Topic for this week: Past climate change - we're looking at both ancient, and more recent natural climate change.
Section 2.1
First off is a video about ancient climate change, and the two conflicting feedback cycles that act against each other.
One is the fact that the sun is getting brighter, and hence the planet should be warming - but it isn't. This is due to chemical erosion over land, taking carbon dioxide out of the atmosphere in rain, forming carbonic acid, and this then becoming trapped at the bottom of oceans by organisms absorbing this in their shells, and eventually being deposited at the bottom of the ocean as carbonate rocks (sedimentary?). This is a negative feedback loop as the increase in temperature due to the increasing brightness of the sun has been counter-acted by the increase in the rate of weather erosion taking more carbon dioxide out of the atmosphere.
The other feedback cycle explains the concept of "snowball earth", the Earth being on two occasions completely covered in ice. As the polar ice caps extend, the albedo goes up, the earth cools more, until a tipping point is reached (when the ice caps go beyond 30 deg. latitude) and a positive feedback cycle takes place, causing the Earth to cool even quicker. Once the Earth is completely covered in ice though, chemical erosion tails off, and volcanic activity releases more CO2 into the atmosphere, which then starts to heat the planet up again, leading to the end of the "snowball" period.
Section 2.2
The second section this week is a page and series of snippets from an episode of Horizon from 2001 showing the development of the snowball earth hypothesis.
I watched the TV programme in it's entirety. It was fascinating, showing how the global distribution of "drop stones" in the geological record from 600 million years ago indicated that there been global glacial activity. Mathematics indicated that this was possible, but would have been catastrophic and would have wiped out all life on the planet at that time. Then came the realisation that volcanic activity would continue, pumping out lots of CO2, and a severe greenhouse effect would cause the planet to go from one of it's coldest periods to one of it's hottest. The violence of this change caused extensive deposits of cap carbonate rocks immediately above the levels containing the drop stones - explaining this geological phenomenon. The whole theory was then shot-down by biologists pointing out that an extended global ice-age would have mean the extinction of all life, until it was discovered that certain forms of life are capable of survival under ice sheets, which would have been formed very slowly, and hence are nearly transparent and free of impurities. The explosion in the evolution of life forms into multi-cell organisms following a sudden end of snowball earth also fits in with this.
A fascinating programme, and I feel I've learn a lot this evening, it't taken a bit more time than I hoped though.
Section 2.3
Exercise to produce some feedback diagrams - I didn't do this, thinking my text above explains this. I looked at a couple of examples posted in the discussions to check my understanding, and registered on the conceptboard web site - but didn't put any time into having a play with this tool (which I think I ought to check out some other time).
Section 2.4
Recent past climate change video with Prof. Dan Charman and Dr. Ewan Woodley.
I liked the fact that parts of this video were on nearby Hound Tor and the Medieval settlement nearby as well as on Exeter University campus.
Main parts of this are - natural variation in solar forcing is due to three factors identified by Milutin Milankovic, these are:
- E - Eccentricity - variation between circular and elliptical of the Earth's orbit - period 100,000 years
- T - Obliquity - variation in the tilt of the Earth's axis - period 41,000 years
- P - Procession - variation or wobble of Earth's axis - period 23,000
Each of these means that the Earth gets different amounts and distributions of solar energy, leading to natural climate variability, and differences in levels of seasonality.
There was a demonstration of taking a tree ring sample, and explanation that aligning these with other tree sources, including those preserved in peat bogs meant it was possible create a climatology going back in time. Reading the transcript it says the last millennia and even further, and that we can recreate variables such as temperature and precipitation - I wonder how? That impresses me!
The next section was about the influence of volcanic activity on the atmosphere. Big volcanic eruptions throw lots of ash and aerosols into the atmosphere, which generally lead to global cooling in the short-term, as seen by the 0.5 deg C drop in temperature following the eruption of Mt. Pinatubo in 1991.
Then pulling it all together - these factors explain the main causes in climate variation, but don't explain what has happened in the last 100-150 years - when man-made CO2 has been pumped into the atmosphere.
Section 2.5
Was some recommended reading to answer one of the following 6 questions:
- What are climate change records?
- The Met Office has a useful link summary
A mixture of direct measurements - which need to be quality controlled for biases in the methods and tools used, plus "proxies" which are other measurements that can be used to infer properties of the climate. E.g. tree rings, coral growth, ice cores and temperature profiles of oil-drilling holes.
