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Messages - GeoffBeacon

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Antarctica / Re: Ice Apocalypse - MULTIPLE METERS SEA LEVEL RISE
« on: October 28, 2020, 03:53:03 PM »
The linked reference confirms that Atlantification of the Arctic Ocean not only reduces sea ice cover along the Russian continental shelves, but increasing in other regions of the Arctic Ocean.  This trend will not only increase Arctic Amplification beyond what is currently assumed by consensus climate science, but also increases the risk of potentially meaningful amount of methane emissions from hydrate decomposition in both Arctic continental shelf and slope regions.

Belter, H. J., Krumpen, T., von Albedyll, L., Alekseeva, T. A., Frolov, S. V., Hendricks, S., Herber, A., Polyakov, I., Raphael, I., Ricker, R., Serovetnikov, S. S., Webster, M., and Haas, C.: Interannual variability in Transpolar Drift ice thickness and potential impact of Atlantification, The Cryosphere Discuss.,, in review, 2020.

Abstract. Changes in Arctic sea ice thickness are the result of complex interactions of the dynamic and variable ice cover with atmosphere and ocean. Most of the sea ice exits the Arctic Ocean through Fram Strait, which is why long-term measurements of ice thickness at the end of the Transpolar Drift provide insight into the integrated signals of thermodynamic and dynamic influences along the pathways of Arctic sea ice. We present an updated time series of extensive ice thickness surveys carried out at the end of the Transpolar Drift between 2001 and 2020. Overall, we see a more than 20 % thinning of modal ice thickness since 2001. A comparison with first preliminary results from the international Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) shows that the modal summer thickness of the MOSAiC floe and its wider vicinity are consistent with measurements from previous years. By combining this unique time series with the Lagrangian sea ice tracking tool, ICETrack, and a simple thermodynamic sea ice growth model, we link the observed interannual ice thickness variability north of Fram Strait to increased drift speeds along the Transpolar Drift and the consequential variations in sea ice age and number of freezing degree days. We also show that the increased influence of upward-directed ocean heat flux in the eastern marginal ice zones, termed Atlantification, is not only responsible for sea ice thinning in and around the Laptev Sea, but also that the induced thickness anomalies persist beyond the Russian shelves and are potentially still measurable at the end of the Transpolar Drift after more than a year. With a tendency towards an even faster Transpolar Drift, winter sea ice growth will have less time to compensate the impact of Atlantification on sea ice growth in the eastern marginal ice zone, which will increasingly be felt in other parts of the sea ice covered Arctic.

Eventually the heat stored in the deep ocean comes back to the surface.  If we can lower the greenhouse gas concentrations in the atmosphere before it comes back to the surface, the stored heat can radiate out to space when the warmer water upwells to the surface.

Only a small portion of the warm water comes into contact with the ice sheets.  Most of it circulates around the globe for centuries.

Here are a couple of studies that discuss the Southern Ocean (where most of the excess heat gets stored) and how it interacts with the Antarctic Ice Sheet.

The Southern Ocean and its interaction with the Antarctic Ice Sheet
David M. Holland, Keith W. Nicholls and Aurora Basinski
DOI: 10.1126/science.aaz5491 (6484), 1326-133

The Southern Ocean exerts a major influence on the mass balance of the Antarctic Ice Sheet, eitherindirectly, by its influence on air temperatures and winds, or directly, mostly through its effects on iceshelves. How much melting the ocean causes depends on the temperature of the water, which in turn is controlled by the combination of the thermal structure of the surrounding ocean and local ocean circulation, which in turn is determined largely by winds and bathymetry. As climate warms and atmospheric circulation changes, there will be follow-on changes in the ocean circulation and temperature. These consequences will affect the pace of mass loss of the Antarctic Ice Sheet.

