̽��ֱ�� of Cambridge - trees

Wrong trees in the wrong place can make cities hotter at night, study reveals

ta385 — Tue, 10 Dec 2024 09:45:00 +0000

Temperatures in cities are rising across the globe and urban heat stress is already a major problem causing illness, death, a surge in energy use to cool buildings down, heat-related social inequality issues and problems with urban infrastructure.

Some cities have already started implementing mitigation strategies, with tree planting prominent among them. But a ̽��ֱ�� of Cambridge-led study now warns that planting the wrong species or the wrong combination of trees in suboptimal locations or arrangements can limit their benefits.

̽��ֱ��study, published today in Communications Earth & Environment found that urban trees can lower pedestrian-level air temperature by up to 12°C. Its authors found that the introduction of trees reduced peak monthly temperatures to below 26°C in 83% of the cities studied, meeting the ‘thermal comfort threshold’. However, they also found that this cooling ability varies significantly around the world and is influenced by tree species traits, urban layout and climate conditions.

“Our study busts the myth that trees are the ultimate panacea for overheating cities across the globe,” said Dr Ronita Bardhan, Associate Professor of Sustainable Built Environment at Cambridge's Dept. of Architecture.

“Trees have a crucial role to play in cooling cities down but we need to plant them much more strategically to maximise the benefits which they can provide.”

Previous research on the cooling effects of urban trees has focused on specific climates or regions, and considered case studies in a fragmented way, leaving major gaps in our knowledge about unique tree cooling mechanisms and how these interact with diverse urban features.

To overcome this, the authors of this study analysed the findings of 182 studies – concerning 17 climates in 110 global cities or regions – published between 2010 and 2023, offering the first comprehensive global assessment of urban tree cooling.

During the day, trees cool cities in three ways: by blocking solar radiation; through evaporation of water via pores in their leaves; and by foliage aerodynamically changing airflow. At night, however, tree canopies can trap longwave radiation from the ground surface, due to aerodynamic resistance and ‘stomatal closure’ – the closing of microscopic pores on the surface of leaves partly in response to heat and drought stress.

Variation by climate type

̽��ֱ��study found that urban trees generally cool cities more in hot and dry climates, and less in hot humid climates.

In the ‘tropical wet and dry or savanna’ climate, trees can cool cities by as much as 12 °C, as recorded in Nigeria. However, it was in this same climate that trees also warmed cities most at night, by up to 0.8°C.

Trees performed well in arid climates, cooling cities by just over 9°C and warming them at night by 0.4 °C.

In tropical rainforest climates, where humidity is higher, the daytime cooling effect dropped to approximately 2°C while the nighttime heating effect was 0.8 °C.

In temperate climates, trees can cool cities by up to 6°C and warm them by 1.5°C.

Using trees more strategically

̽��ֱ��study points out that cities which have more open urban layouts are more likely to feature a mix of evergreen and deciduous trees of varying sizes. This, the researchers found, tends to result in greater cooling in temperate, continental and tropical climates.

̽��ֱ��combined use of trees in these climates generally results in 0.5 °C more cooling than in cities where only deciduous or evergreen trees feature. This is because mixed trees can balance seasonal shading and sunlight, providing three-dimensional cooling at various heights.

In arid climates, however, the researchers found that evergreen species dominate and cool more effectively in the specific context of compact urban layouts such as Cairo in Egypt, or Dubai in UAE.

In general, trees cooled more effectively in open and low-rise cities in dry climates. In open urban layouts, cooling can be improved by about 0.4 °C because their larger green spaces allow for more and larger tree canopies and a greater mix of tree species.

“Our study provides context-specific greening guidelines for urban planners to more effectively harness tree cooling in the face of global warming,” Dr Ronita Bardhan said.

“Our results emphasize that urban planners not only need to give cities more green spaces, they need to plant the right mix of trees in optimal positions to maximize cooling benefits.”

“Urban planners should plan for future warmer climates by choosing resilient species which will continue to thrive and maintain cooling benefits,” said Dr Bardhan, a Fellow of Selwyn College, Cambridge.

