Climate reconstructions based on tree rings have become a major point of contention. In 1998, a climate scientist by the name of Michael Mann had the misfortune of publishing a Northern Hemisphere temperature reconstruction dubbed “the hockey stick,” due to the appearance of the abrupt warming of the last century. He quickly became a favorite target for several high-profile contrarians, which has brought him a great deal of harassment.

Lost in the noise of this blogosphere-generated public controversy is the fact that real debates do go on between climate scientists. Recently, Michael Mann has gotten himself into one of those, as well.

It started with a paper published last year by Mann, Jose Fuentes, and Scott Rutherford, one that focused on a curious feature of some reconstructions of temperature based on the width of annual tree rings. When you look at the tree rings that were laid down at times of large volcanic eruptions, the cooling in the reconstruction is much smaller than the dip in temperatures predicted by climate models.

Mann and his colleagues found that this could be explained by a failure of some trees to grow at all in that year, producing no ring whatsoever. Many of the trees used for temperature reconstructions grow at high altitudes, where their growth is limited by temperature. For some, a large, sudden cooling could push the tree below the minimum temperature threshold for growth.

If that were to occur, any residual growth associated with the year of the eruption may actually have been laid down during the following year (as the cold temperatures ease up), and the date assigned to each ring after that would be off by one year. Merge the rings from the year of the eruption a subsequent (warmer) year, and the result will indicate a warmer temperature than the tree actually experienced. The data become “smeared” because some tree chronologies shift, while others, like those from trees at lower altitudes, don’t.

By using a simple model of tree ring growth that simulates artificial records, Mann and his colleagues found that taking this into account could produce hypothetical reconstructions that better matched the climate model predictions.

Many of the dendrochronologists who compile these tree rings records took offense to the idea that they hadn’t noticed such an important error. After all, researchers always cross-check tree ring records with other trees in the area to look for issues like skipped rings and growth variation between individual trees.

Recently, twenty-three of those scientists submitted a comment to Nature Geoscience about the Mann, Fuentes, and Rutherford paper. In it, they outlined what they felt were flaws in the methods of the paper. They questioned the values chosen for certain parameters in the model of tree ring growth that was used, claiming that they were out of line with best estimates. The comment also pointed to uncertainties in the climate model simulations of volcanic eruptions that were compared to the tree ring reconstructions. Finally, they emphasize that the paper presents no actual evidence of missing years in individual records.

In the published reply by Mann, Fuentes, and Rutherford (as well as a blog post written by Michael Mann), the authors note that their paper presents a hypothesis to explain the perceived mismatch in tree ring reconstructions, but does not claim to demonstrate that the hypothesis is correct.

But they do push back against most of the criticisms of their work. They justify the use of their tree ring growth model, explaining the rationale behind their chosen parameter values. In addition, they claim that the uncertainty in climate model simulations of eruptions is not great enough to erase the discrepancy with the reconstructed temperatures.

Most importantly, the researchers explain why they think it’s possible that missing rings could have slipped past dendrochronologists, despite their cross-checking. They believe that the cold temperatures associated with the eruption would prevent any of the trees in a region from growing. To detect the missing ring, the trees would have to be cross-checked with trees sufficiently far away to have experienced warmer temperatures.

The debate is not without significance. Some researchers have tried to estimate the sensitivity of the climate to change by examining these records of volcanic eruptions. You can compare the amount of solar radiation reflected by volcanic emissions to the change in temperature that resulted. Because the tree ring reconstructions don’t show much cooling during eruptions, the estimates produced with this method have been lower than those made using other approaches. If the reconstructions are hiding a greater response to eruptions, it's possible that all the estimates may fall in line.

While researchers draw conclusions from data sets like these tree ring temperature reconstructions, they also understand that the data are imperfect and are constantly trying to ferret out those imperfections. So while some who reject climate science have repeatedly accused Michael Mann of manipulating data or covering up flaws (accusations he’s been cleared of by multiple investigations), here he is getting into a bit of a dust-up by looking for problems.

As Mann and his colleagues point out, their idea is just a hypothesis awaiting testing. To them, the hypothesis is attractive because it’s capable of explaining an interesting discrepancy. To the dendrochronologists who submitted their comment, that’s irrelevant until there’s solid evidence for it (which they don’t believe will turn up).

Eventually, if Mann's hypothesis is found to be correct, we'll be able to improve the temperature reconstructions. If, instead, the dendrochronologists are vindicated, that could lead to a different understanding of what the discrepancies mean. Since this is a scientific debate, the data will get the last word—and, right now, the data aren't in yet.

While we're waiting for that data, however, the argument shows why most of the actual scientific debates don't make the news: they're generally slow-moving, exceedingly technical, and, individually, they don't change much about our overall understanding of a field.

Nature Geoscience, 2012. DOI: 10.1038/ngeo1394, 10.1038/ngeo1645, and 10.1038/ngeo1646 (About DOIs).