Here the two concluding sections of J. Hansen et alii‘s paper: Ice melt, sea level rise and superstorms: evidence from paleoclimate data, climate modeling, and modern observations that 2 ◦C global warming is highly dangerous. You can find the whole paper, footnotes, bibliography & figures here.
7 The Anthropocene
The Anthropocene (Crutzen and Stoermer, 2000), the era in which humans have contributed to global climate change, is usually assumed to have begun in the past few 25 centuries. Ruddiman (2003) suggested that it began earlier, with deforestation affecting CO2 about 8000 years ago. Southern Ocean feedbacks considered in our present paper are relevant to that discussion.
Ruddiman (2003) assumed that 40 ppm of human-made CO2 was needed to explain a 20 ppm CO2 increase in the Holocene (Fig. 24c), because CO2 decreased ∼ 20 ppm, on average, during several prior interglacials. Such a large human source should have left an imprint on δ 13CO2 that is not observed in ice core CO2 (Elsig et al., 2009). Ruddiman (2013) suggests that 13 5 C was taken up in peat formation, but the required peat formation would be large and no persuasive evidence has been presented to support such a dominant role for peat in the glacial carbon cycle.
We suggest that Ruddiman overestimated the anthropogenic CO2 needed to prevent decline of Antarctic temperature. The CO2 decline in interglacial periods is a climate 10 feedback: declining Southern Ocean temperature slows the ventilation of the deep ocean, thus sequestering CO2 . Avoidance of the cooling and CO2 decline requires only human-made CO2 forcing large enough to counteract the weak natural forcing trend, not the larger feedback-driven CO2 changes in prior interglacials, because, if the natural forcings are counteracted, the feedback does not occur. The required human- 15 made contribution to atmospheric CO2 would seem to be at most ∼ 20 ppm, but less if human-made CO2 increased deep ocean ventilation. The smaller requirement on the human source and the low δ 13C content of deep-ocean CO2 make the Ruddiman hypothesis more plausible, but recent carbon cycle models (Kleinen et al., 2015) have been able to capture CO2 changes in the Holocene and earlier interglacials without an 20 anthropogenic source.
Even if the Anthropocene began millennia ago, a fundamentally different phase, a Hyper-Anthropocene, was initiated by explosive 20th century growth of fossil fuel use. Human-made climate forcings now overwhelm natural forcings. CO2 , at 400 ppm in 2015, is off the scale in Fig. 24c. CO2 climate forcing is a reasonable approximation 25 of the net human forcing, because forcing by other GHGs tends to offset negative human forcings, mainly aerosols (IPCC, 2013). Most of the forcing growth occurred in the past several decades, and two-thirds of the 0.9 ◦C global warming (since 1850) has occurred since 1975 (update of Hansen et al., 2010, available at http://www.columbia. edu/~mhs119/Temperature/).
Our analysis paints a different picture than IPCC (2013) for how this HyperAnthropocene phase is likely to proceed if GHG emissions grow at a rate that continues to pump energy at a high rate into the ocean. We conclude that multi-meter sea level rise would become practically unavoidable. Social disruption and economic con- 5 sequences of such large sea level rise could be devastating. It is not difficult to imagine that conflicts arising from forced migrations and economic collapse might make the planet ungovernable, threatening the fabric of civilization.
This image of our planet with accelerating meltwater includes growing climate chaos and storminess, as meltwater causes cooling around Antarctica and in the North At- 10 lantic while the tropics and subtropics continue to warm. Rising seas and more powerful storms together are especially threatening, providing strong incentive to phase down CO2 emissions rapidly.
8 Summary implications
Humanity faces near certainty of eventual sea level rise of at least Eemian proportions, 15 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 25 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 5 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. 10 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 15 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 20 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 25 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 5 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 10 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 15 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 20 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 25 fee or tax, an approach that has the potential to be near-global, as opposed to national caps or goals for emission reductions. Although a carbon fee is the sine qua non for phasing out emissions, the urgency of slowing emissions also implies other needs including widespread technical cooperation in clean energy technologies (Hansen et al., 2013a).
The task of achieving a reduction of atmospheric CO2 is formidable, but not impossible. Rapid transition to abundant affordable carbon-free electricity is the core requirement, as that would also permit production of net-zero-carbon liquid fuels from electricity. The rate at which CO2 emissions must be reduced is about 6 % yr−1 to reach 5 350 ppm atmospheric CO2 by about 2100, under the assumption that improved agricultural and forestry practices could sequester 100 GtC (Hansen et al., 2013a). The amount of CO2 fossil fuel emissions taken up by the ocean, soil and biosphere has continued to increase (Fig. S23), thus providing hope that it may be possible to sequester more than 100 GtC. Improved understanding of the carbon cycle and non-CO2 10 forcings are needed, but it is clear that the essential requirement is to begin to phase down fossil fuel CO2 emissions rapidly. It is also clear that continued high emissions are likely to lock-in continued global energy imbalance, ocean warming, ice sheet disintegration, and large sea level rise, which young people and future generations would not be able to avoid. Given the inertia of the climate and energy systems, and the grave 15 threat posed by continued high emissions, the matter is urgent and calls for emergency cooperation among nations.