Sommes nous sûrs qu’il faisait plus chaud il y a 6000 ans ?

par Michael Mann (traduit par Thibault de Garidel)

Cette assertion est très souvent répétée mais reste problématique en raison de la mauvaise caractérisation de cette période appelée ““Optimum climatique de l’Holocène moyen” ou “période chaude de l’Holocène moyen”. Les experts des paléoclimats, savent maintenant que cette phase chaude à l’Holocène (il y a approximativement 8000 à 6000 ans) était probablement limitée aux hautes latitudes et à certaines saisons (été dans l’hémisphère nord et hiver dans l’hémisphère sud). Comme la majorité des indicateurs du climat passé disponibles (comme par exemple, assemblages de pollens fossiles) provenaient des latitudes moyennes à hautes de l’hémisphère Nord, et étaient diagnostiques des condition climatiques d’été, certains scientifiques ont cru que cette période de temps était plus chaude globalement. Cependant, désormais, de nombreuses études montrent que les régions tropicales étaient plus fraîches pendant la plus grande partie de l’année.


Tous ces changements sont conformes à la réponse des températures de surface aux changements bien documentés de la géométrie orbitale de la Terre par rapport au soleil pendant cette période de temps et aux rétroactions associées du climat, comme détaillé dans la littérature scientifique (voir Ganopolski et al., 1998; Hewitt et Mitchell, 1998). L’étude de Kitoh et Murakami fournit une des meilleures preuves disponibles que la température moyenne globale était pendant cette période de temps probablement semblable aux conditions prédatant le 20ie siècle, mais plus faible que celle du 20ie siècle (Kitoh et Murakami, 2002).

Pour trouver plus d’informations sur la période chaude de l’Holocène moyen, le site de la NOAA fournit une bonne référence (en anglais) :

Références :

Ganopolski, A., C. Kubatzki, M. Claussen, V. Brovkin, and V. Petoukhov, The Influence of Vegetation-Atmosphere-Ocean Interaction on Climate During the Mid-Holocene, Science, 280, 1916-1919, 1998.

Hewitt, C.D. and J.F.B. Mitchell, A Fully Coupled GCM Simulation of the Climate of the Mid-Holocene, Geophys. Res. Lett., 25, 361-364, 1998.

Kitoh, A., and S. Murakami, Tropical Pacific Climate at the mid-Holocene and the Last Glacial Maximum simulated by a coupled ocean-atmosphere general circulation model, Paleoceanography, 17, 1-13, 2002.

8 comments on this post.
  1. Dian Deevey:

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  2. Jerome S. Thaler:

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  3. Jerome S. Thaler:

    A very much needed service, keep it up
    For regional interest see Catskill Weather, Weather History and Climate Guide
    to the Lower Hudson Valley and Adirondack Weather

  4. Roger Jones:

    Good work on the site. Following is an Antipodean perspective on palaeoclimatic reconstructions.

    The emphasis on northern hemisphere palaeoclimate records as representing a global record is very problematic for those of us who live and work in the rest of the world.

    In southern Australia, the period 6-8 ka (thousand years) before present, crater lakes that now show falling water levels, were overflowing. Reconstructions of climate indicate that the lake evaporation/rainfall ration was above 1 and could hve been greater than 1.2 (Palaeoclimates, 3, 51â??82). The current ratio in the region is about 0.8. If temperatures and solar radiation were at current levels at 6-8 ka, this would require rainfall at >150% above today’s volumes.

    It is therefore likely that the climate at that time was cooler, particularly in summer (though the cloudy winters may have been warmer, suggesting a more uniform seasonal cycle), suppressing evaporation. Rainfall was certainly higher at that time. An abrupt shift at about 5.5 ka saw some 2,000 years of sustained drying.

    In northern Australia at around 6-7 ka (eastern Queensland), reconstructions indicate warmer and drier conditions in southern Queensland and warmer and wetter conditions in the north ( AbstractBook/Grindrod119_122.pdf), the former from one site and the latter from multiple sites. At that time, animals in the montane tropics were restricted in their distribution due to higher temperatures, suggesting that similar restrictions may occur under global warming (Proceedings of the Royal Society: Biological Sciences, 270, 1887-1892; track back through refs for palaeo work). The past 200 years of land-use change in Australia has compromised most hydrological proxies, making reconstruction difficult. We still don’t have a good idea of what was going on in the Holocene in Australia, but words like “optimum”, developed to describe conditions in the cooler regions of the northern hemisphere, confuse when applied globally.

    Note also in reference to the climate of the past 2000 years, south-eastern Australia was relatively cool and wet until about the mid 19th century (Journal of Hydrology, 246, 158-179). It is now as dry in the region as it was in the early Holocene and drier than it has been for most of the Holocene. These changes precede the enhanced greenhouse effect. If changes in rainfall projected by climate models occur it could become as dry in southern Australia as it was about 12.5 ka before present.

    Simple pictures of how Earth’s climate has behaved in the past rarely stand up to regional scrutiny. We should not jump to the conclusion that largely northern hemisphere reconstructions show some kind of globally homogenised picture. Palaeoclimatic reconstructions are sorely needed from the three southern continents: Africa, South America and Australia and from southern islands (the latter mentioned to keep our Kiwi and Pacific friends happy).

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