Matching trees to urban forms

̽��ֱ��study goes further, arguing that species selection and placement needs to be compatible with urban forms. ̽��ֱ��orientation of the ‘street canyon’, local climate zones, aspect ratio, visible sky ratio and other urban features that influence the effects of trees all need to be carefully considered.

Although a higher degree of tree canopy cover in street canyons generally results in more cooling effects, excessively high cover may trap heat at the pedestrian level, especially in compact urban zones in high temperature climates. In such locations, narrow species and sparse planting strategies are recommended.

̽��ֱ��researchers emphasise that we cannot rely entirely on trees to cool cities, and that solutions such as solar shading and reflective materials will continue to play an important role.

̽��ֱ��researchers have developed an interactive database and map to enable users to estimate the cooling efficacy of strategies based on data from cities with similar climates and urban structures.

Reference

H Li et al., ‘Cooling efficacy of trees across cities is determined by background climate, urban morphology, and tree trait’, Communications Earth & Environment (2024). DOI: 10.1038/s43247-024-01908-4

While trees can cool some cities significantly during the day, new research shows that tree canopies can also trap heat and raise temperatures at night. ̽��ֱ��study aims to help urban planners choose the best combinations of trees and planting locations to combat urban heat stress.

Trees have a crucial role to play in cooling cities down but we need to plant them much more strategically to maximise the benefits which they can provide

Ronita Bardhan

hannahisabelnic via Flikr (Public domain)

Trees in an Indian city street. Photo: hannahisabelnic via Flikr (Public domain)

̽��ֱ��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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Licence type:

Public Domain

Planting trees in the Arctic could make global warming worse, not better, say scientists

sc604 — Thu, 07 Nov 2024 13:41:21 +0000

But, writing in the journal Nature Geoscience, an international group of scientists, led by the ̽��ֱ�� of Cambridge and the ̽��ֱ�� of Århus, argue that tree planting at high latitudes will accelerate, rather than decelerate, global warming.

As the climate continues to warm, trees can be planted further and further north, and large-scale tree-planting projects in the Arctic have been championed by governments and corporations as a way to mitigate the worst effects of climate change.

However, when trees are planted in the wrong places - such as normally treeless tundra and mires, as well as large areas of the boreal forest with relatively open tree canopies - they can make global warming worse.

According to lead author Assistant Professor Jeppe Kristensen from Aarhus ̽��ֱ�� in Denmark, the unique characteristics of Arctic and sub-Arctic ecosystems make them poorly suited for tree planting for climate mitigation.

“Soils in the Arctic store more carbon than all vegetation on Earth,” said Kristensen. “These soils are vulnerable to disturbances, such as cultivation for forestry or agriculture, but also the penetration of tree roots. ̽��ֱ��semi-continuous daylight during the spring and early summer, when snow is still on the ground, also makes the energy balance in this region extremely sensitive to surface darkening, since green and brown trees will soak up more heat from the sun than white snow.”

In addition, the regions surrounding the North Pole in North America, Asia and Scandinavia are prone to natural disturbances - such as wildfires and droughts - that kill off vegetation. Climate change makes these disturbances both more frequent and more severe.

“This is a risky place to be a tree, particularly as part of a homogeneous plantation that is more vulnerable to such disturbances,” said Kristensen. “ ̽��ֱ��carbon stored in these trees risks fuelling disturbances and getting released back to the atmosphere within a few decades.”

̽��ֱ��researchers say that tree planting at high latitudes is a prime example of a climate solution with a desired effect in one context but the opposite effect in another.

“ ̽��ֱ��climate debate is very carbon-focused because the main way humans have modified the Earth’s climate in the last century is through emitting greenhouse gases from burning fossil fuels,” said Kristensen. “But at the core, climate change is the result of how much solar energy entering the atmosphere stays, and how much leaves again – Earth’s so-called energy balance.”

Greenhouse gases are one important determinant of how much heat can escape our planet’s atmosphere. However, the researchers say that at high latitudes, how much sunlight is reflected back into space, without being converted into heat (known as the albedo effect), is more important than carbon storage for the total energy balance.

