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Smashing Through 490 — Fragmenting Prospects for Avoiding 2 C Warming
“The
IPCC indicated in its fourth assessment report that achieving a 2 C
target would mean stabilizing greenhouse gas concentrations in the
atmosphere at around 445 to 490 ppm CO2 equivalent or lower. Higher
levels would substantially increase the risks of harmful and
irreversible climate change.”
(NOAA’s
greenhouse gas index shows that CO2e concentration for 2015 averaged
485 ppm. Given recent rates of rise, the 2016 average should be near
490 ppm CO2e. At the latest, this key threshold will be crossed some
time during 2017. Image source: NOAA’s
Earth Systems Research Laboratory.)
*****
12
August, 2016
There
are a few things we know about climate change that should really keep
us up at night. The first is that the world is warming, and this
warming of the Earth, in so many ways, is dangerous to human beings
and all the other innocent creatures living here.
The
second is that, over recent years, this warming has been very rapid.
In the three years from 2014 through 2016, the Earth’s atmospheric
temperature is likely to have increased by 0.2 degrees Celsius or
more to around 1.2 C above 1880s levels. When thinking about this in
absolute terms, it doesn’t sound like much. But in geological
terms, this is very rapid warming, especially when you consider that,
at the end of the last ice age, it took about 400 years to produce a
similar amount of atmospheric temperature gain.
What
all this boils down to is that as global temperatures have spiked,
we’ve rapidly crossed an established climate threshold into a far
more geophysically dangerous time.
Surging
Levels of Heat-Trapping Gasses
405
parts per million carbon dioxide.
That’s about the average level of CO2 accumulation the Earth’s
atmosphere will see by the end of 2016, due primarily to fossil-fuel
burning. It’s a big number. The Earth hasn’t seen a number like
that in millions of years. But 405 ppm CO2 doesn’t tell the whole
story of heat-trapping gasses in the atmosphere. To do that, we have
to look at another number — carbon dioxide equivalent or CO2e.
(During
a typical September and October, daily or weekly values may briefly
dip below 400 ppm CO2, as detected at the Mauna Loa Observatory. But
after September-October 2016, it’s unlikely that you or I will ever
see such low levels of CO2 from that measure again in our lifetimes.
Image source: Scripps
Institution of Oceanography.)
490
ppm CO2e.
That’s about the total amount of CO2-equivalent heat forcing from
all the human-added greenhouse gasses like CO2, methane, various
nitrogen compounds, and other gaseous chemical waste that the Earth’s
atmosphere will see by late 2016 to early 2017.
Why
is this a big deal?
Four
reasons —
First,
hitting 490 CO2e crosses the Intergovernmental
Panel on Climate Change‘s
(IPCC) lowest climate threshold. If this were a highway, and climate
change were a collision, we’d now be careening through the first
guardrail.
Second,
490 CO2e represents significant current and future warming (and
there’s good reason to believe that IPCC’s
estimates of that warming may
be a bit conservative).
Third,
it signifies that we have now fully entered the era of
catastrophic climate change, with some bad climate outcomes almost
certainly locked in as a result. We see a number of these instances
now in the form of extreme rainfall events, extreme drought, coral
bleaching, sea ice and glacial melt, threatened crops, ocean anoxia
and dead zones, widespread harmful algae blooms, ocean acidification,
and expanding infectious disease ranges. However, what we are
experiencing now is just the tip of the (melting) climate change
iceberg if we do not rapidly respond.
Fourth,
if we were never really aware before that we very urgently need to
get serious about swiftly cutting fossil-fuel emissions, protecting
and regrowing forests, and working to help people to adapt to climate
change, then this is our wake-up call.
Crossing
the First Climate Threshold — 490 ppm CO2e
How
did 490 ppm CO2e become a climate milestone? In short, it represents
the threshold at which the first of four global-warming scenarios is
basically locked in.
To
understand this more, we need to take a closer look at these four
scenarios, which were established by the IPCC in 2007. The IPCC calls
these scenarios Representative
Concentration Paths or RCPs.
