L. D. Danny Harvey

Energy implications of future stabilization of atmospheric CO2 content

The United Nations Framework Convention on Climate Change calls for “stabilization of greenhouse-gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system . . . ”. A standard baseline scenario, that assumes no policy intervention to limit greenhouse-gas emissions has 10 TW (10 × 1012 watts) of carbon-emission-free power being produced by the year 2050, equivalent to the power provided by all todays energy sources combined. Here we employ a carbon-cycle/energy model to estimate the carbon-emission-free power needed for various atmospheric CO2 stabilization scenarios. We find that CO2 stabilization with continued economic growth will require innovative, cost-effective and carbon-emission-free technologies that can provide additional tens of terawatts of primary power in the coming decades, and certainly by the middle of the twenty-first century, even with sustained improvement in the economic productivity of primary energy. At progressively lower atmospheric CO2-stabilization targets in the 750–350 p.p.m.v. range, implementing stabilization will become even more challenging because of the increasing demand for carbon-emission-free power. The magnitude of the implied infrastructure transition suggests the need for massive investments in innovative energy research.

Evaluation of the potential impact of methane clathrate destabilization on future global warming

Future global warming due to anthropogenic emissions of greenhouse gases has the potential to destabilize methane clathrates, which are found in permafrost regions and in continental slope sediments worldwide, resulting in the release of methane gas. Since methane is a strong greenhouse gas, this could produce a potentially important positive feedback. Here, the coupled heat transfer and methane destabilization process in oceanic sediments is modeled in a series of one-dimensional, vertical columns on a 1°×1° global grid. Terrestrial permafrost is divided into 11 columns based on mean annual surface air temperature. Our base case clathrate distribution results in about 24,000 Gt C as methane clathrate in marine sediments and about 800 Gt C in terrestrial sediments, only a small fraction of which could be destabilized by future global warming. Scenarios of anthropogenic CO2 and CH4 emission are used to drive a simple model of the carbon cycle, yielding scenarios of CO2 and CH4 concentration increase. These increases drive a one-dimensional coupled atmosphere-ocean climate model. Globally averaged temperature changes as a function of time and ocean depth are used as upper boundary conditions to drive the heat transfer/methane clathrate release models. Three versions of the ocean model are used which result in different temperature perturbations at the sediment-water interface: a purely diffusive ocean model, an upwelling-diffusion ocean model with fixed temperature of bottom water formation, and an upwelling-diffusion ocean model with a feedback between surface temperature and the upwelling velocity. Methane release from clathrate destabilization is added to the anthropogenic CH4 emission, leading to stronger increases in both CH4 and CO2 concentration. Based on a wide variety of parameter input assumptions and anthropogenic emission scenarios, it appears that the potential impact on global warming of methane clathrate destabilization is smaller than the difference in warming between low and medium, or medium and high anthropogenic CO2 emission scenarios, or arising from a factor of two variation in climate sensitivity. Global warming increases by 10–25% compared to the case without clathrate destabilization for our scenarios using what, in many respects, are worst case assumptions.

Climatic impact of ice-age aerosols

Data from Antarctic1–3, Greenland4,5 and Devon Island6 ice cores indicate that large increases in the atmospheric aerosol loading occurred during the Last Glacial Maximum (LCM) of about 18,000 yr BP. The aerosol content of the atmosphere is important in the present climate, causing the global mean temperature to be about 2–3 °C cooler than it would be in the absence of aerosols7. Here I use an energy balance climate model to show that plausible increases in the atmospheric aerosol optical depth during the LGM could have caused a further global mean cooling of 2–3 °C, thereby making a significant contribution to the climatic cooling of the LGM.

Simultaneously Constraining Climate Sensitivity and Aerosol Radiative Forcing

Abstract An energy balance climate model with latitudinal, surface–air, and land–sea resolution is coupled to a two-dimensional (latitude–depth) ocean model and used to simulate changes in surface and surface air temperature since 1765. The climate model sensitivity can be prescribed by adjusting the parameterization of infrared radiation to space, and sensitivities corresponding to an equilibrium, global average warming to a CO2 doubling (ΔT2×) of 1.0° to 5.0°C are used here. The model is driven with various combinations of greenhouse gas (GHG), fossil fuel aerosol, biomass aerosol, solar, and volcanic forcings. The fossil fuel aerosol forcing is concentrated in the NH, while the biomass aerosol forcing is centered near the equator. The variation in the global mean air temperature, and in the NH minus SH temperature, is examined over the period 1856–2000, in order to simultaneously constrain both climate sensitivity and aerosol forcing. The model performance, compared to observations, is evaluated using ...

