"John Hall" wrote in message

Very different, because it was for a /particular/ three month period.
They said something along the lines of: "we expect this coming winter to
have a 70% chance of being warmer than the 1971-2000 average". That
makes it a weather forecast. If they had said "we expect 70% of winters
in the next decade to be warmer than the 1971-2000 average" then in
would have been a climate forecast.


Yes, but forecasting climate in the long-term can be easier than
forecasting weather in the shorter term. Take those two example
forecasts above. With the first, if the winter wasn't warmer than
average most people would view it as having been wrong. It certainly
wouldn't have provided them with useful guidance. But the second
forecast would have provided useful guidance by the end of the decade if
most winters were warmer than average, even if the number was not
precisely 7 out of 10 but 6 or 8 (or arguably even if it was 9 or 10).


That's because, averaged over the whole globe, there seems very little
possibility of that happening unless the understanding embodied in the
models is fatally flawed. I suppose that a sudden unpredicted - and
possibly unpredictable - increase in volcanic activity or a reduction in
solar activity could lead to a reduction, though, at least temporarily
--
John Hall

Thanks for some good answers there John. The difference between a seasonal
forecast and a climate forecast is now somewhat clearer in my mind.

A few points of concern though; When seasonal forecasts are made (whether or
not by the met office), I presume they are based on recent 30 year averages
or perhaps longer and recent trends within that - or they should be anyway.
And are usually made only one season in advance of course. IE I haven't seen
a forecast yet for the winter or summer of 2014....

Is it wrong to therefor say that decadal forecasts should be using 30
decades worth of data (which obviously they can't) as well as recent trends
to forecast with similar accuracy the next decade? And that using less
decadal data hinders the accuracy or value of that forecast? And likewise,
is it sensible to make a forecast for 4 decades from now with such a limited
database and a plethora of potential variables, most of which are not new
and have influenced the climate greatly in the past? IE is a current decadal
climate forcast for the 2050's not akin to making a seasonal forecast now
for 2014, but with even less data?

Let's assume the met office issued the exact same winter forecast for the
next nine winters, and alongside that, global climate predictions say that 7
out of 10 winters should be warmer than average in north west europe. And in
the event nine winters were warmer than average, Haven't they both got it
wrong? Conversely of course, I could have said only half of the winter's
were warmer than average. Who is less wrong on that occasion, and are they
more wrong than if there had been 9?

I think my wider point is that some people, very mistakenly, take for
granted that climate forecasts for 2050 are as accurate as 2 day weather
forecasts and hugely more reliable than a seasonal forecast. And perhaps
worse than this, some scientists advocate that line of thinking and won't
hear a word against it. I would like to understand why this has became such
a common assumption? Some variables such as volcanic or solar activity,
cannot of course be forecast. But the one variable that is a consequence of
continued global warming is an increase in water vapour in the atmosphere,
which strangely people forget about, or even ignore, and will likely have a
massive effect compared to current climate change not only upon the climate,
but equally on humanity. Indeed, I wonder to what extent this variable has
impacted on the incorrect met office seasonal forecast?

As you can tell, I'm far from convinced that forecasting the climate in
decadal terms is any easier than a seasonal weather forecast, but one thing
is for sure, it takes a hell of a lot longer to find out if it's achievable.

And if anyone else would like to chip in, please try to do so in the same
intellectual fashion as John has :P

Alex.