Ensemble Mean Changes Research Articles

Future precipitation changes in California: Comparison of CMIP5 and CMIP6 intermodel spread and its drivers

AbstractCalifornia is one of the major uncertainty hotspots for climate change, as climate models have historically been split between projecting wetter and drier future conditions over the region. We analysed the future (mid‐century and end‐century) projections of California's winter precipitation changes from the latest Coupled Model Intercomparison Project Phase 6 (CMIP6), and studied its respective model agreement in comparison to the previous CMIP5 projections. Over northern California more than two thirds of the models in each ensemble agree on wetter future conditions. However, over southern California both ensembles show highly uncertain precipitation changes, with model projections almost equally divided between wetter or drier conditions. Projected end‐century precipitation changes range from −30% to +70% in CMIP5 and −20% to +80% in CMIP6. The CMIP6 ensemble mean changes are generally wetter and show larger model disagreement compared to CMIP5. Distribution of year‐to‐year precipitation indicates more extremely wet or dry years over southern California in CMIP6 compared to CMIP5, with some models suggesting that the five wettest years account for as much as ~55% of the 20‐year rainfall, and the five driest for as little as ~5%. Dynamically, both ensembles project weakened subsidence over Baja California that is stronger in CMIP6 than in CMIP5, in line with the wetter mean conditions in CMIP6. In the western tropical Pacific we find strengthening of the Hadley circulation in CMIP6 that is not seen in CMIP5, and more El Niño than La Niña conditions in the equatorial Pacific. More CMIP6 models also project an increase in ENSO events compared to CMIP5, and a stronger impact of ENSO on California's precipitation is found in CMIP6 than in CMIP5. These factors also contribute to larger model disagreement and more extremely wet or dry years over southern California in CMIP6.

Understanding recent tropospheric ozone trends in the context of large internal variability: a new perspective from chemistry-climate model ensembles

Observational records of meteorological and chemical variables are imprinted by an unknown combination of anthropogenic activity, natural forcings, and internal variability. With a 15-member initial-condition ensemble generated from the CESM2-WACCM6 chemistry-climate model for 1950–2014, we extract signals of anthropogenic (‘forced’) change from the noise of internally arising climate variability on observed tropospheric ozone trends. Positive trends in free tropospheric ozone measured at long-term surface observatories, by commercial aircraft, and retrieved from satellite instruments generally fall within the ensemble range. CESM2-WACCM6 tropospheric ozone trends are also bracketed by those in a larger ensemble constructed from five additional chemistry-climate models. Comparison of the multi-model ensemble with observed tropospheric column ozone trends in the northern tropics implies an underestimate in regional precursor emission growth over recent decades. Positive tropospheric ozone trends clearly emerge from 1950 to 2014, exceeding 0.2 DU yr−1 at 20–40 N in all CESM2-WACCM6 ensemble members. Tropospheric ozone observations are often only available for recent decades, and we show that even a two-decade record length is insufficient to eliminate the role of internal variability, which can produce regional tropospheric ozone trends oppositely signed from ensemble mean (forced) changes. By identifying regions and seasons with strong anthropogenic change signals relative to internal variability, initial-condition ensembles can guide future observing systems seeking to detect anthropogenic change. For example, analysis of the CESM2-WACCM6 ensemble reveals year-round upper tropospheric ozone increases from 1995 to 2014, largest at 30 S–40 N during boreal summer. Lower tropospheric ozone increases most strongly in the winter hemisphere, and internal variability leads to trends of opposite sign (ensemble overlaps zero) north of 40 N during boreal summer. This decoupling of ozone trends in the upper and lower troposphere suggests a growing prominence for tropospheric ozone as a greenhouse gas despite regional efforts to abate warm season ground-level ozone.

