Reassessing the Global Dataset of Wave Climate Projections

A snapshot of significant wave height (defined as the average of the highest 33% of waves) from a simulated realization of the future, submitted to COWCLIP 2.0. Energetic waves (yellow) originating from tropical cyclones can be seen in the North Pacific. Large waves in the southern hemisphere are due to extra-tropical storms, of large spatial extent, that continuously blast the Southern Ocean. Credit: Ben Timmermans

Wind-generated ocean waves — “wind waves” — can be major disruptors of coastal communities, marine ecosystems, offshore industries, and shipping, causing considerable environmental, geophysical, and socioeconomic impacts across the globe. Large waves during past winter storms, for example, stripped volumes of sand from Monterey, California beaches, attacked vulnerable marine terraces, and ultimately caused steep cliffs near Big Sur to crash into the sea. And around the world, these kinds of extreme weather events are becoming increasingly frequent and intense.

So it is critical to understand how global and regional wave conditions may evolve under climate change. This knowledge can then be integrated into comprehensive assessments of future coastal hazards and vulnerabilities to guide climate adaptation strategies.

Waves are generated from wind stress on the ocean surface — stronger storms generate larger waves. However, factors like storm size, intensity, translation speed, and structure combine to create different wave conditions. Modeling how atmospheric wind fields can lead to different spatial patterns of surface waves is critical for forecasts on weather time scales, but predicting how climate change alters wave conditions is much more complicated. Addressing this problem requires high performance computing.

Researchers at Lawrence Berkeley National Laboratory (Berkeley Lab) are tackling this challenge by generating and analyzing ocean wave climate projections using supercomputing resources at the National Energy Research Scientific Computing Center (NERSC), a Department of Energy user facility located at Berkeley Lab. They are also helping compile the results of international wave climate studies, creating a global ensemble dataset for widespread use by stakeholders, governments, and the research community. In the past year, two community-wide papers covering this research — including work done at Berkeley Lab — were published, one in Nature Climate Change and, more recently, in Scientific Data, also a Nature publication.

Modeling Wind-Wave Climate

Scientists use numerical general circulation models (GCMs) to simulate the dynamics and thermodynamics of the Earth’s atmosphere. Increasingly, they employ coupled GCMs to simulate the atmosphere and ocean simultaneously — and even include other components such as land hydrology — allowing feedback between the various systems. These models enable scientists to investigate the properties of the Earth’s weather and climate, both in the past and possible futures.

Only the most recent coupled atmosphere-ocean climate models include in-line wind-driven wave calculations. In addition, many climate models generate data at fairly coarse resolution, which prevents the identification of more intense storms such as tropical cyclones. The research published in Climate Change and Scientific Data compared multiple off-line wind-driven wave calculations.

Among the co-authors on these papers is Ben Timmermans, a researcher at the National Oceanography Centre in the United Kingdom and a former post-doctoral fellow in Berkeley Lab’s Climate and Ecosystems Science Division. As a postdoc, Timmermans worked with Michael Wehner, a senior scientist in Berkeley Lab’s Computational Research Division, to develop a high-resolution climate projection of average wave conditions across the globe. This work relied on simulations that Wehner had previously generated based on atmospheric data, which were collected in three-hour increments with a spatial resolution of either 25-kilometer squared or 100-kilometer squared. Simulating a 25-kilometer dataset took 64 times more computational resources than a 100-kilometer one.

“These atmospheric model calculations would have been impossible without NERSC’s computers, scratch disk, and high performance storage system,” said Wehner. “We used several hundred million hours and about 7,000 processors on Hopper and Cori for the project.”

However, Wehner’s original atmospheric simulations did not model how the atmosphere interacts with wind waves. So Timmermans extended this work to also model and analyze global wave conditions, which represented both present and possible future wave climate. NERSC again played a critical role, supplying three million core hours that ran concurrently on 20 nodes of Edison. Using the high-resolution data and NERSC computing power, the Berkeley team was able to identify tropical storms and extreme waves that the lower-resolution data lacked.

