Effects Of Anthropogenic Sulfate Aerosols Research Articles

Anthropogenic and natural causes of climate change

We test for causality between radiative forcing and temperature using multivariate time series models and Granger causality tests that are robust to the non-stationary (trending) nature of global climate data. We find that both natural and anthropogenic forcings cause temperature change and also that temperature causes greenhouse gas concentration changes. Although the effects of greenhouse gases and volcanic forcing are robust across model specifications, we cannot detect any effect of black carbon on temperature, the effect of changes in solar irradiance is weak, and the effect of anthropogenic sulfate aerosols may be only around half that usually attributed to them.

A climate sensitivity estimate using Bayesian fusion of instrumental observations and an Earth System model

Current climate model projections are uncertain. This uncertainty is partly driven by the uncertainty in key model parameters such as climate sensitivity (CS), vertical ocean diffusivity (Kv), and strength of anthropogenic sulfate aerosol forcing. These parameters are commonly estimated using ensembles of model runs constrained by observations. Here we obtain a probability density function (pdf) of these parameters using the University of Victoria Earth System Climate Model (UVic ESCM) ‐ an intermediate complexity model with a dynamic three‐dimensional ocean. Specifically, we run an ensemble of UVic ESCM runs varying parameters that affectCS, ocean vertical diffusion, and the effects of anthropogenic sulfate aerosols. We use a statistical emulator that interpolates the UVic ESCM output to parameter settings where the model was not evaluated. We adopt a Bayesian approach to constrain the model output with instrumental surface temperature and ocean heat observations. Our approach accounts for the uncertainties in the properties of model‐data residuals. We use a Markov chain Monte Carlo method to obtain a posterior pdf of these parameters. The mode of the climate sensitivity estimate is 2.8°C, with the corresponding 95% credible interval ranging from 1.8 to 4.9°C. These results are generally consistent with previous studies. TheCS pdf is sensitive to the assumptions about the priors, to the effects of anthropogenic sulfate aerosols, and to the background vertical ocean diffusivity. Our method can be used with more complex climate models.

Contrasts in the effects on climate of anthropogenic sulfate aerosols between the 20th and the 21st century

In this study, we examine the time evolution of the relative contribution of sulfate aerosols and greenhouse gases to anthropogenic climate change. We use the new IPSL‐CM4 coupled climate model for which the first indirect effect of sulfate aerosols has been calibrated using POLDER satellite data. For the recent historical period the sulfate aerosols play a key role on the temperature increase with a cooling effect of 0.5 K, to be compared to the 1.4 K warming due to greenhouse gas increase. In contrast, the projected temperature change for the 21st century is remarkably independent of the effects of anthropogenic sulfate aerosols for the SRES‐A2 scenario. Those results are interpreted comparing the different radiative forcings, and can be extended to other scenarios. We also highlight that the first indirect effect of aerosol strongly depends on the land surface model by changing the cloud cover.

Twentieth‐century temperature and precipitation trends in ensemble climate simulations including natural and anthropogenic forcing

We present results from a series of ensemble integrations of a global coupled atmosphere‐ocean model for the period 1865–1997. Each ensemble consists of three integrations initialized from different points in a long‐running GFDL R30 coupled model control simulation. The first ensemble includes time‐varying forcing from greenhouse gases only. In the remaining three ensembles, forcings from anthropogenic sulfate aerosols, solar variability, and volcanic aerosols in the stratosphere are added progressively, such that the fourth ensemble uses all four of these forcings. The effects of anthropogenic sulfate aerosols are represented by changes in surface albedo, and the effects of volcanic aerosols are represented by latitude‐dependent perturbations in incident solar radiation. Comparisons with observations reveal that the addition of the natural forcings (solar and volcanic) improves the simulation of global multidecadal trends in temperature, precipitation, and ocean heat content. Solar and volcanic forcings are important contributors to early twentieth century warming. Volcanic forcing reduces the warming simulated for the late twentieth century. Interdecadal variations in global mean surface air temperature from the ensemble of experiments with all four forcings are very similar to observed variations during most of the twentieth century. The improved agreement of simulated and observed temperature trends when natural climate forcings are included supports the climatic importance of variations in radiative forcing during the twentieth century.

