As oceans waves rise and fall, they apply forces to the sea floor below and generate seismic waves. These seismic waves are so powerful and widespread that they show up as a steady thrum on seismographs, the same instruments used to monitor and study earthquakes.

That wave signal has been getting more intense in recent decades, reflecting increasingly stormy seas and higher ocean swell.

In a new study in the journal Nature Communications, colleagues and I tracked that increase around the world over the past four decades. These global data, along with other ocean, satellite and regional seismic studies, show a decadeslong increase in wave energy that coincides with increasing storminess attributed to rising global temperatures.

What seismology has to do with ocean waves

Global seismographic networks are best known for monitoring and studying earthquakes and for allowing scientists to create images of the planet’s deep interior.

These highly sensitive instruments continuously record an enormous variety of natural and human-caused seismic phenomena, including volcanic eruptions, nuclear and other explosions, meteor strikes, landslides and glacier-quakes. They also capture persistent seismic signals from wind, water and human activity. For example, seismographic networks observed the global quieting in human-caused seismic noise as lockdown measures were instituted around the world during the coronavirus pandemic.

However, the most globally pervasive of seismic background signals is the incessant thrum created by storm-driven ocean waves referred to as the global microseism.

Two types of seismic signals

Ocean waves generate microseismic signals in two different ways.

The most energetic of the two, known as the secondary microseism, throbs at a period between about eight and 14 seconds. As sets of waves travel across the oceans in various directions, they interfere with one another, creating pressure variation on the sea floor. However, interfering waves aren’t always present, so in this sense, it is an imperfect proxy for overall ocean wave activity.

A second way in which ocean waves generate global seismic signals is called the primary microseism process. These signals are caused by traveling ocean waves directly pushing and pulling on the seafloor. Since water motions within waves fall off rapidly with depth, this occurs in regions where water depths are less than about 1,000 feet (about 300 meters). The primary microseism signal is visible in seismic data as a steady hum with a period between 14 and 20 seconds.

What the shaking planet tells us

In our study, we estimated and analyzed historical primary microseism intensity back to the late 1980s at 52 seismograph sites around the world with long histories of continuous recording.

We found that 41 (79%) of these stations showed highly significant and progressive increases in energy over the decades.

The results indicate that globally averaged ocean wave energy since the late 20th century has increased at a median rate of 0.27% per year. However, since 2000, that globally averaged increase in the rate has risen by 0.35% per year.

We found the greatest overall microseism energy in the very stormy Southern Ocean regions near the Antarctica peninsula. But these results show that North Atlantic waves have intensified the fastest in recent decades compared to historical levels. That is consistent with recent research suggesting North Atlantic storm intensity and coastal hazards are increasing. Storm Ciarán, which hit Europe with powerful waves and hurricane-force winds in November 2023, was one record-breaking example.

The decadeslong microseism record also shows the seasonal swing of strong winter storms between the Northern and Southern hemispheres. It captures the wave-dampening effects of growing and shrinking Antarctic sea ice, as well as the multi-year highs and lows associated with El Niño and La Niña cycles and their long-range effects on ocean waves and storms.

Together, these and other recent seismic studies complement the results from climate and ocean research showing that storms, and waves, are intensifying as the climate warms.

A coastal warning

The oceans have absorbed about 90% of the excess heat connected to rising greenhouse gas emissions from human activities in recent decades. That excess energy can translate into more damaging waves and more powerful storms.

Our results offer another warning for coastal communities, where increasing ocean wave heights can pound coastlines, damaging infrastructure and eroding the land. The impacts of increasing wave energy are further compounded by ongoing sea level rise fueled by climate change and by subsidence. And they emphasize the importance of mitigating climate change and building resilience into coastal infrastructure and environmental protection strategies.

Richard Aster receives funding from the U.S. National Science Foundation.

In 1976, beloved chef, cookbook author and television personality Julia Child returned to WGBH-TV’s studios in Boston for a new cooking show, “Julia Child & Company,” following her hit series “The French Chef.” Viewers probably didn’t know that Child’s new and improved kitchen studio, outfitted with gas stoves, was paid for by the American Gas Association.

While this may seem like any corporate sponsorship, we now know it was a part of a calculated campaign by gas industry executives to increase use of gas stoves across the United States. And stoves weren’t the only objective. The gas industry wanted to grow its residential market, and homes that used gas for cooking were likely also to use it for heat and hot water.

The industry’s efforts went well beyond careful product placement, according to new research from the nonprofit Climate Investigations Center, which analyzes corporate efforts to undermine climate science and slow the ongoing transition away from fossil fuels. As the center’s study and a National Public Radio investigation show, when evidence emerged in the early 1970s about the health effects of indoor nitrogen dioxide exposure from gas stove use, the American Gas Association launched a campaign designed to manufacture doubt about the existing science.

As a researcher who has studied air pollution for many years – including gas stoves’ contribution to indoor air pollution and health effects – I am not naïve about the strategies that some industries use to avoid or delay regulations. But I was surprised to learn that the multipronged strategy related to gas stoves directly mirrored tactics that the tobacco industry used to undermine and distort scientific evidence of health risks associated with smoking starting in the 1950s.

