Transcript Retrospective analysis of NE Atlantic weather (especially storms)

MPI reunion, 28. August 2006
Hans von Storch:
Die Veränderung unserer heimischen Stürme
– jetzt und später im 21ten Jahrhundert
In the early 1990s,
MPI spokespeople told the public ...
• Storms are getting more severe.
• The reason is global warming.
• The evidence is
given by storms counts.
• Dynamical explanation:
in a warmer world
there is more „fuel“
for storms, which are
getting more energetic.
The evidence was poor …
• Key problem: data availability and homogeneity;
better observations describe more extreme
cases.
• Cultural construction – man is deteriorating
climate; one of the apocalyptic riders is – storms.
• „News“ about deteriorating heimische
Sturmtätigkeit sells well – drama & consistency
with pre-scientific knowledge.
• Two options – either join the bandwagon, which
supports a politically correct worldview – or openmindedly examine the claims, which is the task of
science.
10-yearly sum
of events with
winds stronger
than 7 Bft in
Hamburg
Time series of frequency of stormy days
in Kullaberg (south-western Sweden),
number of days per year with wind speed
V≥21 m/s, after Blomgren (1999).
Damaging storm event
Counting storms in weather
maps – steady increase of NE
Atlantic storms since the
1930s ….
99%iles of annual
geostrophic wind speeds
for a series of station
triangles in the North Sea
regions and in the Baltic
Sea region.
Alexandersson et al., 2002
t≤T
Change of # Bft 8/year
t≥T
Weisse et al., J. Climate, 2005
Stormcount 1958-2001
Stockholm
Lund
Time series of pressure-based storminess indices derived from pressure readings in Lund (blue) and Stockholm
(red). From top to bottom: Annual number of pressure observations below 980 hPa (Np980), annual number of
absolute pressure differences exceeding 16 hPa/12 h (NDp/Dt),
Intra-annual 95-percentile and 99-percentile of the pressure differences (P95 and P99) in units of hPa. From
Bärring and von Storch, 2005 (GRL)
Testign the “more fuel” argument
• Simulation with climate model exposed to
estimated volcanic, solar and GHG forcing,
year 1550-2000.
• Model is Atmosphere-Ocean GCM ECHO-G
with Atmospheric Model ECHAM4 (T30)
(~3.75°x 3.75°  ~300 km x 300 km) and
Ocean Model HOPE-G (T43) (~2.8°x 2.8° 
~200 km x 200 km)
• Number of strong wind events per season
(wind at 10 m;  8 Bft, gales) were counted.
Fischer-Bruns et al., 2005: Clim Dyn.
Fischer-Bruns et al., 2005
Warming
N Atlanticand Storms
in the N Atlantic
Temp & storm count:
No correlation in
pre-industrial period
(11-yr running means)
Storm frequency per season
A2 –
pre-industrial
industrial –
pre-industrial
DJF
JJA
Fischer-Bruns et al., 2005
How is the extratropical storm climate
variability linked to hemispheric
temperature variations?
• During pre-industrial times, no obvious link
between extra-tropical storminess and
hemispheric mean temperatures exist.
• In the scenarios describing the effect of
increasing GHG concentrations, storminess
and temperature develop in parallel.
• If we believe the GCM scenario for the end
of the 21st century and assume a linear
development, then we should not be able to
detect a change now and in the near future.
What do we have to tell the public?
1. Anthropogenic climate change is real.
2. Anthropogenic climate change is presently
detectable in temperature and related
parameters.
3. A detectable change will occur in other
variables at a later time.
4. No significant changes in heimischer
Sturmtätigkeit is detectable; An increase
by 10-20% in strong wind speeds is plausible
for the end of the 21st century. For the
coming decades this change is cmall compard
ot the natural variability.