Introduction


The Earth's climate underwent massive changes from the end of the Last Glacial Maximum (LGM) to the current warm period or Holocene, approximately 19,000 to 11,000 years ago. During the LGM temperatures in East Antarctica were approximately 9 to 10 °C lower than today. In Greenland average temperatures were 15 °C lower. About 17,500 years ago CO2 concentrations began to increase. By the end of the deglacial period around 11,700 years ago atmospheric CO2 concentrations had increased by 80 to 100 parts per million by volume (ppmv). Methane levels also began to rise about 17,500 years ago but did not follow the pattern of atmospheric CO2 increase indicating CO2 and methane had different sources.

The last deglaciation was punctuated by many short term warming and cooling events which averaged about 1,500 years in duration. During these events the climates of the Northern and Southern Hemispheres behaved in opposite ways. This is referred to as the hemispheric see-saw. During northern cooling events, the Southern Hemisphere warmed and during northern warming periods the south cooled. The largest northern events 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 at the end of the Oldest Dryas and beginning of Bølling-Allerød raised average temperature in Greenland by about 9 °C. At the end of the Younger Dryas temperatures increased by about 10 °C.

Observations


Sites used in global surface temperature reconstruction
Sites used in global surface temperature reconstruction
A network of high-resolution proxy temperature records with broad geographical coverage was compiled and an area-weighted mean was used to reconstruct global surface temperature during the last deglaciation. To investigate regional effects, surface temperatures reconstructions were developed for each hemisphere.

Results


The global temperature reconstruction exhibits a two-step rise with most warming occurring during and right after the Oldest Dryas and Younger Dryas intervals. The atmospheric CO2 record from the EPICA Dome C ice core has a similar two-step structure and is strongly correlated with the temperature reconstruction.

CO2 concentration and global surface temperature
CO2 concentration and global surface temperature

Global surface temperature (blue), Antarctic ice-core temperature (red), and atmospheric CO2 concentration (yellow dots).
Holocene, Younger Dryas (YD), Bølling–Allerød (B–A), Oldest Dryas (OD) and Last Glacial Maximum (LGM) intervals are indicated.
The phasing of CO2 concentration and temperature for the global (grey), Northern Hemisphere (NH; blue) and Southern Hemisphere (SH; red) based on lag correlations from 20–10,000 years ago.

The results reveal a strong correlation between global surface temperature and atmospheric CO2 concentration throughout the last deglaciation. The evidence shows that CO2 led global surface temperature throughout the deglaciation. The temperature lag behind CO2 is ascribed to the thermal inertia of the climate system owing to ocean heat uptake and ice melting. Including methane and nitrous oxide leaves the correlation with the global temperature reconstruction essentially unchanged and slightly decreases the temperature lag behind the rising greenhouse gases. Based on this evidence it is concluded that rising CO2 concentration preceding global temperature is consistent with CO2 acting as a primary driver, though not the trigger, of global warming.

Regional warming


Each hemispheric temperature shows the same two-step warming as in the global surface temperature reconstruction. however, comparing atmospheric CO2 with Southern Hemisphere surface temperature reveals that the rise in atmospheric CO2 lagged the rise in the southern regional temperature by roughly a thousand years. This finding suggests that CO2 was not the trigger of the initial warming at the beginning of the deglacial period.

The other key finding relates to the seesaw effect where the Northern Hemisphere exhibits moderate cooling coincident with the onset of Southern Hemisphere warming. Protactinium/thorium ratios (Pa/Th) from sea sediments are a proxy for the strength of the Atlantic south/north current or AMOC. A strong correlation was found between the hemispheric temperature changes and sea sediment Pa/Th ratios. The results indicate that temperature decreased during the Oldest Dryas and Younger Dryas intervals, when the Pa/Th record indicates that the Atlantic south/north current was weak. On the other hand, temperature increases were observed during the Bølling–Allerød and the Holocene periods, when Pa/Th record indicate the Atlantic current was stronger. This implies that when the Atlantic south/north current is strong, heat is transported from the south to the north, warming the north and cooling the south.

The evidence from the temperature proxies at 80 sites reveals that global warming before 17,500 years ago occurred in two phases. There was a gradual increase between 21,500 and 19,000 years ago, and then a steeper increase between 19,000 and 17,500 years ago. The data shows that the first increase is associated with warming of the northern mid to high latitudes, particularly in Greenland. The second increase occurred during a hemispheric seesaw event, associated with a reduction in strength of the Atlantic south/north current as recorded in the Pa/Th record, in which southern warming coincided with northern cooling.

A plausible account of the last deglaciation


A plausible mechanism was suggested to explain this sequence of events. Rising northern summer insolation initiated northern warming. This led to the retreat of Northern Hemisphere ice sheets and the associated increase in sea level commencing about 19,000 years ago. Fresh water from melting glaciers pouring into the North Atlantic reduced the strength of the Atlantic south/north current. The resulting see-saw led to cooling in the north and warming in the southern ocean. About a thousand years after warming began in the south, reduced ocean stratification in the Southern Ocean resulted in CO2 ventilation from southern ocean deep water. Rising CO2 levels amplified the warming ultimately leading to the present warm period or Holocene.

Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation, Jeremy D. Shakun, Peter U. Clark,F eng He, Shaun A. Marcott, Alan C. Mix, Zhengyu Liu, Bette Otto-Bliesner, Andreas Schmittner & Edouard Bard, Nature, 484, 49–54 (05 April 2012) doi:10.1038/nature10915