Have you ever wondered when the next ice age will begin? According to Physicist and fellow blogger Clive Best we may already be past the optimum temperature of the Holocene Interglacial and be sliding back towards the next ice age. Clive has fitted the harmonics of combined Earth orbit cycles to a high resolution temperature record derived from carbonate microfossils from 57 ocean drilling sites. A combination of the 100,000 year eccentricity cycle and 41,000 year obliquity cycle provides an excellent fit to the ocean microfossil temperature record (Figure 1). Since Earth’s orbital cycles are known with precision, this can be used to forecast what comes next. His conclusion is rather chilling.
Figure 1 The black line is based on a stack of 57 d18O temperature records for carbonate microfossils from Atlantic, Pacific and Indian Oceans . The way this is plotted, cold is up and warm (interglacials) is down. The blue line is combined 100,000 year and 41,000 year orbit cycles [2,3]. The excellent fit supports the theory, first proposed by Milankovitch, that variations in orbital parameters have controlled the onset and termination of glacial cycles on Earth for the last 2.6 million years . Projecting this into the future shows that Earth is close to the turning point of the cycle and it is downhill from here towards the next ice age. Chart from Clive Best.
- The LR04 stack of 57 temperature records published by Lisiecki and Raymo  provides one of the highest resolution records for global ocean temperature variation over the last 5.3 million years (Ma). The data provide overwhelming evidence for a major cooling trend that resulted in the onset of extensive N hemisphere glaciation about 2.7 Ma ago .
- Cyclical variations in the LR04 stack bear witness to complex interactions of geological (plate tectonic) and orbital cycles upon Earth’s climate and the relative importance of individual components vary with time.
- Milankovitch orbital cycles have three components 1) eccentricity 100,000 years, 2) obliquity 41,000 years, 3) precession 23,000 years. Clive Best finds the precession cycle is absent from the marine LR04 stack but a combination of eccentricity and obliquity matches the observed LR04 record with some precision [2,3]
- Global cooling resulted in the onset of glaciation about 2.6 million years ago. For 1.7 million years, glacial cycles were dominated by the 41,000 year obliquity orbital cycle. But then, about 900,000 years ago the 100,000 year eccentricity cycle kicked in resulting in longer ice ages, much larger oscillations in temperature with brief and fragile interglacials that last 10 to 20 thousand years. We are 12,000 years into the Holocene interglacial.
- The empirical relationship between orbital cycles and climate change in the past can be used to forecast what will happen next and the result is chilling. It is possible that we reached peak temperature 2000 years ago in the Roman Warm Period  and that we are already heading towards the next ice age [2,3].
- The overall situation is made more complex by the superposition of shorter time scale cycles of approximately 1500±500 years duration in the Holocene known as Bond cycles . Bond cycles are modulated by solar geomagnetic activity and give rise to what we know about more recent historical climate cycles such as The Roman Warm Period.
- The most recent of these cycles is The Modern Warm Period. With solar geomagnetic activity in steep decline, the cold phase of the Bond cycle may now be aligned with the cold phase of the eccentricity and obliquity cycles although it is too early to say yet if the Modern Warm Period is over.
The LR04 benthic foraminifera stack
The Lisiecki-Raymo (LR) 04 stack of benthic foraminifera temperatures is one of the amazing data sets to emerge in recent decades that records details of Earth history. The data come from 57 records of variable duration from the Pacific, Atlantic and Indian Oceans (Figure 2).
Figure 2 The 57 d18O records analysed by Lisiecki and Raymo  which when added together produce the summary stack shown in Figure 3.
The records go back 5.3 million years, but it is really the last 500,000 years that are of greatest interest for understanding recent ice age cycles. Oxygen has three isotopes – 16O, 17O and 18O. In creatures with calcium carbonate skeletons the ratio of 16O and 18O measured relative to a standard (d18O) varies in response to changes in seawater composition and temperature. Temperature dominates. The d18O composition for calcium carbonate shells, therefore, provide a proxy temperature record (Figure 3).
Figure 3 The d18O temperature record for the benthic carbonate stack in blue, left hand scale  and the dD derived temperature record for ice from the Vostok ice core in red, right hand scale [6,7]. The alignment is quite striking but prior to 250,000 years there are clearly time scale calibration issues.
The records shown in Figure 2 have been merged to produce the composite stack shown in Figure 3 (blue line) that may be compared with the temperature record from the Vostok ice core (red line). The Vostok temperature record is based on the hydrogen and deuterium isotope composition of the ice (dD). The similarities of these two records in broad terms is striking although earlier than 250,000 years ago there are clearly some time scale calibration problems resulting in data offset on the x-axis.
Figure 4 The dD records for ice from the Vostok (and Epica) ice cores give the impression of a fairly flat temperature evolution for the Holocene. This observation is partly responsible for the notion that the Holocene climate has been uniform and that recent deviations from static climate may be attributed to Man. The evidence from LR04 is distinctly different with gradually warming ocean temperatures for the last 12,000 years. Note that the resolution on LR04 is 1,000 year intervals, too coarse to capture any recent warming that may have occured.