Sallée, J.-B. 2018. Southern Ocean warming.
Oceanography 31(2):52–62,

Article Abstract

The Southern Ocean plays a fundamental role in global climate. With no continental barriers, it distributes climate signals among the Pacific, Atlantic, and Indian Oceans through its fast-flowing, energetic, and deep-reaching dominant current, the Antarctic Circumpolar Current. The unusual dynamics of this current, in conjunction with energetic atmospheric and ice conditions, make the Southern Ocean a key region for connecting the surface ocean with the world ocean’s deep seas. Recent examinations of global ocean temperature show that the Southern Ocean plays a major role in global ocean heat uptake and storage. Since 2006, an estimated 60%–90% of global ocean heat content change associated with global warming is based in the Southern Ocean. But the warming of its water masses is inhomogeneous. While the upper 1,000 m of the Southern Ocean within and north of the Antarctic Circumpolar Current are warming rapidly, at a rate of 0.1°–0.2°C per decade, the surface sub­polar seas south of this region are not warming or are slightly cooling. However, subpolar abyssal waters are warming at a substantial rate of ~0.05°C per decade due to the formation of bottom waters on the Antarctic continental shelves. Although the processes at play in this warming and their regional distribution are beginning to become clear, the specific mechanisms associated with wind change, eddy activity, and ocean-ice interaction remain areas of active research, and substantial challenges persist to representing them accurately in climate models.

A vs B

No because A is not really happening and B is bad enough and is also not happening.

We will C which means overshooting 1,5 and then stabilizing some time later but all that before 2100.

Our goal though would be to decrease the forcings over time and bring the temperatures down to avoid losing too much of the Greenland and Antarctic ice sheets.

If we are really honest we have to admit that 1C with no overshoot might have worked.

If the world had aimed for 1C with max overshoot of 1.5C we could still be where we are now. So about to lose the arctic ice in a decade or two and the permafrost being a source not a sink since early this century.

The situation in Antarctica also deteriorated the recent years and it is not certain we can stop this process at all.

The Climate & Clean Air Coalition discusses Short Lived Climate Pollutants (SLCP), including methane, often in the context of the Sustainable Development Goals framework.

The linked article about the differences between the short-term and long-term global warming potentials of methane indicates that focusing exclusively on methane reductions at the expense of reducing carbon dioxide emissions results in higher long term temperature increases.

Demonstrating GWP*: a means of reporting warming-equivalent emissions that captures the contrasting impacts of short- and long-lived climate pollutants
John Lynch et al 2020 Environ. Res. Lett.15 044023

The atmospheric lifetime and radiative impacts of different climate pollutants can both differ markedly, so metrics that equate emissions using a single scaling factor, such as the 100-year Global Warming Potential (GWP100), can be misleading. An alternative approach is to report emissions as ‘warming-equivalents’ that result in similar warming impacts without requiring a like-for-like weighting per emission. GWP*, an alternative application of GWPs where the CO2-equivalence of short-lived climate pollutant emissions is predominantly determined by changes in their emission rate, provides a straightforward means of generating warming-equivalent emissions. In this letter we illustrate the contrasting climate impacts resulting from emissions of methane, a short-lived greenhouse gas, and CO2, and compare GWP100 and GWP* CO2-equivalents for a number of simple emissions scenarios. We demonstrate that GWP* provides a useful indication of warming, while conventional application of GWP100 falls short in many scenarios and particularly when methane emissions are stable or declining, with important implications for how we consider ‘zero emission’ or ‘climate neutral’ targets for sectors emitting different compositions of gases. We then illustrate how GWP* can provide an improved means of assessing alternative mitigation strategies. GWP*allows warming-equivalent emissions to be calculated directly from CO2-equivalent emissions reported using GWP100, consistent with the Paris Rulebook agreed by the UNFCCC, on condition that short-lived and cumulative climate pollutants are aggregated separately, which is essential for transparency. It provides a direct link between emissions and anticipated warming impacts, supporting stocktakes of progress towards a long-term temperature goal and compatible with cumulative emissions budgets

We can demonstrate the utility of multi-gas cumulative CO2-w.e. totals in a decision making context by considering how they would describe alternative mitigation pathways, as infigure8. In this scenario, the emissions of one gas cease in year 50, and then the emissions of the remaining gas in year 100. Stopping methane first results in a large initial reversal of recent warming, but temperatures then start to rise again due to the ongoing CO2 emissions. Temperature then stabilises at the temperature reached in year 100 when CO2 emissions are also stopped. Stopping CO2 first,we see that the rate of warming declines, and then when methane emissions stop in year 100 we have a significant reversal of warming, stabilising at a lower long-term temperature than in the methane-first scenario. Cumulative CO2-w.e. provides a clear indication of these dynamics, while cumulative CO2e suggests either strategy would lead to the same response, but which represents neither scenario.

