Some energy services and industrial processes-such as long-distance freight transport, air travel, highly reliable electricity, and steel and cement manufacturing-are particularly difficult to provide without adding carbon dioxide (CO) to the atmosphere. Rapidly growing demand for these services, combined with long lead times for technology development and long lifetimes of energy infrastructure, make decarbonization of these services both essential and urgent. We examine barriers and opportunities associated with these difficult-to-decarbonize services and processes, including possible technological solutions and research and development priorities. A range of existing technologies could meet future demands for these services and processes without net addition of CO to the atmosphere, but their use may depend on a combination of cost reductions via research and innovation, as well as coordinated deployment and integration of operations across currently discrete energy industries.

) in an analysis of potential "dangerous anthropogenic interference" with climate.Detailed diagnostics for several of these simulations are available from the repository for IPCC runs (www-pcmdi.llnl. gov/ipcc/about_ipcc.php). Diagnostics for all of these runs, including convenient graphics, are available at data.giss.nasa.gov/ modelE/transient.Sect. 2 defines the climate model and summarizes principal known deficiencies. Sect. 3 defines time-dependent climate forcings and discusses uncertainties. Sect. 4 considers alternative ways of sampling the model's simulated temperature change for comparison with imperfect observations. Sect. 5 compares simulated and observed climate change for 880-2003, focusing on temperature change but including other climate variables. Sect. 6 summarizes the capabilities and limitations of the current simulations and suggests efforts that are needed to improve future capabilities. Climate Model Atmospheric ModelThe atmospheric model employed here is the 20-layer version of GISS modelE (2006) with 4°×5° horizontal resolution. This resolution is coarse, but use of second-order moments for numerical differencing improves the effective resolution for the transport of tracers. The model top is at 0. hPa. Minimal drag is applied in the stratosphere, as needed for numerical stability, without gravity wave modeling. Stratospheric zonal winds and temperature are generally realistic ( Ocean RepresentationsWe find it instructive to attach the identical atmospheric model to alternative ocean representations. We make calcula- AbstractWe carry out climate simulations for 880-2003 with GISS modelE driven by ten measured or estimated climate forcings. An ensemble of climate model runs is carried out for each forcing acting individually and for all forcing mechanisms acting together. We compare side-by-side simulated climate change for each forcing, all forcings, observations, unforced variability among model ensemble members, and, if available, observed variability. Discrepancies between observations and simulations with all forcings are due to model deficiencies, inaccurate or incomplete forcings, and imperfect observations. Although there are notable discrepancies between model and observations, the fidelity is sufficient to encourage use of the model for simulations of future climate change. By using a fixed well-documented model and accurately defining the 1880-2003 forcings, we aim to provide a benchmark against which the effect of improvements in the model, climate forcings, and observations can be tested. Principal model deficiencies include unrealistically weak tropical El Nino-like variability and a poor distribution of sea ice, with too much sea ice in the Northern Hemisphere and too little in the Southern Hemisphere. Greatest uncertainties in the forcings are the temporal and spatial variations of anthropogenic aerosols and their indirect effects on clouds.

