sue mcnab

Climate change course blog

Climate Change course – week 2

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A summary of learning this week so far:

Early Earth warmer than today in spite of sun being fainter. Thicker blanket of gases, especially water vapour & CO2 kept planet warm. Levels of CO2 gradually reduced through weathering by which it dissolved in rainwater to form weak carbonic acid which washed into the hydrosphere, got used by sea creatures to form shells & in turn got deposited on ocean floor becoming sedimentary rock. As sun warms up thus increasing temperature on Earth this negative feedback accelerates, cooling the planet by removing CO2 from the atmosphere & reducing its effect as a greenhouse gas.

Ice sheets at the poles grow over time due to decreasing temperatures & this increases their albedo effect. If the ice sheets reach a tipping point where they cover a large part of the Earth’s surface reaching down to the tropics – or where Texas is today – there is not enough of the Earth’s surface free of light-reflecting ice to warm the planet up & so it completely freezes over.

CO2 pumped into the atmosphere by volcanic activity cannot get back into the lithosphere via weathering due to the total ice cover of snowball Earth. A blanket of CO2 builds up in the atmosphere & gradually causes the Earth’s temperature to warm up. This starts to melt the ice sheets, decrease the albedo effect & allow weathering to happen all over again.

What are climate change records?

Data collected in the past from a variety of sources – ships, weather stations & more recently satellites, together with examination of tree ring growth, ice cores, rock sediment layers etc.  Data has to be adjusted to take account of different measuring techniques, errors, bias etc & allowance has to be made for local conditions where the measurements were made such as urban development, rainfall patterns, tree growth etc.

How do volcanoes affect climate change?

Volcanoes pump huge amounts of particles (aerosols) into the atmosphere.  These include ash, sulphur dioxide & other gases, yielding sulphates.  These aerosols seed clouds which together with the aerosols themselves reflect about a quarter of the sun’s energy back into space.  Some aerosols also absorb sunlight & different aerosols reflect or absorb sunlight in varying ways depending mainly on their colour & composition.  Lighter particles tend to reflect light & darker ones to absorb it.  As the particles erupted from volcanoes can remain in the upper atmosphere for several years before settling back to Earth they can have a major influence on temperature, not only because of their reflecting or absorbing properties but also because they have an albedo effect.  When the particles do fall back to Earth, where they land on ice & snow they can decrease the albedo effect by darkening the surface.   Finally the clouds seeded by particles contain a large number of smaller water droplets compared to clouds formed by clean air & these clouds are denser, whiter & very reflective of sunlight which has a cooling effect on the planet.  The effect of particles in the atmosphere is extremely complex because of all the different factors involved & the difficulty of measuring these particles.  Techniques are improving all the time.

How is today’s warming different from the past?

Using all the data collected from the past (described above) scientists can see that although global temperature has always varied by several degrees in either direction, today the rate of change is much faster.  Whereas in the past million years as Earth came out of ice ages temperatures rose by 4 to 7 degC over a period of about 5000 years, in the past century temperature has risen by 0.7 degC – about ten times faster than the rate of ice age recovery warming.  The predicted rate of warming for the next century is about 20 times faster.

What is the role of isotopes in determining temperatures from the past?

Examination of the calcium carbonate in the shells of small animals which have been deposited in sediment to determine the ratio of the two naturally-occurring stable isotopes of oxygen (16O & 18O)  gives an indication of the temperature at the time the carbonate was dissolved in the oceans.   This is because the isotropic ratio in the oxygen varies slightly depending on the temperature of the surrounding water, but this is complicated by the fact that the isotropic ratio in the oxygen in the shells depends on the same ratio in the surrounding sea water which also varies.  This variation is due to the fact that as water evaporates, the lighter molecules of water – those with atoms of oxygen (16O) – tend to evaporate before those with the 18O atoms.  When water then condenses, the heavier molecules (ie. with 18O oxygen atoms) condense first.  This makes the data very hard to interpret.

How have trees been used to reconstruct different climate variables across the world?

By examining tree rings scientists can work out the climatic conditions at the time – thicker rings indicate more growth due to factors such as rainfall, temperature etc.

How can ice cores provide a record of atmospheric composition?

Layers in ice created by change in texture of the ice surface during summer (24 hours of sunlight) making it different from layers of snow beneath it.  Examining thickness of ice over the years shows climatic conditions such as amount of snow, temperature, composition of the atmosphere & wind patterns (by taking readings from different ice cores in an area).    The ratio of oxygen isotopes (as above in sediment) shows temperature (this time of the air) at the time.  Can also measure the temperature of the ice directly in the middle sections (away from influence of surface temp & temp of the earth’s core) where it will barely have changed since it was formed.  By examining the bubbles of gas in the ice you can see the composition of gases in the atmosphere at the time.  Lastly any particles blown onto the ice settle in the layer & can be analysed to determine wind patterns & volcanic activity.  However, ice core analysis only shows what conditions were like on the parts of the Earth’s surface where they were taken from, not globally, although they do hint at global conditions.

What are the most important themes you have learned this week?   Self-regulating nature of Earth’s climate in the past, feedback mechanisms (+ & -), long & short term forcing factors.

What aspect of this week did you find difficult?  The discussion on isotopes & trying to relate the climate in the Pliocene to today’s in order to examine what might happen in the future – getting very involved!

What did you find most interesting? And why? Snowball Earth topic – all new to me, & beautifully explained in the video.

Was there something that you learned this week that prompted you to do your own research?  Tried to read up about the Pliocene climate etc but got really bogged down in techy articles & gave up.

Are there any web sites or other online resource that you found particularly useful in furthering your knowledge and understanding?  Nothing in particular because so many were rather too technical.  The Met Office & NASA sites are interesting & tend to be easier to follow.  If there was more time to spend on this…

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Author: suemcnab

I've been in IT (developing & supporting my own software) for a long time & now want to get involved in new activities! I am very interested in & concerned about the issue of climate change & want to learn more about it.

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