Found the paper.  It was published in PNAS in 2004.

Greenhouse gas growth rates
James Hansen* and Makiko Sato

We posit that feasible reversal of the growth of atmospheric CH4 and other trace gases would provide a vital contribution toward averting  dangerous  anthropogenic  interference  with  global  cli-mate. Such trace gas reductions may allow stabilization of atmospheric CO2 at an achievable level of anthropogenic CO2 emissions, even if the added global warming constituting dangerous anthropogenic  interference  is  as  small  as  1°C.  A  1°C  limit  on  global warming, with canonical climate sensitivity, requires peak CO2 ~440 ppm if further non-CO2 forcing is ~0.5 W/m2, but peak CO2 ~520 ppm if further non-CO2 forcing is ~0.5 W/m2. The practical result is that a decline of non-CO2 forcings allows climate forcing to be stabilized with a significantly higher transient level of CO2 emissions. Increased ‘‘natural’’ emissions of CO2, N2O, and CH4 are expected  in  response  to  global  warming.  These  emissions,  an indirect  effect  of  all  climate  forcings,  are  small  compared  with human-made climate forcing and occur on a time scale of a few centuries,  but  they  tend  to  aggravate  the  task  of  stabilizing atmospheric composition.

We have suggested (13) that a concerted effort to reduce CH4 emissions could yield a negative forcing, which would be amplified ~40% by the indirect effects of CH4 on stratospheric H2O and tropospheric O3. CH4by itself could yield a forcing change of ~0.25 W/m2 if it were reduced from today’s 1,755 ppb to 1,215 ppb, which would require reducing anthropogenic CH4 emissions by 40–50% (ref. 14 and Drew Shindell, personal communication). Conversely, CH4 could provide large positive forcing if emissions grow, e.g.,CH4 increases to 3,140 ppb in 2100 in the IPCC (3) IS92a scenario,yielding ~0.5 W/m2 forcing.

James Hansen wrote a paper that emphasized the benefits of decreasing methane concentrations in the short term while we worked on bringing down CO2 (a much harder task).  I can't find it thought (he's written a ton of papers).

The linked study provides a good assessment of our ability to reduce methane emissions over the next few decades.

Lena Höglund-Isaksson et al 2020 Environ. Res. Commun. 2 025004
Technical potentials and costs for reducing global anthropogenic methane emissions in the 2050 timeframe –results from the GAINS model


Methane is the second most important greenhouse gas after carbon dioxide contributing to human-made global warming. Keeping to the Paris Agreement of staying well below two degrees warming will require a concerted effort to curb methane emissions in addition to necessary decarbonization of the energy systems. The fastest way to achieve emission reductions in the 2050 timeframe is likely through implementation of various technical options. The focus of this study is to explore the technical abatement and cost pathways for reducing global methane emissions, breaking reductions down to regional and sector levels using the most recent version of IIASA's Greenhouse gas and Air pollution Interactions and Synergies (GAINS) model. The diverse human activities that contribute to methane emissions make detailed information on potential global impacts of actions at the regional and sectoral levels particularly valuable for policy-makers. With a global annual inventory for 1990–2015 as starting point for projections, we produce a baseline emission scenario to 2050 against which future technical abatement potentials and costs are assessed at a country and sector/technology level. We find it technically feasible in year 2050 to remove 54 percent of global methane emissions below baseline, however, due to locked in capital in the short run, the cumulative removal potential over the period 2020–2050 is estimated at 38 percent below baseline. This leaves 7.7 Pg methane released globally between today and 2050 that will likely be difficult to remove through technical solutions. There are extensive technical opportunities at low costs to control emissions from waste and wastewater handling and from fossil fuel production and use. A considerably more limited technical abatement potential is found for agricultural emissions, in particular from extensive livestock rearing in developing countries. This calls for widespread implementation in the 2050 timeframe of institutional and behavioural options in addition to technical solutions.

