Hexbyte Glen Cove Review evaluates the evidence for an intensifying Indian Ocean water cycle thumbnail

Hexbyte Glen Cove Review evaluates the evidence for an intensifying Indian Ocean water cycle

Hexbyte Glen Cove

Co-author Sujata Murty retrieving a coral core piece during the underwater drilling process. Credit: Justin Ossolinski.

The Indian Ocean has been warming much more than other ocean basins over the last 50-60 years. While temperature changes basin-wide can be unequivocally attributed to human-induced climate change, it is difficult to assess whether contemporary heat and freshwater changes in the Indian Ocean since 1980 represent an anthropogenically-forced transformation of the hydrological cycle. What complicates the assessment is factoring in natural variations, regional-scale trends, a short observational record, climate model uncertainties, and the ocean basin’s complex circulation.

A new review paper takes a broad look at whether heat and freshwater changes in the Indian Ocean are consistent with the increase in rainfall that is expected in response to anthropogenic global warming or whether these changes are due to natural variability on multi-decadal and other timescales along with other factors. That distinction has “big implications for climate risk assessment and for the densely populated regions around the Indian Ocean that are vulnerable to the effects of ,” says Caroline Ummenhofer, lead author of the paper, Heat and freshwater changes in the Indian Ocean region, published in Nature Reviews Earth & Environment.

The paper brings together various scientific expertise, tools, and data sources to address key questions regarding climate change in the Indian Ocean, says Ummenhofer, associate scientist in the Physical Oceanography Department at the Woods Hole Oceanographic Institution (WHOI). “The different scientific communities need to come together and have very open discussions about what we can tell from our data, how we can compare apples and oranges, and how we can bring all of this information together to have a better understanding of the entire Indian Ocean system,” she says.

“Rather than rely on climate models that struggle to accurately represent the complex circulation, we look at many different observational records including measurements of sea level, and the and subsurface temperature and salinity,” says co-author Janet Sprintall, a research oceanographer at the Scripps Institution of Oceanography, University of California San Diego.

While some changes in the Indian Ocean appear to be a consistent response to anthropogenic global warming, “in general our ocean observational records are still far too short to distinguish the naturally driven variability from the man-made changes,” says Sprintall. “This tells us that we need to continue measuring our oceans—particularly below the surface—so that we can better understand these long-term changes and their causes, and so that we can improve our prediction and response to them.”

Recovery of the South Ombai mooring, topped with an Acoustic Doppler Current Meter (ADCP) to measures ocean currents, aboard the Indonesian Research Vessel Baruna Jaya I. Observational data in the Indian Ocean is sparse and in situ observations are key to determining heat input to the Indian Ocean. Photo credit: Janet Sprintall. Credit: Janet Sprintall

Quantifying the changes in the Indian Ocean heat and freshwater balance warrants a multi-pronged approach across temporal and spatial scales that integrates in situ observations (including Argo floats robotically programmed to measure ocean temperature, salinity, and other properties; moorings; and buoys), by satellites to measure rainfall and sea surface salinity, improved numerical modeling simulations, and paleoclimate proxy networks, the authors note.

Corals are an important paleoclimate archive in the ocean because their calcium carbonate skeletons incorporate the chemical properties of past oceans and so reflect past climate and environmental conditions. “Corals are unique environmental archives that allow us to extend our understanding of Indian Ocean variability centuries farther back in time than the observational record,” says co-author Sujata Murty, WHOI adjunct scientist and assistant professor in the Department of Atmospheric and Environmental Sciences at the University at Albany, State University of New York. “Including the long-term perspective provided by corals alongside that of observations and remote sensing data enriches our understanding of complex climate and ocean systems and improves our ability to anticipate future changes in a warming world.”

Maintaining and expanding current remote sensing, in situ observations, and a network of paleo proxies is “crucial” for “disentangling the effects of multi-decadal natural variability and anthropogenic change on heat and freshwater changes” in the Indian Ocean and the Maritime Continent region between the Indian and Pacific oceans, according to the paper.

The Indian Ocean, the paper notes, “is particularly vulnerable to anthropogenic climate change,” in part because the ocean is bounded to the north by the Asian continent. This means that heat from the Pacific Ocean that enters the Indian Ocean through the Indonesian Seas cannot easily exit the basin.

The basin “could be a kind of canary in a coal mine,” says Ummenhofer, because those changes now being observed in the Indian Ocean also could happen in other oceans. “We can all benefit from having better observations and a better understanding of the so that we can know whether the changes are a change signal or part of a natural cycle.”

More information:
Ummenhofer, C.C. et al. Heat and freshwater changes in the Indian Ocean region. Nat Rev Earth Environ (2021). DOI: 10.1038/s43017-021-00192-6

Review evaluates the evidence for an intensifying Indian Ocean water cycle (2021, July 20)
retrieved 21 July 2021
from https://phys.org/news/2021-07-evidence-indian-ocean.html

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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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