Background


The global climate underwent massive changes from the end of the Last Glacial Maximum (LGM) approximately 19 thousand years ago to 11 thousand years ago. During the LGM temperatures in East Antarctica were approximately 9–10 °C lower than today. In Greenland average temperatures were 15 °C lower. Small variations in Earth's orbit and tilt caused the amount of solar radiation (insolation) hitting the Northern Hemisphere in the summer to begin to increase about 19,000 years ago. This increase in summer insolation reached a maximum about 11,000 years ao. Both temperature and atmospheric CO2 concentration began to increase about 17,500 years ago. Over the period of the deglaciation, atmospheric CO2 concentrations increased by 80 to 100 parts per million by volume (ppmv). CH4 levels also began to rise starting at 17,500 years ago. Atmospheric warming due to greenhouse gases (GHGs) is dominated by CO2, but abrupt changes ascribable to changes in CH4 and N2O concentrations can be discerned in the paleoclimate record.

The last deglaciation was punctuated by several short term warming and cooling events which averaged about 1,500 years in duration. The largest were the Oldest Dryas (19-15,000 years ago), Younger Dryas (13-12,500 years ago) and intervening Bølling-Allerød period. The very abrupt warmings in Greeenland at the end of the Oldest Dryas and beginning of Bølling-Allerød raised average temperature on Greenland by about 9 °C. At the end of the Younger Dryas temperatures increased by about 10 °C.

The most important north/south Atlantic current brings warm surface waters from the Southern Ocean to the north, where increasing saltiness (salinity) causes it to drop to the ocean floor to form deep water which then flows south. The current, called the Atlantic Meridional Overturning Circulation or AMOC, was affected by large inflows of meltwater and by iceberg calving from melting northern ice sheets. The paleoclimate record reveals a number periods of massive rafting of icebergs across the Northern Atlantic called Heinrich events (HS). Sedimentary protactinium/thorium ratios (Pa/Th) ratios provide a proxy for the strength of past ocean circulation.

Evidence of climate change during the last deglaciation
alt="Evidence of climate change during the last deglaciation"
Evidence of climate change during the last deglaciation
(A) Delta-oxygen-18 records from Greenland - GISP2 (dark-blue line) and GRIP (light-blue line)
(B) Delta-oxygen-18 record (dark-green line) and delta-deuterium record (light-green line) from EPICA Dome C.
(C) July insolation at 65°N (orange line) and January insolation at 65°S (light-blue line).
(D) Combined atmospheric heating (red line) from CO2 (blue dashed line), CH4 (green dashed line), and N2O (purple dashed line).
(E) Relative sea-level data from Bonaparte Gulf (green crosses), Barbados (gray and dark-blue triangles), New Guinea (light-blue triangles), Sunda Shelf (purple crosses), and Tahiti (green triangles) and sea level (gray line).
(F) Rate of change of area of Laurentide Ice Sheet (LIS) and Scandinavian Ice Sheet (SIS).
(G) Freshwater flux to the global oceans.
(H) Record of ice-rafted detritus in North Atlantic identifying times of Heinrich events 1 and 0.
(I) Freshwater flux from continental runoff through the St. Lawrence and Hudson rivers (filled blue time series). Runoff through the St. Lawrence River during the Younger Dryas (solid blue line).
LGM Last Glacial Maximum; OD Oldest Dryas; BA Bølling–Allerød; ACR Antarctic Cold Reversal; YD Younger Dryas.

Observations


Sea and land surface temperature and precipitation patterns were analyzed from 166 geographically distributed paleoclimate records of proxies for either temperature (sea surface or continental) or precipitation for the interval 20-11,000 years ago. Proxies came from a variety of sources including ice cores, sea floor sediments, pollen, cave calcite records, and sea phytoplankton records. For example, alkenones, which are organic compounds produced by a particular type of phytoplankton, are used as a proxy for surface sea temperature. The ratio of oxygen 18 to oxygen 16 (delta-oxygen-18) is used as a proxy for land surface temperature. Other proxies include ice core delta-oxygen-18, pollen, cave calcite delta-oxygen-18, sea floor sediments, and others. Only records of with resolution better than 500 years and with dating from at least several radiometric sources were included. Radiometric dating, including radiocarbon, potassium-argon, and uranium-lead dating, is based on the decay on naturally occurring radioisotopes and is considered the most reliable form of dating.

Geo-temporal analysis


The analytical technique used to analyze the geographically distributed sea surface temperature data to identify geospatial and temporal patterns during the last deglaciation is called principal component or empirical orthogonal function (EOF) analysis. It is a way of decomposing geospatial-temporal data sets to separate physical processes from background noise. By analogy to Fourier analysis of radio signals, it is conceptually similar to extracting low frequencies that carry a message from high frequencies that represent noise. The EOF analysis identifies the dominant spatial patterns and principal time series which explain the greatest amount of variability in the sea surface temperature (SST) record during the last deglaciation.

