MAGICC projects

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Out of the dozens of scientific publications that used MAGICC, this page provides you with a small sample of those. They are listed in chronological order.

  1. 1992: T. M. L. Wigley and S. C. B. Raper "Implications for climate and sea level of revised IPCC emissions scenarios", Nature, 357 293-300 Abstract: A new set of greenhouse gas emissions scenarios has been produced by the Intergovernmental Panel on Climate Change (IPCC). Incorporating these into models that also include the effects of C02 fertilization, feedback from stratospheric ozone depletion and the radiative effects of sulphate aerosols yields new projections for radiative forcing of climate and for changes in global-mean temperature and sea level. Changes in temperature and sea level are predicted to be less severe than those estimated previously, but are still far beyond the limits of natural variability.
  2. 1993: T. M. L. Wigley "Balancing the Carbon Budget - Implications for Projections of Future Carbon-Dioxide Concentration Changes", Tellus Series B-Chemical and Physical Meteorology, 45 (5), 409-425 Abstract: A carbon cycle model is described incorporating CO2 fertilization feedback and a convolution ocean model that allows the atmosphere-to-ocean flux to be varied. The main parameters controlling the model's behaviour are a fertilization feedback parameter (r) and an ocean flux scaling factor (characterized by the mean carbon flux into the ocean over the 1980s, F(1980s)). Since the model's 1980s-mean net land-use-change flux (D(n)(1980s)) is a unique function of r and F(1980s), the model's behaviour can also be characterized by specifying D(n)(1980s) (instead of r) and F(1980s). The history of past land-use fluxes, D(n)(t), is derived by inverse modelling for a range of values of F(1980s) (1.0-3.0 GtC/yr) and D(n)(1980s) (0.6-2.6 GtC/yr). Even with this flexibility, the resultant D(n)(t) differs markedly from the observationally-based record of Houghton, particularly before 1950. The inverse calculations are used to determine the history of the so-called missing sink, as implied directly by the model and by the observationally-based record of D(n)(t), for a range of ocean uptake efficiencies as defined by F(1980s). Projections of future CO2 concentration changes are made for the 6 emissions scenarios recently produced by the Intergovernmental Panel on Climate Change (IS92a-f). The ability to specify F(1980s) and D.(1980s) allows one to account for the missing sink in a variety of ways, and to account for uncertainties in the amount of missing carbon. This leads to a range of projections and provides some insights into the uncertainties surrounding these projections.
  3. 1995: T. M. L. Wigley "Global Mean Temperature and Sea-Level Consequences of Greenhouse-Gas Concentration Stabilization", Geophysical Research Letters, 22 (1), 45-48 Abstract: The Intergovernmental Panel on Climate Change (IPCC) has defined a set of scenarios for future CO2 concentrations stabilizing at levels of 350 to 750 ppmv. Using models previously employed by IPCC, the implied global-mean temperature and sea level changes are calculated out to 2500. While uncertainties are large, the results show that even with concerted efforts to stabilize concentrations of greenhouse gases, substantial temperature and sea level increases can be expected to occur over the next century. Increases ia sea level are likely to continue for many centuries after concentration stabilization because of the extremely long time scales associated with the deep ocean (which influences thermal expansion) and with the large ice sheets of Greenland and Antarctica.
  4. 1996: S. C. B. Raper and U. Cubasch "Emulation of the results from a coupled general circulation model using a simple climate model", Geophysical Research Letters, 23 (10), 1107-1110 Abstract: An upwelling diffusion (UD) model has been fitted to the results of a globally coupled ocean atmosphere general circulation model (O/AGCM). In order to adequately simulate the results of the O/AGCM, the differential heating over land and ocean, the change of the upwelling rate with rising mixed-layer temperature and the climate adjustment of the O/AGCM evident in the control simulation had to be taken into account. Comparisons using the results of four O/AGCM perturbation experiments show that the timing of the adjustment in the O/AGCM control run differs from that in the perturbed runs. The UD model simulates the global mean temperature evolution, as calculated by the O/AGCM reasonably well. However, when time dependent upwelling is included, the UD model substantially overestimates the thermal expansion compared to this O/AGCM. Possible reasons for this overestimation have been identified.
