The Climate Epochs That Weren't

Medieval Warm Period? Little Ice Age? Not So Fast.

by |July 24, 2019

When climate scientists talk about natural climate swings that came before humans started messing with the system, many invoke two epochs. During the Medieval Warm Period, roughly from 800 to 1200 AD, temperatures rose a few degrees above average. That warming has been connected to improved crop yields in parts of Europe, and the temporary Viking occupation of Greenland. During the following Little Ice Age, which lasted roughly from 1300 to 1850, the Greenland Vikings disappeared, glaciers from California to the European Alps advanced, and New York harbor froze, enabling people to walk from Manhattan to New Jersey without benefit of the George Washington Bridge.

For a long time, many took on faith the idea that these phenomena were global. But that assumption has been undermined in the past decade or so by studies from widespread areas (including parts of Greenland) suggesting that in fact temperatures in many places did not line up with one or the other periods. Some regions appear to have been warming when they were supposed to be cooling, and vice versa. The same goes for two lesser-known, more vaguely defined earlier swings, known as the Roman Warm Period (ca. 100-300 AD) and the Dark Ages Cold Period (ca.400-800).

A new study of climate swings during the past 2,000 years uses data from many different sources across the globe, including tree rings, glacier ice, lake sediments and corals. (Courtesy Raphael Neukom et al.)

A new study puts together the evidence on a global scale for the first time. Based on this, the authors say that the supposed warm and cold epochs may represent, more than anything, regional variations that can be explained by random variability. Published in the leading journal Nature this week, the study analyzes paleoclimate data from across the world, using multiple statistical methods and many sources: tree rings, glacial ice cores, corals, lake sediments. It does not suggest that the periods of high or low temperatures observed during the named epochs did not exist in certain places; rather that they did not exist everywhere at the same time, and thus probably were not caused by some kind of planetary driver.

That said, the study does find one very coherent period: an unprecedented warm one extending over 98 percent of the globe, starting in the 20th century. This is almost certainly caused by us.

We spoke with coauthor Nathan Steiger of Columbia University’s Lamont-Doherty Earth Observatory about climates of the past and present, and what we can learn from them.

What are the most important conclusions of your study?

We show that previously named climate epochs of the Common Era were not coherent phenomena across the globe. This goes against the widespread notion that periods like the Little Ice Age or Medieval Warm Period were global periods of cold or warmth. We’re not the first to point out that there are problems with this idea, but our study is the first to rigorously test the hypothesis on a global scale. In contrast to this, we see that current global warming is remarkably coherent.

How do you tell what temperatures were doing in various parts of the world during these past times?

We rely on proxies. Trees, for example, can be very sensitive to annual changes in temperature and moisture, and the width and density of their annual rings reflect those year-to-year changes. We can then sample hundreds of trees all over the world along with other natural archives to infer what climate was like in the past. For this study in particular we used several different statistical methods that combine all of these proxies to produce global maps of temperature change going back 2,000 years.

During the so-called Little Ice Age, European glaciers advanced. Here,18th-century Swiss painter Caspar Wolf depicted Switzerland’s Lower Grindelwald Glacier.

Have scientists been too narrow-minded in their geographical focus? I mean, the very name “Medieval” calls up part of European history–a period that didn’t exist in Asia, the Americas or Africa.

Paleoclimate is like many fields of study. There are historical biases in where data is collected, and how the stories about the data are developed. The first paleoclimate data were largely collected from Europe by Europeans, and so it’s not terribly surprising that the stories that try to make sense of such data are Euro-focused. Another problem is that until recently, people have been reluctant to share data and to create narratives that include more than a single, or perhaps a few, time series. If you’re a scientist who has spent a lot of time and money in producing a particular proxy time series, then there’s a tendency to emphasize the importance of that particular time series and to develop a story explaining it. The simplest story to develop is one that corresponds to a traditional understanding of what the climate “should” be doing going back in time. It’s only been in the past few years that scientists from across the paleoclimate community have begun to publicly collate a wide range of data types from all over the globe.

Are you recommending that scientists stop using terms like the “Little Ice Age”?

Not necessarily. In general, having simplified conceptual models of natural phenomena can be very useful and even essential in the pursuit of scientific understanding. It’s when the conceptual models get in the way of accurate science that problems arise. For example, when one labels any Common Era proxy time series with terms like the “Medieval Climate Anomaly,” they are usually implicitly assuming that such epochs were global, and over well-defined time intervals. Our results show that both of these assumptions are incorrect.