Impact of Climate Change on Precipitation Extremes over Ho Chi Minh City, Vietnam

In the context of climate change, the impact of hydro-meteorological extremes, such as floods and droughts, has become one of the most severe issues for the governors of mega-cities. The main purpose of this study is to assess the spatiotemporal changes in extreme precipitation indices over Ho Chi Minh City, Vietnam, between the near (2021–2050) and intermediate (2051–2080) future periods with respect to the baseline period (1980–2009). The historical extreme indices were calculated through observed daily rainfall data at 11 selected meteorological stations across the study area. The future extreme indices were projected based on a stochastic weather generator, the Long Ashton Research Station Weather Generator (LARS-WG), which incorporates climate projections from the Coupled Model Intercomparison Project 5 (CMIP5) ensemble. Eight extreme precipitation indices, such as the consecutive dry days (CDDs), consecutive wet days (CWDs), number of very heavy precipitation days (R20mm), number of extremely heavy precipitation days (R25mm), maximum 1 d precipitation amount (RX1day), maximum 5 d precipitation amount (RX5day), very wet days (R95p), and simple daily intensity index (SDII) were selected to evaluate the multi-model ensemble mean changes of extreme indices in terms of intensity, duration, and frequency. The statistical significance, stability, and averaged magnitude of trends in these changes, thereby, were computed by the Mann-Kendall statistical techniques and Sen’s estimator, and applied to each extreme index. The results indicated a general increasing trend in most extreme indices for the future periods. In comparison with the near future period (2021–2050), the extreme intensity and frequency indices in the intermediate future period (2051–2080) present more statistically significant trends and higher growing rates. Furthermore, an increase in most extreme indices mainly occurs in some parts of the central and southern regions, while a decrease in those indices is often projected in the north of the study area.

Conceptual deconstruction of the simulated precipitation response to climate change

State-of-the-art climate change projections of the CMIP5 simulations suggest a fairly complex pattern of global precipitation changes, with regions of reduced and enhanced precipitation. Conceptual understanding of these projected precipitation changes is difficult if only based on coupled general circulation model (CGCM) simulations, due to the complexity of these models. In this study we describe a simple deconstruction of the ensemble mean CMIP5 projections based on sensitivity simulations with the globally resolved energy balance (GREB) model. In a series of sensitivity experiments we force the GREB model with four different CMIP5 ensemble mean changes in: surface temperature, evaporation and the vertical atmospheric velocities mean and its standard deviation. The resulting response in the precipitation of the GREB model is very close to the CMIP5 ensemble mean response, suggesting that the precipitation changes can be well represented by a linear combination of these four forcings. The results further provide good insights into the drivers of precipitation change. The GREB model suggests that not one forcing alone can be seen as the main driver, but only the combination of all four changes results in the complex response pattern. However, the dominant forcings are the changes in the large-scale circulation, rather than the pure thermodynamic warming effect. Here, it is interesting to note that changes in high-frequency atmospheric variability of vertical air motion (weather), that are partly independent of the changes in the mean circulation, have a control on the pattern of the time-mean global precipitation changes. The approach presented here provides a powerful basis on which the hydrological cycles of CGCM simulations can be analysed.

Reconciling opposing Walker circulation trends in observations and model projections

A strengthening of the Pacific Walker circulation (PWC) over recent decades triggered an intense debate on the validity of model-projected weakening of the PWC in response to anthropogenic warming. However, limitations of in situ observations and reanalysis datasets have hindered an unambiguous attribution of PWC changes to either natural or anthropogenic causes. Here, by conducting a comprehensive analysis based on multiple independent observational records, including satellite observations along with a large ensemble of model simulations, we objectively determine the relative contributions of internal variability and anthropogenic warming to the emergence of long-term PWC trends. Our analysis shows that the satellite-observed changes differ considerably from the model ensemble-mean changes, but they also indicate substantially weaker strengthening than implied by the reanalyses. Furthermore, some ensemble members are found to reproduce the observed changes in the tropical Pacific. These findings clearly reveal a dominant role of internal variability on the recent strengthening of the PWC. The Pacific Walker circulation is predicted to weaken, however, observations suggest a current strengthening. Satellite humidity observations show a weaker response than reanalyses; coupled with model simulations, this suggests that internal variability dominates recent strengthening.

Future climate change impacts on streamflow and nitrogen exports based on CMIP5 projection in the Miyun Reservoir Basin, China

The projection of streamflow and nutrient exports are essential for future water resource and environmental management plans under climate change. This study investigated the responses of streamflow and total nitrogen (TN) loading to future climate scenarios in the Miyun Reservoir Basin (MRB) by coupling a physical process-based hydrologic and water quality model, Soil and Water Assessment Model (SWAT) and 42 statistical downscaled climate projections on the basis of CMIP5 models. Future climatic projections during the two future periods (2021–2035 and 2051–2065) under RCP4.5 and RCP8.5 emission scenarios, primarily precipitation and temperature, were used to drive the calibrated SWAT model. Results show that the MRB would be warmer and more humid. The ensemble mean changes in average annual precipitation (mean temperature) are expected to be more than 5.4% (0.6°C) during the period of 2021–2035 and 12.5% (1.6°C) during 2051–2065. Future streamflow and TN loading projection are expected to increase in two future periods. Meanwhile, the changes in streamflow and TN loading would be higher in summer than in other seasons. The uncertainty ranges in TN loading projection is larger than that in streamflow projection. The probability that streamflow and TN loading increase would be higher in the period of 2021–2035 than in 2051–2065. This study could be of use for providing an insight into the availability of future streamflow and pollution control for the MRB.

