IPCC A1B Research Articles

A double-resolution transient RCM climate change simulation experiment for near-coastal eastern zone of the Eastern Mediterranean region

A double-resolution regional experiment on hydrodynamic simulation of climate over the eastern Mediterranean (EM) region was performed using an International Center for Theoretical Physics, Trieste RegCM3 model. The RegCM3 was driven from the lateral boundaries by the data from the ECHAM5/MPI-OM global climate simulation performed at the MPI-M, Hamburg and based on the A1B IPCC scenario of greenhouse gases emission. Two simulation runs for the time period 1960-2060, employing spatial resolutions of 50 km/14 L and 25 km/18 L, are realized. Time variations of the differences in the space distributions of simulated climate parameters are analyzed to evaluate the role of smaller scale effects. Both least-square linear and non-linear trends of several characteristics of the EM climate are evaluated in the study. One of the key findings with regard to linear trends is a notable and statistically significant precipitation drop over the near coastal EM zone during December-February and September-November. Statistically significant positive air temperature trends are projected over the entire EM region during the four seasons. Also projected are increases in air temperature extremes and the relative contribution of convective processes in the Southern Mediterranean coastal zone (ECM) region. A notable sensitivity of projected larger-scale climate change signals to smaller-scale effects is also demonstrated.

Changes in weather extremes: Assessment of return values using high resolution climate simulations at convection-resolving scale

Global and regional climate models are currently employed on horizontal resolutions down to 10 km. State-of-the-art numerical weather predicition (NWP) models are already used at the kilometer-scale. At this resolution explicit calculation of convection becomes feasible and effects of small-scale topographic features and land use structures can be accounted for. It is assumed that consequently extremes like wind gusts, thunderstorms or heavy rain will be modeled more realistically. COSMO-CLM is the climate version of the NWP model of the COSMO consortium. It has been employed in simulations at 1,3 km resolution over the region of Rhineland-Palatinate. Two time slices of 10 years (1960-69 and 2015-24) show changes in extremes for the IPCC A1B scenario. A peaks over threshold (POT) extreme value analysis gives information about changes in extremes of near-surface wind speed, screen level temperature and precipitation. Moving block bootstrapping is used for the assessment of the stability of the POT method. Regionalization of the extreme value analysis shows that mountaineous regions will experience the strongest change in daily minimum temperature extremes while in flat and lowland region daily maximum temperature extremes change most. The changes in wind speed tend around zero, in the mean as well as in the extremes. Our study shows that there is an added value through the better resolution of the meteorological variability.

Orographic cirrus in a future climate

Abstract. A cloud resolving model (CRM) is used to investigate the formation of orographic cirrus clouds in the current and future climate. The formation of cirrus clouds depends on a variety of dynamical and thermodynamical processes, which act on different scales. First, the capability of the CRM in realistically simulating orographic cirrus clouds has been tested by comparing the simulated results to aircraft measurements of an orographic cirrus cloud. The influence of a warmer climate on the microphysical and optical properties of cirrus clouds has been investigated by initializing the CRM with vertical profiles of horizontal wind, potential temperature and equivalent potential temperature, respectively. The vertical profiles are extracted from IPCC A1B simulations for the current climate and for the period 2090–2099 for two regions representative for North and South America. The influence of additional moisture in a future climate on the propagation of gravity waves and the formation of orographic cirrus could be estimated. In a future climate, the increase in moisture dampens the vertical propagation of gravity waves and the occurring vertical velocities in the moist simulations. Together with higher temperatures fewer ice crystals nucleate homogeneously. Assuming that the relative humidity does not change in a warmer climate the specific humidity in the model is increased. This increase in specific humidity in a warmer climate results in a higher ice water content. The net effect of a reduced ice crystal number concentration and a higher ice water content is an increased optical depth. However, in some moist simulations dynamical changes contribute to changes in the ice water content, ice crystal number concentration and optical depth. For the corresponding dry simulations dynamical changes are more pronounced leading to a decreased optical depth in a future climate in some cases.

