Réf. Barnett & al. 2005 - A

Référence bibliographique complète
BARNETT T. P. , ADAM J. C.& LETTENMAIER D. P. Potential impacts of a warming climate on water availability in snow-dominated regions. Nature, 2005, 438, 303-309.

Abstract: 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 sufficent, 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 the future water availability–predicted with high confidence and already diagnosed in some regions–are likely to be severe.

Mots-clés
Global warming, hydrological cycle, evaporation, aerosols, snow-dominated regions, regional impacts

Organismes / Contacts
T. P. BARNETT: Climate Research Division, Scripps Institution of Oceanography, La Jolla, California 92093, USA.
J. C. ADAM, D. P. LETTENMAIER: Department of Civil and Environmental Engineering, University of Washington, Seattle, Washington 98195-2700, USA.

(1) - Paramètre(s) atmosphérique(s) modifié(s)
(2) - Elément(s) du milieu impacté(s)
(3) - Type(s) d'aléa impacté(s)
(3) - Sous-type(s) d'aléa
Temperature Runoff - Evapotranspiration - River discharge - Snowfall amounts - Snow/ice melt rates    

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Etage / Tranche d'altitude
Période(s) d'observation
World / USA / Europe Rhine river        

(1) - Modifications des paramètres atmosphériques
Reconstitutions
 
Observations
Aerosols are found to alter cloud physics in a manner that reduces precipitation downstream from the pollution source (61, 62). This also reduces the snow particle rime growth, resulting in lower snow water equivalent, a result obtained from direct field measurements (62-64).
Modélisations

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. 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.

There is relatively little agreement among the global models as to the magnitude (and even direction of) precipitation changes regionally (7-10), 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.

Hypothèses
Aerosols are thought to cool the planet's surface through increased scattering and cloud cover and re-radiation of solar energy to space. The representation of clouds in GCMs carries a large uncertainty all by itself, but the joint interaction of clouds and aerosols represents one of the major challenges to climate modellers today. Virtually all climate models have some representation of direct aerosol effects (that is, reflectivity of the particles) in them, but none have yet fully included the indirect effects (for example, the effect of aerosols on cloud distributions via their role as cloud condensation nuclei, or other effects discussed below). A preliminary study (60) suggests that indirect aerosol impacts on clouds are important but, even given the uncertainty in estimating these impacts, this mechanism is not strong enough to counter greenhouse warming effects.

Recent observational studies (58, 60) show that locally, over India, the total aerosol effect (direct plus indirect) has been associated with a surface cooling of 0.3°C over the last three decades. This is close to the warming expected from greenhouse gases. However, the aeroslos are observed to be associated with warming in the lower to middle troposphere—the regions inhabited by the glacier fields. In this case the aerosols may be enhancing the direct temperature forcing by contributing to the melting of the higher glaciers of the HKH (Himalaya-Hinu Kush) region.

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

 


(2) - Effets du changement climatique sur le milieu naturel
Reconstitutions
 
Observations
 
Modélisations

The projected changes in temperature strongly imply future changes of seasonal runoff patterns in snowmelt-dominated regions. In a warmer climate, snow will melt earlier in the year than it did before and in some places this has already happened (3, 11, 12). Taken together, these impacts mean less snow accumulation in the winter and an earlier peak runoff in the spring.

On a global scale, the largest changes in the hydrological cycle due to warming are predicted for the snow-dominated basins of mid- to higher latitudes, because adding or removing snow cover fundamentally changes the snow pack's ability to act as a reservoir for water storage (13). Studies in various regions of the globe indicate that the stream-flow regime in snowmelt-dominated river basins is most sensitive to wintertime increases in temperature (12, 13).

Hypothèses

Rhine River : Climate-change simulations project a warming in the Rhine River basin of 1.0-2.4°C over present values by the middle of the century (IPCC, 2001). Hydrological simulations suggest that this warming will shift the Rhine river basin from a combined rainfall and snowmelt regime to a more rainfall-dominated regime, resulting in an increase in winter discharge, a decrease in summer discharge, increases in the frequency and height of peak flows, and a longer and more frequent periods of low flow during the summer (33).

Snow/ice melt rates:
A common aerosol found in the atmosphere over many regions of the earth is black carbon. This substance absorbs sunlight. It is scrubbed from the atmosphere by precipitation and, because it is ubiquitous, is likely to end up in the snow and ice fields of the planet. There it could decrease the surface albedo, causing the snow/ice to absorb solar energy more readily and thereby melt sooner. Measurments of black carbon amounts and its budgets are only now being made. By whatever means, darkening the surface of a snow/ice field will enhance melt rates. Again, it seems that proper inclusion of aerosols in global climate models will increase early melting of snow packs and, especially, glaciers and sea ice (65).


