Darrell S. Kaufman / Northern Arizona University

Kristi Wallace of the Alaska Volcano Observatory examines a lake sediment core from southern Alaska that shows intricate layering indicating environmental and climatic changes over centuries.

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.

Read: Warming fastest since dawn of civilization, study shows

"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.

Argo

A system of buoys that record ocean temperatures to a depth of 6,500 feet helped scientists determine where the excess heat is stored.

The deep oceans have recently been soaking up much of the excess heat trapped under the ever-thickening blanket of greenhouse gases that humans pump into the atmosphere, according to a recent study.

The finding may help explain why the pace of global warming at the surface has slowed in recent years compared to the 1990s, a phenomenon that has left members of the climate science community scratching their heads.

"The warming at the surface hasn't stopped, but it has been less than most of the climate models have been predicting," David Pierce, a climate researcher with the Scripps Institution of Oceanography, explained to NBC News. "So the question is: Where is that extra heat going?"

Kevin Trenberth and colleagues at the National Center for Atmospheric Research reanalyzed ocean temperature records between 1958 and 2009. They found that about 30 percent of the extra heat has been absorbed by the oceans and mixed by winds and currents to a depth below about 2,300 feet.

Oceans are well-known to absorb more than 90 percent of the excess heat, but its presence in the deep ocean "is fairly new, it is not there throughout the record," Trenberth said during a teleconference with reporters on Thursday. "So the question is: What happened to produce this?"

To find out, the team used a model that accounts for variables including ocean temperature, surface evaporation, salinity, winds and currents, and tweaked the variables to determine what causes the warming at depth.

"It turns out there is a spectacular change in the surface winds which then get reflected in changing ocean currents that help to carry some of the warmer water down to this greater depth," Trenberth said. "This is especially true in the tropical Pacific Ocean and subtropics."

The change in winds and currents, he added, appears related to a pattern of climate variability called the Pacific Decadal Oscillation which in turn is related to the frequency and intensity of the El Niño/La Niña phenomenon, which impacts weather patterns around the world.

The oscillation shifted from a positive stage to a negative stage at the end of the extraordinarily large El Niño in 1997 and 1998. The negative stage of the oscillation is associated more with La Niñas, which is when the tropical Pacific Ocean is cooler and absorbs heat more readily, Trenberth explained.

"So, some of this heat may come back in the next El Niño event … but some of it is probably contributing to the warming of the overall planet, the warming of the oceans. … It means that the planet is really warming up faster than we might have otherwise expected," he said.

While this ocean mixing has been suggested by some of the models scientists use to simulate the global climate, the new study is the first to re-analyze the observational record to get at an answer, noted Pierce, who was not involved in the study.

This new work, he said, should compel the climate science community to incorporate the mixing into the full suite of models, which in turn could improve climate forecasts in the 5- to 10-year time frame most relevant to planning agencies.

"What people are getting more and more interested in is what's the actual trajectory going to be …this sort of exchange between the surface and the deep they found in this paper really affects the actual trajectory you'll see," explained Pierce.

For example, knowing when the Pacific Decadal Oscillation will switch back to the warm phase could benefit planners on the U.S. West Coast. That's because sea level rise there has been suppressed for the past two decades, Joshua Willis, a project scientist at NASA's Jet Propulsion Laboratory, noted during the teleconference.

"In California," he said, "I like to say we are running a sea level deficit of about 6 centimeters and over the next 10 or 20 years we'll probably make that up and then some."

The findings were reported online earlier this month in the journal Geophysical Research Letters. 

John Roach is a contributing writer for NBC News. To learn more about him, visit his website.