Note: This program first aired on May 10, 2014.
I promised last week that we would look at a time in Earth’s
climate history when climate changed significantly due to the same mechanism as
we are experiencing now, that being, increased atmospheric carbon dioxide.
Recall that carbon dioxide is a green house gas, and green house gasses absorb
outgoing infrared radiation, which is the form of energy by which the Earth loses
heat. By absorbing the radiation, these gasses reradiate that energy back out,
some of it back to Earth. The higher the concentration of green house gasses,
the more energy is absorbed in the atmosphere and radiated back to Earth,
rather than simply being lost to space. That is what is happening right now,
and like I said, it has happened before too.
The last time that green house gasses were the primary
driver for large scale climate change on Earth was about 56 million years ago.
The warming occurred at the end of a geologic epoch called the Paleocene, and
in fact caused a significant enough change in the geologic and fossil records
that Earth scientists recognize the beginning of a new epoch during this time,
the Eocene. Epochs are simply units of geochronological time, and the
boundaries between them are marked by significant changes in layers of rock,
because these differences in the rocks indicate that conditions on Earth were
changing too. This warming event was significant enough to change conditions on
Earth, and marks the boundary between the Paleocene and the Eocene, which has
earned it the name the Paleocene Eocene Thermal Maximum, or PETM for short.
It isn’t 100% clear what initiated the PETM, but the leading
theory is that there was major volcanism associated with the tail end of the
break up of Pangea, and that this released enough carbon dioxide to raise the
global average temperature 1 to 2 degrees centigrade. The Paleocene was already
a much warmer place than Earth is today; for example there were no glaciers on
Antarctica, though it was cold, and there was permafrost there. Based on
evidence from the stratigraphic record, scientists believe that this small
initial warming kicked off several positive feedback loops which then
dramatically raised atmospheric carbon dioxide and global temperatures. The
number one positive feedback that is hypothesized to have had the largest
impact was the melting or vaporization of methane hydrates from the continental
shelves.
And what are methane hydrates you might ask? Methane is the
shortest of all the hydrocarbons, it is a single carbon atom bonded to four
hydrogen atoms. It forms from biological activity, the break down of organic
material by bacteria, but it can also be formed abiotically, which is why we
can find it in outer space. At the right temperatures and pressures, methane
molecules can get trapped inside a lattice work of water molecules. This
usually happens in underwater sediment and in deep permafrost. The temperature
has to be cold and the pressure has to be high. The thought is that in the
Paleocene, Earth was already much warmer than it is now, so perhaps the
conditions required for methane hydrates were already near their limit. The
initial relatively mild warming then pushed the climate system rapidly over the
edge and warmed the water enough to vaporize the methane hydrates. That is the
most widely accepted hypothesis, though the exact mechanisms are definitely
still debated by climate scientists. What isn’t debatable is the fact that methane
is a green house gas. It has some different properties than carbon dioxide to
be sure, mostly having to do with its reactivity, and it infact eventually
degrades into carbon dioxide, but a steady input of methane into the atmosphere
will have a warming effect on the climate which is exactly what mostl likely happened
in the PETM.
The thing about the PETM that is raising so many alarms is,
that while it is our best and most recent analog for the current greenhouse gas
driven warming, there are some important differences as well, and they have to
do with timing. During the PETM the rate of increase of green house gasses
(mostly methane) into the atmosphere is estimated to be on average about 5
billion tons per year. That translates into a warming trend of 0.025 degrees
centigrade per 100 years. It warms, but slowly. You get big warming events if
this slow warming goes on consistently (which it did in the PETM, for nearly
200,000 years). Conditions on Earth change, but again, slowly. They changed
enough to show up in the rock record, and they also induced migration of
various early mammals, driving the evolution of most of the mammal orders we
see on Earth today. Contrast that with today’s scenario. Our average green
house gas input is something more like 30 to 35 billion tons per year,
remember, in the PETM it was 5 billion. That input translates into a 1 to
perhaps as much as 4 degree centigrade increase in temperature in 100 years.
That is what keeps climate scientists and conservation biologists up at night,
the potential speed of the current warming is exactly the kind of abrupt
climate change event that triggers mass extinctions, the Earth’s reset button.
There is a great deal of uncertainty about what the future will hold, but most
of that uncertainty has to do with which feedback loops will get triggered
when, like the warming feedback that melted the methane hydrates in the PETM,
turning a mild warming event to a climatic anomaly that stamped its signature
on the geologic record and left a biological legacy that shaped the world we
enjoy today.
References:
Weather Underground, of all places, is a great climate
change resource: http://www.wunderground.com/climate/PETM.asp?MR=1
Curious list of articles about the PETM, compiled after a
scientific paper was published in 2009; the paper got a lot of press, most of
it off the mark. http://tugpullpushstop.blogspot.com/2009/07/resources-on-palaeocene-eocene-thermal.html
Climate scientist Gavin Schmitt on the PETM: http://www.realclimate.org/index.php/archives/2009/08/petm-weirdness/
Nice video from the American Museum of Natural History on
the PETM: http://www.amnh.org/explore/science-topics/climate-change/pinboard-column-1/petm-unearthing-ancient-climate-change
US Geological Survey on Methane Hydrates: http://woodshole.er.usgs.gov/project-pages/hydrates/primer.html
A scientific journal article about the PETM and attempts to
model it: http://rsta.royalsocietypublishing.org/content/368/1919/2395.long
