The learning curve has long been known to be able to predict the rate of progress in many areas. If one applies it to climate, the results are startling and suggest many centuries until we can predict even the yearly climate forecast as well as we can predict the weather today.
Note: this was written after the Royal Society meeting on climate and any references to “speakers” or similar refers to this meeting.
Scales of Variability
As fig 1 shows, global mean temperature variation in the instrument record increases rapidly when longer periods are considered approximately as follows:
For any change seen over periods of one decade, much greater scale changes are expected over longer periods of centuries but much less change is expected over the year to year scale.
Fig 1: Variability of observed global mean temperature as a function of time-scale (°C2 yr–1)
from figure 9.7 IPCC (2007)
Whilst the causation of this change in scale is not discernible from this graph, it does suggest an upper limit to natural variation which over the climate forecasting time-scale shows a strong increasing upper limit as we approach the century to century scale of forecast and strongly decreasing effect on shorter range forecasts. As Prof Palmer highlighted, Lorenz makes clear that “each scale of motion possesses an intrinsic finite range of “predictability” and the knowledge and skill drawn from day-to-day or even month-to-month month may reflect entirely difference physical phenomena than are present at the decade-to-decade and century-to-century scale. There is therefore little rational based on these different scales of behaviour to suggest that lessons learnt about modelling the physical processes that effect day-to-day changes in weather will provide much insight into the longer-term processes effecting climate. Climate models must be based on appropriate scale data. Month-to-month on month-to-month changes. Year-to-year on year-to-year changes, decade-to-decade on decade-to-decade changes and century-to-century on century-to-century changes.
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