Øyvind Seland

Aerosol-climate interactions in the CAM-Oslo atmospheric GCM and investigation of associated basic shortcomings

The paper discusses some challenges in aerosol-climate modelling. CAM-Oslo, extended from NCAR-CAM3, employs an aerosol module for sea-salt, dust, sulphate, black carbon (BC) and particulate organic matter (OM). Primary aerosol size-distributions are modified by condensation, coagulation and wet-phase processes. Aerosol optics and cloud droplet numbers use look-up tables constructed from first principles. Ground level sulphate and sea-salt are generally well modelled, BC and OM are slightly underestimated (uncertain), and dust is considerably (factor ∼2) underestimated. Since non-desert dust, nitrate, anthropogenic secondary organics, and biological particles are omitted, aerosol optical depths (0.12) are underestimated by 10–25%. The underestimates are large in areas with biomass burning and soil dust. The direct and indirect forcing of aerosol increments since pre-industrial time are estimated at +0.031 Wm−2 and −1.78 Wm−2, respectively. Although the total absorption AOD probably is slightly underestimated, the BC contributes to DRF with double strength compared to the AeroCom average. Main reasons for this include: internal BC-mixing (+0.2 Wm−2), accumulation mode BC-agglomerates (+0.05 Wm−2), assumed aitken-mode OM-BC mixture (+0.1 Wm−2), large BC fraction (36%) above 500 hPa, and high low-level cloudiness. Using a prognostic CDNC and process parametrized CCN activation instead of assuming CDNC are equal to CCN, the indirect forcing is 36% smaller.

High-latitude volcanic eruptions in the Norwegian Earth System Model : the effect of different initial conditions and of the ensemble size

Large volcanic eruptions have strong impacts on both atmospheric and ocean dynamics that can last for decades. Numerical models have attempted to reproduce the effects of major volcanic eruptions on climate; however, there are remarkable inter-model disagreements related to both short-term dynamical response to volcanic forcing and long-term oceanic evolution. The lack of robust simulated behaviour is related to various aspects from model formulation to simulated background internal variability to the eruption details. Here, we use the Norwegian Earth System Model version 1 to calculate interactively the volcanic aerosol loading resulting from SO2 emissions of the second largest high-latitude volcanic eruption in historical time (the Laki eruption of 1783). We use two different approaches commonly used interchangeably in the literature to generate ensembles. The ensembles start from different background initial states, and we show that the two approaches are not identical on short-time scales (<1 yr) in discerning the volcanic effects on climate, depending on the background initial state in which the simulated eruption occurred. Our results also show that volcanic eruptions alter surface climate variability (in general increasing it) when aerosols are allowed to realistically interact with circulation: Simulations with fixed volcanic aerosol show no significant change in surface climate variability. Our simulations also highlight that the change in climate variability is not a linear function of the amount of the volcanic aerosol injected. We then provide a tentative estimation of the ensemble size needed to discern a given volcanic signal on surface temperature from the natural internal variability on regional scale: At least 20–25 members are necessary to significantly detect seasonally averaged anomalies of 0.5°C; however, when focusing on North America and in winter, a higher number of ensemble members (35–40) is necessary.

Investigating the recent apparent hiatus in surface temperature increases: 2. Comparison of model ensembles to observational estimates

To assess published hypotheses surrounding the recent slowdown in surface warming (hiatus), we compare five available global observational surface temperature estimates to two 30-member ensembles from the Norwegian Earth System Model (NorESM). Model ensembles are initialized in 1980 from the transient historical runs and driven with forcings used in the CMIP5 experiments and updated forcings based upon current observational understanding, described in Part 1. The ensembles’ surface temperature trends are statistically indistinguishable over 1998–2012 despite differences in the prescribed forcings. There is thus no evidence that forcing errors play a significant role in explaining the hiatus according to NorESM. The observations fall either toward the lower portion of the ensembles or, for some observational estimates and regions, outside. The exception is the Arctic where the observations fall toward the upper ensemble bounds. Observational data set choices can make a large difference to findings of consistency or otherwise. Those NorESM ensemble members that exhibit Nino3.4 Sea Surface Temperature (SST) trends similar to observed also exhibit comparable tropical and to some extent globalmean trends, supporting a role for El Nino Southern Oscillation in explaining the hiatus. Several ensemble members capture the marked seasonality observed in Northern Hemispheremidlatitude trends, with cooling in the wintertime and warming in the remaining seasons. Overall, we find that we cannot falsify NorESM as being capable of explaining the observed hiatus behavior. Importantly, this is not equivalent to concluding NorESM could simultaneously capture all important facets of the hiatus. Similar experiments with further, distinct, Earth System Models are required to verify our findings.

