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ScienceWeek
AIR POLLUTION, CLIMATE FORCING, AND A GLOBAL HYDROGEN ECONOMY
The following points are made by M.G. Schultz et al (Science 2003 302:624):
1) It is now widely accepted that the increased combustion of fossil fuels since the industrialization of the Western world has led to unprecedented changes in the chemical composition of Earth's atmosphere, with multiple consequences for regional air quality and the global climate system (1,2). For example, despite efforts in several countries to control emissions from car exhaust and stationary sources, the hemispheric background concentrations of tropospheric ozone did not decline after 1985, when catalytic converters were introduced in the United States and Europe (3,4). On the contrary, many regions in the world have observed a serious degradation of air quality over the past decade, due to increased motorized traffic and industrial emissions (5).
2) Since the 1980s, alternative options for fulfilling the global energy demand have been developed. Because of the requirements of a mobile society and the recent advances in fuel cell technology, the widespread use of hydrogen (H2) produced with renewable energy sources currently appears to be the most promising option, in particular for nonstationary energy uses. Although H2 fuel cells themselves are a clean technology, producing only water vapor as exhaust, emissions of greenhouse gases and ozone precursors associated with the production of H2 must be considered. Furthermore, the release of molecular hydrogen may increase because of leakage attributable to the production, transport, storage, and end use of H2.
3) At present, the average leak rate to be expected in a full-scale hydrogen-driven economy is very uncertain. Existing systems such as the regional H2 distribution network that has operated for more than 50 years in Germany or a fully designed intercontinental liquid hydrogen transport chain demonstrate that leak rates below 0.1% can be achieved in industrial applications. Loss rates from natural gas supply chains from the well to the end user typically range between 0.5 and 1.5% and can be reduced even further. If these existing grids were used for transporting hydrogen without further adaptations (such as new fittings or inline coating), the leak rates would increase by a factor of 3 because of the higher diffusivity of H2 as compared to natural gas. In extreme cases, leak rates of 10 to 20% are possible (such rates could be produced by uncontrolled evaporation from liquid hydrogen storage tanks, for example). Such losses are, however, very unlikely to occur on a large scale because of safety and economic considerations.
4) In summary: If today's surface traffic fleet were powered entirely by hydrogen fuel cell technology, anthropogenic emissions of the ozone precursors nitrogen oxide (NO[sub-x]) and carbon monoxide could be reduced by up to 50%, leading to significant improvements in air quality throughout the Northern Hemisphere. Model simulations of such a scenario predict a decrease in global OH and an increased lifetime of methane, caused primarily by the reduction of the NO[sub-x] emissions. The sign of the change in climate forcing caused by carbon dioxide and methane depends on the technology used to generate the molecular hydrogen. A possible rise in atmospheric hydrogen concentrations is unlikely to cause significant perturbations of the climate system.
References (abridged):
1. Climate Change 2001: The Scientific Basis, Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge Univ. Press, Cambridge, 2001)
2. Scientific Assessment of Ozone Depletion, Report No. 47, World Meteorological Organization Global Ozone Research and Monitoring Project (World Meteorological Organization, Geneva, 2002)
3. J. A. Logan et al., J. Geophys. Res. 104, 26373 (1999)
4. A. Volz-Thomas et al., in Towards Cleaner Air for Europe–Science, Tools, and Applications, Results from the Eurotrac-2 Synthesis and Integration Project, Part I (Margraf, Weikersheim, Germany, 2003), pp. 73–122
5. World Health Organization, Fact Sheet No. 187 (September 2000) Available at www.who.int/inf-fs/en/fact187.html
Science http://www.sciencemag.org
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CONSTRAINTS ON RADIATIVE FORCING AND FUTURE CLIMATE CHANGE FROM OBSERVATIONS AND CLIMATE MODEL ENSEMBLES
The following points are made by R. Knutti et al (Nature 2002 416:719):
1) The assessment of uncertainties in global warming projections is often based on expert judgment, because a number of key variables in climate change are poorly quantified. In particular, the sensitivity of climate to changing greenhouse-gas concentrations in the atmosphere and the radiative forcing effects by aerosols are not well constrained, leading to large uncertainties in global warming simulations(1).
