A simple four-dimensional assimilation technique, called Newtonian relaxation, has been applied to the Hamburg climate model (ECHAM), to enable comparison of model output with observations for short periods of time. The prognostic model variables vorticity, divergence, temperature, and surface pressure have been relaxed toward European Center for Medium-Range Weather Forecasts (ECMWF) global meteorological analyses. Several experiments have been carried out, in which the values of the relaxation coefficients have been varied to find out which values are most usable for our purpose.To be able to use the method for validation of model physics or chemistry, good agreement of the model simulated mass and wind field is required. In addition, the model physics should not be disturbed too strongly by the relaxation forcing itself. Both aspects have been investigated. Good agreement with basic observed quantities, like wind, temperature, and pressure is obtained for most simulations in the extratropics. Derived variables, like precipitation and evaporation, have been compared with ECMWF forecasts and observations. Agreement for these variables is smaller than for the basic observed quantities. Nevertheless, considerable improvement is obtained relative to a control run without assimilation. Differences between tropics and extratropics are smaller than for the basic observed quantities. Results also show that precipitation and evaporation are affected by a sort of continuous spin-up which is introduced by the relaxation: the bias (ECMWF-ECHAM) is increasing with increasing relaxation forcing. In agreement with this result we found that with increasing relaxation forcing the vertical exchange of tracers by turbulent boundary layer mixing and, in a lesser extent, by convection, is reduced.

Studies on the impact of climate change and sea level rise usually take climate scenarios as their starting point. To support long-term water management planning in the Netherlands, we carried out a study that started at the opposite end of the effect chain. In the study we refer to three aspects of water management, flood defense, drinking water supply, and protection of the Rotterdam Harbour. We examined whether, and for how long, current water management strategies will continue to be effective under different climate change scenarios. We did this by applying the concept of 'adaptation tipping points', and reached it if the magnitude of change is such that the current management strategy can no longer meet its objectives. Beyond the tipping points, an alternative adaptive strategy is needed. By applying this approach, the following basic questions of decision makers are answered: what are the first issues that we will face as a result of climate change and when can we expect this. The results show, for instance, that climate change and the rise in sea level are more likely to cause a threat to the fresh water supply in the west of the Netherlands than flooding. Expressing uncertainty in terms of the period that the existing strategy is effective (when will a critical point be reached) was found to be useful for the policy makers. 

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The short‐lived radionuclide Rn222 is emitted at a fairly constant rate from the continents and is a good surrogate for studying the transport of “air pollution” from polluted continental areas to clean, remote regions. The large concentration gradients of 2–3 orders of magnitude which exist between the continents and the remote atmosphere present a major challenge to the modelling of horizontal and vertical atmospheric transport. We use the global off‐line tracer transport model TM3 at 3 different resolutions. Input to the model consists of meteorological data for the year 1993 obtained from the European Centre for Medium Range Weather Forecasts (ECMWF). The same meteorological data is used to constrain the climate model ECHAM4‐T42‐L19. Using these meteorological data, Rn222 simulations are used to evaluate and document model performance and associated uncertainties. High time‐resolution measurements made at 2 continental stations, 2 stations under continental influence and 4 remote sites, and aircraft measurements obtained during the NARE aircraft campaign are used for a detailed comparison. Although in specific regions there are inter‐model differences of up to a factor of 2 in the calculated boundary layer concentrations, these differences are not translated into a better performance of either model for the stations used for comparison. We generally obtain high correlations of model results and measurements; these range from r= 0.6–0.8 for the continental and coastal stations and 0.5–0.6 for the remote sites. Calculated mean concentrations and corresponding standard deviations generally agree favourably with observations, lending credibility to the usefulness of our models for evaluating transport of air pollutants from continental sources to remote regions. The main cause of model deviations is probably related to uncertainties in the meteorological input data set provided by the ECMWF model and to a lesser extent by our knowledge of the spatial distribution of Rn222 emissions and uncertainties involving sub‐grid scale parameterization of vertical transport, e.g., diffusion and convection.

