Elliott et. al. (2002) examine two high resolution records of the benthic Foraminifera δ13C C. wuellerstorfi, which is depleted in cold times associated with HEs. These records give insights on the role of NADW ventilation changes associated with DO cycles. The results show drastic decreases of δ13C associated with the HEs reflecting a northward migration of 13C-depleted southern waters to the Irminger Basin (62ºN), and deposition rates alter δ13C values. Changes of δ13C associated with the cold stadials observed in Greenland ice cores, and of NADW formation, are of lesser amplitude in inter-Heinrich stadials than those during an event. However the cooling in the Greenland ice core is much the same between HEs. Therefore ‘…there is need for an amplifying mechanism to generate both the widespread imprint and amplitude of the climatic changes associated with events’ (Elliott et. al 2002), questioning theories that the THC is the sole explanation for millennial-scale cooling events.
Ice Sheet/Glacier Fluctuations and Climate Change
When ice sheets develop, northerly winds take heat from the oceans and freeze them at high latitudes, and when the sheets decay, this system breaks down. It seems that changes in ice sheet morphology and extent have affected albedo feedbacks and therefore atmospheric circulation in high latitudes, causing major regional climate changes. Oxygen Isotope (OI) records have shown that North American ice sheets chilled and seasonally froze the North Atlantic Ocean over
41 ka cycles referred to in Milankovitch’s longer term solar forcing theory. The OI record is widely used in climate studies, and shows climate changes in OI ‘stages’ (figure 2).
(Figure 2 – OI changes for the last 2.6 Ma including some numbering of OI stages, Lowe and Walker, 1997 p. 12)
Ice sheets independent of Milankovitch may have caused climate change, for example a surge of the West Antarctic Ice Sheet at the end of the last interglacial is said to have caused widespread glaciation. Heinrich Events are also evidence of ice sheet fluctuations. Ocean cores, biostratigraphic evidence and Dansgaard-Oeschger cycles in Greenland ice cores (e.g. GRIP).
Shifts in ocean-atmosphere temperatures are grouped into ‘Bond cycles’ (Lowe and Walker, 1997 p. 365) of cooling lasting 10-15 years including many DO cycles. Each cycle ended with a cold stadial then rapid warming after a massive discharge of icebergs, Lowe and Walker (1997) argue this warming results directly from ice sheet behaviour as salinity rises when ice sheets retreat, increasing NADW formation. The warmer phases of DO cycles are also correlated between Heinrich Events, strengthening support for ice sheet forcing of climate change.
Volcanic Activity and Climate Change
Bell and Walker (1992) show that the coldest and wettest summers in Britain appear to have coincided, over the last 3 centuries, with volcanic activity. Dust particles (tephra) temporarily screen incoming solar radiation, and increase cloud formation, for a few days then precipitate quickly because of their size and weight (Zielinski, G.A. 2001 p. 418)). Dawson (1992:10) adds that if dust is forced into the stratosphere, a veil may form, back-scattering or absorbing some isolation. Sulphur, also erupted, is converted to sulphuric acid, being dominant (Bell and Walker, 1992 p. 77) amongst the aerosols of all gases released which back-scatter incoming radiation. Zielinski (2001) shows that erupted sulphuric acid (0.6ºC change) and Carbon Dioxide (1.2ºC change) have strong impacts on climate.
Global winds circulate these aerosols, which have a mean residence time of 1-5 years. As high winds blow poleward from the equator, eruptions in lower latitudes have a greater effect on climate. There is ‘…considerable empirical evidence’ (Bell and Walker, 1992 p. 77) for short term climate change associated with volcanism, as for example the eruptions of Tambara (1851), Krakatoa (1883) and Santa Maria (1902) may have caused a decrease of 0.2-0.3ºC for up to 5 years following the eruptions. As seen in figure 3, the 2000 BP-present Greenland acidity profile appears to negatively correlate to temperature changes over the same period in the Northern Hemisphere.
(Figure 3 – Acidity and temperature changes, Bell and Walker, 1992 p. 79)
There has been cooling after nearly every eruption, although there are also ‘other forces’ (Zielinski, 2001 p. 420) at work, the effects of which may have been accentuated by volcanism. Satellite evidence and other technology, for example the Total Ozone Mapping Spectrometer (TOMS), is used to estimate the quantity of sulphur dioxide erupted.
