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
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.
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Sophie Webb
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
18
O. 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.
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
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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.
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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.
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Fig. 2.
δ18O
records
from
Devil’s
Hole in
Nevada are inconsistent with other records and show clear discrepancies between δ 18O and
insolation at 60°N. By the time insolation values begin to rise at 135 ka, δ 18O 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.
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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. The implication that ice age cycles
are driven by variations in solar radiation in the Southern Hemisphere is that there is
an alternative driving mechanism for the cycles.
The authors suggest that an
underlying process in the Southern Hemisphere could cause either tropical oceanatmosphere dynamics and feedbacks to alter, or changes in atmospheric CO2
concentration, although this speculative process is not identified. A supply of warm
southern water to the North Atlantic could diminish production of the NADW,
leading to a weakening of the THC and consequent global climate changes without a
prolonged influence of insolation.
Atmospheric CO2 Concentration
The beginning of crop agriculture corresponds very closely in time with a sudden
warming event at the beginning of the Holocene. Without such anthropogenic inputs,
methane and CO2 levels would have decreased naturally following this warming, as
observed in previous interglacial-glacial cycles, but instead levels remained relatively
high. Ruddiman (2007) has attributed the prolonged warm period of the last 8000
years to the rise of agriculture, and brief periods of minor cooling within the warm
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Fig. 4. Oxygen isotope records for planktonic foraminifera from a) ODP Site 677 and b) ODP Site
1063. Signals from ODP Site 1063 are shown to lag slightly behind those from Site 677. Particular
points of comparison are indicated by red lines. The green line highlights the record at the approximate
time of the mid-Pleistocene transition where there is an obvious time lag between the signals at each
site. Adapted from Ferretti et al. (2005).
period to pandemics leading to massive human mortality. Sudden decreases in human
population resulted in vast areas of agricultural land being abandoned and
subsequently naturally reforested.
Reforestation sequestered carbon and allowed
atmospheric concentrations of CO2 to fall and global temperatures to cool.
Considering the enormity of astronomical variations in Earth’s nature, it seems
unlikely that an increase of 40 ppm of CO2 over the last 8000 years would be the sole
cause of climate change. At an assumed average increase of 0.005 ppm per year,
feedback mechanisms between earth, atmosphere and oceans may have been capable
of mitigating the effect of CO2 by restoring equilibrium in the global system.
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Albedo
The transition between glacial and interglacial conditions could result from snowalbedo feedback. Albedo can be increased or decreased as a consequence of internal
forcing factors. Large volcanic eruptions may expel vast quantities of dust and ash
into the atmosphere. Deposition of such particles over snow can considerably lower
the albedo, which may cause significant melting of ice cover in the Northern
Hemisphere. Large influxes of fresh water into the North Atlantic are believed to
cause the weakening or shutdown of the thermohaline circulation, the consequences
of which would initiate a glaciation.
Initial reductions in insolation due to orbital forcing allow for snow and ice
accumulation in the Northern Hemisphere. Such an increase in ice surface area, and
therefore in albedo, may alter the ambient environment following reflection of
incident solar radiation, leading to reduced local temperatures. The positive feedback
between ice accumulation and albedo would then ensure that temperatures continued
to decline and promote glacial inception, until another factor, such as those previously
described, altered, thus allowing any threshold to change.
Conclusion
Extended and dateable terrestrial and marine records of climate change are required in
order to identify on what timescales past climate change has occurred and also to
identify a suitable analogue of the present climate system. Understanding controls on
global climate is important if impacts and response to future global climate change
can be predicted and prepared for. If glacial cycles are chaotic and not controlled by
orbital forcing mechanisms, predictions of future glacial cycles are rendered highly
uncertain.
Ice albedo feedbacks are a significant influence on global climate due to the global
extent of ice and the links between ice deposition and NADW formation. Although
changes in insolation may be the initial trigger of glacial cycles and may set threshold
boundaries to the global climate system, it is not unlikely that other factors internal to
the Earth’s climate system are the real pacemakers of the ice ages.
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References
Henderson, G.M. and Slowey, N.C., 2000. Evidence from U-Th dating against
Northern Hemisphere forcing of the penultimate deglaciation. Nature, 404, 61-66.
Huybers, P., 2009. Pleistocene glacial variability as a chaotic response to obliquity
forcing. Climate of the Past, 5, 481-488.
Maslin, M.A. and Ridgwell, A.J., 2005. In Head, M.J. and Gibbard, P.L. (eds), EarlyMiddle Pleistocene Transitions: The Land Ocean Evidence. Geological Society,
London, Special Publications, pp 131-145.
Ruddiman, W.F., 2007.
The early anthropogenic hypothesis: Challenges and
response. Reviews of Geophysics, 45, 1232-1243.
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