Environment Counts | Evidence that rising CO2 concentrations led rising surface temperatures during the last deglaciation
Evidence that rising CO2 concentrations led rising surface temperatures during the last deglaciation
Author: Geoff Zeiss – Published At: 2012-12-28 07:41 – (925 Reads)
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There is a solid body of climate paleohistoric evidence that CO2 concentration and Earth surface temperature are correlated. There is evidence that CO2 concentration amplifies the effects of relatively small external factors such as variations in solar radiation due to eccentricities in Earth’s orbit (Milankovitch cycles). But the time resolution of observations from ice cores and other sources of information about the Earth’s paleoclimate has not been able to determine whether CO2 concentration leads or lags Earth’s surface temperature. In this article the authors construct a record of global surface temperature (blue line) from temperature proxy observations and show that during the last deglaciation (20,000 to 6,000 years ago) temperature is correlated with and generally lags CO2 concentration (red line). However, they find that a global warming of about 0.3 °C precedes the initial increase in CO2 concentration at 17.5  kyr ago, suggesting that rising CO2 concentrations did not initiate deglacial warming. But, based on a combination of observation and transient global climate model simulations, they conclude that ocean circulation changes superimposed on global warming driven by increasing CO2 concentrations provides an explanation for much of the temperature change during the most recent deglaciation. Nature 484, 49–54 (05 April 2012)
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Surface temperatures and CO2 concentation
The authors calculated the area-weighted mean of 80 globally distributed, high-resolution proxy temperature records (alkenone, Mg/Ca and TEX86 Antarctic ice-core, pollen, microfossil assemblage and MBT/CBT) to reconstruct global surface temperature during the last deglaciation.
They find a strong correlation with the atmospheric CO2 record from the EPICA Dome C ice core data accounting for 94% of the variation. This supports previous findings from Antarctic ice cores. They find also that globally CO2 concentrations leads warming, but there are important differences between the northern and southern hemispheres.
Comparison of northern and southern hemisphere temperatures
The authors calculated separate temperature stacks for the Northern Hemisphere and Southern Hemisphere. They found that the magnitude of deglacial warming in the two hemispheric stacks is nearly identical. But the hemispheric stacks differ in two main ways.
- Lag correlations suggest that the Southern Hemisphere temperature probably leads CO2 (consistent with the Antarctic ice-core results) and the Northern Hemisphere temperature lags CO2.
- Second, the Northern Hemisphere shows coolings coincident with the onset of Southern Hemisphere warmings.
Temperature trends in northern and southern hemmispheres
a, Linear temperature trends in the proxy records from 21.5–19 kyr ago (red) and 19–17.5 kyr ago (blue) averaged in 10° latitude bins. b, Proxy temperature stacks for 30° latitude bands. The stacks have been normalized by the glacial–interglacial (G–IG) range in each time series to facilitate comparison.
The authors suggest that variations in the Atlantic meridional overturning circulation (AMOC) causes a seesawing of heat between the hemispheres and is responsible for the Antarctic warming leading Arctic warming. They suggest 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 kyr ago, as being the trigger for global deglacial warming. They point out that more data is required to confirm the magnitude and extent of early warming at northern higher latitudes.
A possible forcing model to explain this sequence of events starts with rising summer insolation driving northern warming, which leads to the observed retreat of Northern Hemisphere ice sheets and an increase in sea level starting about 19 kyr ago. This causes freshwater forcing in the Northern Hemisphere which reduces the strength of the AMOC, leading to a warming of the Southern Hemisphere through the bipolar seesaw effect.
Nature 484, 49–54 (05 April 2012)
