Global Climate Model Experiments Research Articles

Will climate change drive 21st century burn rates in Canadian boreal forest outside of its natural variability: collating global climate model experiments with sedimentary charcoal data

Natural ecosystems have developed within ranges of conditions that can serve as references for setting conservation targets or assessing the current ecological integrity of managed ecosystems. Because of their climate determinism, forest fires are likely to have consequences that could exacerbate biophysical and socioeconomical vulnerabilities in the context of climate change. We evaluated future trends in fire activity under climate change in the eastern Canadian boreal forest and investigated whether these changes were included in the variability observed during the last 7000 years from sedimentary charcoal records from three lakes. Prediction of future annual area burned was made using simulated Monthly Drought Code data collected from an ensemble of 19 global climate model experiments. The increase in burn rate that is predicted for the end of the 21st century (0.45% year–1 with 95% confidence interval (0.32, 0.59) falls well within the long‐term past variability (0.37 to 0.90% year–1). Although our results suggest that the predicted change in burn rates per se will not move this ecosystem to new conditions, the effects of increasing fire incidence cumulated with current rates of clear‐cutting or other low‐retention types of harvesting, which still prevail in this region, remain preoccupying.

Characteristics of the Largest Recorded Annual Maximum Monthly Precipitation in an Atmospheric Global Climate Model Experiment

本研究では，大気全球気候モデル（AGCM）の長期積分結果を用いて，数値実験で再現された既往最大月降水量が，観測をどの程度再現し，またどのような統計的性質を持つかを調査した．AGCMから得られた既往最大月降水量の再現期間は，統計学的には何ら地理的分布を示さなくてよいが，ある地域では500（50）年を上（下）回る再現期間が現れるという特徴的な地理分布を示し，全球で再現期間を集計した確率分布関数は，モデル値と理論値とで良く一致したが，観測値とは定量的には一致しなかった．また，既往最大月降水量を10位の年最大月降水量で割った値（極値比）は，再現期間と同様，地域分布が見られ，極端に大きい月降水量が生じる地域は決まっていることが示された．400年以上の再現期間を持つ年最大月降水量が推定される原因を調べたところ，極値比が大きい （小さい）と，極値統計の一手法であるLモーメント法の3次モーメント（L-skewness）と4次モーメント（L-kurtosis）が共に大きい（小さい）分布となり，極値比と分布形の二つの要素に原因があることが示された．これらの結果は，AGCMで再現された既往最大値を含む極値を水文・水資源の研究に利用する際，十分な注意が必要であることを示唆している．

Regional Differences in the Influence of Irrigation on Climate

Abstract A global climate model experiment is performed to evaluate the effect of irrigation on temperatures in several major irrigated regions of the world. The Community Atmosphere Model, version 3.3, was modified to represent irrigation for the fraction of each grid cell equipped for irrigation according to datasets from the Food and Agriculture Organization. Results indicate substantial regional differences in the magnitude of irrigation-induced cooling, which are attributed to three primary factors: differences in extent of the irrigated area, differences in the simulated soil moisture for the control simulation (without irrigation), and the nature of cloud response to irrigation. The last factor appeared especially important for the dry season in India, although further analysis with other models and observations are needed to verify this feedback. Comparison with observed temperatures revealed substantially lower biases in several regions for the simulation with irrigation than for the control, suggesting that the lack of irrigation may be an important component of temperature bias in this model or that irrigation compensates for other biases. The results of this study should help to translate the results from past regional efforts, which have largely focused on the United States, to regions in the developing world that in many cases continue to experience significant expansion of irrigated land.

