Hi Ross,
See debate unfolding below
To attempt to understand this we need to look at paleoclimate to see
how methane has responded to previous warm periods. An interglacial
that was very similar to our Holocene is the MIS 11 interglacial from
420,000 years BP until 370,000 BP. During MIS 11 Milankovich forcing
was similar to our current Holocene. It was a very long and very warm
interglacial.
Here is a comparison of our current interglacial with MIS 11 from the
EPICA ice core:
http://www.nature.com/nature/journal/v429/n6992/images/nature02599-f5.2.jpg
As you can see global temperatures rose for thousands of years to be
much warmer that modern times. Sea level was much higher and the
Greenland ice sheet and West Antarctica Ice Sheet were much smaller at
that time. Greenland was so warm it had Spruce forrests (Steig and
Wolfe 2008).
So with a warmer and longer period how did methane react?
Here is a comment by Edward Brook from Tiny Bubbles Tell All:
"During MIS 11, an interglacial period between about 420,000 to
370,000 years ago, methane reached typical maximum levels, fell by
about 100 parts per billion over 5000 years, then rose again toward
the end of the interglacial period. This is similar to the pattern
over the past 10,000 years (14), which has been the subject of an
interesting argument over the impact of early human activities on the
atmosphere (15, 16)."
Tiny Bubbles Tell All
http://geo.oregonstate.edu/people/faculty/publications/brooke/Brook_TinyBubbles.pdf
Also from Dr. Brook is this graph from his paper 'Palaeoclimate:
Windows on the greenhouse'.
http://www.nature.com/nature/journal/v453/n7193/images/453291a-f1.2.jpg
Note that during previous warm periods methane increased but despite
a climate that was warmer than today for an extended period there was
no spike in methane.
Another example is the last interglacial period known as the Eemian or
MIS 5. This interglacial 123,000 BP was warmer than today with sea
levels 3 to 5 meters higher than today. Looking at the graph in the
link above the green methane line responds to temperature but despite
the very warm Eemian stayed in a small range, with a maximum of ~750
ppbv only slightly exceeding preindustrial levels of methane in our
atmosphere.
So from recent paleoclimate (the past half million years) we find no
examples of a warm climate causing a massive release in methane.
Methane is currently 3 times the normal from previous interglacials
with no reason it should significantly increase.
There are also methane clathrates, a solid form of water with methane
that are on the ocean floor and buried in some bogs. This is
potentially a huge source of methane, but again there has never been a
case when a warm climate triggered a release. It is thought that there
was one case of a massive release of methane clathrates, at the
Paleocene-Eocene Thermal Maximum 55 million years ago.
This was a huge event. According to Gavin Schmidt:
"The change at the PETM was so large that it would have required a
decrease in biological activity equivalent to roughly three times the
total present-day terrestrial biosphere. In other words, if all of the
terrestrial carbon today (in forests, animals, soils, etc.) were
converted to carbon dioxide and returned to the global inorganic
carbon pool, the change in the global carbon isotopic ratio would only
be a third as big as that observed during the PETM!"
http://www.giss.nasa.gov/research/briefs/schmidt_02/
To attempt to understand the potential for such an event to occur
again we need to look at what caused this release of methane
clathrates. According to Storey et al 2007, 'Palocene-Eocene Thermal
Maximum and the Opening of the Northeast Atlantic' the cause was
"volcanic eruptions that created the North Atlantic Ocean when
Greenland and northwestern Europe separate".
http://news.mongabay.com/2007/0426-petm.html
The oceans were dramatically heated, huge amounts of CO2 were emitted
and this destabilized methane clathrate deposits on the ocean floor.
Looking back over the historical evidence methane emissions do respond
to increased temperature but there is no case where this has caused a
massive feedback and during the interglacials of the past half million
years that were warmer than today we see no sign of increased methane
release. Our atmosphere already has 3 times the previous natural
levels of methane showing that anthropogenic emissions dwarf natural
emissions, the only exception to this was an extreme example where the
ocean floor was literally splitting apart during the PETM.
Based on this I would characterize the talk of methane as a major
feedback to be alarmism.
Science based comments are welcome.
Cheers
Jim
The previous warmings are not comparable because the temperature
increases were much much more gradual. With gradual warming, only
small sections of permafrost in any one decade would have begun to
release methane, and with about a 12 year half-life for oxidation to
carbon dioxide, and slow increases in carbon dioxide so that the sinks
would work well, it is logical that the methane level would not have
gotten very high.
