrichment of the oceans. Hesselbo and co-workers isotopically analysed Toarcian mudstones exposed on the Yorkshire coast in England, which contain both marine matter and fragments of wood formed in terrestrial ecosystems. Both show 12C enrichment, and that means that the entire carbon cycle at the time was somehow perturbed. With no sign of massive extinction, the signature pointed either to a methane release or to hugely increased volcanism.
Although there was strong volcanic activity on Earth during the Toarcian, it came nowhere near being able to generate the anomaly. Also the ‘spike’ occupies at most a period of a few tens of thousand years. Only catastrophic release of methane from clathrates, equivalent to 20% of those estimated to be present today, is able to account for the anomaly. Why it happened is nor certain; it may have been a result of either increased temperature of deep-ocean water by general global warming, to which it added, or perhaps too great a release of methane by decay in organic-rich sediments to be taken in by clathrates. Another trigger, for which evidence is lacking at present, is through a comet impact in an ocean basin.
Methane hydrate layers in the oceans pose an ever-present threat today, because of their extreme sensitivity to temperature and pressure. Some scientists believe that small releases may lie behind inexplicable disappearances of ships due to the drop in bulk density of seawater frothed by bubbles. Also many areas of shallow seas are pockmarked by vents marking methane release when sea level stood lower during glacial epochs, and at least one methane spike in ice-core records can be correlated with a massive submarine landslide off western Norway.
Silica as a control over atmospheric CO2 levels
Today the oceans far from land are the equivalent of deserts, having very low biological productivity. This is not due to a lack of the main nutrients, potassium, nitrogen and phosphorus, or to too little sunlight for photosynthesis. For some time, marine specialists have suggested that the culprit is too little soluble iron – a micronutrient at the core of pigments and the enzyme RuBisCO, on which photosynthesis and the fundamental Calvin cycle depend. The halving of atmospheric CO2 levels during glacial maxima is widely believed to reflect more efficient ocean bioproductivity and thus burial of dead organic matter. The idea that general dryness and windiness during glacial epochs delivered soluble iron to the remote ocean surface is one means of explaining this. However, it took CO2 8 000 years to rise to pre-industrial levels after the last dusty period when ice sheets reacehd their maximum extent, whereas iron lingers in seawater for only a few tens of years at most. Dust carries far more silica than soluble iron, and SiO2 resides for 15 000 years or so. This encourages the blooming of silica secreting diatoms in competition with calcium-carbonate secreting plankton. Carbonate production by cells actually generates CO2, so less carbonate secreters relative to those producing silica shells means that tendency is offset by a greater contribution to buried carbon from dead silica secreters (Source: Tréguer, P and Pndaven, P., 2000. Silica control of carbon dioxide. Nature, v. 406, p. 358-359)