̽��ֱ��researchers are calling for a more holistic view of ecosystems to identify truly meaningful nature-based solutions that do not compromise the overall goal: slowing down climate change.

“A holistic approach is not just a richer way of looking at the climate effects of nature-based solutions, but it’s imperative if we’re going to make a difference in the real world,” said senior author Professor Marc Macias-Fauria, from the ̽��ֱ�� of Cambridge’s Scott Polar Research Institute.

However, the researchers recognise that there can be other reasons for planting trees, such as timber self-sufficiency, but these cases do not come with bonuses for climate mitigation.

“Forestry in the far North should be viewed like any other production system and compensate for its negative impact on the climate and biodiversity,” said Macias-Fauria. “You can’t have your cake and eat it, and you can’t deceive the Earth. By selling northern afforestation as a climate solution, we’re only fooling ourselves.”

So how can we moderate global warming at high latitudes? ̽��ֱ��researchers suggest that working with local communities to support sustainable populations of large herbivores, such as caribou, could be a more viable nature-based solution to climate change in Arctic and subarctic regions than planting millions of trees.

“There is ample evidence that large herbivores affect plant communities and snow conditions in ways that result in net cooling,” said Macias-Fauria. “This happens both directly, by keeping tundra landscapes open, and indirectly, through the effects of herbivore winter foraging, where they modify the snow and decrease its insulation capacity, reducing soil temperatures and permafrost thaw.”

̽��ֱ��researchers say it’s vital to consider biodiversity and the livelihoods of local communities in the pursuit of nature-based climate solutions.

“Large herbivores can reduce climate-driven biodiversity loss in Arctic ecosystems and remain a fundamental food resource for local communities,” said Macias-Fauria. “Biodiversity and local communities are not an added benefit to nature-based solutions: they are fundamental. Any nature-based solutions must be led by the communities who live at the front line of climate change.”

More about this story

Reference:
Jeppe Å Kristensen et al. ‘Tree planting is no climate solution at northern high latitudes.’ Nature Geoscience (2024). DOI: 10.1038/s41561-024-01573-4

Explore more discoveries, innovations and research on climate and nature at the ̽��ֱ�� of Cambridge: www.cam.ac.uk/climate-and-nature

Tree planting has been widely touted as a cost-effective way of reducing global warming, due to trees’ ability to store large quantities of carbon from the atmosphere.

Pierre Longnus via Getty Images

Emerald Lake, Yukon

Yes

Earth’s earliest forest revealed in Somerset fossils

sc604 — Thu, 07 Mar 2024 10:27:02 +0000

̽��ֱ��oldest fossilised forest known on Earth – dating from 390 million years ago – has been found in the high sandstone cliffs along the Devon and Somerset coast of South West England.

̽��ֱ��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.

Experts predict ‘catastrophic ecosystem collapse’ of UK forests within the next 50 years if action not taken

cg605 — Wed, 08 Nov 2023 00:05:54 +0000

A team of experts from across Europe has produced a list of 15 over-looked and emerging issues that are likely to have a significant impact on UK forests over the next 50 years.

This is the first ‘horizon scanning’ exercise – a technique to identify relatively unknown threats, opportunities, and new trends – of UK forests. ̽��ֱ��aim is to help researchers, practitioners, policymakers, and society in general, better prepare for the future and address threats before they become critical.

Dr Eleanor Tew, first author, visiting researcher at Cambridge’s Department of Zoology and Head of Forest Planning at Forestry England said: “ ̽��ֱ��next 50 years will bring huge changes to UK forests: the threats they face, the way that we manage them, and the benefits they deliver to society.”

Forestry England, a part of the Forestry Commission, collaborated with the ̽��ֱ�� of Cambridge on the study, which was published today in the journal, Forestry.

A panel comprising 42 experts, who represented a range of professions, organisations, and geographies, reached out to their networks to seek over-looked and emerging issues that were likely to affect UK forests over the next half a century. ̽��ֱ��resulting 180-item longlist was then whittled down through a series of review exercises to a shortlist of 30 issues. In a final workshop, panellists identified the top 15 issues they believed were likely to have the greatest impact on UK forests in the next 50 years.

