Editor's commentsFor the past 30 years, the Milankovitch hypothesis, which posits that the Earth's climate is controlled by variations in incoming solar radiation which are determined by small, predictable changes in the Earth's orbit about the sun, has been widely accepted by the scientific community.
Orbital forcing is believed to initiate transitions between glacials and interglacials. Although astronomical frequencies corresponding to Milankovitch cycles are found in almost all paleoclimatic records, the different orbital configurations make each glacial and interglacial period unique. The climatic system does not respond linearly to changes in the Sun's radiation impinging on Earth. There is still little understanding of how exactly CO2 concentration and ice sheet growth and melting are influenced by changes in the Sun's radiation (insolation) impinging on Earth.
  • ‘100,000 year problem’. The intervals between glacial terminations over the past million years range from 84,000 to 120,000 years, or roughly every 100,000 years. But the variation in insolation computed from the astronomical model does not does not predict a strong signal at this frequency.
  • There is no simple relation between the amplitudes of the insolation extrema and the corresponding ice volume extrema.
  • The ‘400,000 year problem’. The amplitude of summer high latitude insolation variations is maximum every 400,000 years, due to the dominance of this periodicity in the eccentricity modulation of the precessional forcing, but this frequency is not found in paleoclimatic records.
  • The phase relationship between terminations and the corresponding insolation extremum may not be constant through time. For example, Termination II was in advance of the insolation maximum, the opposite seems to be the case for termination III.

Greenhouse gas concentrations from the paleoclimate record

Antarctica ice coring locations
Past changes in atmospheric temperatures and greenhouse-gas concentrations can be determined with very high confidence from polar ice cores. AR5 states that it is a fact that the ice core data shows that present-day (2011) concentrations of the atmospheric greenhouse gases carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) exceed the range of concentrations recorded in ice cores during the past 800,000 years.

The 2007 Fourth Assessment Report (AR4) stated it was very likely that current atmospheric concentrations of CO2 (379 ppm) and CH4 (1,774 ppb) exceeded by far the natural range of the last 650,000 years. Ice core data indicate that CO2 varied within a range of 180 to 300 ppm (parts per million) and CH4 within 320 to 790 ppb (parts per billion) over this period. Since AR4 these ice core records for the Antarctic have been extended. For example, the records from EPICA Dome C have been extended from 650,000 years to 800,000 years ago. The atmospheric abundance of CO2 was 390 ppm in 2011. Atmospheric N2O was 324 ppb in 2011. Atmospheric CH4 was 1800 ppb in 2011.

Temperature and CO2 concentration for the past 800 000 years from EPICA Dome C ice cores (Antarctica)

Glacial terminations

Black step curveTemperature anomaly record with respect to the mean temperature of the last millennium
italic Arabic numerals Marine Isotope Stages (MIS)
Solid circles CO2 concentration from ice cores from Dome C, Taylor Dome, Vostok
Roman numerals in subscript Glacial terminations
Horizontal lines Mean values of temperature and CO2 for the time periods 799–650,000, 650–450,000, 450–270,000 and 270–50,000 years ago

Changes in greenhouse gases during last deglaciation

During the interval of global warming from the end of the Last Glacial Maximum (LGM) about 20,000 ago to about 10,000 ago, the Earth's climate system experienced large-scale change. The global mean temperature increased very likely by 3°C to 8°C. It is believed that this dramatic global change was triggered by changes in solar radiation (orbital forcing) and amplified by changes in ice sheets and increased greenhouse gas concentrations.

Considerable ice-sheet melting and sea-level rise occurred after about 10,000 years ago, but otherwise by that time the world had entered the current interglacial period with near-preIndustrial greenhouse gas concentrations and relatively stable climates.

During the last deglaciation, there were several episodes of large and rapid sea-level rise and abrupt climate change.

During last deglaciationReliabilityRate of warming
MeanVery likely0.3°C to 0.8°C per thousand years
HighestLikely1°C and 1.5°C per thousand years

AR5 states with medium confidence that the current rate of the observed greenhouse gas concentration increase is unprecedented compared with the lower resolution records from ice cores of the past 800,000 years.

AR5 states with very high confidence that based on the the highest resolution ice core records of the last 22,000 years during the last deglaciation the rates of CO2, CH4 and N2O increasing concentration never reached current rates.

Periods when CO2 concentration exceeded present levels

According to AR5 with medium confidence the paleoclimate record shows that there were two earlier periods with higher than present atmospheric CO2 concentrations that were warmer than present.

