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Old permafrost is loaded with frozen vegetation. As it now melts, it is rotting and releasing methane.

Doesn't that mean that it froze quickly, centuries ago, and stayed frozen until now?

What process caused such a sudden, centuries-long freeze? Don't we need to know this before we understand why it's reversing now? Presumably we humans had nothing to do with the freeze!

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  • $\begingroup$ Your title suggest, you want to know about the origin of permafrost, yet your text itself mainly refers to/asks about vegetation. Please clarify this. $\endgroup$
    – Erik
    Commented Sep 11, 2019 at 8:06

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What caused a centuries long freeze, the beginning of the current ice age. Yes we are in an ice age, as long as greenland and antarctica have large ice sheets we are technically in an ice age) Which in turn was caused by the movement of continents close enough to the poles to create permanent ice caps. the freezing of organic matter into permafrost is not a fast occurrence, it is a slow build up of organics as soil is built and added too, as long as it remains cold enough for permafrost to persist.

The strange thing about the warming now is every major natural factor and most minor factors point towards cooling conditions yet we have warming. Human activity however is producing strong warming factors that are overriding the natural prevailing conditions, which makes you wonder what will happen when some of the natural factors stary cycling back to warming.

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Doesn't that mean that it froze quickly, centuries ago, and stayed frozen until now?

Not necessarily. In places where permafrost exists, the thickness of the 'active layer' (the layer above permafrost that thaws seasonally) is relatively stable. This means that if you add material and raise the ground surface by burying organic material and soil, the top of permafrost will raise. So you can accumulate organic material in permafrost in a stable climate!

Also, consider that the process of organic material decaying once buried can take quite a lot of time, meaning that burial rates don't have to be as rapid as you might think. Modern-day sphagnum peat bogs accumulate a lot of organic material in them even in temperate climates.

What process caused such a sudden, centuries-long freeze? Don't we need to know this before we understand why it's reversing now? Presumably we humans had nothing to do with the freeze!

As stated above, the drop in temperatures doesn't have to be rapid because organic material can accumulate in permafrost even in a stable climate.

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Vegetation in Arctic and sub-Arctic bogs is not necessarily rotting. After a while the vegetation in a bog creates an acidic environment that preserves plant material and animal remains as though they were pickled in vinegar. That is why in Britain and Ireland there are extensive peat bogs many thousands of years old, and coal deposits which began in a similar way. In dryer places it is possible for plants and animals to be suddenly overwhelmed by winter conditions and to stay frozen for thousands of years, and these might generate CO2 and methane when they are frozen out.

A famous example of quick freezing is the bronze age warrior known as the Ice Man, who was found about 15 years ago frozen in the Italian Alps where he was overcome by a snow storm when he tried to cross in about 3,000 BC. So the answer is that some of this vegetation froze quickly, but most of it didn't, or at least was already preserved when it froze. It is possible that there have been brief episodes of thawing since then, so the freezing was not necessarily continuous. Some will decompose when it thaws completely, and some won't.

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  • $\begingroup$ organics in permfrost is a slow build up not a fast quick freeze. $\endgroup$
    – John
    Commented Sep 12, 2019 at 3:35

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