̽��ֱ�� of Cambridge - Ulf Büntgen

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.

Yes

How a drought led to the rise of skateboarding in 1970s California

sc604 — Tue, 12 Dec 2023 13:00:11 +0000

Why did professional skateboarding arise in southern California in the 1970s? Was it a coincidence, or was it a perfect storm of multiple factors?

̽��ֱ��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.

Drought encouraged Attila’s Huns to attack the Roman empire, tree rings suggest

ta385 — Thu, 15 Dec 2022 09:00:00 +0000

Hungary has just experienced its driest summer since meteorological measurements began, devastating the country’s usually productive farmland. Archaeologists now suggest that similar conditions in the 5th century may have encouraged animal herders to become raiders, with devastating consequences for the Roman empire.

̽��ֱ��study, published in the Journal of Roman Archaeology, argues that extreme drought spells from the 430s – 450s CE disrupted ways of life in the Danube frontier provinces of the eastern Roman empire, forcing Hunnic peoples to adopt new strategies to ‘buffer against severe economic challenges’.

̽��ֱ��authors, Associate Professor Susanne Hakenbeck from Cambridge’s Department of Archaeology and Professor Ulf Büntgen from the ̽��ֱ��’s Department of Geography, came to their conclusions after assessing a new tree ring-based hydroclimate reconstruction, as well as archaeological and historical evidence.

̽��ֱ��Hunnic incursions into eastern and central Europe in the 4th and 5th centuries CE have long been viewed as the initial crisis that triggered the so-called ‘Great Migrations’ of ‘Barbarian Tribes’, leading to the fall of the Roman empire. But where the Huns came from and what their impact on the late Roman provinces actually was unclear.

New climate data reconstructed from tree rings by Prof Büntgen and colleagues provides information about yearly changes in climate over the last 2000 years. It shows that Hungary experienced episodes of unusually dry summers in the 4th and 5th centuries. Hakenbeck and Büntgen point out that climatic fluctuations, in particular drought spells from 420 to 450 CE, would have reduced crop yields and pasture for animals beyond the floodplains of the Danube and Tisza.

Büntgen said: “Tree ring data gives us an amazing opportunity to link climatic conditions to human activity on a year-by-year basis. We found that periods of drought recorded in biochemical signals in tree-rings coincided with an intensification of raiding activity in the region.”

Recent isotopic analysis of skeletons from the region, including by Dr Hakenbeck, suggests that Hunnic peoples responded to climate stress by migrating and by mixing agricultural and pastoral diets.

Hakenbeck said: “If resource scarcity became too extreme, settled populations may have been forced to move, diversify their subsistence practices and switch between farming and mobile animal herding. These could have been important insurance strategies during a climatic downturn.”

But the study also argues that some Hunnic peoples dramatically changed their social and political organization to become violent raiders.

From herders to raiders

Hunnic attacks on the Roman frontier intensified after Attila came to power in the late 430s. ̽��ֱ��Huns increasingly demanded gold payments and eventually a strip of Roman territory along the Danube. In 451 CE, the Huns invaded Gaul and a year later they invaded northern Italy.

Traditionally, the Huns have been cast as violent barbarians driven by an “infinite thirst for gold”. But, as this study points out, the historical sources documenting these events were primary written by elite Romans who had little direct experience of the peoples and events they described.

“Historical sources tell us that Roman and Hun diplomacy was extremely complex,” Dr Hakenbeck said. “Initially it involved mutually beneficial arrangements, resulting in Hun elites gaining access to vast amounts of gold. This system of collaboration broke down in the 440s, leading to regular raids of Roman lands and increasing demands for gold.”

̽��ֱ��study argues that if current dating of events is correct, the most devastating Hunnic incursions of 447, 451 and 452 CE coincided with extremely dry summers in the Carpathian Basin.

Hakenbeck said: “Climate-induced economic disruption may have required Attila and others of high rank to extract gold from the Roman provinces to keep war bands and maintain inter-elite loyalties. Former horse-riding animal herders appear to have become raiders.”

Historical sources describe the Huns at this time as a highly stratified group with a military organization that was difficult to counter, even for the Roman armies.

̽��ֱ��study suggests that one reason why the Huns attacked the provinces of Thrace and Illyricum in 422, 442, and 447 CE was to acquire food and livestock, rather than gold, but accepts that concrete evidence is needed to confirm this. ̽��ֱ��authors also suggest that Attila demanded a strip of land ‘five days’ journey wide’ along the Danube because this could have offered better grazing in a time of drought.

“Climate alters what environments can provide and this can lead people to make decisions that affect their economy, and their social and political organization," Hakenbeck said. "Such decisions are not straightforwardly rational, nor are their consequences necessarily successful in the long term.”

“This example from history shows that people respond to climate stress in complex and unpredictable ways, and that short-term solutions can have negative consequences in the long term.”

By the 450s CE, just a few decades of their appearance in central Europe, the Huns had disappeared. Attila himself died in 453 CE.