  5. 1996: S. C. B. Raper, T. M. L. Wigley and R. A. Warrick "Global Sea-level Rise: Past and Future", Sea-Level Rise and Coastal Subsidence: Causes, Consequences and Strategies, 11-45
  6. 1997: T. M. L. Wigley "Implications of recent CO2 emission-limitation proposals for stabilization of atmospheric concentrations", Nature, 390 267-270
  7. 1998: T. M. L. Wigley "The Kyoto Protocol: CO2, CH4 and climate implications", Geophysical Research Letters, 25 (13), 2285-2288 Abstract: Kyoto Protocol implications for CO2, temperature and sea level are examined. Three scenarios for post-Kyoto emissions reductions are considered. In all cases, the longterm consequences are small. The limitations specified under the Protocol are interpreted in terms of both CO2 and CH4 emissions reductions and a new emissions comparison index, the Forcing Equivalence Index (FEI), is introduced. The use of GWPs to assess CO2-equivalence is assessed.
  8. 2000: M. Hulme, T. Wigley, E. Barrow, S. Raper, A. Centella, S. Smith and A. Chipanshi "Using a Climate Scenario Generator for Vulnerability and Adaptaion Assessments: MAGICC and SCENGEN Version 2.4 Workbook", 52, available online
  9. 2001: S. C. B. Raper, J. M. Gregory and T. J. Osborn "Use of an upwelling-diffusion energy balance climate model to simulate and diagnose A/OGCM results", Climate Dynamics, 17 (8), 601-613 Abstract: We demonstrate that a hemispherically averaged upwelling- diffusion energy-balance climate model (UD/EBM) can emulate the surface air temperature change and sea-level rise due to thermal expansion, predicted by the HadCM2 coupled atmosphere- ocean general circulation model, for various scenarios of anthropogenic radiative forcing over 1860-2100. A climate sensitivity of 2.6 degreesC is assumed, and a representation of the effect of sea-ice retreat on surface air temperature is required. In an extended experiment, with CO2 concentration held constant at twice the control run value, the HadCM2 effective climate sensitivity is found to increase from about 2.0 degreesC at the beginning of the integration to 3.85 degreesC after 900 years. The sea-level rise by this time is almost 1.0 m and the rate of rise fairly steady, implying that the final equilibrium value (the 'commitment') is large. The base UD/EBM can fit the 900-year simulation of surface temperature change and thermal expansion provided that the time-dependent climate sensitivity is specified, but the vertical profile of warming in the ocean is not well reproduced. The main discrepancy is the relatively large mid- depth warming in the HadCM2 ocean, that can be emulated by (1) diagnosing depth-dependent diffusivities that increase through time; (2) diagnosing depth-dependent diffusivities for a pure- diffusion (zero upwelling) model; or (3) diagnosing higher depth-dependent diffusivities that are applied to temperature pertubarions only. The latter two models can be run to equilibrium, and with a climate sensitivity of 3.85 degreesC, they give sea-level rise commitments of 1.7 m and 1.3 m, respectively.
  10. 2001: T. M. L. Wigley and S. C. B. Raper "Interpretation of high projections for global-mean warming", Science, 293 (5529), 451-454 Abstract: The Intergovernmental Panel on Climate Change (IPCC) has recently released its Third Assessment Report (TAR), in which new projections are given for global-mean warming in the absence of policies to limit climate change. The full warming range over 1990 to 2100, 1.4 degrees to 5.8 degreesC, is substantially higher than the range given previously in the IPCC Second Assessment Report. Here we interpret the new warming range in probabilistic terms, accounting for uncertainties in emissions, the climate sensitivity, the carbon cycle, ocean mixing, and aerosol forcing. We show that the probabilities of warming values at both the high and Low ends of the TAR range are very Low. In the absence of climate- mitigation policies, the 90% probability interval for 1990 to 2100 warming is 1.7 degrees to 4.9 degreesC.