2) The expected future warming of the climate system and its potential consequences increase the need for climate projections with clearly defined uncertainties and likelihood estimates(2). The IPCC provides these probabilities for most of their findings in the recently published Third Assessment Report(1). However, some of the most important uncertainties -- such as the projected surface warming -- are still based on expert judgment, and are only given as ranges derived from different models. The evidence that part of the observed warming of both atmosphere and ocean(3,4) is caused by anthropogenic emissions of greenhouse gases and aerosols(1,5) may help assess climate models, and has been used to scale model projections for the next few decades.
3) The authors present a Monte Carlo approach to produce probabilistic climate projections, using a climate model of reduced complexity. The uncertainties in the input parameters and in the model itself are taken into account, and past observations of oceanic and atmospheric warming are used to constrain the range of realistic model responses. The authors obtain a probability density function for the present-day total radiative forcing, giving 1.4 to 2.4 W m^(-2) for the 5 to 95 per cent confidence range, narrowing the global-mean indirect aerosol effect to the range of 0 to –1.2 W m^(-2). Ensemble simulations for two illustrative emission scenarios suggest a 40 per cent probability that global-mean surface temperature increase will exceed the range predicted by the Intergovernmental Panel on Climate Change (IPCC), but only a 5 per cent probability that warming will fall below that range.
References (abridged):
1. Houghton, J. T. et al. (eds) Climate Change 2001: The Scientific Basis (Cambridge Univ. Press, Cambridge, 2001).
2. Schneider, S. H. What is "dangerous" in climate change? Nature 411, 17-19 (2001).
3. Jones, P. D., New, M., Parker, D. E., Martin, S. & Rigor, I. G. Surface air temperature and its changes over the past 150 years. Rev. Geophys. 37, 173-199 (1999).
4. Levitus, S., Antonov, J. I., Boyer, T. P. & Stephens, C. Warming of the world ocean. Science 287, 2225-2229 (2000).
5. Santer, B. D. et al. A search for human influences on the thermal structure of the atmosphere. Nature 382, 39-46 (1996)
Nature http://www.nature.com/nature
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CARBON DIOXIDE AND CLIMATE CHANGE
The term "greenhouse effect" refers to the blockade of longwave (infrared) radiation from the Earth into space by trace constituents of the atmosphere, primarily water vapor, carbon dioxide, ozone, methane, nitrous oxide, and various chlorofluorocarbons -- gases referred to collectively as the "greenhouse gases". The sensitivity of the biosphere to the greenhouse effect has existed throughout the history of life on Earth: the infrared radiation-blocking gases return radiated heat to the ground, accounting for approximately 70 percent of the net input of energy to the Earth's surface.
In this context, the term "radiative forcing" refers in general to climate changes produced by changes in incoming or outgoing radiative heat to and from the Earth's surface.
The term "tectonic" refers in general to deformations of the Earth's crust and consequent structural changes, and "tectonic timescales" are the long timescales (e.g., tens of millions of years) over which such deformations occur.
The following points are made by T. J. Crowley and R.A. Berner (Science 2001 292:870):
1) The authors point out that it has long been known that on timescales of tens of millions of years, intervals of continental glaciation were interspersed with intervals of little or no ice. The magnitude of warmth during these warm intervals is impressive: for example, at times during the Cretaceous period (approximately 65 to 145.6 million years ago) duck-billed dinosaurs roamed the northern slope of Alaska, and deep and bottom waters of the ocean, now near freezing, could reach a balmy 15 degrees Celsius.
2) The authors point out that in the 1980s, a convergence of results from paleoclimate data and geochemical and climate models suggested that such long-term variations in climate were strongly influenced by natural variations in the carbon dioxide content of the atmosphere. Recently, some geochemical results have raised concerns about the validity of this conclusion, since carbon dioxide concentrations over the past 65 million years appear to have reached low levels well before the most recent phase (the past 3 million years) of Northern Hemisphere glaciation. A study spanning the Phanerozoic time-frame (the past 570 million years) also suggests some decoupling between times of predicted high carbon dioxide and some climate indices.