This article summarizes the study:

Three workable strategies for putting a big dent in methane, the “other” greenhouse gas
April 16, 2020
Andrew Urevig

Improve Waste Management

Yard waste and uneaten food decomposing in landfills vent methane into the air, so the study finds lots of potential in improved garbage management. The researchers estimate that separating waste by source, with better recycling and schemes to capture energy from some trash — plus a ban on organic waste in landfills — could help the world avoid emitting 778 million metric tons (858 million tons) of methane that would otherwise make its way into the air between now and 2050.

Repair Leaks

Ultimately, fossil fuels will also need to be phased out, Höglund Isaksson writes. But in the meantime, the study finds that we could slow the growth of methane emissions by taking steps such as implementing programs to detect and repair leaks in oil production and the extraction and transportation of natural gas. Coal mines could consistently implement degasification and improve ventilation, and oil drillers could try to recover associated gas. Such steps — with leakage detection and repair being the biggest — could prevent 2.35 billion metric tons (2.57 billion tons) of methane emissions by 2050.

Modify Agricultural Practices

Methane emissions from agriculture, the study finds, will be the hardest area for technical improvements. Rice cultivation’s footprint could decrease if farmers used alternative hybrids, improved water management and added materials to improve soil properties. These steps could avoid 335 million metric tons (370 million tons) of emitted methane by 2050. Livestock breeders could continue efforts to raise more productive animals: If farmers could use fewer cows to produce the same amount of milk, for example, that would cut back on emissions. This approach could yield different emissions results in cows, pigs, sheep and other livestock.

Just posted: Cut methane emissions soon, Sea level, methane and a false assumption

I'd still like to know if anyone agrees/disagrees.
Interesting post .

Never thought of it that way.

Basically that provides another reason to cut what methane emissions we can a.s.a.p.

But of course  we should already do that because we are probably already committed to a bad outcome.

while the contribution from ice sheets may not be reversible under any plausible future scenario (see below).

vs (or maybe just and...)

‘Teetering at the edge’: Scientists warn of rapid melting of Antarctica’s ‘Doomsday glacier’
Thwaites glacier is losing ice at an accelerating rate, threatening catastrophic sea-level rise


“The big question is how quickly it becomes unstable. It seems to be teetering at the edge,” Paul Cutler, programme director for Antarctic glaciology at America’s National Science Foundation told the Financial Times this week.


Recent research shows that after we hit reductions of CO2 we still have 15 years of land warming. For seas there is often a 40 year lag. Not sure if it is applicable to CDW but if you figure in a 40 year delay then the damage done now is from 1980.

And the eventual peak we committed too is at least this year.


The European Union is formulating policies to address methane emissions.

EC consults on methane leaks in push to clean up EU gas imports
13 Jul 2020
Siobhan Hall

Brussels — The European Commission is seeking views on how to reduce leaks of potent greenhouse gas methane from oil, gas and agricultural sectors as part of the EU's efforts to become climate-neutral by 2050.

Most of the methane leaks from fossil gas production and transport happen before the natural gas or LNG reaches the EU, so a new EU policy on methane emissions could have far-reaching impacts on the global gas market.

The key challenge is how to improve measuring, reporting and verifying emissions at the level of private entities, it said.

On average, 5% of sources account for 50% of the leaks, known as "super-emitters".

Leak detection and repair programs, as well as finding and addressing these "super-emitters, can be a very effective action," the EC said.

The IPCC Special Report on the Oceans and Cryosphere covers this topic in section and includes the reference on short lived greenhouse gases (Zickfeld et. al., 2017).

