State of the Planet

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The Climate Epochs That Weren’t

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 JosĂ© Gomez-Navarro, University of Murcia, Spain; Jianghao Wang, MathWorks, Natick, Mass.; and Johannes Werner,  Bjerknes Center for Climate Research, Norway.

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David
David
4 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.

Candid One
Candid One
Reply to  David
8 months ago

Typo? NAOO? The “Hockey Stick” curve existed before Michael Mann was born.

Patrick Charles Trombly
Patrick Charles Trombly
Reply to  Candid One
3 months ago

? No, it didn’t. The MWP and much cooler LIA existed before Michael Mann’s grandparents’ grandparents were born. The hockey stick wasn’t even in the first IPCC TAR.

Alexander
Alexander
4 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.

Candid One
Candid One
Reply to  Alexander
8 months ago

What’s God got to do with it?

Kelly
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.

Candid One
Candid One
Reply to  Kelly
8 months ago

Global uniformity has never been implied. No evidence of such generality has ever been proposed. All studies of global processes infer latitude zonation at most. Not even during supercontinent eras was any blanketing climatic process proposed, except in Hollywood. The earth’s axial tilt is what makes cross-equatorial uniformity incongruous. While magnet reversals and modest axial wobbles may be indicated in sea floor spreading models, no suggestion of much change of spin axis has been made.

Ray Tomes
3 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.

Candid One
Candid One
Reply to  Ray Tomes
8 months ago

Comparing N/S hemispheres puts the onus on you. Land and ocean mass ratios differ between N & S. Current AGW is not uniform in the northern hemisphere. It’s not tempting to assume the southern hemisphere will mirror the north’s variability.

Patrick Charles Trombly
Patrick Charles Trombly
Reply to  Candid One
3 months ago

No, rewriting the climate history, or any history, puts the onus on the person rewriting it. There are similar examples of what happened when and where around the world, that had, before the climate became a political issue, been considered to support a MWP and LIA strong enough to materially impact human activity, and contribute to the rise and fall of civilizations. I.e., more than a half a degree. Long before there was a climate change debate, in fact before modern climate change, the concept of a strong and global MWP and LIA evolved due to the fact that such events happened at the same time across most of human civilization.

“My proxy data plugged into my computer model says otherwise” does not refute that.

An alternative theory as to how all these things didn’t actually happen, or happened for some other reason, and how observations made at the time were misguided, would refute it.

That has never been done, and the few attempts to do that are so feeble as to MAKE someone who wasn’t skeptical skeptical.

The Anasazi civilization didn’t collapse in drought, but because of religious and political strife? Yeah – “hey chief, praying to your rain god isn’t working, it’s not raining, the crops ran dry again, and we’re leaving.”

There are vineyards in England now, after 1000 years of technology and cultivation of hardier grape varieties, and so the rise of English viticulture during the MWP doesn’t prove anything? Really? There was more demand for wine in England in the MWP because the Normans settled there and because of Eleanor of Aquitaine? Wouldn’t it have been easier to import the wine, like the Brits imported Port? Do you know how costly and time consuming an endeavor it is to plant a vineyard and develop it? It takes a fortune and a generation – that’s not done on a whim or after a couple of warm years – it has to be reliably warm for so many people to even attempt that.

How’d Lake Naivasha run dry for 200 years, at the same time that the Vikings sailed around in wooden boats in the North Atlantic, and settled areas not suitable for human habitation or production of a land-based diet either before or after?

There are dozens of other examples.

Those examples have, since they happened, been attributed to climate change on a scale, if not speed, at least as great as today’s.

Maybe there’s another logical explanation – a cohesive one that explains all of it.

If and when there is, then we can ditch the MWP and LIA, neither of which was invented by “deniers” or “denialists” or “fossil fuel industry funded cretins.”

There was no fossil fuel industry when these concepts evolved and became understood to have been the global climate timeline.

Trying to pretend that there wasn’t such a timeline or that it was developed after the fact, during the present debate, by people with an agenda to alter the timeline, is absurd – that’s what YOU’RE doing.