3) In order to reevaluate the validity of the assumed carbon dioxide-climate link, the authors compared estimates of Phanerozoic carbon dioxide variations and net radiative forcing with the continental glaciation record and low-latitude temperature estimates. The authors report that the first order agreement between the carbon dioxide record and continental glaciation continues to support the conclusion that carbon dioxide has played an important role in long-term climate change.
4) The authors conclude that to weigh the merits of the carbon dioxide paradigm, it may be necessary to expand the scope of climate modeling. For factors responsible for the presence or absence of continental ice, the carbon dioxide model works very well. In contrast, there are substantial gaps in our understanding of how climate models distribute heat on the planet in response to carbon dioxide changes on tectonic time scales. "Given the need for better confidence in some of the paleoclimate data, and unanticipated complications arising from altered tectonic boundary conditions, it may be hazardous to infer that existing discrepancies between models and data cloud interpretations of future anthropogenic greenhouse gas projections."
Science http://www.sciencemag.org
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EARTH SCIENCES: AN ALTERNATIVE SCENARIO FOR GLOBAL WARMING
Earth's global surface temperature has increased by approximately 0.5 degrees centigrade since 1975, a relative "burst" of warming that has apparently taken the global temperature to its highest level in the past 1000 years, and there is a growing consensus that the warming is at least in part a consequence of increasing anthropogenic *greenhouse gases. These gases cause a global "climate forcing", i.e., an imposed perturbation of the energy balance of the Earth with space. There are many competing natural and anthropogenic climate forcings, but increased greenhouse gases are estimated to be the largest forcing and to result in a net positive forcing, especially during the past few decades. Evidence supporting this interpretation has been provided by observed heat storage in the ocean, which is positive and which is of the magnitude of the energy imbalance estimated from climate forcings for recent decades.
The following points are made by J. Hansen et al (Proc. Nat. Acad. Sci. 2000 97:9875):
1) The authors point out that a common view is that the current global warming rate will continue or accelerate. The authors, however, argue that rapid warming in recent decades has been driven mainly by non-carbon dioxide greenhouse gases such as chlorofluorocarbons, methane, and N(sub2)O, and not by the products of fossil fuel burning, carbon dioxide, and *aerosols, the positive and negative climate forcings of which are partially offsetting.
2) The authors point out that the growth rate of non-carbon dioxide greenhouse gases has declined in the past decade. If sources of methane and O(sub3) precursors were reduced in the future, the change in climate forcing by non-carbon dioxide greenhouse gases in the next 50 years could be near zero. Combined with a reduction of *black carbon emissions and plausible success in slowing carbon dioxide emissions, this reduction of non-carbon dioxide greenhouse gases could lead to a decline in the rate of global warming, reducing the danger of dramatic climate change.
3) The authors suggest that such a focus on air pollution has practical benefits that unite the interests of developed and developing countries, although assessment of ongoing and future climate change requires composition-specific long-term global monitoring of aerosol properties.
[Editor's note: After its publication, this paper became controversial and received considerable publicity. The senior author, James Hansen, is noted for helping to alert the world to global warming in 1988, and this recent paper has been interpreted as a reversal of his ideas concerning the dangers of fossil fuel, carbon dioxide, and aerosol emissions, and publicized by those opposed to the Kyoto Protocol on climate change. For an account of reaction to this paper, see: Nature 2000 407:7.]
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Notes:
greenhouse gases: The physical basis of the so-called "*greenhouse effect" is essentially simple: carbon dioxide gas is transparent to visible light but relatively opaque to infrared radiation. The same is true of glass. Relatively high-energy visible light radiation from the sun passes inward through the atmosphere, warms the surface of the Earth, which then radiates lower energy in the form of infrared radiation (heat) back to the atmosphere. But if the atmosphere has a concentration of infrared impenetrable gases such as carbon dioxide, the infrared radiation cannot pass out, and the surface of the Earth underlying the atmosphere cannot cool, and the surface of the Earth thus will continue to grow hotter.
aerosols: The term "aerosol" refers to a dispersion in which a finely divided solid is suspended in air and the particles are of colloidal dimensions. The term "colloidal dimensions" refers to the range approximately 1 nanometer to 100 nanometers in diameter.
black carbon: (carbon black) Amorphous (i.e., non-crystalline) carbon.
Proc. Nat. Acad. Sci. http://www.pnas.org
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