Reference

S.E. Hakenbeck & U. Büntgen, ‘ ̽��ֱ��role of drought during the Hunnic incursions into central-east Europe in the 4th and 5th centuries CE’, Journal of Roman Archaeology (2022). DOI: 10.1017/S1047759422000332

Hunnic peoples migrated westward across Eurasia, switched between farming and herding, and became violent raiders in response to severe drought in the Danube frontier provinces of the Roman empire, a new study argues.

People respond to climate stress in complex and unpredictable ways

Susanne Hakenbeck

Stefan Lefnaer

Devínska Kobyla Forest steppe in Slovakia

̽��ֱ��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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UK plants flowering a month earlier due to climate change

sc604 — Wed, 02 Feb 2022 00:08:32 +0000

Using a citizen science database with records going back to the mid-18th century, a research team led by the ̽��ֱ�� of Cambridge has found that the effects of climate change are causing plants in the UK to flower one month earlier under recent global warming.

̽��ֱ��researchers based their analysis on more than 400,000 observations of 406 plant species from Nature’s Calendar, maintained by the Woodland Trust, and collated the first flowering dates with instrumental temperature measurements.

They found that the average first flowering date from 1987 to 2019 is a full month earlier than the average first flowering date from 1753 to 1986. ̽��ֱ��same period coincides with accelerating global warming caused by human activities. ̽��ֱ��results are reported in Proceedings of the Royal Society B.

While the first spring flowers are always a welcome sight, this earlier flowering can have consequences for the UK’s ecosystems and agriculture. Other species that synchronise their migration or hibernation can be left without the flowers and plants they rely on – a phenomenon known as ecological mismatch – which can lead to biodiversity loss if populations cannot adapt quickly enough.

̽��ֱ��change can also have consequences for farmers and gardeners. If fruit trees, for example, flower early following a mild winter, entire crops can be killed off if the blossoms are then hit by a late frost.

While we can see the effects of climate change through extreme weather events and increasing climate variability, the long-term effects of climate change on ecosystems are more subtle and are therefore difficult to recognise and quantify.

“We can use a wide range of environmental datasets to see how climate change is affecting different species, but most records we have only consider one or a handful of species in a relatively small area,” said Professor Ulf Büntgen from Cambridge’s Department of Geography, the study’s lead author. “To really understand what climate change is doing to our world, we need much larger datasets that look at whole ecosystems over a long period of time.”

̽��ֱ��UK has such a dataset: since the 18th century, observations of seasonal change have been recorded by scientists, naturalists, amateur and professional gardeners, as well as organisations such as the Royal Meteorological Society. In 2000, the Woodland Trust joined forced with the Centre for Ecology & Hydrology and collated these records into Nature’s Calendar, which currently has around 3.5 million records going back to 1736.

“Anyone in the UK can submit a record to Nature’s Calendar, by logging their observations of plants and wildlife,” said Büntgen. “It’s an incredibly rich and varied data source, and alongside temperature records, we can use it to quantify how climate change is affecting the functioning of various ecosystem components across the UK.”

For the current study, the researchers used over 400,000 records from Nature’s Calendar to study changes in 406 flowering plant species in the UK, between 1753 and 2019. They used observations of the first flowering date of trees, shrubs, herbs and climbers, in locations from the Channel Islands to Shetland, and from Northern Ireland to Suffolk.

̽��ֱ��researchers classified the observations in various ways: by location, elevation, and whether they were from urban or rural areas. ̽��ֱ��first flowering dates were then compared with monthly climate records.

To better balance the number of observations, the researchers divided the full dataset into records until 1986, and from 1987 onwards. ̽��ֱ��average first flowering advanced by a full month, and is strongly correlated with rising global temperatures.

“ ̽��ֱ��results are truly alarming, because of the ecological risks associated with earlier flowering times,” said Büntgen. “When plants flower too early, a late frost can kill them – a phenomenon that most gardeners will have experienced at some point. But the even bigger risk is ecological mismatch. Plants, insects, birds and other wildlife have co-evolved to a point that they’re synchronised in their development stages. A certain plant flowers, it attracts a particular type of insect, which attracts a particular type of bird, and so on. But if one component responds faster than the others, there’s a risk that they’ll be out of synch, which can lead species to collapse if they can’t adapt quickly enough.”

Büntgen says that if global temperatures continue to increase at their current rate, spring in the UK could eventually start in February. However, many of the species that our forests, gardens and farms rely on could experience serious problems given the rapid pace of change.

“Continued monitoring is necessary to ensure that we better understand the consequences of a changing climate,” said co-author Professor Tim Sparks from Cambridge’s Department of Zoology. “Contributing records to Nature’s Calendar is an activity that everyone can engage in.”

̽��ֱ��research was supported in part by the European Research Council, the Fritz and Elisabeth Schweingruber Foundation, and the Woodland Trust.

Reference:
Ulf Büntgen et al. ‘Plants in the UK flower a month earlier under recent warming.’ Proceedings of the Royal Society B (2022). DOI: 10.1098/rspb.2021.2456

Climate change is causing plants in the UK to flower a month earlier on average, which could have profound consequences for wildlife, agriculture and gardeners.

