Prof. Gerbrand Komen

(ex-) Director Climate Research KNMI

20 November 2008KNGMG Conference

Climate changefacts - uncertainties - myths

Variability

•Many mechanism with characteristic patterns and characteristic time scales

•Example: North Atlantic Oscillation

NAO Index - measuresair pressure difference between Iceland and the Acores

1. Use all available observations

2. Quantify processes, transports, interactions and feedbacks

Scientific approach

Natural archives (tree rings, ice cores, oceanic sediments, etc):

Instrumental observations (20th century)

Longer time scales provide important perspective

Because of the limited time I will focus on the past century

Observations: many time scales

Instrumental observations

Surface stations in the Global Observing System (GOS) of the Word Meteorological Organisation (www.wmo.ch)

Upper air observations inthe Global Observing System (GOS) of the World Meteorological Organisation (www.wmo.ch)

Voluntary Observing Ships in GOS

In the year 2007 : 383 ppm (37% above pre-industrial)

Example: annual mean precipitation

Atmosphere Atmosphere Atmosphere Atmosphere Atmosphere Atmosphere

Land surfaceLand surfaceLand surfaceLand surfaceLand surface

Ocean & sea-ice Ocean & sea-ice Ocean & sea-ice Ocean & sea-ice

Sulphateaerosol

Sulphateaerosol

Sulphateaerosol

Non-sulphateaerosol

Non-sulphateaerosol

Carbon cycle Carbon cycle

Atmosphericchemistry

1975 1985 1992 1997

Earth System Models

Th

e M

t.O

ffice

Had

ley

Cen

tre

Concerted efforts in Climate Model Diagnosis and Intercomparison

http://www-pcmdi.llnl.gov/

Average annual precipitation

Observations

Multi-model average

Global mean temperature in the 20th century model simulations vs observations

Natural causes: chaos, sun, ocean, orbital variations, volcanic dust . .

How does one establish a human influence: Greenhouse gases, Aerosols, Land surface change . .

a. Forcing with all known forcings

b. Forcing with natural forcings only

Compare model simulations with observations

Observations

All forcing

Solar + volcanic

Complex en unique system:

Observations

Model limitations

Sources of uncertaintySources of uncertainty

Limited understanding

Limited predictability

Most warming over the past 50 years is very likely due to anthropogenic GHG increases. Part of the variation can be accounted for by natural causes.

Δ T in 2100 (relative to 1990) is likely to lie between 1.1 en 6.4 °C

‘‘Quantifying’ uncertaintiesQuantifying’ uncertainties

Note: These probabilities involve a certain amount of (subjective) expert judgement

Robust: Global warming1, increase CO2

1, human influence2, expected future rise of mean temperature3, . .

Key uncertainties:Magnitude of expected change, trends in extremes,

regional change, feedbacks, role aerosols, Greenland (sea level rise!), . .

1. Based on observations2. Based on observation and interpretation with the

help of models3. Based on Greenhouse gas theory and models

Robust

1. Impacts on human society and biosphere

2. Reduction of vulnerability requires more adaptation - but there are limits to adaptability

3. Mitigation is possible

Key uncertainties

1. Costs: Unmitigated change, adaptation, mitigation

2. Best possible development path (energy innovation speed, adaptive capacity, governance, . .)

IPCC, possible responsesIPCC, possible responses

Forget uncertainties

Open mind Try to refute

Just curious . . . . . .

Policy relevant but not policy

prescriptive

. . . . . .

Influencepolicy

. . . . .

Taxonomy of actors in the climate Taxonomy of actors in the climate debatedebate

Alarmists Sceptics

Pure scientists

Honest broker

Climate science will never provide absolute certainty A description of current understanding is rather

complex (see summary in my summary) This complex message is often deformed in the

media and in the public debate Values play an important role in decision making Science should proceed as usual

Thank you !