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

The climate record definitively shows that the Earth's climate is sensitive to various drivers, including greenhouse gases, orbital variations and continental shifts. Unfortunately, there are no past analogs for the anticipated 21st century climate changes, and so a principal challenge in applying these constraints to the future is to interpret these changes quantitatively.


Figure 1: Climate sensitivities are a function of what feedbacks are included and what timescales are being considered.

At the global scale, the framework of radiative forcing and response is a powerful method to constrain sensitivity, however, there are many nuances. First, the system being described needs to be defined - what are the forcings, and what are the responses? This might seem clear at first glance, but actually depends on the availability of data and what timescales are being considered (Fig. 1). Second, there needs to be clarity in how the calculated sensitivity relates to either the “climate sensitivity” determined by models, or the related concept of the transient climate response.

The commonly used “Charney sensitivity” - the equilibrium surface temperature response to 2xCO2 allowing most atmospheric processes to react, but holding ice sheets, vegetation, atmospheric composition and ocean circulation constant - is a useful climate model metric. Constraining this from paleo-data requires information on all the “constant” components, most notably for the Last Glacial Maximum (LGM), where many (though not all) the elements are available (Köhler et al. 2010; Schmittner et al. 2011), and perhaps the last millennium, where many aspects are not fundamentally different from today (Hegerl et al. 2006). However, while the Charney sensitivity is a useful characterization of the any particular atmospheric model, it is not the same as what would actually occur if 2xCO2 were reached and maintained for a long time.

There are important nuances: climate sensitivity to cooler conditions might not be equivalent to climate sensitivity to warmer ones (indeed evidence suggests it is 80 to 90% smaller; Hargreaves et al. 2007; Crucifix 2006; Hansen et al. 2005) and some forcings just can't be fitted into a global forcing/response framework at all (such as orbital variations). Furthermore, there is often substantial uncertainty in the forcings - whether it is the size of ice sheets at the LGM, or solar forcing in medieval times, that must be taken into account in assessing the uncertainties in any estimates.

Constraining any sensitivities from the paleo-record is thus still a work in progress. Predominantly data-driven approaches (like Köhler et al. 2010 or Lorius et al. 1990, for the LGM) suggest a Charney sensitivity of around 3ºC (with a 2σ range of ~1-5ºC). Synthesis estimates that use a combination of intermediate models constrained by LGM paleo-data have given ranges of 1.2-4.3ºC (5-95%) (Schneider von Deimling et al. 2006) and 1.7-2.6ºC (17-83% range) (Schmittner et al. 2011). Note however, that the latter estimate includes a vegetation feedback, not included in the standard definition of the Charney sensitivity. A correction for this reduces the estimated sensitivity by about 0.2ºC. There is a large (and as yet barely quantified) sensitivity to model structure in these calculations since the models used to date do not give a very good fit to the regional details of the proxy data.

By expanding the framework to incorporate excluded fast and slow feedback elements, it is possible to estimate the long-term “Earth System Sensitivity” (ESS) (Lunt et al. 2010; Hansen et al. 2008), i.e. the temperature realized after all the feedbacks have worked themselves out. For instance, Lunt et al. (2010) found that the addition of ice sheet and vegetation responses (derived from Pliocene proxy data), increased their model sensitivity to CO2 by ~50%. However, this will apply only at very long timescales (many tens of thousands of years or even longer). Intermediate definitions of the sensitivity might also be calculated - for instance, taking dust, aerosol and ozone changes (fast atmospheric responses) or ocean circulation changes as feedbacks as well, but still holding ice sheets and vegetation constant.

Linking estimates of climate drivers in the past, estimates of the climate response, and the prospects for future change is however a crucial task (Schmidt 2010). To a large extent it requires the use of climate models, and the incorporation of a paleo-climate modeling component in CMIP5 will serve as a good testbed for using the paleo-record to assess the credibility of many aspects of the future projections (not simply the global mean temperature sensitivity).

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