  • During the mid-Pliocene (3.3 to 3.0 million years ago), atmospheric CO2 concentrations between 350 ppm and 450 ppm occurred when global mean surface temperatures were approximately 2°C to 3.5°C higher than for pre-industrial climate.

  • During the Early Eocene (52 to 48 million years ago), atmospheric CO2 concentrations exceeded ~1000 ppm when global mean surface temperatures were 9°C to 14°C higher than for pre-industrial conditions.

In each of these two cases, warming was likely strongly amplified in the Arctic.

CO2 Concentration for the Past 65 Million Years

AR5 states that it is virtually certain that variations in solar radiation hitting the Earth will be unable to trigger widespread glaciation during the next 1,000 years because for orbital configurations close to present-day, the paleoclimate record shows that ice ages have only started when atmospheric CO2 concentrations were significantly lower than pre-industrial levels.

CO2 concentration for the past 65 million years

Symbols with error bars Observed CO2 concentration for the past 65 million years
long-dashed grey line Present (2012) CO2 concentrations
short-dashed grey line Pre-industrial CO2 concentrations
light blue shading Uncertainty band (1-standard deviation)

Glacial-interglacial cycles

One of the questions that has as yet been unanswered is what causes the glacial-interglacial cycles (roughly every 100 000 years) of the past million years. The most important external factors that are considered potential candidates are variations in solar radiation, volcanic activities and changes in the Earth's orbit and orientation relative to the Sun ("orbital forcing").

Orbital forcing refers to the changes in the incoming solar radiation caused by variations in Earth´s orbital parameters as well as changes in its axial tilt. Orbital forcing can be precisely computed from astronomical calculations for the past and future. Changes in the Earth's orbital parameters affect the seasonal and latitudinal distribution and magnitude of solar energy received at the top of the atmosphere, and the durations and intensities of local seasons.

AR4 reported that over the past 650,000 years, antarctic temperature and CO2 concentrations are closely correlated, indicating a close relationship between climate and the carbon cycle. AR5 states with high confidence that changes in atmospheric CO2 concentration play an important role in glacial-interglacial cycles. (Ice core data indicate that CO2 concentrations varied within a range of 180 to 300 ppm and CH4 within 320 to 790 ppb over the past 650 000 years.) It is generally accepted that while glacial-interglacial CO2 variations have strongly amplified climate variations, there is little evidence that CO2 variations have triggered the end of glacial periods.

It is generally accepted that while the primary driver of glacial interglacial cycles is the seasonal and latitudinal distribution of incoming solar energy driven by changes in the geometry of the Earth’s orbit around the Sun (orbital forcing), the full magnitude of glacial interglacial temperature changes cannot be explained without accounting for changes in atmospheric CO2 content and the associated climate feed backs including the dynamics of ice sheets.

Comparison of variations in solar radiation, EPICA Dome C ice core data (Antarctica) with other palaeoclimatic records

Glacial terminations

a Solar radiation records.
upper blue curve Mid-July at 65°N - left axis
lower black curve Annual mean at 75°S (the latitude of Dome C) - right axis
b Temperature proxy (δD) from EPICA Dome C (3,000 yr averages).
red Vostok δD is shown for comparison
c Marine oxygen isotope (δO18) record, a proxy for temperature
solid blue line From sites MD900963 and ODP6773
dashed red line From seven other sites for the last 400.000 years ago but from only one site before 400,000 years ago
d Dust from EPICA Dome C

Sea level rise and ice sheet melting

The paleoclimate record shows that mean sea level was above the present during periods of the past few million years that were warmer than present.

Comparison of atmospheric CO2 concentration, tropical surface temperature, Antarctica surface temperature, deep-ocean temperature, and sea level.

d Atmospheric concentration of CO2 from Antarctic ice cores
e Tropical surface temperature
f Antarctic temperature based on different ice cores
g Sea floor δ18O (radioisotope oxygen 18), a proxy for global ice volume and deep-ocean temperature
h Reconstructed sea level
i Rate of change of global mean temperature during the last deglaciation
For other details of the chart see AR5 Chapter 5.

According to AR5 there is high confidence that the volumes of the Greenland and West Antarctic ice sheets were reduced during those periods of the past few million years that were globally warmer than present. For comparison total deglaciation of these ice sheets would result in sea level rise of about 70 meters.

A sea level rise corresponding to total deglaciation of these two ice sheets has not been observed in the paleoclimate record of the past few million years. However partial deglaciation has occurred at least twice.

