ESDD - Latest Articles

Critical impacts of global warming on land ecosystems

Thu, 16 May 2013 00:00:00 +0200

Critical impacts of global warming on land ecosystems

Earth System Dynamics Discussions, 4, 541-565, 2013

Author(s): S. Ostberg, W. Lucht, S. Schaphoff, and D. Gerten

Globally increasing temperatures may have unmanageable impacts on terrestrial, aquatic and marine ecosystems. Quantifying impacts worldwide and systematically as a function of global warming is critical to substantiate the ongoing international negotiations on climate mitigation targets. Here we present a macro-scale analysis of climate change impacts on terrestrial ecosystems based on newly developed sets of climate scenarios featuring a step-wise sampling of global mean temperature increase between 1.5 and 5 K by 2100. These are processed by a biogeochemical model (LPJmL) to derive an aggregated metric of simultaneous biogeochemical and structural shifts in land surface properties which we interpret as a proxy for the risk of shifts and possibly disruptions in ecosystems.

Our results show a substantial risk of climate change to transform terrestrial ecosystems profoundly. Nearly no area of the world is free from such risk, unless strong mitigation limits warming to around 2 degrees above preindustrial level. Even then, most climate models agree that up to one fifth of the land surface may experience at least moderate change, primarily at high latitudes and high altitudes. If countries fulfill their current emissions pledges, resulting in roughly 3.5 K of warming, this area expands to cover half the land surface, including the majority of tropical forests and savannas and the boreal zone. Due to differences in regional patterns of climate change the area potentially at risk of severe ecosystem change considering all AOGCMs is up to 2.5 times as large as for a single AOGCM.

The impact of nitrogen and phosphorous limitation on the estimated terrestrial carbon balance and warming of land use change over the last 156 yr

Wed, 08 May 2013 00:00:00 +0200

The impact of nitrogen and phosphorous limitation on the estimated terrestrial carbon balance and warming of land use change over the last 156 yr

Earth System Dynamics Discussions, 4, 507-539, 2013

Author(s): Q. Zhang, A. J. Pitman, Y. P. Wang, Y. Dai, and P. J. Lawrence

We examine the impact of land use and land cover change (LULCC) over the period from 1850 to 2005 using an Earth System Model that incorporates nitrogen and phosphorous limitation on the terrestrial carbon cycle. We compare the estimated CO₂ emissions and warming from land use change in a carbon only version of the model with those from simulations including nitrogen and phosphorous limitation. If we omit nutrients, our results suggest LULCC cools on the global average by about 0.1 °C. Including nutrients reduces this cooling to ~ 0.05 °C. Our results also suggest LULCC has a major impact on total land carbon over the period 1850–2005. In carbon only simulations, the inclusion of LULCC decreases the total additional land carbon stored in 2005 from around 210 Pg C to 85 Pg C. Including nitrogen and phosphorous limitation also decreases the scale of the terrestrial carbon sink to 80 Pg C. In particular, adding LULCC on top of the nutrient limited simulations changes the sign of the terrestrial carbon flux from a sink to a source (12 Pg C). The CO₂ emission from LULCC from 1850 to 2005 is estimated to be 130 Pg C for carbon only simulation, or 97 Pg C if nutrient limitation is accounted for in our model. The difference between these two estimates of CO₂ emissions from LULCC largely results from the weaker response of photosynthesis to increased CO₂ and smaller carbon pool sizes, and therefore lower carbon loss from plant and wood product carbon pools under nutrient limitation. We suggest that nutrient limitation should be accounted in simulating the effects of LULCC on the past climate and on the past and future carbon budget.

Agnotology: learning from mistakes

Fri, 03 May 2013 00:00:00 +0200

Agnotology: learning from mistakes

Earth System Dynamics Discussions, 4, 451-505, 2013

Author(s): R. E. Benestad, H. O. Hygen, R. van Dorland, J. Cook, and D. Nuccitelli

Replication is an important part of science, and by repeating past analyses, we show that a number of papers in the scientific literature contain severe methodological flaws which can easily be identified through simple tests and demonstrations. In many cases, shortcomings are related to a lack of robustness, leading to results that are not universally valid but rather an artifact of a particular experimental set-up. Some examples presented here have ignored data that do not fit the conclusions, and in several other cases, inappropriate statistical methods have been adopted or conclusions have been based on misconceived physics. These papers may serve as educational case studies for why certain analytical approaches sometimes are unsuitable in providing reliable answers. They also highlight the merit of replication. A lack of common replication has repercussions for the quality of the scientific literature, and may be a reason why some controversial questions remain unanswered even when ignorance could be reduced. Agnotology is the study of such ignorance. A free and open-source software is provided for demonstration purposes.

