The Tripati study
A couple of history lessons ...
important questions answered by paleoclimatology
As the Keeling curve climbs steadily toward 400 ppm, with a couple of years to go, it's a bit strange you don't often hear this ...
It's a heck of a question.
We know that once it gets there, it's going to be a very long time before it comes down on its own.
We know the warming due to it will come over a few centuries and last for many more.
And we know that 400 isn't the end - only determined actions of ours can stop it rising much higher - maybe 500 before mid-century.
So you'd think we'd be pretty interested.
Well, students of climate history have found some answers.
When was the last time on Earth CO2 was 400, and what was it like then?
Here's another question we should be pretty interested in:
Holocene sea-level has been pretty stable, but the seas are rising now.
Yes, we do. It wasn't very long ago ... the last interglacial before the present one, usually called the Eemian (about 131-114,000 years ago - boundaries are a bit arbitrary, as you can tell from the chart).
Being geologically recent, the Eemian is a good subject for study. Evidence of sea-level during the period can be found in old elevated coral reefs & wave-notched terraces in several parts of the world, specially the Red Sea, the Bahamas, and Western Australia - tectonically stable regions with abundant coastal features left by Eemian seas. And atmospheric CO2 is known with high precision.
The level of the global ocean depends on the volume of ice on land. That, in turn depends on temperatures; and that varies with greenhouse gases. In a recent study designed to see just how CO2 and sea-level varied over 40 million years, it was discovered that CO2 is indeed the major influence, although other geophysical properties that affect heat storage and transport have an effect too.
Do we have good sea-level records for any period when it was a degree or two warmer?
Eemian sea level
CO2 and sea-level for the middle Miocene, showing a sea-level peak near 40m for CO2 400 ppmv.
The time scale on the bottom is in millions of years. The sea-level scale on the right is in metres relative to present.
Has this ever happened on Earth before ... a lot of carbon getting into the atmosphere fast? If so, what were the consequences?
1. It seems clear that at least 2-3,000 Gt of carbon entered the air during the event - an amount that compares roughly with the quantity of recoverable fossil fuel reserves we could burn this century. With this thought in mind, it is possible to conceive a human-induced PETM. Some scholars think the amount of PETM carbon must have been higher - a lot depends on what we think the climate "sensitivity" to greenhouse forcing was at the time - something we can't really know - for now.
2. It looks as if a lot of the carbon was released in the first few thousand years - maybe a lot quicker. Some evidence suggests it took place in at least two discrete episodes, each containing rapid phases. It probably lasted about 170,000 years - that is, before the global carbon cycle returned to where it was.
3. One of the earliest suggestions for a source was the large deposits of methane "ice", known to geologists & oceanographers as clathrates, buried under the sea bed in high latitudes, especially the Arctic. A big clathrate melt would fit the evidence very well. Physical disruption or melting would saturate the water with methane. It would then bubble up to the air and eventually oxidise to CO2. The deposits could have been disturbed by tectonic events associated with the opening of the North Atlantic ocean ... there was a major volcanic episode near Greenland at just this time. However, some researchers don't think there was enough frozen methane at the time to produce the observed effects.
4. There was a marked acidification of the ocean, as you'd expect, but the effects on marine life have been hard to unravel, apart from a significant extinction of foraminifera. Responses on land are even less clear.
5. It appears the PETM was not unique. A couple of other sudden, smaller "hyperthermal events" have been detected in the record, suggesting that, whatever the cause, it was repeatable. On the face of it, this should strengthen the clathrate hypothesis, because these deposits are accumulating all the time (they are just frozen un-decayed organic debris) and anything at all that can destabilise them could trigger such an event. It should also make us think about the size and permanence of these deposits today.
In March 2010, a team of Russian & American scientists reported on what they had found during six years of field work in the Arctic. The report raised plenty of eyebrows, and its significance is still being debated.
The scientists had been measuring methane in the water and air at many places in this large area of shallow Arctic sea. During ice ages, what is now sea floor is tundra, so it accumulates lots of peaty stuff - the source of methane. It had been believed that this carbon was sealed underneath a frozen layer of mud - but the results showed otherwise.
Average methane concentration in the Arctic turned out to be 1850 parts per billion by volume (ppbv). The global mean is 1400 ppbv. 150 years ago it was 700 ppbv. Plumes of methane escaping from the sea floor were observed in large numbers. Methane is soluble in water, so a lot of this doesn't enter the air directly; on the other hand, methane is a potent greenhouse gas - about 25 times as powerful as CO2 over a century, and about 70 times as potent over 20 years.
These scientists expressed great concern about the potential for future massive methane releases ... in their own words:
They found that more than 80 percent of the deep water and more than 50 percent of surface water had methane levels more than eight times that of normal seawater. In some areas, the saturation levels reached more than 250 times that of background levels in the summer and 1,400 times higher in the winter. They found corresponding results in the air directly above the ocean surface. Methane levels were elevated overall and the seascape was dotted with more than 100 hotspots.
Study area in the shallow sea off the Siberian coast.
Below (R) is a map of the methane venting to the atmosphere, reported in 2010
Other scientists are trying to assess the quantity of methane released from thawing tundra. Below is a newly formed tundra melt lake, emitting methane in summer.
How fast could the sea actually rise, if these huge masses of ice "collapsed"?
Meltwater pulse 1A
Is it true that modern warming is nothing special; that it was warmer in the middle ages?
Figure 11, from Heaven & Earth, Prof Ian Plimer's book, which he uses to prove it was warmer in the middle ages. Trouble is, the graph shows no such thing. It is a sketch of a diagram from a work by Hubert Lamb in 1965 concerning the climate history of central England. It has nothing to do with global temperature; it was compiled from old weather records, not modern proxies - and both graphs have been distorted in favour of Plimer's point ... that the "hockey stick" reconstruction of global temperature over a thousand years or so is false.
But it isn't false. Since Michael Mann first published it, his findings have been independently confirmed over and over by different investigators using different methods . He published a major update himself in 2008, using improved techniques.
The medieval warming was not "suppressed" as Plimer and others claim; it is represented in proxy studies exactly as it occurred.
The Holocene temperature record
If you want to know more about the "HOCKEY STICK CONTROVERSY" there's a decent article at RealClimate, here