Prof. Gerbrand Komen (ex-) Director Climate Research KNMI 20 November 2008 KNGMG Conference Climate change facts - uncertainties - myths

Variability Many mechanism with characteristic patterns and characteristic time scales Example: North Atlantic Oscillation NAO Index - measures air 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 (

Upper air observations inthe Global Observing System (GOS) of the World Meteorological Organisation (

Voluntary Observing Ships in GOS

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

Example: annual mean precipitation

Atmosphere Land surface Ocean & sea-ice Sulphate aerosol Sulphate aerosol Sulphate aerosol Non-sulphate aerosol Non-sulphate aerosol Carbon cycle Atmospheric chemistry  Earth System Models The Mt.Office Hadley Centre  Concerted efforts in Climate Model Diagnosis and Intercomparison /

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

Robust : Global warming 1, increase CO 2 1, human influence 2, expected future rise of mean temperature 3,.. Key uncertainties: Magnitude of expected change, trends in extremes, regional change, feedbacks, role aerosols, Greenland (sea level rise!),.. 1.Based on observations 2.Based on observation and interpretation with the help of models 3.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 responses

Forget uncertainties Open mindTry to refute Just curious.... Policy relevant but not policy prescriptive. Influence policy.. Taxonomy of actors in the climate debate 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 !