Immediately upon its publication, the anti-global warming community pounced. "New Study Thoroughly Debunks Global Warming" reads the headline.
But wait. TAE is here, with two degrees in "science" to help the layman understand this highly scientific article. The summary of the article references a term called "orbital forcing" and suggests it is the cause of the temperature shift over the last couple millennia:
Over recent millennia, orbital forcing has continually reduced summer insolation in the Northern Hemisphere5. Peak insolation changes in Northern Hemisphere high latitudes, at ~65° N between June–August (JJA), have been identified as the prime forcing of climate variability over the past million years1. Together with long-term CO2 variability resulting from biogeochemical feedbacks of the marine and terrestrial ecosystems14, these insolation cycles have initiated the interplay between glacial and interglacial periods.What is this orbital forcing? What is solar insolation?
Orbital forcing, basically, is the change in the tilt of the Earth over time. Most people know that the Earth is tilted on its axis. This tilt, combined with the year-long orbit around the sun, causes our seasons. When the Earth's tilt and orbit point the Northern Hemisphere towards the sun, we get summer in Kansas. 6 months later, we have winter as the tilt of the Earth and its orbit around the sun now direct more solar energy at the Southern Hemisphere.
Solar insolation is a measure of the total radiation, be it heat, light, UV, gamma rays, etc that the sun outputs that is hitting the Earth. So in a nutshell, orbital forcing is a change in the tilt of the Earth, which in turn amplifies or lessens the amount of solar insolation, i.e. heat, hitting the Earth.
These scientists have measured the size of tree rings dating back as far as 138 BC and found that solar insolation has been slowly decreasing over that period, as a result of orbital forcing. Therefore, the temperature of the Earth has been decreasing, though ever so slightly.
So is global warming debunked? Well yes, and no. Certainly the Earth is cooling off, if this study is true. But is it cooling off as fast as it would without the anthropogenic climate changes? If humans weren't raping and pillaging the Earth as fast as they possibly can, would the Earth have even more noticeable cooling effects?
Saying this "debunks" global warming is an over-simplification. This study alone does indicate that the global average temperature over the last 2,000 years has not increased - in fact it has gotten lower - but it makes no presumptions about the post-industrial revolution era, nor do the authors explicitly (or even implicitly) suggest that humans played a factor in their data.
The climate is a complex thing. Using a single tree ring study to "debunk" global warming is just as bad as the reactionists who point to the current hot/dry summer as "proof" of global warming.