  11. 2002: S. C. B. Raper, J. M. Gregory and R. J. Stouffer "The Role of Climate Sensitivity and Ocean Heat Uptake on AOGCM Transient Temperature Response", Journal of Climate, 15 124-130 Abstract: The role of climate sensitivity and ocean heat uptake in determining the range of climate model response is investigated in the second phase of the Coupled Model Intercomparison Project (CMIP2) AOGCM results. The fraction of equilibrium warming that is realized at any one time is less in those models with higher climate sensitivity, leading to a reduction in the temperature response range at the time of CO2 doubling [transient climate response (TCR) range]. The range is reduced by a further 15% because of an apparent relationship between climate sensitivity and the efficiency of ocean heat uptake. Some possible physical causes for this relationship are suggested.
  12. 2002: T. M. L. Wigley and S. C. B. Raper "Reasons for larger warming projections in the IPCC Third Assessment Report", Journal of Climate, 15 (20), 2945-2952 Abstract: Projections of future warming in the Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report (TAR) are substantially larger than those in the Second Assessment Report (SAR). The reasons for these differences are documented and quantified. Differences are divided into differences in the emissions scenarios and differences in the science (gas cycle, forcing, and climate models). The main source of emissions-related differences in warming is aerosol forcing, primarily due to large differences in SO2 emissions between the SAR and TAR scenarios. For any given emissions scenario, concentration projections based on SAR and TAR science are similar, except for methane at high emissions levels where TAR science leads to substantially lower concentrations. The new (TAR) science leads to slightly lower total forcing and slightly larger warming. At the low end of the warming range the effects of the new science and the new emissions scenarios are roughly equal. At the high end, TAR science has a smaller effect and the main reason for larger TAR warming is the use of a different high-end emissions scenario, primarily changes in SO2 emissions.
  13. 2004: S. C. B. Raper "Interpretation of Model Results that Show Changes in the Effective Climate Sensitivity with Time", IPCC Workshop on Climate Sensitivity, 131-133
  14. 2005: T. M. L. Wigley "The climate change commitment", Science, 307 (5716), 1766-1769 Abstract: Even if atmospheric composition were fixed today, global-mean temperature and sea level rise would continue due to oceanic thermal inertia. These constant-composition (CC) commitments and their uncertainties are quantified. Constant-emissions (CE) commitments are also considered. The CC warming commitment could exceed 1 degrees C. The CE warming commitment is 2 degrees to 6 degrees C by the year 2400. For sea level rise, the CC commitment is 10 centimeters per century (extreme range approximately 1 to 30 centimeters per century) and the CE commitment is 25 centimeters per century (7 to 50 centimeters per century). Avoiding these changes requires, eventually, a reduction in emissions to substantially below present levels. For sea level rise, a substantial long-term commitment may be impossible to avoid.
  15. 2005: T. M. L. Wigley and S. C. B. Raper "Extended scenarios for glacier melt due to anthropogenic forcing", Geophysical Research Letters, 32 (5) Abstract: The IPCC Third Assessment Report (TAR) developed a formula for the global meltwater contribution to sea level rise from Glaciers and Small Ice Caps (GSICs) that is applicable out to 2100. We show that, if applied to times beyond 2100 (as is necessary to assess sea level rise for concentration-stabilization scenarios), the formula imposes an unrealistic upper bound on GSIC melt. A modification is introduced that allows the formula to be extended beyond 2100 with asymptotic melt equal to the initially available ice volume (V-0). The modification has a negligible effect on the original TAR formulation out to 2100 and provides support for the IPCC method over this time period. We examine the sensitivity of GSIC melt to uncertainties in V0 and mass balance sensitivity, and give results for a range of CO2 concentration stabilization cases. Approximately 73-94% of GSIC ice is lost by 2400.
