April 22, 2013 at 3:41 PM ET
A long-term global cooling trend ended in the late 19th century, a reversal in temperature that cannot be explained by natural variability alone, according to a new study.
The finding stems from 2,000-year-long continental-scale temperature records inferred from tree rings, ice cores, lake sediments and other so-called proxies from around the world.
The records show variations in temperature caused by changes in Earth's orbit, output of solar energy, and volcanic eruptions, noted Nicholas McKay, a climate scientist at Northern Arizona University and study co-author. Volcanic eruptions, for example, inject particles in the atmosphere that reflect some of the solar radiation back out to space.
"The 18th and 19th centuries would probably have been colder than the 20th century no matter what just because there has been a bit less volcanism in this century, but the amount of warming we've seen is extremely unlikely to have happened solely due to natural processes," he told NBC News.
In fact, he and colleagues note in the study — published Sunday in Nature Geoscience — that the natural factors that drove the Earth's long-term cooling are still present today, despite the fact that we are in a period of rising global temperatures.
The "hockey stick"
The record is consistent with other recent temperature reconstructions that show the reversal in long-term cooling coinciding with the acceleration of greenhouse gas emissions from human activity during the industrial revolution at the end of the 19th century.
Gerald North, a climate scientist at Texas A&M University, told NBC News in an email that the new study seems to fit the emerging consensus of a gradual cooling of the past 1,000 to 2,000 years followed by "an abrupt warming since 1900."
"Each year we have more evidence corroborating these same findings," he said. "It is 15 years since the first paper ... known as the 'hockey stick' paper. We have no credible evidence that they got it wrong."
The researcher behind the iconic 1999 "hockey stick" graph, Pennsylvania State University climate scientist Michael Mann, was not part of the new study, but he told NBC News in an emailed statement that the work of McKay and his team "adds to the growing body of scientific evidence that the recent warming is likely unprecedented even further back in time."
Mann added, "While the study doesn't attribute causality to the warming, there is an extensive body of research that shows that we can only explain the anomalous recent warming with human impacts, i.e. burning of fossil fuels and resulting increase in greenhouse gas concentrations."
Regional temperature variations
One distinguishing feature of the new study, noted McKay, is that it highlights variability in temperature around the globe at any one time. For example, a rise in temperatures known as the Medieval Warm Period followed by cooling during the Little Ice Age was pronounced in Europe and North America, less so in the Southern Hemisphere, he said.
While the paper isn't the first to look at regional climate reconstructions, it is the first so well organized, noted David Anderson, a paleoclimatologist with the National Oceanic and Atmospheric Administration's National Climatic Data Center. And, collectively, the regions show the end to the cooling trend on a global scale. "It is truly no debate," he told NBC News.
The ability to see the regional variability in response to forces on the global climate — from human burning of fossil fuels to volcanic eruptions — will be increasingly important as humans try to mitigate and adapt to future climate change, McKay added.
John Roach is a contributing writer for NBC News. To learn more about him, visit his website.