The UN Framework Convention on Climate Change calls for “stabilization of greenhouse gas concentrations at a level that would prevent dangerous anthropogenic interference with the climate system.” Even if we could determine a “safe” level of interference in the climate system, the sensitivity of global mean temperature to increasing atmospheric CO 2 is known perhaps only to a factor of three or less. Here we show how a factor of three uncertainty in climate sensitivity introduces even greater uncertainty in allowable increases in atmospheric CO 2 concentration and allowable CO 2 emissions. Nevertheless, unless climate sensitivity is low and acceptable amounts of climate change are high, climate stabilization will require a massive transition to CO 2 emission–free energy technologies.

Stabilizing the carbon dioxide–induced component of climate change is an energy problem. Establishment of a course toward such stabilization will require the development within the coming decades of primary energy sources that do not emit carbon dioxide to the atmosphere, in addition to efforts to reduce end-use energy demand. Mid-century primary power requirements that are free of carbon dioxide emissions could be several times what we now derive from fossil fuels (∼10 13 watts), even with improvements in energy efficiency. Here we survey possible future energy sources, evaluated for their capability to supply massive amounts of carbon emission–free energy and for their potential for large-scale commercialization. Possible candidates for primary energy sources include terrestrial solar and wind energy, solar power satellites, biomass, nuclear fission, nuclear fusion, fission-fusion hybrids, and fossil fuels from which carbon has been sequestered. Non–primary power technologies that could contribute to climate stabilization include efficiency improvements, hydrogen production, storage and transport, superconducting global electric grids, and geoengineering. All of these approaches currently have severe deficiencies that limit their ability to stabilize global climate. We conclude that a broad range of intensive research and development is urgently needed to produce technological options that can allow both climate stabilization and economic development.

The effects of parameterizations of subgrid-scale mixing on simulated distributions of natural C-14, temperature, and salinity in a three-dimensional ocean general circulation model are examined. The parameterizations studied are 1) the Gent-McWilliams parameterization of lateral transport of tracers by isopycnal eddies; 2) horizontal mixing; 3) a parameterization of vertical mixing in which the amount of mixing depends on the local vertical density gradient; and 4) prescribed vertical mixing. The authors perform and analyze four ocean GCM simulations that use different combinations of these parameterizations. It is confirmed that the Gent-McWilliams parameterization largely eliminates the tendency of GFDL-based models to overestimate temperatures in the thermocline. However, in the authors' simulations with the Gent-McWilliams parameterization the deep ocean is too cold, in places by more than 3 degrees. Our results are the first known to assess the effects of the Gent-McWilliams parameterization on the simulated distribution of natural C-14. The most important change (compared to results obtained with horizontal mixing) is that interior ocean C-14 values are lower; that is, the water is ''older'' with Gent-McWilliams. In most locations in the deep North Atlantic, simulated Delta(14)C values are much too low with horizontal mixing and are even lower with Gent-McWilliams. Both this problem and the problem of the simulated deep ocean being too cold are probably due, at least in part, to insufficient downward penetration of NADW, resulting in the deep North Atlantic in the model being ventilated primarily via AABW. This problem exists when Gent-McWilliams is not used, but Gent-McWilliams makes the symptoms it presents (an overly cold and old deep North Atlantic) worse. Gent-McWilliams also results in a dramatic reduction in convective adjustment in the model, compared to results obtained with horizontal mixing; as a result, simulated tracer distributions are improved at high latitudes. Finally, Gent-McWilliams increases the susceptibility of the authors' model to some types of numerical problems. The stability-dependent vertical mixing parameterization causes relatively small changes in simulated distributions of temperature and natural C-14 (compared to results with a prescribed uniform vertical diffusivity), but these changes tend to improve agreement with observations. Assuming they are based on correct physical premises and are properly calibrated, both the stability-dependent vertical mixing parameterization and the Gent-McWilliams parameterization should give the model more predictive capability than simpler parameterizations do in that they allow the amount or direction of mixing to change in response to changes in ocean density.

Accurate global carbon cycle models are needed to estimate the future change of atmospheric CO2 for specified scenarios of CO2 emissions. Model accuracy cannot be tested directly because of the difficulty in estimating the carbon flux to the oceans and the terrestrial biosphere. However, one test of model consistency is the requirement that the model reproduce past changes and spatial distributions of 14C. A model for carbon exchange within and among the atmosphere, oceans, and terrestrial biosphere is found to satisfy this test. The ocean is modeled as an upwelling‐diffusion column capped by a mixed layer with recirculation of the polar bottom water to complete the thermohaline circulation. This ocean advection scheme contains only two key dynamic parameters, the vertical eddy diffusivity κ and the upwelling velocity w, which are calibrated to match the vertical distribution of preanthropogenic 14C. The thermocline depth scale κ/w = 1343 m found by calibration is considerably deeper than that required to match the steady vertical temperature profile (500 m). This is consistent with the hypothesis that isopycnal mixing, which is much more rapid than diapycnal mixing, has a stronger effect on 14C than on temperature since isopycnals are nearly isothermal. This model is found to match measured values, within measurement error, of the prebomb decrease in 14C in the atmosphere and the mixed layer due to the Suess effect, the bomb 14C in the mixed layer, the bomb 14C penetration depth, the bomb 14C ocean inventory, and the vertical distribution of total carbon. Results are compared to those of other schematic carbon cycle models as well as those of ocean general circulation models.