Impacts of aerosol-cloud interactions on past and future changes in tropospheric composition

Abstract. The development of effective emissions control policies that are beneficial to both climate and air quality requires a detailed understanding of all the feedbacks in the atmospheric composition and climate system. We perform sensitivity studies with a global atmospheric composition-climate model to assess the impact of aerosols on tropospheric chemistry through their modification on clouds, aerosol-cloud interactions (ACI). The model includes coupling between both tropospheric gas-phase and aerosol chemistry and aerosols and liquid-phase clouds. We investigate past impacts from preindustrial (PI) to present day (PD) and future impacts from PD to 2050 (for the moderate IPCC A1B scenario) that embrace a wide spectrum of precursor emission changes and consequential ACI. The aerosol indirect effect (AIE) is estimated to be −2.0 Wm−2 for PD-PI and −0.6 Wm−2 for 2050-PD, at the high end of current estimates. Inclusion of ACI substantially impacts changes in global mean methane lifetime across both time periods, enhancing the past and future increases by 10% and 30%, respectively. In regions where pollution emissions increase, inclusion of ACI leads to 20% enhancements in in-cloud sulfate production and ~10% enhancements in sulfate wet deposition that is displaced away from the immediate source regions. The enhanced in-cloud sulfate formation leads to larger increases in surface sulfate across polluted regions (~10–30%). Nitric acid wet deposition is dampened by 15–20% across the industrialized regions due to ACI allowing additional re-release of reactive nitrogen that contributes to 1–2 ppbv increases in surface ozone in outflow regions. Our model findings indicate that ACI must be considered in studies of methane trends and projections of future changes to particulate matter air quality.

A method for deriving a future temporal spectrum of heavy precipitation on the basis of weather patterns in low mountain ranges

AbstractA weather‐pattern‐based multiple regression model to derive future possible changes in the level of the higher temporal resolution spectrum of heavy precipitation has been developed. The temporal spectrum was described using statistical precipitation amounts as a function of the event's duration (1–24 h) and return period (once in 5 yr to once in 100 yr). The principle of the method consists in projecting a statistical relationship between the parameters of a transformed Gumbel distribution (theoretical extreme value distribution) and the distribution of classes of objective weather patterns to time slices in the near future of climate. Changes in distribution parameters were constructed in the model from changes in the distribution of weather patterns. Possible change signals were calculated for the catchment of the Weißeritz River (Ore Mountains, Germany) for the time slices centred around 2025 (2011–2040) and 2050 (2036–2065) as changes versus the reanalyses of the reference period 1961–2000 (May–September). For the climate conditions to be expected in the near future (IPCC A1B scenario), increases in the amounts of heavy precipitation, i.e. decreases in the return periods of equal amounts of heavy precipitation from the reference period, were obtained for the entire temporal spectrum covered by this paper. Overall, the change signals derived on the basis of a concept of weather patterns seem plausible because they represent a possible continuation of the already observed increase in frequency and intensification of events of heavy precipitation in the extended study area. Copyright © 2009 Royal Meteorological Society

Quantification of the impact of climate uncertainty on regional air quality

Abstract. Uncertainties in calculated impacts of climate forecasts on future regional air quality are investigated using downscaled MM5 meteorological fields from the NASA GISS and MIT IGSM global models and the CMAQ model in 2050 in the continental US. Differences between three future scenarios: high-extreme, low-extreme and base case, are used for quantifying effects of climate uncertainty on regional air quality. GISS, with the IPCC A1B scenario, is used for the base case simulations. IGSM results, in the form of probabilistic distributions, are used to perturb the base case climate to provide the high- and low-extreme scenarios. Impacts of the extreme climate scenarios on concentrations of summertime fourth-highest daily maximum 8-h average ozone are predicted to be up to 10 ppbV (about one-seventh of the current US ozone standard of 75 ppbV) in urban areas of the Northeast, Midwest and Texas due to impacts of meteorological changes, especially temperature and humidity, on the photochemistry of tropospheric ozone formation and increases in biogenic VOC emissions, though the differences in average peak ozone concentrations are about 1–2 ppbV on a regional basis. Differences between the extreme and base scenarios in annualized PM2.5 levels are very location dependent and predicted to range between −1.0 and +1.5 μg m−3. Future annualized PM2.5 is less sensitive to the extreme climate scenarios than summertime peak ozone since precipitation scavenging is only slightly affected by the extreme climate scenarios examined. Relative abundances of biogenic VOC and anthropogenic NOx lead to the areas that are most responsive to climate change. Overall, planned controls for decreasing regional ozone and PM2.5 levels will continue to be effective in the future under the extreme climate scenarios. However, the impact of climate uncertainties may be substantial in some urban areas and should be included in assessing future regional air quality and emission control requirements.