̽��ֱ��research method did not support the overall ranking of the 15 issues in order of importance or likelihood of occurrence. However, when the issues were scored individually by the panel of experts, it was notable that ‘catastrophic forest ecosystem collapse’ was the most highly ranked issue, with 64% of experts ranking it as their top issue and 88% ranking it within their top three.

‘Catastrophic forest ecosystem collapse’ refers to multiple interrelated hazards that have a cascading effect on forests, leading to their total or partial collapse. This has already been witnessed in continental Europe and North America.

Tew said: “We hope the results from this horizon scanning exercise serve as an urgent call to action to build on, and dramatically upscale, action to increase forest resilience.”

Another issue identified was that droughts caused by climate change may lead to competition for water resources between forests and society. On the other hand, forests may help to mitigate the impact of floods caused by climate change.

Tree viral diseases were also identified as an issue. In the UK, pests and pathogens are increasing due to globalisation and climate change, with viruses and viroids (RNA molecules) being the largest group on the UK Plant Health Risk Register. However, little is known about how viral diseases affect forest tree species and indeed the wider ecosystem.

A further issue was the effect of climate change on forest management, with extreme weather leading to smaller windows of time when forestry can be carried out. Experts warn that the seasons for carrying out work such as harvesting and thinning are getting narrower as we see wetter winters and scorching summers.

However not all emerging issues are threats – some are new opportunities. For example, trees will be at the heart of future urban planning. Experts predict that ‘forest lungs’ will be created thanks to an increased understanding of the benefits of trees for society. They say there will likely be a greater blurring of boundaries between urban and rural areas, with an increase in green infrastructure and connectivity.

International commitments around nature are also likely to have repercussions at the local level. For example, the mandatory reporting of companies’ supply chain impacts on nature, such as through the new framework being developed by the Taskforce on Nature-related Financial Disclosures (TNFD), could create additional incentives for nature-friendly forest management.

Tew concluded: “These results are both concerning and exciting. However, we should be optimistic, remembering that these are possibilities and not certainties. Crucially, we have time to act ‒ by responding to the threats and embracing the opportunities, future generations can have resilient forests with all the benefits they offer.”

Senior author and pioneer of horizon scanning, Professor Bill Sutherland, from the Department of Zoology at the ̽��ֱ�� of Cambridge said: “We are already seeing dramatic events in Europe’s forests whether fires, disease or bark beetles, whilst the importance of trees is increasingly recognised. Horizon scanning to identify future issues is key, especially as trees planted now will face very different circumstances as they mature in scores of years.”

This research was funded by Forestry England. ̽��ֱ��Forestry Commission is bringing the sector together in 2024 to look at next steps.

Tew et al, A horizon scan of issues affecting UK forest management within 50 years, Forestry DOI: 10.1093/forestry/cpad047

Other threats to UK forests include competition with society for water, viral diseases, and extreme weather affecting forest management.

̽��ֱ��next 50 years will bring huge changes to UK forests: the threats they face, the way that we manage them, and the benefits they deliver to society.

Dr Eleanor Tew, visiting researcher at Cambridge’s Department of Zoology and Head of Forest Planning at Forestry England

Graham Custance Photography / Moment via Getty Images

Ashridge, Hertfordshire, UK

̽��ֱ��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 – 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.

Yes

Roadside hedges can reduce harmful ultrafine particle pollution around schools

cmm201 — Wed, 13 Sep 2023 14:19:33 +0000

̽��ֱ��research, a collaboration with Lancaster ̽��ֱ��, found that hedges can act as protective barriers against air pollution from major city roads by soaking up significant quantities of harmful particles emitted by traffic.

̽��ֱ��researchers applied a new type of pollution analysis, using magnetism to study particles trapped by a hedge separating a major 6-lane road from a primary school in Manchester, UK. They found that the hedge was especially successful in removing ultrafine particle pollution, which can be more damaging to health.

“Our findings show that hedges can provide a simple, cheap and effective way to help reduce exposure to local sources of pollution,” said lead author Hassan Sheikh from Cambridge’s Department of Earth Sciences.

