Here's the issue: In recent decades, some — although not all — trees have stopped responding positively to higher temperatures. How do we know? For the past 150 years, we've been measuring temperatures directly with various instruments. And, indeed, trees seem to follow temperatures faithfully, growing more during warm years and less during cold, until around 50 years ago. Then, even as scientific instruments continued to register rising temperatures, some trees started growing less.
If you were to go solely by these tree rings — and if you were looking at just ring density and width — you'd erroneously conclude that temperatures were falling when, in fact, they were rising. That's why scientists sometimes omit tree-ring data from recent decades in favor of the more accurate instrumental data.
Here are two paragraphs from Jones's response to the recent controversy. One shows the tree-ring data separate from the instrumental data. The other shows a graph with instrumental data inserted for the last two decades. One key point: For more than 100 years, tree rings and instrumental data track each other closely. They only diverge significantly in the past 20 years. Why?
Scientists have several possible explanations for this divergence, none of them mutually exclusive, and all of which — drought, global dimming, ozone holes — fault human activity for the slowing growth rate of some trees. In other words, scientists generally take the divergence as further evidence that humans are changing Earth's climate and that the warming is stressing various life forms — including trees in environments that are already extreme.
This conclusion is precisely the opposite of that reached by authors of many climate-skeptic opinion pieces and blogs, who argue that if tree rings show cooling, earth cannot be warming.