̽��ֱ�� of Cambridge - tree rings

Extreme drought contributed to barbarian invasion of late Roman Britain, tree-ring study reveals

ta385 — Thu, 17 Apr 2025 06:00:00 +0000

̽��ֱ��‘Barbarian Conspiracy’ of 367 CE was one of the most severe threats to Rome’s hold on Britain since the Boudiccan revolt three centuries earlier. Contemporary sources indicate that components of the garrison on Hadrian’s wall rebelled and allowed the Picts to attack the Roman province by land and sea. Simultaneously, the Scotti from modern-day Ireland invaded broadly in the west, and Saxons from the continent landed in the south.

Senior Roman commanders were captured or killed, and some soldiers reportedly deserted and joined the invaders. Throughout the spring and summer, small groups roamed and plundered the countryside. Britain’s descent into anarchy was disastrous for Rome and it took two years for generals dispatched by Valentian I, Emperor of the Western Roman Empire, to restore order. ̽��ֱ��final remnants of official Roman administration left Britain some 40 years later around 410 CE.

̽��ֱ�� ̽��ֱ�� of Cambridge-led study, published today in Climatic Change, used oak tree-ring records to reconstruct temperature and precipitation levels in southern Britain during and after the ‘Barbarian Conspiracy’ in 367 CE. Combining this data with surviving Roman accounts, the researchers argue that severe summer droughts in 364, 365 and 366 CE were a driving force in these pivotal events.

First author Charles Norman, from Cambridge’s Department of Geography, said: “We don’t have much archaeological evidence for the ‘Barbarian Conspiracy’. Written accounts from the period give some background, but our findings provide an explanation for the catalyst of this major event.”

̽��ֱ��researchers found that southern Britain experienced an exceptional sequence of remarkably dry summers from 364 to 366 CE. In the period 350 to 500 CE, average monthly reconstructed rainfall in the main growing season (April–July) was 51 mm. But in 364 CE, it fell to just 29mm. 365 CE was even worse with 28mm, and 37mm the following year kept the area in crisis.

Professor Ulf Büntgen, from Cambridge’s Department of Geography, said: “Three consecutive droughts would have had a devastating impact on the productivity of Roman Britain’s most important agricultural region. As Roman writers tell us, this resulted in food shortages with all of the destabilising societal effects this brings.”

Between 1836 and 2024 CE, southern Britain only experienced droughts of a similar magnitude seven times – mostly in recent decades, and none of these were consecutive, emphasising how exceptional these droughts were in Roman times. ̽��ֱ��researchers identified no other major droughts in southern Britain in the period 350–500 CE and found that other parts of northwestern Europe escaped these conditions.

Roman Britain’s main produce were crops like spelt wheat and six-row barley. Because the province had a wet climate, sowing these crops in spring was more viable than in winter, but this made them vulnerable to late spring and early summer moisture deficits, and early summer droughts could lead to total crop failure.

̽��ֱ��researchers point to surviving accounts written by Roman chroniclers to corroborate these drought-driven grain deficits. By 367 CE, Ammianus Marcellinus described the population of Britain as in the ‘utmost conditions of famine’.

“Drought from 364 to 366 CE would have impacted spring-sown crop growth substantially, triggering poor harvests,” Charles Norman said. “This would have reduced the grain supply to Hadrian’s Wall, providing a plausible motive for the rebellion there which allowed the Picts into northern Britain.”

̽��ֱ��study suggests that given the crucial role of grain in the contract between soldiers and the army, grain deficits may have contributed to other desertions in this period, and therefore a general weakening of the Roman army in Britain. In addition, the geographic isolation of Roman Britain likely combined with the severity of the prolonged drought to reduce the ability of Rome to alleviate the deficits.

Ultimately the researchers argue that military and societal breakdown in Roman Britain provided an ideal opportunity for peripheral tribes, including the Picts, Scotti and Saxons, to invade the province en masse with the intention of raiding rather than conquest. Their finding that the most severe conditions were restricted to southern Britain undermines the idea that famines in other provinces might have forced these tribes to invade.