̽��ֱ��new study differs from conventional air pollution studies because the researchers specifically measured magnetic particles, which originate from vehicle exhaust and the wearing of brake pads and tyres. That allowed them to distinguish local traffic pollution from other sources of air pollution.

In England alone, epidemiological studies estimate that 26,000 to 38,000 deaths and thousands of NHS hospital admissions are linked to dust-like particles carried in air pollution — much of which is generated by heavy traffic in urban environments.

This particle pollution — or particulate matter — is made up of a variety of chemical compounds, metals and other materials, some of which are toxic. ̽��ֱ��bigger particles (which are still tiny) measure less than 10 microns in diameter (called PM10) and are easily inhaled. Finer particles of less than 2.5 microns across (PM2.5) can penetrate deeper into the lungs and are small enough to enter the bloodstream.

Children attending schools next to busy roads are especially vulnerable to the effects of air pollution because their airways are still developing and they breath faster than adults.

Sheikh and the team studied magnetic particles captured by a western red cedar ‘tredge’ (trees managed at head-height), which was previously installed outside St Ambrose Primary School as part of a trial led by Lancaster ̽��ֱ��.

“Western red cedar does a great job in 'capturing ' particulate pollution because it has abundant, fine, evergreen leaves into which airborne particles bump and then settle from the roadside air,” said study co-author Professor Barbara Maher from the Lancaster ̽��ֱ��, who led the previous research.

Sheikh and the team measured particles of varying sizes on the leaves of the tredge and used air filters to measure particle abundance at intervals downwind toward the school playground.

They also developed a new experiment that used a tracer gas to understand how ultrafine particles (measuring less that 2.5 microns) moved through and were trapped by the tredge.

Their results revealed that there was a substantial reduction in particle pollution downwind of the tredge. “ ̽��ֱ��tredge acts as a permeable barrier, intercepting and capturing particles effectively on its leaves,” said Sheikh.

In the school playground, 30 metres from the road, they measured a 78% decrease in PM10 relative to roadside air.

They noticed that this removal was even more efficient for ultrafine PM2.5 particles. “What was remarkable was just how efficiently the tredge hoovered up the very finest particles,” said senior author Professor Richard Harrison, also from Cambridge’s Department of Earth Sciences. They measured an 80% reduction in the ultrafine particles just behind the tredge.

They think the ultrafine particles are preferentially filtered out by the tredge because they have a higher likelihood of being captured on the ridged surfaces of the red cedar leaves than coarser particles.

However, they did note a slight uptick in levels of magnetic PM2.5 in the playground, although they were still 63% below roadside air. “ ̽��ֱ��ultrafine particles were very effectively removed, but this shows that some air still goes over or around the tredge,” said Sheikh. Less is currently known about how particulate matter moves and disperses at this higher level, where air mixes around buildings and trees.

“That means the design and placement of tredges near playgrounds and schools should be carefully considered so that their ability to soak up particles can be used to maximum effect,” said Harrison.

Cllr Tracey Rawlins, Executive Member for Environment for Manchester City Council, said: "We were keen to be part of this study as Manchester seeks to embrace innovation in our efforts to become a greener city with cleaner air and tackle climate change.

" ̽��ֱ��findings underline the contribution which nature-based innovations can make to rising to that challenge. We look forward to delivering more green screens as well as many trees at school sites, complementing our education climate change strategy," said Rawlins.

Previously, Sheikh and Harrison used their new magnetic analysis to identify high levels of ultrafine particles polluting the London Underground. They now plan on working with colleagues at the MRC Toxicology Unit in Cambridge to find out what happens when cells are exposed to this type of ultrafine particle pollution.

Reference

Sheikh, H A, Maher, B A, Woods, A W, Tung, P Y, and Harrison, R J (2023). Efficacy of green infrastructure in reducing exposure to local, traffic-related sources of airborne particulate matter (PM). Science of the Total Environment, 166598.

A new study led by Cambridge ̽��ֱ�� confirms that planting hedges between roadsides and school playgrounds can dramatically reduce children’s exposure to traffic-related particle pollution.

