It is possible to demonstrate that the impending collapse of sea ice extent alone is enough to cause very grave existential threats to civilisation. Unfortunately this is by no means the only problem we are presented with by climate change. There are numerous other positive feedback processes acting over differing time scales and sea is merely one of the most immediate and certain of these. It is also easy to explain and well documented and understood.
Many people are ignorant of the role methane plays in climate change. It is a greenhouse gas that is often quoted as being 25 times as bad as carbon dioxide, but this figure is unhelpful and misleading as it takes the effects over a timescale of 100 years. Methane breaks down in the atmosphere in around 10 years and therefore the vast majority of the effects occur in a much shorter timeframe. Over 20 years the effects of methane are 72 times as bad as carbon dioxide - or by some calculations (that include effects on aerosols) 105 times as bad.
Although carbon dioxide is currently the majority player in terms of greenhouses gases we can control methane is significant. This though is not enough in itself to merit inclusion on a site that details the threat to civilisation in the very immediate future.
The reason methane is significant is that there is a region of the world that contains billions of tonnes of methane as free gas that could be released abruptly if a severe enough tectonic event occurred and which is already showing signs of escalating chronic release regardless of abrupt triggers. The East Siberian Arctic Shelf (ESAS) is the area of key concern although by no means the only potential source of large methane emissions. This is a shallow ocean shelf off the coast of Siberia.
Anaerobic decay of organic matter swept into the sea and buried in sediments produces methane. Under sufficient pressure or at low enough temperatures this methane can be locked up in ice to form methane clathrate. In shallow water like the ESAS this is kept stable by temperature - if the temperature rises the ice melts and releases the methane. It should also be noted that some of the methane can persist as free gas.
Until recently the temperature in this region was low enough to keep the accumulated methane clathrates stable and the free gas component was securely trapped under a solidly frozen impermeable permafrost cap on the seabed. However warming in this region is now perforating the cap and enabling the release of methane. Already multiple methane emission sites up to 1km in diameter have been observed and there is speculation that a submarine landslide or earthquake would provide a trigger to fracture the remaining sea bed sufficient for a large and immediate release of large quantities of methane.
There is an estimated 50 billion tonnes of methane thought eligible for release at any time. If this were abruptly released (though likely at some point within a decade) it would be equivalent over 20 years to over 5 trillion tonnes of carbon dioxide. We emit 35 billion tonnes of carbon dioxide annually and therefore this would be equivalent to approximately 150 years of our carbon dioxide emissions - all dumped into the system rapidly. Even the release of a small fraction of this would be catastrophic. It should be noted that there is thought to be in excess of 1400 billion tonnes of methane clathrates in the ESAS.
It is both true that scientists do not completely understand the conditions within which this system would undergo final failure and also true that there is very little to nil political will to investigate and research this issue as urgently and adequately as it merits. This is not a good reason to ignore it however, since whether or not we know about it makes no difference to whether or not it subjects us to an unexpected kick into catastrophe or not.
The theoretical long term effects of a catastrophe driven by methane are almost unimaginably bad and we can only hope that the system does not destabilise to the maximum thought possible.
Ignoring this is analogous to having a tea party next to a ticking bomb and saying this is fine because we do not know how many ticks are left on the bomb, nor how powerful the explosive inside it.