Sea surface temperature (SST) analysis

69 high-resolution sea-surface temperature proxy records spanning the period 20–11,000 years ago were compiled. The data shows that warming trends were smallest at low latitudes (1–3°C)and higher at higher latitudes (3–6°C). This data was subjected to a EOF analysis to extract the dominant trends, temporal and geospatial, that accounted for the greatest sea surface temperature variability. The analysis revealed that two trends accounted for 78% of the variability in the global sea surface temperature during the period 20-11,000 years ago.

  • The first trend is geographically uniform. Its associated principal temporal component (PC1) displays a two-step warming pattern with the first extending 18–14,300 years ago, followed by a plateau, and the second warming resuming during 12,800–11,000 years ago. It is the the most important component and accounts for nearly 50% of the variance in the sea surface temperature.

  • The second trend is more complex geographically, but its associated principal temporal component (PC2) oscillates with decreases during the Oldest and Younger Dryas cooling events and separated by an increase during the Bølling–Allerød warming period. It accounts for less of the variance than PC1, explaining less than a third of the variance.

Principal component analysis for SST
Principal component analysis for SST

(A) Principal component (PC1) based on all of the SST records (solid blue line). PC1s based only on alkenone (dashed light-blue line) and Mg/Ca records (dashed orange line) are also shown. The percentage of variance explained by PC1 is 49%, by PC1 (Mg/Ca) is 59%, and by PC1 (UK037)is 64%.
(B) PC2 based on all of the SST records (solid blue line). PC2s based only on alkenone (dashed light-blue line) and Mg/Ca records (dashed orange line) are also shown. The percentage of variance explained by PC2 is 29%, by PC2 (Mg/Ca) is 13%, and by PC2 (UK037) is 15%.

Analysis for sea and land surface temperature and precipitation

Applying a similar analysis to land surface temperature and precipitation data sets result in principal components for land surface temperature and precipitation during the last deglaciation.

Principal component analysis for sea and land temperature and precipitation
Principal component analysis for sea and land temperature and precipitation

Global principal components for temperature (T) and precipitation (P). PC1s are shown as blue lines, PC2s as red lines

The analysis indicates that, as in the case of sea surface temperature, two trends explains much of the variability (64–100%) in regional and global climate during the last deglaciation.

Principal component analysis and comparison with other sources
Principal component analysis and comparison with other sources

(A) Comparison of the global temperature PC1 (blue line) with atmospheric CO2 from EPICA Dome C ice core (red line)
(B) Comparison of the global temperature PC2 (blue line) with Pa/Th record (proxy for AMOC strength) (green and purple symbols). Also shown are freshwater fluxes from ice-sheet meltwater, Heinrich events, and routing events.
(C) Comparison of the global precipitation PC1 (blue line) with atmospheric methane (green line) and atmospheric heating from greenhouse gases (red line)

Results


The EOF analysis identified two major physical trends that influenced surface temperatures and precipitation during the last deglaciation. The most important is a global warming trend that started at about the time of increasing Northern Hemisphere insolation due to orbital forcing. Two warming periods between 18,000–14,300 and 12,800–11,000 years ago brought global temperatures to pre-industrial levels. This trend correlates strongly with increasing greenhouse gas levels. The data revealed that the beginning of rising atmospheric CO2 concentration lagged the start of Antarctic warming by 800 or fewer years.

The second physical trend was found to be remarkably similar to a North Atlantic Pa/Th record that has been interpreted as a proxy for the strength of the primary north/south Atlantic current. This suggests a strong coupling between sea surface temperature and ocean circulation. It is thought that large inflows of meltwater in the north led to reduced current strength in the Atlantic north/south current which increased cooling in the north and warming in the south - referred to as the hemispheric sea-saw. For example, the large reduction in the north/south Atlantic current during the Oldest Dryas can be explained as a response to a large meltwater pulse about 19,000 years ago from Northern-Hemisphere ice sheets. This is associated with Heinrich event HS-1. The reduction in the strength of the current during the Younger Dryas was likely caused by a freshwater pulse through the St. Lawrence River and is associated with Heinrich event HS-0.

Interpretation


The low concentrations of atmospheric CO2 during the LGM are thought to have been caused by greater storage of carbon in the deep ocean through stratification of the Southern Ocean. Southern Ocean deep waters were the source of the most dense and salty waters in the LGM deep ocean. Deglacial warming is thought to have begun in the Southern, Indian, and equatorial Pacific oceans. Ventilation of the sequestered carbon into the atmosphere may have occurred due to deep Southern Ocean overturning caused by increased wind-driven upwelling, sea-ice retreat and reduced albedo associated with Antarctic warming. As a result of the hemispheric sea-saw these periods of warming in the south coincided with the Oldest and Younger Dryas cold events in the north. The North Atlantic and North Pacific ocean temperatures began to increase later and experienced more pronounced millennial-scale variability due to variations in the north/south Atlantic current corresponding to the Oldest Dryas-Bølling-Allerød-Younger Dryas events.

Supplementary Notes

Measuring climate change during the last deglaciation

Sources

Global climate evolution during the last deglaciation, Peter U. Clark, Jeremy D. Shakun, et al., Proceedings of the National Academy of Sciences 2012 vol. 109 no. 19 E1134–E1142