- How do volcanoes affect climate change?
- NASA’s Earth Observatory website link
- How is today’s warming different from the past?
- NASA’s Earth Observatory website link
- What is the role of isotopes in determining temperatures from the past?
- NASA’s Goddard Institute for Space Studies link
- How have trees been used to reconstruct different climate variables across the world?
- Science of Tree Rings website link
- This page outlines the main principles of dendrochronology, and even provides an answer to my question above... the trees/wood sampled is chosen such that it most sensitive to the environmental factor that you want to measure. E.g. to measure the variation in the amount of precipitation trees that will suffer most in droughts are selected, i.e. those on rocky ground or hill edges, their tree rings will vary the most with the amount of rain in a season. These are "proxies" and the full record is constructed by collating with periods with measurements available. There is also the principle of crossdating to line up one time period so that it overlaps with another, hence a longer record is made up.
- How can ice cores provide a record of atmospheric composition?
- NASA’s Earth Observatory link
Section 2.6- The Met Office has a useful link summary
- A mixture of direct measurements - which need to be quality controlled for biases in the methods and tools used, plus "proxies" which are other measurements that can be used to infer properties of the climate. E.g. tree rings, coral growth, ice cores and temperature profiles of oil-drilling holes.
- NASA’s Earth Observatory website link
- NASA’s Earth Observatory website link
- NASA’s Goddard Institute for Space Studies link
- Science of Tree Rings website link
- This page outlines the main principles of dendrochronology, and even provides an answer to my question above... the trees/wood sampled is chosen such that it most sensitive to the environmental factor that you want to measure. E.g. to measure the variation in the amount of precipitation trees that will suffer most in droughts are selected, i.e. those on rocky ground or hill edges, their tree rings will vary the most with the amount of rain in a season. These are "proxies" and the full record is constructed by collating with periods with measurements available. There is also the principle of crossdating to line up one time period so that it overlaps with another, hence a longer record is made up.
- NASA’s Earth Observatory link
400 parts per million - discussion about crossing this threshold. Didn't participate, again due to time pressures!
Reflection
1. What are the most important themes you have learned this week?
Wow, I feel we covered a lot of material this week, but I learnt a lot about the natural causes of global climate variability, and was introduced to a number of concepts that I wasn't previously familiar with, such as "snowball earth", the long-term carbon cycle (I was previously under the impression this cycled through vegetation and fossil fuels only, not aware of the significance of chemical weathering), and the explanation of the three types of variation in the movement of the Earth around the sun.
2. What aspect of this week did you find difficult?
I deliberately set about do this in little chunks rather than in one go this week, and seem to have spent about two or three times as much time on it as last week, definitely exceeding the 2-3 hours I thought I'd signed up for! I've had a really busy week, so there were moments when I didn't know if I was going to get it all done. Having said that, I've made some time for other things, and maybe writing these rather extensive notes is a bit of overkill. Not sure what my approach to week three will be.
3. What did you find most interesting? and why?
The Horizon episode was simply fascinating - I knew nothing of the concept before, and loved the way the programme was constructed. Particularly with new theories and evidence countering those theories leading to an evolution in the theory - I suppose this is a demonstration of the scientific method.
I also enjoyed reading the article on dendrochronology - mainly because it answered some questions that sprung to my mind whilst watching the video. I'd always knew this was done, just not how.
The long-term carbon cycle was also new to me, so that was a key thing learnt this week.
4. Was there something that you learned this week that prompted you to do your own research?
Well, I've had a quick read up on chemical weathering here.
5. Are there any web sites or other online resource that you found particularly useful in furthering your knowledge and understanding?
Haven't ventured that far looking due to lack of time.
Summary for the week: learnt lots of new stuff and as an aside, I also realised that although I did Geography O'Level, I've never had any exposure to Geology in any previous learning context, so maybe that's something I should learn about some time in the future.
Links to external documents used this week:
BBC video about snowball Earth - http://www.bbc.co.uk/nature/ancient_earth/Snowball_Earth and a full episode of Horizon from 2001 showing the development of the hypothesis is available on Youtube here http://www.youtube.com/watch?v=ydLNrTzMIgc
Articles I've read this week of related interest:
http://www.theguardian.com/business/2014/jan/15/bp-predicts-greenhouse-emissions-rise-third