Properly represented aerosols in climate models will apparently also work together with increasing temperature to reduce snow/ice in regions where heavy air pollution exists (for example, China, the western USA and Europe).


Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
Runoff seasonality and intensity sensitivity to temperature and precipitation

The discharge of 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 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.

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 runoff, is much more strongly related to temperature than to precipitation changes.

Global distribution of snowmelt-dominated and runoff

Use of a spatially distribued macroscale hydrology model (14) to identify the regions of the globe where snowmelt plays a dominant role in the seasonal patterns of stream-flow. The model was run over all global land areas (excluding Antartica and Greenland) at a spatial resoluion of 0.5° latitude/longitude for a twenty-year (1980-1999) period. The importance of snow to annual runoff was approximated. This allowed to determine wether or not runoff for each grid cell is snowmelt-dominated by using criterion that R > 0,5 for these cells.

For each of the world's major river basins, the simulated annual runoff was compared to the estimated reservoir storage capacity (15, 16) in order to determine cases where reservoir storage capacity is adequate to buffer large seasonal stream-flow shifts (and hence exclude basins that, in spite of being snowmelt-dominated, would be insensitive to shifts in runoff timing). Watersheds within the snowmelt-dominated domain that meet these criteria include the Colorado River, the Churchill River and the Grand River (all in North America), and the Angara River (a tributary of the Yenisei River) in Asia.

In general, the snowmelt-dominated regions occupy parts of the globe that are at latitudes greater than ~ 45° (North and South), with some exceptions : Mountainous regions are generally snowmelt-dominated.


(3) - Effets du changement climatique sur l'aléa
Reconstitutions
 
Observations
 
Modélisations
 
Hypothèses
 

Paramètre de l'aléa
Sensibilité du paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 
 

(4) - Remarques générales

All of the future climate predictions have uncertainties (...) In some cases, the uncertainties have to do with the models' inability to reproduce today's climate, casting doubt on on future climate predictions. Predictions using regional, high-spatial-resolution models , of the type needed for regional water studies, are only now starting to come into their own in the greenhouse arena, but they carry a whole set of problems in addition to those associated with the coupled atmosphere-ocean general circulation models (CGCMs). for instance, they often have different physics from the CGCMs— there are scale-dependence issues, and new levels of parametrizations are required. however, such regional models will be required for good quantitative estimates of potential future water problems [...].

One of the greatest uncertainties in future prediction has to do with how the models are forced. Stated more directly, what are the implications of omitting forcings that we strongly suspect (or know) are important but cannot yet reliably by included in the model physics? Of these, the most important is thougt to be the incomplete inclusion of aerosols and their impacts, especially on clouds. [...] discussions of the current state of the aerosol problem [...] shows the sensitivity of climate model predictions to uncertainties in indirect aerosol forcing.

The key question for this paper is: Can aerosol/cloud problem overwhelm the direct greenhouse-gas-induced temperature forcing that affects the regional hydrological cycle, giving net cooling as opposed to warming?


(5) - Syntèses et préconisations
In this review, we suggest that the simplest of changes associated with global warming (a model increase in near-surface air temperature) will be responsible for alterations of the hydrological cycle in snowmelt-dominated regions via seasonal shifts in stream-flow. Without adequate water storage capacity, these changes will lead to a serious reduction in dry-season water availability in many regions of the Earth within the next few decades.

The physical principles found to apply in snowmelt-dominated regions (for example, the western USA) are one of the probable causes of the observed early snowmelt and, more importantly, deglaciation that is now occurring in most mountainous regions of the world. The serious situations developing inthe HKH (Himalaya-Hindu Kush) region and South America have been briefly presented. It is clear that both regions, as well as others not mentioned, are headed for a water-supply crisis. Better water management techniques can help, but cannot solve the problem without significant changes to agriculture, industry and lifestyle. Detailed studies of the future impact of global warming on water ressources in thes regions are long overdue.

We have discussed briefly here some of the major uncertainties in the models, in particular the impacts of aerosols and clouds, as well as their suspected impacts on the aspects of the hydrological cycle having to do with snow and ice. In all the cases considered, current scientific evidence suggests that these processes, which are currently either not included, or are marginally included, in IPCC scenario runs, willact to increase the impact of mere temperature increase on the snow and ice fields of the planet.

Time is running out for nations in the sensitive areas we have evaluated, particularly those whose water supplues are dependent on mid-latitude glaciers, to understand just what the future might hold for them. How much they can do is uncertain given the several decades of warming that will occur as a result of past actions, even if greenhouse emissions were halted at today's levels, but perhaps the initiation of strategic planning will be motivated by the prospect (and what is rapidly becoming the reality) of diminished water supplies..

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