We're in uncharted territory: A new study says the warming we've seen in the past 100 years is unprecedented when compared with the past 2,000 years.
From roughly the year "zero" to the late 1800s, warm and cool periods would happen in different parts of the world at different times because of natural weather cycles, solar activity and volcanic eruptions.
But since the late 1800s, when humans started burning fossil fuels for energy, the entire world has warmed consistently in a way that it didn't for almost all of the previous 2,000 years.
The study was published Wednesday in the peer-reviewed British journal Nature.
"The warming of the climate system that we've seen in the past 100 years is fundamentally different than what we've seen in the prior 1,900 years," study co-author Nathan Steiger, a Columbia University climate scientist, told USA TODAY.
"Climate epochs of the past 2,000 years, which have been popularized with names such as the 'Little Ice Age' and the 'Medieval Warm Period,' were only regional or continental-scale phenomena. These cold and warm decades can be explained by natural climate variability.
"But it's only been since humans starting burning fossil fuels that the entire Earth has warmed together."
In fact, the study said that the fast temperature increases toward the end of the 1900s impacted over 98% of the surface of the Earth.
It also said that modern climate change cannot be explained by random fluctuations, but by human-caused emissions of carbon dioxide and other greenhouse gases.
To determine the Earth's temperature from the past 2,000 years, scientists pored over nearly 700 records from trees, ice, sediment, corals, cave deposits, documentary evidence and other archives.
"It's true that during the Little Ice Age it was generally colder across the whole world," explained study co-author Raphael Neukom of the University of Bern, "but not everywhere at the same time. The peak periods of pre-industrial warm and cold periods occurred at different times in different places."
Thus, the research results show the current global warming is unusual not only in magnitude, but also in terms of its geography. Scott St. George, a paleoclimatologist at the University of Minnesota who was not involved in the study, told Science magazine that "no matter where you go, you can’t avoid the dramatic march toward warmer temperatures.”
In a commentary that accompanied the study, St. George wrote that "the familiar maxim that the climate is always changing is certainly true. But even when we push our perspective back to the earliest days of the Roman Empire, we cannot discern any event that is remotely equivalent ... to the warming over the past few decades. Today’s climate stands apart in its torrid global synchrony."
This article originally appeared on USA TODAY: Global warming: Climate is warming faster than it has in the last 2,000 years