Dendroclimatology is the science of estimating past climatic conditions by checking the growth-rings found in sections of tree wood.

Because trees tend to grow more when the temperatures are higher, the width of their growth rings can be used as a rough guide to the climatic conditions in a particular year. In general, it correlates very well across trees of different species.

The technique correlates well with actual measured temperatures - within half a degree (C) or so since the 1880s. Since the 1950s, however, the correlation has drifted - the error now being as much as 3°C. According to the correlation factor, the trees are not growing as quickly as the temperature would suggest.

The measured error, known as the 'Divergence Problem' casts doubt on the accuracy of past measurements - which in some cases go back as far as 10,000 years.

The divergence problem has potentially significant implications for large-scale patterns of forest growth, the development of paleoclimatic reconstructions based on tree-ring records from northern forests, and the global carbon cycle.

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There is currently no explanation for the drift. Theories include:

• Changes in the ozone layer
• 'Global dimming' (changes in the amount of sunlight reaching the Earth's surface)
• Changes in insect activity

See : Global and Planetary Change Journal Vol. 60 (3–4): 289–305 (archived)

Note: The divergence has implications for the accuracy of calculations regarding (future) climate change. If the current observed tend continues, then forests will not grow as quickly as would be expected, and so will absorb less CO₂ from the atmosphere.

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