Sensitivity of the climate system
Terje Koren Berntsen1,2
With contributions from:
Ragnhild Skeie2, Marit Holden3, Magne Aldrin3 and Gunnar Myhre2
1 University
of Oslo, 2Center for International Climate and Environmental
Research – Oslo (CICERO), 3Norwegian Computing Center
Daily Net Radiation
Top of the atmosphere. ERBE satellite data
Temperature Profile of Atmosphere
Adding Greenhouse Gases. What happens?
LWout from higher up (lower temperature, E=σT4)
Less radiation to space
Positive energy balance
The system warms until LWout and energy balance is restored
Simple example:
Double CO2 concentrations  New equilibrium
Keep other factors constant
Water vapor in the air
Cloud distribution (4-D)
Surface properties (snow cover, vegetation, etc.)
Biogeochemical cycles (e.g. carbon cycle)
Ocean currents
Definition of ECS:
 New equilibrium with ΔTs = 1.1 ± 0.1K
Equilibrium Climate Sensitivity (ECS):
Global mean surface warming due to a
sustained doubling of CO2 in the
atmosphere when the system has
reached a new equilibrium.
Feedbacks in the climate system
How can we estimate the overall climate sensitivity?
1. Design laboratory experiment
2. Attempt to understand and quantify all
relevant sub-processes  Build global
Earth System Models (ESMs)
Estimating climate sensitivity: Two approaches
Historic forcing
Climate system response
Observed response
Estimated climate
sensitivity
1200
CO2 (ppm)
1000
800
600
CO2 (ppm)
400
200
0
-100 -50 0
50 100 150 200 250
Year
Idealized Stepwise forcing in
ESMs
Calculated response in ESMs.
CCSM3, Jonko et al.
Range from CMIP5 (2.1-4.7K),
Andrews et al., GRL, 2012
Bayesian estimation of equilibrium
climate sensitivity
1. Process model
gt
- gt: True state of the climate system
- mt: Modelled state of system
- ECS: Eq. Climate sensitivity
β1·et: Impact of ENSO variability
ntliv: Longtern internal variability
- ntm: Residual model error
2. Data model
- yt: Observational data
- β0: Shift of reference period
- nt0: Observational error
Global Climate Models
Previous estimates of ECS (IPCC, 2007)
Radiative forcing since1750 (IPCC, 2007)
Radiative forcing since 1850 (Skeie et al., 2011)
http://data.giss.nasa.gov/gistemp/graphs_v3/
Figure 10.30
Observed trends in Ocean Heat Content
Levitus et al., GRL, 2012
Kouketsu et al. (JGR, 2011).
Skeie et al. 2013, in review in Journal of Climate
Including Ocean data below 700 m
• Levitus et al. (GRL, 2012). OHC 0-2000m
• Kouketsu et al. (JGR, 2011). OHC below 3000 m
• Adding the data from the deep
ocean does not change the ECS
estimate
ECS from global climate models in CMIP5
Andrews et al. Geophys. Res. Lett. 2012
Relation between ECS and Transient Climate
Response
Meinshausen et al., Nature, 2009
Knowledge of ECS  pdf of future
warming and allowable emissions
Meinshausen et al., Nature, 2009
Summary
• Observed climate change over the last decade
leads to a lower and better constrained estimate
of the climate sensitivity
• Global climate models indicate higher sensitivity
• A lower estimate of the ECS allows cumulative
carbon emissions (2000-2050) to increase from
1400 Gt to 1700.
• Stabilizing at current emissions  1600-1700 Gt