However, the temperature rise is now very rapid, so large tracts of
permafrost are being rapidly warmed to release large amounts of
methane. We are also overwhelming our sinks with the fossil fuel burning.
We need a scientific group to try to quantify this difference with
appropriate computer models.
But in the meantime, there is no adequate scientific reason to say
that just because there was not a major increase in methane in
previous slow warming periods, there will not be a major increase in
our current extremely rapid warming period.
Jim
Hi Jim,
--- In ClimateConcern@yahoogroups.com, "James F. Newell"
>
> The previous warmings are not comparable because the temperature
> increases were much much more gradual. With gradual warming, only
> small sections of permafrost in any one decade would have begun to
> release methane, and with about a 12 year half-life for oxidation
> to carbon dioxide, and slow increases in carbon dioxide so that the
> sinks would work well, it is logical that the methane level would
> not have gotten very high.
>
Actually methane changes are consistently seen in the paleoclimate
history. Methane may have a 12 year half-life but when it is released
it tends to be released in quantities. Also climate tends to cool in
slow steps but warm quite rapidly.
A good example is at the Younger Dryas transition, a very rapid
cooling and then after a 1,000 years of near ice age warmed more than
10 degrees C in less than 50 years. Methane did increase, roughly 350
ppbv during that period of actual rapid climate change and returned to
typical interglacial levels.
Going back further, Dansgaard-Oeschger Events during the past ice age
were also rapid climate events accompanied by a release of methane.
Concentrations jumped as much as 200 ppmv at that time.
A few references:
Ice-core evidence of abrupt climate changes
http://www.pnas.org/content/97/4/1331.full
Vostok Temperature and Methane Graph
http://www.giss.nasa.gov/research/features/methane/core1.gif
Also interesting is this study which shows that the large methane
increase was not from "tropical wetlands or from plant" and not
seafloor methane deposits:
http://www.eurekalert.org/pub_releases/2006-08/osu-sro082106.php
This similar study agrees:
http://www.eurekalert.org/pub_releases/2008-04/haog-mso041508.php
"The isotopic measurements show no signs of CH4 emissions by a
destabilization of marine gas hydrate reservoirs when climate was
warming."
So there is no evidence of a methane clathrate release of any
significance since the PETM and to forecast any such release due to
climate change would not be based on anything factual but simply alarmism.
It is also important to look at the historical range of methane when
compared to today. Looking at the Vostok ice core (above) methane
moved in a range of ~ 400 ppbv to ~ 800 ppbv with the changes driven
by the warming and cooling of the earth (primarily though sea level
changes and increased/decreased wetlands). Today methane is 1760ppbv,
far above any natural methane release ever seen.
http://www.giss.nasa.gov/research/features/methane/core2.gif
So with mankind already increasing methane far above any time of the
past 600,000 years there is nothing to support a further increase due
to warming climate and any natural release would be a small percentage
of our already historically high methane level.
> However, the temperature rise is now very rapid, so large tracts of
> permafrost are being rapidly warmed to release large amounts of
> methane.
We frequently see talk of how rapid climate change is, or that past
climate change was much more gradual but it is simply not true. This
decade particularly we are not seeing continued warming. But let's
look at the recent warming as a whole.
Here is a graphic from the Hadley Centre CRU.
http://www.metoffice.gov.uk/corporate/pressoffice/myths/Forcing-Diagram.gif
The purpose of this diagram is to show that models can reproduce the
early 20th century warming by natural forcing alone but not the
warming of the latte half of the 20th century.
Notice that the natural warming trend of the first half of the 20th
century is of the same slope as the only warming period attributed to
AGW, the period 1979 to 1998. This shows us that the period attributed
to AGW is no more rapid than the most recent natural warming phase and
definitely cannot be considered rapid when compared to actual rapid
warming periods of the past such as the Younger Dryas transition.
The last decade we definitely cannot talk about how rapid the
temperature rise is with the relatively steady temperature we have seen.
http://www.globalwarmingart.com/images/a/af/Short_Instrumental_Temperature_Record.png
So with research showing no release of methane clathrates from
previous climate warming and with the relatively small releases of
methane that did exist being dwarfed by methane from our civilization
I will again state that the talk of a methane release as a major
feedback to AGW is pure alarmism with no basis in sound science.
Again science based responses are welcome, however please don't simply
insist something without any scientific basis in fact.
Cheers
Jim