We present gas/aerosol partitioning calculations of multicomponent aerosols and aerosol associated water on a global scale. We have coupled a computationally efficient gas‐aerosol scheme (EQSAM) to a global atmospheric chemistry‐transport model (TM3). Our results show that gas/aerosol partitioning strongly affects the gas‐phase concentrations at relatively low temperatures. During winter and at night during all seasons the calculated aerosol load, including water, is considerably higher than without accounting for gas/aerosol partitioning. The reason is that gaseous nitric acid near the surface is often neutralized by ammonia and therefore partitions almost completely into the aerosol phase to yield ammonium nitrate (NH4NO3). The aerosol NH4NO3 has a longer atmospheric residence time compared to the corresponding precursor gases (NH3 and HNO3) and can therefore be transported over larger distances, for instance from India to Africa and Europe. These modeling results are intriguing; however, verification requires in situ measurements. A comparison with a limited set of ground‐based measurements indicates that our model yields realistic results for the ammonium‐sulfate‐nitrate‐water aerosol system in relatively polluted locations where ammonium nitrate is important. For remote locations for which we underestimate the total aerosol load, however, it will be necessary to also account for other aerosol species such as sea salt, mineral dust and organic compounds. We further show that assumptions on turbulent mixing and model resolution have a much stronger effect on aerosol calculations than the uncertainties resulting from the simplifications made in EQSAM.

The comparison of large‐scale sulphate aerosol models study (COSAM) compared the performance of atmospheric models with each other and observations. It involved: (i) design of a standard model experiment for the world wide web, (ii) 10 model simulations of the cycles of sulphur and 222Rn/210Pb conforming to the experimental design, (iii) assemblage of the best available observations of atmospheric SO=4, SO2 and MSA and (iv) a workshop in Halifax, Canada to analyze model performance and future model development needs. The analysis presented in this paper and two companion papers by Roelofs, and Lohmann and co‐workers examines the variance between models and observations, discusses the sources of that variance and suggests ways to improve models. Variations between models in the export of SOx from Europe or North America are not sufficient to explain an order of magnitude variation in spatial distributions of SOx downwind in the northern hemisphere. On average, models predicted surface level seasonal mean SO=4 aerosol mixing ratios better (most within 20%) than SO2 mixing ratios (over‐prediction by factors of 2 or more). Results suggest that vertical mixing from the planetary boundary layer into the free troposphere in source regions is a major source of uncertainty in predicting the global distribution of SO=4 aerosols in climate models today. For improvement, it is essential that globally coordinated research efforts continue to address emissions of all atmospheric species that affect the distribution and optical properties of ambient aerosols in models and that a global network of observations be established that will ultimately produce a world aerosol chemistry climatology.

Abstract. A chemical transport model has been extended with an aerosol model describing processes which determine the mass distribution of sulfate, nitrate, ammonium, and aerosol associated water. A specific summer episode is simulated, and the results have been compared to surface concentration measurements and with the aerosol optical depth (AOD) retrieved from satellite measurements, with a focus on the European continent. This study is one of the first to use satellite retrievals over land for this purpose. An average difference in AOD between model and satellite measurements of 0.17-0.19 is calculated, and on average only 40-50% of the observed satellite signal can be explained by our modeled aerosol. In contrast, the observed patterns of optical thickness are well simulated by the model. Also, surface concentrations of simulated aerosol components are in close agreement with measurements. Errors in the vertical distribution of sulfate, ammonium, and nitrate, and hence in the vertical distribution of hygroscopic growth, and errors in modeled optical parameters may partly account for the observed differences in AOD. However, we argue that the most important reason for the large difference is due to the fact that organic and mineral aerosol are not taken into account in this model simulation. A sensitivity study with reduced SO2 emissions in Europe showed that reduction of the emissions of SO2 in the model leads to a better agreement with surface measurements of SO2; however, calculated sulfate was less strongly influenced.

Tipping points have become a key concept in research on climate change, indicating points of abrupt transition in biophysical systems as well as transformative changes in adaptation and mitigation strategies. However, the potential existence of tipping points in socio-economic systems has remained underexplored, whereas they might be highly policy relevant. This paper describes characteristics of climate change induced socio-economic tipping points (SETPs) to guide future research on SETPS to inform climate policy. We review existing literature to create a tipping point typology and to derive the following SETP definition: a climate change induced, abrupt change of a socio-economic system, into a new, fundamentally different state. Through stakeholder consultation, we identify 22 candidate SETP examples with policy relevance for Europe. Three of these are described in higher detail to identify their tipping point characteristics (stable states, mechanisms and abrupt change): the collapse of winter sports tourism, farmland abandonment and sea-level rise-induced migration. We find that stakeholder perceptions play an important role in describing SETPs. The role of climate drivers is difficult to isolate from other drivers because of complex interplays with socio-economic factors. In some cases, the rate of change rather than the magnitude of change causes a tipping point. The clearest SETPs are found on small system scales. On a national to continental scale, SETPs are less obvious because they are difficult to separate from their associated economic substitution effects and policy response. Some proposed adaptation measures are so transformative that their implementations can be considered an SETP in terms of 'response to climate change'. Future research can focus on identification and impact analysis of tipping points using stylized models, on the exceedance of stakeholder-defined critical thresholds in the RCP/SSP space and on the macro-economic impacts of new system states.