  16. 2005: Wigley, T. M. L. "The climate change commitment." Science 307(5716): 1766-1769. available online Abstract: Even if atmospheric composition were fixed today, global-mean temperature and sea level rise would continue due to oceanic thermal inertia. These constant-composition (CC) commitments and their uncertainties are quantified. Constant-emissions (CE) commitments are also considered. The CC warming commitment could exceed 1°C. The CE warming commitment is 2° to 6°C by the year 2400. For sea level rise, the CC commitment is 10 centimeters per century (extreme range approximately 1 to 30 centimeters per century) and the CE commitment is 25 centimeters per century (7 to 50 centimeters per century). Avoiding these changes requires, eventually, a reduction in emissions to substantially below present levels. For sea level rise, a substantial long-term commitment may be impossible to avoid.
  17. 2006: T. J. Osborn, S. C. B. Raper and K. R. Briffa "Simulated climate change during the last 1000 years: comparing the ECHO-G general circulation model with the MAGICC simple climate model. ", Climate Dynamics, 27 185-197, doi:10.1007/s00382-006-0129-5 available online Abstract: An intercomparison of eight climate simulations, each driven with estimated natural and anthropogenic forcings for the last millennium, indicates that the so-called “Erik” simulation of the ECHO-G coupled ocean-atmosphere climate model exhibits atypical behaviour. The ECHO-G simulation has a much stronger cooling trend from 1000 to 1700 and a higher rate of warming since 1800 than the other simulations, with the result that the overall amplitude of millennial-scale temperature variations in the ECHO-G simulation is much greater than in the other models. The MAGICC (Model for the Assessment of Greenhouse-gas-Induced Climate Change) simple climate model is used to investigate possible causes of this atypical behaviour. It is shown that disequilibrium in the initial conditions probably contributes spuriously to the cooling trend in the early centuries of the simulation, and that the omission of tropospheric sulphate aerosol forcing is the likely explanation for the anomalously large recent warming. The simple climate model results are used to adjust the ECHO-G Erik simulation to mitigate these effects, which brings the simulation into better agreement with the other seven models considered here and greatly reduces the overall range of temperature variations during the last millennium simulated by ECHO-G. Smaller inter-model differences remain which can probably be explained by a combination of the particular forcing histories and model sensitivities of each experiment. These have not been investigated here, though we have diagnosed the effective climate sensitivity of ECHO-G to be 2.39±0.11 K for a doubling of CO2.
  18. 2007: T. M. L. Wigley, R. Richels and J. Edmonds "Overshoot pathways to CO2 stabilization in a multi-gas context", Human Induced Climate Change: An Interdisciplinary Assessment, 84-92
  19. 2009: M. Meinshausen, N. Meinshausen, W. Hare, S. C. B. Raper, K. Frieler, R. Knutti, D. J. Frame and M. R. Allen "Greenhouse-gas emission targets for limiting global warming to 2°C", Nature, 458 (7242), 1158 Abstract: More than 100 countries have adopted a global warming limit of 2?°C or below (relative to pre-industrial levels) as a guiding principle for mitigation efforts to reduce climate change risks, impacts and damages1, 2. However, the greenhouse gas (GHG) emissions corresponding to a specified maximum warming are poorly known owing to uncertainties in the carbon cycle and the climate response. Here we provide a comprehensive probabilistic analysis aimed at quantifying GHG emission budgets for the 2000–50 period that would limit warming throughout the twenty-first century to below 2?°C, based on a combination of published distributions of climate system properties and observational constraints. We show that, for the chosen class of emission scenarios, both cumulative emissions up to 2050 and emission levels in 2050 are robust indicators of the probability that twenty-first century warming will not exceed 2?°C relative to pre-industrial temperatures. Limiting cumulative CO2 emissions over 2000–50 to 1,000?Gt CO2 yields a 25% probability of warming exceeding 2?°C—and a limit of 1,440 Gt CO2 yields a 50% probability—given a representative estimate of the distribution of climate system properties. As known 2000–06 CO2 emissions3 were ~234?Gt CO2, less than half the proven economically recoverable oil, gas and coal reserves4, 5, 6 can still be emitted up to 2050 to achieve such a goal. Recent G8 Communiqués7 envisage halved global GHG emissions by 2050, for which we estimate a 12–45% probability of exceeding 2?°C—assuming 1990 as emission base year and a range of published climate sensitivity distributions. Emissions levels in 2020 are a less robust indicator, but for the scenarios considered, the probability of exceeding 2?°C rises to 53–87% if global GHG emissions are still more than 25% above 2000 levels in 2020.