Projections of twenty-first century climate over Europe

We present an assessment of climate change pro jections over the European region for the 21 st century from the ensembles of CMIP3 global model experiments and PRUDENCE regional climate model experiments. The A2, A1B, and B1 IPCC emission scenarios are considered. A brief review is also presented of the literature available on future European climate pro jections. In all emission scenarios the European region shows maximum warming of up to several degrees C over the Mediterranean region in summer and over northeastern Europe in winter. The precipitation change signal shows a north-south dipolar structure, with increasing precipitation over Northern Europe and decreasing over southern Europe. This structure migrates northward from the winter to the summer and is tied to the north-south motion of an increasing anticyclonic circulation cell over the North Atlantic-European sector. Temperature interannual variability decreases in winter over central and northern Europe and increases in summer throughout Europe. Precipitation interannual variability shows a predominant increase, most pronounced in summer. The seasonal temperature anomaly probability density functions (PDFs) show a shift and a widening and flattening in future climate conditions, especially in summer, which is indicative of pronounced increases of extreme hot seasons. The seasonal precipitation anomaly PDFs show pronounced changes over Southern Europe in summer, with a strong increase of very dry seasons. In general, the magnitude of future climate change increases with the greenhouse gas forcing. A broad consensus is found between the pro jections obtained with the CMIP3 and PRUDENCE ensembles, as well as between the present analysis and previous generations of model pro jections. The climate change signal over Europe exhibits a consistent latitudinal and seasonal evolution identified as the European Climate change Oscillation (ECO) by Giorgi and Coppola [F. Giorgi and E. Coppola, Geophys. Res. Lett. 34, L21703 (2007)]. The changes of temperature and precipitation over Europe are pronounced, making this region highly vulnerable to global warming.

Transient response of severe thunderstorm forcing to elevated greenhouse gas concentrations

We investigate the transient response of severe‐thunderstorm forcing to the time‐varying greenhouse gas concentrations associated with the A1B emissions scenario. Using a five‐member ensemble of global climate model experiments, we find a positive trend in such forcing within the United States, over the period 1950–2099. The rate of increase varies by geographic region, depending on (i) low‐level water vapor availability and transport, and (ii) the frequency of synoptic‐scale cyclones during the warm season. Our results indicate that deceleration of the greenhouse gas emissions trajectory would likely result in slower increases in severe thunderstorm forcing.

Increased tropical Atlantic wind shear in model projections of global warming

To help understand possible impacts of anthropogenic greenhouse warming on hurricane activity, we assess model‐projected changes in large‐scale environmental factors tied to variations in hurricane statistics. This study focuses on vertical wind shear (Vs) over the tropical Atlantic during hurricane season, the increase of which has been historically associated with diminished hurricane activity and intensity. A suite of state‐of‐the‐art global climate model experiments is used to project changes in Vs over the 21st century. Substantial increases in tropical Atlantic and East Pacific shear are robust features of these experiments, and are shown to be connected to the model‐projected decrease in the Pacific Walker circulation. The relative changes in shear are found to be comparable to those of other large‐scale environmental parameters associated with Atlantic hurricane activity. The influence of these Vs changes should be incorporated into projections of long‐term hurricane activity.

Pan‐oceanic response to increasing anthropogenic aerosols: Impacts on the Southern Hemisphere oceanic circulation

Examinations of the impact of anthropogenic aerosols on oceanic heat content have focused largely on the global average response. Given that aerosol‐induced cooling is greater in the Northern Hemisphere (NH) than in the Southern Hemisphere (SH), do aerosols induce a greater impact on NH oceanic heat content? Sea level rise over the past 50 years has shown little hemispheric differentiation. Using a set of global climate model experiments forced with and without anthropogenic aerosols, we show that increasing aerosols in the 20th century induce a pan‐oceanic heat redistribution. This leads to a reduction in the SH oceanic heat content comparable to that in the NH oceans. The process includes a strengthening of the northward cross‐equatorial heat transport in the Atlantic and Pacific Oceans, with the majority taking place in the Atlantic Ocean via the most effective pathway: the globally interconnected ocean current system associated with the Atlantic overturning.

