The gravitating perfect fluid-scalar field equations: quintessence and tachyonic

The system consisting of a self gravitating perfect fluid and scalar field is considered in detail. The scalar fields considered are the quintessence and ``tachyonic'' forms which have important application in cosmology. Mathematical properties of the general system of equations are studied including the algebraic and differential identities as well as the eigenvalue structure. The Cauchy problem for both quintessence and the tachyon is presented. We discuss the initial constraint equations which must be satisfied by the initial data. A Cauchy evolution scheme is presented in the form of a Taylor series about the Cauchy surface. Finally, a simple numerical example is provided to illustrate this scheme.Comment: 15 pages, 2 figures. Revised version contains more references. Accepted for publication in General Relativity and Gravitatio

The evolution of the global aerosol system in a transient climate simulation from 1860 to 2100

The evolution of the global aerosol system from 1860 to 2100 is investigated through a transient atmosphere-ocean General Circulation Model climate simulation with interactively coupled atmospheric aerosol and oceanic biogeochemistry modules. The microphysical aerosol module HAM incorporates the major global aerosol cycles with prognostic treatment of their composition, size distribution, and mixing state. Based on an SRES A1B emission scenario, the global mean sulfate burden is projected to peak in 2020 while black carbon and particulate organic matter show a lagged peak around 2070. From present day to future conditions the anthropogenic aerosol burden shifts generally from the northern high-latitudes to the developing low-latitude source regions with impacts on regional climate. Atmospheric residence- and aging-times show significant alterations under varying climatic and pollution conditions. Concurrently, the aerosol mixing state changes with an increasing aerosol mass fraction residing in the internally mixed accumulation mode. The associated increase in black carbon causes a more than threefold increase of its co-single scattering albedo from 1860 to 2100. Mid-visible aerosol optical depth increases from pre-industrial times, predominantly from the aerosol fine fraction, peaks at 0.26 around the sulfate peak in 2020 and maintains a high level thereafter, due to the continuing increase in carbonaceous aerosols. The global mean anthropogenic top of the atmosphere clear-sky short-wave direct aerosol radiative perturbation intensifies to −1.1 W m^−2 around 2020 and weakens after 2050 to −0.6 W m^−2, owing to an increase in atmospheric absorption. The demonstrated modifications in the aerosol residence- and aging-times, the microphysical state, and radiative properties challenge simplistic approaches to estimate the aerosol radiative effects from emission projections

Cloud microphysics and aerosol indirect effects in the global climate model ECHAM5-HAM

The double-moment cloud microphysics scheme from ECHAM4 has been coupled to the size-resolved aerosol scheme ECHAM5-HAM. ECHAM5-HAM predicts the aerosol mass and number concentrations and the aerosol mixing state. This results in a much better agreement with observed vertical profiles of the black carbon and aerosol mass mixing ratios than with the previous version ECHAM4, where only the different aerosol mass mixing ratios were predicted. Also, the simulated liquid, ice and total water content and the cloud droplet and ice crystal number concentrations as a function of temperature in stratiform mixed-phase clouds between 0 and –35°C agree much better with aircraft observations in the ECHAM5 simulations. ECHAM5 performs better because more realistic aerosol concentrations are available for cloud droplet nucleation and because the Bergeron-Findeisen process is parameterized as being more efficient. The total anthropogenic aerosol effect includes the direct, semi-direct and indirect effects and is defined as the difference in the top-of-the-atmosphere net radiation between present-day and pre-industrial times. It amounts to –1.8 W m^−2 in ECHAM5, when a relative humidity dependent cloud cover scheme and present-day aerosol emissions representative for the year 2000 are used. It is larger when either a statistical cloud cover scheme or a different aerosol emission inventory are employed

Fire emission heights in the climate system - Part 1: Global plume height patterns simulated by ECHAM6-HAM2