Beyond the 21st century, the relative importance of the long-term contributions of the various components of SLR changes markedly. For glaciers, the long-term is of limited importance, because the sea level equivalent of all glaciers is restricted to 0.32 ± 0.08 m when taking account of ice mass above present day sea level (Farinotti et al., 2019). Hence, there is high confidence that the contribution of glaciers to SLR expressed as a rate will decrease over the 22nd century under RCP8.5 (Marzeion et al., 2012). For thermal expansion the gradual rate of heat absorption in the ocean will lead to a further SLR for several centuries (Zickfeld et al., 2017).

While the IPCC SR1.5 didn't make it super-clear, they did discuss this issue in brief.  The reference that I bolded discusses the same issue for CO2.

Sea level

Policy decisions related to anthropogenic climate change will have a profound impact on sea level, not only for the remainder of this century but for many millennia to come (Clark et al., 2016). On these long time scales, 50 m of sea level rise (SLR) is possible (Clark et al., 2016). While it is virtually certain that sea level will continue to rise well beyond 2100, the amount of rise depends on future cumulative emissions (Church et al., 2013) as well as their profile over time (Bouttes et al., 2013; Mengel et al., 2018) . Marzeion et al. (2018) found that 28–44% of present-day glacier volume is unsustainable in the present-day climate and that it would eventually melt over the course of a few centuries, even if there were no further climate change. Some components of SLR, such as thermal expansion, are only considered reversible on centennial time scales (Bouttes et al., 2013; Zickfeld et al., 2013) , while the contribution from ice sheets may not be reversible under any plausible future scenario (see below).

Could you add some links like for:
According to IPCC AR5 (WG1 SPM), 90% of the "methane heat" remains, as Ocean Heat Content (OHC) [including the latent heat difference between ice mass and melt water?]. This remaining heat dissipates over a few centuries.

And the SIDS quote.


Policy and solutions / Re: Cars, cars and more cars Part Deux
« on: February 07, 2020, 09:32:47 PM »
Subsidizing free busses (E-Busses), cheap Light Rail and modern HSR - as opposed to paying the rich to buy expensive EV's is such an obvious move in the right direction that I'm astonished that there is any opposition.

We subsidize bridges rather than helping the wealthy purchase sailing yachts because everyone can walk across the damn bridge. Well, everyone can hop on a free bus, if you persist in demanding door to door service then I must insist that you pick up the tab.

If you insist on living far from the madding crowd, then I insist that you finance your own transportation rather than whining that what works in my city isn't suited to your more bucolic estate.

Millions have learned to live with their neighbors & close to their shops & jobs. If we're not forced to pay for your prefered lifestyle we may be able to provide free mass transportation for those masses that would gladly share a seat with a stranger.

The private automobile was an aberration that brought us to our knees. It had a short and inglorious history. Rather than reinventing the car we need to leave them in the smog filled miasma of our recent past.

Policy and solutions / Re: Cars, cars and more cars Part Deux
« on: February 07, 2020, 06:01:41 AM »
Desperately clinging to last straws ;)
Please try to imagine a future without private cars. Can you?

That's interesting GeoffBeacon, thanks.
"have not dared" is very likely: ESLD (erring on side of least drama, economical/governing/keep-dreams-alive)

Policy and solutions / Re: Cars, cars and more cars Part Deux
« on: February 07, 2020, 12:22:38 AM »
We can have cars to drive or a planet to live in.

But not both.

What if... we recycled ICE cars into EVs?  (And reduced the total number of cars overall, via car sharing, robotaxis, improved public transportation, etc.)... until more population centers are designed and built to minimize transportation needs?

Consequences / Re: Quantifying increased costs of living and CO2 output
« on: October 01, 2019, 01:18:49 AM »
Follow the money:  climate risk is underpriced

"The market's failure to integrate climate science with investment analysis has created a mispricing phenomenon that is possibly larger than the mortgage credit bubble of the mid-2000s"

"a key culprit for the mispriced risk in the U.S. mortgage market is outdated flood maps drawn by the federal government."