Trend of standardized precipitation index during Indian summer monsoon season in agroclimatic zones of India

Thu, 18 Apr 2013 00:00:00 +0200

Trend of standardized precipitation index during Indian summer monsoon season in agroclimatic zones of India

Earth System Dynamics Discussions, 4, 429-449, 2013

Author(s): S. Jha, V. K. Sehgal, R. Raghava, and M. Sinha

Standardized precipitation index (SPI) was computed with CRU TS3.0 gridded 0.5 × 0.5° monthly precipitation dataset for each of the 14 mainland agroclimatic zones (ACZs) of India for individual months (June, July, August and September) and season (JJAS) of summer monsoon for 56 yr (1951–2006). Mann Kendall Trend Test with the representative SPI of the ACZs shows that only six out of 14 mainland ACZs have a significant trend during summer monsoon. Trans-Gangetic plain significantly gains wetness during the month of June. West coast plain and hill has a typical feature of significant increasing trend of wetness during June and increasing dryness during July. In general Upper Gangetic plain, Middle Gangetic plain, Central plateau and hill and Eastern plateau and hill have a significantly increasing drying trend during the whole duration of summer monsoon season.

The sensitivity of the energy budget and hydrological cycle to CO₂ and solar forcing

Mon, 18 Mar 2013 00:00:00 +0100

The sensitivity of the energy budget and hydrological cycle to CO₂ and solar forcing

Earth System Dynamics Discussions, 4, 393-428, 2013

Author(s): N. Schaller, J. Cermak, M. Wild, and R. Knutti

The transient responses of the energy budget and the hydrological cycle to CO₂ and solar forcings of the same magnitude in a global climate model are quantified in this study. Idealized simulations are designed to test the assumption that the responses to forcings are linearly additive, i.e. whether the response to individual forcings can be added to estimate the response to the combined forcing, and to understand the physical processes occurring as a response to a surface warming caused by CO₂ or solar forcing increases of the same magnitude. For the global climate model considered, the responses of most variables of the energy budget and hydrological cycle, including surface temperature, do not add linearly. A separation of the response into a forcing and a feedback term shows that for precipitation, this non-linearity arises from the feedback term, i.e. from the non-linearity of the temperature response and the changes in the water cycle resulting from it. Further, changes in the energy budget show that less energy is available at the surface for global annual mean latent heat flux, and hence global annual mean precipitation, in simulations of transient CO₂ concentration increase compared to simulations with an equivalent transient increase in the solar constant. On the other hand, lower tropospheric water vapor increases more in simulations with CO₂ compared to solar forcing increase of the same magnitude. The response in precipitation is therefore more muted compared to the response in water vapor in CO₂ forcing simulations, leading to a larger increase in residence time of water vapor in the atmosphere compared to solar forcing simulations. Finally, energy budget calculations show that poleward atmospheric energy transport increases more in solar forcing compared to equivalent CO₂ forcing simulations, which is in line with the identified strong increase in large-scale precipitation in solar forcing scenarios.

Global modeling of withdrawal, allocation and consumptive use of surface water and groundwater resources

Wed, 27 Feb 2013 00:00:00 +0100

Global modeling of withdrawal, allocation and consumptive use of surface water and groundwater resources