Effect of changes in climate and emissions on future sulfate‐nitrate‐ammonium aerosol levels in the United States

Global simulations of sulfate, nitrate, and ammonium aerosols are performed for the present day and 2050 using the chemical transport model GEOS‐Chem. Changes in climate and emissions projected by the IPCC A1B scenario are imposed separately and together, with the primary focus of the work on future inorganic aerosol levels over the United States. Climate change alone is predicted to lead to decreases in levels of sulfate and ammonium in the southeast U.S. but increases in the Midwest and northeast U.S. Nitrate concentrations are projected to decrease across the U.S. as a result of climate change alone. In the U.S., climate change alone can cause changes in annually averaged sulfate‐nitrate‐ammonium of up to 0.61 μg/m3, with seasonal changes often being much larger in magnitude. When changes in anthropogenic emissions are considered (with or without changes in climate), domestic sulfate concentrations are projected to decrease because of sulfur dioxide emission reductions, and nitrate concentrations are predicted to generally increase because of higher ammonia emissions combined with decreases in sulfate despite reductions in emissions of nitrogen oxides. The ammonium burden is projected to increase from 0.24 to 0.36 Tg, and the sulfate burden to increase from 0.28 to 0.40 Tg S as a result of globally higher ammonia and sulfate emissions in the future. The global nitrate burden is predicted to remain essentially constant at 0.35 Tg, with changes in both emissions and climate as a result of the competing effects of higher precursor emissions and increased temperature.

Climate Change and water resources: Scenarios of low-flow conditions in the Upper Danube River Basin

Global Climate Change will have regional impacts on the water resources and will force water resources managers and farmers to adapt. Both low-flow and its duration are critical hydrological parameters, which strongly influence the state of aquatic ecosystems as well as power production, reservoir management and industry. Impacts of future climate change is analysed using scenarios for the change of meteorological drivers and regional hydrological simulation models. The project GLOWA-Danube (www.glowa-danube.de) develops integrative modelling techniques combining process knowledge from both natural and social sciences to examine the sustainability of regional water systems as well as water management alternatives in the Upper Danube watershed (A = 77000 km2). Special emphasis is given to changes in low-flow condition. DANUBIA describes the regional water cycle both physical and spatially distributed. It consists of a collection of tightly coupled models, which strictly preserve energy and matter and are not calibrated to maximise their overall predictive abilities. The paper demonstrates that DANUBIA can reproduce the daily discharge for the time period from 1971-2003 with a Nash-Suttcliffe coefficient of 0.84 (gauge Achleiten). Based on a statistical climate simulator 12 realisations of the IPCC A1B climate scenario were used to investigate impacts of climate change during the simulation period of 2011-2060. The change in discharge and frequency of occurrences of low-flow in the watershed for the scenario ensemble were analysed for the outlet gauge. The analysis shows that strong changes were simulated in the frequency of occurrences of low-flow conditions. The changing climate gradually reduces a 50-years NM7Q discharge of today to less than half of its discharge in the year 2060. These results clearly indicate that the expected climate change will strongly alter the low-flow conditions in the Upper Danube watershed.