Andreas Rzepecki, from the Generaldirektion Kulturelles Erbe Rheinland-Pfalz, said: “Our findings align with the accounts of Roman chroniclers and the seemingly coordinated nature of the ‘Conspiracy’ suggests an organised movement of strong onto weak, rather than a more chaotic assault had the invaders been in a state of desperation.”

“ ̽��ֱ��prolonged and extreme drought seems to have occurred during a particularly poor period for Roman Britain, in which food and military resources were being stripped for the Rhine frontier, while immigratory pressures increased.”

“These factors limited resilience, and meant a drought induced, partial-military rebellion and subsequent external invasion were able to overwhelm the weakened defences.”

̽��ֱ��researchers expanded their climate-conflict analysis to the entire Roman Empire for the period 350–476 CE. They reconstructed the climate conditions immediately before and after 106 battles and found that a statistically significant number of battles were fought following dry years.

Tatiana Bebchuk, from Cambridge’s Department of Geography, said: “ ̽��ֱ��relationship between climate and conflict is becoming increasingly clear in our own time so these findings aren’t just important for historians. Extreme climate conditions lead to hunger, which can lead to societal challenges, which eventually lead to outright conflict.”

Charles Norman, Ulf Büntgen, Paul Krusic and Tatiana Bebchuk are based at the Department of Geography, ̽��ֱ�� of Cambridge; Lothar Schwinden and Andreas Rzepecki are from the Generaldirektion Kulturelles Erbe Rheinland-Pfalz in Trier. Ulf Büntgen is also affiliated with the Global Change Research Institute, Czech Academy of Sciences and the Department of Geography, Masaryk ̽��ֱ�� in Brno.

Reference

C Norman, L Schwinden, P Krusic, A Rzepecki, T Bebchuk, U Büntgen, ‘Droughts and conflicts during the late Roman period’, Climatic Change (2025). DOI: 10.1007/s10584-025-03925-4

Funding

Charles Norman was supported by Wolfson College, ̽��ֱ�� of Cambridge (John Hughes PhD Studentship). Ulf Büntgen received funding from the Czech Science Foundation (# 23-08049S; Hydro8), the ERC Advanced Grant (# 882727; Monostar), and the ERC Synergy Grant (# 101118880; Synergy-Plague).

Three consecutive years of drought contributed to the ‘Barbarian Conspiracy’, a pivotal moment in the history of Roman Britain, a new Cambridge-led study reveals. Researchers argue that Picts, Scotti and Saxons took advantage of famine and societal breakdown caused by an extreme period of drought to inflict crushing blows on weakened Roman defences in 367 CE. While Rome eventually restored order, some historians argue that the province never fully recovered.

Our findings provide an explanation for the catalyst of this major event.

Charles Norman

Adam Cuerden

Milecastle 39 on Hadrian's Wall

̽��ֱ��text in this work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. Images, including our videos, are Copyright © ̽��ֱ�� of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.

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2023 was the hottest summer in two thousand years

sc604 — Tue, 14 May 2024 15:00:00 +0000

Although 2023 has been reported as the hottest year on record, the instrumental evidence only reaches back as far as 1850 at best, and most records are limited to certain regions.

Now, by using past climate information from annually resolved tree rings over two millennia, scientists from the ̽��ֱ�� of Cambridge and the Johannes Gutenberg ̽��ֱ�� Mainz have shown how exceptional the summer of 2023 was.

Even allowing for natural climate variations over hundreds of years, 2023 was still the hottest summer since the height of the Roman Empire, exceeding the extremes of natural climate variability by half a degree Celsius.

“When you look at the long sweep of history, you can see just how dramatic recent global warming is,” said co-author Professor Ulf Büntgen, from Cambridge’s Department of Geography. “2023 was an exceptionally hot year, and this trend will continue unless we reduce greenhouse gas emissions dramatically.”

̽��ֱ��results, reported in the journal Nature, also demonstrate that in the Northern Hemisphere, the 2015 Paris Agreement to limit warming to 1.5C above pre-industrial levels has already been breached.

Early instrumental temperature records, from 1850-1900, are sparse and inconsistent. ̽��ֱ��researchers compared early instrumental data with a large-scale tree ring dataset and found the 19th century temperature baseline used to contextualise global warming is several tenths of a degree Celsius colder than previously thought. By re-calibrating this baseline, the researchers calculated that summer 2023 conditions in the Northern Hemisphere were 2.07C warmer than mean summer temperatures between 1850 and 1900.