The jurisdictional framework for municipal action to reduce greenhouse gas emissions: Case studies from Canada, the USA and Germany

Abstract This paper addresses two questions: (1) Given a commitment at the national level to reduce greenhouse gas (GHG) emissions, what tools are available to national‐level governments to induce complimentary actions required at subnational levels? (2) In the absence of a serious commitment at national and regional levels to reduce GHG emissions, what is the scope for, and jurisdictional rights of, cities to undertake actions? In this context, federal, regional and municipal legislation relevant for GHG emissions is examined in Canada, the USA and Germany. Regarding the first question, different national governments find themselves in considerably different positions to implement climate initiatives at subnational levels, with the German government in the strongest position and the Canadian government in the weakest. The implications of this for a nations willingness to adopt emission reduction targets could be serious. Regarding the second question, there are few significant differences among Canadian...

A guide to global warming potentials (GWPs)

Abstract In order to quantitatively compare the greenhouse effect of different greenhouse gases a global warming potential (GWP) index has been used which is based on the ratio of the radiative forcing of an equal emission of two different gases, integrated either over all time or up to an arbitrarily determined time horizon. The GWP index is analogous to the ozone depleting potential (ODP) index. However, the GWP index is subject to major conceptual difficulties arising from the fact that the atmospheric lifespan for part of the emitted CO 2 is, for all practical purposes, infinite. In addition, there are major uncertainties in the atmospheric lifespans and indirect heating effects of the important greenhouse gases, which are reviewed here. An alternative GWP index is proposed which explicitly takes into account the duration of capital investments in the energy sector and is less sensitive to uncertainties in atmospheric lifespans and radiative heating than the usual GWP index for time horizons longer than the lifespan of the capital investment. The effect of the GWP index proposed here, compared with previous indices, is to shift attention away from short lived gases such as methane and toward CO 2 .

On the role of high latitude ice, snow, and vegetation feedbacks in the climatic response to external forcing changes

A seasonal energy balance climate model containing a detailed treatment of surface and planetary albedo, and in which seasonally varying land snow and sea ice amounts are simulated in terms of a number of explicit physical processes, is used to investigate the role of high latitude ice, snow, and vegetation feedback processes. Feedback processes are quantified by computing changes in radiative forcing and feedback factors associated with individual processes. Global sea ice albedo feedback is 5–8 times stronger than global land snowcover albedo feedback for a 2% solar constant increase or decrease, with Southern Hemisphere cryosphere feedback being 2–5 times stronger than Northern Hemisphere cryosphere feedback.In the absence of changes in ice extent, changes in ice thickness in response to an increase in solar constant are associated with an increase in summer surface melting which is exactly balanced by increased basal winter freezing, and a reduction in the upward ocean-air flux in summer which is exactly balanced by an increased flux in winter, with no change in the annual mean ocean-air flux. Changes in the mean annual ocean-air heat flux require changes in mean annual ice extent, and are constrained to equal the change in meridional oceanic heat flux convergence in equilibrium. Feedback between ice extent and the meridional oceanic heat flux obtained by scaling the oceanic heat diffusion coefficient by the ice-free fraction regulates the feedback between ice extent and mean annual air-sea heat fluxes in polar regions, and has a modest effect on model-simulated high latitude temperature change.Accounting for the partial masking effect of vegetation on snow-covered land reduces the Northern Hemisphere mean temperature response to a 2% solar constant decrease or increase by 20% and 10%, respectively, even though the radiative forcing change caused by land snowcover changes is about 3 times larger in the absence of vegetational masking. Two parameterizations of the tundra fraction are tested: one based on mean annual land air temperature, and the other based on July land air temperature. The enhancement of the mean Northern Hemisphere temperature response to solar constant changes when the forest-tundra ecotone is allowed to shift with climate is only 1/3 to 1/2 that obtained by Otterman et al. (1984) when the mean annual parameterization is used here, and only 1/4 to 1/3 as large using the July parameterization.The parameterized temperature dependence of ice and snow albedo is found to enhance the global mean temperature response to a 2% solar constant increase by only 0.04 °C, in sharp contrast to the results of Washington and Meehl (1986) obtained with a mean annual model. However, there are significant differences in the method used here and in Washington and Meehl to estimate the importance of this feedback process. When their approach is used in a mean annual version of the present model, closer agreement to their results is obtained.