Long-term effects of biogeophysical and biogeochemical interactions between terrestrial biosphere and climate under anthropogenic climate change

A complex earth system model, simulating atmosphere and ocean dynamics, marine biogeochemistry, terrestrial vegetation and ice sheets, was used to study feedbacks between the terrestrial biosphere and climate with a set of long-term climate change ensemble experiments. CO 2 emissions were assigned according to historical data and the IPCC SRES scenarios B1, A1B and A2, followed by an exponential decay of the emissions for the period 2100–3000. The experiments give a reasonable reconstruction of the measured CO 2 concentrations between 1750 and 2000. Maximum atmospheric CO 2 concentrations of 520 ppm (B1), 860 ppm (A1B) and 1680 ppm (A2) were reached between 2200 and 2500. Additional experiments were performed with CO 2 emissions and suppressed climate change, as well as an experiment with a prescribed land surface. The experiments were repeated with the vegetation model driven offline, to investigate the effects of climate and CO 2 changes separately. The biogeochemical and biogeophysical interactions between terrestrial biosphere and atmosphere were quantified and compared. A decrease of albedo at high latitudes was the most important biogeophysical change. For the A2 scenario experiment, it causes an additional temperature increase of 1 to 2 K for some high latitude regions by the year 3000, but the changes are minor compared to the heating due to CO 2 increase. The terrestrial biosphere takes up between 15 and 30% of the CO 2 emissions, depending on the scenario and the period considered. The carbon is stored in the tropics and subtropics, where carbon is stored fast, and in the high latitudes, where carbon storage, partly due to forest expansion, is much slower. By the year 3000, the storage of terrestrial carbon results in a decrease of atmospheric CO 2 concentration of 400 ppm, which in turn decreases the global temperature increase by 0.4 K.

The role of climate and emission changes in future air quality over southern Canada and northern Mexico

Abstract. Potential impacts of global climate and emissions changes on regional air quality over southern (western and eastern) Canada and northern Mexico are examined by comparing future summers' (i.e., 2049–2051) average regional O3 and PM2.5 concentrations with historic concentrations (i.e., 2000–2002 summers). Air quality modeling was conducted using CMAQ and meteorology downscaled from the GISS-GCM using MM5. Emissions for North America are found using US EPA, Mexican and Canadian inventories and projected emissions following CAIR and IPCC A1B emissions scenario. Higher temperatures for all sub-regions and regional changes in mixing height, insolation and precipitation are forecast in the 2049-2051 period. Future emissions are calculated to be lower over both Canadian sub-regions, but higher over northern Mexico. Global climate change, alone, is predicted to affect PM2.5 concentrations more than O3 for the projections used in this study: average daily maximum eight (8) hour O3 (M8hO3) concentrations are estimated to be slightly different in all examined sub-regions while average PM2.5 concentrations are estimated to be higher over both Canadian sub-regions (8% over western and 3% over eastern) but 11% lower over northern Mexico. More days are forecast where M8hO3 concentrations are over 75 ppb in all examined sub-regions but the number of days where PM2.5 concentration will be over 15 μg/m3 is projected higher only over western Canada. Climate change combined with the projected emissions lead to greater change in pollutant concentrations: average M8hO3 concentrations are simulated to be 6% lower over western Canada and 8% lower over eastern Canada while average PM2.5 concentrations are simulated to be 5% lower over western Canada and 11% lower over eastern Canada. Although future emissions over northern Mexico are projected higher, pollutant concentrations are simulated to be lower due to US emissions reductions. Global climate change combined with the projected emissions will decrease average M8hO3 4% and PM2.5 17% over northern Mexico. Significant reductions in the number of days where M8hO3 concentrations are over 75 ppb and PM2.5 concentration over 15 μg/m3 are also projected with a significant reduction in peak values.

Linking global to regional models to assess future climate impacts on surface ozone levels in the United States

We investigate the impact of climate change on future air quality in the United States with a coupled global/regional scale modeling system. Regional climate model scenarios developed by dynamically downscaling outputs from the GISS GCM are used by CMAQ to simulate present air pollution climatology, and modeled surface ozone mixing ratios are compared with recent observations. Though the model accurately simulates ozone in the northeast U.S. and in central California, a positive bias of 10–15 ppb exists throughout most of the central and southeast U.S. The model is also applied to a simulated 2050 climate based on the IPCC A1B greenhouse gas scenario. Two future simulations are conducted, one with anthropogenic emissions held at 2001 levels, and one with anthropogenic emissions reduced in accordance with the A1B scenario. Without ozone precursor emissions changes, increases from 2–5 ppb in summer mean 8‐h ozone mixing ratios are projected in Texas and parts of the eastern U.S., while high ozone episodes become more frequent. Increases of 2–8 ppb during the autumn are predicted over a large area in the central and southwest U.S., suggesting a lengthening of the ozone season. These increases within the regional modeling domain are predicted despite large decreases in the future global background ozone mixing ratio. Substantial decreases exceeding 15 ppb during the summer are predicted for the future reduced emissions case. A sensitivity test conducted with 30% higher methane mixing ratio yields widespread ozone increases of 0.5–2 ppb, an effect larger than that of climate‐induced increases in isoprene emissions, demonstrating the need to consider changes in methane levels alongside climate change when simulating future air quality.