Nancy
Nancy
2 years 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

https://www.nature.com/articles/s41598-018-26038-8

Medieval Temperature Trends in Africa and Arabia
https://eos.org/research-spotlights/medieval-temperature-trends-in-africa-and-arabia

Climate and Culture Change in North America AD 900–1600
https://www.jstor.org/stable/10.7560/737419

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
https://www.sciencedirect.com/science/article/abs/pii/S0277379115301542

The Little Ice Age and medieval warming in South Africa
https://www.researchgate.net/publication/230751443_The_Little_Ice_Age_and_medieval_warming_in_South_Africa

The Effects of the Little Ice Age (c. 1300-1850)
http://www.science.smith.edu/climatelit/the-effects-of-the-little-ice-age/

Richard
Richard
Reply to  Nancy
1 year ago

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

Serv
Serv
Reply to  Nancy
10 months ago

Sorry but that doesn’t make sense. The reason why scientists look at the “global” is because there’s a difference between regional effects and global ones. CO2 changes temperature on a global level. This means that the overall average of surface temperature across the globe is expected to change with respect to how CO2 changes.

Candid One
Candid One
Reply to  Serv
8 months ago

Global “effects” are largely statistical. Regional effects have measurable comparisons and contrasts. BTW, as AGW effects have passed the tipping point, the atmosphere’s actual primary “greenhouse gas”, water vapor, is now the major factor, while non-anthropogenic CO2 has become a more significant factor.

As atmospheric heat is exchanged with the oceans, warming oceans emit water vapor into an atmosphere which increases its moisture capacity as it warms. Meanwhile, the oceans, as the planet’s primary CO2 sink is decarbonating as they warm, much like our warming carbonated beverages. Both above processes constitute a major planetary compound feedback loop, with self-sustaining momentum, beyond what human posturing, preaching, denying, procrastinating, and fingerpointing can do about it.

Skeeter McCluskey
Skeeter McCluskey
1 year 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?

Candid One
Candid One
Reply to  Skeeter McCluskey
8 months ago

Wrong cycle? The planet is too dynamic, virtually, for cycles to be likely. Plate tectonics are always moving the goal posts, virtually. We marvel at the extinction of the dinosaurs, allegedly, but little mention exists of how the north Altantic was barely opening in the present migration of continental plates, as they move toward the next supercontinent aggregation.

Ri hard
Ri hard
1 year ago

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

Candid One
Candid One
Reply to  Ri hard
8 months ago

As noted in the actual published article, link in 3rd paragraph of this one, in its “Acknowledgements” section, this was a polyglot collaboration from throughout Europe and the US, with separate listed funding sources among the listed contributors.

John Monaghan
John Monaghan
1 year 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.

Candid One
Candid One
Reply to  John Monaghan
8 months ago

Your view needs adjustment. Europeans have been in North American for 500 years. In the 1520s, Spain’s Narváez expedition, explored the current US Gulf Coast, from Tampa Bay to the Rio Grande. A decade later, Spanish exploration, led by Hernando de Soto, across much of the lower half of North America followed. Oops?

Tom
Tom
10 months ago

The sampling points primarily are tree rings and coral, based on the map of samples and locations. The sample points of tree rings are mostly in the northern hemisphere and often mid-continent. They rarely overlap sample points of coral taken in coastal areas of the southern hemisphere and equatorial regions. Why do you believe such dissimilar sources can be used to infer a difference in climate between these regions? Shouldn’t you use the same sampling method at all locations?

Candid One
Candid One
Reply to  Tom
8 months ago

Trees and coral in same locations? Their hemispheric generalizations aren’t simplistic. Trees and their aerial environments can’t be simple comparisons to marine coral in oceanic environments. Their assumptions and conclusions can’t be linear. No assumptions of climatic continuity are valid without some basis in environmental indicators, which remain cited in the published article in Nature…see link in 3rd paragraph of this article.

Chris
Chris
3 months ago

Are these 2,000 years, with their ups and downs of temperature (more up than down now for sure) representative of the 4.5 billion years of ourplanet, a geological time where ice caps were not the norm ?

Last edited 3 months ago by Chris
Patrick Charles Trombly
Patrick Charles Trombly
3 months ago

Hold on. These were independent observations, from across the globe, of events and phenomena, and human activity either known to have been undertaken or long believed to have been undertaken, because of these shifts – shifts large enough to cause these phenomena (which have not happened yet during this warm period) and to have induced significant changes in human behavior. I.e., not shifts of half a degree C, and not preceded by an even warmer period than the MWP.

The concept of a history including a global MWP and LIA was not a result of extrapolation from local (UK or North Atlantic) to global but from the fact that there were similar events each difficult to explain as having occurred but-for warmer temps, around the world – from megadroughts causing the collapse of the Anasazi and other civilizations in the American Southwest, to tree lines in Russia and Scandinavia, to Vikings sailing small wooden boats to far reaching places with far less fear of sea ice, and settling previously – and subsequently – uninhabitable areas – and eating a land based diet, then a sea based diet, then abandoning the site…., to Lake Naivasha, which is west of Nairobi (so please don’t blame it on ocean currents and somehow tie it to the North Atlantic) drying up for 200 years (in a location where hot and dry correlate), to climate shifts in China impacting the rise and fall of dynasties, to crop yields, planting altitudes and latitudes, and tree lines, from around the world.

