ScienceWeek

CLIMATOLOGY: IMPACTS OF WARMING CLIMATE ON WATER AVAILABILITY

The following points are made by T.P. Barnett et al (Nature 2005 438:303):

1) Water is essential to human sustenance. Well over half of the world's potable water supply is extracted from rivers, either directly or from reservoirs. The discharge of these rivers is sensitive to long-term changes in both precipitation and temperature, particularly in the snowmelt-dominated parts of the world. Changes in the amount of precipitation tend to affect the volume of runoff and particularly the maximum snow accumulation, which usually occurs near the end of the winter at the onset of the melt season. On the other hand, temperature changes mostly affect the timing of runoff. Increasing temperatures lead to earlier runoff in the spring or winter, and reduced flows in summer and autumn -- at least in the absence of changes in precipitation.

2) In general, the direction and (to a lesser extent) the magnitude of surface temperature changes are much more consistent among climate models than are precipitation changes[1]. Near-surface air-temperature predictions from existing global climate models that are forced with anthropogenic increases in atmospheric greenhouse gas concentrations imply a high degree of confidence that future changes to the seasonality in water supply will occur in snowmelt-dominated regions. Even for models with temperature sensitivities near the lower end of the predicted range, impacts on snowmelt-dominated regional water resources are substantial[2]. Indeed, such changes are already obvious in the observational records of key components of the hydrological cycle, such as snow pack in the western USA[3-5]. Taken together, the predictions and observations portend important issues for the water resources of a substantial fraction of the world's population.

3) It is generally thought that increasing greenhouse gases will cause the global hydrological cycle to intensify[1], with benefits for water availability[1], although a possible exacerbation of hydrological extremes may counteract the benefits to some degree. However, in regions where the land surface hydrology is dominated by winter snow accumulation and spring melt, the performance of water management systems such as reservoirs, designed on the basis of the timing of runoff, is much more strongly related to temperature than to precipitation changes. Even though there is relatively little agreement among the global models as to the magnitude (and even direction of) precipitation changes regionally, there is no indication for a seasonal shift of precipitation to the summer and autumn. The projected changes in temperature therefore strongly imply future changes of seasonal runoff patterns in snowmelt-dominated regions.

4) In summary: All currently available climate models predict a near-surface warming trend under the influence of rising levels of greenhouse gases in the atmosphere. In addition to the direct effects on climate -- for example, on the frequency of heatwaves -- this increase in surface temperatures has important consequences for the hydrological cycle, particularly in regions where water supply is currently dominated by melting snow or ice. In a warmer world, less winter precipitation falls as snow and the melting of winter snow occurs earlier in spring. Even without any changes in precipitation intensity, both of these effects lead to a shift in peak river runoff to winter and early spring, away from summer and autumn when demand is highest. Where storage capacities are not sufficient, much of the winter runoff will immediately be lost to the oceans. With more than one-sixth of the Earth's population relying on glaciers and seasonal snow packs for their water supply, the consequences of these hydrological changes for future water availability -- predicted with high confidence and already diagnosed in some regions -- are likely to be severe.

References (abridged):

1. The International Panel for Climate Change (IPCC) Climate Change 2001: The Scientific Basis (eds Houghton, J. T. et al.) (Cambridge Univ. Press, Cambridge, UK, 2001)

2. Barnett, T. P. & Pennell, W. (eds) Impact of global warming on Western US water supplies. Clim. Change 62 (Spec. Vol.) (2004)

3. Mote, P. W. , Hamlet, A. F. , Clark, M. P. & Lettenmaier, D. P. Declining mountain snow pack in western North America. Bull. Am. Met. Soc. 86, 39 49 (2005)

4. Dettinger, M. D. , Cayan, D. R. , Meyer, M. K. & Jeton, A. E. Simulated hydrologic responses to climate variations and change in the Merced, Carson, and American River Basins, Sierra Nevada, California, 1900 2099. Clim. Change 62, 283 317 (2004)

5. Hamlet, A. F. , Mote, P. W. , Clark, M. P. & Lettenmaier, D. P. Effects of temperature and precipitation variability on snow pack trends in the western U.S. J. Clim. (in the press)

Nature http://www.nature.com/nature

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OCEANOGRAPHY: GREENHOUSE EFFECT AND WARMING OF THE OCEANS

The following points are made by G.C. Hegerl and N.L. Bindoff (Science 2005 309:254):

1) Rising greenhouse gas concentrations in the atmosphere are trapping more infrared radiation near Earth's surface. This extra radiation is expected to warm Earth's surface and lower atmosphere, but observations indicate that most of the heat is transported into the oceans. New work [1] substantially strengthens the evidence that human activities are indeed warming the world's oceans.