Earth System Dynamics Discussions, 4, 355-392, 2013

Author(s): Y. Wada, D. Wisser, and M. F. P. Bierkens

To sustain growing food demand and increasing standard of living, global water withdrawal and consumptive water use have been increasing rapidly. To analyze the human perturbation on water resources consistently over a large scale, a number of macro-scale hydrological models (MHMs) have been developed over the recent decades. However, few models consider the feedback between water availability and water demand, and even fewer models explicitly incorporate water allocation from surface water and groundwater resources. Here, we integrate a global water demand model into a global water balance model, and simulate water withdrawal and consumptive water use over the period 1979–2010, considering water allocation from surface water and groundwater resources and explicitly taking into account feedbacks between supply and demand, using two re-analysis products: ERA-Interim and MERRA. We implement an irrigation water scheme, which works dynamically with daily surface and soil water balance, and include a newly available extensive reservoir data set. Simulated surface water and groundwater withdrawal show generally good agreement with available reported national and sub-national statistics. The results show a consistent increase in both surface water and groundwater use worldwide, but groundwater use has been increasing more rapidly than surface water use since the 1990s. Human impacts on terrestrial water storage (TWS) signals are evident, altering the seasonal and inter-annual variability. The alteration is particularly large over the heavily regulated basins such as the Colorado and the Columbia, and over the major irrigated basins such as the Mississippi, the Indus, and the Ganges. Including human water use generally improves the correlation of simulated TWS anomalies with those of the GRACE observations.

Can bioenergy cropping compensate high carbon emissions from large-scale deforestation of mid to high latitudes?

Wed, 20 Feb 2013 00:00:00 +0100

Can bioenergy cropping compensate high carbon emissions from large-scale deforestation of mid to high latitudes?

Earth System Dynamics Discussions, 4, 317-354, 2013

Author(s): P. Dass, C. Müller, V. Brovkin, and W. Cramer

Numerous studies have concluded that deforestation of mid to high latitudes result in a global cooling. This is mainly because of the increased albedo of deforested land which dominates over other biogeophysical and biogeochemical mechanisms in the energy balance. This dominance however may be due to an underestimation of the biogeochemical response, as carbon emissions are typically at or below the lower end of estimates. Here, we use the dynamic global vegetation model LPJmL for a better estimate of the carbon cycle under such large-scale deforestation. These studies are purely academic to understand the role of vegetation in the energy balance and the earth system. They must not be mistaken as possible mitigation options, because of the devastating effects on pristine ecosystems. We show that even optimistic assumptions on the manageability of these areas and its utilization for bioenergy crops could not make up for the strong carbon losses in connection with the losses of vegetation carbon and the long-term decline of soil carbon stocks. We find that the global biophysical bioenergy potential is 78.9 ± 7.9 EJ yr⁻¹ of primary energy at the end of the 21st century for the most plausible scenario. Due to avoided usage of fossil fuels over the time frame of this experiment, the cooling due to the biogeophysical feedback could be supplemented by an avoided warming of approximately 0.1 to 0.3 °C. However, the extensive deforestation simulated in this study causes an immediate emission of 182.3 ± 0.7 GtC followed by long term emissions. In the most plausible scenario, this carbon debt is not neutralized even if bioenergy production is assumed to be carbon-neutral other than for the land use emissions so that global temperatures would increase by ~0.2 to 0.6 °C by the end of the 21st century. The carbon dynamics in the high latitudes, especially with respect to permafrost dynamics and long-term carbon losses, require additional attention in the role for the Earth's carbon and energy budget.

Comparing projections of future changes in runoff and water resources from hydrological and ecosystem models in ISI-MIP

Wed, 13 Feb 2013 00:00:00 +0100

Comparing projections of future changes in runoff and water resources from hydrological and ecosystem models in ISI-MIP

Earth System Dynamics Discussions, 4, 279-315, 2013

Author(s): J. C. S. Davie, P. D. Falloon, R. Kahana, R. Dankers, R. Betts, F. T. Portmann, D. B. Clark, A. Itoh, Y. Masaki, K. Nishina, B. Fekete, Z. Tessler, X. Liu, Q. Tang, S. Hagemann, T. Stacke, R. Pavlick, S. Schaphoff, S. N. Gosling, W. Franssen, and N. Arnell

Projections of future changes in runoff can have important implications for water resources and flooding. In this study, runoff projections from ISI-MIP (Inter-sectoral Impact Model Intercomparison Project) simulations forced with HadGEM2-ES bias-corrected climate data under the Representative Concentration Pathway 8.5 have been analysed. Projections of change from the baseline period (1981–2010) to the future (2070–2099) from a number of different ecosystems and hydrological models were studied. The differences between projections from the two types of model were looked at globally and regionally. Typically, across different regions the ecosystem models tended to project larger increases and smaller decreases in runoff than the hydrological models. However, the differences varied both regionally and seasonally. Sensitivity experiments were also used to investigate the contributions of varying CO₂ and allowing vegetation distribution to evolve on projected changes in runoff. In two out of four models which had data available from CO₂ sensitivity experiments, allowing CO₂ to vary was found to increase runoff more than keeping CO₂ constant, while in two models runoff decreased. This suggests more uncertainty in runoff responses to elevated CO₂ than previously considered. As CO₂ effects on evapotranspiration via stomatal conductance and leaf-area index are more commonly included in ecosystems models than in hydrological models, this may partially explain some of the difference between model types. Keeping the vegetation distribution static in JULES runs had much less effect on runoff projections than varying CO₂, but this may be more pronounced if looked at over a longer timescale as vegetation changes may take longer to reach a new state.

