Hexbyte Glen Cove Experiment evaluates the effect of human decisions on climate reconstructions thumbnail

Hexbyte Glen Cove Experiment evaluates the effect of human decisions on climate reconstructions

Hexbyte Glen Cove

Credit: Unsplash/CC0 Public Domain

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

The experiment, designed and run by researchers from the University 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 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 University 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 has to make when building a reconstruction or model.”

To improve the reliability of reconstructions, the researchers suggest that teams make multiple reconstructions at once so that they can be seen as an ensemble. The 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. The challenge with a climate reconstruction is that—absent a —there is no way to confirm it is correct.

“While the information contained in 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 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.

The 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 3rd century, as well as between the 10th and 12th century; they all showed abrupt summer cooling following clusters of large volcanic eruptions in the 6th, 15th and 19th century; and they all showed that the recent warming since the 20th and 21st 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 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 , 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.”

More information:
The influence of decision-making in tree ring-based climate reconstructions, Nature Communications (2021). DOI: 10.1038/s41467-021-23627-6

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Hexbyte Glen Cove The DALI experiment: Searching for the axion, a proposed component of dark matter thumbnail

Hexbyte Glen Cove The DALI experiment: Searching for the axion, a proposed component of dark matter

Hexbyte Glen Cove

The boxes show how filaments and superclusters of galaxies grow over time, from billions of years after the Big Bang to current structures. Credit: Modification of work by CXC/MPE/V. Springel

The detection of the axion would mark a key episode in the history of science. This hypothetical particle could resolve two fundamental problems of Modern Physics at the same time: the problema of Charge and Parity in the strong interaction, and the mystery of dark matter. However, in spite of the high scientific interest in finding it, the search at high radio frequency—above 6 GHz—has been almost left aside for the lack of the high sensitivity technology which could be built at reasonable cost. Until now.

The Instituto de Astrofísica de Canarias (IAC) will participate in an to develop the DALI (Dark-photons & Axion-Like particles Interferometer) experiment, an astro-particle telescope for dark matter whose scientific objective is the search for axions and paraphotons in the 6 to 60 GHz band. The prototype, proof of concept, is currently in the design and fabrication phase at the IAC. The white-paper describing the experiment has been accepted for publication in the Journal of Cosmology and Astroparticle Physics (JCAP).

Predicted by theory in the 1970’s, the axion is a hypothetical low mass particle which interacts weakly with standard particles such as nucleons and electrons, as well as with photons. These proposed interactions are studied to try to detect the axion with different types of instruments. One promising technique is to study the interaction of axions with standard photons.

“Axions ‘mix’ with photons under the action of a strong external magnetic field, such as those produced by the in particle detectors or those used for medical diagnostics by magnetic resonance, and produce a weak radio or microwave signal. This signal has been looked for in a variety of experiments since the end of the 80’s, and it is just the signal that we want to detect now with DALI, although in a new almost unexplored range of parameters, which will be accessible for the first time thanks to this experiment”, explains Javier De Miguel, an IAC researcher and the first author of the study.

The first axion detectors, made in the 80’s and 90’s, used a resonant cavity which, inside a super-magnet, amplified the weak predicted from the axion, trying to bring it into a power range detectable by scientific instruments. Unfortunately, the size of the cavity is inversely proportional to the scanning frequency and, for the axion, the cavies were too small to be made for frequencies greater than some 6 GHz.

For this reason, the new experiment brings together the most promising techniques for scanning at high frequencies, and includes it in a practical design to which is also added the capacity of astro- for axionic dark matter. In this way, DALI comprises a powerful superconducting magnet, an axion detector with a novel resonator to make the weak signal caused by the axions detectable, and an altazimuth mount to allow it to scan objects and regions in the sky looking for dark matter.

This way, DALI could help in the detection of the , a pseudo-scalar particle whose nature is similar to that of the Higgs boson, discovered in 2012 at CERN, and a promising candidate for . Dark matter is a fundamental constituent of the Universe which interacts very weakly with ordinary matter, and so is very difficult to detect directly, but whose discovery would allow us to explain the rotation curves of the spiral galaxies, and why the formation of structure in the Universe has developed in the way it has until now, among other mysteries.

More information:
Javier De Miguel, A dark matter telescope probing the 6 to 60 GHz band, Journal of Cosmology and Astroparticle Physics (2021). DOI: 10.1088/1475-7516/2021/04/075

The DALI experiment: Searching for the axion, a proposed component of dark matter (2021, April 30)
retrieved 1 May 2021
from https://phys.org/news/2021-04-dali-axion-component-dark.html

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