2) Observations have shown that 84% of the total heating of the Earth system since the 1950s is in the oceans [2]. This increased ocean heat content has led to thermal expansion of the ocean, contributing at least 25% of the global sea-level rise observed over the same period [3]. Ocean warming may also lead to greater stratification of the ocean, causing a weakening of the global overturning circulation in most model projections of future climates [4]. Furthermore, the oceans are a key element in the global carbon cycle and are estimated to be storing roughly half of the total carbon released through human activities since preindustrial times [5]. For all these reasons, the oceans are an important place to look for changes expected due to greenhouse warming ("fingerprints").

3) Many of the changes observed at Earth's surface and in the free atmosphere in the 20th century can be reproduced by climate models that account for the increase in greenhouse gases, aerosols associated with pollution, changes in solar radiation, and reflection by volcanic aerosols. Fingerprint methods use detailed information about the climate response to these external influences in order to separate them from each other and from natural variability within the climate system. Studies using such methods have shown with high confidence that much of the temperature change observed at Earth's surface and in the free atmosphere over the past 50 years has been caused by increases in greenhouse gas concentrations.

4) Barnett et al [1] applied a fingerprint method to the temperature history of the upper 700 m of each ocean basin since 1960. In this depth range, the greatest temperature changes are found [1,2]; it is also where we have the best knowledge of ocean behavior. The authors compare the best available ocean observation data set to simulations using two different climate models. They find strong evidence that the anthropogenic fingerprint anticipated by the models is present in the observations. The results show that changes in solar radiation and volcanic forcing cannot explain the observed pattern of ocean changes. The fact that the findings are robust for two different climate models indicates that the results are not affected significantly by differences in model formulation.

References (abridged):

1. T. P. Barnett et al., Science 309, 284 (2005)

2. S. Levitus, J. I. Antonov, T. P. Boyer, Geophys. Res. Lett. 32, L02604 (2005)

3. J. I. Antonov, S. Levitus, T. P. Boyer, Geophys. Res. Lett. 32, L12602 (2005)

4. U. Cubasch et al., in Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, J. T. Houghton et al., Eds. (Cambridge Univ. Press, New York, 2001), pp. 525-582

5. C. L. Sabine et al., Science 305, [367] (2004)

Science http://www.sciencemag.org

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CLIMATE SENSITIVITY UNCERTAINTY AND THE NEED FOR ENERGY WITHOUT CO2 EMISSION

The following points are made by K. Caldeira et al (Science 2003 299:2052):

1) "Climate sensitivity" (T2X) is the global mean climatological temperature change resulting from a doubling of atmospheric CO2 content. Climate sensitivity is thought, based primarily on models, to lie in the range of 1.5ø to 4.5 degrees C (1,2). Cloud feedbacks remain the greatest source of uncertainty in model predictions of global mean warming (3). Aerosols, non-CO2 greenhouse gases, internal variability in the climate system, and land use change also affect Earth's temperature (2). Uncertainty in aerosol radiative forcing precludes a more accurate, observationally based estimate of climate sensitivity to a CO2 doubling (4,5).

2) In summary: 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 CO2 is known perhaps only to a factor of three or less. The authors demonstrate how a factor of three uncertainty in climate sensitivity introduces even greater uncertainty in allowable increases in atmospheric CO2 concentration and allowable CO2 emissions. Nevertheless, unless climate sensitivity is low and acceptable amounts of climate change are high, climate stabilization will require a massive transition to CO2 emission-free energy technologies.

References (abridged):

1. J. T. Houghton et al., Eds., Climate Change 1995: The Science of Climate Change (Cambridge Univ. Press, UK, 1996)

2. U. Cubasch et al., in Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, J. T. Houghton et al., Eds. (Cambridge Univ. Press, UK, 2001), pp. 525-582

3. R. D. Cess, et al., J. Geophys. Res. 101, 12791 (1996)

4. N. G Andronova and M. E. Schlesinger, J. Geophys. Res. 106, 22605 (2001)

5. C. E. Forest, P. H. Stone, A. P. Sokolov, M. R. Allen, M. D. Webster, Science 295, 113 (2002)

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