Wednesday, February 15, 2023

Before global warming, was the Earth cooling down or heating up?

NORTHERN ARIZONA UNIVERSITY

Accurate climate models play a critical role in climate science and policy, helping to inform policy- and decision-makers throughout the world as they consider ways to slow the deadly effects of a warming planet and to adapt to changes already in progress.

To test their accuracy, models are programmed to simulate past climate to see if they agree with the geologic evidence. The model simulations can conflict with the evidence. How can we know which is correct?

A review article published today in Nature addresses this conflict between models and evidence, known as the Holocene global temperature conundrum. Lead author Darrell Kaufman, a Regents’ professor in the School of Earth and Sustainability, and University of Arizona postdoctoral researcher Ellie Broadman, a co-author who worked on this study while earning her Ph.D. at NAU, analyzed a broad swath of available data from the last 12,000 years to break down the conundrum. The study builds on work Kaufman did that was included in the latest major climate report by the Intergovernmental Panel on Climate Change (IPCC) and looks at whether the global average temperature 6,500 years ago was warmer, as indicated by proxy evidence from natural archives of past climate information, or colder, as simulated by models, in comparison to the late 19th century, when the Industrial Revolution led to a significant increase in human-caused warming.

This comprehensive assessment concludes that the global average temperature about 6,500 years ago was likely warmer and was followed by a multi-millennial cooling trend that ended in the 1800s. But, they cautioned, uncertainty still exists despite recent studies that claimed to have resolved the conundrum.

“Quantifying the average temperature of the earth during the past, when some places were warming while others were cooling, is challenging, and more research is needed to firmly resolve the conundrum,” Kaufman said. “But tracing changes in global average temperature is important because it’s the same metric used to gauge the march of human-caused warming and to identify internationally negotiated targets to limit it. In particular, our review revealed how surprisingly little we know about slow-moving climate variability, including forces now set into motion by humans that will play out as sea level rises and permafrost thaws over coming millennia.”

What we know

We know more about the climate of the Holocene, which began after the last major ice age ended 12,000 years ago, than any other multi-millennial period. There are published studies from a variety of natural archives that store information about historical changes that occurred in the atmosphere, oceans, cryosphere and on land; studies that look at the forces that drove past climate changes, such as Earth’s orbit, solar irradiance, volcanic eruptions and greenhouse gases; and climate model simulations that translate those forces into changing global temperatures. All these types of studies were included in this review.

The challenge up to now has been that our two significant lines of evidence point in opposite directions. Paleo-environmental “proxy” data, which includes evidence from oceans, lakes, and other natural archives, point to a peak global average temperature about 6,500 years ago and then a global cooling trend until humans started burning fossil fuels. Climate models generally show global average temperatures increasing in the last 6,500 years.

If the proxy data are correct, that points to deficiencies in the models and specifically suggests that climate feedbacks that can amplify global warming are underrepresented. If the climate models are correct, then the tools for reconstructing paleotemperatures need to be sharpened.

We also know that, whether the numbers trend up or down, the change in global average temperature in the past 6,500 years has been gradual—probably less than 1 degree Celsius (1.8 degrees Fahrenheit). This is less than the warming already measured in the last 100 years, most of which humans have caused. However, because global temperature change of any magnitude is significant, especially in response to changing greenhouse gases, knowing whether temperatures were higher or lower 6,500 years ago is important to our knowledge of the climate system and improving forecasts of future climate.

What we don’t know

This study highlighted uncertainties in the climate models. If the authors’ preferred interpretation—that recent global warming was preceded by 6,500 years of global cooling—is correct, then scientists’ understanding of natural climate forcings and feedbacks, and how they are represented in models, needs improvement. If they’re incorrect, then scientists need to improve their understanding of the temperature signal in proxy records and further develop analytical tools to capture these trends on a global scale.

Attempting to resolve the Holocene global temperature conundrum has been a priority for climate scientists in the last decade; Broadman remembers reading the initial paper on this topic when she started her Ph.D. in 2016. All the studies since have added to the understanding of this issue, which gets scientists in the field closer to a comprehensive understanding. Recent studies on this topic have tried adjusting proxy data to account for their presumed weaknesses, inserting plausible forcings into climate models and blending proxy data with climate-model output, all arriving at different conclusions about the cause of the conundrum. This review takes a step back to revisit the issue with a comprehensive global-scale assessment, showing that we don’t yet know the solution to this conundrum.

