Top-down versus bottom-up · Top-down versus bottom-up 1999-2008 to 2024-2033 temperature...

Top-down versus bottom-up

Daithı Stone ([email protected]) 1


Test of progress in modelling of climate change

• GCMs have improved through

generations in terms of

absolute climatology

• But what about transient

climate change?

• Focus on adaptation (Africa)

Reichler and Kim (2008)



Progress in estimating climate sensitivity

• Estimated 7-million-fold increase in computing power over 25 years



1999-2008 to 2024-2033 temperature difference over

SADC

• 77 simulations from 35 GCMs

• Estimated 13000-fold increase in computing power over 15 years



1999-2008 to 2024-2033 precipitation change over SADC

• 77 simulations from 35 GCMs

• Estimated 13000-fold increase in computing power over 15 years



Forget the complicated models?

• The bottom-up approach does not seem to be getting us higher

precision.

• It also does not given us any obvious way of estimating accuracy.

• What about a “top-down” approach?

• Let’s take a simple linear relaxation model

dXi(t)

dt=

1

αi

(Fi(t) − βiXi(t))

X(t) =∑

i

Xi(t)

and impose constraints from the observational record.



Comparison of extrapolation of observed relation



An alternate road to more precise (and accurate?)

climate change predictions

• Maybe we should invert the modelling-monitoring hierarchy:

– Use monitoring to constrain simple models, GCMs for evaluation

of predictions.

– GCM simulations provide surrogates of the future, and we can

make lots of them for tests.

– Our estimate of the past and present comes directly from the

observations, no middle-man.

• Monitoring foundation of the predictions.

– Brings us more in line with other sciences.

– Predictions become more convincing to non-specialists.


Developing a weather risk attribution forecast system


With Chris Lennard and Mark Tadross

Daithı Stone ([email protected]), Chris Lennard, Mark Tadross 10


http://www.csag.uct.ac.za/∼daithi/forecast

• Two primary goals:

– To gain experience with this sort of product (both provider and

user)

– To find out how people interpret this sort of product

• Two secondary goals:

– To determine sensitivity of estimates to model, forecast/hindcast,

region, variable

– To provide information required for adaptation (Africa)



The forecast system



The chance of having an unusually wet November 2009

Real world Non-GHG world FAR

Had

AM

3H

adA

M3P



The chance of having an unusually wet November 2009

Real world Non-GHG world FAR

Had

AM

3Pfo

reca

stH

adA

M3P

hind

cast



But there may be issues when ENSO is important...



And temperature... is boring



Current main issues

• How to condense data to usable information

• How to deal with selection bias

• Whether to worry about Non-GHG scenario uncertainty

• Whether to do non-ANT scenario

• When to launch


Top-down versus bottom-up · Top-down versus bottom-up 1999-2008 to 2024-2033 temperature...

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Transcript of Top-down versus bottom-up · Top-down versus bottom-up 1999-2008 to 2024-2033 temperature...