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To What Extent Can Orbital Forcing Still Be Seen As the “Pacemaker Of Sophie Webb To what extent can orbital forcing still be seen as the main driver of global climate change? Introduction The Quaternary refers to the last 2.6 million years of geological time. During this period, there have been many oscillations in global climate resulting in episodes of glaciation and fluctuations in sea level. By examining evidence from a range of sources, palaeoclimatic data across different timescales can be considered. The recovery of longer, better preserved sediment and ice cores in addition to improved dating techniques have shown that climate has changed, not only on orbital timescales, but also on shorter scales of centuries and decades. Such discoveries have lead to the most widely accepted hypothesis of climate change, the Milankovitch hypothesis, being challenged and the emergence of new explanations. Causes of Climate Change Milankovitch Theory Orbital mechanisms have little effect on the amount of solar radiation (insolation) received by Earth. However, they do affect the distribution of this energy around the globe and produce seasonal variations which promote the growth or retreat of glaciers and ice sheets. Eccentricity is an approximate 100 kyr cycle and refers to the shape of the Earth’s orbit around the Sun. The orbit can be more elliptical or circular, altering the time Earth spends close to or far from the Sun. This in turn affects the seasons which can initiate small climatic changes. Seasonal changes are also caused by the Earth’s precession which runs on a cycle of around 23 kyr. The effect of precession is influenced by the eccentricity cycle: when the orbit is round, Earth’s distance from the sun is constant so there is not a hugely significant precessional effect. The obliquity, or tilt, of the Earth ranges from approximately 22º to 24º and back every 41 kyr and is currently at around 23.5º. The tilt controls the global distribution of insolation and the 1 Sophie Webb ratio of energy at the equator to energy at the poles. The biggest effect of obliquity is observed at 66ºN, the southernmost boundary of the Arctic Circle. Milankovitch Theory predicts that it is these orbital mechanisms that drive global climate change. Although the 41- and 100 kyr cycles can be clearly observed in climatic proxy data compiled from various locations around the world (Fig. 1), there is increasing opposition to Milankovitch Theory and a number of emerging alternative theories to explain the causes of global climate change. Fig. 1. Benthic δ 18O records from 57 locations around the globe, combined. The amplitude and duration of glacial cycles has increased since the beginning of the record, as shown in fluctuations of δ 18O. Cycles completed during the last 0.8Ma last for around 100 ka, the same duration as the eccentricity cycle. The black line indicates the timing of this Mid-Pleistocene Transition from 41- to 100 ka cycles. Adapted from Lisiecki and Raymo (2005). Challenges to Milankovitch Theory Milankovitch theory has been generally accepted as the most likely cause of growth and retreat of glaciers and studies show consistencies between long-term insolation patterns and variations in global climate. However, there are issues with dating geological events and emerging inconsistencies which have lead to the Milankovitch hypothesis is being challenged. Palaeoclimatic records have revealed a tendency for climatic changes to occur on timescales of centuries or decades which can not be attributed to orbital forcing. Other possible influences on global climate are now thought to include internal mechanisms such as tectonic events, which encompass carbon dioxide concentrations, presence of volcanic dust in the atmosphere and changes in Earth’s topography. 2 Sophie Webb Doubts have been cast over the relative importance of the three orbital mechanisms. The duration of glacial cycles since 0.8Ma is approximately 100 kyr, suggesting that insolation variations due to eccentricity are a dominant forcing factor of global climate. However, variations caused by eccentricity are too small to be the direct cause of Ice Ages suggesting other cycles or factors are significant in driving climate change. Consequently, the obliquity and precession cycles have been explored as the underlying influence on climate due to their effects on solar radiation. As obliquity changes the ratio of solar energy received by the mid and high latitudes, it is plausible that this cycle could be the cause of global climate change. In opposition to the hypothesis that precession is a driver of climate change, other evidence suggests that the deglaciation leading up to MIS 5e, named Termination II, was initiated before precession could cause a significant increase in high latitude summer insolation, raising the possibility that different combinations of forcing mechanisms were in operation. This is referred to as the causality problem. The Causality Problem. Controversial evidence of the MIS 6-5 deglaciation occurring before a peak in insolation has been observed in