Could the Azolla Event Happen in Reverse?

Yes, it could; there are two basic ways, and they could happen either together or separately.  The Azolla Event put enormous reserves of ‘fossil fuels’ in the Arctic region.  This probably includes coal, oil and natural gas.  There are certainly massive reserves of Methane Hydrates.  The slow way for the Azolla Event to be reversed is for some of these reserves to be used, and in being used, produce Carbon Dioxide.  The increased greenhouse effect from this would warm the atmosphere and slowly melt the icecaps.

However, there is another and potentially much faster way for it to happen:

The Methane Gun

When a gun fires, the explosive in the bullet is set off.  The reaction spreads throughout the explosive so fast that to us it appears instantaneous, and the bullet is accelerated so fast that it comes out of the end of the barrel, in some cases, faster than the speed of sound.

In the Arctic there are huge amounts of Methane.  Most of it is combined with water molecules to form ‘Methane Hydrates’.  These Methane Hydrates are quite unstable in many conditions.  They are stable if they are cold enough or held under high pressure. In many oceans there are deposits of Methane Hydrates held stable by the pressure of the water above. 

In the Arctic there are Methane Hydrates on land as well as in the Ocean.  The ones on land, in places like Alaska and Siberia, are held in the hydrate form by the ‘permafrost’. This is a layer of ‘soil’ which is ‘permanently’ frozen.  In this case, we should remember that this ‘permafrost’ has not always been there.  It formed on top of huge deposits of vegetation, including Azolla (See “The Azolla Event”).  This is mostly not frozen and has decayed over millions of years, producing Methane which has combined with water to form Methane Hydrates.

If permafrost melted, the Methane Hydrates would not longer be held and the Methane released.  Of course it would not all melt at once.  If part of the permafrost melted, some of the Methane would be released.

We hear a lot about greenhouse gases.  The one we hear about most is Carbon Dioxide.  This greenhouse gas is the second most important one in keeping the Earth nice and warm.  The most important one is Water Vapour.  As the temperature of the Earth’s atmosphere increases, generally, the amount of Water Vapour in it increases.  So the Water Vapour tends to increase the effect of any increase from other causes.

The third most important of the greenhouse gases is Methane.  This is many times as effective as either Carbon Dioxide or Water Vapour, but because of its low concentration in the atmosphere it is less important at the moment.  However, it is the one that is increasing at the fastest rate:

In 1750 the Methane concentration of the atmosphere was about 700 parts per billion (ppb).  This increased to 1754 ppb in 1998.  This increase appeared to be levelling out around 2005, but in 2007 it started to increase again.  We do not know what it will do next although there are many (often opposing) ideas about this.  At present, the increase in Methane concentration is responsible for about a third of the effect of the increase in Carbon Dioxide concentration.

Methane does not last very long in the atmosphere.   It gets oxidised to Carbon Dioxide and water. However although an often quoted average length of time before this happens is seven years, this depends on the concentration in the atmosphere.  As the concentration increases, the rate of oxidation goes down.  So if more is released, the rate of oxidation slows.

As the Earth warms, some of the permafrost will melt, not only on land, but also in the Arctic Ocean.  The Arctic Ocean is mostly not deep enough for the pressure to keep the Methane Hydrates stable; in this case it is mainly cold that is doing it.  There is evidence that the ice on the surface of the Arctic Ocean is melting.  The temperature of the water has started rising, may be allowing the Methane Hydrates to release Methane which will lead to further rises in temperature.  This will also increase the amount of Water Vapour in the atmosphere and further increase the temperature; releasing more Methane.  Also, less ice means less reflection of the Sun’s rays, meaning more warming.

These effects are a ‘positive reinforcement’.  If it happens it will be hard to stop unless we catch it very early on.  In geological terms this could be almost ‘instantaneous’ in the sense of someone looking back in the distant future.  If we are alive when this happens it will not seem so fast.  However, in geological terms the effect will be like the firing of a gun.

Has this happened before in the Earth’s (Geological) history?

Yes, it has, quite a few times, although, as usual, people will argue about this statement.  There is evidence that for a period of over 100 million years the Earth was very cold.  The surface of land and sea was covered by ice over the whole Earth.  Then it got warmer.  Probably by some of the ice sheets becoming unstable and releasing Methane.  The Earth ‘suddenly’ changed from one of its coldest periods to a very warm one.

And how long did this take to happen?  No one really knows.  Some people who have studied this think it happened in as little as one hundred years.  Others think it took longer.

Could it happen again?

Yes, of course it could.

Is there evidence that it has already started?

It depends on who is interpreting the evidence.  There have been places on the floor of the Arctic Ocean where large amounts of Methane are being released, but it is not clear if there has been a recent large increase in the amount being released.

Could it happen in, say, the next 20 years?