Model calculations of non-cloud radiative forcing due to anthropogenic sulphate aerosol

Anthropogenic sulphate aerosol particles scatter incoming solar radiation thereby perturbing the radiative budget, hence climate. We have used a three dimensional radiative transfer model together with the sulphate concentration fields simulated by two independent chemistry-transport models to calculate the annual cycle of the radiative forcing due to anthropogenic sulphate aerosol. The calculated forcing pattern shows large peaks over the eastern United States, southeast Europe and eastern China. The calculated annual global-mean radiative forcing is -0.50 Wm-2 for Langner and Rodhe (1991) data and -0.49 Wm-2 for Penner et el. [1994 (a&b)] data. The forcing was found to vary with season, with a larger forcing during northern hemispheric summer than winter. Sulphate aerosol also appreciably perturbs the lower tropospheric heating rates over northern hemispheric mid-latitudes. The forcing was also found to be sensitive to the global cloud cover and to the optical properties of the aerosol. The possible sources of the differences in magnitude with previous estimates are discussed. Over northern hemispheric mid-latitudes, the negative radiative forcing due to the direct effect of aerosols appreciably offsets the positive forcing due to increase in greenhouse gases. A 26-layer radiative-convective model (RCM) was also used to examine the equilibrium temperature profiles due to sulphate aerosols and increase in greenhouse gases. It was found that the effect of sulphate aerosols is the cooling of surface-troposphere system. Sulphate aerosols reduce the tropospheric warming and enhance the stratospheric cooling caused by increase in greenhouse gases.

The blue suns of 1831: was the eruption of Ferdinandea, near Sicily, one of the largest volcanic climate forcing events of the nineteenth century?

One of the largest climate forcing eruptions of the nineteenth century was, until recently, believed to have taken place at Babuyan Claro volcano, in the Philippines, in 1831. However, a recent investigation found no reliable evidence of such an eruption, suggesting that the 1831 eruption must have taken place elsewhere. A newly compiled dataset of reported observations of a blue, purple and green sun in August 1831 is here used to reconstruct the transport of a stratospheric aerosol plume from that eruption. The source of the aerosol plume is identified as the eruption of Ferdinandea, which took place about 50 km off the south-west coast of Sicily (lat. 37.1° N., long. 12.7° E.), in July and August 1831. The modest magnitude of this eruption, assigned a Volcanic Explosivity Index (VEI) of 3, has commonly caused it to be discounted or overlooked when identifying the likely source of the stratospheric sulphate aerosol in 1831. It is proposed, however, that convective instability in the troposphere contributed to aerosol reaching the stratosphere and that the aerosol load was enhanced by addition of a sedimentary sulphur component to the volcanic plume. One of the largest climate forcing volcanic eruptions of the nineteenth century would thus effectively have been hiding in plain sight, arguably ‘lowering the bar’ for the types of eruptions capable of having a substantial climate forcing impact. Prior estimates of the mass of stratospheric sulphate aerosol responsible for the 1831 Greenland ice-core sulphate deposition peaks which have assumed a source eruption at a low-latitude site will therefore have been overstated. The example presented in this paper serves as a useful reminder that VEI values were not intended to be reliably correlated with eruption sulphur yields unless supplemented with compositional analyses. It also underlines that eye-witness accounts of historical geophysical events should not be neglected as a source of valuable scientific data.

Return to different climate states by reducing sulphate aerosols under future CO2 concentrations

It is generally believed that anthropogenic aerosols cool the atmosphere; therefore, they offset the global warming resulting from greenhouse gases to some extent. Reduction in sulphate, a primary anthropogenic aerosol, is necessary for mitigating air pollution, which causes atmospheric warming. Here, the changes in the surface air temperature under various anthropogenic emission amounts of sulphur dioxide (SO2), which is a precursor of sulphate aerosol, are simulated under both present and doubled carbon dioxide (CO2) concentrations with a climate model. No previous studies have conducted explicit experiments to estimate the temperature changes due to individual short-lived climate forcers (SLCFs) in different climate states with atmosphere–ocean coupled models. The simulation results clearly show that reducing SO2 emissions at high CO2 concentrations will significantly enhance atmospheric warming in comparison with that under the present CO2 concentration. In the high latitudes of the Northern Hemisphere, the temperature change that will occur when fuel SO2 emissions reach zero under a doubled CO2 concentration will be approximately 1.0 °C, while this value will be approximately 0.5 °C under the present state. This considerable difference can affect the discussion of the 1.5 °C/2 °C target in the Paris Agreement.

The effect of anthropogenic aerosols on the Aleutian Low

Past studies have suggested that regional trends in anthropogenic aerosols can influence the Pacific Decadal Oscillation (PDO) through modulation of the Aleutian Low. However, the robustness of this connection is debated. This study analyses changes to the Aleutian Low in an ensemble of climate models forced with large, idealised global and regional black carbon (BC) and sulphate aerosol perturbations. To isolate the role of ocean feedbacks, the experiments are performed with an interactive ocean and with prescribed sea surface temperatures. The results show a robust weakening of the Aleutian Low forced by a global 10-fold increase in BC in both experiment configurations. A linearised steady-state primitive equation model is forced with diabatic heating anomalies to investigate the mechanisms through which heating from BC emissions influences the Aleutian Low. The heating from BC absorption over India and east Asia generates Rossby wave trains that propagate into the North Pacific sector, forming an upper tropospheric ridge. Sources of BC outside of east Asia enhance the weakening of the Aleutian Low. The responses to a global 5-fold and regional 10-fold increase in sulphate aerosols over Asia show poor consistency across climate models, with a multi-model mean response that does not project strongly onto the Aleutian Low. These findings for a large, idealised step increase in regional sulphate aerosol differ from previous studies that suggest the transient increase in sulphate aerosols over Asia during the early 21st century weakened the Aleutian Low and induced a transition to a negative PDO phase.