Our findings show that hedges can provide a simple, cheap and effective way to help reduce exposure to local sources of pollution

Hassan Sheikh

H A Sheikh

Monitoring particle air pollution either side of the tredge installed at St Ambrose primary school, Manchester

̽��ֱ��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 – 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.

Yes

Phone-based measurements provide fast, accurate information about the health of forests

sc604 — Tue, 07 Mar 2023 01:21:40 +0000

̽��ֱ��researchers, from the ̽��ֱ�� of Cambridge, developed the algorithm, which gives an accurate measurement of tree diameter, an important measurement used by scientists to monitor forest health and levels of carbon sequestration.

̽��ֱ��algorithm uses low-cost, low-resolution LiDAR sensors that are incorporated into many mobile phones, and provides results that are just as accurate, but much faster, than manual measurement techniques. ̽��ֱ��results are reported in the journal Remote Sensing.

̽��ֱ��primary manual measurement used in forest ecology is tree diameter at chest height. These measurements are used to make determinations about the health of trees and the wider forest ecosystem, as well as how much carbon is being sequestered.

While this method is reliable, since the measurements are taken from the ground, tree by tree, the method is time-consuming. In addition, human error can lead to variations in measurements.

“When you’re trying to figure out how much carbon a forest is sequestering, these ground-based measurements are hugely valuable, but also time-consuming,” said first author Amelia Holcomb from Cambridge’s Department of Computer Science and Technology. “We wanted to know whether we could automate this process.”

Some aspects of forest measurement can be carried out using expensive special-purpose LiDAR sensors, but Holcomb and her colleagues wanted to determine whether these measurements could be taken using cheaper, lower-resolution sensors, of the type that are used in some mobile phones for augmented reality applications.

Other researchers have carried out some forest measurement studies using this type of sensor, however, this has been focused on highly-managed forests where trees are straight, evenly spaced and undergrowth is regularly cleared. Holcomb and her colleagues wanted to test whether these sensors could return accurate results for non-managed forests quickly, automatically, and in a single image.

“We wanted to develop an algorithm that could be used in more natural forests, and that could deal with things like low-hanging branches, or trees with natural irregularities,” said Holcomb.

̽��ֱ��researchers designed an algorithm that uses a smartphone LiDAR sensor to estimate trunk diameter automatically from a single image in realistic field conditions. ̽��ֱ��algorithm was incorporated into a custom-built app for an Android smartphone and is able to return results in near real time.

To develop the algorithm, the researchers first collected their own dataset by measuring trees manually and taking pictures. Using image processing and computer vision techniques, they were able to train the algorithm to differentiate trunks from large branches, determine which direction trees were leaning in, and other information that could help it refine the information about forests.

̽��ֱ��researchers tested the app in three different forests – one each in the UK, US and Canada – in spring, summer and autumn. ̽��ֱ��app was able to detect 100% of tree trunks and had a mean error rate of 8%, which is comparable to the error rate when measuring by hand. However, the app sped up the process significantly and was about four and a half times faster than measuring trees manually.

“I was surprised the app works as well as it does,” said Holcomb. “Sometimes I like to challenge it with a particularly crowded bit of forest, or a particularly oddly-shaped tree, and I think there’s no way it will get it right, but it does.”

Since their measurement tool requires no specialised training and uses sensors that are already incorporated into an increasing number of phones, the researchers say that it could be an accurate, low-cost tool for forest measurement, even in complex forest conditions.

̽��ֱ��researchers plan to make their app publicly available for Android phones later this spring.

̽��ֱ��research was supported in part by the David Cheriton Graduate Scholarship, the Canadian National Research Council, and the Harding Distinguished Postgraduate Scholarship.

Reference:
Amelia Holcomb, Linzhe Tong, and Srinivasan Keshav. ‘Robust Single-Image Tree Diameter Estimation with Mobile Phones.’ Remote Sensing (2023). DOI: 10.3390/rs15030772

Researchers have developed an algorithm that uses computer vision techniques to accurately measure trees almost five times faster than traditional, manual methods.

Ground-based measurements are hugely valuable, but also time-consuming. We wanted to know whether we could automate this process.

Amelia Holcomb

Baac3nes via Getty Images

Treetops seen from a low angle

̽��ֱ��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.

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