The Divergence Problem or 'Hiding the Decline'

Examining the Complexities of Tree-Ring Data and its Implications for Past Climate Reconstructions.

The "divergence problem" is an interesting phenomenon that has been a topic of much debate in the field of paleoclimatology. It refers to the unexpected divergence between tree ring growth and temperature data, particularly in the latter half of the 20th century. This divergence has been observed in some high-latitude regions, leading to questions about the reliability of tree ring data as a proxy for temperature reconstruction. Here, I will explore the divergence problem, its importance, how it affects reconstructions of past climate, and its relationship to Dr. Mann's ‘hide the decline’ issue.

The divergence problem refers to the discrepancy between temperature records derived from tree rings (proxies) and those directly measured by thermometers (instrumental records). Traditionally, dendroclimatology, or the study of tree rings to infer past climate conditions, relies on the principle that tree growth is often limited by climatic conditions. In temperate and cold environments, tree-ring width and density typically correlate with temperature, allowing dendroclimatologists to reconstruct past temperatures.

However, since the late 20th century, some tree-ring series have shown a decline or 'divergence' in ring width and density, despite concurrent temperature records indicating rising temperatures. This decoupling between tree-ring data and temperature records poses a challenge to the reliability of tree-ring-based climate reconstructions.

The importance of the divergence problem lies in its potential to undermine our understanding of past climate change and future climate projections. If tree rings, traditionally considered reliable proxies, fail to accurately reflect recent warming, their ability to reconstruct past climates comes into question. This, in turn, throws a wrench into efforts to model future climate scenarios as these are based on our understanding of past climatic states.

The divergence problem complicates the interpretation of tree-ring data, not only for the recent period when divergence is observed but for the past as well. It casts doubt on the assumption that tree growth is consistently and linearly related to temperature. This issue is especially pertinent when reconstructing temperature records for the last few millennia, a period crucial for understanding the context of modern climate change and the basis for outrageous claims like ‘2023 was the hottest year in the last 125,000 years’.

Researchers face methodological challenges in dealing with the divergence problem. They must determine whether to include divergent data in reconstructions, how to statistically treat this data, and how to interpret periods where tree-ring data might not be a reliable proxy for temperature in the past.

In scientific research, the integrity of knowledge hinges on an unwavering commitment to impartiality, especially in the treatment of data. It is imperative not to discard or downplay data simply because it does not align with preconceived narratives or hypotheses. Such selective handling of evidence can lead to skewed understandings and misrepresentations of reality, impeding the progress of science. The true strength of scientific methodology lies in its ability to challenge and refine our understanding of the world, a process that demands the inclusion and examination of all data, irrespective of its congruence with existing theories.

Not showing this divergence became known as ‘hiding the decline’ or ‘Mike’s Nature trick’. In scientific contexts, ‘trick’ often means a clever method or technique, and in this case,