Implications of accounting for land use in simulations of ecosystem services and carbon cycling in Africa

Thu, 07 Feb 2013 00:00:00 +0100

Implications of accounting for land use in simulations of ecosystem services and carbon cycling in Africa

Earth System Dynamics Discussions, 4, 235-278, 2013

Author(s): M. Lindeskog, A. Arneth, A. Bondeau, K. Waha, J. Seaquist, S. Olin, and B. Smith

Dynamic global vegetation models (DGVMs) are important tools for modelling impacts of global change on ecosystem services. However, current models often do not take full account of human land management and land use and land cover changes (LULCC). We integrated croplands and their management and natural vegetation recovery and succession following land use abandonment into the LPJ-GUESS DGVM. The revised model was applied to Africa as a case study to investigate the implications of accounting for land use on agricultural production, net ecosystem carbon balance (NECB) and on the general skill of the model in reproducing trends and patterns in vegetation structure and function. The seasonality of modelled monthly fraction of absorbed photosynthetically active radiation (FPAR) was shown to agree well with satellite-inferred normalised difference vegetation index (NDVI). In regions with a large proportion of cropland, the managed land addition improved the FPAR vs. NDVI fit significantly. Modelled 1991–1995 average yields for the seven most important African crops, representing potential optimal yields limited only by climate forcings, were generally higher than reported FAO yields by a factor of 2–6, similar to previous yield gap estimates. Modelled inter-annual yield variations during 1971–2005 generally agreed well with FAO statistics, especially in regions with pronounced climate seasonality. Modelled land-atmosphere carbon fluxes for Africa associated with land use change (0.09 PgC yr⁻¹ release to the atmosphere for the 1980s) agreed well with previous estimates. Cropland management options (residue removal, grass as cover crop) were shown to be of similar importance to the land-atmosphere carbon flux as land use change for the 20th century.

Comment on "Polynomial cointegration tests of anthropogenic impact on global warming" by Beenstock et al. (2012) – Some fallacies in econometric modelling of climate change

Wed, 06 Feb 2013 00:00:00 +0100

Comment on "Polynomial cointegration tests of anthropogenic impact on global warming" by Beenstock et al. (2012) – Some fallacies in econometric modelling of climate change

Earth System Dynamics Discussions, 4, 219-233, 2013

Author(s): D. F. Hendry and F. Pretis

We demonstrate major flaws in the statistical analysis of Beenstock et al. (2012), discrediting their initial claims as to the different degrees of integrability of CO₂ and temperature.

A theoretical framework for the net land-to-atmosphere CO₂ flux and its implications in the definition of "emissions from land-use change"

Thu, 31 Jan 2013 00:00:00 +0100

A theoretical framework for the net land-to-atmosphere CO₂ flux and its implications in the definition of "emissions from land-use change"

Earth System Dynamics Discussions, 4, 179-217, 2013

Author(s): T. Gasser and P. Ciais

We develop a theoretical framework and analysis of the net land-to-atmosphere CO₂ flux in order to discuss possible definitions of "emissions from land-use change". The terrestrial biosphere is affected by two perturbations: the perturbation of the global Carbon-Climate-Nitrogen system (CCN) with elevated atmospheric CO₂, climate change and nitrogen deposition; and the Land-Use Change perturbation (LUC). Here, we progressively establish mathematical definitions of four generic components of the net land-to-atmosphere CO₂ flux. The two first components are the fluxes that would be observed if only one perturbation occurred. The two other components are due to the coupling of the CCN and LUC perturbations, which shows the non-linear response of the terrestrial carbon cycle. Thanks to these four components, we introduce three possible definitions of "emissions from land-use change", that are indeed used in the scientific literature, often without clear distinctions, and we draw conclusions as for their absolute and relative behaviors. Thanks to the OSCAR v2 model, we provide quantitative estimates of the differences between the three definitions, and we find that comparing results from studies that do not use the same definition can lead to a bias of up to 20% between estimates of those emissions. After discussion of the limitations of the framework, we conclude on the three major points of this study that should help the community to reconcile modeling and observation of emissions from land-use change. The Appendix mainly provides more detailed mathematical expressions of the four components of the net land-to-atmosphere CO₂ flux.

