BatteriesNotIncluded
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MediocrityNET: because people died for this!
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8 Summary implications
Humanity faces near certainty of eventual sea level rise of at least Eemian proportions,
5–9 m, if fossil fuel emissions continue on a business-as-usual course, e.g., IPCC scenario
A1B that has CO2 ∼ 700 ppm in 2100 (Fig. S21). It is unlikely that coastal cities
or low-lying areas such as Bangladesh, European lowlands, and large portions of the
United States eastern coast and northeast China plains (Fig. S22) could be protected
against such large sea level rise.
Rapid large sea level rise may begin sooner than generally assumed. Amplifying
feedbacks, including slowdown of SMOC and cooling of the near-Antarctic ocean surface
with increasing sea ice, may spur nonlinear growth of Antarctic ice sheet mass
loss. Deep submarine valleys in West Antarctica and the Wilkes Basin of East Antarctica,
each with access to ice amounting to several meters of sea level, provide gateways
to the ocean. If the Southern Ocean forcing (subsurface warming) of the Antarctic ice
sheets continues to grow, it likely will become impossible to avoid sea level rise of
several meters, with the largest uncertainty being how rapidly it will occur.
The Greenland ice sheet does not have as much ice subject to rapid nonlinear disintegration,
so the speed at which it adds to 21st century sea level rise may be limited.
However, even a slower Greenland ice sheet response is expected to be faster than
carbon cycle or ocean thermal recovery times. Therefore, if climate forcing continues
to grow rapidly, amplifying feedbacks will assure large eventual mass loss. Also with
present growth of freshwater injection from Greenland, in combination with increasing
North Atlantic precipitation, we already may be on the verge of substantial North
Atlantic climate disruption.
Storms conjoin with sea level rise to cause the most devastating coastal damage.
End-Eemian and projected 21st century conditions are similar in having warm tropics
and increased freshwater injection. Our simulations imply increasing storm strengths
for such situations, as a stronger temperature gradient caused by ice melt increases
baroclinicity and provides energy for more severe weather events. A strengthened
Bermuda High in the warm season increases prevailing northeasterlies that can help
account for stronger end-Eemian storms. Weakened cold season sea level pressure
south of Greenland favors occurrence of atmospheric blocking that can increase wintertime
Arctic cold air intrusions into northern midlatitudes.
Effects of freshwater injection and resulting ocean stratification are occurring sooner
in the real world than in our model. We suggest that this is an effect of excessive small
scale mixing in our model that limits stratification, a problem that may exist in other
models (Hansen et al., 2011). We encourage similar simulations with other models,
with special attention to the model’s ability to maintain realistic stratification and perturbations.
This issue may be addressed in our model with increased vertical resolution,
more accurate finite differencing method in ocean dynamics that reduces noise, and
use of a smaller background diffusivity.
There are many other practical impacts of continued high fossil fuel emissions via climate
change and ocean acidification, including irreplaceable loss of many species, as
reviewed elsewhere (IPCC, 2013, 2014; Hansen et al., 2013a). However, sea level rise
sets the lowest limit on allowable human-made climate forcing and CO2
, because of the
extreme sensitivity of sea level to ocean warming and the devastating economic and
humanitarian impacts of a multi-meter sea level rise. Ice sheet response time is shorter
than the time for natural geologic processes to remove CO2
from the climate system,
so there is no morally defensible excuse to delay phase-out of fossil fuel emissions as
rapidly as possible.
We conclude that the 2 ◦C global warming “guardrail”, affirmed in the Copenhagen
Accord (2009), does not provide safety, as such warming would likely yield sea level
rise of several meters along with numerous other severely disruptive consequences for
human society and ecosystems. The Eemian, less than 2 ◦C warmer than pre-industrial
Earth, itself provides a clear indication of the danger, even though the orbital drive for
Eemian warming differed from today’s human-made climate forcing. Ongoing changes
in the Southern Ocean, while global warming is less than 1 ◦C, provide a strong warning,
as observed changes tend to confirm the mechanisms amplifying change. Predicted
effects, such as cooling of the surface ocean around Antarctica, are occurring
even faster than modeled.
Our finding of global cooling from ice melt calls into question whether global temperature
is the most fundamental metric for global climate in the 21st century. The first
order requirement to stabilize climate is to remove Earth’s energy imbalance, which is
now about +0.6 W m−2
, more energy coming in than going out. If other forcings are
unchanged, removing this imbalance requires reducing atmospheric CO2
from ∼ 400
to ∼ 350 ppm (Hansen et al., 2008, 2013a).
The message that the climate science delivers to policymakers, instead of defining
a safe “guardrail”, is that fossil fuel CO2 emissions must be reduced as rapidly as
practical. Hansen et al. (2013a) conclude that this implies a need for a rising carbon
fee or tax, an approach that has the potential to be near-global, as opposed to national
caps or goals for emissio..
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