  20. 2010: J. Rogelj, C. Chen, J. Nabel, K. Macey, W. Hare, M. Schaeffer, K. Markmann, N. Hohne, K. K. Andersen and M. Meinshausen "Analysis of the Copenhagen Accord pledges and its global climatic impacts-a snapshot of dissonant ambitions", Environmental Research Letters, 5 (3) Abstract: This analysis of the Copenhagen Accord evaluates emission reduction pledges by individual countries against the Accord's climate-related objectives. Probabilistic estimates of the climatic consequences for a set of resulting multi-gas scenarios over the 21st century are calculated with a reduced complexity climate model, yielding global temperature increase and atmospheric CO2 and CO2-equivalent concentrations. Provisions for banked surplus emission allowances and credits from land use, land-use change and forestry are assessed and are shown to have the potential to lead to significant deterioration of the ambition levels implied by the pledges in 2020. This analysis demonstrates that the Copenhagen Accord and the pledges made under it represent a set of dissonant ambitions. The ambition level of the current pledges for 2020 and the lack of commonly agreed goals for 2050 place in peril the Accord's own ambition: to limit global warming to below 2 degrees C, and even more so for 1.5 degrees C, which is referenced in the Accord in association with potentially strengthening the long-term temperature goal in 2015. Due to the limited level of ambition by 2020, the ability to limit emissions afterwards to pathways consistent with either the 2 or 1.5 degrees C goal is likely to become less feasible.
  21. 2010: A. Reisinger, M. Meinshausen, M. Manning and G. Bodeker "Uncertainties of global warming metrics: CO2 and CH4", Geophys. Res. Lett., 37 (14), L14707, 10.1029/2010gl043803 Abstract: We present a comprehensive evaluation of uncertainties in the Global Warming Potential (GWP) and Global Temperature Change Potential (GTP) of CH4, using a simple climate model calibrated to AOGCMs and coupled climate-carbon cycle models assessed in the IPCC Fourth Assessment Report (AR4). In addition, we estimate uncertainties in these metrics probabilistically by using a method that does not rely on AOGCMs but instead builds on historical constraints and uncertainty estimates of current radiative forcings. While our mean and median GWPs and GTPs estimates are consistent with previous studies, our analysis suggests that uncertainty ranges for GWPs are almost twice as large as estimated in the AR4. Relative uncertainties for GTPs are larger than for GWPs, nearly twice as high for a time horizon of 100 years. Given this uncertainty, our results imply the possibility for substantial future adjustments in best-estimate values of GWPs and in particular GTPs.
  22. 2011: A. Hof, C. Hope, J. Lowe, M. Mastrandrea, M. Meinshausen and D. van Vuuren "The benefits of climate change mitigation in integrated assessment models: the role of the carbon cycle and climate component", Climatic Change, 1-21, 10.1007/s10584-011-0363-7 (online first) Abstract: Integrated Assessment Models (IAMs) are an important tool to compare the costs and benefits of different climate policies. Recently, attention has been given to the effect of different discounting methods and damage estimates on the results of IAMs. One aspect to which little attention has been paid is how the representation of the climate system may affect the estimated benefits of mitigation action. In that respect, we analyse several well-known IAMs, including the newest versions of FUND, DICE and PAGE. Given the role of IAMs in integrating information from different disciplines, they should ideally represent both best estimates and the ranges of anticipated climate system and carbon cycle behaviour (as e.g. synthesised in the IPCC Assessment reports). We show that in the longer term, beyond 2100, most IAM parameterisations of the carbon cycle imply lower CO 2 concentrations compared to a model that captures IPCC AR4 knowledge more closely, e.g. the carbon-cycle climate model MAGICC6. With regard to the climate component, some IAMs lead to much lower benefits of mitigation than MAGICC6. The most important reason for the underestimation of the benefits of mitigation is the failure in capturing climate dynamics correctly, which implies this could be a potential development area to focus on.
