Relationship between decadal climate variability and climate sensitivity

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this relationship is across different model ensembles, what mechanisms are responsible for it, and whether it can be used as an emergent constraint on climate sensitivity.We examine the relationship between decadal variability and ECS using models of varying complexity, including CMIP5 control runs and a range of conceptual energy balance models for which analytical solutions are presented.Based on these results, a general mechanism becomes apparent and the shape of the relationship is determined to be more quadratic than linear.The nonlinearity has implications for using this relationship as an emergent constraint, where an incorrect assumption of linearity might lead to biased estimates.A further surprising implication of the study is that a slowdown in global warming does not necessarily imply that climate sensitivity is lower than previously estimated.Models with a higher sensitivity, but which broadly reproduce the long-term record of global warming, are actually more likely to have slow-down periods than models with lower sensitivity.Here we examine them together.• Decadal variability chosen as scale relevant to humans.

Results
Assumptions include:

Acknowledgements and References
Why more sensitive systems have more variability Histogram of decadal trends found in a set of climate models.Again, the high sensitive models show higher decadal variability.
Overall, the Pearson's r between ECS and the standard deviation of decadal trends is 0.82.

Probability of a decade without warming.
Here a background warming independent of ECS was assumed, which corresponds to a model ensemble that is tuned to match historical warming.Using a ECS-dependent background warming, the relationship becomes weaker.Each dot corresponds to one member of the CMIP5 ensemble.
Chance of a hyperwarming decade in the RCP8.5 scenario.Hyperwarming is defined here as >10 times the mean warming rate over the 20 th century.
Possibility that one decade of 21 st century warming equals entire 20 th century's.
• Under RCP8.5 and with high climate sensitivity: 1 in 12 decades will be decades of hyperwarming.Virtually impossible in a low ECS world.
Cooling decade begin 21 st century more likely in high ECS climate.

Femke•
Major question in climate science to determine "safe climate" is finding out climate sensitivity: How much does the earth warm under doubling of CO2?• But: safe climate also depends on climate variability.

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Use the control simulations of the CMIP5 model ensemble: (Control because of long record) 2. Compute all temperature trends of 10 years 3. Compute the standard deviation and fit a normal distribution 4. Combine this information with the background information of historical simulations and projections.Sketch of a system of a high sensitivity (left) versus low sensitivity (right).Giving the left-hand system a small perturbation will lead to a big temperature change and a slow recovery rate.Common model of Earth's temperature: Hasselmann model: Here C is the heat capacity, T temperature anomaly, 1 λ proportional to Equilibrium Climate Sensitivity (ECS) and Q internal noise (forcing) and external forcing.Comparison of Global Mean Surface Temperatures (GMST) timeseries for control simulation.The orange is the HadGEM2-ES model with ECS = 4.6 K, while the purple is the GISS-E2-R model with ECS = 2.1 K.The high sensitivity has a larger typical decadal trend.