Development of a Sea Ice Model for Use in Zonally Averaged Energy Balance Climate Models

Abstract A sea ice model for use in zonally averaged energy balance climate models is presented which includes the following processes: surface melting, basal freezing and melting, lateral melting from ice-flee water or growth of new ice in leads, snowfall and the formation of white ice, ice advection, and a parameterized ice and snow thickness distribution which represents the effects of small-scale dynamics. The ice growth equations of Hibler are solved analytically, thereby permitting a gradual increase in zonal ice fraction in fall and winter. Both lateral and vertical melting lead to a continuous decrease of ice fraction during ice decay. The correlation between ice thickness and ice thickness sensitivity to the upward heat flux at the ice base is of opposite sign seasonally and latitudinally. The parameterized feedback between ice thickness and the minimum permitted lead fraction is found to be very important to the ice simulation, and is a process which needs to be studied using higher resolution, ...

Managing atmospheric CO2

A coupled carbon cycle-climate model is used to compute global atmospheric CO2 and temperature variation that would result from several future CO2 emission scenarios. The model includes temperature and CO2 feedbacks on the terrestrial biosphere, and temperature feedback on the oceanic uptake of CO2. The scenarios used include cases in which fossil fuel CO2 emissions are held constant at the 1986 value or increase by 1% yr−1 until either 2000 or 2020, followed by a gradual transition to a rate of decrease of 1 or 2% yr−1. The climatic effect of increases in non-CO2 trace gases is included, and scenarios are considered in which these gases increase until 2075 or are stabilized once CO2 emission reductions begin. Low and high deforestation scenarios are also considered. In all cases, results are computed for equilibrium climatic sensitivities to CO2 doubling of 2.0 and 4.0 °C.Peak atmospheric CO2 concentrations of 400–500 ppmv and global mean warming after 1980 of 0.6–3.2 °C occur, with maximum rates of global mean warming of 0.2–0.3 °C decade−1. The peak CO2 concentrations in these scenarios are significantly below that commonly regarded as unavoidable; further sensitivity analyses suggest that limiting atmospheric CO2 to as little as 400 ppmv is a credible option.Two factors in the model are important in limiting atmospheric CO2: (1) the airborne fraction falls rapidly once emissions begin to decrease, so that total emissions (fossil fuel + land use-induced) need initially fall to only about half their present value in order to stabilize atmospheric CO2, and (2) changes in rates of deforestation have an immediate and proportional effect on gross emissions from the biosphere, whereas the CO2 sink due to regrowth of forests responds more slowly, so that decreases in the rate of deforestation have a disproportionately large effect on net emission.If fossil fuel emissions were to decrease at 1–2% yr−1 beginning early in the next century, emissions could decrease to the rate of CO2 uptake by the predominantly oceanic sink within 50–100 yrs. Simulation results suggest that if subsequent emission reductions were tied to the rate of CO2 uptake by natural CO2 sinks, these reductions could proceed more slowly than initially while preventing further CO2 increases, since the natural CO2 sink strength decreases on time scales of one to several centuries. The model used here does not account for the possible effect on atmospheric CO2 concentration of possible changes in oceanic circulation. Based on past rates of atmospheric CO2 variation determined from polar ice cores, it appears that the largest plausible perturbation in ocean-air CO2 flux due to changes of oceanic circulation is substantially smaller than the permitted fossil fuel CO2 emissions under the above strategy, so tieing fossil fuel emissions to the total sink strength could provide adequate flexibility for responding to unexpected changes in oceanic CO2 uptake caused by climatic warming-induced changes of oceanic circulation.

A Semianalytic Energy Balance Climate Model with Explicit Sea Ice and Snow Physics

Abstract An energy balance climate model (EBCM) is presented having 1) a seasonal cycle; 2) surface-air, land-sea, and latitudinal resolution; 3) simulation of sea ice in terms of a number of explicit physical processes and in such a way that the sea ice fraction in any given zone changes continuously during the course of the seasonal cycle; 4) simulation of a continuously varying land-snow fraction in terms of explicit physical processes; and 5) a detailed treatment of surface and planetary albedo. A semianalytic solution is used which permits use of 6 day time steps, with very little dependence of the simulated climate on the choice of time step length for time steps of 1 to 6 days. Model sensitivity to internal parameter changes is investigated. The temperature response to a doubling of the drag coefficients for the vertical fluxes of latent and sensible heat is complex, and involves radiative constraints, the effect of stronger coupling to the large thermal inertia of the mixed layer, and ice and snow...