To really understand what climate change is doing to our world, we need much larger datasets that look at whole ecosystems over a long period of time

Ulf Büntgen

Crab apple tree in bloom

Yes

Experiment evaluates the effect of human decisions on climate reconstructions

sc604 — Mon, 07 Jun 2021 09:21:35 +0000

̽��ֱ��experiment, designed and run by researchers from the ̽��ֱ�� of Cambridge, had multiple research groups from around the world use the same raw tree-ring data to reconstruct temperature changes over the past 2,000 years.

While each of the reconstructions clearly showed that recent warming due to anthropogenic climate change is unprecedented in the past two thousand years, there were notable differences in variance, amplitude and sensitivity, which can be attributed to decisions made by the researchers who built the individual reconstructions.

Professor Ulf Büntgen from the ̽��ֱ�� of Cambridge, who led the research, said that the results are “important for transparency and truth – we believe in our data, and we’re being open about the decisions that any climate scientist has to make when building a reconstruction or model.”

To improve the reliability of climate reconstructions, the researchers suggest that teams make multiple reconstructions at once so that they can be seen as an ensemble. ̽��ֱ��results are reported in the journal Nature Communications.

Information from tree rings is the main way that researchers reconstruct past climate conditions at annual resolutions: as distinctive as a fingerprint, the rings formed in trees outside the tropics are annually precise growth layers. Each ring can tell us something about what conditions were like in a particular growing season, and by combining data from many trees of different ages, scientists are able to reconstruct past climate conditions going back hundreds and even thousands of years.

Reconstructions of past climate conditions are useful as they can place current climate conditions or future projections in the context of past natural variability. ̽��ֱ��challenge with a climate reconstruction is that – absent a time machine – there is no way to confirm it is correct.

“While the information contained in tree rings remains constant, humans are the variables: they may use different techniques or choose a different subset of data to build their reconstruction,” said Büntgen, who is based at Cambridge’s Department of Geography, and is also affiliated with the CzechGlobe Centre in Brno, Czech Republic. “With any reconstruction, there’s a question of uncertainty ranges: how certain you are about a certain result. A lot of work has gone into trying to quantify uncertainties in a statistical way, but what hasn’t been studied is the role of decision-making.

“It’s not the case that there is one single truth – every decision we make is subjective to a greater or lesser extent. Scientists aren’t robots, and we don’t want them to be, but it’s important to learn where the decisions are made and how they affect the outcome.”

Büntgen and his colleagues devised an experiment to test how decision-making affects climate reconstructions. They sent raw tree ring data to 15 research groups around the world and asked them to use it to develop the best possible large-scale climate reconstruction for summer temperatures in the Northern hemisphere over past 2000 years.

“Everything else was up to them – it may sound trivial, but this sort of experiment had never been done before,” said Büntgen.

Each of the groups came up with a different reconstruction, based on the decisions they made along the way: the data they chose or the techniques they used. For example, one group may have used instrumental target data from June, July and August, while another may have only used the mean of July and August only.

̽��ֱ��main differences in the reconstructions were those of amplitude in the data: exactly how warm was the Medieval warming period, or how much cooler a particular summer was after a large volcanic eruption.

Büntgen stresses that each of the reconstructions showed the same overall trends: there were periods of warming in the 3^rd century, as well as between the 10^th and 12^th century; they all showed abrupt summer cooling following clusters of large volcanic eruptions in the 6^th, 15^th and 19^th century; and they all showed that the recent warming since the 20^th and 21^st century is unprecedented in the past 2000 years.

“You think if you have the start with the same data, you will end up with the same result, but climate reconstruction doesn’t work like that,” said Büntgen. “All the reconstructions point in the same direction, and none of the results oppose one another, but there are differences, which must be attributed to decision-making.”

So, how will we know whether to trust a particular climate reconstruction in future? In a time where experts are routinely challenged, or dismissed entirely, how can we be sure of what is true? One answer may be to note each point where a decision is made, consider the various options, and produce multiple reconstructions. This would of course mean more work for climate scientists, but it could be a valuable check to acknowledge how decisions affect outcomes.

Another way to make climate reconstructions more robust is for groups to collaborate and view all their reconstructions together, as an ensemble. “In almost any scientific field, you can point to a single study or result that tells you what to hear,” he said. “But when you look at the body of scientific evidence, with all its nuances and uncertainties, you get a clearer overall picture.”

Reference:
Ulf Büntgen et al. ' ̽��ֱ��influence of decision-making in tree ring-based climate reconstructions.’ Nature Communications (2021). DOI: 10.1038/s41467-021-23627-6

̽��ֱ��first double-blind experiment analysing the role of human decision-making in climate reconstructions has found that it can lead to substantially different results.

Scientists aren’t robots, and we don’t want them to be, but it’s important to learn where the decisions are made and how they affect the outcome

Ulf Büntgen

Hrafn Óskarsson

Subfossil trees preserved in Iceland

Yes

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.