Temperature Variations in Past Centuries and the so-called "Hockey Stick"

Instrumental data describing large-scale surface temperature changes 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temperature changes further back in time must therefore make use of the few long available instrumental records or historical documents and natural archives or ‘climate proxy’ indicators, such as tree rings, corals, ice cores and lake sediments, and historical documents to reconstruct patterns of past surface temperature change.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.

The term “Hockey Stick” was coined by the former head of NOAA’s 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” (broadly speaking, the 10th-mid 14th centuries) through the “Little Ice Age” (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” can be found on various non-peer reviewed websites and other non-scientific venues.

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 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.

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, 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.

Reviews of past scientific research in this area can be found in the following peer-reviewed journal articles:

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