In contrast to pre-industrial #climate fluctuations, current, anthropogenic climate change is occurring across the whole world at the same time & is warming the planet faster than in the last 2,000 years: https://t.co/wYmpvpyg1c @CentreOeschger @nature @NatureGeosci @NatureNews pic.twitter.com/5if3Lq7Bi5

— Universität Bern (@unibern) July 24, 2019

"The main takeaway is that climate variability in the contemporary period is very different than what's happened in the past 2,000 years," Columbia University climate scientist Nathan Steiger, who co-authored one of the papers published in Nature Wednesday, told Reuters. "This is definitely further evidence that fossil fuels and anthropogenic activity actually has fundamentally changed the climate," he added.

Some climate deniers have pointed to past climate shifts to argue that there is nothing new or human-caused about the current warming period. The Medieval Climate Anomaly warmed temperatures from 800 to 1200 AD and the Little Ice Age saw temperatures fall from the 1300s to the 1850s, but the new research shows that these and other events were much more localized than current climate change. None of them covered more than half the globe at any one time, according to The Guardian.

Based on 6 different probabilistic reconstruction methods using the PAGES2k proxy database, we show that Common Era climate epochs were not globally coherent phenomena. Warmth and cold happened in different places at different times.

— Nathan Steiger (@njsteiger) July 24, 2019

In the past, researchers had taken evidence for these warming and cooling periods in Europe and North America and assumed they applied to the rest of the globe, the University of Bern explained in a press release. But for this week's papers, researchers consulted the PAGES (Past Global Changes) database of worldwide climate data from tree rings, ice cores, lake sediments and corals.

"It's true that during the Little Ice Age it was generally colder across the whole world," research team leader Raphael Neukom of the Oeschger Center for Climate Change Research at the University of Bern said, "but not everywhere at the same time. The peak periods of pre-industrial warm and cold periods occurred at different times in different places."

For example, the Little Ice Age was coldest in the Pacific in the 15th century, Europe in the 17th century and elsewhere in the 19th century, The Guardian reported.

"This paper should finally stop climate change deniers claiming that the recent observed coherent global warming is part of a natural climate cycle. This paper shows the truly stark difference between regional and localised changes in climate of the past and the truly global effect of anthropogenic greenhouse emissions," University College London climatology professor Mark Maslin told The Guardian.

The Nature paper was accompanied by a second, published in Nature Geoscience, that focused on the impacts of five large volcanic eruptions during the early 19th century that caused cooler temperatures, drought in Africa and the advance of glaciers in the Alps, the University of Bern further explained.

5 large volcanic eruptions in the early 19th century led to dry monsoon regions and growing glaciers in the Alps, according to a study led by the @CentreOeschger of the #unibern. https://t.co/0eBV0TdW7z #climate @NatureGeosci pic.twitter.com/oWAXo0fKcq

— Universität Bern (@unibern) July 24, 2019

Because these eruptions occurred at the start of the industrial revolution, researchers say it is difficult to determine how volcanism interacted with the increasing burning of greenhouse gasses to impact climate at that time.

"It does sort of mask the effect of industrial processes, where they are starting to omit more CO2, because they counteract each other," Steiger told Smithsonian.com. "So volcanoes could cool, and humans would warm by the release of greenhouse gasses. It's tricky to parse out what's what."

This matters because international targets like the Paris agreement seek to limit warming to certain amounts above a pre-industrial norm.

"Given the large climatic changes seen in the early 19th century, it is difficult to define a pre-industrial climate, a notion to which all our climate targets refer," lead author and University of Bern climatologist Stefan Brönnimann said in the university release.

But what is clear is that today's warming is unprecedented during the study period. A final paper, also published in Nature Geoscience, concluded that the rates of warming documented in the second half of the 20th century were the fastest in 2,000 years.

That finding "highlights the extraordinary character of current climate change," Neukom told Smithsonian.com.