The response of the climate system to the indirect effects of anthropogenic sulfate aerosol

The indirect effects of anthropogenic sulfate aerosols on the albedo and lifetime of clouds may produce a significant impact on the climate system. A `state of the art' general circulation model (GCM) which includes an interactive sulfur cycle and a physically based cloud microphysics scheme is coupled to a mixed-layer ocean model in order to study the impact of the indirect effects on the coupled climate system. The linearity of the two indirect effects on the model response is also investigated by including each effect separately in the model. The response of the sea surface temperatures (SSTs) and sea ice is found to provide an important feedback on the cooling at high latitudes and the change in meridional SST gradient results in a southward shift of the inter-tropical convergence zone (ITCZ). The sensitivity of the model to the forcing from the indirect effects of sulfate aerosol is found to be similar to, but slightly weaker than that obtained from a doubling of CO2.

Discriminants of Twentieth-Century Changes in Earth Surface Temperatures

An approach to identifying climate changes is presented that does not hinge on simulations of natural climate variations or anthropogenic changes. Observed interdecadal climate variations are decomposed into several discriminants, mutually uncorrelated spatiotemporal components with a maximal ratio of interdecadal-to-intradecadal variance. The dominant discriminants of twentieth-century variations in surface temperature exhibit large-scale warming in which, particularly in the Northern Hemisphere summer months, localized cooling is embedded. The structure of the large-scale warming is consistent with expected effects of increases in greenhouse gas concentrations. The localized cooling, with maxima on scales of 1000–2000 km over East Asia, eastern Europe, and North America, is suggestive of radiative effects of anthropogenic sulfate aerosols.

Reflectivity variations off the Peru Coast: Evidence for indirect effect of anthropogenic sulfate aerosols on clouds

Using reflectivity measurements from the Total Ozone Mapping Spectrometer (TOMS), we show that over the months when stratocumulus clouds are prominent off the Peru Coast, the ultraviolet (UV) reflectance of two marine sites is consistently higher than that of the surroundings. The regions of reflectivity enhancement coincide with large anthropogenic sulfate aerosol emission sources, and the magnitude of the enhancement has a strong seasonal dependence that is related to the seasonal cloud movement. We propose the indirect aerosol effect as a plausible explanation for the reflectivity observations.

Model assessment of regional surface temperature trends (1949–1997)

Analyses are conducted to assess whether simulated trends in SST and land surface air temperature from two versions of a coupled ocean‐atmosphere model are consistent with the geographical distribution of observed trends over the period 1949–1997. The simulated trends are derived from model experiments with both constant and time‐varying radiative forcing. The models analyzed are low‐resolution (R 15, ∼4°) and medium‐resolution (R30, ∼2°) versions of the Geophysical Fluid Dynamics Laboratory (GFDL) coupled climate model. Internal climate variability is estimated from long control integrations of the models with no change of external forcing. The radiatively forced trends are based on ensembles of integrations using estimated past concentrations of greenhouse gases and direct effects of anthropogenic sulfate aerosols (G+S). For the regional assessment, the observed trends at each grid point with adequate temporal coverage during 1949–1997 are first compared with the R15 and R30 model unforced internal variability. Nearly 50% of the analyzed areas have observed warming trends exceeding the 95th percentile of trends from the control simulations. These results suggest that regional warming trends over much of the globe during 1949–1997 are very unlikely to have occurred due to internal climate variability alone and suggest a role for a sustained positive thermal forcing such as increasing greenhouse gases. The observed trends are then compared with the trend distributions obtained by combining the ensemble mean G+S forced trends with the internal variability “trend” distributions from the control runs. Better agreement is found between the ensemble mean G+S trends and the observed trends than between the model internal variability alone and the observed trends. However, the G+S trends are still significantly different from the observed trends over about 30% of the areas analyzed. Reasons for these regional inconsistencies between the simulated and the observed trends include possible deficiencies in (1) specified radiative forcings, (2) simulated responses to specified radiative forcings, (3) simulation of internal climate variability, or (4) observed temperature records.

