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 "C 4 MIP", 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 22% 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 CO 2 doubling for the AOGCMs is on average 288 °C, about 033 °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

Abstract: . Improving observations of ocean heat content show that Earth is absorbing more energy from the Sun than it is radiating to space as heat, even during the recent solar minimum. The inferred planetary energy imbalance, 0.58 ± 0.15 W m −2 during the 6-yr period 2005–2010, confirms the dominant role of the human-made greenhouse effect in driving global climate change. Observed surface temperature change and ocean heat gain together constrain the net climate forcing and ocean mixing rates. We conclude that most climate models mix heat too efficiently into the deep ocean and as a result underestimate the negative forcing by human-made aerosols. Aerosol climate forcing today is inferred to be −1.6 ± 0.3 W m −2 , implying substantial aerosol indirect climate forcing via cloud changes. Continued failure to quantify the specific origins of this large forcing is untenable, as knowledge of changing aerosol effects is needed to understand future climate change. We conclude that recent slowdown of ocean heat uptake was caused by a delayed rebound effect from Mount Pinatubo aerosols and a deep prolonged solar minimum. Observed sea level rise during the Argo float era is readily accounted for by ice melt and ocean thermal expansion, but the ascendency of ice melt leads us to anticipate acceleration of the rate of sea level rise this decade.

Abstract: While many studies of the effects of global warming on hurricanes predict an increase in various metrics of Atlantic basin-wide activity, it is less clear that this signal will emerge from background noise in measures of hurricane damage, which depend largely on rare, high-intensity landfalling events and are thus highly volatile comparedtobasin-widestormmetrics.Usingarecentlydevelopedhurricanesynthesizerdrivenbylarge-scale meteorological variables derived from global climate models, 1000 artificial 100-yr time series of Atlantic hurricanes that make landfall along the U.S. Gulf and East Coasts are generated for four climate models and for current climate conditions as well as for the warmer climate of 100 yr hence under the Intergovernmental Panel on Climate Change (IPCC) emissions scenario A1b. These synthetic hurricanes damage a portfolio of insured property according to an aggregate wind-damage function; damage from flooding is not considered here. Assumingthat the hurricane climate changes linearly with time, a 1000-member ensemble of time series of propertydamagewas created.Three of thefour climatemodels usedproduce increasing damagewith time, with the global warming signal emerging on time scales of 40, 113, and 170 yr, respectively. It is pointed out, however, that probabilities of damage increase significantly well before such emergence time scales and it is shownthatprobabilitydensitydistributionsofaggregatedamagebecomeappreciablyseparatedfromthoseof the control climateon time scales as shortas 25 yr. For the fourthclimatemodel, damagesdecreasewith time, but the signal is weak.

Abstract: [1] Future anthropogenic climate change in the Southern Hemisphere is likely to be driven by two opposing effects, stratospheric ozone recovery and increasing greenhouse gases. We examine simulations from two coupled climate models in which the details of these two forcings are known. While both models suggest that recent positive summertime trends in the Southern Annular Mode (SAM) will reverse sign over the coming decades as the ozone hole recovers, climate sensitivity appears to play a large role in modifying the strength of their SAM response. Similar relationships are found between climate sensitivity and SAM trends when the analysis is extended to transient CO2 simulations from other coupled models. Tropical upper tropospheric warming is found to be more relevant than polar stratospheric cooling to the intermodel variation in the SAM trends in CO2‐only simulations. Citation: Arblaster, J. M., G. A. Meehl, and D. J. Karoly (2011), Future climate change in the Southern Hemisphere: Competing effects of ozone and greenhouse gases, Geophys. Res. Lett., 38, L02701,

Abstract: Anthropogenic greenhouse gas emissions are changing the Earth's cli- mate and impose substantial risks for current and future generations What are scientifically sound, economically viable, and ethically defendable strategies to manage these climate risks? Ratified international agreements call for a reduction of greenhouse gas emissions to avoid dangerous anthropogenic interference with the climate system Recent proposals, however, call for a different approach: to geoengineer climate by injecting aerosol precursors into the stratosphere Published economic studies typically neglect the risks of aerosol geoengineering due to (i) the potential for a failure to sustain the aerosol forcing and (ii) the negative impacts associated with the aerosol forcing Here we use a simple integrated assessment model of climate change to analyze potential economic impacts of aerosol geo- engineering strategies over a wide range of uncertain parameters such as climate sensitivity, the economic damages due to climate change, and the economic damages due to aerosol geoengineering forcing The simplicity of the model provides the

Abstract: A growing body of economics research projects the effects of global climate change on economic outcomes. Climate scientists often criticize these articles because nearly all ignore the well-established uncertainty in future temperature and rainfall changes, and therefore appear likely to have downward biased standard errors and potentially misleading point estimates. This paper incorporates climate uncertainty into estimates of climate change impacts on U.S. agriculture. Accounting for climate uncertainty leads to a much wider range of projected impacts on agricultural profits, with the 95% confidence interval featuring drops of between 17% to 88%. An application to African agriculture yields similar results.