Response in extremes of daily precipitation and wind from a downscaled multi-model ensemble of anthropogenic global climate change scenarios

Time-slices of eight global climate model (GCM) response simulations of the IPCC IS92a, CMIP2, SRES A2, B2 and A1B greenhouse gas scenarios have been downscaled using the HIRHAM atmospheric regional climate model (RCM). The area covers Central and Northern Europe, adjacent sea-areas and Greenland. The GCM data were provided from the Max Planck Institute, Germany (MPI), the Hadley Centre, U.K. (HC), the Bjerknes Centre, Norway (BCCR) and University of Oslo, Norway (UiO). The resulting ensemble covers a range of future climate realizations from different global models, different greenhouse gas scenarios and natural climate variability. In order to present trends in statistical parameters including extreme events and their return periods, the downscaled response data are combined as an ensemble of equally valid possible realizations. The combined statistics is obtained after adjustments accounting for (i) different set-ups of the respective GCMs in producing the control climate and (ii) the variable range of time between the control and scenario periods. We find that annual extreme events of daily precipitation and wind speed in the control climate become more frequent in the scenario period over large areas in Northern Europe. The variability in the regional result appears sensitive to the phase of the Scandinavian pattern.

Predicted change in global secondary organic aerosol concentrations in response to future climate, emissions, and land use change

The sensitivity of secondary organic aerosol (SOA) concentration to changes in climate and emissions is investigated using a coupled global atmosphere‐land model driven by the year 2100 IPCC A1B scenario predictions. The Community Atmosphere Model (CAM3) is updated with recent laboratory determined yields for SOA formation from monoterpene oxidation, isoprene photooxidation and aromatic photooxidation. Biogenic emissions of isoprene and monoterpenes are simulated interactively using the Model of Emissions of Gases and Aerosols (MEGAN2) within the Community Land Model (CLM3). The global mean SOA burden is predicted to increase by 36% in 2100, primarily the result of rising biogenic and anthropogenic emissions which independently increase the burden by 26% and 7%. The later includes enhanced biogenic SOA formation due to increased emissions of primary organic aerosol (5–25% increases in surface SOA concentrations in 2100). Climate change alone (via temperature, removal rates, and oxidative capacity) does not change the global mean SOA production, but the global burden increases by 6%. The global burden of anthropogenic SOA experiences proportionally more growth than biogenic SOA in 2100 from the net effect of climate and emissions (67% increase predicted). Projected anthropogenic land use change for 2100 (A2) is predicted to reduce the global SOA burden by 14%, largely the result of cropland expansion. South America is the largest global source region for SOA in the present day and 2100, but Asia experiences the largest relative growth in SOA production by 2100 because of the large predicted increases in Asian anthropogenic aromatic emissions. The projected decrease in global sulfur emissions implies that SOA will contribute a progressively larger fraction of the global aerosol burden.

Response in extremes of daily precipitation and wind from a downscaled multi-model ensemble of anthropogenic global climate change scenarios

Time-slices of eight global climate model (GCM) response simulations of the IPCC IS92a, CMIP2, SRES A2, B2 and A1B greenhouse gas scenarios have been downscaled using the HIRHAM atmospheric regional climate model (RCM). The area covers Central and Northern Europe, adjacent sea-areas and Greenland. The GCM data were provided from the Max Planck Institute, Germany (MPI), the Hadley Centre, U.K. (HC), the Bjerknes Centre, Norway (BCCR) and University of Oslo, Norway (UiO). The resulting ensemble covers a range of future climate realizations from different global models, different greenhouse gas scenarios and natural climate variability. In order to present trends in statistical parameters including extreme events and their return periods, the downscaled response data are combined as an ensemble of equally valid possible realizations. The combined statistics is obtained after adjustments accounting for (i) different set-ups of the respective GCMs in producing the control climate and (ii) the variable range of time between the control and scenario periods. We find that annual extreme events of daily precipitation and wind speed in the control climate become more frequent in the scenario period over large areas in Northern Europe. The variability in the regional result appears sensitive to the phase of the Scandinavian pattern.