While the broad alignment of LR04 with Vostok is striking there is also one striking difference and that is during the all important last 12,000 years of the Holocene. Where Vostok (and Epica) show a largely flat temperature evolution for the Holocene, LR04 does not (Figure 4). The benthic carbonate record from 57 sites around the globe shows rapid ocean warming beginning 18,000 years ago. 12,000 years ago the rate of warming slowed but did not stop. The peak (so far) was during the Roman Warm period and there are signs that natural warming has slowed to a near stop in anticipation of the next turn downwards.
Milankovitch orbital cycles
Serbian physicist Milutin Milanković was among the first to suggest that temporal changes in Earth’s orbital geometry around The Sun gave rise to the cyclic rythm of ice ages and interglacials. Clive Best has found that of three orbital parameters it is the 100,000 year eccentricity cycle and the 41,000 year obliquity cycle that have greatest impact on the pattern of ice ages over the last 900,000 years.
Figure 5 Chart slightly adapted from Clive Best [2,3]. On this chart the present day is to the right. The green line shows 100,000 year eccentricity cycle. Zero eccentricity (left hand scale) = circular orbit. Note the 100,000 year cycle is superimposed upon a 400,000 year super-cycle. The black line is the LR04 benthic stack. The arrows show where we are on the eccentricity cycle today and where we were 400,000 years ago. The stage is set for a rapid decline into the next ice age.
Figure 6 Chart slightly adapted from Clive Best [2,3]. On this chart it is the lower blue line that traces out the obliquity cycle (left hand scale). Close scrutiny shows that rapid glacial terminations correspond to rising obliquity (increasing tilt of Earth’s axis) and that ice ages begin with falling obliquity. Note that the current obliquity cycle has already begun to fall.
In addition to the orbital cycle modulation of ice ages and interglacials there are second order impacts upon Earth’s climate, potentially caused by cyclical change in solar activity itself on a time scale of 1500±500 years (i.e. a frequency that varies from 1,000 to 2,000 years). Cyclic variability to climate caused by the Sun is widely contested. These cycles are identified in Greenland ice cores where they are called Dansgaard-Oescheger events  and in the detrital content of North Atlantic ocean sediments where they are called Bond events 
Bond et al  measured the clastic component of deep ocean sediments from 4 sites in the North Atlantic, recognising 3 specific types of debris derived from specific localities and deposited by melting drift ice as it passed “overhead” (Figure 7).
Figure 7 Bond et al  recognise three types of lithic detritus in N Atlantic bore holes that have specific provenance, i.e. Icelandic volcanic glass, detrital carbonate and hematitie stained grains, that are interpreted to have been transported by drift ice. The stack of 4 wells is shown in the lowermost panel.
A picture of cyclic change in drift ice emerges from the combined stack shown as the lowermost panel in Figure 7. In recent centuries, the 4 localities have been receiving virtually no ice rafted debris consistent with the observation that drift ice is largely absent from the North Atlantic today. An intriguing aspect of Bond et al’s work is that the observed cycles correlate with cycles in the cosmogenic nuclide 14C whose production is modulated by the strength of the solar wind and solar geomagentic activity.
This work has for many years intrigued me especially since Bond cycles are easily linked to what we know about historical climate change for the last 2,000 years or more as illustrated in Figure 8.
Figure 8 The Bond stack compared with CO2 from ice cores and Mauna Loa and a number of historical episodes. Skara Brae is a famous Neolithic archaeological site on the Orkney Islands that overlaps with the warm period between Bond cycles 4 and 3. RWP = the Roman Warm Period maps out as an extended anomalous warm period that is perhaps also picked out as a blip on the LR04 benthic stack (Figure 4). Deterioration of climate as NW Europe descended into the Dark Ages (DA) cold period may have contributed to the fall of The Roman Empire. The down pointing arrows mark the expedition of Eric the Red to found Viking settlements in southern Greenland (985 AD) and the loss of the Greenland Knarr in 1380 AD that marked the end of the Greenland settlements. The habitation of Greenland by the Vikings corresponded with the Medieval Warm Period (MWP) that gave way to The Little Ice Age (LIA) and ultimately the Modern Warm Period.
Bond et al interpret their data in terms of cyclical change in the pattern of atmospheric circulation and ocean currents that are linked to cyclical change in solar geomagnetic activity. The activity of the Gulf Stream is particularly important and Bond et al suggest that it is periodically cut off by the cold Labrador current allowing drift ice, blown by northerly winds, to advance much further south into the N Atlantic than occurs at the present time. A larger than previously understood variance in the spectral output from The Sun, linked to changes in geomagnetic activity, may begin to provide understanding of the physical process .
The data and findings of Bond et al have been contested [10, 11]. The data shown in Figure 7 are either real in which case their findings are extremely important, or they are somehow false. Unfortunately Bond is dead and his co-workers have thus far failed to respond to my emails.
