Effective radiative forcing and adjustments in CMIP6 models
oleh: C. J. Smith, C. J. Smith, R. J. Kramer, R. J. Kramer, G. Myhre, K. Alterskjær, W. Collins, A. Sima, O. Boucher, J.-L. Dufresne, P. Nabat, M. Michou, S. Yukimoto, J. Cole, D. Paynter, H. Shiogama, H. Shiogama, F. M. O'Connor, E. Robertson, A. Wiltshire, T. Andrews, C. Hannay, R. Miller, L. Nazarenko, A. Kirkevåg, D. Olivié, S. Fiedler, A. Lewinschal, C. Mackallah, M. Dix, R. Pincus, R. Pincus, P. M. Forster
| Format: | Article |
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| Diterbitkan: | Copernicus Publications 2020-08-01 |
Deskripsi
<p>The effective radiative forcing, which includes the instantaneous forcing plus adjustments from the atmosphere and surface, has emerged as the key metric of evaluating human and natural influence on the climate. We evaluate effective radiative forcing and adjustments in 17 contemporary climate models that are participating in the Coupled Model Intercomparison Project (CMIP6) and have contributed to the Radiative Forcing Model Intercomparison Project (RFMIP). Present-day (2014) global-mean anthropogenic forcing relative to pre-industrial (1850) levels from climate models stands at 2.00 (<span class="inline-formula">±0.23</span>) W m<span class="inline-formula"><sup>−2</sup></span>, comprised of 1.81 (<span class="inline-formula">±0.09</span>) W m<span class="inline-formula"><sup>−2</sup></span> from <span class="inline-formula">CO<sub>2</sub></span>, 1.08 (<span class="inline-formula">±</span> 0.21) W m<span class="inline-formula"><sup>−2</sup></span> from other well-mixed greenhouse gases, <span class="inline-formula">−1.01</span> (<span class="inline-formula">±</span> 0.23) W m<span class="inline-formula"><sup>−2</sup></span> from aerosols and <span class="inline-formula">−0.09</span> (<span class="inline-formula">±0.13</span>) W m<span class="inline-formula"><sup>−2</sup></span> from land use change. Quoted uncertainties are 1 standard deviation across model best estimates, and 90 % confidence in the reported forcings, due to internal variability, is typically within 0.1 W m<span class="inline-formula"><sup>−2</sup></span>. The majority of the remaining <span class="inline-formula">0.21</span> W m<span class="inline-formula"><sup>−2</sup></span> is likely to be from ozone. In most cases, the largest contributors to the spread in effective radiative forcing (ERF) is from the instantaneous radiative forcing (IRF) and from cloud responses, particularly aerosol–cloud interactions to aerosol forcing. As determined in previous studies, cancellation of tropospheric and surface adjustments means that the stratospherically adjusted radiative forcing is approximately equal to ERF for greenhouse gas forcing but not for aerosols, and consequentially, not for the anthropogenic total. The spread of aerosol forcing ranges from <span class="inline-formula">−0.63</span> to <span class="inline-formula">−1.37</span> W m<span class="inline-formula"><sup>−2</sup></span>, exhibiting a less negative mean and narrower range compared to 10 CMIP5 models. The spread in <span class="inline-formula">4×CO<sub>2</sub></span> forcing has also narrowed in CMIP6 compared to 13 CMIP5 models. Aerosol forcing is uncorrelated with climate sensitivity. Therefore, there is no evidence to suggest that the increasing spread in climate sensitivity in CMIP6 models, particularly related to high-sensitivity models, is a consequence of a stronger negative present-day aerosol forcing and little evidence that modelling groups are systematically tuning climate sensitivity or aerosol forcing to recreate observed historical warming.</p>