400 ppm CO2 and beyond

Humankind’s Last Days Below 400 PPM CO2?

Robertscribbler,

25 November, 2015

Another ominous milestone fades away in the smog behind us…

By mid November of 2015, atmospheric carbon dioxide levels as measured at the Mauna Loa Observatory had again risen above 400 parts per million. Over the past two weeks, these levels maintained. And even though we may see a few days during which CO2 levels drop below that key threshold during late November and, perhaps, early December, those days could well be the last.

The last days of below 400 parts per million CO2 may then be behind us. It’s likely that none of us now living will ever see such ‘low’ levels of this critical atmospheric greenhouse gas. And it’s possible that humankind itself will never again see a day in which CO2 levels fall into this range (please see Is This the Last Year Below 400 PPM CO2?).

(By November 13 of 2015, atmospheric CO2 levels at the Mauna Loa Observatory had risen above the 400 ppm CO2 threshold. During 2015, just four months of the year have been below that level. For 2016, it’s possible that no month will average below 400 ppm CO2. And after that time, we’ll likely never see below 400 ppm CO2 levels again. Image source: The Keeling Curve.)

Dr. Ralph Keeling, in a blog over at the Keeling Curve site back in October clearly explained why 2015 may be the last year with monthly readings below 400 ppm CO2. In short, the added global heat from El Nino tends to squeeze more carbon out of the Earth’s lands and oceans. As a result, strong El Nino years have tended to show higher rates of atmospheric CO2 increase when these Earth System feedbacks are added in to the carbon coming from an insane global rate of fossil fuel burning. The last time a strong El Nino emerged during 1998, global CO2 levels rose by 3.7 parts per million during a single year. Since the rate of human fossil fuel burning has substantially increased over the 1998 to 2015 period and as the current El Nino is peaking at or above 1997-1998 intensities, it’s possible that the annual increase in CO2 could match or exceed that seen during 1998. We could see annual CO2 levels rise by 3.5, 3.75, or even 4 parts per million or more during the 2015 to 2016 interval.

If CO2 levels rise so rapidly, 2015 will be the last year seeing any significant period below 400 ppm.

(During 2016, CO2 levels as measured at Mauna Loa will peak at between 405 and 408 parts per million CO2. These levels are the highest seen at any time during the last 3-15 million years and are more typical to the early Pliocene or Middle Miocene climate epochs. Such amazingly high levels of CO2 beg equally amazing and harmful geophysical changes to follow. Image source: The Keeling Curve.)

Unfortunately, due to CO2’s long residence time in the atmosphere and also due to the fact that the added heat provided by that CO2 tends to generate long-lasting carbon feedbacks from the global environment, it is likely that this period of above 400 parts per million CO2 will last for a very, very long time. With a single molecule of CO2 having a warming impact period of at least 500 years so long as the oceans can eventually draw the net carbon increase down (a dubious proposition in the context of warming world), even if human fossil fuel emissions halted after just one or two additional decades, it would mean that above 400 parts per million CO2 levels last for centuries to come. On the other hand, if human fossil fuel burning and related carbon emissions continue even into the middle of this Century, then it’s possible that such heightened CO2 levels could last for millennia — possibly outlasting the human race itself. In such an event, the only hope for bringing CO2 back to levels below 400 parts per million is a very significant change in land use, farming, and technological practices to directly draw carbon out of the atmosphere.

Atmospheric CO2 levels remaining at 400 parts per million for any significant period will push the Earth climate to warm by between 2 and 3 degrees Celsius. It will push for sea levels to rise by at least 75 feet. In other words, a world at 400 parts per million is a world radically changed. A world that human beings have never seen before. And as a cautionary note, the total forcing from all greenhouse gasses currently emitted by humans is now in the range of 485 parts per million of CO2 equivalent. A level well beyond the current 400 parts per million threshold and one that likely equates to around 4 degrees Celsius worth of long term warming.

These are the stark consequences of fossil fuel burning. A burning, that if it continues, will almost certainly ensure that human beings living after 2015 or 2016 never again see CO2 levels below 400 parts per million.

Links:

The Keeling Curve

Is This the Last Year Below 400?

Pliocene Climate

Miocene Climate

Skeptical Science on CO2 Residence Times