Abstract: We present a comprehensive assessment of the present and expected future pulse of the Indian monsoon climate based on observational and global climate model projections. The analysis supports the view that seasonal Indian monsoon rains in the latter half of the 21th century may not be materially different in abundance to that experienced today although their intensity and duration of wet and dry spells may change appreciably. Such an assessment comes with considerable uncertainty. With regard to temperature, however, we find that the Indian temperatures during the late 21st Century will very likely exceed the highest values experienced in the 130-year instrumental record of Indian data. This assessment comes with higher confidence than for rainfall because of the large spatial scale driving the thermal response of climate to greenhouse gas forcing. We also find that monsoon climate changes, especially temperature, could heighten human and crop mortality posing a socio-economic threat to the Indian subcontinent.

Abstract: Given the inherent uncertainties in predicting how climate and environments will respond to anthropogenic emissions of greenhouse gases, it would be beneficial to society if science could identify geological analogues to the human race’s current grand climate experiment. This has been a focus of the geological and palaeoclimate communities over the last 30 years, with many scientific papers claiming that intervals in Earth history can be used as an analogue for future climate change. Using a coupled ocean–atmosphere modelling approach, we test this assertion for the most probable pre-Quaternary candidates of the last 100 million years: the Mid- and Late Cretaceous, the Palaeocene–Eocene Thermal Maximum (PETM), the Early Eocene, as well as warm intervals within the Miocene and Pliocene epochs. These intervals fail as true direct analogues since they either represent equilibrium climate states to a long-term CO2 forcing—whereas anthropogenic emissions of greenhouse gases provide a progressive (transient) forcing on climate—or the sensitivity of the climate system itself to CO2 was different. While no close geological analogue exists, past warm intervals in Earth history provide a unique opportunity to investigate processes that operated during warm (high CO2) climate states. Palaeoclimate and environmental reconstruction/modelling are facilitating the assessment and calculation of the response of global temperatures to increasing CO2 concentrations in the longer term (multiple centuries); this is now referred to as the Earth System Sensitivity, which is critical in identifying CO2 thresholds in the atmosphere that must not be crossed to avoid dangerous levels of climate change in the long term. Palaeoclimatology also provides a unique and independent way to evaluate the qualities of climate and Earth system models used to predict future climate.

Abstract: Climate change is often characterized in terms of climate sensitivity, the globally averaged temperature rise associated with a doubling of the atmospheric CO2 (equivalent) concentration. In this study, we develop and apply two new ecological sensitivity metrics, analogs of climate sensitivity, to investigate the potential degree of plant community changes over the next three centuries. We use ten climate simulations from the Intergovernmental Panel on Climate Change Fourth Assessment Report, with climate sensitivities from 2–4°C. The concept of climate sensitivity depends upon the continuous nature of the temperature field across the Earth’s surface. For this research, the bridge between climate change and biospheric change predictions is provided by the Equilibrium Vegetation Ecology model (EVE), which simulates a continuous description of the Earth’s terrestrial plant communities as a function of climate. The ecosensitivity metrics applied to the results of EVE simulations at the end of the twenty-first century result in 49% of the Earth’s land surface area undergoing plant community changes and 37% of the world’s terrestrial ecosystems undergoing biome-scale changes. EVE is an equilibrium model, and, although rates of ecological change are not addressed, the resultant ecological sensitivity projections provide an estimate of the degree of species turnover that must occur for ecosystems to be in equilibrium with local climates. Regardless of equilibrium timescales, the new metrics highlight the Earth’s degree of ecological sensitivity while identifying ecological “hotspots” in the terrestrial biosphere’s response to projected climate changes over the next three centuries.