“The abundance of resources at NERSC allowed me to push the wave model almost to its limits in terms of parallel computing capability,” Timmermans said. 

Assembling Wave Climate Projections

This Berkeley Lab project is part of a new generation of global wind-wave studies completed by several international modeling groups. These individual studies, however, use various statistical approaches, dynamical wind-wave models, and data structures, making comparisons between the analyses difficult. In addition, single studies alone cannot be used to quantify total uncertainty given the range and diversity of available wind-wave modeling methods. Without a broader research effort, it remains unclear why the standalone studies sometimes differ in their projected changes in wind-wave characteristics across the world’s oceans.

The Coordinated Ocean Wave Climate Project (COWCLIP) is trying to overcome this problem by creating a consistent multivariate dataset of global wave climate projections for widespread use. Berkeley Lab is one of ten contributing institutions to COWCLIP phase 2, as described in the Scientific Data paper; all ten contributing institutions validated their global wave projection datasets with respect to observations, an important part of the production process.

For example, Timmermans’ validation involved a comparison of his projections of wind speed and wave height distributions against observations from fixed-position oceanic data buoys maintained by the National Oceanic and Atmospheric Administration. However, the COWCLIP2 team also conducted validation on the entire ensemble of datasets, comparing against 26 years of global satellite measurements of significant wave height on a global and regional scale.

“COWCLIP is a coordinated community effort to gather and explore output from state-of-the-art simulations of ocean wave climate, identifying and quantifying the key sources of uncertainty,” Timmermans said. “This new dataset will support future broad-scale coastal hazard and vulnerability assessments and climate adaptation studies in many offshore and coastal engineering applications.”

Berkeley Lab’s high-resolution climate model output used to drive the wave model is suitable for many other types of analyses and is freely available at https://portal.nersc.gov/c20c/.

This is a reposting of my news feature, courtesy of Lawrence Berkeley National Laboratory. 

The Threat of Terrestrial Carbon

Dr. Margaret Torn at an NGEE field test site near Council, Alaska. Lawrence Berkeley National Laboratory – Roy Kaltschmidt, photographer.
Dr. Margaret Torn at an NGEE field site near Council, Alaska. Photo: Lawrence Berkeley National Laboratory – Roy Kaltschmidt, photographer.

In the northernmost city of the United States – Barrow, Alaska – the treeless flat tundra looks stark and forbidding to many people. The permanently frozen soil (permafrost) is only capable of supporting plants like moss, heather and lichen and the temperatures can drop as low as -60 °F.

However, this tundra is a mecca for climate scientists like Dr. Margaret Torn, co-lead of the Climate and Carbon Sciences Program at Lawrence Berkeley National Laboratory (Berkeley Lab). Torn just returned from performing field experiments near Barrow. She is part of the 10-year Next-Generation Ecosystem Experiment, a large collaboration of scientists and engineers who are trying to better understand the Arctic terrestrial ecosystem so they can improve vital climate predictions. These scientists are finding new ways to study the complex ecosystem of the Arctic landscape, including looking deep into the soil.

“Soil is a big mystery,” explained Torn. “We don’t understand why soil holds so much carbon. And we don’t understand how a warming climate will affect soils. The question being whether a warming climate will result in carbon transferring from soils to the atmosphere as greenhouse gases, creating additional global warming.”

Soils are an important part of the carbon cycle. In the natural carbon cycle, carbon dioxide is taken up by plants and photosynthesized. If the plants aren’t harvested for food or fuel, they decay and their organic matter makes its way to the soil where it is processed by tiny microbes – bacteria and fungi – that release the carbon dioxide back into the atmosphere.

Soils are critical because they store about 2.3 trillion tons of carbon – more than twice as much as the atmosphere or vegetation. In comparison, burning fossil fuels releases about 9 billion tons of carbon dioxide per year.

