Past reconstructions: problems, pitfalls and progress

Many people hold the mistaken belief that reconstructions of past climate are the sole evidence for current and future climate change. They are not. However, they are very interesting and useful for all sorts of reasons: for modellers to test out theories of climate change, for geographers, archaeologists and historians to examine the impact of climate on past civilizations and ecosystems, and for everyone to get a sense of what climate is capable of doing, how fast it does it and why.

As a small part of that enterprise, the climate of the medieval period has received a very high (and sometimes disproportionate) profile in the public discourse – due in no small part to the mistaken notion that it is an important factor for the attribution of current climate change. Its existence as a period of generally warmer temperatures (at least in the Northern hemisphere) than the centuries that followed is generally accepted. But the timing, magnitude and spatial extent are much more uncertain. All previous multiproxy reconstructions indicate a Northern Hemisphere mean temperature less than current levels, though possibly on a par with the mid- 20th century. But there are only a few tenths of a degree in it, and so the description that it is likely to have been warmer now (rather than virtually certain) is used to express the level of uncertainty.

A confounding factor in discussions of this period is the unfortunate tendency of some authors to label any warm peak prior to the 15th Century as the ‘Medieval Warm Period’ in their record. This leads to vastly different periods being similarly labelled, often giving a misleading impression of coherence. For instance, in a recent paper it was defined as 1200-1425 CE well outside the ‘standard’ definition of 800-1200 CE espoused by Lamb.

Since a new ‘reconstruction’ of the last 2000 years from Craig Loehle is currently doing the rounds, we thought it might be timely to set out what the actual issues are in making such reconstructions (as opposed to the ones that are more often discussed), and how progress is being made despite the pitfalls.

The Loehle paper was published in Energy and Environment – a journal notable only for its rather dubious track record of publishing contrarian musings. The reconstruction itself is based on a network of 18 records that are purportedly local temperature proxies, and we will use those as examples in the points below. More discussion of this paper is available here (via the wayback machine).

Issue 1: Dating Nothing is more important than chronology in paleo-climate. If you can’t line up different records accurately, you simply can’t say anything about cause and effect or coherence or spatial patterns. Records where years can be accurately counted are therefore at a premium in most reconstructions. These encompass tree ring width, density and isotopes, some ice cores, corals, and varved lake sediments. The next most useful set of data are sources that have up to decadal resolution but that can still be dated relatively accurately. High- resolution ocean sediment cores can sometimes be found that fit this, as can some cave (speleothem) records and pollen records etc.

There are nonetheless more problems with the decadal data – they may have been smoothed by non-climate processes, and their dating may be off by a decade or two. But there are enough records that are widely enough dispersed to make them a useful adjunct in a reconstruction that hopes to capture decadal to multi-decadal variability.

Using data that has significantly worse resolution than that in reconstructions of recent centuries is asking for trouble. The age models tend to have errors in the 100’s of years, and the density of points rarely allows one to reach the modern instrumental period.

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