Climate sensitivity - How sensitive is Earth’s climate to CO2 [Present]


Figure 1: A comparison of estimates of the Transient Climate Response from complex models evaluated against modern observations from versions 3 and 5 of the Coupled Model Intercomparison Project (Meehl et al. 2007; Taylor et al. 2011) shown as red and blue dots respectively. A PDF of the TCR computed from a simple model with parameters constrained by observations is also shown (Gregory and Forster 2008).

The Climate Sensitivity (CS) is a key parameter for assessing future climate change, as is its counterpart the Transient Climate Response (TCR – see figure 1). The TCR is defined as the 20-year global, annual mean temperature change averaged around the time of CO2 doubling, under a forcing scenario of CO2 increasing at a rate of 1% per year compounded. Just like the CS, the TCR quantifies physical feedbacks in the climate system associated with the surface, clouds, water vapor, sea ice, etc., but it is more relevant for transient climate change in the near future and does not suffer from the ”long tail” evident in estimates of the CS (Frame et al. 2005).

A notable feature of the latest version of the Coupled Model Intercomparison Project (CMIP5) is that atmosphere models coupled to simple slab or mixed-layer oceans are not included in the design, limiting a direct comparison of the range of CSs with previous versions of CMIP (although the CS and TCR tend to be well correlated in models and the effective CS can be calculated from experiments included in CMIP5).

We may use modern observations to aid in building complex models of the climate system from “first principles” i.e. by solving the dynamical equations of the atmosphere and ocean and parameterizing sub-grid-scale processes in as much detail as possible. Multiple data sources may be used to evaluate both the individual building blocks and the emergent properties of the model; data from process-based observations, possibly gathered during dedicated field campaigns, historical in situ measurements, remotely sensed data, etc. We can then interpret measures such as the CS and TCR computed from complex modes as estimates that integrate our understanding of climate (embodied in the laws of physics) and modern day observations.

The range of CS and TCR has not changed much in successive generations of models. The example in the figure shows a range of 1.2-2.6ºC for the CMIP3 models and 1.3-2.4ºC for the CMIP5 models available at the time of writing.

An alternative approach comes from using simple climate models that may only simulate aggregate variables such as global mean temperature. Simple models can be run many times and statistical approaches can be used to formally estimate the parameters of the model based on constraints from observations/estimates of e.g. recent ocean heat uptake and radiative forcing. Measures such as CS and TCR then come with likelihood estimates and the uncertainty may be expressed as a probability density function (PDF – see Fig. 1).

Unfortunately, using different observational data sources from different modern (and paleo) time periods, have not produced tight constraints on variables such as the TCR. The 5-95% range in the example from the figure from (Gregory and Forster 2008) is 1.3-2.3ºC, comparable with the ad hoc range from CMIPs. CMIP ranges of CS are also comparable with observationally constrained PDFs (Knutti and Hegerl 2008).

As the signal of climate change emerges from the noise of natural variability, PDFs based on simple-model constraints should narrow. Collection of new and more detailed modern observations, particularly of climate processes such as clouds, should allow us to better improve and evaluate our complex models. One recent approach combines complex modeling with formal parameter estimation to produce PDFs of global and regional change (Sexton et al., in press). This allows multiple modern observational records to be used to constrain projections, although the cost of implementation is high. There is still scope for much research in quantifying how sensitive Earth’s climate is to CO2 change using modern models and data.

Category: Science Highlights | PAGES Magazine articles

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