  23. 2011: M. Meinshausen, S. C. B. Raper and T. M. L. Wigley "Emulating coupled atmosphere-ocean and carbon cycle models with a simpler model, MAGICC6: Part I – Model Description and Calibration", Atmospheric Chemistry and Physics 11 1417-1456, available online Abstract: Current scientific knowledge on the future response of the climate system to human-induced perturbations is comprehensively captured by various model intercomparison efforts. In the preparation of the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC), intercomparisons were organized for atmosphere-ocean general circulation models (AOGCMs) and carbon cycle models, named "CMIP3" and "C4MIP", respectively. Despite their tremendous value for the scientific community and policy makers alike, there are some difficulties in interpreting the results. For example, radiative forcings were not standardized across the various AOGCM integrations and carbon cycle runs, and, in some models, key forcings were omitted. Furthermore, the AOGCM analysis of plausible emissions pathways was restricted to only three SRES scenarios. This study attempts to address these issues. We present an updated version of MAGICC, the simple carbon cycle-climate model used in past IPCC Assessment Reports with enhanced representation of time-varying climate sensitivities, carbon cycle feedbacks, aerosol forcings and ocean heat uptake characteristics. This new version, MAGICC6, is successfully calibrated against the higher complexity AOGCMs and carbon cycle models. Parameterizations of MAGICC6 are provided. The mean of the emulations presented here using MAGICC6 deviates from the mean AOGCM responses by only 2.2% on average for the SRES scenarios. This enhanced emulation skill in comparison to previous calibrations is primarily due to: making a "like-with-like comparison" using AOGCM-specific subsets of forcings; employing a new calibration procedure; as well as the fact that the updated simple climate model can now successfully emulate some of the climate-state dependent effective climate sensitivities of AOGCMs. The diagnosed effective climate sensitivity at the time of CO2 doubling for the AOGCMs is on average 2.88 °C, about 0.33 °C cooler than the mean of the reported slab ocean climate sensitivities. In the companion paper (Part 2) of this study, we examine the combined climate system and carbon cycle emulations for the complete range of IPCC SRES emissions scenarios and the new RCP pathways.
  24. 2011: M. Meinshausen, S. Smith, K. Calvin, J. Daniel, M. Kainuma, J. F. Lamarque, K. Matsumoto, S. Montzka, S. Raper, K. Riahi, A. Thomson, G. Velders and D. P. van Vuuren "The RCP greenhouse gas concentrations and their extensions from 1765 to 2300", Climatic Change, 109 (1), 213-241, 10.1007/s10584-011-0156-z available online. Abstract: We present the greenhouse gas concentrations for the Representative Concentration Pathways (RCPs) and their extensions beyond 2100, the Extended Concentration Pathways (ECPs). These projections include all major anthropogenic greenhouse gases and are a result of a multi-year effort to produce new scenarios for climate change research. We combine a suite of atmospheric concentration observations and emissions estimates for greenhouse gases (GHGs) through the historical period (1750–2005) with harmonized emissions projected by four different Integrated Assessment Models for 2005–2100. As concentrations are somewhat dependent on the future climate itself (due to climate feedbacks in the carbon and other gas cycles), we emulate median response characteristics of models assessed in the IPCC Fourth Assessment Report using the reduced-complexity carbon cycle climate model MAGICC6. Projected ‘best-estimate’ global-mean surface temperature increases (using inter alia a climate sensitivity of 3°C) range from 1.5°C by 2100 for the lowest of the four RCPs, called both RCP3-PD and RCP2.6, to 4.5°C for the highest one, RCP8.5, relative to pre-industrial levels. Beyond 2100, we present the ECPs that are simple extensions of the RCPs, based on the assumption of either smoothly stabilizing concentrations or constant emissions: For example, the lower RCP2.6 pathway represents a strong mitigation scenario and is extended by assuming constant emissions after 2100 (including net negative CO 2 emissions), leading to CO 2 concentrations returning to 360 ppm by 2300. We also present the GHG concentrations for one supplementary extension, which illustrates the stringent emissions implications of attempting to go back to ECP4.5 concentration levels by 2250 after emissions during the 21 st century followed the higher RCP6 scenario. Corresponding radiative forcing values are presented for the RCP and ECPs.