Hydrological cycle over south and southeast Asian river basins as simulated by PCMDI/CMIP3 experiments

Tue, 22 Jan 2013 00:00:00 +0100

Hydrological cycle over south and southeast Asian river basins as simulated by PCMDI/CMIP3 experiments

Earth System Dynamics Discussions, 4, 109-177, 2013

Author(s): S. Hasson, V. Lucarini, and S. Pascale

We investigate how CMIP3 climate models describe the hydrological cycle over four major South and Southeast Asian river basins (Indus, Ganges, Brahmaputra and Mekong) for the XX, XXI, and XXII centuries. For the XX century, models' simulated water balance and total runoff quantities are neither consistent with the observed mean river discharges nor among the models. Most of the models underestimate the water balance for the Ganges, Brahmaputra and Mekong basin and overestimate it for the Indus basin. The only modest inter-model agreement is found for the Indus basin in terms of precipitation, evaporation and the strength of the hydrological cycle and for the Brahmaputra basin in terms of evaporation. While some models show inconsistencies for the Indus and the Ganges basins, most of the models seem to conserve water at the river basin scale up to a good degree of approximation. Models agree on a negative change of the water balance for Indus and a positive change in the strength of the hydrological cycle, whereas for Brahmaputra, Mekong and Ganges, most of the models project a positive change in both quantities. Most of the models foresee an increase in the inter-annual variability of the water balance for the Ganges and Mekong basins which is consistent with the projected changes in the Monsoon precipitation. No considerable future change in the inter-annual variability of water balance is found for the Indus basin, characterized by a more complex meteorology, because its precipitation regime is determined not only by the summer monsoon but also by the winter mid-latitude disturbances.

Radon monitoring as a possible indicator of tectonic events

Mon, 21 Jan 2013 00:00:00 +0100

Radon monitoring as a possible indicator of tectonic events

Earth System Dynamics Discussions, 4, 93-107, 2013

Author(s): V. I. Outkin, I. A. Kozlova, A. K. Yurkov, P. K. Dutta, O. P. Mishra, and M. K. Naskar

The proposed work describes research into the behavior of radon (VAR-volume activity of radon) excreted from the array. Radon migration and its selection from the array depends on the porosity, permeability and fractures in the array. A drastic change in the strength of an array and reset the elastic stresses by external forces (earthquake) occurs when certain changes in the structure of the array as the compressive and tensile strength of the array.

A trend-preserving bias correction – the ISI-MIP approach

Mon, 21 Jan 2013 00:00:00 +0100

A trend-preserving bias correction – the ISI-MIP approach

Earth System Dynamics Discussions, 4, 49-92, 2013

Author(s): S. Hempel, K. Frieler, L. Warszawski, J. Schewe, and F. Piontek

Statistical bias correction is commonly applied within climate impact modeling to correct climate model data for systematic deviations of the simulated historical data from observations. Methods are based on transfer functions generated to map the distribution of the simulated historical data to that of the observations. Those are subsequently applied to correct the future projections. Thereby the climate signal is modified in a way not necessarily preserving the trend of the original climate model data.

Here, we present the bias correction method that was developed within ISI-MIP, the first Inter-Sectoral Impact Model Intercomparison Project. ISI-MIP is designed to synthesise impact projections in the agriculture, water, biome, health, and infrastructure sectors at different levels of global warming. However, bias-corrected climate data that are used as input for the impact simulations could be only provided over land areas. To ensure consistency with the global (land + ocean) temperature information the bias correction method has to preserve the warming signal. Here we present the applied bias correction method that preserves the absolute changes in monthly temperature, and relative changes in monthly values of precipitation and the other variables needed for ISI-MIP.

The proposed methodology represents a modification of the transfer function approach applied in the Water Model Intercomparison Project (Water-MIP). Correction of the monthly mean is followed by correction of the daily variability about the monthly mean.