We consider the relative effectiveness of systems for collection and marine disposal Of CO2 from fossil fuel fired power plants using comparisons of the trend with time Of CO2 concentration in the atmosphere from systems with and without marine disposal. The retention time for CO2 increases rapidly with depth of disposal, from a few years in the mixed layer up to several centuries as the depth increases beyond 1000 m, suggesting that deep oceans should be considered as potential storage sites for CO2. However, CO2 collection and disposal consumes energy and produces extra CO2. We show that some of this extra this CO2 reaches the atmosphere, so that atmospheric concentrations from systems with marine disposal ultimately exceed those from systems without controls. In some circumstances they do so rapidly, making marine disposal less favorable than direct atmospheric release. Global Warming Potential can be used to compare systems. This highlights the difficult issues involved in determining what time scales are important in considering options to reduce concerns about global warming.

To assess the future impact of anthropogenic greenhouse gases on global climate, we need a reliable estimate of the sensitivity of the Earth's climate to changes in radiative forcing. Climate sensitivity is conventionally defined as the equilibrium surface temperature increase for carbon dioxide doubling, ΔT2x. Uncertainties in cloud processes spread general circulation model (GCM) estimates of this parameter over the range 1.5< ΔT2x <4.5°C (refs 1, 2). An alternative to model-based estimates is in principle available from the reconstruction of past climates3–6, which implicitly includes cloud feedback. Here we retrieve the sensitivity of two palaeoclimates, one colder and one warmer than present, by independently reconstructing both the equilibrium surface tem-perature change and the radiative forcing. Our results yield ΔT2x = 2.3 ±0.9 °C. This range is comparable with estimates from GCMs and inferences from recent temperature observations and ocean models7,8. Future application of the method to additional climates in the geological record might constrain climate sensitivity enough to narrow the model uncertainties of global warming predictions.

The vertical structure of total carbon, alkalinity, nutrients, and dissolved oxygen in the world oceans is examined with a one-dimensional equatorial ocean/polar ocean box model. Photosynthesis/respiration cycles affect and are affected by fluxes in the ocean and the structure of the profiles. Marine biota produce by-products that lead to organic and inorganic (calcareous) sediments. In steady state, rates of phosphorous and alkalinity runoff from land are linked to surface nutrient supply, the rates of particulate rain, the degree of anoxia near sediments, the lysocline depth, and thereby control rates of sedimentation. These, in turn, are influenced by internal mixing dynamics and the action of the marine biota. The interdependence of ocean composition and rates of organic and inorganic carbon burial is found to be sensitive to the traits of the marine biosphere.

Energy is needed to produce wealth, and an increasing world population will need increasing amounts of energy to improve its standard of living. Through the use of a carbon cycle model, it is shown that continued reliance on fossil fuels will cause a global greenhouse warming. An energy-CO2-economics model is used to project future demand for fossil-fuel-generated energy. When this demand is compared with the fossil fuel use that is permissible if a global warming is to be avoided, a shortfall in energy becomes evident. Terrestrial photovoltaics, nuclear fission, nuclear fusion, and the solar power satellite (SPS) are examined as means of making up this energy shortfall.On comparing these technologies, the SPS appears to be the most feasible means of providing the required energy and preventing a global warming. Laser, 2.45 GHz, and 35 GHz SPS technologies are intercompared, and results indicate that the 2.45 GHz technology remains the most feasible SPS option.

A new space power concept incorporating Earth-to-satellite microwave power transmission coupled to onboard regenerative electrochemical energy storage is proposed for energizing defensive satellite constellations. The system addresses housekeeping, orbital maneuvering, and burst-mode power requirements, and offers an attractive alternative to the nuclear space power systems currently being considered for this application. This energy-conversion system incorporates six steps: 1) generate primary dc power at surface stations along the satellite ground track, 2) convert to microwave (rf) frequencies, 3) transmit in a narrow beam to spacecraft using phased-array antennas that track and lock on to satellite receivers as they pass in range during a fraction of their orbit, 4) receive the energy and convert to dc in space using lightweight and inexpensive rectennas, 5) store the energy onboard as chemical energy by electrolysis of water to oxygen and hydrogen, and 6) recover free energy onboard the spacecraft during the balance of the orbit continuously or on demand as pulsed power with a high power-density fuel cell. Component and overall systems considerations of this scheme are discussed, outstanding research problems are defined, and preliminary analyses are described.

