Henning Rodhe

A safe operating space for humanity

Identifying and quantifying planetary boundaries that must not be transgressed could help prevent human activities from causing unacceptable environmental change, argue Johan Rockstrom and colleagues.

Dust sources and deposition during the last glacial maximum and current climate: A comparison of model results with paleodata from ice cores and marine sediments

Mineral dust aerosols in the atmosphere have the potential to affect the global climate by influencing the radiative balance of the atmosphere and the supply of micronutrients to the ocean. Ice and marine sediment cores indicate that dust deposition from the atmosphere was at some locations 2–20 times greater during glacial periods, raising the possibility that mineral aerosols might have contributed to climate change on glacial-interglacial time scales. To address this question, we have used linked terrestrial biosphere, dust source, and atmospheric transport models to simulate the dust cycle in the atmosphere for current and last glacial maximum (LGM) climates. We obtain a 2.5-fold higher dust loading in the entire atmosphere and a twenty-fold higher loading in high latitudes, in LGM relative to present. Comparisons to a compilation of atmospheric dust deposition flux estimates for LGM and present in marine sediment and ice cores show that the simulated flux ratios are broadly in agreement with observations; differences suggest where further improvements in the simple dust model could be made. The simulated increase in high-latitude dustiness depends on the expansion of unvegetated areas, especially in the high latitudes and in central Asia, caused by a combination of increased aridity and low atmospheric [CO2]. The existence of these dust source areas at the LGM is supported by pollen data and loess distribution in the northern continents. These results point to a role for vegetation feedbacks, including climate effects and physiological effects of low [CO2], in modulating the atmospheric distribution of dust.

A Comparison of the Contribution of Various Gases to the Greenhouse Effect

The current concern about an anthropogenic impact on global climate has made it of interest to compare the potential effect of various human activities. A case in point is the comparison between the emission of greenhouse gases from the use of natural gas and that from other fossil fuels. This comparison requires an evaluation of the effect of methane emissions relative to that of carbon dioxide emissions. A rough analysis based on the use of currently accepted values shows that natural gas is preferable to other fossil fuels in consideration of the greenhouse effect as long as its leakage can be limited to 3 to 6 percent.

Transient Climate Change Simulations with a Coupled Atmosphere–Ocean GCM Including the Tropospheric Sulfur Cycle

Abstract The time-dependent climate response to changing concentrations of greenhouse gases and sulfate aerosols is studied using a coupled general circulation model of the atmosphere and the ocean (ECHAM4/OPYC3). The concentrations of the well-mixed greenhouse gases like CO2, CH4, N2O, and CFCs are prescribed for the past (1860–1990) and projected into the future according to International Panel on Climate Change (IPCC) scenario IS92a. In addition, the space–time distribution of tropospheric ozone is prescribed, and the tropospheric sulfur cycle is calculated within the coupled model using sulfur emissions of the past and projected into the future (IS92a). The radiative impact of the aerosols is considered via both the direct and the indirect (i.e., through cloud albedo) effect. It is shown that the simulated trend in sulfate deposition since the end of the last century is broadly consistent with ice core measurements, and the calculated radiative forcings from preindustrial to present time are within th...

A global three-dimensional model of the tropospheric sulfur cycle

The tropospheric part of the atmospheric sulfur cycle has been simulated in a global three-dimensional model. The model treats the emission, transport, chemistry, and removal processes for three sulfur components; DMS (dimethyl sulfide), SO2 and SO42− (sulfate). These processes are resolved using an Eulerian transport model, the MOGUNTIA model, with a horizontal resolution of 10° longitude by 10° latitude and with 10 layers in the vertical between the surface and 100 hPa. Advection takes place by climatological monthly mean winds. Transport processes occurring on smaller space and time scales are parameterized as eddy diffusion except for transport in deep convective clouds which is treated separately. The simulations are broadly consistent with observations of concentrations in air and precipitation in and over polluted regions in Europe and North America. Oxidation of DMS by OH radicals together with a global emission of 16 Tg DMS-S yr−1 from the oceans result in DMS concentrations consistent with observations in the marine boundary layer. The average turn-over times were estimated to be 3, 1.2–1.8, and 3.2–6.1 days for DMS, SO2, and SO42− respectively.

Brown Clouds over South Asia: Biomass or Fossil Fuel Combustion?

