Climate Abyss

Weather and climate issues with John Nielsen-Gammon

Dispatch from AGU: How Fast Is Fast?



You have to go back 50 million years to find anything similar.  Scientists call it the Paleocene-Eocene Thermal Maximum, or PETM.  We know from the change in carbon isotopes that the event was caused by a massive carbon release, either carbon dioxide or methane. The global average temperature spiked 5 C, which is more than what’s predicted today from CO2 doubling.

Naturally, the PETM is a subject of intense investigation, as scientists look for concrete evidence of what happens when the Earth warms suddenly.

One important question is how rapidly temperatures rose, which in turn is tied to how rapidly the carbon-containing gases were added to the atmosphere. Current estimates vary from 750 years to over 10,000 years.

I listened to a talk today that was so far outside of what previous studies had found, it must either be groundbreaking or wrong. Time will tell!

The work was by Morgan Schaller and James Wright, of Rutgers University. Conveniently for them, a layer of mud deposited during the PETM lies near the surface from New Jersey to Virginia. For research purposes, several cores have been drilled, making the mud record available for study.

Wright and Schaller noticed that a few of the cores have a long series of regular layers about 2 cm thick. This is the sort of thing that marks a regular natural cycle, and this cycle was so regular that it was almost certainly astronomical in origin. There were also regular isotopes variations that implied that these cycles involved temperature changes of up to 7 C.

There seems to be only two possibilities. One would be Milankovitch cycles, the same things that are responsible for Ice Age cycles. But the time scale was wrong: with 750 distinct cycles on the core would require the core to be deposited over 15 million years, which doesn’t fit with the other evidence.

The only other obvious possibility is the annual cycle, the changing of the seasons. The strength is right, the regularity is right. The sedimentation rate would be a bit large, but not outlandish. The frequency of plankton appearing in the core also seems way large, but oh well.

Finding a core from 50 million years ago with data at better than annual resolution is great. Finding one that actually includes the PETM would be the Mother Lode.

Well, they found the Mother Lode. With one core, they could extract carbon isotopes data every year, and really pin down the rate of carbon release. And they discovered that the ramp-up time for carbon in the atmosphere was a mere 13 years. The initial rise took place effectively instantaneously, much less than a year!

If this turns out to be right, the present rate of CO2 rise is not the fastest ever. The PETM, with its mass extinctions, may serve in many ways as a worst case, or near worst case, scenario.

Time will tell.

Categories: General
John Nielsen-Gammon

9 Responses

  1. JohnD says:

    Thirteen years? What mechanism would release CO2 on that scale at that speed?

    • john n-g says:

      There are two possibilities: massive volcanic eruptions or an impact from a comet-sized object.

    • Dr. J says:

      Has it been proven to be CO2??? For the last 40 years or so CH4 was the preferred gas to use for temp increases in the PETM.

    • john n-g says:

      It’s one or the other.

    • JohnD says:

      massive volcanic eruptions or an impact from a comet-sized object

      Neither of which is well substantiated in the geologic record. We’d need something about ten times larger than the Yellowstone mega-eruption 2.1 million years ago to put that much CO2 into the atmosphere, and we should see both the pyroclastic evidence and some evidence for short-term cooling from the SO4 inherent to such an eruption. Similarly, any impactor would leave some sort of mark other than just CO2. We should see an astrobleme or an iridium layer coincident with the PETM.

      There almost has to be something more going on than meets the eye.

    • Dr. J says:

      Here’s a paper I recall being presented last year on the issue:

  2. Steve Bloom says:

    Perhaps extensive burning of the Antarctic tundra?

    In any case, a comet/asteroid or volcano trigger remains unlikely due to the need to explain the subsequent hyperthermals and their steady decline in magnitude.

    Just to be clear, we’re talking about the triggering event. Feedbacks to it would have added more carbon, as the paper J linked discusses..

    If it was burning tundra, note that we have a rather large supply currently.

  3. thingsbreak says:

    There are a number of other cycle timescales between orbital and annual. Reading the abstract, I can only hope that there is some as of yet unstated justification for assuming the annual timescale for the cycles, because it is, on its face, simply not plausible (and a number of alternatives are).

  4. Steve Bloom says:

    things, I mentioned the Antarctica tundra/permafrost because it seems to be the current leading candidate for the initial carbon trigger. If it burned extensively, the carbon pulse could have been that fast. Note that a soot feedback would be involved, as is the case in the Arctic today. So on its face it does seem plausible, albeit far from proven.