Location of sites used for global temperature reconstruction over last deglaciation Shakun et al Nature

Rising CO2 concentration drove but did not trigger warming during last deglaciation

There is a solid body of paleoclimatic evidence that over 800,000 years CO2 concentration and the Earth’s surface temperature have been closely correlated.  But the time resolution of observations from ice cores and other sources of information about the Earth’s paleoclimate have not been able to determine whether CO2 concentration leads or lags Earth’s surface temperature in particular at glacial terminations. In this article the authors construct a record of global surface temperature from temperature proxy observations and show that during the last deglaciation global surface temperature is correlated with but generally lags CO2 concentration. However, they find that at the beginning of the deglaciation a global warming of about 0.3 °C preceded the initial increase in CO2 concentration.  This suggests that rising CO2 concentration amplified but did not initiate deglacial warming. To investigate regional effects separate temperature reconstructions were developed for the Northern and Southern Hemispheres. It was found that in the Southern Hemisphere the rise in temperature preceded rising CO2, whereas in the Northern Hemisphere increasing temperature lagged CO2.

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 characterized 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.

Surface temperatures and CO2 concentation

Location of sites used for global temperature reconstruction over last deglaciation Shakun et al Nature
Location map of sites used for global surface temperature reconstruction over last deglaciation. Distribution of the records by latitude (grey histogram) and areal fraction of the planet in 5° steps (blue line) is also shown.

The authors calculated the area-weighted mean of 80 globally distributed, high-resolution proxy temperature records to reconstruct global surface temperature during the last deglaciation.  To investigate regional effects, surface temperatures reconstructions were developed for each hemisphere.  This is compared to Antarctic surface temperature and atmospheric CO2 concentration reconstructed from the EPICA Dome C ice core record.

The global temperature reconstruction shows 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 and temperature during the last deglaciation
Comparison of global surface temperature (blue), Antarctic surface temperature (red) and atmospheric CO2 concentration (yellow dots). The Holocene, Younger Dryas (YD), Bølling–Allerød (B–A), Oldest Dryas (OD) and Last Glacial Maximum (LGM) intervals are indicated.

The analysis revealed that CO2 probably leads global warming over the course of the deglaciation. A comparison of global temperature with Antarctic temperature shows that although the shape of the global temperature curve is similar to the pattern of Antarctic temperature change, it lags Antarctica by several centuries to a millennium throughout most of the deglaciation. The small apparent lead of Antarctic temperature over CO2 in the ice-core records does not apply to global temperature.  Further support for is provided by an analysis of the inflection points in the CO2 and global temperature records suggests that changes in CO2 concentration were either synchronous with or led global warming during the various steps of the deglaciation. An important exception is the onset of deglaciation about 17.5 thousand years ago where about 0.3 °C of global warming is observed before the initial increase in CO2 . This finding suggests that CO2 was not the cause of initial warming.

Comparison of northern and southern hemisphere temperatures

Ocean currents NASA
Major ocean currents NASA

The authors calculated separate temperature stacks for the Northern Hemisphere and Southern Hemisphere.   Each hemispheric temperature shows the same two-step warming as in the global surface temperature reconstruction.  The analysis found that

  • The Southern Hemisphere temperature probably leads CO2 (consistent with the Antarctic ice-core results) and the Northern Hemisphere temperature lags CO2.
  • The Northern Hemisphere shows coolings coincident with the onset of Southern Hemisphere warmings.

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.

Another 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.

It is posited that variations in the Atlantic meridional overturning circulation (AMOC) were responsible for a seesawing of heat between the hemispheres and is responsible for the Antarctic warming leading Arctic warming. This suggests that warming at northern mid to high latitudes, leading to a reduction in the AMOC and thus warming the southern hemisphere occurring at about 19 thousand years ago, as being the trigger for global deglacial warming.

Possible mechanism for 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