  • Mean sea level was above the present during some warm intervals of the mid-Pliocene (3.3 to 3.0 million years ago). AR5 states with high confidence that the best estimates from various methods suggest that sea level rise did not exceed +20 m during the warmest periods of the Pliocene implying partial deglaciation of the Greenland and Antarctica ice sheets.

  • AR5 states with high confidence that the maximum global mean sea level rise during the interglacial period (Eemian, 129,000 to 116,000 years ago) prior to the last ice age was at least 5 m but did not exceed 10 m above present. The best estimate is 6 m higher than present. Since the paleoclimate record suggests that warming is amplified in northern latitudes this implies a significant deglaciation of the Greenland Ice Sheet.

To put this in context at the last glacial maximum (LGM), sea level was about 120 m below present. For further comparison total melting of these ice sheets would result in about 70 m sea level rise.

Ice sheetIce volume (km**3)Maximum sea level rise (m)

Ocean currents

AR% states that another important internal factor affecting climate is ocean currents. The meridional overturning circulation is a system of surface and deep currents encompassing all ocean basins. It transports large amounts of water, heat, salt, carbon, nutrients and other substances around the globe, and connects the surface ocean and atmosphere with the huge reservoir of the deep sea. As such, it is of critical importance to the global climate system.

Ocean currents play an important role in supplying heat to the polar regions, and thus in regulating the amount of sea ice in these regions. Changes in Ocean circulation, such as the Atlantic Meridional Overturning Circulation (AMOC) which forms the ocean's deep water, are thought to have significant impacts on the Earth's radiation budget. Changes in temperature and salinity governs the rate at which deep waters are exposed to the surface and it may also play an important role in determining the concentration of carbon dioxide in the atmosphere.

One of the important areas of reserach is the extent to which the AMOC can be disrupted by events such as a massive influx of freash water from a suddenly melting glacier. The AR5 report states with high confidence that during a deglaciation period the paleoclimate record shows that the Atlantic Ocean meridional overturning circulation(AMOC) can recover from a short-term freshwater input into the subpolar North Atlantic.

Lake Agassiz and Lawrentide Ice Sheet collapse
The basis for this is an analysis of a cold event about 8,200 years ago which enabled researchers to study the recovery time of the AMOC as a result of massive fresh water flow (meltwater pulse MWP-1C) into the North Atlantic freshwater under near-modern conditions.

It is believed that the cold event resulted from the sudden drainage of glacial Lakes Agassiz and Ojibway which releasing a large volume of fresh water into the North Atlantic within a very short period of time.

A rapid sea level rise event occurred 8,200-7,600 years ago. Though it only produced about 1 meter of global sea level rise, it was first identified by a hiatus in coral growth in the Caribbean. Additional support has come from stratigraphic data from several sources.

The records from ice cores in Greenland indicate that a cold event occurred around 8,200 years ago. It was the most significant climate-change event of the past 10,000 years. At its maximum it produced a 5°C drop in mean temperature. A variety of terrestrial and aquatic proxies indicate that this was a Northern Hemisphere event.

Meltwater event at 8.2 ka

Paleoenvironmental and climate model data for the abrupt Holocene cold event at 8,200 years ago (8.2 ka).
Vertical blue bar brackets published age constraints for the period of release of freshwater from glacier lakes Agassiz and Ojibway.
Vertical grey bar denotes the time of the main cold event as found in Greenland ice-core records.
(a) Black curve: Greenland ice-core temperature proxy
(b) North Atlantic/Nordic Seas sea surface temperature (SST) reconstructions.
Blue curve: Nordic Seas.
Black curve: Gardar Drift south of Iceland.
(c) Deep- and intermediate-water records.
Black curve: overflow strength proxy (silt record) from Gardar Drift south of Iceland, Atlantic intermediate water temperature reconstruction.
(d) Black curve: deep water ventilation proxy at 3.4 km water depth south of Greenland
For details of other curves see AR5 Chapter 5.

The additional freshwater that entered the North Atlantic during the 8.2-ka event is estimated between 160,000 and 800,000 cubic km. The duration of the melt water release was very rapid on the order of two centuries or less.

AR5 reports on new proxy records that confirm that the pattern of surface-ocean and atmospheric climate anomalies is consistent with a reduction in the strength of the AMOC as a result of freshwater flow into the North Atlantic. Available proxy records from the North Atlantic support the hypothesis that freshwater input into the North Atlantic reduced the amount of deep and central water-mass formation. A concomitant decrease of sea surface temperature (SST) and atmospheric temperatures in the North Atlantic and in Greenland has been observed. The climate anomaly associated with the event lasted 100–160 years.