  25. 2011: M. Meinshausen, T. M. L. Wigley and S. C. B. Raper "Emulating atmosphere-ocean and carbon cycle models with a simpler model, MAGICC6: Part 2– Applications", Atmospheric Chemistry and Physics 11 1457-1471, available online. Abstract: Intercomparisons of coupled atmosphere-ocean general circulation models (AOGCMs) and carbon cycle models are important for galvanizing our current scientific knowledge to project future climate. Interpreting such intercomparisons faces major challenges, not least because different models have been forced with different sets of forcing agents. Here, we show how an emulation approach with MAGICC6 can address such problems. In a companion paper (Meinshausen et al., 2011a), we show how the lower complexity carbon cycle-climate model MAGICC6 can be calibrated to emulate, with considerable accuracy, globally aggregated characteristics of these more complex models. Building on that, we examine here the Coupled Model Intercomparison Project's Phase 3 results (CMIP3). If forcing agents missed by individual AOGCMs in CMIP3 are considered, this reduces ensemble average temperature change from pre-industrial times to 2100 under SRES A1B by 0.4 °C. Differences in the results from the 1980 to 1999 base period (as reported in IPCC AR4) to 2100 are negligible, however, although there are some differences in the trajectories over the 21st century. In a second part of this study, we consider the new RCP scenarios that are to be investigated under the forthcoming CMIP5 intercomparison for the IPCC Fifth Assessment Report. For the highest scenario, RCP8.5, relative to pre-industrial levels, we project a median warming of around 4.6 °C by 2100 and more than 7 °C by 2300. For the lowest RCP scenario, RCP3-PD, the corresponding warming is around 1.5 °C by 2100, decreasing to around 1.1 °C by 2300 based on our AOGCM and carbon cycle model emulations. Implied cumulative CO2 emissions over the 21st century for RCP8.5 and RCP3-PD are 1881 GtC (1697 to 2034 GtC, 80% uncertainty range) and 381 GtC (334 to 488 GtC), when prescribing CO2 concentrations and accounting for uncertainty in the carbon cycle. Lastly, we assess the reasons why a previous MAGICC version (4.2) used in IPCC AR4 gave roughly 10% larger warmings over the 21st century compared to the CMIP3 average. We find that forcing differences and the use of slightly too high climate sensitivities inferred from idealized high-forcing runs were the major reasons for this difference.
  26. 2011: A. Reisinger, M. Meinshausen, M. Manning and G. Bodeker "Future changes in global warming potentials under representative concentration pathways", Environmental Research Letters, 6 (2), 024020 Abstract: Global warming potentials (GWPs) are the metrics currently used to compare emissions of different greenhouse gases under the United Nations Framework Convention on Climate Change. Future changes in greenhouse gas concentrations will alter GWPs because the radiative efficiencies of marginal changes in CO 2 , CH 4 and N 2 O depend on their background concentrations, the removal of CO 2 is influenced by climate–carbon cycle feedbacks, and atmospheric residence times of CH 4 and N 2 O also depend on ambient temperature and other environmental changes. We calculated the currently foreseeable future changes in the absolute GWP of CO 2 , which acts as the denominator for the calculation of all GWPs, and specifically the GWPs of CH 4 and N 2 O, along four representative concentration pathways (RCPs) up to the year 2100. We find that the absolute GWP of CO 2 decreases under all RCPs, although for longer time horizons this decrease is smaller than for short time horizons due to increased climate–carbon cycle feedbacks. The 100-year GWP of CH 4 would increase up to 20% under the lowest RCP by 2100 but would decrease by up to 10% by mid-century under the highest RCP. The 100-year GWP of N 2 O would increase by more than 30% by 2100 under the highest RCP but would vary by less than 10% under other scenarios. These changes are not negligible but are mostly smaller than the changes that would result from choosing a different time horizon for GWPs, or from choosing altogether different metrics for comparing greenhouse gas emissions, such as global temperature change potentials.