The cause of Pliocene-Pliestocene glaciations
The reason why the Earth entered a period of cyclic glacial activity 2.6 million years ago is poorly understood although as shown in Figure 9 this was in part due to a longer term cooling trend.
Figure 9 From Clive Best . On this chart cold is up and warm is down, the present day is to the left. In black is the remarkable LR04 d18O record. Hence this chart shows long term cooling and increasing amplitude of glacial oscillation. In blue are variations in insolation due to orbital parameters. From 5.3 million years to 900,000 years ago, the 100,000 year oscillation is all but absent in the LR04 data which oscillates solely in response to the 41,000 year obliquity cycle. But since 900,000 years ago the 100,000 year cycle kicks in together with the 41,000 year cycle.
One of the enigmatic aspects of the orbital coherence of ice ages is that the changes in insolation are too small to account for the substantial changes in temperatures that occur and so there needs to be some form of amplification mechanism. In ice cores, CO2 concentration, trapped in air bubbles in ice, vary semi-synchronously with temperature. Warmists argue that this is the amplification mechanism hence imparting importance to CO2 as a controlling variable on Earth’s climate. Sceptics point out that CO2 lags temperature and is therefore a response to temperature change and not the cause of it. They also point out that turning points in the temperature record occur at maximum CO2, i.e. cooling phases begin when CO2 is at a maximum. Furthermore, the small changes in CO2 concentration are also insufficient to account for the large swings in temperature between ice ages and interglacials.
It has been suggested by a number of sources that the underlying geological causes responsible for the onset of glaciation are likely to be a combination of plate tectonic events: 1) the closure of the Panama Isthmus that created the American “super continent” believed to have a profound effect on ocean circulation and leading to the establishment of the Gulf Stream; 2) the drift polewards of Antarctica that will have impacted the amount of land ice, sea level and the distribution of sea ice in the southern hemisphere (Figure 9); 3) the uplift of the Tibetan Plateau. It is beyond the scope of this post to delve deeper into the underlying causes of the ice age cycles but informed input in comments would be welcome. Clive has already offered his explanation .
One observation I would make is that the 41,000 year temperature oscillation signal in the oceans is present all the way through the 5.3 million year LR04 record (Figure 9). Hence, rhythmic temperature oscillations were taking place long before the onset of N hemisphere glaciation 2.7 million years ago. Despite major plate tectonic adjustments over the past 5 million years this 41,000 year heart beat is pervasive.
When will the next ice age begin?
Clive Best has combined the eccentricity and obliquity cycles to provide the excellent fit to the observed LR04 stack shown in Figure 1. Since future orbital cycles are known with precision this algorithm can be used to forecast when the next ice age is due to begin. Looking at the detail (Figure 10) suggests we are right on the turning point of the combined orbital cycles. We are at the pinnacle of the climatic optimum and should be grateful for that.
Figure 10 Detail of the recent past and future from Figure 1. Note that in this case time is passing from left to right, cold is up and warm is down. Chart from Clive Best [2,3]
Clive has this to say:
The current glacial cycle has similar orbital parameters to the one occuring 5 glaciations ago. That interglacial warm period lasted 10,000 years or about the length of the current one. We are due for another ice age… Next Ice Age due to start in ~1000 years time.
There is no immediate need to panic since natural climate variability proceeds as a series of bumps that last 100s to 1,000s of years. Northern Europeans should be concerned by the current solar slumber that is reminiscent of recent cold periods such as The Little Age and should be prepared for perhaps 30 years or more of periodic extreme cold winters and have the power generation infrastructure in place that is fit for that purpose.
 Lorraine E. Lisiecki and Maureen E. Raymo 2005. A Pliocene-Pleistocene stack of 57 globally distributed benthic D18O records. PALEOCEANOGRAPHY, VOL. 20
 Clive Best: A challenge for climate models
 Clive Best: Phenomenology of Ice Ages
 Mark Maslin & Jonathan Adams: The onset of Northern Hemisphere Glaciation during the Tertiary and Quaternary
 Gerard Bond et al. 2001. Persistent Solar Influence on
North Atlantic Climate During the Holocene. SCIENCE VOL 294
 Petit, J.R et al, 1999. Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica. Nature 399: 429-436
 NOAA NCDC Vostok Ice Core
 Sarah Ineson et al 2011. Solar forcing of winter climate variability in the Northern Hemisphere. NATURE GEOSCIENCE | ADVANCE ONLINE PUBLICATION
 JOHN T.ANDREWS 2009. Seeking a Holocene drift ice proxy: non-clay mineral variations from the SW to N-central Iceland shelf: trends, regime shifts, and periodicities. JOURNAL OF QUATERNARY SCIENCE (2009) 24(7) 664–676
 Andrews et al 2009. A robust, multisite Holocene history of drift ice off northern Iceland: implications for North Atlantic climate. The Holocene 19,1 (2009) pp. 71–77
 Clive Best: The real cause of Ice Ages – Resonant dust clouds?