cc: keith.alverson@pages.unibe.ch, pedersen@eos.ubc.ca, rbradley@geo.umass.edu, mhughes@ltrr.arizona.edu, whitlock@oregon.uoregon.edu, jto@u.arizona.edu date: Mon, 10 Jul 2000 13:34:37 +0200 from: Frank Oldfield subject: the ghost of futures past to: k.briffa@uea.ac.uk Salut mes amis, I've lost sleep fussing about the figure coupling Mann et al. (or any alternative climate-history time series) to the IPCC scenarios. It seems to me to encapsulate the whole past-future philosophical dilemma that bugs me on and off (Ray - don't stop reading just yet!), to provide potentially the most powerful peg to hang much of PAGES future on, at least in the eyes of funding agents, and, by the same token, to offer more hostages to fortune for the politically motivated and malicious. It also links closely to the concept of being inside or outside 'the envelope' - which begs all kinds of notions of definition. Given what I see as its its prime importance, I therefore feel the need to understand the whole thing better. I don't know how to help move things forward and my ideas, if they have any effect at all, will probably do the reverse. At least I might get more sleep having unloaded them, so here goes...... The questions in my mind centre round the following issues. If I've got any one of them wrong, what follows in each section can be disregarded or (more kindly) set straight for my benefit. 1. How can we justify bridging proxy-based reconstruction via the last bit of instrumental time series to future model-based scenarios. 2. How can the incompatibilities and logical inconsistencies inherent in the past-future comparisons be reduced? 3. More specifically, what forms of translation between what we know about the past and the scenarios developed for the future deal adequately with uncertainty and variability on either side of the 'contemporary hinge' in a way that improves comparability across the hinge. 4. Which, if any, scenarios place our future in or out of 'the envelope' in terms of experienced climate as distinct from calculated forcing? This idea of an envelope is an engaging concept, easy to state in a quick and sexy way (therefore both attractive and dangerous); the future could leave us hoisted by our own petard unless it is given a lot more thought. 1. I am more or less assuming that this can already be addressed from data available and calculations completed, by pointing to robust calibration over the chosen time interval and perhaps looking separately at variability pre 1970, if the last 3 decades really do seem to have distorted the response signatures for whatever reasons. I imagine developing this line of argument could feed into the 'detection' theme in significant ways. 2 & 3. This is where life gets complicated. For the past we have biases, error bars that combine sources of uncertainty, and temporal variability. For the future we have no variability, simply a smooth, mean, monotonic trend to a target 'equilibrium' date. Bandwidths of uncertainty reflect model construction and behaviour. So we are comparing apples and oranges when we make any statement about the significance of the past record for the future on the basis of the graph. Are there ways of partially overcoming this by developing different interactions between past data and future models? My own thinking runs as follows: Take variability. Do we need to wait for models to capture this before building it into future scenarios? This seems unnecessary to me, especially since past variability will be the validation target for the models. Is there really no way of building past variability into the future projections? One approach would be to first smooth the past record on the same time-span as the future scenarios. This would get us to first base in terms of comparability, but a very dull and pretty useless first base in and of itself. It would, however, allow all kinds of calculations of inter-annual variability relative to a mean time line of the 'right' length. This in turn could be used in several ways, for example: - build the total range of past variability into the uncertainty bands of each future scenario. - take the 30,50 or 100 year period (depending on the scenario for comparison) during which there was the greatest net variability, or the greatest net fall in Temperature, or the greatest net increase in T. and superimpose/add this data-based variability on the mean trends. - take the n-greatest positive anomalies relative to the trend and use them to define an upper limit of natural variability to compare with the (to my mind) more realistic future scenarios. These and cleverer variants I cannot begin to think up seem to me to hold out the possibility of linking future projections of GHG forcing with what we know about natrual variability in reasonably realistic ways and perhaps even of redefining the 'past data-future scenario' relationship in ways that benefit both the paleo-community and the quality of future projections. 4. I also think the above kinds of exercise might eventually lead us towards a better definition of 'the envelope' and more confidence in deciding what is outside and what is not. The same sort of approach can be taken towards projections of P/E I imagine and, more particularly, at regional rather than global or hemispheric level. Sorry if all this sounds stupid or obvious. I got afflicted with the 'need to share' bug. Frank ____________________________________________ Frank Oldfield Executive Director PAGES IPO Barenplatz 2 CH-3011 Bern, Switzerland e-mail: frank.oldfield@pages.unibe.ch Phone: +41 31 312 3133; Fax: +41 31 312 3168 http://www.pages.unibe.ch/pages.html