date: Fri, 28 Feb 1997 10:39:59 +0000 (GMT) from: "N.W.Arnell" subject: Re: thresholds paper.. to: m.hulme@uea.ac.uk (Mike Hulme) Mike, How likely is tolerable climate change... I think this is a very good - and revealing - paper. I like the approach taken, and methods proposed. My comments fall into two areas. 1. Signal/noise ratios and defining climate change scenarios I very much like the comparison of 30-year time slices from the HADCM2 transient run. This is exactly the sort of information I was seeking at the DoE Global meeting. Have you done it for other seasons, or for months or locations? I guess the problem is even worse with decades! The variability you show emphasises the dangers of simply comparing two n-year periods or, more worringly, comparing results from scenarios based on different periods (eg rescaled 2080s compared to 2020s). The only solution appears to lie in the use of ensembles... I'm not so concerned about the similarities (or otherwise) between future changes and past n-year variability. In practice, most impact assessments use (or should use) a well-defined baseline period, and it is change relative to that which is important, even if it is small (but systematic). Managers are planning for the future, not the past, although an idea of future vs past variability would give them some clues about their ability to manage future changes. One significant point that you have highlighted, however, is that changes in estimated system performance due to climate change may be of a similar magnitude to changes which appear simply because different data periods are used (as a footnote, water companies in Britain are currently reviewing their system yields, supposedly using consistent baseline data - they are in for some surprises!!). 2. Identification of critical thresholds Although thresholds appear attractive in water management and are reasonably easy to define (eg implementation of a hosepipe ban / water level in a reservoir below 10% of usable storage...), it can be very difficult to work out or generalise the risk of them being exceeded. In water resources terms, for example, it is not just summer temperature and precipitation that is important, but rather the state of storages at the beginning of the summer season: these are an integration, in Britain, of climate over winter. Unfortunately, the winter-end/summer start storage can depend very much on how climate was distributed over winter. It is not just the rainfall total that is important, but also (although probably to a lesser extent) how it was distributed over the winter. A full assessment of the risk of threshold exceedance would therefore be multivariate, with axes defining, for example, cumulative winter net rainfall, the distribution of that rainfall through winter (eg December total as % of the DJF total??), summer precipitation and summer temperature. The "danger zone" would be multi-dimensional. ...but this is a quibble, and perhaps reflects problems peculiar to the water system, which is by nature integrative over time. I promised Ferenc Toth that I would prepare a paper on thresholds and water - even though I had to withdraw from the workshop at the last minute....! - and I am attempting to work some of these issues up. I will try to include some numerical examples. I'll let you see a draft once I have made some proper progress. Regards Nigel Arnell