Abstract: In recent decades, there have been a number of debates on climate warming and its driving forces. Based on an extensive literature review, we suggest that (1) climate warming occurs with great uncertainty in the magnitude of the temperature increase; (2) both human activities and natural forces contribute to climate change, but their relative contributions are difficult to quantify; and (3) the dominant role of the increase in the atmospheric concentration of greenhouse gases (including CO2) in the global warming claimed by the Intergovernmental Panel on Climate Change (IPCC) is questioned by the scientific communities because of large uncertainties in the mechanisms of natural factors and anthropogenic activities and in the sources of the increased atmospheric CO2 concentration. More efforts should be made in order to clarify these uncertainties.

Abstract: Glaciers respond to long-term climate changes and also to the year-to-year fluctuations inherent in a constant climate. Differentiating between these factors is critical for the correct interpretation of past glacier fluctuations and for the correct attribution of current changes. Previous work has established that century-scale, kilometre-scale fluctuations can occur in a constant climate. This study asks two further questions of practical significance: how likely is an excursion of a given magnitude in a given amount of time, and how large a trend in length is statistically significant? A linear model permits analytical answers wherein the dependencies on glacier geometry and climate setting can be clearly understood. The expressions are validated with a flowline glacier model. The likelihood of glacier excursions is well characterized by extreme-value statistics, although probabilities are acutely sensitive to some poorly known glacier properties. Conventional statistical tests can be used for establishing the significance of an observed glacier trend. However, it is important to determine the independent information in the observations which can be effectively estimated from the glacier geometry. Finally, the retreat of glaciers around Mount Baker, Washington State, USA, is consistent with, but not independent proof of, the regional climate warming that is established from the instrumental record.

Abstract: 1. Overview of climate variability and climate science 2. Basics of global climate 3. Physical processes in the climate system 4. El Nino and year-to-year climate prediction 5. Climate models 6. The greenhouse effect and climate feedbacks 7. Climate model scenarios for global warming References Index.

Abstract: The future climate change projections are essentially based on coupled general circulation model (CGCM) simulations, which give a distinct global warming pattern with arctic winter amplification, an equilibrium land-sea warming contrast and an inter-hemispheric warming gradient. While these simulations are the most important tool of the Intergovernmental Panel on Climate Change (IPCC) predictions, the conceptual understanding of these predicted structures of climate change and the causes of their uncertainties is very difficult to reach if only based on these highly complex CGCM simulations. In the study presented here we will introduce a very simple, globally resolved energy balance (GREB) model, which is capable of simulating the main characteristics of global warming. The model shall give a bridge between the strongly simplified energy balance models and the fully coupled 4-dimensional complex CGCMs. It provides a fast tool for the conceptual understanding and development of hypotheses for climate change studies, which shall build a basis or starting point for more detailed studies of observations and CGCM simulations. It is based on the surface energy balance by very simple representations of solar and thermal radiation, the atmospheric hydrological cycle, sensible turbulent heat flux, transport by the mean atmospheric circulation and heat exchange with the deeper ocean. Despite some limitations in the representations of the basic processes, the models climate sensitivity and the spatial structure of the warming pattern are within the uncertainties of the IPCC models simulations. It is capable of simulating aspects of the arctic winter amplification, the equilibrium land-sea warming contrast and the inter-hemispheric warming gradient with good agreement to the IPCC models in amplitude and structure. The results give some insight into the understanding of the land-sea contrast and the polar amplification. The GREB model suggests that the regional inhomogeneous distribution of atmospheric water vapor and the non-linear sensitivity of the downward thermal radiation to changes in the atmospheric water vapor concentration partly cause the land-sea contrast and may also contribute to the polar amplification. The combination of these characteristics causes, in general, dry and cold regions to warm more than other regions.

Abstract: Rising concentrations of anthropogenic greenhouse gases in the atmosphere may already be influencing the intensity of rainfall and increasing the risk of substantial damage from the associated flooding. See Letters p.378 & p.382 A significant effect of anthropogenic activities has already been detected in observed trends in temperature and mean precipitation. But to date, no study has formally identified such a human fingerprint on extreme precipitation — an increase in which is one of the central theoretical expectations for a warming climate. Seung-Ki Min and colleagues compare observations and simulations of rainfall between 1951 and 1999 in North America, Europe and northern Asia. They find a statistically significant effect of increased greenhouse gases on observed increases in extreme precipitation events over much of the Northern Hemisphere land area.