Projected changes in future climate over the Midwest and Great Lakes region using downscaled CMIP5 ensembles

ABSTRACTDespite an increasing body of evidence from observed data that climate change is having a significant impact on different types of biogeophysical systems in the Midwest and Great Lakes region, there still remain critical questions of how quickly and how much climate will be altered over this region in the future. For this evaluation, we make use of 31 global climate model (GCM) projections from the Coupled Model Intercomparison Project, Phase 5 (CMIP5). Based on changes in temperature (T) and precipitation (P) over the Midwest, we selected ten GCM scenarios which (1) simulate historical climate well and (2) successfully capture the range of future climate from the entire CMIP5 ensemble. We then downscaled T and P projections to 1/16° gridded data sets for two different emission scenarios (RCP4.5 and RCP8.5) for three 30‐year future periods using the Hybrid Delta (HD) statistical downscaling approach which was proven to be applicable for daily‐scale application by a validation work using historical data. T is projected to increase across all seasons, with ensemble mean changes up to 6.5 °C by 2100 for the RCP8.5 scenarios. P increases up to 30% in spring and winter with decreasing snowfall to precipitation ratio, while summer P decreases moderately (−15%) by the 2080s. Changes in daily extreme events show similar seasonal patterns including increasing daily extreme P events in winter and decreasing P in summer. Growing season P may actually increase, however, despite projected P reductions in the warmest summer months. Regional warming results in decreased heating degree days (−1639 °C days, −32%) and increasing cooling degree days (+318 °C days, +957%) by 2080s, with overall net reductions in energy demand.

A new mechanism for warm-season precipitation response to global warming based on convection-permitting simulations

Climate models project increasing precipitation intensity but decreasing frequency as greenhouse gases increase. However, the exact mechanism for the frequency decrease remains unclear. Here we investigate this by analyzing hourly data from regional climate change simulations with 4 km grid spacing covering most of North America using the Weather Research and Forecasting model. The model was forced with present and future boundary conditions, with the latter being derived by adding the CMIP5 19-model ensemble mean changes to the ERA-interim reanalysis. The model reproduces well the observed seasonal and spatial variations in precipitation frequency and histograms, and the dry interval between rain events over the contiguous US. Results show that overall precipitation frequency indeed decreases during the warm season mainly due to fewer light-moderate precipitation (0.1 10 mm/h) events increase. Dry spells become longer and more frequent, together with a reduction in time-mean relative humidity (RH) in the lower troposphere during the warm season. The increased dry hours and decreased RH lead to a reduction in overall precipitation frequency and also for light-moderate precipitation events, while water vapor-induced increases in precipitation intensity and the positive latent heating feedback in intense storms may be responsible for the large increase in intense precipitation. The size of intense storms increases while their number decreases in the future climate, which helps explain the increase in local frequency of heavy precipitation. The results generally support a new hypothesis for future warm-season precipitation: each rainstorm removes ≥7% more moisture from the air per 1 K local warming, and surface evaporation and moisture advection take slightly longer than currently to replenish the depleted moisture before the next storm forms, leading to longer dry spells and a reduction in precipitation frequency, as well as decreases in time-mean RH and vertical motion.

Hadley Circulation Strength Change in Response to Global Warming: Statistics of Good Models

In this study, we examine future changes in the Hadley cell (HC) strength using CMIP5 climate change simulations. The current study is an extension of a previous study by Seo et al. that used all 30 available models. Here, we select 18-23 well-performing models based on their significant internal sensitivity of the interannual HC strength variation to the latitudinal temperature gradient variation. The model projections along with simple scaling analysis show that the inter-model variability in the HC strength change is a result of the inter-model spread in the meridional temperature gradient across the subtropics for both DJF and JJA, not by the tropopause height or gross static stability change. The HC strength is expected to weaken significantly during DJF, while little change is expected in the JJA HC strength. Compared to the calculations with all model members, selected model statistics increase the linear correlation between the changes in HC strength and meridional temperature gradient by 13~23%, confirming the robust sensitivity of the HC strength to the meridional temperature gradient. Two scaling equations for the selected models predict changes in HC strength better than all-member predictions. In particular, the prediction improvement in DJF is as high as 30%. The simple scaling relations successfully predict both the ensemble-mean changes and model-to-model variations in the HC strength for both seasons.

