• What kinds of proxy evidence can be used to estimate surface temperatures for the last 2,000 years?

  • How are proxy data used to reconstruct surface temperatures over different geographic regions and time periods?

  • What is our current understanding of how the hemispheric mean or global mean surface temperature has varied over the last 2,000 years?

  • What conclusions can be drawn from large-scale surface temperature reconstructions?

  • What are the limitations and strengths of large-scale surface temperature reconstructions?

  • What do climate models and forcing estimates tell us about the last 2,000 years?

  • How central are large-scale surface temperature reconstructions to our understanding of global climate change?

  • What comments can be made on the value of exchanging information and data?

  • What might be done to improve our understanding of climate variations over the last 2,000 years?

What kinds of proxy evidence can be used to estimate surface temperatures for the last 2,000 years?

Instrumental Records

Combining instrumental records to calculate large-scale surface temperatures requires including a sufficient number of instrumental sites with wide geographic distribution to get a representative estimate. Instrumental temperature records extend back over 250 years in some locations, but only since the middle of the 19th century has there been a sufficient number of observing stations to estimate the average temperature over the Northern Hemisphere or over the entire globe. Tropical measurements are particularly useful for estimating global mean temperature because tropical temperature variations tend to track global mean variations more closely.

Documentary and Historical Records

In many parts of the world, the surface temperature record can be extended back several centuries by examining historical documents such as logbooks, journals, court records, and the dates of wine harvests. This evidence shows that several regions were relatively cool from about 1500 to 1850, a period sometimes referred to as the Little Ice Age. Historical evidence also suggests that Europe and East Asia, in particular, experienced periods of relative warmth during the medieval interval from roughly A.D. 900 to 1300. In contrast to the widespread warming of the 20th century, the timing of these earlier warm episodes appears to have varied from location to location, but the sparseness of data precludes certainty on this point.

In areas where writing was not widespread or preserved, archeological evidence such as excavated ruins can also sometimes offer clues as to how climate may have been changing at certain times in history and how human societies may have responded to those changes. However, the interpretation of historical, documentary, and archeological evidence is often confounded by factors such as disease outbreaks and societal changes. Hence, climatologists more often rely on natural proxy evidence to

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