Estimation of the climate feedback parameter by using radiative fluxes from CERES EBAF

Wed, 09 Jan 2013 00:00:00 +0100

Estimation of the climate feedback parameter by using radiative fluxes from CERES EBAF

Earth System Dynamics Discussions, 4, 25-47, 2013

Author(s): P. Björnbom

Top-of-the-Atmosphere (TOA) net radiative flux anomalies from Clouds and Earth's Radiant Energy Systems (CERES) Energy Balanced and Filled (EBAF) and surface air temperature anomalies from HadCRUT3 were compared for the time interval September 2000–May 2011. In a phase plane plot with the radiative flux anomalies lagging the temperature anomalies with 7 months the phase plane curve approached straight lines during about an eight months long period at the beginning and a five year period at the end of the interval. Both of those periods, but more clearly the latter one, could be connected to the occurrence of distinct El Niño Southern Oscillation (ENSO) episodes. This result is explained by using a hypothesis stating that non-radiative forcing connected to the ENSO is dominating the temperature changes during those two periods and that there is a lag between the temperature change and the radiative flux feedback. According to the hypothesis the slopes of the straight lines equal the value of the climate feedback parameter. By linear regression based on the mentioned five year period the value of the climate feedback parameter was estimated to 5.5 ± 0.6 W m⁻² K⁻¹ (± two standard errors).

Consistent increase in Indian monsoon rainfall and its variability across CMIP-5 models

Mon, 07 Jan 2013 00:00:00 +0100

Consistent increase in Indian monsoon rainfall and its variability across CMIP-5 models

Earth System Dynamics Discussions, 4, 1-24, 2013

Author(s): A. Menon, A. Levermann, J. Schewe, J. Lehmann, and K. Frieler

The possibility of an impact of global warming on the Indian monsoon is of critical importance for the large population of this region. Future projections within the Coupled Model Intercomparison Project Phase 3 (CMIP-3) showed a wide range of trends with varying magnitude and sign across models. Here the Indian summer monsoon rainfall is evaluated in 20 CMIP-5 models for the period 1850 to 2100. In the new generation of climate models a consistent increase in seasonal mean rainfall during the summer monsoon periods arises. All models simulate stronger seasonal mean rainfall in the future compared to the historic period under the strongest warming scenario RCP-8.5. Increase in seasonal mean rainfall is the largest for the RCP-8.5 scenario compared to other RCPs. The interannual variability of the Indian monsoon rainfall also shows a consistent positive trend under unabated global warming. Since both the long-term increase in monsoon rainfall as well as the increase in interannual variability in the future is robust across a wide range of models, some confidence can be attributed to these projected trends.

Dynamical and biogeochemical control on the decadal variability of ocean carbon fluxes

Fri, 21 Dec 2012 00:00:00 +0100

Dynamical and biogeochemical control on the decadal variability of ocean carbon fluxes

Earth System Dynamics Discussions, 3, 1347-1389, 2012

Author(s): R. Séférian, L. Bopp, D. Swingedouw, and J. Servonnat

Several recent observation-based studies suggest that ocean anthropogenic carbon uptake has slowed down due to the impact of anthropogenic forced climate change. However, it remains unclear if detected changes over the recent time period can really be attributed to anthropogenic climate change or to natural climate variability (internal plus naturally forced variability). One large uncertainty arises from the lack of knowledge on ocean carbon flux natural variability at the decadal time scales. To gain more insights into decadal time scales, we have examined the internal variability of ocean carbon fluxes in a 1000-yr long preindustrial simulation performed with the Earth System Model IPSL-CM5A-LR. Our analysis shows that ocean carbon fluxes exhibit low-frequency oscillations that emerge from their year-to-year variability in the North Atlantic, the North Pacific, and the Southern Ocean. In our model, a 20-yr mode of variability in the North Atlantic air-sea carbon flux is driven by sea surface temperature variability and accounts for ~40% of the interannual regional variance. The North Pacific and the Southern Ocean carbon fluxes are also characterized by decadal to multi-decadal modes of variability (10 to 50 yr) that account for 30–40% of the interannual regional variance. But these modes are driven by the vertical supply of dissolved inorganic carbon through the variability of Ekman-induced upwelling and deep-mixing events. Differences in drivers of regional modes of variability stem from the coupling between ocean dynamics variability and the ocean carbon distribution, which is set by large-scale secular ocean circulation.

