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Did Early Agricultural CO2 Save Earth From An Ice Age?

13.01.2016
13.01.2016 15:06 Age: 1 year

Earth has had a narrow escape from a new ice age possibly due to humanity's early CO2 emissions, according to a paper in Nature. Modelling studies looking at the link between insolation and CO2 confirm that we would now be entering an ice age had atmospheric CO2 concentrations prior to the industrial revolution not risen above 240ppm

Click to enlarge. From the paper. Best-fit logarithmic relation (black line) between the maximum summer insolation at 65° N and the CO2threshold for glacial inception; grey shaded area indicates ±1s.d. Blue dots correspond to the coldest model version and red dots to the warmest. Courtesy: authors and Nature

Click to enlarge. From the paper. The locations of previous glacial inceptions in insolation–CO2 phase space relative to the best-fit logarithmic curve from the top image. Glacial inception is only possible when the point is located below the insolation–CO2 curve. Courtesy: authors and Nature

Click to enlarge. From the paper. The timing of past and future glacial inceptions can be explained by the CO2 concentration and the insolation–CO2 relation. The thin grey line depicts the CO2 threshold value for glacial inception, derived as a function of the maximum summer insolation at 65° N. The CO2 concentration from ice core data for the past 800,000 years is shown (blue line), along with the CO2 scenarios of 0 Gt C cumulative anthropogenic emissions (blue line), 500Gt C (orange line), 1,000Gt C (red line) and 1,500Gt C (dark red line). Pale blue vertical bars indicate the time periods when the reconstructed value is below the critical CO2 concentration, and the light blue bar shows the timing of a possible next glacial inception. The horizontal dotted line indicates the present-day CO2 level. The lower curve depicts a proxy for the global ice volume (thick grey line). Courtesy: authors and Nature

 

New research implicating ice age onset with a critical relationship between solar radiation and atmospheric carbon dioxide (CO2) concentrations suggests that Earth came close to entering a new ice age before the beginning of the Industrial Revolution.

Ice ages are believed to be triggered when levels of solar radiation reaching the ground, known as insolation, are reduced at boreal latitudes around 650N. The new study suggests a trade-off relationship between insolation and atmospheric CO2 concentrations which means that the insolation trigger point for ice age onset reduces as atmospheric CO2 concentrations rise.

This would explain why, with boreal latitude insolation close to a minimum there is no sign of any ice age. The relationship also explains how Earth missed entering into a new ice age in the last few thousand years. This is because atmospheric CO2 concentrations rose above 240 parts per million (ppm) to reach around 280ppm just before the Industrial Revolution began in the eighteenth century.

This concentration of CO2 was critical given the relatively low levels of insolation at boreal latitudes seen during the last few thousand years - a period known as the late Holocene. With CO2 levels of just 240 ppm there would have been a rapid ice build up over the last few thousand years, according to the Nature paper. "This means that the Earth system would already be well on the way towards a new glacial state if the pre-industrial CO2 level had been merely 40 p.p.m. lower than it was during the late Holocene, which is consistent with previous results," the authors write. 

"Whether this narrow escape from glacial inception was natural remains debatable," say the authors of the paper. "It has been proposed that pre-industrial land-use at least partly contributed to the high Holocene CO2 level, but the magnitude of this contribution is very uncertain," they write.

Human CO2 emissions are likely to delay the next ice age for some time, according to the researchers. Their paper states that moderate anthropogenic cumulative CO2 emissions of 1,000 to 1,500 gigatonnes of carbon will postpone the next glacial inception by at least 100,000 years.

News release

Here is the text of a news release from the Potsdam Institute for Climate Impact Research (PIK) concerning this study:

Human-made climate change suppresses the next ice age

Humanity as a geological force

End of news release.

Abstract

The past rapid growth of Northern Hemisphere continental ice sheets, which terminated warm and stable climate periods, is generally attributed to reduced summer insolation in boreal latitudes. Yet such summer insolation is near to its minimum at present, and there are no signs of a new ice age. This challenges our understanding of the mechanisms driving glacial cycles and our ability to predict the next glacial inception. Here we propose a critical functional relationship between boreal summer insolation and global carbon dioxide (CO2) concentration, which explains the beginning of the past eight glacial cycles and might anticipate future periods of glacial inception. Using an ensemble of simulations generated by an Earth system model of intermediate complexity constrained by palaeoclimatic data, we suggest that glacial inception was narrowly missed before the beginning of the Industrial Revolution. The missed inception can be accounted for by the combined effect of relatively high late-Holocene CO2concentrations and the low orbital eccentricity of the Earth. Additionally, our analysis suggests that even in the absence of human perturbations no substantial build-up of ice sheets would occur within the next several thousand years and that the current interglacial would probably last for another 50,000 years. However, moderate anthropogenic cumulative CO2 emissions of 1,000 to 1,500 gigatonnes of carbon will postpone the next glacial inception by at least 100,000 years. Our simulations demonstrate that under natural conditions alone the Earth system would be expected to remain in the present delicately balanced interglacial climate state, steering clear of both large-scale glaciation of the Northern Hemisphere and its complete deglaciation, for an unusually long time.

Citation

here.

Source

Nature.

PIK news release here.

Note: PIK news release was added to this report at 0900 on 14 January 2016.