A
Faustian Bargain on the Short Road to Hell: Living in a World at
480 CO2e
On
the highway to a smokestack hell, Faust met a devil who said to him:
“Give
me all your tomorrows, all your children and all your children’s
children, and I will make this one day, for you, a paradise.”
5
March, 2014
Understanding
how much warming may be in store from all the CO2, methane, N2O and
other greenhouse gasses humans have pumped into the atmosphere can be
a bit problematic. First, definitions have tended to be confused due
to the fact that equilibrium climate sensitivity measures (Charney)
used to project warming for this century by the IPCC only take into
account about half of long-term (slow feedback) warming should CO2
and other greenhouse gas levels remain high.
For
example, equilibrium climate sensitivity measures show an effective
rate of warming by about 3 degrees Celsius (C) for every doubling of
CO2 from 1880 onward. By this measure, we get about 3 C worth of
warming over this century once we hit 550 ppm CO2 and about 6 C worth
of warming at levels around 1100 ppm. It is important to stress that
these short term warming projections do not take into account
long-term ‘slow’ feedbacks to a given rise of CO2 that are strong
enough to double the ultimate temperature increase. This
larger Earth Systems Sensitivity
(ESS) measure is both observable in paleoclimate and in the various
model runs that project a given level of atmospheric CO2 out through
the centuries.
(Fast feedback equilibrium climate sensitivity over one century vs long term sensitivity over multiple centuries to a given greenhouse gas forcing. Note that approximately double the amount of warming occurs after ‘slow feedbacks’ like ice sheet response and environmental ghg emissions are taken into account. Image source: Leeds.)
So
both paleoclimate and most model runs end up with a long term warming
of about 6 C at 550 ppm CO2 and of about 12 C at 1100 ppm CO2.
It
is here that we run into an additional difficulty. We don’t
ultimately know how long, long-term will really be. We hope, and our
climate models seem to support this hope, that such ‘long term’
warming from the so-called slow feedbacks like ice sheet albedo
response and natural carbon emissions won’t appear in force this
century. But given the stunning pace of human greenhouse gas build-up
combined with a number of observed ‘slow feedback’ responses
going on now, we don’t really know for certain. And there is some
reason to believe that the ‘slow feedbacks’ might not be so slow
after all.
In
this context, the current level of CO2, at around 400 ppm, results in
a warming this century of around 1 to 1.5 degrees Celsius (if the
slow feedbacks are as slow as expected) and a long-term warming of
about 2-3 degrees Celsius. And it is at this point where an already
complex dynamic begins to break down, taking on a number of, yet more
complex, factors.
A
Host of Extra Gasses No-One Really Talks About
At
issue is the fact that humans have emitted a massive volume of
additional greenhouse gas into the atmosphere. These gasses have
grown in proportion and heating effect alongside the, admittedly
larger and more significant, CO2 emission. And each has made their
own additional contribution to human warming.
Some
of these gasses, like methane, have been a typical part of natural
atmospheres for millions of years. At times, methane concentrations
are observed to have spiked to levels even higher than those seen
today. But the periods during which such levels were apparent were
also times of global crisis — the hothouse mass extinction events.
(Atmospheric
methane concentrations since 1984 as observed at the Mauna Loa
Observatory. Image source: NOAA
ESRL.)
But
the other gasses: nitrous oxide, CFCs, HFCs, nitrogen triflouride,
and a host of nearly 50 other industrial chemicals that contribute to
warming were either never in the atmosphere before or were present at
much lower levels than what is seen today. The result of this added
pollution is yet more potential warming, in addition to a number of
other difficult to deal with impacts. A pollution impact that is
outside the context of past global crises and that puts current day
greenhouse gas forcing at a critical and unstable level.
Methane
levels alone have more than doubled since the start of the Industrial
Revolution, rising from about 750 parts per billion to about 1835
parts per billion today. This value, depending on how it’s
calculated over time, is equivalent to an additional CO2 forcing of
between 22 and 110 parts per million. And though methane is the
strongest non-CO2 warming agent, adding them all together can result
in a value that is quite a bit higher than the base CO2 level would
indicate.
(Atmospheric nitrous oxide levels since 1997 as observed at the Mauna Loa Observatory. Image source: NOAA ESRL.)
In
addition, on the negative side of the ledger, human fossil fuel
burning (primarily coal) burning emits sulfur dioxide, other sulfates
and various aerosols which, overall, create strong negative feedbacks
in the climate system by reflecting incoming sunlight. The net result
is a temporary suppression of a portion of human-caused warming. The
reason this suppression is temporary is due to the fact that the
sulfur dioxide and related sulfates rapidly wash out of the
atmosphere. So if coal burning ceases, the reflective particles
rapidly fall away and we readily come to witness the full strength of
the human greenhouse gas emission.
Which
brings us to the question — what is the full strength of the
current human emission and how long will it last? There’s a term
for this number: CO2e. In other words — the equivalent CO2 forcing
of all greenhouse gasses added together.
Fortunately
for our exploration, there’s been a bit of work done on just this
subject. Last
year, MIT’s Advanced Global Atmospheric Gasses Experiment issued a
report describing model data that determined the current CO2
equivalent forcing from all of the more than 50 greenhouse
contributing trace gasses in the atmosphere.
