In article ,
Alex Stephens Jr writes:
Thanks for some good answers there John.
Thanks.
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.
I think the Met Office seasonal forecasts specifically said that they
were relative to the 1971-2000 average.
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....
Yep. A /weather/ forecast - as distinct from a /climate/ forecast - that
far in advance could be no better than guesswork, I think. (And a
climate forecast for 2014 would have to say no more than that the
climate would be much as it is now, as four years is too short a period
for it to change much. Which doesn't rule out the possibility of 2014
having either unusually warm or unusually cold weather in the UK.)
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?
Did you really mean 30 decades? I don't think that as long a record as
that is necessary. What you need to put in the models as a starting
point is as accurate a representation of the current climate as possible
- for which thirty years of data should be enough. There's a trade off:
use too short a reference period, and the short term "noise" of weather
can distort the picture, but too long a period and the underlying
climate may have changed appreciably during that time. I suspect that's
why conditions for 1971-2000 are usually used as a base-line. Even then,
some warming is evident during that period.
Having said that, you would obviously want to check, and maybe
calibrate, your model by running it for the past, to compare its results
with what actually happened. Clearly the longer period that you can do
that for, the better. Since climate is a global thing, and you can't
really look at one part of the world in isolation, you ideally need
global data, which I suspect would limit you to the last 100-150 years.
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?
I think you get into areas where it's hard to know what is meant by
"right" and "wrong" unless you define your criteria precisely in
advance. And even then a scientist might view a forecast as having been
right where a typical member of the public might view it as wrong.
Personally I would say that a forecast of 7 out of 10 winters warmer
than average would be pretty successful if 9 out of 10 were warmer,
since the /weather/ can't be forecast that far ahead so there's always
the chance of essentially random factors causing rather more - or rather
fewer - warm winters than expected.
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?
About the same, I would say.
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.
If you look at 2050 in isolation, then you are effectively turning the
climate forecast into a weather forecast, and it is likely to be worse -
less accurate and much less detailed - than an attempt to forecast the
next season in 2010. But if you regard it as a climate forecast for the
period 2045-2055, then the picture should be different. It won't tell
you about individual years, but it will tell you about the
characteristics of the period. Forty years is a far way away, but it
should be possible to say something like 95% confidence that mean
temperature for 2045-55 will be between 1 and 2 degrees (say) higher
than it is now. And for that far ahead, that's really all governments
and big corporations need for long-term planning. It's not like seasonal
forecasts, where you might want to know if you should take your holiday
at home or abroad, or as a council whether you should stockpile more
salt.
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.
It's certainly taken account of in climate modelling.
Indeed, I wonder to what extent this variable has
impacted on the incorrect met office seasonal forecast?
Not at all, I would have thought, since the amount of water vapour
currently in the atmosphere is known and the effect of GW isn't going to
noticeably affect it over a period as short as three months.
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
--
John Hall
"Acting is merely the art of keeping a large group of people
from coughing."
Sir Ralph Richardson (1902-83)