Abstract: Despite the continued efforts of skeptics motivated by a desire for attention or short-term economic interests (Oreskes and Conway 2010), we can be certain about a number of basic facts: human activities have resulted in dramatic increases in the atmospheric concentration of carbon dioxide and a number of other greenhouse gasses; those increased concentrations are changing the climate and will continue to do so; one of those changes will be average warming on a planetary scale. Another unambiguous consequence of rising atmospheric concentrations of carbon dioxide will be the continued acidification of the world’s oceans, which are already 30% more acidic today than they were in pre-industrial times. While we still do not understand all the details of the physics, we can also be certain that factors such as clouds and water vapor, aerosol loadings, the extent of ice cover, and the strength of ocean circulation all play a key role in shaping the climate of today and of the future. In its periodic reviews, the IPCC routinely discusses and provides consensus judgments about the nature and extent of scientific uncertainty about these and other factors. Thanks largely to the work of Moss and Schneider (2000) these discussions are no longer couched only in terms of general uncertainty words such as “likely” or “unlikely.” Indeed, the climate science community has made more progress in understanding the importance of linking such words to quantitative statements about probability ranges than has any other community I know that is engaged in performing scientific assessments. Climatic Change (2011) 108:707–721 DOI 10.1007/s10584-011-0184-8

Abstract: [1] Estimates of climate sensitivity are typically characterized by highly asymmetric probability density functions (pdfs). The reasons are well known, but the situation leaves open an uncomfortably large possibility that climate sensitivity might exceed 4.5°C. In the contexts of (1) global-mean observations of the Earth's energy budget and (2) a global-mean feedback analysis, we explore what changes in the pdfs of the observations or feedbacks used to estimate climate sensitivity would be needed to remove the asymmetry, or to substantially reduce it, and demonstrate that such changes would be implausibly large. The nonlinearity of climate feedbacks is calculated from a range of studies and is shown also to have very little impact on the asymmetry. The intrinsic relationship between uncertainties in the observed climate forcing and the climate's radiative response to that forcing (i.e., the feedbacks) is emphasized. We also demonstrate that because the pdf of climate forcing is approximately symmetric, there is a strong expectation that the pdf of climate feedbacks should be symmetric as well.

Abstract: The importance of decadal climate variability (DCV) research is being increasingly recognized, including by the World Climate Research Program (WCRP) and the Intergovernmental Panel on Climate Change (IPCC). An improved understanding of DCV is very important because stakeholders and policymakers want to know the likely climate trajectory for the coming decades for applications to water resources, agriculture, energy, and infrastructure development. Responding to this demand, many climate modeling groups in the United States, Europe, Japan, and elsewhere are gearing up to assess the potential for decadal climate predictions. The magnitudes of regional DCV often exceed those associated with the trends resulting from anthropogenic changes. Therefore, differentiating between the two is also very important for planning, implementation, and national and international treaties.

Abstract: Well informed decisions on climate policy necessitate simulation of the climate system for a sufficiently wide range of emissions scenarios. While recent literature has been devoted to low emissions futures, the potential for very high emissions has not been thoroughly explored. We specify two illustrative emissions scenarios that are significantly higher than the A1FI scenario, the highest scenario considered in past IPCC reports, and simulate them in a global climate model to investigate their climate change implications. Relative to the A1FI scenario, our highest scenario results in an additional 2 K of global mean warming above A1FI levels by 2100, a complete loss of arctic summer sea-ice by 2070 and an additional 43% sea level rise due to thermal expansion above A1FI levels by 2100. Regional maximum temperature increases from late 20th century values are 50–100% greater than A1FI increases, with some regions such as the Central US, the Tibetan plateau and Alaska showing a 300–400% increase above A1FI levels.

Abstract: [1] In this paper, we breakdown the temperature response of coupled ocean-atmosphere climate models into components due to radiative forcing, climate feedback, and heat storage and transport to understand how well climate models reproduce the observed 20th century temperature record. Despite large differences between models' feedback strength, they generally reproduce the temperature response well but for different reasons in each model. We show that the differences in forcing and heat storage and transport give rise to a considerable part of the intermodel variability in global, Arctic, and tropical mean temperature responses over the 20th century. Projected future warming trends are much more dependent on a model's feedback strength, suggesting that constraining future climate change by weighting these models on the basis of their 20th century reproductive skill is not possible. We find that tropical 20th century warming is too large and Arctic amplification is unrealistically low in the Geophysical Fluid Dynamics Laboratory CM2.1, Meteorological Research Institute CGCM232a, and MIROC3.2(hires) models because of unrealistic forcing distributions. The Arctic amplification in both National Center for Atmospheric Research models is unrealistically high because of high feedback contributions in the Arctic compared to the tropics. Few models reproduce the strong observed warming trend from 1918 to 1940. The simulated trend is too low, particularly in the tropics, even allowing for internal variability, suggesting there is too little positive forcing or too much negative forcing in the models at this time. Over the whole of the 20th century, the feedback strength is likely to be underestimated by the multimodel mean.

