Hans von Storch:

Die Veränderung unserer heimischen Stürme

– jetzt und später im 21ten Jahrhundert

MPI reunion, 28. August 2006

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 aregetting 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 open-mindedly 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

Weis

se et

al.,

J. C

limate

, 200

5

Change of # Bft 8/year

t ≤ T t ≥ T

Stormcount 1958-2001

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)

Stockholm

Lund

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.

(11-yr running means)

Temp & storm count:No correlation in pre-industrial periodN AtlanticWarming and Storms

in the N Atlantic

Fis

cher-

Bru

ns

et

al.,

20

05

industrial – pre-industrial

Storm frequency per season

DJF

JJA

A2 – pre-industrial

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.