Haze, Hunger, Hesitation: Disaster Aid after the 1783 Lakagígar Eruption

The 1783-1784 Laki eruption was one of the most severe natural catastrophes to occur in Iceland in historical times (since 1140 years). Vegetation damage by sulphate aerosol and fluorine poisoning caused a massive decimation of livestock. The impact of fluorine poisoning and sulphate aerosol on human mortality is uncertain, but the loss of animals caused a famine which took many lives. The vulnerability of the Icelandic society to famine is discussed. 18th Century Iceland was a Danish dependency and, despite the abundance of fish in the surrounding waters, a subsistence farming community and thus highly dependent on livestock. On the other hand, the farming community possessed coping strategies which mitigated the impact of livestock loss. During the famine, the Danish government was in principle willing to provide relief. However, local authorities in Iceland were slow to ask for help, and did not dare to exploit the means at their disposal (e.g. the right to ban the export of Icelandic foodstuff) without consent from Copenhagen. The Danish officials in turn were unwilling to act decisively upon incomplete information. These two factors prevented timely measures. While 4.4 × 10^5kg of grain were provided for famine relief in summer 1784, the merchants exported 1.2 × 10^6kg of fish, which greatly aggravated the hunger in the second winter. The effects of this ‘natural’ catastrophe could therefore have been significantly reduced by efficient government.

What drives the Arctic response to mid-latitude sulphate aerosol emissions?

<p><span>Modeling studies have shown that changes in sulphate aerosol emissions from both Europe and North America can have an impact on remote Arctic climate. The bulk of this response is driven by atmospheric changes, rather than through changes in meridional ocean heat transport. However, these simulations have focused on the Arctic response from an equilibrium perspective; i.e. the simulations are run for 200 years and the analyses are based on means of the last 50 years. While these simulations are useful to analyze the extent of contribution of mid-latitude aerosol emission changes, they cannot be used to investigate the mechanistic processes that initiate and drive the high-latitude response. We approach this problem by conducting two sets of initial condition ensemble simulations with >30 members for each set and focus our analysis on the first 30 years. Having a large number of ensemble members improves the signal-noise ratio and allows us to distinguish the model response to emission changes from internal variability. In the first set of simulations (control set), the aerosol emissions are set to year 2000. In the second set of simulations (perturbed set), we increase the European sulphate aerosol emissions to seven times the year 2000 value. We compare the two sets of simulations to evaluate the dynamical response of the atmosphere to the change in aerosol emissions. One of the key parameters that link the mid- and high-latitudes in the equilibrium response is the change in sea-ice area in the sub-polar latitudes. Reduced sea-ice coverage and greater open ocean area with lower mid-latitude aerosol emissions leads to increased ocean-atmosphere energy exchange and impacts the atmospheric meridional heat and energy budgets in the Arctic. We present the extent and seasonality of sea-ice changes for the first 30 years of our ensemble simulations and discuss their implications in the context of the mechanistical links between the mid- and high-latitudes.</span></p><p> </p>

Impact of ship emission controls recorded by cloud properties

<p>The impact of aerosols on cloud properties is one of the largest uncertainties in the anthropogenic forcing of the climate system. As large, isolated sources of aerosol, ships provide the ideal opportunity to investigate aerosol-cloud interactions. However, their use for quantifying the aerosol impact on clouds has been limited by a lack on information on the aerosol perturbation generated by the ship.</p><p>In this work, satellite cloud observations are combined with ship emissions estimated from transponder data. Using over 17,000 shiptracks during the implementation of emission controls, the central role of sulphate aerosol in controlling shiptrack properties is demonstrated. Meteorological factors are shown to have a significant impact on shiptrack formation, particularly cloud-top relative humidity. Accounting for this meteorological variation, this work also demonstrates the potential for satellite retrievals of ship sulphate emissions, providing a pathway to the use of cloud observations for monitoring air pollution.</p>

Re-evaluating the link between the Laacher See volcanic eruption and the Younger Dryas

The Younger Dryas is the most well-documented millennial-scale cooling event of the Quaternary, but the mechanisms responsible for its initiation remain elusive. Here we use a recently revised chronology for the GISP2 ice core ion dataset to identify a large volcanic sulphur spike coincident with both the sulphur-rich Laacher See volcanic eruption and the onset of Younger Dryas-related cooling (GS-1) in Greenland. Lake sediment and stalagmite records confirm that the eruption's timing was indistinguishable from the onset of cooling across the North Atlantic, but that it preceded westerly wind repositioning over central Europe by ~ 200 years. We suggest that the initial short-lived volcanic sulphate aerosol cooling was amplified by oceanic circulation shifts or sea ice expansion, gradually cooling the North Atlantic region and incrementally shifting the mid-latitude westerlies to the south. The aerosol-related cooling probably only lasted 2–4 years, and the majority of Younger Dryas-related cooling was instead due to this positive feedback, which was particularly effective during the intermediate ice volume conditions characteristic of ~ 13 ka BP. We conclude that the large and sulphur-rich Laacher See eruption should be considered a viable trigger for the Younger Dryas.