From Goddard: “Forcings in GISS Climate Models”

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NASA Goddard Space Flight Center

2016-02-05
Dr. Makiko Sato
Dr. Gavin Schmidt.

We summarize here forcing datasets used in GISS global climate models over the years. Note that the forcings are estimates that may be revised as new information or better understandings of the source data become available. We archive both our current best estimates of the forcings, along with complete sets of forcings used in specific studies. All radiative forcings are with respect to a specified baseline (often conditions in 1850 or 1750).

Forcings can be specified in a number of different ways. Traditionally, forcings have been categorised based on specific components in the radiative transfer calculation (concentrations of greenhouse gases, aerosols, surface albedo changes, solar irradiance, etc.). More recently, attribution of forcings have been made via specific emissions (which may have impacts on multiple atmospheric components) or by processes (such as deforestation) that impact multiple terms at once (e.g., Shindell et al., 2009).

Additionally, the definition of how to specify a forcing can also vary. A good description of these definitions and their differences can be found in Hansen et al. (2005). Earlier studies tend to use either the instantaneous radiative imbalance at the tropopause (Fi), or very similarly, the radiative imbalance at the Top-of-the-Atmosphere (TOA) after stratospheric adjustments — the adjusted forcing (Fa). More recently, the concept of an ‘Effective Radiative Forcing’ (Fs) has become more prevalent, a definition which includes a number of rapid adjustments to the imbalance, not just the stratospheric temperatures. For some constituents, these differences are slight, but for some others (particularly aerosols) they can be significant.

In order to compare radiative forcings, one also needs to adjust for the efficacy of the forcing relative to some standard, usually the response to increasing CO2. This is designed to adjust for particular geographical features in the forcing that might cause one forcing to trigger larger or smaller feedbacks than another. Applying the efficacies can then make the prediction of the impact of multiple forcings closely equal the net impact of all of them. This is denoted Fe in the Hansen description. Efficacies can depend on the specific context (i.e. they might be different for a very long term simulation, compared to a short term transient simulation) and don’t necessarily disappear by use of the different forcing definitions above.

Quantifiying the actual forcing within a global climate model is quite complicated and can depend on the baseline climate state. This is therefore an additional source of uncertainty. Within a modern complex climate model, forcings other than solar are not imposed as energy flux perturbations. Rather, the flux perturbations are diagnosed after the specific physical change is made. Estimates of forcings for solar, volcanic and well-mixed GHGs derived from simpler models may be different from the effect in a GCM. Forcings from more heterogenous forcings (aerosols, ozone, land use, etc.) are most often diagnosed from the GCMs directly.
Forcings in the CMIP5 Simulations

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Fig. Instantaneous radiative forcing at the tropopause (W/m2) in the E2-R NINT ensemble. (a) Individual forcings and (b) Total forcing, along with the separate sums of natural (solar, volcanic and orbital) and anthropogenic forcings. (Updated: 3/12/2016)

Calculations and descriptions of the forcings in the GISS CMIP5 simulations (1850-2012) can be found in Miller et al. (2014). Data for these figures are available here and here. (Note the iRF figure and values were corrected on 3/12/2016) to account for a missing forcing in the ‘all forcings’ case. Fig. 4 in Miller et al (2014) was also updated). Snapshots of the ERF (Fs) and adjusted forcings (Fa) from these simulations. Note that the forcings from 2000 (or 2005 in some cases) are extrapolations taken from the RCP scenarios, and the real world has diverged slightly from them.

Further estimates of the responses, including temperatures and the ocean heat content changes, and efficacies are available in the supplementary material associated with Marvel et al. (2016).

Forcings in Hansen et al. (2011)

The following chart of forcings from 1880-2011 is taken from Hansen et al. (2011):

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Data is updated from the CMIP3 studies below (e.g., Hansen et al. 2007a, b) and extended to 2011 using assumptions outlined in the paper. The separate radiative forcing data (Fe) are available here (Net forcing). The figures are also available as PDFs here and here.

Forcings in the CMIP3 simulations

The following chart of forcings from 1750-2000 is taken from Hansen et al. (2005):

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Figure is also available in PDF format. (Source: Figure 28 of Hansen et al. (2005). More details, including maps and timeseries of individual forcings are available on the Efficacy web pages.

See the full article here.

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