The four potential pathways are informed by the amount of fossil
fuels potentially burned through the year 2100, the levels of CO2e
heat-trapping gasses in the atmosphere as a result, and how much the
world consequently warms over this timeframe.
RCPs
range from 2.6 to 8.5 watts per meter squared, with these
measurements indicating the amount of added heat from the greenhouse
gas additions trapped at the top of the atmosphere. A more
direct measure is to look at the total greenhouse gas thresholds for
each scenario. Broken down, the four RCP pathways represent 490 ppm
CO2e (RCP 2.6), 650 ppm CO2e (RCP 4.5), 850 ppm CO2e (RCP 6.0), and
1370 ppm CO2e (RCP 8.5). For reference, atmospheric CO2e levels just
prior to the start of large-scale fossil fuel burning were around 300
ppm. By comparison, 1370 ppm CO2e is about equivalent to the levels
during some of the worst hothouse mass extinctions the Earth has
experienced.
In
a nutshell, RCPs represent potential warming scenarios. A
middle-range temperature increase estimate by the year 2100 for each
scenario can be seen below in this table provided by Skeptical
Science:
Developed
at the IPCC’s 2007 meeting, these RCPs also describe a range of
potential human civilization responses to global warming. RCP 2.6
allows for fast emissions cuts beginning at the time of the 2007
meeting. These cuts would swiftly level off and then reduce
fossil-fuel emissions and ultimately generate one of the milder
warming scenarios. The IPCC envisioned that warming would remain near
1.5 C this century under these emissions cuts. Scientists hoped this
scenario would allow the avoidance of most of climate change’s bad
outcomes.
RCP
4.5 assumes somewhat less aggressive emissions cuts, with
fossil-fuel burning and related carbon emissions peaking near 15
billion tons per year by the mid-2040s. Stronger warming is locked in
with this scenario — about 2.4 C according to IPCC — and
scientists were doubtful that serious climate impacts could be
avoided.
(We’ve
pretty much missed the window for the IPCC’s mildest possible
climate scenario, RCP 2.6, which would have required strong policies
and policy support almost immediately following the IPCC’s 2007
meeting. Image source: Skeptical
Science.)
RCP
6.0 shows emissions cuts that are slow to unfold. Global carbon
emissions would peak around 19 billion tons per year by 2060 and then
rapidly drop off. Warming under this scenario is considerable,
hitting 3 C by the end of this century. So much warming and such high
levels of greenhouse gasses would result in some seriously bad
outcomes.
The
final pathway, RCP 8.5, represents an absolute nightmare climate
scenario. Under this path, real emissions cuts are not achieved.
Despite growth in renewable energy, all energy
use continues to grow as well, including fossil fuels. As a result,
in this scenario, the IPCC expects the Earth to warm by a
catastrophic 4.9 C by 2100.
In
the context of understanding climate change, particularly for someone
interested and patient enough to read the IPCC reports, the various
RCP scenarios were a real help in exploring climate change options
and outcomes. They helped many scientists and policymakers provide
clear warnings and rewards for action by governments, the public, and
business leaders.
(Climate
change produces multiple difficult-to-predict impacts. As
temperatures rise, conditions grow ever more extreme. In the graph
above, it’s worth noting that sea-level rise is already an issue
for many cities and regions including numerous Pacific islands,
Bangladesh, the Indus Delta region, South Florida, New Orleans, New
York, and the various low-lying coastal and river delta regions
around the world. Image source: Federal
Highway Administration.)
But
despite very clear communication and activism from scientists like
Dr. James Hansen, policy bloggers like Joe Romm, and climate
activists like Bill McKibben, overall global emissions policy has not
moved swiftly enough to stay within the RCP 2.6 pathway in the 9
years since its creation. In fact, decent global emissions reduction
policies didn’t begin to universally take hold until recently, in
2014 and 2015, and those implemented were often ardently opposed by
fossil fuel-related political interests in countries like Australia,
Great Britain, Germany, Canada, and the United States.