“Many of the conversations we have around global warming are tied to a baseline temperature from the mid-19th century, but why is this the baseline? What is normal, in the context of a constantly-changing climate, when we’ve only got 150 years of meteorological measurements?” said Büntgen. “Only when we look at climate reconstructions can we better account for natural variability and put recent anthropogenic climate change into context.”

Tree rings can provide that context, since they contain annually-resolved and absolutely-dated information about past summer temperatures. Using tree-ring chronologies allows researchers to look much further back in time without the uncertainty associated with some early instrumental measurements.

̽��ֱ��available tree-ring data reveals that most of the cooler periods over the past 2000 years, such as the Little Antique Ice Age in the 6th century and the Little Ice Age in the early 19th century, followed large-sulphur-rich volcanic eruptions. These eruptions spew huge amounts of aerosols into the stratosphere, triggering rapid surface cooling. ̽��ֱ��coldest summer of the past two thousand years, in 536 CE, followed one such eruption, and was 3.93C colder than the summer of 2023.

Most of the warmer periods covered by the tree ring data can be attributed to the El Niño climate pattern, or El Niño-Southern Oscillation (ENSO). El Niño affects weather worldwide due to weakened trade winds in the Pacific Ocean and often results in warmer summers in the Northern Hemisphere. While El Niño events were first noted by fisherman in the 17th century, they can be observed in the tree ring data much further back in time.

However, over the past 60 years, global warming caused by greenhouse gas emissions are causing El Niño events to become stronger, resulting in hotter summers. ̽��ֱ��current El Niño event is expected to continue into early summer 2024, making it likely that this summer will break temperature records once again.

“It’s true that the climate is always changing, but the warming in 2023, caused by greenhouse gases, is additionally amplified by El Niño conditions, so we end up with longer and more severe heat waves and extended periods of drought,” said Professor Jan Esper, the lead author of the study from the Johannes Gutenberg ̽��ֱ�� Mainz in Germany. “When you look at the big picture, it shows just how urgent it is that we reduce greenhouse gas emissions immediately.”

̽��ֱ��researchers note that while their results are robust for the Northern Hemisphere, it is difficult to obtain global averages for the same period since data is sparse for the Southern Hemisphere. ̽��ֱ��Southern Hemisphere also responds differently to climate change, since it is far more ocean-covered than the Northern Hemisphere.

̽��ֱ��research was supported in part by the European Research Council.

Reference:
Jan Esper, Max Torbenson, Ulf Büntgen. ‘2023 summer warmth unparalleled over the past 2,000 years.’ Nature (2024). DOI: 10.1038/s41586-024-07512-y

Researchers have found that 2023 was the hottest summer in the Northern Hemisphere in the past two thousand years, almost four degrees warmer than the coldest summer during the same period.

When you look at the long sweep of history, you can see just how dramatic recent global warming is

Ulf Büntgen

trekandshoot via Getty Images

Morning sun over Los Angeles, USA.

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̽��ֱ��Fens of eastern England once held vast woodlands

sc604 — Fri, 24 Nov 2023 05:23:59 +0000

̽��ֱ��Fens of eastern England, a low-lying, extremely flat landscape dominated by agricultural fields, was once a vast woodland filled with huge yew trees, according to new research.

European summer droughts since 2015 unprecedented in past two millennia

sc604 — Mon, 15 Mar 2021 14:37:48 +0000

Recent summer droughts in Europe are far more severe than anything in the past 2,100 years, according to a new study.

̽��ֱ��testimony of trees: how volcanic eruptions shaped 2000 years of world history

sc604 — Mon, 28 Sep 2020 03:00:00 +0000

̽��ֱ��researchers, led by the ̽��ֱ�� of Cambridge, used samples from more than 9000 living and dead trees to obtain a precise yearly record of summer temperatures in North America and Eurasia, dating back to the year 1 CE. This revealed colder and warmer periods that they then compared with records for very large volcanic eruptions as well as major historical events.

Crucial to the accuracy of the dataset was the use of the same number of data points across the entire 2000 years. Previous reconstructions of climate over this extended period have been biased by over-representation of trees from more recent times.