"Due to budget cuts, more than three-quarters of the maps have not been updated in at least five years"

"Outdated maps mean far fewer people are required to have flood insurance than are at risk,"

"The gaps are evident: About 70% of all damages to homes that were flooded during Harvey were not covered by insurance"

"the gaps mean the risk is not properly priced. The cost for an average policy in low-risk Green Bay, Wisconsin, for example, is three times that in Gulfport, Mississippi, a town devastated by Hurricane Katrina"

"The Federal Emergency Management Agency has said it aims to fix some of these problems with a major risk re-rating on Oct. 1, 2020."

"Burt�??s bet is that the move will result in significant cost increases. That in turn will lead to home price declines and mortgage losses, which would increase volatility in RMBS prices."

"He expects a correction beginning in the next 6-18 months. "

"Investors also have been taking on more risk. Some RMBS issued by Freddie Mac and Fannie Mae since 2017, called credit risk transfer (CRT) deals, move the risk  of default to the investors. In traditional agency RMBS, Fannie and Freddie cover those losses. "


Arctic sea ice / Re: When will the Arctic Go Ice Free?
« on: August 01, 2018, 07:02:31 PM »
A scenario of year-round ice-free Arctic can only be reached (IMO) by further a northward reach of the warm ocean currents.
I keep reading this dreamy misconception everywhere. People seem to be forgetting about the fact that the quantity of heat energy required to melt 1kg of ice (of just below freezing) to 1kg of water (of just above freezing) would raise the temperature of that same 1 kg of water to 80 degrees Celsius. This means that as soon as ice is gone, and there is heat energy (i.e. Sunlight), the oceans will be very hot at the surface (provided that surface T will also keep on rising as it does) all around the Arctic circle. It already is super anomalously warm, by the way. So when the sun is gone at the polar caps, all it needs is a little flow from warmer lower ocean currents to keep it from freezing up, and/or surface winds blowing the warmer (sun-heated) waters Northwards. Considering all the additional feedbacks, I'd say year round ice free poles could be a reality around 2035 at the very latest.

But water is not just stagnant in the Arctic and waiting to warm. While its true that a lot more heat can go into the water once there's no ice to melt, its also the case that the worlds oceans are very large, very deep, and circulating. I find it hard to believe that mainstream science is so wrong on the timescales for a year-round BOE. I'm not saying the current mainstream predictions are gospel and won't change, but 2035 is so at odds with the mainstream view that I find it hard to accept. No ice in winter also means more can escape, does it not? (No really, correct me if I'm wrong - I'm no expert!)

Arctic sea ice / Re: When will the Arctic Go Ice Free?
« on: August 01, 2018, 06:39:02 PM »
It is madness. Worse than pure denial of the entire situation. I'm less bothered by those who think it is all an elite globalist ploy to enslave the masses, than I am by those who engage with the data on a daily basis but come to the conclusion that mild solutions will be sufficient to save civilization.  OR for that matter those who think that it is no big deal to change the climate drastically and kill off humanity cuz the earth will bounce back.  Are we really going to successfully prevent nuclear war as everything falls apart? Are we really going to successfully decommission the hundreds of nuclear power plants around the world? Even if you aren't bothered by the collapse of civilization and the horrible deaths of billions of people, the possibility of turning the earth into a planet like venus or mars should give you some pause.

While I agree with your whole post, the quoted graf really resonated with me.  I find it astonishing how many people, many of them real, honest-to-Pete scientists, see the data and the (often conservative yet terrifying) projections, and aren't standing on tables screaming about this mess we've created.  Just as bad are the environmental activists who will argue that you can't tell mainstreamers the full truth about CC or "you'll scare them away".  (I've had that argument numerous times with local enviros.)

Barring some nearly miraculous ramping up of carbon removal and sequestration, there is no way we'll avoid something between horrific consequences and a full-blown, worldwide catastrophe.

