IPCC (WG1) Overview

Thomas StockerOeschger Centre for Climate Change ResearchPhysics InstituteUniversity of Bern

~3,900 H2 O molecules ~ 280 CO2 molecules~ <1 CH4 molecule

this raises temperature

from –18°C to +15°C

before year 1750 1,000,000 molecules of air contained:

+195 H2 O molecules +105 CO2 molecules

+1 CH4 molecule

in year 2007 1,000,000 molecules of air contain:

how much more warming

and other changes ?

CO2 : higher levels and faster rise

(IPCC, 2007, Fig. SPM-1)

Sie

gent

hale

r et a

l., (2

005)

Joos

& S

pahn

i (20

08)

last 10,000 years

last 200 years

last 50 years

CO2 : higher levels and faster rise

(Fig. 2.3, IPCC 2007)

CO2 increase is due to fossil fuel burning

UN Intergovernmental Panel on Climate Change

• 152 Authors• 450 Contributors• 40 Countries• ~600 Reviewers• 4 years of work

Report (WG1): 996 p.

SPM (WG1): 18 p.

www.ipcc.ch

very high confidence that the [] net effect of human activities since 1750 has been one of warming

(IPCC, 2007, Fig. SPM-2)

Change in many climate variables

The warming in the

climate system is

unequivocal, ...

(IPCC, 2007, Fig. SPM-3)

Arctic sea ice reduces rapidly

(Stro

eve

et a

l., 2

007)

Warmth of the last half century is unusual

(IPC

C, 2

007,

Fig

. TS-

20)

Warming rates similar throughout atmosphere

°C per decade (1979-2005)

(IPC

C, 2

007,

Fig

. TS-

6)

Fingerprint of climate change in atmosphere

2080-2099

60°S 60°NEQ

800

600

400

200

1000

Climate sensitivity now better constrained

IPCC TAR IPCC AR4

Equilibrium Climate Sensitivity

range:1.5 to 4.5°C

most likely valueabout 3°C

very unlikely <1.5°C

likely range:2.0 to 4.5°C

(IPC

C, 2

007,

Box

10.

2, F

ig. 1

c)

Equilibrium Climate Sensitivity (°C)

The key question:

Has the warming anything to

do with the increase of

greenhouse gases ?

Causal link between increasing CO2 and warming

(IPCC, 2007, Fig. SPM-4)

How good are IPCC projections ?

(IPC

C, 2

007,

Fig

. TS-

26)

Global warming depends on scenario

Emissions „high"

(IPCC, 2007, Fig. SPM-5)

Emissions „low"

„low

"„h

igh"

Gre

enho

use

gas

emis

sion

s

(IPCC, 2007, Fig. SPM-6)

Global warming depends on scenario

(IPCC, 2007, Fig. 10.19)

Heat waves very likely to become more frequent

Sea level rise: Very long-term – large uncertainties

(IPCC, 2007, Fig. 10.33)

(IPCC, 2007, Fig. 10.38)

Very likely reduction of the Atlantic MOC

(IPC

C, 2

007,

Fig

. 10.

15)

Water resources will change worldwide

(IPC

C, 2

007,

Fig

. SP

M-7

)

Conclusions 1: Robust findings

• Geographic patterns of temperature increase

• Long-term sea level rise

• Large-scale changes of precipitation

• Significant decrease of glaciers

• Acceleration of snow cover reduction in Arctic

• Slow down of the Atlantic MOC

• More frequent heat waves, fewer frost days

• Higher risk of droughts

Conclusions 2: Key Uncertainties

• Abrupt change, „tipping points“

• Extremes in precipitation

• Frequency and strength of tropical cyclones

• Response of climate modes (ENSO, NAO, AMO)

• Carbon cycle feedbacks

• Ocean acidification

• Ice sheet instabilities: Greenland, WAIS

• Possible irreversible changes

Switzerland: Great Aletsch Lake in 2100 ?

M. Bader, Sonntagszeitung 31.12. 2006