Soils are also a long-term reservoir of carbon. Carbon cycles very slowly deep in the soil, where it can remain for 50,000 years. So a critical question is how long will soils contain these rich deposits of carbon? Will the carbon stay put? Or will it enter the atmosphere in the near future, greatly amplifying climate change?

The Arctic tundra is an area that is particularly worrisome. Cold temperatures suppress microbial growth, which helps trap the vast stores of carbon in the soil. But global warming is causing the Arctic permafrost to thaw, triggering the microbes to become active and respire carbon dioxide into the atmosphere.

Equipment Ron and her team use to sample greenhouse gases flowing from the land to the atmosphere. They later determine how old the carbon is in these gas samples using carbon-14 dating. Photo: Margaret Torn.
Equipment Torn and her team use to sample greenhouse gases flowing from the land into the atmosphere. They later determine how old the carbon is in these gas samples using carbon-14 dating. Photo: Margaret Torn.

Torn’s group drills wells in the Alaskan ground to directly measure the flow of carbon dioxide and methane from the land to the atmosphere. They measure these gas flows in areas where the permafrost is intact and where it is thawing, trying to understand the environmental variables that are controlling the release of greenhouse gases.

They see very high methane concentrations in areas where the permafrost is thawing. However, this summer they found that in some areas specialized microbes consume this methane before it is released, so carbon dioxide is released into the atmosphere instead. This is good news for the environment, because carbon dioxide is a less potent greenhouse gas than methane.

They also take soil core samples from different regions in Barrow, and then incubate them at different temperatures back home at Berkeley Lab. They find that one handful of soil has thousands of different kinds of microbes and billions of cells, which respond to the environment differently. They also determine how old the carbon is in the samples using carbon-14 dating.

“One thing we’ve seen this summer is that the carbon that is being decomposed just above the permafrost is more than 2500 years old,” described Torn. “So this place that we’re studying has been storing carbon for a long, long time. But that carbon can be decomposed and released as carbon dioxide very quickly when the conditions are right.”

These results have been validated by other recent experiments, but they contradict the old belief that carbon hidden deep in soil will remain there forever due to the soil’s material properties. “The field is evolving rapidly. We’re trying to unravel the mystery of why we see older carbon in the soil, trying to create a more realistic view,” explained Torn. “It is more complex. It’s the interaction between the entire ecosystem and the material properties that’s important.”

Of course the more complicated, realistic view makes climate modeling more challenging. Climate models are computer programs that simulate how the climate has changed in the past and how it will change in the future. They are critical to understanding our planet and how to limit the impact of human activity upon it. But scientists know that their climate models are wrong when it comes to soil carbon. This is why scientists need new data, like they are acquiring in Alaska, to test and improve their models.

“We can do so much better than we’re doing,” exclaimed Torn. “So we feel pretty confident that we can make improvements. It may not be perfect, but our work is going to make predictions more robust and believable.”

This is a repost of my KQED QUEST blog titled, “The Great Escape: How Soil Protects Us from Carbon Emissions.”

How Hot Will It Get?

Muir Glacer melt, Alaska. 1882 photo taken by G.D. Hazard; 2005 photo taken by Bruce F. Molnia. Courtesy of the Glacier Photograph Collection, National Snow and Ice Data Center/World Data Center for Glaciology.
Muir Glacer melt, Alaska. 1882 photo taken by G.D. Hazard; 2005 photo taken by Bruce F. Molnia. Courtesy of the Glacier Photograph Collection, National Snow and Ice Data Center/World Data Center for Glaciology.

Stay tuned for the next Science at the Theater, a free public lecture hosted by Lawrence Berkeley National Laboratory. It will be held on Monday April 22 at 7 pm at the Berkeley Repertory Theater.

Scientists will talk about their latest research findings on how the earth’s climate is changing, from the arctic to the rainforest. Participating speakers will address critical questions: What happens when the permafrost thaws? What do computer models predict about our future climate – floods, droughts, hurricanes and heat waves? What role do our forests play in carbon absorption? What kind of carbon tax might actually work?

Come find out what to expect and if there is anything you can do about it!