Even with his more rudimentary form of analysis, Lovejoy notes, his results are pretty much in line with estimates by the IPCC and National Academies of Science. The study predicts that doubling carbon dioxide levels in the atmosphere would cause the climate to warm by between 2.5 and 4.2 degrees Celsius; the IPCC's projection was for an increase of 1.5 to 4.5 degrees Celsius under the same scenario.
"Increases in CO2 is strongly linked to global economic activity," Lovejoy says. "To a good approximation, if you double the economy, you double the emissions -- and, therefore, you double the effects."
There was some criticisms of Lovejoy's research from the climate skeptic camp for his inclusion of tree ring data for climate reconstructions, which some studies suggest can deviate widely from thermometer-based temperature readings, and for not considering other warmer periods in the planet's recent history, such as the so-called "Medieval Warming Period" of 1,000 years ago.
Lovejoy notes that tree ring records make up only one very small facet of his review among many other factors, and that his results would be more or less the same if tree ring data were exempted. As for other warming events in the past several thousand years, he points out that it is important to understand that those were probably more regional or continental in nature rather than global. But even global scale temperatures can change by large amounts as long as their rate of increase is small enough. In other words, what makes the current climate change we are experiencing so remarkable is that it is happening very quickly over an extremely short time and over the entire globe.
Maureen Raymo, a paloeclimatologist and marine scientist from Columbia University and the Lamont Doherty Earth Observatory who has spent decades studying our planet's natural climate variability, agrees.
"What we're seeing right now is not natural," says Raymo on a Yale Forum video. "Right now, we're living in a world of a Pliocene atmosphere. But the whole rest of the climate system -- the oceans are trying to catch up, the ice sheets are waning and everything is trying to catch up to this Pliocene atmosphere."
Raymo clarified that none of the recent changes in the climate can be explained by the Earth's distance to the sun, solar output or our planet's elliptical orbit. If anything, the climate community has often underestimated and understated the rate of climate change and its projected impacts, she says.
As Lovejoy puts it, "How bad [climate change] is going to be, we don't exactly know, whether it's really bad or somewhat bad. But we should be starting to wean ourselves off of fossil fuels and begin investing in carbon-free technologies immediately."