Developing widely applicable methods of quantifying past temperature is a high priority for climate scientists already. For example, Kaufman’s lab is testing the use of chemical reactions involving amino acids preserved in lake sediment as a new method for studying past temperature changes. Combined with new radiocarbon dating technology from the Arizona Climate and Ecosystem lab at NAU, this technique could help determine whether global warming reversed a long-term cooling trend.

Why it matters

Broadman, whose work includes a focus on science communication, created the figures that accompany the research. This is a critical way of communicating hard-to-understand results to audiences—and in climate science, the audiences are diverse and include educators, policymakers, nonprofits and scientists throughout the world.

“One interesting takeaway is that our findings demonstrate the impact that regional changes can have on global average temperature. Environmental changes in some regions of the Earth, like declining Arctic sea ice or changing vegetation cover in what are now vast deserts, can cause feedbacks that influence the planet as a whole,” Broadman said. “With current global warming, we already see some regions changing very quickly. Our work highlights that some of those regional changes and feedbacks are really important to understand and capture in climate models.”

Additionally, Kaufman said, accurately reconstructing the details of past temperature change offers insights into climate’s response to various causes of both natural and anthropogenic climate change. The responses serve as benchmarks to test how well climate models simulate the Earth’s climate system.

“Climate models are the only source of detailed quantitative climate predictions, so their fidelity is critical for planning the most effective strategies to mitigate and adapt to climate change,” he said. “Our review suggests that climate models are underestimating important climate feedbacks that can amplify global warming.”

JOURNAL

Nature

DOI

10.1038/s41586-022-05536-w

METHOD OF RESEARCH

Systematic review

SUBJECT OF RESEARCH

Not applicable

ARTICLE TITLE

Revisiting the Holocene global temperature conundrum

ARTICLE PUBLICATION DATE

15-Feb-2023

New compound that withstands extreme heat and electricity could lead to next-generation energy storage devices

Flexible polymers made with a new generation of the Nobel-winning “click chemistry” reaction find use in capacitors and other applications

Peer-Reviewed Publication

DOE/LAWRENCE BERKELEY NATIONAL LABORATORY

IMAGE: A NEW TYPE OF POLYSULFATE COMPOUND CAN BE USED TO MAKE POLYMER FILM CAPACITORS THAT STORE AND DISCHARGE HIGH DENSITY OF ELECTRICAL ENERGY WHILE TOLERATING HEAT AND ELECTRIC FIELDS BEYOND THE LIMITS OF EXISTING POLYMER FILM CAPACITORS. view more
CREDIT: YI LIU AND HE (HENRY) LI/BERKELEY LAB

– By Rachel Berkowitz

Society’s growing demand for high-voltage electrical technologies – including pulsed power systems, cars and electrified aircraft, and renewable energy applications – requires a new generation of capacitors that store and deliver large amounts of energy under intense thermal and electrical conditions. Researchers at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and Scripps Research have now developed a new polymer-based device that efficiently handles record amounts of energy while withstanding extreme temperatures and electric fields. The device is composed of materials synthesized via a next-generation version of the chemical reaction for which three scientists won the 2022 Nobel Prize in Chemistry.

Polymer film capacitors are electrical components that store and release energy within an electric field using a thin plastic layer as the insulating layer. They make up 50% of the global high voltage capacitor market and offer advantages including light weight, low cost, mechanical flexibility, and robust cyclability. But state-of-the-art polymer film capacitors decrease dramatically in performance with increasing temperature and voltages. Developing new materials with improved tolerance for heat and electric fields is paramount; and creating polymers with near-perfect chemistry offers a way to do so.

“Our work adds a new class of electrically robust polymers to the table. It opens many possibilities to the exploration of more robust, high performing materials,” said Yi Liu, a chemist at Berkeley Lab and senior author on the Joule study reporting the work. Liu is the Facility Director of Organic and Macromolecular Synthesis at the Molecular Foundry, a DOE Office of Science user facility at Berkeley Lab.

In addition to remaining stable when subjected to high temperatures, a capacitor needs to be a strong “dielectric” material, meaning that it remains a strong insulator when subjected to high voltages. However, few known materials systems exist that deliver both thermal stability and dielectric strength. This scarcity is due to a lack of reliable and convenient synthesis methods, as well as a lack of fundamental understanding of the relationship between polymer structure and properties. “Improving the thermal stability of existing films while retaining their electrical insulating strength is an ongoing materials challenge,” said Liu.

