It’s almost a truism that many elements of the climate system respond to forcings and feedbacks in a non-linear fashion. There’s plenty of comment around the various blogs about the recent paper by Hansen et. al., which explores the possibility of rapid shifts in the ice sheets and the implications of this.
One of the important questions to be asked, is how rapidly can things change? Just out in the Journal of Climate is this, by Baines & Folland: Evidence for a rapid global climate shift across the late 1960′s. Apart from the intrinsic interest in the paper’s findings, that a lot of things changed before we were using satellites to systematically measure them, there is the additional interest of inferring both that there is an interconnectedness of elements within the system which respond in a concerted (monotonic?) manner, and that shifts in pattern can occur on a decadal time-scale.
The old man has commented on sea ice and sea level before, suggesting that a sea level rise of 1-1.5 metres by 2100, in response to increased loss of ice from the GIS and the WAIS, is not implausible. Hansen’s paper goes beyond this, suggesting that a BAU scenario of emissions is likely to result in a sudden collapse of the WAIS and a concomitant sea level rise in the range of some metres. The paper also implies that such an event could occur on the decadal, rather than centennial time-scale.
I don’t know what to make of the Hansen paper; it isn’t clear what it is telling us that is new or original, except the proposal that we should rethink the time-scale of ice sheet degradation, or the proposal that 3C of warming from pre-industrial levels is a plausible ‘tipping point’ for the onset of such a collapse.
A lot of time is spent by so-called ‘sceptics’ complaining that climate scientists are ‘alarmists’ (often based on media reporting of papers, rather than the papers themselves). This is to miss the point. What is known, or inferred, about the current climate and likely future trends, is a cause for concern insofar as it impacts on human society, in particular, relating to climate system shifts which change precipitation patterns or induce climate feedbacks. In this is included a concern about the rate at which change occurs, as this affects our ability to adapt effectively to changing circumstances in such ways as to minimise human loss or suffering.
But it is what is not known which is, arguably, the most significant object of ‘alarmism’ within the climate science community (inasmuch as there are some genuinely ‘alarmed scientists). If important elements of the system are non-linear in their response to progressive changes in temperature (and, by inference, concentrations of CO2), and if all that is known is that there is a point at which a rapid ‘quantum jump’ from one system state to another is initiated, then clearly, understanding what this ‘point’ is becomes critical. But we don’t know either whether such a ‘tipping point’ really does exist, or whether, if it does, how it relates to current and projected future global average temperatures.
What these two papers serve to illustrate is that it is likely that several changes will occur almost simultaneously, within a period of decades, once the stone at the top of the hill starts to roll. Secondly, they point out that we simply cannot be sure how close we are to such a point, but that it may be closer than we thought. There are those who will argue that worrying about such things is akin to worrying about what hides in the shadows, but in this case, we can be fairly sure that there is something in the shadows.
Totally sideways, but hopefully a cause for thought; recently, I’ve been wondering about 800 years. This number comes up fairly frequently. If you allow a little leeway, and say 750 years, you can also fit in D-O-type, or Bond-type patterns (half of a 1500 year pattern). My intuition tells me that this time-scale is important. My memory tells me that 750-800 years has passed (give or take) since the ‘MWP’. 800 years is also the ‘standard’ measure of lag in ice cores; it is also the estimate of time involved in a complete ‘rotation’ of the global oceanic circulation. It feels like it is significant, but, whilst a ‘pattern’ is plausible, its meaning is not clear. Is there anyone out there who can enlighten us on this? Is there a known cyclical pattern of climate change which places us at an 800 (or 750) year turning point? If so, is this significant? And, if so, to what extent is this exacerbated or overtaken by the impact of human forcings on the climate?