As for the topic of this thread, while I missed the voting window, I would definitely have voted for the 2020-2025 period.  Trends plus variability means we won't need some wildly improbable set of events in a given summer to hit <1M km^2.  I would also predict it gets no more than 60 seconds of, "Golly, look at that!" coverage on TV news, with the usual suspects talking about the economic benefits of newly-opened shipping routes every summer.

Arctic sea ice / Re: When will the Arctic Go Ice Free?
« on: August 01, 2018, 12:41:00 AM »
A quick glance says that is a model study.  Care to demonstrate any skill at all in the models?
Assuming you can read; Care to read it again?

Arctic sea ice / Re: When will the Arctic Go Ice Free?
« on: July 30, 2018, 09:04:11 PM »
There certainly is a lot of doom and gloom around here, as usual.  I'll dissent from that, with some predictions that don't involve the near-future collapse of civilization:

1. The first year with a sub-1 million km ice extent day will probably occur in the late 2020s or 2030s. 

2. Insofar as the past 40 years' reduction in September ice extent has induced some fairly subtle changes in northern hemisphere climate during the fall months, those changes will continue and get bigger as the ice extent at minimum shrinks, but there won't be any sudden game-changing effect from crossing the purely arbitrary 1,000,000 km2 threshold.

3. Subsequent years will bounce back (as 2013 did after the 2012 low) but extreme low-ice (under 1,000,000 km2) years will become more and more common until they are the rule, rather than the exception, probably by 2040 or so. 

4. The duration of that annual very-low-ice-extent period will expand during the second half of the century to produce first ice-free Septembers, then ice-free summers.  There won't be an ice-free year in this century, and probably not in the next, either.

5. There won't be any 50-GT "methane bomb".   There was none in the early Holocene when the Arctic Ocean was ice-free during summers.  There was none during the previous interglacial (MIS 5e), when the Arctic was quite warm.  There was none during interglacial MIS-11, when the Arctic was so warm for so long that virtually all the land ice in Greenland melted. 

6. An ice-free Arctic Ocean won't lead to the collapse of civilization. The Arctic is already halfway ice-free in September now, and the effects of that are not particularly civilization-imperiling.

1 Human kind is pushing the process in a way that the world has never experienced before. There is no scientific paper showing any time in paleoclimate when the greenhouse gases have increased so quickly. As James Hansen often say, we are doing a unique experiment at an extremely high speed

"Global mean atmospheric carbon dioxide (CO2) concentration has now passed 400 ppm, a level that last occurred about 3 million years ago, when global average temperature and sea level were significantly higher than today (high confidence). Continued growth in CO2 emissions over this century and beyond would lead to an atmospheric concentration not experienced in tens of millions of years (medium confidence). The present-day emissions rate of nearly 10 GtC per year suggests that there is no climate analog for this century any time in at least the last 50 million years (medium confidence)."

And I think we are now in the process of tipping 410 ppm

2 therefore modelling the tipping points and when and how they are going to happen is dangerously uncertain

3 climate change is coming on top of other major environmental impacts very well described by Elizabeth Kolbert in the 6th extinction published in 2015

4 the tipping points are numerous (see below, and the Postdam Institut website) , and some are "modelled" (with high uncertainty) to be triggered at 2.5°c (e.g. the end of carbone sink role from the rain forest in the amazon... ocean acidification...) this could almost already be in the pipeline due to climate change hysteresis and the evident BAU path that we are continuing

5 There is a risk of multiple or sequential/cascading  tipping points could happen  in close sequences, this was pinpointed in the US climate assessment (chapter 15) to the US congress at the end of last year

6 As an exemple, the synchronicity of blue ocean Arctic sea triggering a much faster Greenland ice sheet melting and rapid permafrost melting increasing further greenhouse gases, with a potential rapid collapse of West Antartic ice sheet, is not to excluded post 2050. While simultaneous collapse of Amazon rain forest and ocean carbon sink is not to be excluded...

Tipping points:

(even if I don't necessarily agree with all the interpretation from Paul Beckwith particularly SRM solar radiation management -probably not feasable-, this somewhat  tie with the great uncertainty pinpoint the US climate report)

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