A long-term collaboration between researchers at the Molecular Foundry and Scripps Research Institute has now met that challenge. They used a simple and quick chemical reaction developed in 2014 that swaps out fluorine atoms in compounds that contain sulfur-fluoride bonds, to yield long polymer chains of sulfate molecules called polysulfates. This Sulfur-Fluoride Exchange (SuFEx) reaction is a next-generation version of the click chemistry reaction pioneered by K. Barry Sharpless, a chemist at Scripps Research and two-time Nobel laureate in Chemistry, along with Peng Wu, also a chemist at Scripps Research. The near-perfect yet easy-to-run reactions join separate molecular entities through strong chemical bonds that form between different reactive groups. Liu’s team had originally used a variety of thermal analysis tools to examine the basic thermal and mechanical properties of these new materials.

As part of a Berkeley Lab program to synthesize and identify novel materials that could be useful in energy storage, Liu and his colleagues now find that, surprisingly, the polysulfates have outstanding dielectric properties, especially at high electric fields and temperatures. “Several commercial and lab-generated polymers are known for their dielectric properties, but polysulfates had never been considered. The marriage between polysulfates and dielectrics is one of the novelties here,” said He Li, a postdoctoral researcher in the Molecular Foundry and in Berkeley Lab’s Materials Sciences Division, and lead author of the study.

Inspired by the excellent baseline dielectric properties offered by polysulfates, the researchers deposited extremely thin layers of aluminum oxide (Al2O3) onto thin films of the material to engineer capacitor devices with enhanced energy storage performance. They discovered that the fabricated capacitors exhibited excellent mechanical flexibility, withstood electric fields of more than 750 million volts per meter, and performed efficiently at temperatures up to 150 degrees Celsius. In comparison, today’s benchmark commercial polymer capacitors only function reliably at temperatures lower than 120 degrees Celsius. Above that temperature, they can only withstand electric fields smaller than 500 million volts per meter, and the energy efficiency severely drops by over half.

The work opens new possibilities for exploring robust, high performing materials for energy storage. “We have provided deep insight into the underlying mechanisms that contribute to the material’s excellent performance,” said Wu.

The polymer strikes a balance of electrical, thermal, and mechanical properties, likely due to the sulfate linkages introduced by the click chemistry reaction. Because modular chemistry accommodates extraordinary structural diversity and scalability, the same route could offer a viable path to new polymers with higher performance that meet even more demanding operational conditions.

The polysulfates are strong contenders to become new state-of-the-art polymer dielectrics. Once researchers overcome barriers in large-scale manufacturing processes for thin film materials, the devices could greatly improve the energy efficiency of integrated power systems in electric vehicles and enhance their operational reliability.

“Who could have imagined that a wispy sulfate polymer film could fend off lightning and fire, two of the most destructive forces in the universe?!” said Sharpless.

“We’re continuously pushing the envelope of thermal and electrical properties, and accelerating the lab-to-market transition,” Liu added.

The technology is now available for licensing by contacting ipo@lbl.gov.

The work received funding from the Department of Energy’s Office of Science, the National Science Foundation, and the National Institute of Health. The work was carried out at the Molecular Foundry.

Polysulfates with excellent thermal properties are casted into flexible free-standing films. High-temperature, high-voltage capacitors based on such films show state-of-the-art energy storage properties at 150 oC. Such power capacitors are promising for improving the energy efficiency and reliability of integrated power systems in demanding applications such as electrified transportation.

CREDIT

Credit: Yi Liu and He (Henry) Li/Berkeley Lab

Founded in 1931 on the belief that the biggest scientific challenges are best addressed by teams, Lawrence Berkeley National Laboratory and its scientists have been recognized with 16 Nobel Prizes. Today, Berkeley Lab researchers develop sustainable energy and environmental solutions, create useful new materials, advance the frontiers of computing, and probe the mysteries of life, matter, and the universe. Scientists from around the world rely on the Lab’s facilities for their own discovery science. Berkeley Lab is a multiprogram national laboratory, managed by the University of California for the U.S. Department of Energy’s Office of Science.

DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science.