Simulating crop net primary production in China from 2000 to 2050 by linking the Crop-C model with a FGOALS’s model climate change scenario

Net primary production (NPP) of crop represents the capacity of sequestrating atmospheric CO2 in agro-ecosystem, and it plays an important role in terrestrial carbon cycling. By linking the Crop-C model with climate change scenario projected by a coupled GCM FGOALS via geographical information system (GIS) techniques, crop NPP in China was simulated from 2000 to 2050. The national averaged surface air temperature from FGOALS is projected to increase by 1.0°C over this period and the corresponding atmospheric CO2 concentration is 535 ppm by 2050 under the IPCC A1B scenario. With a spatial resolution of 10 × 10 km2, model simulation indicated that an annual average increase of 0.6 Tg C yr−1 (Tg=1012 g) would be possible under the A1B scenario. The NPP in the late 2040s would increase by 5% (30 Tg C) within the 98 × 106 hm2 cropland area in contrast with that in the early 2000s. A further investigation suggested that changes in the NPP would not be evenly distributed in China. A higher increase would occur in a majority of regions located in eastern and northwestern China, while a slight reduction would appear in Hebei and Tianjin in northern China. The spatial characteristics of the crop NPP change are attributed primarily to the uneven distribution of temperature change.

Impacts of global climate change and emissions on regional ozone and fine particulate matter concentrations over the United States

Simulated future summers (i.e., 2049–2051) and annual (i.e., 2050) average regional O3 and PM2.5 concentrations over the United States are compared with historic (i.e., 2000–2002 summers and all of 2001) levels to investigate the potential impacts of global climate change and emissions on regional air quality. Meteorological inputs to the CMAQ chemical transport model are developed by downscaling the GISS Global Climate Model simulations using an MM5‐based regional climate model. Future‐year emissions for North America are developed by growing the U.S. EPA CAIR inventory, Mexican and Canadian emissions and by using the IMAGE model with the IPCC A1B emissions scenario that is also used in projecting future climate. Reductions of more than 50% in NOX and SO2 emissions are forecast. Impacts of global climate change alone on regional air quality are small compared to impacts from emission control‐related reductions, although increases in pollutant concentrations due to stagnation and other factors are found. The combined effect of climate change and emission reductions lead to a 20% decrease (regionally varying from −11% to −28%) in the mean summer maximum daily 8‐hour ozone levels (M8hO3) over the United States. Mean annual PM2.5 concentrations are estimated to be 23% lower (varies from −9% to −32%). Major reductions in sulfate, nitrate and ammonium PM2.5 components combined with the limited reduction in organic carbon suggests that organic carbon will be the dominant component of PM2.5 mass in the future. Regionally, the eastern United States benefits more than the rest of the regions from reductions in both M8hO3 and PM2.5, because of both spatial variations in the meteorological and emissions changes. Reduction in the higher M8hO3 concentrations is also estimated for all subregions and fewer days with M8hO3 above the air quality standards in urban sites with Atlanta in the southeast benefiting most.

Simulating long-term Caspian Sea level changes: The impact of Holocene and future climate conditions