a number of data points. δ18O records recovered from Devil’s Hole, Nevada, have provided evidence to suggest that the timing of the penultimate deglaciation is incompatible with the chronology according to Milankovitch. The Milankovitch timescale places the midpoint of the MIS 5 deglaciation at 127 ka, but more recent findings from separate sources (coral terraces and Devil’s Hole cave deposits) suggest a much earlier midpoint of up to 142 ka, implying Termination II had an alternative cause to orbital forcing. δ18O from Devil’s Hole is seen to peak well before insolation at 60°N (Fig. 2) which is widely accepted to be a critical point where insolation drives climate change. However, signals from Devil’s Hole correspond very well with Southern Hemisphere insolation. As this is not a critical location with regard to direct influences of insolation on ice sheets or formation of the North Atlantic Deep Water (NADW), another factor must have indirectly caused the termination. Suggestions of possible factors include changes in Southern Hemisphere CO2 concentration or variations in the tropical ocean-atmosphere system dynamics. 3 Sophie Webb The Mid-Pleistocene Transition (MPT) Approximately 0.8 Ma, glacial cycles began to occur over 100 kyr instead of 41 kyr as during the early Quaternary (2-0.8 Ma). It has been assumed that eccentricity is the primary driver of the 100 kyr cycle, although a major problem with this theory exists: insolation variations attributable to the 100 kyr eccentricity cycle are extremely small (approximately 0.03% of total annual insolation) when compared to insolation associated with the precession and obliquity cycles. The change in periodicity does not coincide with any significant change in external forcing. This implies that either internal climate feedback must have changed or, as suggested by Huybers (2009), the transition from 41- to 100 kyr cycles was spontaneous, independent of variations in atmospheric CO2 or other internal controls. While hypothetically possible, the theory does not address glacial inception or long term trends in glacial cycles. If spontaneous shifting were to be responsible for the MPT, it might be expected that such an event would be observed elsewhere in the palaeoclimatic record considering how much data has been recovered. Also, if the change was spontaneous then the shift from 41- to 100 kyr cycles would be more abrupt, but as indicated by δ18O in Fig. 1, the amplitude and duration of glacial cycles changes relatively gradually between approximately 3Ma and 1Ma. The time of the MPT is accompanied by a change in the oxygen isotope composition of benthic foraminifera that would suggest greater ice volume or decreased deep-water temperatures. However, spectral peaks from benthic foraminiferal oxygen isotopes occur every 100 kyr whereas spectral peaks for eccentricity occur at a range of ages: 95 kyr, 125 kyr and 400 kyr. If eccentricity was to be the cause of global climate change, foraminiferal oxygen isotope data would have split peaks to correspond to the 95- and 125 kyr peaks of eccentricity. However, this is not the case, suggesting eccentricity is not the primary influence on ice volume or deep-water temperatures. It has been suggested by Maslin and Ridgwell (2005) that glacial cycles following the MPT are most closely linked to precession, but are paced by eccentricity which may place thresholds on the climate system. Only when the eccentricity cycle is at a crucial point will there be a significant effect of precession. 4 Sophie Webb Fig. 2. δ18O records from Devil’s Hole in Nevada are inconsistent with other records and show clear discrepancies between δ18O and insolation at 60°N. By the time insolation values begin to rise at 135 ka, δ18O has almost reached a peak (indicated by red line). However, there are definite similarities between the Devil’s Hole record and that of insolation at 60°S. Adapted from Henderson and Slowey (2000). Non-linear Climate Responses The relationship between Earth’s orbit and climate cannot be assumed to be linear due to the effects of internal feedback mechanisms. Various mechanisms involving changes in ocean circulation, atmospheric greenhouse gas concentrations, snow and ice cover (albedo) and the rise of the Himalayas in Tibet have been suggested to explain these sub-Milankovitch variations. 5 Sophie Webb Smaller scale climate change Sub-Milankovitch scale climate fluctuations are evident in palaeoclimatic records. Such fluctuations occurred on shorter time scales and on smaller spatial scales than the long term, global variations associated with orbital forcing mechanisms, implying other causal factors. Changes in the ocean-atmosphere circulation One of the main proposed feedbacks thought to amplify climatic changes caused by eccentricity involves ocean circulation, both directly through surface and sub-surface (thermohaline) heat transport and indirectly through its ability to store and release CO2. Henderson and Slowey (2000) provide evidence of the penultimate deglaciation having been initiated in the Southern Hemisphere.
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