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No More ‘Hiatus’ — Human Emission to Completely Overwhelm Nature by 2030
2
September, 2014
Keep
burning fossil fuels at current rates and you can kiss nature’s
influence over temperature good-bye. That’s the conclusion of two
recent scientific studies.
*****
Humans
are forcing heat trapping substances into the atmosphere at a
terrifying pace. We pump out more than 10 billion tons of carbon on
the back of about 50 billion tons of CO2 equivalent hothouse gasses
into the atmosphere each year. This massive volume is the upshot of
an inexorably rising emission starting during the 19th century and
continuing to this day. By the end of this century, rates of burning
could again increase nearly threefold.
The
current, rampant pace of human emission is now at least six times
faster than at any time during Earth’s geological past. But on our
current path, that rate could exceed 20 times that seen during any of
the previous worst hothouse extinction events.
(Current rate of annual carbon emissions in gigatons [black dots] compared to IPCC projected scenarios. Note that current human emissions are on the worst case emissions path. Image source: Global Carbon Project.)
This
incredible rate of emission was the key factor in two new studies
issued this week investigating the possibility of future hiatuses or
‘pauses’ in global warming due to nature-driven variability (see
the studies here and here).
And what the studies found was that rampant human burning of fossil
fuels removed any possibility for hiatus decades driven by natural
variability after 2030.
In
essence, we are in the process of shutting down nature’s
temperature-related influence entirely.
Understanding
Natural Cycling Between Warm and Cool Periods
Natural
shifts between atmospheric warm and cool spells appear to be
primarily driven by how much heat the oceans uptake or expel.
In
the Pacific, this rate of heat uptake is driven by the strength or
weakness of the trade winds driving across the equator. During
periods in which the trade winds are strong, a great volume of air
contacts the surface water and more atmospheric heat is driven into
the ocean through down-welling. During periods in which the trade
winds are weak, the atmosphere-to-ocean heat transfer shuts down even
as warmer waters rise from the depths and spread out across the ocean
surface. During these times, the ocean is dumping heat back into the
atmosphere.
A
similar process happens in the Atlantic where salty, warm surface
water down-welling transfers atmospheric heat toward the deep ocean.
When that process shuts down, more heat piles up at the ocean surface
and bleeds back into the atmosphere.
The
first of these processes is called Interdecadal Pacific Oscillation
(IPO — which is related to ENSO variations) and is thought to be
the primary governor of this global natural variability. The second
process, Atlantic Multidecadal Oscillation (AMO), is thought to be
the lesser of the two forces.
Past
Variability in the Global Temperature Record
Even
with large-scale human warming proceeding throughout the 20th Century
and into the 21st Century, we can see the effects of this natural
variability on the global temperature record. During naturally driven
warmer periods human-caused warming advances rapidly. During two of
the naturally driven cool periods, human forced warming has a
set-back, and, during the third, only seems to briefly slow down.
(Global temperature record as compiled by NASA. Note how warming has traditionally proceeded in a step-like fashion. Image source: NASA GISS.)
Taking
a closer look, we find that the time from 1880 to 1910 saw global
surface temperatures falling by 0.2 degrees Celsius as ocean heat
uptake increased and IPO went negative. From 1910 to 1940, the IPO
driver switched into positive. As the oceans disgorged their heat,
the first effects of human greenhouse gas heat forcing became evident
as global temperatures jumped by 0.45 degrees Celsius over a 30 year
period. From 1945 to 1975, IPO again switched into negative, but this
time human forcing was in the driver’s seat and temperatures only
fell by around 0.06 degrees Celsius. By 1975, temperatures were again
on the rise and through 2002, the heat spike rocketed fully 0.6
degrees C upward.
From
about 2002 onward, we enter the current ‘hiatus’ period in which
atmospheric warming, during a time when we should have seen cooling,
has proceeded slowly despite major natural variability factors
pushing for cooler atmospheres and warmer oceans.
Reducing
Impacts of Natural Variability
The
term ‘global warming hiatus,’ however inaccurate, is a new
invention. Its use first cropped up over the past couple of years as
human greenhouse gas forced warming seemed to slow somewhat from its
rampant upward pace through the 1980s and 1990s. This brief pause in
atmospheric warming caused some global warming skeptics to declare an
end to human-caused heating. An extraordinary claim in the face of
highest ever heat-trapping gas emissions.
But
what was really happening was that natural variability, which should
have been driving the Earth’s atmosphere to cool, was starting to
take a back seat.
For
two recent studies, mentioned above, found that natural variability
driven temperature change has radically fallen even since the 1980s.
The
first study, headed by Masahiro Wantanabe, found
that, during the 1980s natural variability was responsible for about
47 percent of the observed global temperature change. By the 1990s,
this number had fallen to 37 percent. And as an IPO driven switch
should have led to cooler temperatures during the 2000s, Wantanabe
finds that the effect of natural variability had again plunged to 27
percent.
The
cause for the loss of the temperature driving effect of natural
variability, according to global climate model runs, is a stunning
rate of human greenhouse gas increase. And
a related study led by Nicola Maher found
that if greenhouse gas emissions by humans kept rising at ever more
rapid rates, the natural variability measure is completely
overwhelmed by 2030:
The
likelihood of future hiatus periods is found to be sensitive
to the rate of change of anthropogenic forcing. Under
high rates of greenhouse gas emissions there is little chance of a
hiatus decade occurring beyond 2030, even in the event of a large
volcanic eruption.
Under
the worst case emissions scenario — RCP 8.5 — natural variability
is completely subsumed by human warming by 2030. Continuing on this
track through 2100 means that the human forcing is so strong that
even a volcanic eruption on the scale of Krakatau would not be enough
to generate a warming hiatus.
For
those considering use of solar radiation mitigation through aerosol
inject, this point is a very important one to consider. It is a basis
for proof that such mitigation eventually radically fails to reduce
greenhouse gas heating effects if levels of emissions are not also
drawn down.
Sadly,
we are currently on the RCP 8.5 track. But, according to the studies,
if humans could somehow rapidly reduce greenhouse gas emissions, the
effect of natural variability on climate would be at least somewhat
preserved.
Human-Forced
Variability
The
discussion of natural variability does not include instances in which
human heat forcing produces outcomes outside of natural variability.
The most obvious of these would be a large glacial outburst event in
which enough water is released from Greenland and West Antarctica to
raise seas by 1 meter or more this century. Such an event would have
a temporary cooling effect that could result in an unnatural hiatus
in warming. Such a human-forced variability was not considered in
these global climate model studies, but it is worth considering as
the strength of the now rampant human heat forcing continues to
increase.
Links:
Hat
Tip to Colorado Bob
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