Climate Comparisons and Change Projections for the Northwest Atlantic from Six CMIP5 Models

Key physical variables for the Northwest Atlantic (NWA) are examined in the “historical” and two future Representative Concentration Pathway (RCP) simulations of six Earth System Models (ESMs) available through Phase 5 of the Climate Model Intercomparison Project (CMIP5). The variables are air temperature, sea-ice concentration, surface and subsurface ocean temperature and salinity, and ocean mixed-layer depth. Comparison of the historical simulations with observations indicates that the models provide a good qualitative and approximate quantitative representation of many of the large-scale climatological features in the NWA (e.g., annual cycles and spatial patterns). However, the models represent the detailed structure of some important NWA ocean and ice features poorly, such that caution is needed in the use of their projected future changes. Monthly “climate change” fields between the bidecades 1986–2005 and 2046–2065 are described, using ensemble statistics of the changes across the six ESMs. The results point to warmer air temperatures everywhere, warmer surface ocean temperatures in most areas, reduced sea-ice extent and, in most areas, reduced surface salinities and mixed-layer depths. However, the magnitudes of the inter-model differences in the projected changes are comparable to those of the ensemble-mean changes in many cases, such that robust quantitative projections are generally not possible for the NWA.

Twenty-First-Century Precipitation Changes over the Los Angeles Region*

Abstract A new hybrid statistical–dynamical downscaling technique is described to project mid- and end-of-twenty-first-century local precipitation changes associated with 36 global climate models (GCMs) in phase 5 of the Coupled Model Intercomparison Project archive over the greater Los Angeles region. Land-averaged precipitation changes, ensemble-mean changes, and the spread of those changes for both time slices are presented. It is demonstrated that the results are similar to what would be produced if expensive dynamical downscaling techniques were instead applied to all GCMs. Changes in land-averaged ensemble-mean precipitation are near zero for both time slices, reflecting the region’s typical position in the models at the node of oppositely signed large-scale precipitation changes. For both time slices, the intermodel spread of changes is only about 0.2–0.4 times as large as natural interannual variability in the baseline period. A caveat to these conclusions is that interannual variability in the tropical Pacific is generally regarded as a weakness of the GCMs. As a result, there is some chance the GCM responses in the tropical Pacific to a changing climate and associated impacts on Southern California precipitation are not credible. It is subjectively judged that this GCM weakness increases the uncertainty of regional precipitation change, perhaps by as much as 25%. Thus, it cannot be excluded that the possibility that significant regional adaptation challenges related to either a precipitation increase or decrease would arise. However, the most likely downscaled outcome is a small change in local mean precipitation compared to natural variability, with large uncertainty on the sign of the change.

A mechanism for future changes in Hadley circulation strength in CMIP5 climate change simulations

AbstractThe Coupled Model Intercomparison Project Phase 5 (CMIP5) 21st century climate change simulations exhibit a robust (slight) weakening of the Hadley cell (HC) during the boreal winter (summer, respectively) season in the future climate. Using 30 different coupled model simulations, we investigate the main mechanisms for both the multimodel ensemble mean changes in the HC strength and its intermodel changes in response to global warming during these seasons. A simple scaling analysis relates the strength of the HC to three factors: the meridional potential temperature gradient, gross static stability, and tropopause height. We found that changes in the meridional potential temperature gradients across the subtropics in a warming climate play a crucial role in the ensemble mean changes and model‐to‐model variations in the HC strength for both seasons. A larger reduction in the meridional temperature gradient in the Northern Hemisphere in boreal winter leads to the larger reduction of the HC strength in that season.