Studies of paleoclimate and modern observations indicate that evaporative effects limit thermal response in equatorial regions. We develop a latitude-resolved, steady-state energy balance model which incorporates the effect of an evaporative constraint on the variation of equatorial temperature with solar luminosity. For a diffusive model of surface heat transport the constraint requires the diffusion coefficient to vary with insolation. We find that the movement of the iceline with insolation is four times larger than in standard energy balance models with a constant thermal diffusion coefficient. This is a consequence of the global energy balance which forces temperature changes to occur at high latitudes when they are evaporatively buffered at the equator. Nonlinear temperature-ice albedo feedback at high latitudes then amplifies the response leading to greater sensitivity in the vicinity of current climate.

Geologic evidence of the prior existence of liquid water on Mars suggests surface temperatures Ts were once considerably warmer than at present; and that such a condition may have arisen from a larger atmospheric greenhouse. Here we develop a simple climate model for a CO2/H2O Mars atmosphere including water vapor-longwave opacity feedback in the atmosphere and temperature-albedo feedback at surface icecaps, under the assumption that once the Martian surface pressure was ps ≥ 1 atm CO2. Longwave flux to space is computed as a function of Ts and ps using band-absorption models for the effect of the 15-μm fundamental, and the 10- and 15-μm hot bands, of the CO2 molecule; as well as the pure rotation bands and e continuum of H2O. The derived global radiative balance predicts a global mean surface temperature of 283°K at 1 atm CO2. When the emission model is coupled to a latitudinally resolved energy balance climate model, including the effect of poleward heat transfer by atmospheric baroclinic eddies, the solutions vary, depending on ps. We considered two cases: (1) the present Mars (ps ≅ 0.007 atm) with pressure-buffering by solid CO2 icecaps, and limited poleward heat flux by the atmosphere; and (2) a hypothetical “hot Mars” (ps ≅ 1.0 atm), whose much higher CO2 amount augmented by H2O evaporative feedback yields a theoretical Ts distribution with latitude admitting liquid water over 95% of the surface, water icecaps at the poles, and a diminished equator-to-pole temperature gradient relative to the present.

The influence of the world oceans on climatic response is considered here with emphasis on the heat transferred to waters beneath the well‐mixed surface layer and to polar bottom water forming zones. An upwelling‐diffusing model is formulated to treat this problem whose effective transport properties are calibrated from the steady state vertical profiles of radiocarbon, potential temperature and other tracers measured by chemical oceanographers. The key issue with regard to the question of atmospheric temperature response to external climatic forcing is whether heat is exchanged between the surface mixed layer and deep sea at rates comparable to heat transfer rates between the planetary radiation field and the atmosphere‐mixed layer system. An important model parameter appearing in the analysis is the polar sea warming coefficient ∏ equal to the rate of change of polar sea temperature relative to changes in areally averaged mixed layer temperature. For ∏ values in the range of 0 to 2 the models predicts response times in the range of 8 to 20 years to attain 63% of the equilibrium temperature change for a step function climatic forcing, and 50 to 1000 years to get 90% of the equilibrium response. These may be compared with the roughly 4 year response time one gets with an oceanic mixed layer only model. To study the carbon dioxide climate problem, a more realistic time‐dependent forcing function is used based on the historical growth of fossil fuel CO2 and a logarithmic scaling law for the temperature increment which would obtain at any instant if the system were in radiative‐convective equilibrium. Our results suggest the influence of deep sea thermal storage could delay the full value of temperature increment predicted by equilibrium models by 10 to 20 years in 1980 to 2000 A.D. time frame. Also considered is the model response to periodic forcing, the sensitivity of the results, and the implications of the model results with regard to climatic changes on a decadal to millenial timescale.

If the penetration of tritium into the pycnocline of the Norwegian Sea is assumed to be via vertical eddy diffusion, the magnitude of the diffusivity must be from 3‐5 cm²/s and the buoyancy flux must be on the order of 4‐5 cm²/s³.

A computational technique applicable to analysis of supersonic transport (SST) wake photochemistry and diffusion is presented. Sensitivity studies of SST effluent effects upon ozone depletion are facilitated by the computational rapidity of the method. The article compares results from other studies and predictions of some variables related to global NOx input. Results indicate that the NO/NO2 ratio in an SST wake at photochemical equilibrium is a sensitive function of photolysis rates.