Divergence problem

Twenty-year smoothed plots of averaged ring-width (dashed) and tree-ring density (thick line), averaged across all sites, and shown as standardized anomalies from a common base (1881-1940), and compared with equivalent-area averages of mean April-September temperature anomalies (thin solid line). From Briffa 1998 .

The divergence problem is an anomaly from the field of dendroclimatology, the study of past climate through observations of old trees, primarily the properties of their annual growth rings. It is the disagreement between the temperatures measured by the thermometers (instrumental temperatures) and the temperatures reconstructed from the widths of tree rings in the far northern forests.

While the thermometer records indicate a substantial warming trend, many tree rings do not display a corresponding change in their width. A temperature trend extracted from tree rings alone would not show any substantial warming. The temperature graphs calculated in these two ways thus "diverge" from one another since the 1950s, which is the origin of the term.

Samples from southern forests do not exhibit this divergence, though this could be due to paucity of samples, and not all trees in the northern hemisphere do. Divergence is most common in the far northern hemisphere.

Importance

The divergence problem does not tell us anything interesting about temperatures over the last 150 years - during this period, the instrumental temperature record is far more accurate. However, the fact that during one period the tree-ring temperature proxy deviates from a more accurate record suggests the possibility that this may have happened in the past, too, for which there is no instrumental record to cross-check against. Other palaeo-proxies - for example, ice cores - do not suffer from this effect and to a limited extent suggest that the problem does not occur; but they have less geographical coverage.

However, the real problem is with the reconstructions before the instrumental record begins. Specifically, tree ring data was used to suggest that the current warming is much more than what occurred during the Medieval Warm Period. Using sediment cores (another temperature proxy), there is evidence that the current CO2 induced warming is significantly less than normal climate variability. The divergence problem, and the fact that the key IPCC scientists were trying to hide it, clearly indicates that more research is needed.

Possible explanations

Rosanne D'Arrigo, senior research scientist at the Tree-Ring Lab at Columbia University's Lamont-Doherty Earth Observatory, has a hypothesis that "beyond a certain threshold level of temperature the trees may become more stressed physiologically, especially if moisture availability does not increase at the same time." Signs suggestive of such stress are visible from space, where satellite pictures show "evidence of browning in some northern vegetation despite recent warming."

Another possible explanation advanced for the divergence is global dimming. In this scenario, growth rings are affected by changes in cloud patterns and atmospheric pollution.