Not that there isn’t already plenty of evidence, but does this study add to the argument that humans are causing global warming?

We show that conditions during medieval times or during the Little Ice Age are expected to occur naturally. But the large spatial consistency of the present warm phase cannot be explained by natural variability. This result corroborates many existing studies that have shown that humans are causing global temperatures to rise since the beginning of the industrial period.

Are there limitations to your study?

Yes. Paleoclimate proxies can be used to infer past temperatures, but are not thermometers per se, and so they include non-temperature “noise.” We have therefore tried to use as many proxies as feasible for our study, but we are limited by where the data exists and the quality of the data. Uncertainties are usually largest in places without good quality proxy data. But for the particular hypothesis we’re testing, we don’t think these uncertainties significantly impact the results. We find the same results regardless of which proxy networks or which statistical methodologies we use. So we’re pretty confident in the results.

The study’s lead author is Raphael Neukom, University of Bern, Switzerland. Other coauthors are Juan José Gomez-Navarro, University of Murcia, Spain; Jianghao Wang, MathWorks, Natick, Mass.; and Johannes Werner,  Bjerknes Center for Climate Research, Norway.

Notify of
Inline Feedbacks
View all comments
3 years ago

The NAOO is known to revise its temperature record to magnify the arithmetic of “Global Warming”. It is logical for journalism to erase the Medieval Warm Period and the Little Ice Age to be more in accord with the “Hockey Stick” creation of Michael Mann of climategate fame.

3 years ago

That is very weak point. There is evidence of forest in Antarctica, not mentioning dinosaurs running all over the earth. So whats happened? Meteor or volcanic activity? I do not see the difference between scientist preaching about it or priest saying its the God work so far.

3 years ago

The researchers have missed an incredibly important point that Ice Age is not uniform everywhere. If Doggerland could support mammoths, (Water and grass) it was not a barren wasteland all of the time while not far away ice 1 to 2Km thick was standing upon UK. The North Sea still holds some secrets. Evidence holds proof that Vikings tracks in Norway are only just defrosting after ?hundreds? or ?thousands? of years complete with remains that should have decayed (weathered) if they had not been frozen quickly.

Implications are that heating and cooling can be quicker than a decade.

Ray Tomes
2 years ago

You claim that the medieval warm period that saw the vikings go to Greenland was not global. And yet this is the same time that saw Polynesians come to New Zealand. Perhaps your data is not good because of the extensive use of tree rings as temperature proxies.

1 year ago

Have scientists been too narrow-minded in their geographical focus? I mean, the very name “Medieval” calls up part of European history–a period that didn’t exist in Asia, the Americas or Africa.”
If you’re looking for uniform change worldwide at any point, including the present, you won’t find it. If you looked for a variety of changes worldwide, you’d have found an extensive amount of research. Even now as the climate continues to change, some places are getting wetter, some drier, some warmer, some colder.

The word “global” should be removed from the discussion entirely. It warps regional realities. Why would you expect the same changes in the Amazon as the Arctic Circle? Or New Zealand and Greece? And so, what sense would an average make? There is constant ongoing regional change everywhere, all of the time.

Effects of the Medieval Warm Period on Asia, Africa and the Americas:

East Asian warm season temperature variations over the past two millennia

Medieval Temperature Trends in Africa and Arabia

Climate and Culture Change in North America AD 900–1600

Effects of the Little Ice Age on Asia, Africa and the Americas:

Little Ice Age wetting of interior Asian deserts and the rise of the Mongol Empire

The Little Ice Age and medieval warming in South Africa

The Effects of the Little Ice Age (c. 1300-1850)

Reply to  Nancy
10 months ago

This is extensively thought out and includes a good collection of supporting data. Thank you.

Skeeter McCluskey
Skeeter McCluskey
11 months ago

A recent discovery in Antarctica of a stony beach which was revealed by melting ice had many petrified penguins which were carbon dated to 800 years ago. That would mean that sometime around 1,200 ad the ice melt was equivalent to todays. Why does that not point to the current climate fears as simply being cyclical?

Ri hard
Ri hard
10 months ago

Just curious, but what was the source of funding for this experiment?

John Monaghan
John Monaghan
10 months ago

It is apparent that people do studies with a view to promoting their point of view. Any idiot can do that. The “temporary Scandinavian occupation of Greenland” for example lasted for about 5 centuries. Europeans by contrast have been in North America for perhaps 3 centuries.