This could well be another post that illustrates my ignorance, more than anything else, but maybe I’ll learn something from it, even if noone else does. I have been assuming that it was generally agreed/accepted that periods of major climate change were forced rather than unforced. It’s possible that I’m using these terms incorrectly, so what I mean by forced is that the changes are driven by some kind of external influence. Unforced being internal variability. [Addendum : I had initially used the term natural/internal variability but, as Kevin points out in the comments there can be natural external forcings, so I’m trying to stick with internal – to the climate system – variability when referring to unforced changes].
Examples of forced variability would be volcanic outgassing – hundreds of millions of years ago – allowing greenhouse gases to accumulate in the atmosphere, warming the planet and driving us out of a snowball earth. Asteroid impacts have been associated with other periods of climate change. Orbital variations (Milankovitch cycles) are thought to be the driver for climate variability in the last 500000 years. Variations in solar intensity and variations in volcanic activity (in this case associated with aerosols) have been linked with the Medieval Warm Anomaly (MWA) and with the Little Ice Age (LIA). Today, the driver of climate change today is thought (known?) to be anthropogenic emissions of CO2.
A few recent discussions have, however, made me wonder about the issue of forced versus unforced variability. From discussions on this post it seems that even though millenial reconstructions can tell us a lot about our past climate history, there is much we still don’t know and much more (in particular about forcings) that they could still tell us. From discussion on this post it seems that even though the timings of the orbital variations (Milankovitch cycles) coincide with variations in our climate, we still haven’t shown definitively that they are indeed linked. Science of Doom has an interesting recent post that discusses this issue.
So, could this uncertainty about our past climate history indicate that some past climate changes could have been unforced, rather than forced? Our climate is clearly very complex and can be chaotic (on short timescales at least). It certainly seems plausible that some kind of internal variability could change – for example – a major ocean current, send warm water to the poles, melt polar ice, change the planet’s albedo, and produce a phase of warming that could then be amplified by the release of greenhouse gases. It’s my understanding, though, that there’s no actual evidence for this – at this stage at least. Also, although it may be true that we haven’t definitively shown, for example, that Milankovitch cycle are the drivers for major climate change in the last 500000 years, the timings seem so similar that it, at least, seems plausible that this is the driver, even if we haven’t definitively shown how it actually operated. Maybe not, of course, but I’m not aware of a plausible alternative.
So, I guess the two issues are : is there any evidence for unforced variability (long-term rather than short-term at least), and – in terms of global warming/climate change today – does it matter? I’m well aware that many “sceptics” argue that global warming/climate change today could be due to internal variability (unforced). So, if one could find evidence for unforced variability in our past climate, that would – I suspect – lead some to argue that we can’t rule out a significant unforced influence to climate change today. The problem with that is that we “know” that there is a significant external forcing (from anthropogenic greenhouse gasses). If you want global warming/climate change today to be unforced, you then need to show how the anthropogenic forcings have been cancelled while, independently, internal variations have produced a warming that is consistent with that expected from this external forcing. Anything’s possible – I guess – but some things are just very unlikely.
So, as I said at the beginning, maybe this post is more indicative of my ignorance than anything else. An earlier comment by Arthur Smith partly motivated this post and makes me think that – at least – I’m not alone in wondering about this. If others have thoughts or views on this, feel free to make them through the comments (although taking the new Comments Policy into account would be appreciated).
[Addendum : I’ve edited this slightly to try and better illustrate that unforced means internal variability, while forced applies to external influences, some of which can still be natural.]