Correction to “A scheme for process‐tagged SO4 and BC aerosols in NCAR‐CCM3: Validation and sensitivity to cloud processes”

[1] In the paper ‘‘A scheme for process-tagged SO4 and BC aerosols in NCAR-CCM3: Validation and sensitivity to cloud processes’’ by T. Iversen and O. Seland (Journal of Geophysical Research, 107(A24), 4751, doi:10.1029/ 2001JD000885, 2002), several figures were printed incorrectly. Several figure captions were also incorrect. The correct versions of the Figures 5, 11, 14, 15, and 16 and their captions appear below. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. D16, 4502, doi:10.1029/2003JD003840, 2003

Remote sensing of aerosols in the Arctic for an evaluation of global climate model simulations.

In this study Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua retrievals of aerosol optical thickness (AOT) at 555 nm are compared to Sun photometer measurements from Svalbard for a period of 9 years. For the 642 daily coincident measurements that were obtained, MODIS AOT generally varies within the predicted uncertainty of the retrieval over ocean (ΔAOT = ±0.03 ± 0.05 · AOT). The results from the remote sensing have been used to examine the accuracy in estimates of aerosol optical properties in the Arctic, generated by global climate models and from in situ measurements at the Zeppelin station, Svalbard. AOT simulated with the Norwegian Earth System Model/Community Atmosphere Model version 4 Oslo global climate model does not reproduce the observed seasonal variability of the Arctic aerosol. The model overestimates clear-sky AOT by nearly a factor of 2 for the background summer season, while tending to underestimate the values in the spring season. Furthermore, large differences in all-sky AOT of up to 1 order of magnitude are found for the Coupled Model Intercomparison Project phase 5 model ensemble for the spring and summer seasons. Large differences between satellite/ground-based remote sensing of AOT and AOT estimated from dry and humidified scattering coefficients are found for the subarctic marine boundary layer in summer. KEY POINTS Remote sensing of AOT is very useful in validation of climate models.

Future Response of Temperature and Precipitation to Reduced Aerosol Emissions as Compared with Increased Greenhouse Gas Concentrations

AbstractExperiments with a climate model (NorESM1) were performed to isolate the effects of aerosol particles and greenhouse gases on surface temperature and precipitation in simulations of future climate. The simulations show that by 2025–49 a reduction of aerosol emissions from fossil fuels following a maximum technically feasible reduction (MFR) scenario could lead to a global and Arctic warming of 0.26 and 0.84 K, respectively, as compared with a simulation with fixed aerosol emissions at the level of 2005. If fossil fuel emissions of aerosols follow a current legislation emissions (CLE) scenario, the NorESM1 model simulations yield a nonsignificant change in global and Arctic average surface temperature as compared with aerosol emissions fixed at year 2005. The corresponding greenhouse gas effect following the representative concentration pathway 4.5 (RCP4.5) emission scenario leads to a global and Arctic warming of 0.35 and 0.94 K, respectively. The model yields a marked annual average northward shi...

A Standardized Global Climate Model Study Showing Unique Properties for the Climate Response to Black Carbon Aerosols

AbstractThe climate response to an abrupt increase of black carbon (BC) aerosols is compared to the standard CMIP5 experiment of quadrupling CO2 concentrations in air. The global climate model NorESM with interactive aerosols is used. One experiment employs prescribed BC emissions with calculated concentrations coupled to atmospheric processes (emission-driven) while a second prescribes BC concentrations in air (concentration-driven) from a precalculation with the same model and emissions, but where the calculated BC does not force the climate dynamics. The difference quantifies effects of feedbacks between airborne BC and other climate processes. BC emissions are multiplied with 25, yielding an instantaneous top-of-atmosphere (TOA) radiative forcing (RF) comparable to the quadrupling of atmospheric CO2. A radiative kernel method is applied to estimate the different feedbacks.In both BC runs, BC leads to a much smaller surface warming than CO2. Rapid atmospheric feedbacks reduce the BC-induced TOA forcing...