Global Warming and the Summertime Evapotranspiration Regime of the Alpine Region

Changes of the summer evapotranspiration regime under increased levels of atmospheric greenhouse gases are discussed for three Alpine river basins on the basis of a new set of simulations carried out with a high-resolution hydrological model. The climate change signal was inferred from the output of two simulations with a state-of-the-art global climate model (GCM), a reference run valid for 1961–1990 and a time-slice simulation valid for 2071–2100 under forcing from the A2 IPCC emission scenario. In this particular GCM experiment and with respect to the Alpine region summer temperature was found to increase by 3 to 4°C, whereas precipitation was found to decrease by 10 to 20%. Global radiation and water vapor pressure deficit were found to increase by about 5% and 2 hPa, respectively. On this background, an overall increase of potential evapotranspiration of about 20% relative to the baseline was predicted by the hydrological model, with important variations between but also within individual basins. The results of the hydrological simulations also revealed a reduction in the evapotranspiration efficiency that depends on altitude. Accordingly, actual evapotranspiration was found to increase at high altitudes and to the south of the Alps, but to decrease in low elevation areas of the northern forelands and in the inner-Alpine domain. Such a differentiation does not appear in the GCM scenario, which predicts an overall increase in evapotranspiration over the Alps. This underlines the importance of detailed simulations for the quantitative assessment of the regional impact of climate change on the hydrological cycle.

Analysis of global climate model experiments to elucidate past and future changes in surface insolation and warming in China

Trends in climate variables and their interrelationships over China are examined using a combination of observations and global climate model simulations to elucidate the mechanism for producing an observed 1°C increase in surface temperature despite a significant decrease in surface insolation from 1950 to 2000. For the 21st century, the model simulations suggest that the downward trend in insolation is expected to continue until 2050, primarily forced by the prescribed atmospheric sulfate burden (IPCC SRES A1B). A continuous increase in surface temperature (3°C) and vapor pressure (1mb) is simulated during the 21st century. Our analysis suggests that both the past and the future warming are primarily caused by an increase in downward longwave radiation. This occurs, in part, as a result of both the lower and upper atmospheric water vapor feedbacks, triggered by the increase in anthropogenic greenhouse gases.

Weakening of tropical Pacific atmospheric circulation due to anthropogenic forcing

Since the mid-nineteenth century the Earth's surface has warmed, and models indicate that human activities have caused part of the warming by altering the radiative balance of the atmosphere. Simple theories suggest that global warming will reduce the strength of the mean tropical atmospheric circulation. An important aspect of this tropical circulation is a large-scale zonal (east-west) overturning of air across the equatorial Pacific Ocean--driven by convection to the west and subsidence to the east--known as the Walker circulation. Here we explore changes in tropical Pacific circulation since the mid-nineteenth century using observations and a suite of global climate model experiments. Observed Indo-Pacific sea level pressure reveals a weakening of the Walker circulation. The size of this trend is consistent with theoretical predictions, is accurately reproduced by climate model simulations and, within the climate models, is largely due to anthropogenic forcing. The climate model indicates that the weakened surface winds have altered the thermal structure and circulation of the tropical Pacific Ocean. These results support model projections of further weakening of tropical atmospheric circulation during the twenty-first century.

Interference of extratropical surface climate anomalies induced by El Niño and stratospheric sudden warmings

The El Niño/Southern Oscillation (ENSO) and stratospheric sudden warmings (SSWs) each induce significant surface climate anomalies in Northern latitudes during winter. Nonetheless, possible connections between the impacts of the ENSO and SSWs remain relatively unexplored. Using both observational analysis and global climate model (GCM) experiments, we show that the impacts of El Niño and SSWs interfere over North America. The interference includes constructive interference, or synergistic impacts of the El Niño and SSWs over the southeastern US through northern Mexico, resulting in enhancement of colder and wetter climate when El Niño and SSWs occur in the same winter. The interference is of practical importance such as in extended‐range weather forecasts, since the impacts of the two phenomena affect seasonal averages and increase the probability of extreme weather conditions in the region.