Carbonaceous aerosols cause strong atmospheric heating and large surface cooling that is as important to South Asian climate forcing as greenhouse gases, yet the aerosol sources are poorly understood. Emission inventory models suggest that biofuel burning accounts for 50 to 90% of emissions, whereas the elemental composition of ambient aerosols points to fossil fuel combustion. We used radiocarbon measurements of winter monsoon aerosols from western India and the Indian Ocean to determine that biomass combustion produced two-thirds of the bulk carbonaceous aerosols, as well as one-half and two-thirds of two black carbon subfractions, respectively. These constraints show that both biomass combustion (such as residential cooking and agricultural burning) and fossil fuel combustion should be targeted to mitigate climate effects and improve air quality.

Simulation of the tropospheric sulfur cycle in a global climate model

Abstract Emission, transport, chemistry and rainout of the sulfur species DMS, SO2 and sulfate are calculated on-line with the meteorology in a global atmospheric circulation model. The model simulates the main components of the hydrological cycle, including the liquid water content of clouds, and hence it allows an explicit treatment of cloud transformation processes and precipitation scavenging. The importance of the different oxidation pathways of DMS and SO2 is estimated. About 2 3 of the sulfate is produced within clouds, with H2O2 being the most efficient pathway (59%) and with a minor contribution due to oxidation with O3 (7%). Predicted atmospheric surface concentrations of SO2 and sulfate and the deposition fluxes are compared with the observations. Over most parts of the globe the agreement between simulated and observed annual averages is within a factor of 2. A significant underestimate of the simulated sulfate concentrations was found in high latitudes in winter. This bias may be attributed to a too slow oxidation in clouds. The calculated global mean turn-over times for DMS (2.2 d), SO2 (1.6 d) and sulfate (4.4 d) are within the range of previous estimates.

Mercury in the global troposphere: a three-dimensional model study

The global distributions of elemental mercury (Hg0) and divalent mercury compounds (HgII) were estimated with a climatological transport model (MOGUNTIA). Natural and man-made sources, including re-emission of previously deposited mercury (of man-made origin), oxidation of Hg0 to HgII and wet and dry deposition of HgII were explicitly treated. Comparisons with observations of Hg0 in surface air, HgII in precipitation and trends in mercury deposited in lake sediments and peat bogs show a reasonable agreement if the oxidation rate of Hg0 was chosen to be 1.0/yr. An oxidation time scale outside the range 0.5—1.5 yr is diƒcult to reconcile with observations. A recently measured large decrease in the concentrations of Hg0 over the Atlantic is diƒcult to explain only by a decrease in man-made emissions in Europe and North America. This latter diƒculty indicates either that the man-made emissions have been underestimated or that there are large temporal variations in natural emissions (or re-emissions). We conclude that direct global man-made mercury emissions are likely to be at least 30% as large as the natural emissions, implying that the deposition rate, averaged over the globe, has increased by at least 50% since pre-industrial times. To the extent that re-emission of previously deposited mercury of man-made origin is important, the average deposition rate may well have tripled. In and around the most industrial regions (Europe, North America, Southeastern China) the deposition rate has increased by a factor 2—10 during the past two hundred years. ( 1999 Elsevier Science Ltd. All rights reserved.

Trends and Periodicities in East African Rainfall Data

Abstract Annual rainfall values from 35 stations in parts of East Africa (Kenya, northern Tanzania, and south-eastern Uganda) are analyzed for trend, periodicities, and variability. The data cover time periods ranging from 44 to 83 years. After normalizing the series by dividing by the long-term mean, areal averaged series are constructed. Spectral analysis by the Blackman-Tukey method shows in most cases peaks at frequencies corresponding to the following time periods: 2–2.5, about 3.5, and 5–5.5 years. The area distribution and the statistical significance of these peaks are briefly discussed. Smoothing by binomial coefficients is applied to the data series to filter out short period fluctuations. The resulting series show no definite long term trends, except possibly at some stations in northern Kenya where a trend towards increased precipitation in the recent years is indicated.

Acidification in developing countries: ecosystem sensitivity and the critical load approach on a global scale.

Abstract Acidification represents a growing threat to certain developing country ecosystems in tropical and subtropical climates. A methodology investigating the extent of acidification risks from sulfur emissions on a global scale is presented. Atmospheric transfer models have been used to calculate transfer and deposition of sulfur (using emissions for 1990 and a projection for 2050) and alkaline soil dust. A method to derive the relative sensitivity of terrestrial ecosystems is explained and preliminary critical load values are assigned. A range of values for critical loads and base cation deposition have been used to investigate uncertainty in maps depicting the excess of deposition above critical loads. These show an increasing risk of acidification in 2050 in extended regions of southern and eastern Asia, as well as parts of southern Africa, in comparison to 1990. Certain areas, especially in Asia, are shown at risk even when high values of critical load and base cation deposition are used.