Quote:
Originally Posted by Archy
Thanks for the chart.
It shows, inter alia,
- temperature over the recent past was not as high as 150 Ky ago.
- however, CO2 levels have risen spectacularly.
This is as it should be, and a explained by Fred Hoyle: the relation between Co2 levels and temperature is logarithmic ie. it takes a very large increase in CO2 to induce a small increase in temperature |
Archy,
The time period to which you refer is at 120ka (ka = kilo annum is the correct unit, also Ma = mega annum) and is refered to as Marine Oxygen Isotope Stage 5 (or just Stage 5) or the Last Interglacial. It is well known to be warmer and have higher sea-levels. This attributed to differences in the Earths orbital parameters, look up the Wiki on Milankovic for more details.
If you also look up Beer-Lamberts Law you will see that the relationship between concentration and absorbance of a spectral peak is indeed logarithmic. The situation however is a bit more complex as regards CO2. The relevant spectral line already has 0% transmission (or infinite absorbance) and so increases in CO2 concentration cannot absorb any more radiation at that wavelength. However, while ideal spectra consist of lines in the real world interactions betwen molecules produce an effect call peak broadening which changes the line to bell-shaped peaks. As the concentration rises the width of the 'shoulders' increases and it is this peak broadening which is responsibe for the increased absobance of heat in the atm by CO2.
While it is true that the amount of radiative forcing caused by CO2 increase is small it is significant because of its duration. For instance the sunspot cycle also changes slightly the energy arriving at the Earth but this does not produce a measurable effect because it rises for 5-6 years then drops again over the next 5-6 years. On the other hand the CO2 forcing has been going on for over a century (and is increasing). The difference is a bit like the difference between tugging your ear-lobe and hanging a weight on it for several years.