Climate Variability and Change in Lithuania

The article examines the dynamics of the Lithuanian climate in the 18th–20th centuries. Evaluation of air temperature, precipitation amount and the intensity of cyclonic circulation during the observation period was made. Seasonal trends of different climatological indices were determined. To predict climate change trends in Lithuania in the 21st century, the outputs of Global Climate Model (GCM) experiments from five modelling centres were used. The results of investigation show that current climate change tendencies in Lithuania will remain in the future: against the background of air temperature rise the continentality of the Lithuanian climate will decrease.

The West African dipole in rainfall and its forcing mechanisms in global and regional climate models

The leading mode of 20th century West African rainfall variability represents a well-documented drought tendency in the Sahel and the Guinea Coast region from the 1960s onward. The following three modes describe an out-of-phase relationship between the Sahel Zone (SHZ) and the Guinea Coast region (GCR) to the south, with positive rainfall anomalies in GCR being associated with even more severe drought conditions in SHZ between 1970 and 1998. This West African dipole in rainfall (WDR) has been of high relevance to migration processes in recent decades over the entire subsaharan region.
 WDR is equally revealed in long-term observational data and global climate model output. There is a clear scale-dependence of the anticorrelation, SHZ rainfall changes being decoupled from GCR ones in the low-frequency range. It is found that the high pass filtered interannual WDR fluctuations are closely tied to Atlantic sea surface temperatures (SSTs), particularly in the Gulf of Guinea. Soil moisture is not a dominant player in forcing the dipole anomalies.
 Sensitivity studies with a regional climate model support the physical link between tropical Atlantic SST and WDR. The simulated rainfall response to changing SST is non-linear and in the same order of magnitude as in the long-term observations. The SST impact accounts for up to 40% of total rainfall variability, particularly over the southernmost part of West Africa, and is statistically significant at the 1% level even with respect to the remarkable day-to-day variations. Prescribing late 21st century warmer tropical Atlantic SST as derived from global climate model experiments under increasing greenhouse gas (GHG) concentrations, leads to increasing rainfall amount in GCR (around+300mm) and a reduction in freshwater supply in SHZ (around -150mm) during the July-August main rainy season. Thus, the SHZ-GCR contrast in rainfall amount may rise in the future, inducing ongoing north-to-south migrations in whole subsaharan West Africa.

Flying in the face of climate change: a review of climate change, past, present and future

The distribution and abundance of birds is known to depend critically upon climate variability at a range of temporal and spatial scales. In this paper we review historical changes in climate in the context of what is known about climate variability over the last millennium, with particular reference to the British Isles. The climate of Britain is now warmer than it has been in at least 340 years, with the 1990s decade 0.5 °C warmer than the 1961–1990 average. In addition, the frequency of cold days (mean temperature below 0 °C), particularly during March and November, has declined and there has been a marked shift in the seasonality of precipitation, with winters becoming substantially wetter and summers becoming slightly drier. Current understanding is that the rate of future warming is likely to accelerate with more frequent and more intense summer heatwaves, milder winters, an increase in winter rainfall, an increased risk of winter river floods, and an increase in mean sea‐level and associated coastal flooding. All of these aspects of climate change are likely to impact on coastal birds. A range of potential future climate scenarios for the British Isles are presented derived from recently completed global climate model experiments. For migrant bird species, changes in the British climate have also to be seen within the context of remote climate change in both the breeding and the overwintering grounds.