Climate change impact on available water resources obtained using multiple global climate and hydrology models

Tue, 04 Dec 2012 00:00:00 +0100

Climate change impact on available water resources obtained using multiple global climate and hydrology models

Earth System Dynamics Discussions, 3, 1321-1345, 2012

Author(s): S. Hagemann, C. Chen, D. B. Clark, S. Folwell, S. N. Gosling, I. Haddeland, N. Hanasaki, J. Heinke, F. Ludwig, F. Voß, and A. J. Wiltshire

Climate change is expected to alter the hydrological cycle resulting in large-scale impacts on water availability. However, future climate change impact assessments are highly uncertain. For the first time, multiple global climate (three) and hydrological models (eight) were used to systematically assess the hydrological response to climate change and project the future state of global water resources. The results show a large spread in projected changes in water resources within the climate–hydrology modelling chain for some regions. They clearly demonstrate that climate models are not the only source of uncertainty for hydrological change. But there are also areas showing a robust change signal, such as at high latitudes and in some mid-latitude regions, where the models agree on the sign of projected hydrological changes, indicative of higher confidence. In many catchments an increase of available water resources is expected but there are some severe decreases in central and Southern Europe, the Middle East, the Mississippi river basin, Southern Africa, Southern China and south eastern Australia.

Quantifying drivers of chemical disequilibrium in the Earth's atmosphere

Fri, 23 Nov 2012 00:00:00 +0100

Quantifying drivers of chemical disequilibrium in the Earth's atmosphere

Earth System Dynamics Discussions, 3, 1287-1320, 2012

Author(s): E. Simoncini, N. Virgo, and A. Kleidon

It has long been observed that Earth's atmosphere is uniquely far from its thermochemical equilibrium state in terms of its chemical composition. Studying this state of disequilibrium is important both for understanding the role that life plays in the Earth system, and for its potential role in the detection of life on exoplanets. Here we present a methodology for assessing the strength of the biogeochemical cycling processes that drive disequilibrium in planetary systems. We apply it to the simultaneous presence of CH₄ and O₂ in Earth's atmosphere, which has long been suggested as a sign of life that could be detected from far away. Using a simplified model, we identify that the most important property to quantify is not the distance from equilibrium, but the power required to drive it. A weak driving force can maintain a high degree of disequilibrium if the residence times of the compounds involved are long; but if the disequilibrium is high and the kinetics fast, we can conclude that the disequilibrium must be driven by a substantial source of energy. Applying this to Earth's atmosphere, we show that the biotically-generated portion of the power required to maintain the methane-oxygen disequilibrium is around 0.67 TW, although the uncertainty in this figure is about 50% due to uncertainty in the global CH₄ production. Compared to the chemical energy generated by the biota by photosynthesis, 0.67 TW represents only a very small fraction and, perhaps surprisingly, is of a comparable magnitude to abiotically-driven geochemical processes at the Earth's surface. We discuss the implications of this new approach, both in terms of enhancing our understanding of the Earth system, and in terms of its impact on the possible detection of distant photosynthetic biospheres.

Do GCM's predict the climate... or macroweather?

Thu, 22 Nov 2012 00:00:00 +0100

Do GCM's predict the climate... or macroweather?

Earth System Dynamics Discussions, 3, 1259-1286, 2012

Author(s): S. Lovejoy, D. Schertzer, and D. Varon

We are used to the weather – climate dichotomy, yet the great majority of the spectral variance of atmospheric fields is in the continuous "background" and this defines instead a trichotomy with a "macroweather" regime in the intermediate range ≈10 days to 30 yr. In the weather, macroweather and climate regimes, exponents characterize the type of variability over the entire ranges and it is natural to identify them with qualitatively different synergies of nonlinear dynamical mechanisms that repeat scale after scale. Since climate models are essentially meteorological models (although with extra couplings) it is thus important to determine whether they currently model all three regimes. Using Last Millennium simulations from four GCM's, we show that control runs only reproduce macroweather and that runs with various (reconstructed) climate forcings do somewhat better but have overly weak multicentennial variabilities. A possible explanation is that the models lack – or inadequately treat – important slow "climate" processes such as land-ice or deep ocean dynamics.