Volcanism may itself be forced by climate (Zielinski, 2001), as well as forcing climate. This is explained as the greatest increases in volcanism occur at phases of climatic change, whereby crustal loading/unloading of ice sheets may stress the lithosphere enough to increase activity of magma chambers at climatic transitions. This is supported by ice cores in the poles.
(Figure 4 – Volcanic eruptions and temperature changes since 1740, Zielinski, 2001 p. 420)
However, despite there being information from instrumental and other recent data about the role of volcanism which supports a link with climate change, it is necessary to look into the past to understand variability in the system. As Zielinski (2001, p. 434) states; ‘Future work needs to proceed using a multidisciplinary approach to understand…the nature of the volcanism-climate system’.
Solar Forcing and Climate Change
Two main variables are key in considering the role of solar forcing in climate change. These are the fluctuations in solar output (sunspot activity and solar flares) and variations in the quality of solar energy concerning changes in the UV range of the solar spectrum.
It has been suggested (Bell and Walker, 1992 p. 72) that a change of 1% in solar irradiance could cause a 1-2ºC decline in global air temperatures. Sunspots are ‘…conspicuous dark patches that occur as shallow depressions in the general photospheric level…reflecting convectional activity within the photosphere’ (Bell and Walker, 1992 p. 72), thus indicating variations in solar irradiance. Variations (figure 5) in sunspot numbers follow cyclical patterns. Heinrich Schwabe in 1843 suggested the first, an 11-year periodicity which was confirmed later by astronomers and satellites and is also followed by solar flares and other solar processes. The causes of this are unknown, but changes in thermal structure or solar radius may be involved. Over the Holocene other cycles have been found, a 22-year (double sunspot or ‘Hale’ cycle, Bell and Walker, 1992 p.74; Lowe and Walker, 1997 p. 369) cycle, and 80/200-year cycles. Tree rings, ice cores and varved sediments are indicators of this.
It is difficult to relate weather patterns to the 11-year cycle, and longer cycles appear more informative. 14C records show evidence that a decrease in solar activity leads to increased 14C in the upper atmosphere. 14C is found in tree rings which are closely related to climate change for the past 5 ka, and are closely correlated with the sunspot minima named in figure 5. Reconstructing trends, however, is not easy and much is still to be learnt on the Carbon cycle’s links with solar activity.
(Figure 5 – Sunspot cycles with minimum periods highlighted, Beer 2000 p. 409)
As for UV changes (see Bell and Walker, 1992 p. 75; Lowe and Walker, 1997 p. 369), the flux is only approximately 1.05% of the total photon energy from the sun., but it is significant in heating the upper atmosphere in its reaction with Ozone which absorbs it. There is evidence of an 11-year ozone variation almost in phase with sunspots, linked to UV variations. Satellites recently confirmed temporal changes in UV irradiance. Testing this link though, is difficult, and only when longer term records over many cycles are available will conclusions be possible.
Bond et. al (1997; see van Geel et. al., 1999) downplays solar forcing, and claims ocean-atmosphere processes and ice rafting were the principle mechanisms involved in millennial-scale cycles. Van Geel et. al. (1999) refute this, showing that cosmogenic isotopes 14C and 10Be provide further evidence of solar forcing, decreasing as temperature rises. Major increases of 14C at c.850 (calibrated) years BP and c.1600 AD, could not have been caused by Carbon Cycle changes (van Geel et. al., 1999) claim solar forcing is responsible. Figure 6 shows 10Be and δ18O being strongly correlated as well as 10Be being correlated with Dansgaard-Oeschger cycles and Heinrich Events, and since δ18O is an excellent proxy for climatic changes this suggests 10Be is also a good proxy.
The Maunder Minimum (1645-1725) correlates with the Little Ice Age, where an extraordinary sunspot minimum caused strong cooling. Kilian et. al (1995, see van Geel et. al 1999, p. 334) analysed European peats and found evidence of climatic cooling matching a sharp rise in 10Be and 14C.
Van Geel et. al (1999) conclude that the climatic system is more sensitive to solar forcing than previously thought, and also that additional evidence in paleorecords and instrumental observations is needed.