To improve our understanding of the relationship between climate change and variations in Caspian Sea level (CSL), we performed simulations of annual CSL for the period 8 ka to 2100 CE using a coupled model setup representing climate, hydrology and sea level. We forced our climate model with long-term changes in orbital parameters and atmospheric greenhouse gas concentrations, using the IPCC A1b scenario for the 21st Century. Our simulations produce an orbitally forced, long-term decline in CSL of 5 m from 5.5 to 0 ka, caused by a decrease in river runoff and over-sea precipitation that is not fully compensated by a decrease in over-sea evaporation. Superimposed on this long-term downward CSL trend we simulated centennial-scale fluctuations of up to 4 m and decadal-scale variations of up to 2 m, caused by the internal variations of our modeled climate system, amplified by the sensitivity of CSL to small changes in river runoff and in the over-sea P–E budget. The A1b anthropogenic emission scenario causes a 4.5 m fall in CSL in the 21st Century, due to a pronounced increase in over-sea evaporation that is stronger than the enhanced river discharge. This decline in CSL is of the same order of magnitude as the orbitally-forced millennial-scale downward CSL trend simulated for the last 8000 years. Our results are generally consistent with CSL estimates based on geological, historical and measured data, as well as with most other model studies.

Stabilizing greenhouse gas concentrations at low levels: an assessment of reduction strategies and costs

On the basis of the IPCC B2, A1b and B1 baseline scenarios, mitigation scenarios were developed that stabilize greenhouse gas concentrations at 650, 550 and 450 and – subject to specific assumptions – 400 ppm CO2-eq. The analysis takes into account a large number of reduction options, such as reductions of non-CO2 gases, carbon plantations and measures in the energy system. The study shows stabilization as low as 450 ppm CO2-eq. to be technically feasible, even given relatively high baseline scenarios. To achieve these lower concentration levels, global emissions need to peak within the first two decades. The net present value of abatement costs for the B2 baseline scenario (a medium scenario) increases from 0.2% of cumulative GDP to 1.1% as the shift is made from 650 to 450 ppm. On the other hand, the probability of meeting a two-degree target increases from 0%–10% to 20%–70%. The mitigation scenarios lead to lower emissions of regional air pollutants but also to increased land use. The uncertainty in the cost estimates is at least in the order of 50%, with the most important uncertainties including land-use emissions, the potential for bio-energy and the contribution of energy efficiency. Furthermore, creating the right socio-economic and political conditions for mitigation is more important than any of the technical constraints.

Detecting potential changes in the meridional overturning circulation at 26˚N in the Atlantic

We analyze the ability of an oceanic monitoring array to detect potential changes in the North Atlantic meridional overturning circulation (MOC). The observing array is ‘deployed’ into a numerical model (ECHAM5/MPI-OM), and simulates the measurements of density and wind stress at 26°N in the Atlantic. The simulated array mimics the continuous monitoring system deployed in the framework of the UK Rapid Climate Change program. We analyze a set of three realizations of a climate change scenario (IPCC A1B), in which – within the considered time-horizon of 200 years – the MOC weakens, but does not collapse. For the detection analysis, we assume that the natural variability of the MOC is known from an independent source, the control run. Our detection approach accounts for the effects of observation errors, infrequent observations, autocorrelated internal variability, and uncertainty in the initial conditions. Continuous observation with the simulated array for approximately 60 years yields a statistically significant (p < 0.05) detection with 95 percent reliability assuming a random observation error of 1 Sv (1 Sv = 106 m3 s−1). Observing continuously with an observation error of 3 Sv yields a detection time of about 90 years (with 95 percent reliability). Repeated hydrographic transects every 5 years/ 20 years result in a detection time of about 90 years/120 years, with 95 percent reliability and an assumed observation error of 3 Sv. An observation error of 3 Sv (one standard deviation) is a plausible estimate of the observation error associated with the RAPID UK 26°N array.

Potential regional climate change and implications to U.S. air quality

Regional climate change scenarios were generated by dynamical downscaling to assess the potential effects of climate change on U.S. air quality. Comparing the climate simulation for 2045–2055 based on the IPCC A1B scenario with the control simulation of 1995–2005, large atmospheric changes that could affect air quality were found in several regions. Analyses were based on changes in surface air temperature and downward solar radiation, precipitation frequency, stagnation events, and ventilation. Changes in the Midwest and Texas during summer are of opposite sign, suggesting negative impacts on air quality in Texas and small positive or no impact in the Midwest. During fall, large warming, increased solar radiation, reduced rainfall frequency, increased stagnation occurrence, and reduced ventilation in the western U.S. all suggest negative impacts on regional air quality. These changes are related to an anticyclonic pattern in the 500 hPa height change that is also found in other GCM projections.