As
a result, emissions stayed near or just below worst-case pathway
ranges (RCP 8.5). As of this year, the window for achieving the RCP
2.6 scenario — or the mildest and most optimistic warming scenario
— appears to have closed.
Possibly
More Warming From 490 CO2e Than We Feared
Hitting
490 CO2e in 2016 means that the 1.5 C warming IPCC predicted for this
amount by 2100 is almost certainly locked in. With the world hitting
near 1.2 C above 1880s temperature averages in 2016, some reasonable
questions have been raised, the most relevant being if 490 ppm CO2e
will result in more warming than IPCC predicted.
To
be fair, the 1.5 C figure above is a simplification of model
predictions ranging from about 0.9 C to around 2.3 C during this
century under a 490 ppm CO2e forcing. However, since we’ve already
surpassed the lower portion of this range, and we’re barely into
the beginning of this century, it appears that some of the lower
sensitivity model runs were rather far off the mark.
Moreover, paleoclimate
proxy temperature data indicates
that 490 ppm CO2 during the Middle Miocene produced warming in the
range of 4 C long-term (over hundreds of years). Given this implied
long-term impact, and coupled with annual readings that are already
in the 1.2 C range, it’s possible to infer an ultimate warming
closer to 2 C by 2100 from a maintained 490 ppm CO2e. Hitting such a
mark would only require about 0.11 C warming per decade — a rate of
decadal warming about 40 percent slower than the temperature rise
seen from the late 1970s through the 2010s.
(Amplifying
feedbacks due to loss of sea ice reflectivity in the Arctic and
Antarctic, reduced carbon-store uptake and carbon-store emissions can
result in an overall greater sensitivity to an initial heat forcing
such as the current 490 ppm CO2e. Paleoclimate proxies hint that
these feedbacks may cause the Earth System to be more sensitive than
IPCC models currently indicate. Image source: LANCE
MODIS.)
The
paleoclimate-implied warming from the other climate scenarios is
likely higher as well. RCP 4.5 probably hits closer to 3 C under such
a climate sensitivity range. RCP 6.0 probably sees 4 to 4.5 C warming
by 2100. And the worst-case RCP 8.5 probably achieves closer to 6 C
warming.
It’s
for these and other reasons that some scientists say that avoiding
1.5 C at this time is probably impossible. Meanwhile, it’s pretty
reasonable to say that avoiding 2 C presents a huge challenge
requiring a very rapid response, a goal that will probably require
reducing the atmospheric CO2e levels below their current ranges.
CO2e
Increasing by 3 ppm Per Year
Human
beings are still dumping massive volumes of carbon into the
atmosphere. Carbon emissions are still near record-high levels. As a
result, atmospheric CO2e levels are rising by about 3 ppm or more
each year. For 2016, CO2 alone may rise by 3.4 ppm or more, and CO2e
may jump by more than 4 ppm — to hit near 490 ppm CO2e. This is due
in part to the 2015-2016 El Nino’s cyclical warming of the
Equatorial oceans, forests, and lands on top of the
already-strengthening heat of human warming. And
this added heat reduces the ability of these carbon sinks to take in
CO2.
Even
if this rate of CO2e rise is just maintained, it’s possible that
we’ll see 1.5 C warming not by the end of this century, but by the
early 2030s. And as the world heats up, it’s likely we’ll see
additional emissions coming as carbon
sinks become stressed and stop taking in such high volumes of
greenhouse gasses or even turn into sources.
The
result is that the challenge presented to us now is far greater, far
more urgent than that of 2007. We risk, over the next few
decades, locking in not just 2 C warming, but 3 C warming or more if
we do not act swiftly and seriously. And with 1.5 C warming coming
with almost 100 percent certainty, we need to ramp up climate-change
mitigation strategies as well as provide aid and succor for the
increasing harms, dislocations, and inequalities that will likely
emerge.
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