A multi-model study of impacts of climate change on surface ozone in Europe

Abstract. The impact of climate change on surface ozone over Europe was studied using four offline regional chemistry transport models (CTMs) and one online regional integrated climate-chemistry model (CCM), driven by the same global projection of future climate under the SRES A1B scenario. Anthropogenic emissions of ozone precursors from RCP4.5 for year 2000 were used for simulations of both present and future periods in order to isolate the impact of climate change and to assess the robustness of the results across the different models. The sensitivity of the simulated surface ozone to changes in climate between the periods 2000–2009 and 2040–2049 differs by a factor of two between the models, but the general pattern of change with an increase in southern Europe is similar across different models. Emissions of isoprene differ substantially between different CTMs ranging from 1.6 to 8.0 Tg yr−1 for the current climate, partly due to differences in horizontal resolution of meteorological input data. Also the simulated change in total isoprene emissions varies substantially across models explaining part of the different climate response on surface ozone. Ensemble mean changes in summer mean ozone and mean of daily maximum ozone are close to 1 ppb(v) in parts of the land area in southern Europe. Corresponding changes of 95-percentiles of hourly ozone are close to 2 ppb(v) in the same region. In northern Europe ensemble mean for mean and daily maximum show negative changes while there are no negative changes for the higher percentiles indicating that climate impacts on O3 could be especially important in connection with extreme summer events.

Abrupt CO2 experiments as tools for predicting and understanding CMIP5 representative concentration pathway projections

A fast simple climate modelling approach is developed for predicting and helping to understand general circulation model (GCM) simulations. We show that the simple model reproduces the GCM results accurately, for global mean surface air temperature change and global-mean heat uptake projections from 9 GCMs in the fifth coupled model inter-comparison project (CMIP5). This implies that understanding gained from idealised CO2 step experiments is applicable to policy-relevant scenario projections. Our approach is conceptually simple. It works by using the climate response to a CO2 step change taken directly from a GCM experiment. With radiative forcing from non-CO2 constituents obtained by adapting the Forster and Taylor method, we use our method to estimate results for CMIP5 representative concentration pathway (RCP) experiments for cases not run by the GCMs. We estimate differences between pairs of RCPs rather than RCP anomalies relative to the pre-industrial state. This gives better results because it makes greater use of available GCM projections. The GCMs exhibit differences in radiative forcing, which we incorporate in the simple model. We analyse the thus-completed ensemble of RCP projections. The ensemble mean changes between 1986–2005 and 2080–2099 for global temperature (heat uptake) are, for RCP8.5: 3.8 K (2.3 × 1024 J); for RCP6.0: 2.3 K (1.6 × 1024 J); for RCP4.5: 2.0 K (1.6 × 1024 J); for RCP2.6: 1.1 K (1.3 × 1024 J). The relative spread (standard deviation/ensemble mean) for these scenarios is around 0.2 and 0.15 for temperature and heat uptake respectively. We quantify the relative effect of mitigation action, through reduced emissions, via the time-dependent ratios (change in RCPx)/(change in RCP8.5), using changes with respect to pre-industrial conditions. We find that the effects of mitigation on global-mean temperature change and heat uptake are very similar across these different GCMs.

Multi-model analysis of RCM simulated 1-day to 30-day seasonal precipitation extremes in the Czech Republic

► Statistical analysis of 1-day to 30-day seasonal precipitation extremes. ► Ensemble of 14 transient regional climate model simulations. ► Negative ensemble mean bias in the quantiles in summer, decreasing with duration. ► Extreme quantiles better reproduced than moderate quantiles in the other seasons. ► Increases in the quantiles for all durations, return periods and seasons. This paper presents an analysis of seasonal precipitation extremes in an ensemble of fourteen transient regional climate model (RCM) simulations for durations varying from 1 to 30 days. It is assumed that these precipitation maxima follow a generalized extreme value (GEV) distribution with time-varying parameters. In addition, spatial pooling of the maxima at various grid boxes is applied to reduce the uncertainty of the parameters of the GEV model. Relatively large positive biases in moderate quantiles of long-duration precipitation extremes have been found in autumn, winter and spring. The extreme quantiles are much better reproduced in these seasons. In summer, the ensemble mean bias in the quantiles is negative and decreases (in absolute value) with increasing duration. The ensemble mean changes in the quantiles are positive for all durations, return periods and seasons considered. Large quantiles of the distribution of summer precipitation extremes increase more than moderate quantiles and this increase is relatively large at short durations. The dependence of the changes on duration and return period is much weaker in the other seasons, in particular in winter and spring. Although the spread in the RCM ensemble is considerable, most of the RCM simulations agree on the sign of the changes.