We use the global circulation model ECHAM6 extended by the aerosol module HAM2 to simulate global patterns in wildfire emission heights. Prescribed plume heights in ECHAM6 are replaced by an implementation of a simple, semi-empirical plume height parametrization. In a first step, the global performance of the plume height parametrization is evaluated for plumes reported in the Multiangle Imaging Spectroradiometer (MISR) Plume Height Project (MPHP) data set. Our results show that the parametrization simulates a largely reasonable global distribution of plume heights. While the modeled global mean plume height (1411 ± 646 m) is in good agreement with the observed mean (1382 ± 702 m), the upper and lower tails of the plume height distribution tend to be slightly underrepresented. Furthermore, we compare plume heights simulated by the simple parametrization to a more complex, analytical plume model. Major differences in global plume height distributions are found for the lowest 1.5 km, but reasonable agreement is observed for higher plumes. In a second step, fire radiative power (FRP) as reported in the global fire assimilation system (GFAS) is used to simulate plume heights for observed fires globally for the period 2005–2011. The global fraction of simulated daytime plumes injecting emissions into the free troposphere (FT) ranges from 3.7 ± 0.7 to 5.2 ± 1.0 %. This range is comparable to results from observational studies, but it is much lower than results for prescribed plume heights in the ECHAM6-HAM2 standard setup. Nevertheless, occasionally deep emission injections exceeding 5–7 km in height are simulated for intense fires and favorable meteorological conditions. The application of a prescribed diurnal cycle in FRP turns out to be of minor importance. For a hypothetical doubling in FRP, moderate changes in plume heights of 100–400 m are simulated. These small changes indicate that a potential future increase in fire intensity will only slightly impact the emission heights on a global scale

Controls on fire activity over the Holocene

Changes in fire activity over the last 8000 years are simulated with a global fire model driven by changes in climate and vegetation cover. The changes were separated into those caused through variations in fuel availability, fuel moisture or wind speed, which react differently to changes in climate. Disentangling these controlling factors helps in understanding the overall climate control on fire activity over the Holocene. Globally the burned area is simulated to increase by 2.5% between 8000 and 200 cal yr BP, with larger regional changes compensating nearly evening out on a global scale. Despite the absence of anthropogenic fire ignitions, the simulated trends in fire activity agree reasonably well with continental-scale reconstructions from charcoal records, with the exception of Europe. For some regions the change in fire activity is predominantly controlled through changes in fuel availability (Australia monsoon, Central America tropics/subtropics). For other regions changes in fuel moisture are more important for the overall trend in fire activity (North America, Sub-Saharan Africa, Europe, Asia monsoon). In Sub-Saharan Africa, for example, changes in fuel moisture alone lead to an increase in fire activity between 8000 and 200 cal yr BP, while changes in fuel availability lead to a decrease. Overall, the fuel moisture control is dominating the simulated fire activity for Sub-Saharan Africa. The simulations clearly demonstrate that both changes in fuel availability and changes in fuel moisture are important drivers for the fire activity over the Holocene. Fuel availability and fuel moisture do, however, have different climate controls. As such, observed changes in fire activity cannot be related to single climate parameters such as precipitation or temperature alone. Fire models, as applied in this study, in combination with observational records can help in understanding the climate control on fire activity, which is essential to project future fire activity

Potential climate forcing of land use and land cover change

Pressure on land resources is expected to increase as global population continues to climb and the world becomes more affluent, swelling the demand for food. Changing climate may exert additional pressures on natural lands as present-day productive regions may shift, or soil quality may degrade, and the recent rise in demand for biofuels increases competition with edible crops for arable land. Given these projected trends there is a need to understand the global climate impacts of land use and land cover change (LULCC). Here we quantify the climate impacts of global LULCC in terms of modifications to the balance between incoming and outgoing radiation at the top of the atmosphere (radiative forcing, RF) that are caused by changes in long-lived and short-lived greenhouse gas concentrations, aerosol effects, and land surface albedo. We attribute historical changes in terrestrial carbon storage, global fire emissions, secondary organic aerosol emissions, and surface albedo to LULCC using simulations with the Community Land Model version 3.5. These LULCC emissions are combined with estimates of agricultural emissions of important trace gases and mineral dust in two sets of Community Atmosphere Model simulations to calculate the RF of changes in atmospheric chemistry and aerosol concentrations attributed to LULCC. With all forcing agents considered together, we show that 40% (+/- 16 %) of the present-day anthropogenic RF can be attributed to LULCC. Changes in the emission of non-CO2 greenhouse gases and aerosols from LULCC enhance the total LULCC RF by a factor of 2 to 3 with respect to the LULCC RF from CO2 alone. This enhancement factor also applies to projected LULCC RF, which we compute for four future scenarios associated with the Representative Concentration Pathways. We attribute total RFs between 0.9 and 1.9 W m(-2) to LULCC for the year 2100 (relative to a preindustrial state). To place an upper bound on the potential of LULCC to alter the global radiation budget, we include a fifth scenario in which all arable land is cultivated by 2100. This theoretical extreme case leads to a LULCC RF of 3.9 W m(-2) (+/- 0.9 W m(-2)), suggesting that not only energy policy but also land policy is necessary to minimize future increases in RF and associated climate changes