And the results were somewhat disconcerting. As of June of 2013, that
amount was equal to 478 parts per million CO2. Or a CO2e of 478 parts
per million when all the other greenhouse gasses were added to the
already high and rapidly rising levels of CO2. Adding in the current
rate of CO2 rise, we end up with about 480 parts per million of CO2e
from all greenhouse gasses by this year. So if we’re talking about
the total burden of all greenhouse gasses and the one that will be
with us through the long term, 480 is, unfortunately, the number we
should be dealing with and not 400.
Aerosols
and the Faustian Bargain
Unfortunately,
to determine the current forcing one has to also take into account
those pesky aerosols we mentioned above. And, luckily, we also have a
reliable measure that provides the negative forcing or relative
cooling effect of sulfur dioxide in the current atmosphere. As
of 2013, the IPCC had found that sulfates and other effects due to
aerosols provided a net negative forcing of about .8 Watts per meter
squared or
about 1/2 the positive forcing of CO2 which was, then, at around 390
ppmv (2011), about 1.68 Watts per meter squared. This approximate 1/2
value, when divided by the then observed rise in CO2 since 1880 gives
us a rough equivalent negative forcing value of minus 55 parts per
million CO2e.
So
subtracting out the net effect of sulfates and other aerosols brings
us to a total net forcing from all factors related to human changes
to the atmosphere of about 425 ppm CO2e. A rather disturbing final
number both due to its departure over the current 400 ppm CO2 value
and due to the fact that though most greenhouse gasses have
atmospheric residence times of decades to centuries, the cooling
sulfates would likely last for 1-2 years before falling out entirely.
This means that once fossil emissions stop, we may as well just add
+55 ppm CO2e to the current total.
This
warmth masking factor of human coal emissions was described by James
Hansen as a kind of Faustian bargain in which current burning of the
dirty fuel provides temporary respite to warming at the cost of even
more rapid future temperature increases. And it is just this devil’s
deal in which we are now entangled.
425
CO2e: A Dangerous Interim
So
it is likely that current atmospheric forcing, including all
greenhouse gasses and all human sulfates, is probably at around 425
ppm CO2e. And since the residence times of these gasses are decades
to millennium, while Earth Systems feedbacks appear to be enough to
maintain high methane levels indefinitely, we should probably view
this as an interim figure when considering how much short and
long-term warming is likely locked in.
In
the short term, using equilibrium climate sensitivity measures, we
are likely to end up with between 1.2 and 1.8 C warming over the
course of this century even if all greenhouse gas levels, along with
sulfate levels, were to remain stable and if the slow feedbacks move
along at the expected pace. Meanwhile, long-term warming of between
2.4 and 3.6 C would be expected if all atmospheric gas levels were to
stabilize.
But
unless an ongoing regime of sulfate aerosol spraying of the
stratosphere were put into place, the sulfates would, predictably,
fall out once human emissions stopped. And that rapidly brings us
back to the 480 ppm CO2e number.
480
CO2e: What is Probably Locked in Long-Term
Looking
at the more permanent 480 CO2e value, the fact begins to sink in that
we are already well on the way to extreme climate difficulties. For
480 CO2e, without the reflective aerosols, means that the world
probably ends up warming by between 1.8 and 2.3 C before the slow
feedbacks kick in and between 3.5 and 4.5 C long-term. At these
levels, major ice sheet destabilization and melt is eventually likely
to result in between 50 and 140 feet of sea level rise with the only
remaining glaciers in the end confined to central and eastern
Antarctica.
The
only saving grace to a cold turkey cessation of emissions now is that
most of the worst amplifying feedbacks are likely to be kept in check
and thus prevent rapidly accelerated warming and climate
destabilization. The extra 1.7 to 2.2 C worth of long-term warming
likely comes from a combination of albedo loss, permafrost thaw and
related ghg release keeping currently high levels high long-term,
and, perhaps, a methane belch in the 1-50 gigaton range that spikes
atmospheric levels.
I
say
likely to be kept in check…
but we have to also consider that there is a low, but not out of the
question, risk of setting off a kind of mini-runaway that generates
warming far beyond the expected range and pushes climates to a
hothouse state not seen since the PETM or Permian extinction events.
There is little evidence for such an event in response to current
climate forcings in the models at this time, but we have a number of
scientists, including Peter Wadhams, Natalia Shakhova, and Igor
Simeletov, who have raised the possibility, based on their
observations of Arctic sea ice and carbon stores, that just such an
event could be in the offing. Unfortunately, without more in-depth
research into the potential pace of release of current carbon stores
(permafrost, forest, clathrate, ocean) we don’t have a scientific
oracle that provides a comfortable certainty on this key issue.
It’s
worth noting that this best possible future, where the risk of a
mini-runaway in warming to PETM or Permian levels remains low,
probably won’t happen as business as usual fossil fuel emissions
continue unabated with no sign of being rationally held in check.
Under the current regime, a CO2e of about 550 ppm, enough to warm the
Earth between 5-6 C long term, is locked in within 25-30 years. A
climate state that pushes the risk of a mini-runaway to moderate.
Meanwhile, levels that would almost certainly set off a Permian or
PETM type, anoxic ocean, extinction event, at around 800 ppm CO2e,
become possible under BAU by 2060-2080.
The
situation is, therefore, once again worse than expected…
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