Investigating the recent apparent hiatus in surface temperature increases: 1. Construction of two 30-member Earth System Model ensembles

The recent Intergovernmental Panel on Climate Change report, along with numerous studies since, has suggested that the apparent global warming hiatus results from some combination of natural variability and changes to external forcings. Herein the external forcings for greenhouse gases (GHGs), long-lived trace gases, volcanic and tropospheric aerosols, and solar irradiance have been replaced in the Norwegian Earth System Model using recent observational estimates. The potential impact of these alternative forcings, and by residual the internally generated variability, is examined through two 30-member ensembles covering the period 1980 to 2012. The Reference ensemble uses the Coupled Model Intercomparison Project phase 5 historical forcings extended with the Representative Concentration Pathway 8.5 (RCP8.5) scenario, while the Sensitivity ensemble uses the alternative forcings. Over the hiatus period defined herein as 1998–2012, all of the forcings show some change between the Sensitivity and Reference experiments and have a combined net forcing change of −0.03 W m−2. The GHG forcing is 0.012 W m−2 higher in the Sensitivity forcings. The alternative solar forcing differs from the Reference forcing by −0.08 W m−2, the same as the alternative volcanic forcing that was based on the latest estimates from NASA Goddard Institute for Space Studies. Anthropogenic aerosol emissions were replaced using the EU-EclipseV4a data set and produce a mean forcing change of 0.11 W m−2 over the period. Part 1 details the creation of the two 30-member ensembles and their characterization for parameters of particular relevance to the explanation of the hiatus. A detailed investigation of the two resulting ensembles global surface temperature behavior is given in Part 2, along with comparisons to observational data sets.

CLIMATE EFFECTS OF SULPHATE AND BLACK CARBON ESTIMATED IN A GLOBAL CLIMATE MODEL

The work on climatic effects of aerosols has not come far enough to draw any firm conclusions, mainly due to uncertainties w.r.t. indirect effects. The concentration calculations as well as the forcing estimates need validation. Also organic carbon (OC), a large group of primary and secondary organic particles,should be included.Penner et al.(1996) estimated much larger indirect effects of OC to than the of sulphate, because OC-particles are not produced in clouds as opposed to sulphate. On the other hand, the indirect effects of sulphate estimated by Lohmann and Feichter (1997)is probably too high, since they did not take into account that much sulphate is produced in pre-existing droplets. The anthropogenic OC-burden in the troposphere is poorly known. Table 2 summarises our forcing estimates along with results by others. Despite the uncertainties it appears quite certain that the aerosols’ impacts on radiative forcing is comparable to greenhouse gases. The pronounced regional patterns may affect the climate system more efficiently than indicated by the net global forcing. Climate scenario simulations are needed to reveal this.

The Role of Cumulus Parameterisation in Global and Regional Sulphur Transport

In connection with regional acidification studies and investigation of Arctic haze and hemispheric-scale transport, limited-area chemistry-transport models (CTMs) on limited domains horizontally as well as vertically have been used. More recently, global models are being used for oxidized sulphur components for the purpose of calculating of possible impacts of sulphate particles on climate. To some surprise sulphur modeling has proven more difficult when integrated in global circulation models (GCMs) than experience from limited-area models, and to some extent by global CTMs. In particular the vertical distribution appears to be wrong. This also shows up as a considerable mismatch between ground-level measurements and calculations in source-regions. The inter-comparison exercise by (2001) emphasized this problem, and it was confirmed in experiments with a new scheme by (2002). That paper presented results from using an extended version of the NCAR CCM3 atmospheric GCM to calculate sulphate and black carbon (BC). Also the result of the NCAR-group’s own sulphur model produced these biases (Rasch et al., 2000).