Controls of Global-Mean Precipitation Increases in Global Warming GCM Experiments

Abstract This paper examines the controls on global precipitation that are evident in the transient experiments conducted using coupled climate models collected for the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). The change in precipitation, water vapor, clouds, and radiative heating of the atmosphere evident in the 1% increase in carbon dioxide until doubled (1pctto2x) scenario is examined. As noted in other studies, the ensemble-mean changes in water vapor as carbon dioxide is doubled occur at a rate similar to that predicted by the Clausius–Clapeyron relationship. The ratio of global changes in precipitation to global changes in water vapor offers some insight on how readily increased water vapor is converted into precipitation in modeled climate change. This ratio ɛ is introduced in this paper as a gross indicator of the global precipitation efficiency under global warming. The main findings of this paper are threefold. First, increases in the global precipitation track increase atmospheric radiative energy loss and the ratio of precipitation sensitivity to water vapor sensitivity is primarily determined by changes to this atmospheric column energy loss. A reference limit to this ratio is introduced as the rate at which the emission of radiation from the clear-sky atmosphere increases as water vapor increases. It is shown that the derived efficiency based on the simple ratio of precipitation to water vapor sensitivities of models in fact closely matches the sensitivity derived from simple energy balance arguments involving changes to water vapor emission alone. Second, although the rate of increase of clear-sky emission is the dominant factor in the change to the energy balance of the atmosphere, there are two important and offsetting processes that contribute to ɛ in the model simulations studied: One involves a negative feedback through cloud radiative heating that acts to reduce the efficiency; the other is the global reduction in sensible heating that counteracts the effects of the cloud feedback and increases the efficiency. These counteracting feedbacks only apply on the global scale. Third, the negative cloud radiative heating feedback occurs through reductions of cloud amount in the middle troposphere, defined as the layer between 680 and 440 hPa, and by slight global cloud decreases in the lower troposphere. These changes act in a manner to expose the warmer atmosphere below to high clouds, thus resulting in a net warming of the atmospheric column by clouds and a negative feedback on the precipitation.

Climate of Russia in the 21st century. Part 3. Future climate changes calculated with an ensemble of coupled atmosphere-ocean general circulation CMIP3 models

Probable climate changes in Russia in the 21st century are considered based on the results of global climate simulations with an ensemble of coupled atmosphere-ocean CMIP3 models. The future changes in the surface air temperature, atmospheric pressure, cloud amount, atmospheric precipitation, snow cover, soil water content, and annual runoff in Russia and some of its regions in the early, middle, and late 21st century are analyzed using the A2 scenario of the greenhouse gas and aerosol emission. Future changes in the yearly highest and lowest surface air temperatures and in summer precipitation of high intensity are estimated for Russia. Possible oscillations of the Caspian Sea level associated with the expected global climate warming are estimated. In addition to the estimates of the ensemble mean changes in climatic characteristics, the information about standard deviations and statistical significance of the corresponding climate changes is given.

Changing Northern Hemisphere Storm Tracks in an Ensemble of IPCC Climate Change Simulations

Abstract Winter storm-track activity over the Northern Hemisphere and its changes in a greenhouse gas scenario (the Special Report on Emission Scenarios A1B forcing) are computed from an ensemble of 23 single runs from 16 coupled global climate models (CGCMs). All models reproduce the general structures of the observed climatological storm-track pattern under present-day forcing conditions. Ensemble mean changes resulting from anthropogenic forcing include an increase of baroclinic wave activity over the eastern North Atlantic, amounting to 5%–8% by the end of the twenty-first century. Enhanced activity is also found over the Asian continent and over the North Pacific near the Aleutian Islands. At high latitudes and over parts of the subtropics, activity is reduced. Variations of the individual models around the ensemble average signal are not small, with a median of the pattern correlation near r = 0.5. There is, however, no evidence for a link between deviations in present-day climatology and deviations with respect to climate change.