Cloud factor and seasonality of the indirect effect of anthropogenic sulfate aerosols

A method for evaluation of the indirect shortwave effect of anthropogenic sulfate aerosols based on the empirically derived cloud climatology is described and applied. The method is simple and computationally efficient which allows many sensitivity tests to be conducted. The tests presented focus mainly on the effects of cloud amounts and types as well as on seasonality of the indirect forcing. We found the indirect shortwave forcing estimated from cloud climatology (−1.1 W m−2, with a hemispheric difference of 0.4 W m−2) is close to that obtained from general circulation model simulations in previous studies. The hemispheric forcing has a strong seasonal cycle with the northern hemisphere (NH) forcing exceeding that of the southern hemisphere (SH) during the NH spring and summer, and the SH forcing prevailing during the SH spring and summer. We also estimate that single‐layer St/Sc contribute most to the aerosol indirect forcing (60%) with an additional 25% from St/Sc overlapped with Ci/Cs/Cc and 15% from St/Sc overlapped with As/Ac. The upper and middle level clouds overlapping the low level stratiform cloud layers reduce the indirect forcing by more than a factor of 2 (from −2.5 to −1.1 W m−2). The estimate of the indirect forcing of −1.1 W m−2 is based on climatological mean values of transmittance for Ac/As and Ci/Cs/Cc. The transmittance of these clouds, however, may vary in a wide range resulting in a large regional and seasonal variability of the aerosol indirect forcing.

The effect of anthropogenic sulfate aerosols on marine cloud droplet concentrations

Nonseasalt sulfate (nss SO 4 2- ) mass concentrations, cloud condensation nuclei (CCN) number concentrations, and cloud droplet concentrations in warm cumulus and stratocumulus clouds were simultaneously measured in situ in marine air masses on El Yunque peak in Puerto Rico. Our results show that CNN number concentrations (measured at 0.5% supersaturation) and nss SO 4 2- mass concentrations (in the range of ∼ 400–1700 ng m -3 ) are significantly correlated at this site. Droplet concentrations in the cumulus clouds studied do not show a discernible trend with nss SO 4 2- mass concentrations (in the range of ∼ 300–1400 ng m -3 ). In stratocumulus clouds, a small increase in droplet concentration with nss SO 4 2- mass concentrations in the range of ∼ 300–1100 ng m -3 was observed. We attribute the low sensitivities of the droplet number concentrations to nss SO 4 2- mass concentrations to the entrainment/mixing processes in these clouds. The magnitudes of the empirically derived sensitivities are considerably lower than those assumed in recent assessments of the effect of anthropogenic sulfate aerosols on cloud albedo. DOI: 10.1034/j.1600-0889.1994.t01-1-00005.x

The effect of anthropogenic sulfate aerosols on marine cloud droplet concentrations

Nonseasalt sulfate (nss SO 4 2- ) mass concentrations, cloud condensation nuclei (CCN) number concentrations, and cloud droplet concentrations in warm cumulus and stratocumulus clouds were simultaneously measured in situ in marine air masses on El Yunque peak in Puerto Rico. Our results show that CNN number concentrations (measured at 0.5% supersaturation) and nss SO 4 2- mass concentrations (in the range of ∼ 400–1700 ng m -3 ) are significantly correlated at this site. Droplet concentrations in the cumulus clouds studied do not show a discernible trend with nss SO 4 2- mass concentrations (in the range of ∼ 300–1400 ng m -3 ). In stratocumulus clouds, a small increase in droplet concentration with nss SO 4 2- mass concentrations in the range of ∼ 300–1100 ng m -3 was observed. We attribute the low sensitivities of the droplet number concentrations to nss SO 4 2- mass concentrations to the entrainment/mixing processes in these clouds. The magnitudes of the empirically derived sensitivities are considerably lower than those assumed in recent assessments of the effect of anthropogenic sulfate aerosols on cloud albedo. DOI: 10.1034/j.1600-0889.1994.t01-1-00005.x

Effect of anthropogenic sulfate aerosols on low‐level cloud albedo over oceans

By reducing cloud droplet size, anthropogenic sulfate aerosols are capable of increasing cloud albedo and thus possibly changing the climate. To test the detectability of this effect, we examined satellite‐measured low‐level cloud albedo off the east coasts of North America and Asia at mid‐latitudes where anthropogenic sulfate sources are large and aerosols are transported eastward over the oceans by prevailing westerlies. The satellite data demonstrate enhanced cloud albedo near the coastal boundaries where sulfate concentrations are large. Similar trends are absent over ocean regions of the southern hemisphere that are removed from anthropogenic sulfate sources.