Water balance of Lake Victoria: update to 2000 and climate change modelling to 2100 / Bilan hydrologique du Lac Victoria: mise à jour jusqu’en 2000 et modélisation des impacts du changement climatique jusqu’en 2100

An annual water balance model of Lake Victoria is derived for the period 1925–2000. Regression techniques are used to derive annual inputs to the water balance, based on lake rainfall data, measured and derived inflows and estimated evaporation during the historical period. This approach acknowledges that runoff is a nonlinear function of lake rainfall. A longer inflow series is produced here which is representative of the whole inflow to the lake, rather than just from individual tributaries. The results show a good simulation of annual lake levels and outflows and capture the high lake level in 1997–1998. Climate change scenarios, from a recent global climate model experiment, are applied to the lake rainfall inflow series and evaporation data to estimate future water balances of the lake. The scenarios produce a potential fall in lake levels by the 2030s horizon, and a rise by the 2080s horizon. A discussion of the application of climate change data to this complex hydrological system is presented.

The impact of sea-ice dynamics on the Arctic climate system

Five paired global climate model experiments, one with an ice pack that only responds thermodynamically (TI) and one including sea-ice dynamics (DI), were used to investigate the sensitivity of Arctic climates to sea-ice motion. The sequence of experiments includes situations in which the Arctic was both considerably colder (Glacial Inception, ca 115,000 years ago) and considerably warmer (3 × CO2) than today. Sea-ice motion produces cooler anomalies year-round than simulations without ice dynamics, resulting in reduced Arctic warming in warm scenarios and increased Arctic cooling in cold scenarios. These changes reflect changes in atmospheric circulation patterns: the DI simulations favor outflow of Arctic air and sea ice into the North Atlantic by promoting cyclonic circulation centered over northern Eurasia, whereas the TI simulations favor southerly inflow of much warmer air from the North Atlantic by promoting cyclonic circulation centered over Greenland. The differences between the paired simulations are sufficiently large to produce different vegetation cover over >19% of the land area north of 55°N, resulting in changes in land-surface characteristics large enough to have an additional impact on climate. Comparison of the DI and TI experiments for the mid-Holocene (6000 years ago) with paleovegetation reconstructions suggests the incorporation of sea-ice dynamics yields a more realistic simulation of high-latitude climates. The spatial pattern of sea-ice anomalies in the warmer-than-modern DI experiments strongly resembles the observed Arctic Ocean sea-ice dipole structure in recent decades, consistent with the idea that greenhouse warming is already impacting the high-northern latitudes.

The relationship between the SOI and extended tropical precipitation in simulations of future climate change

[1] The Southern Oscillation Index (SOI), its relationship with precipitation, and how greenhouse gas-induced changes in climate may modify these relationships has been examined in 12 coupled atmosphere-ocean Global Climate Model (GCM) experiments. The model SOI series predominantly show a tendency towards more positive (La Nina-like) SOI values as climate warms, but opposing trends with regard to changes in its interannual variability. EOF analyses of the dominant mode of precipitation reveals that most models display the main observed features of tropical precipitation related to ENSO. Only modest changes in the modelled EOF1 coefficient fields between the simulated periods 1900–1949 and 2050–2099 are found. The model EOF1 amplitude series, in general, correlate highly with the model SOI series for the historic 1900–1949 period. For the future 2050–2099 period, however, this correlation decreases in a number of the simulations, due either to a shift in the centres of ENSO action or to an overall weaker SOI signal.

Prediction of a geographical shift in the prevalence of rice stripe virus disease transmitted by the small brown planthopper, Laodelphax striatellus(Fallen)(Hemiptera: Delphacidae), under global warming.

Global warming may affect crop damage caused by insect pest, by changing the degree of synchronization between pest occurrence and the susceptible stage of crops. The epidemiological system of rice stripe virus disease (RSV disease) transmitted by the small brown planthopper, Laodelphax striatellus (Fallen), is greatly influenced by synchronization, because the susceptible stage for virus infection is within several weeks after transplanting. We calculated how the area potentially vulnerable to RSV disease will change under future global warming by using the results of the Global Climate Model (GCM) experiments reported by the Intergovernmental Panel on Climate Change. For simplicity, assuming that rice seedlings are transplanted from May to June, we made a map, in which the number of generations of the small brown planthopper on June 1 was plotted by calculating the effective cumulative temperature. The influence of solar radiation was also considered in this calculation. We judged that the area located near the boundary of generations is potentially vulnerable to disease prevalence, because planthoppers are in the adult stage there. Generation maps indicated that the Tohoku and Hokuriku districts, which are major districts of rice production in Japan, might be potentially vulnerable to disease infection under future global warming.

