Will Steffen: The Science of Climate Change

As mentioned previously, being back at a university provides opportunities to listen to some well-informed people on topics beyond my own area. The Director of the Research School organises a series of colloquia throughout the year and earlier this month I listened to Will Steffen, Director of the Climate Change Institute and ANU discuss the science of the changing climate. If you wish to hear and see what he presented, an audio recording synchronised with his slides can be found at the bottom of that last link. I found it an informative colloquium and an unedited copy of the notes I took can be found below.

Note that the thermal energy reserve in the oceans is far bigger than that of the atmosphere & change in the energy content of the oceans is a significant indication.

Divergence between sunspot activity & Global Temperature Index (GTI) which were previous correlated.

Correlation between greenhouse gases & GTI but not to cosmic rays.

AIRS satellite observation of emission spectra (blackbody with absorption by H₂O, CO₂ & O₃)

1950 a tipping point in greenhouse gas (GHG) concentrations.

d¹³ CO₂ signature shows burning of fossil fuel. (fossil fuels have depleted d¹³ CO₂ due to being trapped away & not part of the regular cycle).

Albedo ~30%. Ice at poles does a lot of reflecting.

CO₂ absorption is a bending mode of the molecule. N₂ & O₂ are transparent since they have no bending mode being a bi-atomic molecule. The reason CO₂ is important is because 99% of the atmosphere has no effect on absorption of reflected light.

Land use (deforestation & replacement with grasses & crops) cools planet through increase in albedo.

Overall radiative forcing due to human activity.

Increase/decrease in radiative forcing increases/decreases intensity of hydrological cycle. Water vapour is a fast, strong GHG.

CO₂ less soluble in warm water than cold.

Note that system is not in equilibrium (undergoing a perturbation) & so some equations & assumptions are not valid.

Troposphere is warming but stratosphere is cooling -> consistent with GHG warming but not solar (external).

Warming unequivocal but anthropogenic 'very likely'.

Increase in number of 'hot weather' events & extreme weather events.

More deaths from extreme heat in Melbourne (2009?) than in the bushfires.

Earth oscillates between two stable states, glacial and warm with the glacial being slightly more stable.

Natural CO₂ (last 400k years) between 200-280 ppm. Currently ~400+ppm.

Bifurcation point around 3 degrees C -> system moves to a much hotter stable state? Keep around 2 degrees C possible return to Holocene condition.

Previous abrupt changes show classic signs of a complex system.

Previously hotter ~1-2 degrees C & sea levels 5-9m higher than now.

Orbital (earth) variations account for cycling of the system in the past. More circular orbit gives a warmer period (Medieval Warm Period).