The sensitivity of global wildfires to simulated past, present, and future lightning frequency

In this study, components of the Max Planck Institute Earth System Model were used to explore how changes in lightning induced by climate change alter wildfire activity. To investigate how climate change alters global flash frequency, simulations with the atmospheric general circulation model ECHAM6 were performed for the time periods preindustrial, present-day, and three future scenarios. The effect of changes in lightning activity on fire occurrence was derived from simulations with the land surface vegetation model JSBACH. Global cloud-to-ground lightning activity decreased by 3.3% under preindustrial climate and increased by up to 21.3% for the RCP85 projection at the end of the century when compared to present-day, respectively. Relative changes were most pronounced in North America and northeastern Asia. Global burned area was little affected by these changes and only increased by up to 3.3% for RCP85. However, on the regional scale, significant changes occurred. For instance, burned area increases of over 100% were found in high-latitude regions, while also several regions were identified where burned area declined, such as parts of South America and Africa. ©2014. American Geophysical Union. All Rights Reserved

Der DMS-Zyklus im Ozean-Atmosphären-System und dessen Resonanz auf anthropogene Störungen

Dimethylsulfide (DMS) is the main biogenic sulfur compound in the atmosphere. DMS is mainly produced by the marine biosphere and plays an important role in the atmospheric sulfur cycle. It has been proposed that DMS is linked to the global climate through a negative biogeochemical feedback cycle stabilizing the Earth against global warming. This so-called CLAW hypothesis initiated extensive research and improved the understanding of many aspects of the biogeochemical sulfur cycle. However, the overall magnitude and even the sign of this feedback cycle are still open questions in present-day research. The objectives of this work are to study the DMS cycle in the ocean and atmosphere and to investigate its response to anthropogenic perturbations. The novelty of this study is the coupling of the relevant components. A global coupled ocean-atmosphere circulation model was established with a fully coupled prognostic treatment of DMS in the ocean, its flux into the atmosphere, and the resulting sulfur concentrations in the atmosphere. The DMS cycle in the ocean is linked to plankton dynamics simulated in the marine biogeochemistry model HAMOCC5, which is embedded in an ocean general circulation model (MPI-OM). The atmospheric model ECHAM5 is extended by the microphysical aerosol model HAM, treating the sulfur chemistry in the atmosphere and the evolution of the microphysically internally- and externally mixed aerosol population. In order to evaluate the established modeling system, a climatological mean simulation was performed and compared to available measurements. Thereby, aerosol and aerosol-precursor emissions were set to conditions representative for the year 2000. The simulated global annual mean DMS sea surface concentration is 1.8 nmol l-1. The DMS emission amounts to 28 Tg(S) yr-1, resulting in a DMS burden of the atmosphere of 0.077 Tg(S), and a DMS lifetime of 1.0 days. DMS contributes 25 % to the global annually averaged SO2 column burden and 27 % to the SO42- column burden. The global distribution of DMS sea surface concentrations compares reasonably well with measurements. In the marine biological active season SO42- surface concentrations are overestimated in regions were DMS is the main SO42- precursor. As the DMS sea surface concentrations are in agreement with the observations, the most likely explanation is a missing chemical reaction mechanism in the atmosphere preventing the formation of SO42- from DMS oxidation. The response of the DMS cycle to global warming was investigated in a transient climate simulation from 1860 to 2100. The results were analyzed in terms of simulated changes between the periods 1861-1890 and 2061-2090. The global annual mean DMS sea surface concentration and DMS flux decrease by 10% in a warmer climate. Thereby, the response to global warming is largely driven by changes in the ocean dynamics, such as an enhanced ocean stratification causing a reduction in marine net primary production and a decrease in the DMS production in the ocean. The DMS burden in the atmosphere is reduced by 3%, owing to a 7% longer lifetime of DMS in the atmosphere in a warmer climate. The simulated decrease in the DMS emission and atmospheric DMS concentrations in a warmer climate is in contrast to the proposed negative feedback in the CLAW hypothesis, in which increasing DMS emissions in a warmer climate constitute a key mechanism