  27. 2011: T. Wigley "Coal to gas: the influence of methane leakage", Climatic Change, 108 (3), 601-608, 10.1007/s10584-011-0217-3 Abstract: Carbon dioxide (CO 2 ) emissions from fossil fuel combustion may be reduced by using natural gas rather than coal to produce energy. Gas produces approximately half the amount of CO 2 per unit of primary energy compared with coal. Here we consider a scenario where a fraction of coal usage is replaced by natural gas (i.e., methane, CH 4 ) over a given time period, and where a percentage of the gas production is assumed to leak into the atmosphere. The additional CH 4 from leakage adds to the radiative forcing of the climate system, offsetting the reduction in CO 2 forcing that accompanies the transition from coal to gas. We also consider the effects of: methane leakage from coal mining; changes in radiative forcing due to changes in the emissions of sulfur dioxide and carbonaceous aerosols; and differences in the efficiency of electricity production between coal- and gas-fired power generation. On balance, these factors more than offset the reduction in warming due to reduced CO 2 emissions. When gas replaces coal there is additional warming out to 2,050 with an assumed leakage rate of 0%, and out to 2,140 if the leakage rate is as high as 10%. The overall effects on global-mean temperature over the 21st century, however, are small.
  28. 2011: J. Rogelj, W. Hare, J. Lowe, D. P. van Vuuren, K. Riahi, B. Matthews, T. Hanaoka, K. Jiang and M. Meinshausen "Emission pathways consistent with a 2 degree C global temperature limit", Nature Clim. Change, 1 (8), 413-418, Abstract: In recent years, international climate policy has increasingly focused on limiting temperature rise, as opposed to achieving greenhouse-gas-concentration-related objectives. The agreements reached at the United Nations Framework Convention on Climate Change conference in Cancun in 2010 recognize that countries should take urgent action to limit the increase in global average temperature to less than 2?°C relative to pre-industrial levels1. If this is to be achieved, policymakers need robust information about the amounts of future greenhouse-gas emissions that are consistent with such temperature limits. This, in turn, requires an understanding of both the technical and economic implications of reducing emissions and the processes that link emissions to temperature. Here we consider both of these aspects by reanalysing a large set of published emission scenarios from integrated assessment models in a risk-based climate modelling framework. We find that in the set of scenarios with a ‘likely’ (greater than 66%) chance of staying below 2?°C, emissions peak between 2010 and 2020 and fall to a median level of 44?Gt of CO2 equivalent in 2020 (compared with estimated median emissions across the scenario set of 48?Gt of CO2 equivalent in 2010). Our analysis confirms that if the mechanisms needed to enable an early peak in global emissions followed by steep reductions are not put in place, there is a significant risk that the 2?°C target will not be achieved.
  29. 2012: J. Rogelj, M. Meinshausen and R. Knutti "Global warming under old and new scenarios using IPCC climate sensitivity range estimates", Nature Clim. Change, advance online publication Abstract: Climate projections for the fourth assessment report1 (AR4) of the Intergovernmental Panel on Climate Change (IPCC) were based on scenarios from the Special Report on Emissions Scenarios2 (SRES) and simulations of the third phase of the Coupled Model Intercomparison Project3 (CMIP3). Since then, a new set of four scenarios (the representative concentration pathways or RCPs) was designed4. Climate projections in the IPCC fifth assessment report (AR5) will be based on the fifth phase of the Coupled Model Intercomparison Project5 (CMIP5), which incorporates the latest versions of climate models and focuses on RCPs. This implies that by AR5 both models and scenarios will have changed, making a comparison with earlier literature challenging. To facilitate this comparison, we provide probabilistic climate projections of both SRES scenarios and RCPs in a single consistent framework. These estimates are based on a model set-up that probabilistically takes into account the overall consensus understanding of climate sensitivity uncertainty, synthesizes the understanding of climate system and carbon-cycle behaviour, and is at the same time constrained by the observed historical warming.

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