This post is offered for the benefit of recent subscribers and as the result of a self-improvement exercise for me: A few months ago, I published a series of posts under the title ‘Climate Science in a Nutshell’, which started here (on 31 Oct 2011). This was itself contained within a slightly longer series of posts regarding my reading of James Hansen’s Storms of my Grandchildren, which started a few days earlier (on 27 Oct 2011). However, I now hope to go one step further; and reduce the former ‘Nutshell’ series of posts into a single item (about one fifth of the total number of words). [If really pressed for time you can even skip to the footnote, below, for a one-paragraph “nut shaving” (i.e. only 104 words)!] What follows has been derived from my initial attempts to answer some questions about CO2 consistently lagging behind temperature changes in the palaeoclimatic record, as posed by an Engineer, John Kosowski from Illinois, who came to my blog last Thursday. In so doing, therefore, I found myself doing what I have just said I seek to do here. However one prior explanation is required – namely that a climate or radiative “forcing” is any change in energy imbalance (between incoming solar radiation and outgoing long-wave radiation) that tends to cause an atmospheric and thus a surface temperature change in order to restore that balance (e.g. see Wikipedia). So then, here goes:
Nearly all climatologists would accept that natural climate forcings (e.g. the Milankovitch cycles [i.e. “wobbles”] in the Earth’s axis of rotation) are anything up to ten times weaker than anthropogenic forcings (i.e. the addition of fossilised carbon to the atmosphere through human activity). Without the so-called Greenhouse Effect, the average Earth surface temperature would be about 30 Celsius colder than now (i.e. minus 17 Celsius); and complex life would never have appeared on it. The Earth therefore first created conditions suitable for Life by releasing CO2 from volcanoes and trapping them in its atmosphere.
Next, early life forms like Stromatolites converted CO2 to oxygen; with the latter eventually accounting for 20% of our atmosphere before an equilibrium was reached between photosynthesising plants and respiring animals. Ever since then – and this is the key to understanding everything else – the Earth has regulated its temperature by moving CO2 between its oceans and its atmosphere. If and when a natural forcing would tend to make the planet warmer or cooler (by changing the amount of incoming solar radiation), this would give rise to an imbalance between the incoming solar radiation and outgoing radiation (heat loss). In order to restore this energy balance, it is – and always has been – necessary for the Earth to have more or less CO2 in its atmosphere. This is why, in palaeoclimatology, CO2 changes always lag 200 to 1000 years behind natural temperature changes (interspersed with long periods of relative stability in both).
Although not the strongest greenhouse gas (GHG), CO2 is the most abundant and long-lived GHG there is (i.e. water vapour is much more abundant but comes and goes; whereas methane is much less abundant but 23 times more powerful as a GHG). All natural forcings (Milankovitch wobbles, the precession of the equinoxes, and the eccentricity [i.e. non-circularity] of the Earths orbit) are all predictable and fairly constant (i.e. the Earth’s axis of rotation moves between two angles of inclination at predictable intervals and at a predictable speed). These 3 natural forcings have only been dominant in the last 1 million years (hence all the Ice Ages we have had). Prior to that, other natural forcings such as plate tectonics (i.e. continental collisions, burial of limestone sea bed, mountain building) have caused much greater changes in the CO2 content of the atmosphere (primarily via volcanic activity but also the chemical weathering and erosion of mountain ranges) and greater changes in temperature.
However, it must be borne in mind that climate sensitivity to CO2 was lower during the time of the dinosaurs (i.e. requiring greater change in CO2 per unit change in temperature) compared with what it is now. Finally, then, the most important aspect to understanding why we now have a problem: Complex life on Earth is adapted to the conditions that have existed for at least the last 35 million years (when Antarctica first became glaciated). Life can adapt but only if change is slow (as in the Ice Ages); although early humans were almost wiped out in the depths of the last Ice Age (about 70k years ago). Most critically of all, human civilisation (cities and agriculture) have only been possible in the last 10k years (i.e. since the last Ice Age – characterised by relative temperature and sea level stability).
We are therefore already in an inter-glacial warm period and, unfortunately, we have now found a much more effective way to change the CO2 content of the Earth’s atmosphere and thereby induce an unnatural temperature change that will eventually restore the unnatural energy imbalance that we have caused. Therefore, there will never be another Ice Age unless or until humans go extinct.
Meanwhile, inertia in the climate system means we are now headed for 450ppm or more; and the last time CO2 was that high, it was on average 4 to 6 Celsius warmer. Add to that all the positive feedback mechanisms now kicking-in, and you have the spectre of the runaway enhanced greenhouse effect that we now face.
If you want to know more, read Hansen’s book or failing that: How does James Hansen sleep at night? and the posts that follow it (especially those in my Climate Science in a Nutshell mini-series) that summarise the book. Also, if you have not seen it, you should read my subsequent (brief) response to John on the currently-favoured insanity of proceeding to burn all the fossil fuels that we can find here (2nd paragraph onwards).
Footnote: If I were to attempt to go even further and summarise, in one single paragraph, why everyone on Earth should be concerned about ongoing anthropogenic climate disruption, it would read something like this:
Concern over anthropogenic climate disruption (ACD) is not based on computer modelling; it is based on the study of palaeoclimatology. Computer modelling is based on physics we have understood for over 100 years and is used to predict what will happen to the atmosphere for a range of projections for CO2 reductions. As such, the range of predictions is due to uncertainty in those projections; and not uncertainties in climate science. Furthermore, when one goes back 20 years and chooses to look at the projection scenario that most-closely reflects what has since happened to emissions, one finds that the modelled prediction matches reality very closely indeed.
UPDATE: I think all would do well to also read the the Comments below (or at least the first 3).