Climate Data That Sounds Meaningful...But Isn't

June 10, 2011

Today's Seattle Times features a story with the headline "Study of 800-year-old tree rings backs global warming." The article notes that snowpack loss in the Western United States has been more severe in recent decades than in the last millennium based on studies of tree rings. There are two key claims here.

The first claim is that increased temperatures in recent decades have caused snowpack loss. They argue the tree rings demonstrate this. What else might demonstrate this? Thermometers. The graph below shows temperature anomaly (variance from the norm) for the last 130 years. Temperatures have been rising.

This leads to the second key claim of the study, that "that human greenhouse-gas emissions are contributing to the loss of snowpack." This is a difficult claim to make. Note that temperatures began rising long before the concentrations of CO2 in the atmosphere were large enough to have an impact. Indeed, climate models show that even the current level of CO2 in the atmosphere, 392 ppm, has a negligible impact on temperature.

Additionally, the study does not actually show a link between human greenhouse-gas emissions and snowpack. It only says that recent snowpack decline is unusual. The claim that the decline is due to human greenhouse-gas emissions comes by combining the unusual changes in snowpack from the tree-ring study, with separate climate models indicating that increases in CO2 concentrations cause temperature increases.

I spoke with one of the authors of the study and he confirmed that his study does not provide a link to human-caused climate change, although he does argue that the unusual snowpack loss tends to support the notion that human-caused climate change is occurring.

Ultimately, however, the interpretation that this study proves human-caused climate change is an exaggeration. Indeed, climate models indicate that the impact of current CO2 concentrations on the climate is slight, within the "noise level" in the data. In other words, according to the climate models, we are at levels in which it is hard to distinguish the CO2 impacts from natural forces.

Ironically, then, the claim that the impacts seen here are caused by anthropogenic CO2 emissions is both reliant on climate models and discredits those same models because the tree-ring study claims impacts that are beyond what the models claim.

So, does this study actually shed light on the key questions of climate change? I don't think so. First, we don't need tree rings to show temperatures are increasing - we have thermometers.

Second, the study doesn't demonstrate that carbon emissions are causing the snowpack loss. It merely says something is happening. Some may argue climate change is the cause (although the models are certainly not definitive on that), but that is a hypothesis.

Put simply, this study is neither necessary nor sufficient to answer the key questions about climate change. That doesn't mean it is useless. It can be helpful in understanding water issues. But the way the study is being portrayed in the public is misleading.