A multimodel ensemble approach to assessment of climate change impacts on the hydrology and water resources of the Colorado River Basin

Abstract. Implications of 21st century climate change on the hydrology and water resources of the Colorado River Basin were assessed using a multimodel ensemble approach in which downscaled and bias corrected output from 11 General Circulation Models (GCMs) was used to drive macroscale hydrology and water resources models. Downscaled climate scenarios (ensembles) were used as forcings to the Variable Infiltration Capacity (VIC) macroscale hydrology model, which in turn forced the Colorado River Reservoir Model (CRMM). Ensembles of downscaled precipitation and temperature, and derived streamflows and reservoir system performance were assessed through comparison with current climate simulations for the 1950–1999 historical period. For each of the 11 GCMs, two emissions scenarios (IPCC SRES A2 and B1, corresponding to relatively unconstrained growth in emissions, and elimination of global emissions increases by 2100) were represented. Results for the A2 and B1 climate scenarios were divided into three periods: 2010–2039, 2040–2069, and 2070–2099. The mean temperature change averaged over the 11 ensembles for the Colorado basin for the A2 emission scenario ranged from 1.2 to 4.4°C for periods 1–3, and for the B1 scenario from 1.3 to 2.7°C. Precipitation changes were modest, with ensemble mean changes ranging from −1 to −2% for the A2 scenario, and from +1 to −1% for the B1 scenario. An analysis of seasonal precipitation patterns showed that most GCMs had modest reductions in summer precipitation and increases in winter precipitation. Derived April 1 snow water equivalent declined for all ensemble members and time periods, with maximum (ensemble mean) reductions of 38% for the A2 scenario in period 3. Runoff changes were mostly the result of a dominance of increased evapotranspiration over the seasonal precipitation shifts, with ensemble mean runoff changes of −1, −6, and −11% for the A2 ensembles, and 0, −7, and −8% for the B1 ensembles. These hydrological changes were reflected in reservoir system performance. Average total basin reservoir storage and average hydropower production generally declined, however there was a large range across the ensembles. Releases from Glen Canyon Dam to the Lower Basin were reduced for all periods and both emissions scenarios in the ensemble mean. The fraction of years in which shortages occurred increased by approximately 20% by period 3 for both emissions scenarios.

Multimodel ensemble simulations of present‐day and near‐future tropospheric ozone

Global tropospheric ozone distributions, budgets, and radiative forcings from an ensemble of 26 state‐of‐the‐art atmospheric chemistry models have been intercompared and synthesized as part of a wider study into both the air quality and climate roles of ozone. Results from three 2030 emissions scenarios, broadly representing “optimistic,” “likely,” and “pessimistic” options, are compared to a base year 2000 simulation. This base case realistically represents the current global distribution of tropospheric ozone. A further set of simulations considers the influence of climate change over the same time period by forcing the central emissions scenario with a surface warming of around 0.7K. The use of a large multimodel ensemble allows us to identify key areas of uncertainty and improves the robustness of the results. Ensemble mean changes in tropospheric ozone burden between 2000 and 2030 for the 3 scenarios range from a 5% decrease, through a 6% increase, to a 15% increase. The intermodel uncertainty (±1 standard deviation) associated with these values is about ±25%. Model outliers have no significant influence on the ensemble mean results. Combining ozone and methane changes, the three scenarios produce radiative forcings of −50, 180, and 300 mW m−2, compared to a CO2 forcing over the same time period of 800–1100 mW m−2. These values indicate the importance of air pollution emissions in short‐ to medium‐term climate forcing and the potential for stringent/lax control measures to improve/worsen future climate forcing. The model sensitivity of ozone to imposed climate change varies between models but modulates zonal mean mixing ratios by ±5 ppbv via a variety of feedback mechanisms, in particular those involving water vapor and stratosphere‐troposphere exchange. This level of climate change also reduces the methane lifetime by around 4%. The ensemble mean year 2000 tropospheric ozone budget indicates chemical production, chemical destruction, dry deposition and stratospheric input fluxes of 5100, 4650, 1000, and 550 Tg(O3) yr−1, respectively. These values are significantly different to the mean budget documented by the Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report (TAR). The mean ozone burden (340 Tg(O3)) is 10% larger than the IPCC TAR estimate, while the mean ozone lifetime (22 days) is 10% shorter. Results from individual models show a correlation between ozone burden and lifetime, and each model's ozone burden and lifetime respond in similar ways across the emissions scenarios. The response to climate change is much less consistent. Models show more variability in the tropics compared to midlatitudes. Some of the most uncertain areas of the models include treatments of deep tropical convection, including lightning NOx production; isoprene emissions from vegetation and isoprene's degradation chemistry; stratosphere‐troposphere exchange; biomass burning; and water vapor concentrations.