Abstract: As regulatory efforts to curb greenhouse-gas emissions stall, many are seeking legal routes to achieve justice on climate change.

Abstract: Historical records have documented considerable changes to the global climate, with significant health, economic, and environmental consequences. Climate projections predict more intense hurricanes; increased sea level rise; and more frequent and more intense natural disasters such as heat waves, heavy rainfall, and drought in the future (1; 2). The coast along the Gulf of Mexico is particularly vulnerable to many of these environmental hazards and at particular risk when several strike simultaneously—such as a hurricane disrupting electricity transmission during a heat wave. Due to its significant contribution to global greenhouse gas (GHG) emissions, the building sector already plays an important role in climate change mitigation efforts (e.g., reducing emissions). For example, voluntary programs such as the LEED (Leadership in Energy and Environmental Design) Rating System (3), the Architecture 2030 Challenge (4), the American College and University Presidents' Climate Commitment (5), and the ...

Abstract: Earth's surface will continue to warm for decades, and the sea level to rise for centuries, even if the atmospheric concentration of greenhouse gases (GHGs) is held fixed at current levels. This is referred to as “committed” climate change because it is essentially unavoidable. Committed climate change arises due to the large thermal inertia of the oceans and their consequent time lag in adjusting to altered GHG concentrations. This work describes the basic heat balance of the oceans, the physical reasons for the long time lag in ocean temperature and sea-level rise, and the observational evidence for human-induced ocean warming over the past 50 years.

Abstract: Publisher Summary Recent global warming (1978–1998) has pushed climate changes into the forefront of scientific inquiry with a great deal at stake for human populations. This chapter provides a geological evidence of recurring climate cycles and their implication for the cause of global climate change. The Intergovernmental Panel on Climate Change (IPCC) report provides abundant physical evidence from the geologic past and provides a record of former periods of recurrent global warming and cooling that were far more intense than recent warming and cooling. These geologic records provide a clear evidence of global warming and cooling that could not have been caused by increased CO2. There is no tangible physical evidence that CO2 is causing global warming, and computer climate models that assume CO2 is the cause and computer model simulations are all based on assumption. No tangible, physical evidence exists that proves a cause-and-effect relationship between global climate changes and atmospheric CO2. The fact that CO2 is a greenhouse gas and that CO2 has increased doesn't prove that CO2 has caused global warming. Ninety five percent of greenhouse gas warming is due to water vapor and there is no evidence that atmospheric water vapor has increased. Only 3.6% of the greenhouse effect is due to CO2. As shown by isotope measurements from ice cores in Greenland and Antarctica, and by measurements of atmospheric CO2 during El Nino warming, oceans emit more CO2 into the atmosphere during climatic warming. The ice core records indicate that after the last Ice Age, temperatures rose for about 600–800 years before atmospheric CO2 rose, showing that climatic warming caused CO2 to rise, not vice versa.

Abstract: Climate change is likely to be one of the defining global issues of the 21st century. The past decade—the hottest in recorded history—has witnessed countries around the world struggling to deal with drought, heat waves, and extreme weather. The sheer scale of the problem also makes it hard to understand, predict, and solve. Climate science journals regularly publish special issues on specific climate models, typically timed to present results from a major new release of a given model. However, these tend to focus on the new science that the model enables, rather than to describe the software and its development. This special issue of IEEE Software magazine focuses on the "software" behind climate change models.

Abstract: [1] In recent years, climate change has become a major focus of public and political discussion. Ongoing scientific inquiry, revolving predominantly around understanding the anthropogenic effects of rising greenhouse gas levels, coupled with how successfully findings are communicated to the public, has made climate science both contentious and exigent. In the AGU monograph Climate Dynamics: Why Does Climate Vary?, editors De-Zheng Sun and Frank Bryan reinforce the importance of investigating the complex dynamics that underlie the natural variability of the climate system. Understanding this complexity—particularly how the natural variability of climate may enhance or mask anthropogenic warming—could have important consequences for the future. In this interview, Eos talks to De-Zheng Sun.