Abstract. We investigate the issue of "dangerous humanmade interference with climate" using simulations with GISS modelE driven by measured or estimated forcings for 1880-2003 and extended to 2100 for IPCC greenhouse gas scenarios as well as the "alternative" scenario of Hansen and Sato (2004). Identification of "dangerous" effects is partly subjective, but we find evidence that added global warming of more than 1 • C above the level in 2000 has effects that may be highly disruptive. The alternative scenario, with peak added forcing ∼1.5 W/m 2 in 2100, keeps further global warming under 1 • C if climate sensitivity is ∼3 • C or less for doubled CO 2 . The alternative scenario keeps mean regional seasonal warming within 2σ (standard deviations) of 20th century variability, but other scenarios yield regional changes of 5-10σ , i.e. mean conditions outside the range of local experience. We conclude that a CO 2 level exceeding about 450 ppm is "dangerous", but reduction of non-CO 2 forcings can provide modest relief on the CO 2 constraint. We discuss three specific sub-global topics: Arctic climate change, Correspondence to: J. Hansen (jhansen@giss.nasa.gov) tropical storm intensification, and ice sheet stability. We suggest that Arctic climate change has been driven as much by pollutants (O 3 , its precursor CH 4 , and soot) as by CO 2 , offering hope that dual efforts to reduce pollutants and slow CO 2 growth could minimize Arctic change. Simulated recent ocean warming in the region of Atlantic hurricane formation is comparable to observations, suggesting that greenhouse gases (GHGs) may have contributed to a trend toward greater hurricane intensities. Increasing GHGs cause significant warming in our model in submarine regions of ice shelves and shallow methane hydrates, raising concern about the potential for accelerating sea level rise and future positive feedback from methane release. Growth of non-CO 2 forcings has slowed in recent years, but CO 2 emissions are now surging well above the alternative scenario. Prompt actions to slow CO 2 emissions and decrease non-CO 2 forcings are required to achieve the low forcing of the alternative scenario.

Dozens of scenarios are published each year outlining paths to a low carbon global energy system. To provide insight into the relative feasibility of these global decarbonization scenarios, we examine 17 scenarios constructed using a diverse range of techniques and introduce a set of empirical benchmarks that can be applied to compare and assess the pace of energy system transformation entailed by each scenario. In particular, we quantify the implied rate of change in energy and carbon intensity and low-carbon technology deployment rates for each scenario and benchmark each against historical experience and industry projections, where available. In addition, we examine how each study addresses the key technical, economic, and societal factors that may constrain the pace of low-carbon energy transformation. We find that all of the scenarios envision historically unprecedented improvements in energy intensity, while normalized low-carbon capacity deployment rates are broadly consistent with historical experience. Three scenarios that constrain the available portfolio of low-carbon options by excluding some technologies (nuclear and carbon capture and storage) a priori are outliers, requiring much faster low-carbon capacity deployment and energy intensity improvements. Finally, all of the studies present comparatively little detail on strategies to decarbonize the industrial and transportation sectors, and most give superficial treatment to relevant constraints on energy system transformations. To be reliable guides for policymaking, scenarios such as these need to be supplemented by more detailed analyses realistically addressing the key constraints on energy system transformation.

Abstract. We investigate the issue of "dangerous human-made interference with climate" using simulations with GISS modelE driven by measured or estimated forcings for 1880–2003 and extended to 2100 for IPCC greenhouse gas scenarios as well as the "alternative" scenario of Hansen and Sato (2004). Identification of "dangerous" effects is partly subjective, but we find evidence that added global warming of more than 1°C above the level in 2000 has effects that may be highly disruptive. The alternative scenario, with peak added forcing ~1.5 W/m2 in 2100, keeps further global warming under 1°C if climate sensitivity is ~3°C or less for doubled CO2. The alternative scenario keeps mean regional seasonal warming within 2σ (standard deviations) of 20th century variability, but other scenarios yield regional changes of 5–10σ, i.e., mean conditions outside the range of local experience. We discuss three specific sub-global topics: Arctic climate change, tropical storm intensification, and ice sheet stability. We suggest that Arctic climate change has been driven as much by pollutants (O3, its precursor CH4, and soot) as by CO2, offering hope that dual efforts to reduce pollutants and slow CO2 growth could minimize Arctic change. Simulated recent ocean warming in the region of Atlantic hurricane formation is comparable to observations, suggesting that greenhouse gases (GHGs) may have contributed to a trend toward greater hurricane intensities. Increasing GHGs cause significant warming in our model in submarine regions of ice shelves and shallow methane hydrates, raising concern about the potential for accelerating sea level rise and future positive feedback from methane release. Growth of non-CO2 forcings has slowed in recent years, but CO2 emissions are now surging well above the alternative scenario. Prompt actions to slow CO2 emissions and decrease non-CO2 forcings are needed to achieve the low forcing of the alternative scenario.