The Little Ice Age was a period of bitter winters and mild summers that affected Europe and North America between the 14th and 19th centuries. The cold weather is well documented in written records and supported by paleoclimatic records such as tree rings, glacial growth, and lake sediments. These paleoclimatic records serve as proxies that register past temperatures, confirming that it was colder than usual.
Thanks to paleoclimatic records, climate scientists have identified four cold and warm “climate epochs” during the past 2,000 years: the Roman Warm Period, which covered the first centuries of the Common Era; the Dark Ages Cold Period, from 400 to 800; the Medieval Warm Period between 800 and 1200; and, most recently, the Little Ice Age.
The temperature proxies that allowed scientists to define these epochs were mostly from the extratropical Northern Hemisphere, particularly Europe and North America. Lacking information from other regions, researchers had long assumed that these climatic epochs must have happened simultaneously around the entire planet, but that might not be true.
To find out, a team of researchers led by Raphael Neukom, a climate scientist at the University of Bern, Switzerland, has used a database of globally distributed paleoclimatic records recently developed by the PAGES (Past Global Changes) international consortium. PAGES helps climate scientists to share climate proxy measurements accompanied by detailed information about the geographical location, methods used, and other data necessary so other researchers can make use of them. This documentation makes it easier to combine different types of proxies, such as tree ring measurements from the Northern Hemisphere and coral growth from the tropics, to extract information about the past evolution of Earth’s climate.
Feeding this information into computer climate simulations, Neukom and colleagues determined that none of these past climate epochs affected the entire planet at the same time, not even the Little Ice Age.
“We know that it was indeed cold during the 15th to 19th century, so we don’t deny this,” Neukom says. “But what we find is that the actual minimum occurred at different times at different places.”
For instance, during the Little Ice Age, minimum temperatures hit the eastern Pacific during the 15th century, northwestern Europe and southeastern North America during the 17th century, and most remaining regions during the mid-19th century. For the previous climate epochs, the spatial coherence is even less significant.
Data supporting temperature minimums hitting different parts of the world at different times go against the idea of the Little Ice Age as a single incident triggered by one or several events such as volcanism or reduced solar activity.
“If you have a cooling driver, like a volcanic eruption, from our understanding of the climate system it’s unlikely that this causes the whole globe to cool at the same time and for the same time period,” Neukom says.
Even if the study doesn’t test for the influence of specific culprits for each epoch, it points to natural climatic variability as the most likely explanation. These findings are described in an article published on 24 July in Nature.
Anthropogenic Climate Change Is Different
The study also reveals that the current period of climate change is different from previous climatic epochs.
This is the first study that assesses the spatial evolution of human-made global warming, showing that global temperatures haven’t been as high in the past 2,000 years and also that the increase has occurred simultaneously on 98% of Earth’s surface.
“Statistically, the spatial coherence of the warming in the last century is totally different from the spatial coherence of any other period in the past,” says study coauthor Juan José Gómez-Navarro, an environmental physicist at the University of Murcia in Spain.
The study concludes that the current warming pattern cannot be explained by random fluctuations of Earth’s climate.
“The idea that the current warming is not spatially heterogeneous but is spatially uniform I think is an important point to make and puts some context on the current warming in a way that’s new and different,” says Michael Evans, a climate scientist at the University of Maryland who wasn’t involved in the study.
Evans, who has coauthored other studies along with Neukom using PAGES data, notes that although this study sheds light on an important question for the paleoclimate community, its findings also become more uncertain as they delve back into the past. Most trees don’t live for millennia, or they are damaged by natural causes or human activity. The same happens with corals, ice cores, and so on. As a result, for certain areas and time periods studied, the closest temperature proxies are many thousands of kilometers away.
Aware of the problem, the authors used a variety of statistical tests to assess the validity of their claims and gauge the uncertainty in the simulations. “Our final results are solid, as set in stone,” says Gómez-Navarro.
—Javier Barbuzano (@javibarbuzano), Freelance Science Journalist