̽��ֱ��results, reported in the journal Dendrochronologia, show that the effect of volcanoes on global temperature changes is even greater than had been recognised, although the researchers stress that their work in no way diminishes the significance of human-caused climate change.

Instead, the researchers say, the study contributes to our understanding of the natural causes and societal consequences of summer temperature changes over the past two thousand years.

“There is so much we can determine about past climate conditions from the information in tree rings, but we have far more information from newer trees than we do for trees which lived a thousand years or more ago,” said Professor Ulf Büntgen from Cambridge’s Department of Geography, the study’s lead author. “Removing some of the data from the more recent past levels the playing field for the whole 2000-year period we’re looking at, so in the end, we gain a more accurate understanding of natural versus anthropogenic climate change.”

Comparing the data from tree rings against evidence from ice cores, the researchers were able to identify the effect of past volcanic eruptions on summer temperatures.

Large volcanic eruptions can lower global average temperatures by fractions of a degree Celsius, with strongest effects in parts of North America and Eurasia. ̽��ֱ��main factor is the amount of sulphur emitted during the eruption that reaches the stratosphere, where it forms minute particles that block some sunlight from reaching the surface. This can result in shorter growing seasons and cooler temperatures, that lead in turn to reduced harvests. Conversely, in periods when fewer large eruptions occurred, the Earth is able to absorb more heat from the Sun and temperatures rise.

“Some climate models assume that the effect of volcanoes is punctuated and short,” said Büntgen. “However, if you look at the cumulative effect over a whole century, this effect can be much longer. In part, we can explain warm conditions during the 3^rd, 10^th and 11^th centuries through a comparative lack of eruptions.”

Reconstructed summer temperatures in the 280s, 990s and 1020s, when volcanic forcing was low, were comparable to modern conditions until 2010.

Compared with existing large-scale temperature reconstructions of the past 1200–2000 years, the study reveals a greater pre-industrial summer temperature variability, including strong evidence for the Late Antique Little Ice Age (LALIA) in the 6^th and 7^th centuries.

Then, working with historians, the scientists found that relatively constant warmth during Roman and medieval periods, when large volcanic eruptions were less frequent, often coincided with societal prosperity and political stability in Europe and China. However, the periods characterised by more prolific volcanism often coincided with times of conflict and economic decline.

“Interpreting history is always challenging,” said Dr Clive Oppenheimer, the lead volcanologist of the study. “So many factors come into play – politics, economics, culture. But a big eruption that leads to widespread declines in grain production can hurt millions of people. Hunger can lead to famine, disease, conflict and migration. We see much evidence of this in the historical record.

“We knew that large eruptions could have these effects, especially when societies were already stressed, but I was surprised to see the opposite effect so clearly in our data – that centuries with rather few eruptions had warmer summers than the long-term average.”

̽��ֱ��new temperature reconstructions provide deeper insights into historical periods in which climactic changes, and their associated environmental responses, have had an outsized impact on human history. This has clear implications for our present and future. As climate change accelerates, extreme events, such as floods, drought, storms and wildfires, will become more frequent.

“Humans have no effect on whether or not a volcano erupts, but the warming trend we are seeing right now is certainly related to human activity,” said Büntgen. “While nothing about the future is certain, we would do well to learn how climate change has affected human civilisation in the past.”

Reference:
Ulf Büntgen et al. ‘Prominent role of volcanism in Common Era climate variability and human history.’ Dendrochronologia (2020). DOI: 10.1016/j.dendro.2020.125757

Researchers have shown that over the past two thousand years, volcanoes have played a larger role in natural temperature variability than previously thought, and their climatic effects may have contributed to past societal and economic change.

Some climate models assume that the effect of volcanoes is punctuated and short. However, if you look at the cumulative effect over a whole century, this effect can be much longer.

Ulf Büntgen

Clive Oppenheimer

Driftwood in Siberia

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. Images, including our videos, are Copyright © ̽��ֱ�� of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – as here, on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.

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Amount of carbon stored in forests reduced as climate warms

sc604 — Wed, 15 May 2019 09:00:37 +0000

̽��ֱ��team, led by the ̽��ֱ�� of Cambridge, found that as temperatures increase, trees grow faster, but they also tend to die younger. When these fast-growing trees die, the carbon they store is returned to the carbon cycle.

