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Archive for the ‘Meteorology’ Category

The global mean temperature is not constant – it varies over many timescales. Over the last 150 years the temperature of our planet has been increasing. The graph below shows (in red) the mean sea surface temperature over the North Atlantic since 1870. The data comes from the HadISST data set, produced by the Met Office Hadley Centre. A timeline of the kings and queens of Britain is shown along the top – leaving out Edward VIII who was king for less than a year between George V and George VI.

kings_and_queens

So Edward VII had the coldest reign, George V led to warming, George VI managed to maintain these temperatures then Elizabeth II has turned up the heating in the second half of her reign.

The causes of these changes are not fully understood. Edward VII’s cooling is thought to be due to volcanic eruptions – such as the eruption of Santa Maria in 1902. Elizabeth II’s warming is though to be due to anthropogenic causes, although some contribution from internal variability has also been suggested. George V’s warming is least understood – the Intergovernmental Panel on Climate Change (IPCC) state it is very likely due to natural causes, likely some contribution from manmade causes and possible that internal variability is also involved.

 

 

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Global average temperature has increased over the last 150 years, leading to widespread ice melt and rising global average sea level. The increase has not been at a constant rate – the Intergovernmental Panel on Climate Change identified two periods of accelerated warming, 1910 to 1945 and 1976 to 2000.

The earlier warming is poorly understood. Climate scientists disagree over causes suggesting solar activity, volcanic eruptions, and manmade emissions. The later warming has been attributed to anthropogenic causes, though scientists disagree over the proportion contributed by internal variability.

I am studying the earlier warming in the North Atlantic. Analysing historical observations I have found atmospheric circulation changes may be a key mechanism behind the warming. Using climate models I can test which processes could have been involved. Climate models are numerical representations of the climate system. They allow specific processes to be studied and can provide data where observations are not available, such as 3D ocean properties. Initial investigations have found large changes in ocean heat transport at the time of the warming.

Understanding the roles of atmospheric circulation and ocean heat transport will help us separate mankind’s influence on climate from natural processes, allowing better estimations of future climate change.

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Back to the PhD!

The cruise report from JR265 (my trip down south) has now been published. If you want to read about the science, not just the bits I did, here is a link: http://eprints.soton.ac.uk/208991/

I have been back in the UK almost a month now, so my PhD is back in full swing – or would be if I hadn’t caught fresher’s flu. You’d have thought that by my fifth year of being a student I’d have become immune. Apparently not!

I am studying detection and attribution of climate change. By looking at observations of climate variables, such as mean global temperature, climate change can be detected. By using models and statistical methods these changes can be attributed to a variety of factors, some natural and others caused by human influences.

You can read about attribution methods in more details in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report: http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch9.html

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20 minutes on a train, 50 minutes on a bus, 6 hours in air terminals, 16 hours on an aeroplane, and 2 new passport stamps later I have made it to the Falkland Islands! So far it looks like the Welsh mountains or Scottish highlands with a few sheep and low level vegetation, but with far less evidence of humans. The roads are a mixture of solid and loose stones, no road markings, many cattle grids, occasional signs. The buildings generally have colourful, corrugated roofs; the vehicles are mostly 4×4’s. It’s cold. Ski jacket, gloves and hat cold – I hadn’t expected that so soon.

Whilst flying over the South Atlantic watching the layers of clouds between me and the ocean and trying to do some atmospheric physics revision, I started thinking about the air right outside the window. Television screens updated us on the flight conditions – including speed, height and local time. The air outside the window was a chilly -56°C.

What would happen if a parcel of air was brought inside? This high up (10972 m or 36000 feet according to the television screens) the pressure is much lower than at the Earth’s surface, so presuming the pressure inside the aircraft cabin is the Earth’s surface pressure, the pressure of the air parcel would increase.

When pressure is increased the temperature goes up – think what happens when you use a bike pump, the valve gets hot due to the pressure of the air being forced into the tyre.

So how much warmer would the air get? Potential temperature can be used to calculate this, it is the temperature a parcel would become if brought (at constant entropy, or energy state) to a reference pressure.

Potential temperature = T(Po/P)^(R/Cp)

Where T=temperature (in Kelvin), P0 = reference temperature, P=initial pressure, R= 287 Jkg-1K-1 (specific gas constant) and cp= 1004 Jkg-1K-1 (specific heat at constant pressure).

I knew T=-56 °C or 217 K, the aircraft height was 10972 m, which (from the US standard atmosphere look-up chart) corresponds to ~225 Pascals. The reference pressure inside the aircraft cabin is assumed to be surface pressure, ~1000 Pascals.

Substituting these values into the equation to calculate potential temperature:

Pot. T = 217 x (1000/225)^(R/Cp) = 332K or 59°C

So, rather oddly, the air outside the window would become uncomfortably hot if brought inside. Luckily this can only ever be a theoretical calculation, the cabin is pressurised, and so switching the air is not possible!

The same principles can be applied to the ocean, just in reverse. Water at depth is at a greater pressure than water at the surface due to the mass of water above it. If a water parcel could be brought to the surface at constant entropy it would lose temperature as its pressure would be decreased.

(Quick Disclaimer – these calculations have not been confirmed by a grown-up scientist… I think they are correct!)

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