The release of e-mails hacked from University of East Anglia climate scientists in 2009 (unimaginatively dubbed “Climategate” in media scandalese) generated about as much public discussion as any report of the science has. Long after the initial attention died down and a number of independent investigations found no evidence of scientific malpractice, snippets of quotes from the e-mails continue to pop up in conversations and opinion columns.

The most famous snippet related to reconstructions of past climate based on tree rings. One researcher, describing work putting together a graph, mentioned using “Mike’s Nature trick” to “hide the decline." It was exactly the nefarious-sounding sort of language that those who combed the e-mails for dirt wanted to find. Of course, it turned out to simply be a casual description of something much more mundane. “Mike’s Nature trick” was to display the instrumental temperature record and tree ring data on a graph—as climate scientist Michael Mann had done for a paper published in Nature.

So what about “the decline”? It’s no secret that many tree ring climate records from the Arctic diverge from instrumental data around the 1950s, failing to show the warming we’ve observed. So for some reconstructions, data from the second half of the 20th century is known to be inaccurate. In many ways, the divergence itself is much more interesting than arguments about unremarkable e-mails. Tree ring researchers have puzzled over what could explain the odd behavior of these Arctic trees.

A few possibilities have been proposed, including climatic factors, ozone depletion, and air pollution. Testing these possibilities, however, has been tricky. In a new study, San Francisco State University’s Alexander Stine and Harvard’s Peter Huybers think they’ve identified the cause: reductions in sunlight caused by anthropogenic aerosol emissions.

They analyzed tree ring records—both those based on the width of rings and those based on the wood density in the latter part of each annual ring—from around the Arctic. The key was to separate them by the light availability where they grow, determined by the amount of cloud cover in weather records. Trees in low light are much more sensitive to changes in light than trees that are bathed in it.

To look for the signature of light availability in annual tree growth (as archived in their rings), the researchers examined two different kinds of events. First, they examined what happened after the 15 biggest volcanic eruptions between 1300 and 1950, since the sulfur dioxide released produces aerosols (tiny particles) that reflect sunlight.

The tree ring records all showed slower growth following eruptions, but those from the cloudier locations suffered more. It was also true that those from colder locations were hit harder (as the reduced sunlight also causes cooling), but this wasn’t the source of the difference between cloudy and clear regions.

Second, they looked at the period of 1955-1975, when steadily increasing anthropogenic emissions of aerosol pollution reduced Arctic sunlight by around 10 percent. (The major eruption of Mount Pinatubo in 1991, for reference, temporarily reduced sunlight there by about twice that.) For this period, the analysis can actually get a little more sophisticated since we know how temperatures changed. The researchers removed the temperature effect from the tree ring records, meaning that the remaining changes in growth should be the result of other factors.

Again, the pattern was clear. While trees from the sunniest locations showed little to no decline in growth over that time period, the trees from cloudier areas dropped off considerably. In other words, the raw records from trees in cloudier locations would appear to show less warming in the second half of the 20th century—leading to the mismatch with the instrumental record.

Arctic trees make great climate recorders because their growth is limited by the cold temperatures, but light availability also appears to have been limiting over the past few decades in some places. (There’s no evidence for a similar divergence between growth and temperatures prior to that.) There will certainly be more research to follow up on this report, as it’s of great interest to the scientists who work with tree rings. But beyond explaining the recent divergence, this work also opens the door to an intriguing possibility: that tree rings could tell us something about changes in the amount of sunlight reaching the Earth's surface.

Open Access at Nature Communications, 2014. DOI: 10.1038/ncomms4836 (About DOIs).