The effect of rapid adjustments to halocarbons and N2O on radiative forcing

Hodnebrog Ø, Myhre G, Kramer RJ, Shine KP, Andrews T, Faluvegi G, Kasoar M, Kirkevåg A, et al. (2020). The effect of rapid adjustments to halocarbons and N2O on radiative forcing. Climate and Atmospheric Science 3 (1): e43. DOI:10.1038/s41612-020-00150-x.

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Project: Constraining uncertainty of multi decadal climate projections (CONSTRAIN, H2020 820829)


Rapid adjustments occur after initial perturbation of an external climate driver (e.g., CO2) and involve changes in, e.g. atmospheric temperature, water vapour and clouds, independent of sea surface temperature changes. Knowledge of such adjustments is necessary to estimate effective radiative forcing (ERF), a useful indicator of surface temperature change, and to understand global precipitation changes due to different drivers. Yet, rapid adjustments have not previously been analysed in any detail for certain compounds, including halocarbons and N2O. Here we use several global climate models combined with radiative kernel calculations to show that individual rapid adjustment terms due to CFC-11, CFC-12 and N2O are substantial, but that the resulting flux changes approximately cancel at the top-of-atmosphere due to compensating effects. Our results further indicate that radiative forcing (which includes stratospheric temperature adjustment) is a reasonable approximation for ERF. These CFCs lead to a larger increase in precipitation per kelvin surface temperature change (2.2 ± 0.3% K−1) compared to other well-mixed greenhouse gases (1.4 ± 0.3% K−1 for CO2). This is largely due to rapid upper tropospheric warming and cloud adjustments, which lead to enhanced atmospheric radiative cooling (and hence a precipitation increase) and partly compensate increased atmospheric radiative heating (i.e. which is associated with a precipitation decrease) from the instantaneous perturbation.

Ozone-depleting halocarbons and nitrous oxide (N2O) are well-mixed greenhouse gases that have contributed substantially to radiative forcing (RF) since pre-industrial time, by 0.33 ± 0.03 W m−2 (0.18 ± 0.17 W m−2 when including stratospheric ozone depletion) and 0.17 ± 0.03 W m−2, respectively1. A substantial contribution to global warming and Arctic sea-ice loss in the latter half of the 20th century was recently attributed to ozone-depleting substances2,3. Atmospheric lifetimes of chlorofluorocarbons (CFCs), an important group of ozone-depleting halocarbons, are typically several decades or centuries4. Their impact on climate will therefore remain strong for many years to come despite regulations of halocarbon emissions through the Montreal Protocol signed in 1987.

Most halocarbons, such as CFC-12, have their main infrared absorption bands at different spectral wavelengths compared to the two main anthropogenic greenhouse gases CO2 and methane (CH4), in the so-called atmospheric window region around 800–1200 cm−1 where the RF efficiency is strong5,6. The main infrared absorption bands of N2O partly overlap with CH4, but an important difference is that CH4 has more significant shortwave (SW) absorption bands7,8. Although these factors may imply that the climate effects of halocarbons and N2O differ from those of CO2 and CH4, relatively little is known about the details of short and long-term responses of halocarbons and N2O on climate.

While numerous studies exist on radiative transfer modelling of halocarbons, only a few studies have performed global climate model (GCM) experiments to investigate the climate effects of halocarbons separately. Hansen et al.9,10 found that the surface temperature response to a large forcing of CFCs and N2O had very similar geographical patterns as the response to CO2 and CH4 forcing. The efficacies (i.e. the warming per unit RF) for CFCs were, however, around 30% larger than for CO2, but close to unity for CFCs and N2O when rapid adjustments were accounted for, as also found in a recent multi-model study11. Forster and Joshi12 investigated halocarbon contributions to atmospheric temperature change and found a significant warming at the tropical tropopause; e.g. this led to a ~6% weaker surface warming per unit forcing for CFC-12 compared to CO2.

In the 5th Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC), RF was diagnosed as effective radiative forcing (ERF), which includes instantaneous radiative forcing (IRF) and rapid adjustments including stratospheric temperature adjustment1,13,14. These fast responses occur on timescales of days to months, before most of the changes in the global-mean and annual-mean surface temperature occur. ERF is shown to be a better indicator for surface temperature change than the earlier RF definition (RF which includes stratospheric temperature adjustment), and rapid adjustments have recently been quantified for some of the most important climate drivers15,16. However, for halocarbons and N2O, contributions of different rapid adjustment terms remain largely unknown, with the exception of stratospheric temperature adjustment which can be estimated in radiative transfer models17.

The fast responses are typically investigated with GCMs using fixed sea surface temperatures (SSTs) and the total response is normally studied using coupled atmosphere–ocean GCMs. The slow (feedback) response is the difference between the total and fast response. In terms of global mean precipitation, an increase of 2–3% per kelvin global mean surface warming is found for the slow response, independent of the climate drivers studied (CO2, CH4, solar irradiance, black carbon and sulphate)18, but rapid adjustment processes lead to differences in the total precipitation response between drivers15,19. Further, one modelling study has indicated that CFCs would have had a strong impact on the hydrological cycle without the Montreal Protocol20.

Here, our focus is to explore how halocarbons and N2O, some of the most important anthropogenic greenhouse gases, influence climate on various time scales by using a range of GCMs. A main aim is to quantify ERF of these compounds and to determine their impact on the hydrological cycle. The climate effects of these gases through stratospheric ozone depletion is beyond the scope of this study but covered elsewhere (see ref. 4 and references therein), and rapid adjustments due to ozone changes (in both the troposphere and stratosphere) have recently been quantified21. Experiments are performed within the Precipitation Driver and Response Model Intercomparison Project (PDRMIP)22, where a main focus is on how different climate drivers affect various components of the climate system, and the hydrological cycle in particular, on both short and long time scales. PDRMIP studies have been used to better understand results from the more complex Coupled Model Intercomparison Project phase 5 (CMIP5)23 simulations for the historical period19,24,25, where several climate drivers are perturbed at the same time; similarly, PDRMIP results can be used to explain results from the new generation CMIP6 model simulations26, which will serve as input to the upcoming 6th Assessment Report of IPCC.

Results presented here involve GCM simulations of perturbations in CFC-11 (named “CFC11×8”), CFC-12 (“CFC12×9”), and N2O concentrations (named “N2O×3”) (where the multiplier indicates the approximate size of the imposed perturbation—see “Methods” for exact factors and how the experiments are constructed) and complement the core PDRMIP experiments CO2×2, CH4×3, solar irradiance + 2% (Sol + 2%), black carbon×10 (BC×10) and sulphate×5 (Sul×5).

Item Type: Article
Research Programs: Energy (ENE)
Depositing User: Luke Kirwan
Date Deposited: 19 Nov 2020 10:06
Last Modified: 19 Nov 2020 10:06

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