{"id":7,"date":"2004-12-04T18:22:50","date_gmt":"2004-12-04T22:22:50","guid":{"rendered":"\/?p=7"},"modified":"2020-02-03T09:07:29","modified_gmt":"2020-02-03T14:07:29","slug":"temperaturevariations-in-past-centuries-and-the-so-called-hockey-stick","status":"publish","type":"post","link":"https:\/\/www.realclimate.org\/index.php\/archives\/2004\/12\/temperaturevariations-in-past-centuries-and-the-so-called-hockey-stick\/","title":{"rendered":"Temperature Variations in Past Centuries and the so-called "Hockey Stick" Variations de temp\u00e9rature sur les derniers si\u00e8cles et la "crosse de hockey"<\/lang_fr>"},"content":{"rendered":"
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Instrumental data describing large-scale surface temperature changes<\/a> are only available for roughly the past 150 years. Estimates of surface temperature changes further back in time must therefore make use of the few long available instrumental records or historical documents and natural archives or ‘climate proxy’<\/a> indicators, such as tree rings, corals, ice cores and lake sediments, and historical documents to reconstruct patterns of past surface temperature change. Due to the paucity of data in the Southern Hemisphere, recent studies have emphasized the reconstruction of Northern Hemisphere (NH) mean, rather than global mean temperatures over roughly the past 1000 years.<\/p>\n


\npar Mike Mann (traduit par Gilles Delaygue)<\/small>
\nLes donn\u00e9es intrumentales d\u00e9crivant les changements \u00e0 grande \u00e9chelle de la temp\u00e9rature de surface<\/a> ne sont disponibles que pour les derniers 150 ans environ. Pour estimer des changements de temp\u00e9rature de surface ant\u00e9rieurs, il faut ainsi utiliser les quelques longs enregistrements disponibles, ou des documents historiques, ainsi que des archives naturelles ou proxies climatiques<\/a>, comme les cernes d’arbres, coraux, carottes de glace et s\u00e9diments lacustres, pour reconstruire les changements de temp\u00e9rature dans le pass\u00e9. En raison de la raret\u00e9 des donn\u00e9es dans l’h\u00e9misph\u00e8re sud, les \u00e9tudes r\u00e9centes ont \u00e9t\u00e9 focalis\u00e9es sur les reconstructions de moyennes pour l’h\u00e9misph\u00e8re nord, plut\u00f4t que globales, sur les derniers 1000 ans environ.
\n<\/lang_fr><\/p>\n

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The term “Hockey Stick” was coined by the former head of NOAA\u2019s Geophysical Fluid Dynamics Laboratory, Jerry Mahlman, to describe the pattern common to numerous proxy and model-based estimates of Northern Hemisphere mean temperature changes over the past millennium. This pattern includes a long-term cooling trend from the so-called “Medieval Warm Period”<\/a> (broadly speaking, the 10th-mid 14th centuries) through the “Little Ice Age”<\/a> (broadly speaking, the mid 15th-19th centuries), followed by a rapid warming during the 20th century that culminates in anomalous late 20th century warmth (Figure 1). Numerous myths regarding the “hockey stick”<\/a> can be found on various non-peer reviewed websites and other non-scientific venues.<\/p>\n

Estimates of Northern Hemisphere average temperature changes from climate model simulations employing estimates of long-term natural (e.g. volcanic and solar) and modern anthropogenic (greenhouse gas and sulphate aerosol) radiative forcings<\/a> of climate agree well, in large part, with the empirical, proxy-based reconstructions. One notable exception is a study by Gonzalez-Rouco et al (2003) that makes use of a dramatically larger estimate of past natural (solar and volcanic) radiative forcing than is accepted in most studies, and exhibits greater variability than other models (Figure 2). Yet, as in all of the other simulations, even in this case unprecedented warmth is indicated for the late 20th century.<\/p>\n

The simulations all show that it is not possible to explain the anomalous late 20th century warmth without including the contribution from anthropogenic forcing factors<\/a>, and, in particular, modern greenhouse gas concentration increases. A healthy, vigorous debate can be found in the legitimate peer-reviewed climate research literature with regard to the precise details of empirically and model-based estimates of climate changes in past centuries, and it remains a challenge to reduce the substantial uncertainties that currently exist. Despite current uncertainties, it nonetheless remains a widespread view among paleoclimate researchers that late 20th century hemispheric-scale warmth is anomalous in a long-term (at least millennial) context, and that anthropogenic factors likely play an important role in explaining the anomalous recent warmth.<\/p>\n

Reviews of past scientific research in this area can be found in the following peer-reviewed journal articles:<\/p>\n