Both the northern hemisphere and overall global temperatures actually lagged CO2; in other words, for the world as a whole, warming happened after atmospheric CO2 concentrations increased. The reasons for this are complex and are driven in part by changes in ocean currents as ice ages end. The figure below shows the results from the Shakun et al paper for different regions of the world, along with the uncertainties in their estimate across different climate proxy locations and time periods during the end of the last ice age.
The orange values are for the southern hemisphere, the blue values show the northern hemisphere and the grey values show global temperature estimates. The count on the y-axis represents how many of the 1, simulations — which examined the sensitivity of the results to uncertainties in CO2 age datingand proxy temperature estimates — show a lag of that size.
Specifically, Shakun and colleagues argue that changes in orbital cycles triggered initial melting of ice sheets in the northern hemisphere. This caused large amounts of freshwater to pour into the oceans as ice sheets melted, disrupting the Atlantic Meridional Overturning Circulation AMOC , which, in turn, cooled the northern hemisphere and warmed the southern hemisphere.
This southern hemisphere warming caused ocean releases of CO2, which, in turn, warmed the entire planet. Shakun et al suggest that the vast majority of the global warming at the end of the last ice age occurred after CO2 increased, though this warming was driven by a combination of albedo reflectivity changes and the greenhouse effect.
The world of the past ice age was very different from today. It is not necessarily assumed that all the same forces are at work today because human emissions now drive climate changes. For example, the absence of large-scale sea ice cover in the Southern Ocean means that rising temperatures are not expected to drive ocean release of CO2 in the current climate — though the strength of the ocean CO2 sink is expected to decrease as the world continues to warm. If CO2 were the only factor at work, this would imply a very high sensitivity of the climate to CO2 — around 8C per doubling CO2, far higher than current estimates based on climate models and other lines of evidence.
Receive our free Daily Briefing for a digest of the past 24 hours of climate and energy media coverage, or our Weekly Briefing for a round-up of our content from the past seven days. Just enter your email below:. The fact that relatively small changes in external forcings can drive such a large planetary response during ice ages should serve as a cautionary example.
This is because human emissions of CO2 and other greenhouse gases push the Earth further out of the range of climate conditions that have characterised the past few million years. Global temperatures were only around 4C colder during the last ice age than they are today.
Under higher emissions scenarios with limited mitigation efforts, it is quite possible that the world may warm more in just over a century than it warmed over many thousands of years during the end of the last ice age.
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