There’s a tale in the ice. A record of past atmospheres locked away as snowfall trapped air bubbles and then compressed them into thin layers age after age over tens of thousands of years. Over the last few decades, scientists have been drilling holes in the great ice packs of Greenland and Antarctica. Their quest? To unlock this tale and reveal a direct record of global greenhouse gas levels through the deep past.
What their drilling uncovered was both quite informative and rather chilling. First, it showed that, for more than 800,000 years, global CO2 levels had been relatively stable in a range of 180 to 300 parts per million. As the levels of heat trapping CO2 rose, temperatures peaked during brief interglacials. And as levels fell, temperatures plunged back into ice age conditions.
Global temperature flux during these swings from ice age to interglacial were just 4 degrees Celsius. A 100 ppm CO2 rise correlated roughly to a 250 foot rise in sea level and much warmer average conditions globally. A corresponding fall of about the same amount brought temperatures back down and piled ice two miles high over today’s temperate regions such as New York.
What the ice cores also revealed was that human CO2 emissions had pushed global levels of this potent greenhouse gas far out of any climate reckoning comparable to the context of human beings, who have only existed in current form for about 200,000 years.
In fact, what scientists found was that atmospheric CO2 levels were pushing more than 100 parts per million higher than at any time during this vast epochal span:
(Antarctic ice core CO2 record and comparable temperature swings. Note that the difference between ice age and interglacial is about 8 C of local temperature and about 100 ppm of CO2. It is worth considering that, due to polar amplification, Antarctic temperature changes were about double the global average. Current CO2 levels are more than 100 parts per million higher than even the peak value over this 800,000 year period. If an average peak interglacial CO2 average of 275 ppm is considered, then current values are around 127 parts per million higher. Image credit: Havard/Jeremy Shakun.)
This record was a key contribution to climate science. One, it revealed how past CO2 levels compared to past temperatures. And since the data was directly derived from air bubbles trapped beneath hundreds of feet of ice, it also provided us with an exact measure for past atmospheres.
Secondly, and perhaps much more ominously, it showed us how very far beyond any climate comparable to that great span of time we’d already come.
102 ppm higher than at any time in the last 800,000 years
Humans have now pushed the CO2 boundary 102 parts per million higher than the context provided by the last 800,000 years. It’s kind of a big deal when you consider that a mere fluctuation of about 100 parts per million CO2 was enough, when combined with changes in orbital forcing, to set off feedbacks resulting in a 4 C temperature change globally (8 C change for the Antarctic environment) as ice age proceeded to interglacial and back.
Current human forcings through CO2 and other emissions have now entirely over-ridden the natural cycle, eliminating the possibility for future ice ages and putting us on a trajectory for catastrophe. With annual global carbon emissions now exceeding 12 gigatons, not only have we forced ourselves well outside of any past bounds to which we can easily relate, we have also generated an unprecedented velocity of change. For the current human carbon emission now exceeds, by at least six times, the most rapid past level of natural carbon emission.
No vast flood basalt could ever rival the volume and pace at which humans currently emit greenhouse gasses.
This enormous emission continues to have severe effect through an ever-higher ratcheting of global CO2 levels.
As of the past week, global daily CO2 values had rocketed to 402.2 parts per million, well outside anything seen in the ice core record:
(Mauna Loa Observatory CO2 measure for the last six months. Note that daily and weekly values have been mostly above 400 ppm since early March. Image source: The Keeling Curve.)
This an extraordinary measure. One that has no context in direct records such as those available to us through ice core data. But paleoclimate proxy data does provide some corollary. According to isotopic carbon measures found through seabed samples, we can determine that the last time CO2 levels were above 400 parts per million was during the mid-Plieocene between 3 and 3.3 million years ago.
And during that time global average temperatures were 2 to 3 degrees Celsius warmer than they are today (with Antarctic values at least twice that). Both Greenland and West Antarctica were mostly ice free and sea levels were between 15-75 feet higher. These are, likely, the potential low end of the changes we’ve locked in due to human global greenhouse gas forcing long term, even if, somehow, global CO2 levels are brought to a plateau.
(An graphic extrapolation of Antarctica’s ice cover and elevation based on paleoclimate data. Note that the Antarctic ice sheet is greatly diminished at a time when CO2 values remained constant around 400 ppm. Image source: Commons.)
Unfortunately, the global CO2 measure doesn’t tell quite the entire story. For atmospheric levels of gasses like methane, nitrous oxide, and a host of less common industrial chemicals have also all been on the rise in Earth’s atmosphere due to human emissions. As a result, according to research by the Advanced Global Atmospheric Gasses Center at MIT, total heat forcing equal to CO2 when all the other gasses were added in was about 478 ppm CO2e during the spring of 2013. Adding in the high-velocity human greenhouse gas contributions since that time gets us to around 480 ppm CO2e value. In the context of past climates and of near and long term climate changes due to human interference, 480 ppm CO2e is nothing short of fearsome.
The last time the world saw such a measure of comparable atmospheric greenhouse gas heat forcing was during the Miocene around 15-20 million years ago. At that time, global temperatures were 3-4 C warmer, the Antarctic ice sheet was even further diminished, and sea levels were 80-120 higher than today.
This combined forcing is enough to result in a state of current climate emergency. In just a few years, according to the recent work of climate scientist Michael Mann, we will likely lock in a 2 C short term warming this century and a probable 4 C warming long-term. If the current, high-velocity pace of emission continues, we will likely hit 2 C warming by 2036, setting off extraordinary and severe global changes over a very short period.
These are very dangerous and, likely, catastrophic levels. In such a context, the inexorably rising rate of atmospheric CO2 and other greenhouse gas forcings simply adds further insult to a very high risk situation.