past climates

this weeks lessons!

This week has been a journey through time, back to the earth's origins almost 4.5 billion years ago, to examine the way the earth's climate has changed on a geological timescale, and  the techniques used to identify the processes that brought those changes about. Geological specimens can give clues about early climate - sediments as old as 3.8 billion years have been found.

In the early history of the earth the sun was weaker - delivering 25% to 30% less energy than today. Paradoxically, the geological record shows a climate suitable for the evolution of life at a time when the planet should have been frozen. In fact it was probably warmer than today - almost certainly because CO2 concentrations were far higher than those found in our atmosphere now.

Carbon pools in the major reservoirs on earth.^[2]

Pool	Quantity (gigatons)
Atmosphere	720
Oceans (total)	38,400
Total inorganic	37,400
Total organic	1,000
Surface layer	670
Deep layer	36,730
Lithosphere
Sedimentary carbonates	> 60,000,000
Kerogens	15,000,000
Terrestrial biosphere (total)	2,000
Living biomass	600 - 1,000
Dead biomass	1,200
Aquatic biosphere	1 - 2
Fossil fuels (total)	4,130
Coal	3,510
Oil	230
Gas	140
Other (peat)	250

The geological record has allowed insights in natural cyclical changes to the earth's climate. There's a great slow carbon cycle in which carbon dioxide, highly soluble in water,  forms carbonic acid - which reacts with exposed rocks to leach out soluble bi-carbonates.

These eventually find their way to the sea and are used by marine animals as a building-block for their shells - made of insoluble calcium carbonate. Eventually they settled to the sea floor, and, over millennia, gradually lockaway free CO2 into the lithosphere - reducing atmospheric concentrations and cooling the atmosphere.

This process of fixing carbon has removed vast quantities of carbon from the active carbon cycle - If I've interpreted the table correctly, all the carbon in the atmosphere, dissolved in the oceans, stored in the biosphere and, amazingly, in the worlds fossil fuel reserves, is only a tiny fraction of the carbon accumulated in the lithosphere, around 1/20,000.

We learned that at one point so much CO2 was locked away that the earth cooled to a point where an irreversible feedback mechanism tipped the climate into a prolonged frozen phase.

This "snowball earth" may have arisen from a combination of decreased greenhouse gas, leading to more extensive ice cover and higher albedo, and perhaps one of the cyclical variations in the earths's orbit, known as the Milankovitch cycles or an event that increased dust content in the atmosphere, reducing the energy arriving at the earth's surface to a point where so much ice formed that its high albedo triggered an irreversible spiral of cooling.

the process led to "20 million year winters", with global temperatures as low as those of modern Antarctica. In the frozen state all the natural carbon sequestration was halted - there was no liquid water to form carbonic acid and probably little exposed rock. The icy conditions did not stop vulcanism and gradually CO2 levels reached a level where warming restarted a new phase of warming and cooling.

As we came closer to "modern times" we looked at the way climatologists are able to use a range of techniques to work out climatic change in fine detail - using:

•  dendrochronology (study of tree rings)  - involves gathering many cores of trees and relating growth patterns to current and historic weather records. This data can then be compared with tree ring samples from older living trees and also from trees preserved in buildings and in fossils to build a picture of historic weather patterns
• Ice core samples - As snow falls it traps air - year on year new snow falls - compressing earlier layers - eventually turning them into ice - trapping the air permanently. By drilling through deep ice in the places like the Antarctic and  Greenland air samples from ice as old as 800,000 years have been obtained - allowing air samples to be analysed for greenhouse gasses,  and also yielding other variables that can affect climate like ash and dust and other climate indicators like pollen.
• Core samples of sediments - which yield samples of fossil micro flora, pollen etc that can be used as proxies to deduce climatic conditions in the past - the most spectacular example of this is work done was at Lake El'gygytgyn - Known as Lake E, in eastern Siberia, which give an unbroken sediment record of over 3 million years.

This evidence has allowed climatologists and geologists to build a more detailed picture of the cyclical reduction of CO2 levels and periods of glaciation known as ice ages over the last 4 million years. We are technically in a warm "interglacial" at the moment - with ice retreating from the far north of Britain only 12 thousand years ago 

From these very accurate tools we know that the pre industrial CO2 concentrations of around 220 parts per million (ppm)  have been the norm for a  very long time - and that since the late 19th century, levels have risen to 400 ppm due to human activities, mainly burning coal and oil and deforestation. This a CO2 concentration, according to the Lake E results,  not seen for 3.5 million years, and one that lead to Arctic temperatures 8oC warmer than today.

There is a growing body of evidence to suggest that the extra energy high levels of greenhouse gasses pump into the world's climate systems cause more frequent and more severe weather events, reducing sea ice cover, and will lead to sea level rises, that could threaten many of the world's major cities and population centres. 

Learning experiences

I didn't find anything especially difficult this week - probably the hardest thing for me was watching the "Snowball Earth" Video - which I found over-dramatic and a poor way of absorbing information - and sorry course leaders - also populist science 13 years out of date.

It was good to get a more structured view of the tools used to research past climates but the thing I found most interesting was the number of students who were a tad sceptical about climate change - I'm no scientist, but I've been delving deeply into the political context of climate change denial, in particular in the USA, for the last 4 years. There were a lot of people on the course who felt we needed to listen to "the other side of the story" - not realising the "other side" is not an alternate scientific view but  a propaganda exercise. If I have the time I plan to blog about this at some point through the week.

I've been researching and using other web sites through the week - I've linked to a few of them in this and my previous post  - but there are a few links that ~I've found invaluable.

Skeptical Science is a denial myths debunk site but also carries a wealth of info.Science Daily have e-mail a daily bulletin of all environmental stories that report new developments in climate research as they are published and Carbon Brief issue a weekly e-mail bulletin on political and technological developments, news etc.