̽��ֱ��results, reported in the journal Nature Communications, have implications for global carbon cycle dynamics. As the Earth’s climate continues to warm, tree growth will continue to accelerate, but the length of time that trees store carbon, the so-called carbon residence time, will diminish.

During photosynthesis, trees and other plants absorb carbon dioxide from the atmosphere and use it to build new cells. Long-lived trees, such as pines from high elevations and other conifers found across the high-northern latitude boreal forests, can store carbon for many centuries.

“As the planet warms, it causes plants to grow faster, so the thinking is that planting more trees will lead to more carbon getting removed from the atmosphere,” said Professor Ulf Büntgen from Cambridge’s Department of Geography, the study’s lead author. “But that’s only half of the story. ̽��ֱ��other half is one that hasn’t been considered: that these fast-growing trees are holding carbon for shorter periods of time.”

Büntgen uses the information contained in tree rings to study past climate conditions. Tree rings are as distinctive as fingerprints: the width, density and anatomy of each annual ring contains information about what the climate was like during that particular year. By taking core samples from living trees and disc samples of dead trees, researchers are able to reconstruct how the Earth’s climate system behaved in the past and understand how ecosystems were, and are, responding to temperature variation.

For the current study, Büntgen and his collaborators from Germany, Spain, Switzerland and Russia, sampled more than 1100 living and dead mountain pines from the Spanish Pyrenees and 660 Siberian larch samples from the Russian Altai: both high-elevation forest sites that have been undisturbed for thousands of years. Using these samples, the researchers were able to reconstruct the total lifespan and juvenile growth rates of trees that were growing during both industrial and pre-industrial climate conditions.

̽��ֱ��researchers found that harsh, cold conditions cause tree growth to slow, but they also make trees stronger, so that they can live to a great age. Conversely, trees growing faster during their first 25 years die much sooner than their slow-growing relatives. This negative relationship remained statistically significant for samples from both living and dead trees in both regions.

̽��ֱ��idea of a carbon residence time was first hypothesised by co-author Christian Körner, Emeritus Professor at the ̽��ֱ�� of Basel, but this is the first time that it has been confirmed by data.

̽��ֱ��relationship between growth rate and lifespan is analogous to the relationship between heart rate and lifespan seen in the animal kingdom: animals with quicker heart rates tend to grow faster but have shorter lives on average.

“We wanted to test the ‘live fast, die young’ hypothesis, and we’ve found that for trees in cold climates, it appears to be true,” said Büntgen. “We’re challenging some long-held assumptions in this area, which have implications for large-scale carbon cycle dynamics.”

Reference:
Ulf Büntgen et al. ‘Limited capacity of tree growth to mitigate the global greenhouse effect under predicted warming.’ Nature Communications (2019). DOI: 10.1038/s41467-019-10174-4

A bold response to the world’s greatest challenge
̽��ֱ�� ̽��ֱ�� of Cambridge is building on its existing research and launching an ambitious new environment and climate change initiative. Cambridge Zero is not just about developing greener technologies. It will harness the full power of the ̽��ֱ��’s research and policy expertise, developing solutions that work for our lives, our society and our biosphere.

Accelerated tree growth caused by a warming climate does not necessarily translate into enhanced carbon storage, an international study suggests.

We’re challenging some long-held assumptions, which have implications for large-scale carbon cycle dynamics

Ulf Büntgen

Trees in the Spanish Pyrenees

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Silent witnesses: how an ice age was written in the trees

lw355 — Tue, 27 Feb 2018 12:00:35 +0000

Researchers use tree rings to unravel past climates and their impact on civilisations.

READ THE STORY HERE

What connects a series of volcanic eruptions and severe summer cooling with a century of pandemics, human migration and the rise and fall of civilisations? Tree rings, says Ulf Büntgen, who leads Cambridge’s first dedicated tree-ring laboratory in the Department of Geography.

Once you embark on these integrative approaches you can ask questions like how did complex societies cope with climate change? That’s when it starts to get really exciting.

Ulf Büntgen

Hrafn Óskarsson

Subfossil trees preserved in Iceland

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

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