It was not “deniers” going back in time and creating these concepts, and it was not any central body or school of thought with any kind of agenda trying to fit dozens of independent examples into a preconceived version of events, or extrapolating without any basis from what had happened in their particular corner. It was the simple fact that when you map out a timeline of what happened, when and where, from around the world, and for what likely reason when considered individually, you see the same timeline.

“My proxy data plugged into my computer model says otherwise” does not refute that.

An infinite amount of proxy data plugged into an infinite number of computer models does not and cannot refute that.

When someone credibly argues, and supports the argument with evidence and with a logical explanation, that all these things happened for some other reason other than the reason that has been given for decades or centuries, depending on the example, or that somehow they didn’t happen, then you can take MWP and LIA as examples of similar shifts, even if occurring at a slower pace, off the board.

Most of the events are not possible to conceive without not only warmth but consistent warmth. You don’t, for example, invest the time or capital to plant a crop – of any kind, especially a vineyard – unless the conditions vital to it have existed for many years, if not decades. These are not little one-time anecdotes scrapped together. These are massive shifts in human activity, each independently attributed in part or whole to noticeable and long-term shifts in climate, happening at the same time across civilizations many of which were unaware of each other’s existence.

“Deniers” did not go back centuries in time to tell millions of people to start changing their societies to make it look as if it was warmer – and they did not go back fewer centuries, or decades, in time to tell historians to interpret those events as having resulted from shifts in climate. Even if the fossil fuel companies had as much money to throw at this as governments do, they would not be able to pull that off.

None of this means that modern warming isn’t caused by CO2 emissions, or that modern warming isn’t a threat – in fact, one could argue that since whatever caused the MWP isn’t happening now, we’d be screwed if it did happen AND we kept emitting this much CO2 – to this day I don’t understand why your side didn’t take that tack.

But the standard for revising history – not writing, revising – can’t be “proxy plus computer trumps long-held interpretations of countless events and phenomena observed globally,” and denying that that’s what the MWP and LIA are, is a much bigger “denial” than dismissing modern warming as minor or potentially harmless.

Revising history based solely on trying to mathematically validate the new proxy-derived version rather than debunking the long-held old version – explaining away the evidence through some new comprehensive theory – is not sufficient.

Claiming that there was no old version, or that it was concocted recently by “deniers,” detracts from the credibility of the hockey stick team.

I WENT to Vatnajokull last year and was reminded again, by the guides, that, if we’d done the hike when the Vikings discovered it, we’d have had to hike an extra half a mile before putting on crampons and stepping up onto the ice. And as beautiful as it was, on and under the glacier, all I could think of was hockey stick.

I WENT to Kenya last year, SAW Lake Naivasha – and other endorheic lakes in the region – SPOKE with guides who were pretty clear that hot aligns with dry there and that the lake depth and salinity change over multi-year hot and dry periods, which explains flamingo migration – and all I could think of – besides not stepping in nyati poop – was Naivasha being dry for 200 years, and hockey stick.

Every time I visit the Cloisters and see a British carving on walrus ivory, and see tapestries with unicorns – whose existence was believed because so-called unicorn horns – actually narwhal tusks – were sold in markets, all I can think of is Norsemen sailing in what had been and would later be ice-bound areas, and establishing colonies and outposts in what had been and would later be areas where a town couldn’t sustain itself – and hockey stick.

Every time I read about the collapse of the Anasazi…… the rise of the Mongols when there was more grass and a longer warm season that enabled raiders to ride longer and more often, and other historical events, I think hockey stick.

I could list many other examples, but I have to go back to work as my lunch break is over.

It’s not “I’m a denier, ergo I cling to a history that never was.”

It’s “I’m old enough to remember climate not being a political issue, and learning about a MWP and LIA in school, as having been global, and as having been an interpretation based on observations from around the world, of phenomena the most likely explanation for which was temperature, and have since been explained as having resulted from changes in temperature, over the same timeline, and I’m skeptical of anyone who says that that didn’t happen without debunking the basis for it and very skeptical of anyone who says that that was never the history.”

My grade school teachers weren’t “deniers.”

I went to grade school before there was anything to deny.

And I didn’t go to school in the UK – the Vikings were one of many, many examples.

Thank you.