Indirect forcing by anthropogenic aerosols: A global climate model calculation of the effective‐radius and cloud‐lifetime effects

A global climate model (GCM) that includes a physically based cloud scheme is used to calculate the indirect radiative forcing due to the modification of liquid‐water cloud properties by anthropogenic aerosols. The distribution of cloud‐droplet number concentration Nd required by the cloud scheme is estimated empirically from monthly mean fields of sulfate mass generated by a chemical transport model. The effects of anthropogenic changes in Nd are considered in the calculation of precipitation (the “cloud‐lifetime” effect) and of the droplet effective radius used in the shortwave and longwave radiation schemes (the “effective‐radius” effect). The modeled cloud‐droplet effective radii for present‐day conditions agree quite well with satellite‐retrieved values, although the land‐ocean and hemispheric contrasts are weaker in the model than in the observations. The total indirect forcing is −2.1 W m−2, including a small longwave forcing of +0.1 W m−2. The forcing results from a 1% increase in cloudiness, a 6% increase in liquid water path, and a 7% decrease in droplet effective radius. The breakdown of the total indirect forcing into the effective‐radius and cloud‐lifetime effects is estimated by performing separate GCM experiments in which each effect is included individually. The estimated forcings due to the effective‐radius and cloud‐lifetime effects are −1.2 and −1.0 W m−2, respectively. The calculated forcings show some sensitivity to the autoconversion threshold, the sulfate‐Nd relation, and the vertical distribution of sulfate, but in each case the cloud‐lifetime forcing is at least 25% of the total indirect forcing. These results suggest that the cloud‐lifetime effect should not be ignored in future calculations of the indirect forcing due to anthropogenic aerosols.

A physically based scheme for the treatment of stratiform clouds and precipitation in large‐scale models. I: Description and evaluation of the microphysical processes

AbstractA stratiform‐cloud and precipitation scheme, incorporating prognostic variables for cloud liquid water and cloud ice, has been developed for the CSIRO global climate model (GCM). the scheme includes physically based treatments of key microphysical processes, turbulent mixing and semi‐Lagrangian advection of cloud‐water species and interactive cloud radiative properties. Objectives in the development of the scheme were to improve upon the physical realism of parametrizations used in earlier schemes, whilst also trying to provide a scheme with moderate computational overheads.The parametrized microphysical processes are evaluated in relation to observations and theory, and are compared to treatments used in earlier schemes in a series of short GCM experiments. It is argued that the treatment of precipitation formation in warm, and mixed‐phase, stratiform clouds is more realistic in the present scheme than in earlier schemes, which used crude methods for the parametrization of autoconversion, and did not treat key ice‐processes in a consistent way. In the present scheme, accretion processes are more important, whereas autoconversion is less important than in earlier schemes.To determine whether the cloud scheme requires the use of a reduced (split) time‐step, the sensitivity of the various terms to the time‐step is evaluated in another series of short GCM experiments. It is shown that the various terms are not very sensitive to the time‐step, so the scheme can be efficiently implemented without the use of a split time‐step. Overall, analytical or time‐centred treatments perform better than implicit or explicit schemes, especially in the calculation of the precipitation of cloud ice, where only an accurate analytical treatment is found to perform satisfactorily at large time‐steps.As a preliminary validation of the scheme, zonal‐mean fields from a six‐year model‐run are presented for the month of July. the results generally agree well with observations; in particular, the modelled cloudiness and long‐wave cloud‐forcing fields are more realistic than those obtained with the standard version of the CSIRO GCM.