(Figure 6 - 10Be versus δ18O [from GRIP 2 ice core, Greenland] and temperature [from Dye 3 ice core, Greenland], van Geel et. al (1999) p. 333).
Atmospheric Gas Content
Carbon Dioxide, Nitrogen Dioxide and Methane are closely correlated with δ18O values (figure 7), thus showing strong correlations with climate change. Lower concentrations of these gases occur in cold stages (Lowe and Walker, 1997 p. 365-366), and amplitudes of change are seen to be large (e.g. CO2 increase from 200ppmv to 280ppmv in the last glacial-interglacial transition). The mechanisms involved are not completely understood, but a coupling of atmospheric gases and major climate changes over at least the last 130 ka has been suggested (Lowe and Walker, 1997 p. 367). Evidence suggests gas changes continue to be correlated with climate changes. Possible mechanisms must be further investigated.
(Figure 7 – Atmospheric gas content, Lowe and Walker, 1997 p.366)
The Greenhouse Effect
Figure 8 show that the warming in the late 20th century is very rapid indeed. There is considerable debate over the role of CO2, post-industrial revolution, in this. Crowley (2000) argues it is the principal causal mechanism. Chambers et. al. (1999) claim the need for more study into solar forcing, as activity is currently high (figure 5), before the role of the Greenhouse Effect can be understood.
(Figure 8 – Temperature anomaly series, Chambers et. al. 1999 p. 182)
Geomagnetic and Geodynamic Factors in Short Term Climate Change
In geomagnetism, changes in the Earth’s magnetic field are frequently linked to climate change. A weakened magnetic field is thought to correspond (Bell and Walker, 1992) to increased temperature, and a relationship with the 22-year solar cycle has been suspected for a time. The solar wind is said to react with the geomagnetic field, triggering geochemical reactions involving gases of the upper atmosphere, which then control the stratospheric Greenhouse Effect, cloud cover and other factors. However it is difficult to distinguish cause and effect here, therefore the issue is left very open.
Geodynamic factors involve changes in distribution and volume of land ice, causing sea level changes that may have affected the Earth’s angular momentum, causing acceleration as sea level rose and vice-versa. The rates and directions of the Gulf Stream and Labrador/Humboldt currents may have changed. This theory may explain some recent short term climate changes, however it is very difficult to test as there is no proxy for the changes and it cannot explain major Quaternary cyclical changes, although it should not be discounted.
Conclusion
There is still much that is unknown about short term Quaternary climate change. At present there is a growing case for solar and volcanic forcing, alongside arguments in favour of the role of changes in the ocean-atmosphere system, ice sheet fluctuations and other suggested explanations. More recently anthropogenically-produced CO2 from industry and transport has been cited as a major factor in ‘unprecedented’ (Chambers et. al. late-20th century warming, though there is still considerable uncertainty. If a better knowledge of the factors discussed can be achieved, this will strengthen modelling of future changes and provide better support for management strategies.
References
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Beer, J., Mende, W. and Stellmacher, R. (2000) The Role of the Sun in Climate Forcing, Quaternary Science Reviews, 19, 403-415 pp.
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Bell, M. and Walker, M.J.C. (1992) Late Quaternary Environmental Change: Physical and Human Perspectives. London, Longman. 72-79pp.
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Chambers, F.M., Ogle, M. and Blackford, J.J. (1999) Palaeoenvironmental Evidence for Solar Forcing of Holocene Climate: Linkages to Solar Science, Progress in Physical Geography, 23 (2), 181-204 pp.
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Crowley, T.J. (2000) Causes of Climate Change Over the Past 1000 Years, Science, 289, 270-277 pp.
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Dawson, A.G. (1992) Ice Age Earth: Late Quaternary Geology and Climate. London, Routledge. Chapter 10.
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Lowe, J.J. and Walker, M.J.C. (1997) Reconstructing Quaternary Environments. London, Longman. 361-369pp.
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Van Geel, B., Raspopov, O.M., Renssen, H., van der Plicht, J., Dergachev, V.A. and Meijer, H.A.J. (1999) The Role of Solar Forcing Upon Climate Change, Quaternary Science Reviews, 18, 331-338 pp.
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Zielinski, G.A. (2000) Use of Paleo-records in Determining Variability within the Volcanism-Climate System, Quaternary Science Reviews, 19, 417-438 pp.