Manufacturing controversy

The gas industry relied on Hill & Knowlton, the same public relations company that masterminded the tobacco industry’s playbook for responding to research linking smoking to lung cancer. Hill & Knowlton’s tactics included sponsoring research that would counter findings about gas stoves published in the scientific literature, emphasizing uncertainty in these findings to construct artificial controversy and engaging in aggressive public relations efforts.

For example, the gas industry obtained and reanalyzed the data from an EPA study on Long Island that showed more respiratory problems in homes with gas stoves. Their reanalysis concluded that there were no significant differences in respiratory outcomes.

The industry also funded its own health studies in the early 1970s, which confirmed large differences in nitrogen dioxide exposures but did not show significant differences in respiratory outcomes. These findings were documented in publications where industry funding was not disclosed. These conclusions were amplified in numerous meetings and conferences and ultimately influenced major governmental reports summarizing the state of the literature.

This campaign was remarkable, since the basics of how gas stoves affected indoor air pollution and respiratory health were straightforward and well established at the time. Burning fuel, including natural gas, generates nitrogen oxides: The air in Earth’s atmosphere is about 78% nitrogen and 21% oxygen, and these gases react at high temperatures.

Nitrogen dioxide is known to adversely affect respiratory health. Inhaling it causes respiratory irritation and can worsen diseases such as asthma. This is a key reason why the U.S. Environmental Protection Agency established an outdoor air quality standard for nitrogen dioxide in 1971.

No such standards exist for indoor air, but as the EPA now acknowledges, nitrogen dioxide exposure indoors also is harmful.

How harmful is indoor exposure?

The key question is whether nitrogen dioxide exposure related to gas stoves is large enough to lead to health concerns. While levels vary across homes, scientific research shows that the simple answer is yes – especially in smaller homes and when ventilation is inadequate.

This has been known for a long time. For example, a 1998 study that I co-authored showed that the presence of gas stoves was the strongest predictor of personal exposure to nitrogen dioxide. And work dating back to the 1970s showed that indoor nitrogen dioxide levels in the presence of gas stoves could be far higher than outdoor levels. Depending on ventilation levels, concentrations could reach levels known to contribute to health risks.

Despite this evidence, the gas industry’s campaign was largely successful. Industry-funded studies successfully muddied the waters, as I have seen over the course of my research career, and stalled further federal investigations or regulations addressing gas stove safety.

This issue took on new life at the end of 2022, when researchers published a new study estimating that 12.7% of U.S. cases of childhood asthma – about one case in eight – were attributable to gas stoves. The industry continues to cast doubt on gas stoves’ contribution to health effects and fund pro-gas stove media campaigns.

A concern for climate and health

Residential gas use is also controversial today because it slows the ongoing shift toward renewable energy, at a time when the impacts of climate change are becoming alarmingly clear. Some cities have already moved or are considering steps to ban gas stoves in new construction and shift toward electrifying buildings.

As communities wrestle with these questions, regulators, politicians and consumers need accurate information about the risks of gas stoves and other products in homes. There is room for vigorous debate that considers a range of evidence, but I believe that everyone has a right to know where that evidence comes from.

The commercial interests of many industries, including alcohol, tobacco and fossil fuels, aren’t always compatible with the public interest or human health. In my view, exposing the tactics that vested interests use to manipulate the public can make consumers and regulators savvier and help deter other industries from using their playbook.

Jonathan Levy has received funding from the National Institutes of Health, the U.S. Environmental Protection Agency, the U.S. Department of Housing and Urban Development, and the Health Effects Institute for studies on the contribution of outdoor and indoor sources to air pollution levels in homes.

Earth’s boreal forests circle our planet’s far northern reaches, just south of the Arctic’s treeless tundra. If the planet wears an Arctic ice cap, then the boreal forests are a loose-knit headband wrapped around its ears, covering large portions of Alaska, Canada, Scandinavia and Siberia.

The boreal region’s soils have long buffered the planet against warming by storing huge quantities of carbon and keeping it out of the atmosphere. Its remoteness has historically protected its forests and wetlands from extensive human impact.

These two traits rank boreal forests among the most important ecosystems on Earth. In addition, numerous species of mammals, fish, plants, insects and birds make these forests home.

For over two centuries, scientists have recognized that climate plays a key role in determining the geographic zones of plant communities. Because boreal forests and soils face subzero winters and short summers, these forests and the animals that live in them are shifting northward as temperatures rise.

However, boreal forests’ northward advance has been spotty and slower than expected. Meanwhile, their southern retreat has been faster than scientists predicted. As scholars who study northern ecosystems, forests and wetlands, we see concerning evidence that as the world warms, its largest forest wilderness appears to be shrinking.

The largest wilderness on Earth

Boreal forests contain billions of trees. Most are needleleaf, cone-bearing conifers, but there also are patches of broadleaf species, including birch, aspen and poplar. They support millions of migratory birds and iconic mammals like brown bears, moose and lynx.

These trees and the soils around their roots help regulate Earth’s climate, in part by pulling carbon dioxide out of the atmosphere, where it would otherwise act as a greenhouse gas. The trees use this carbon to grow roots, trunks and leaves, which eventually turn into carbon-rich soil once the tree dies. Significant changes to the forests will translate to changes in global climate.

These forests are warming at rates well above the global average. Rising temperatures directly affect the growth and survival of trees and, in turn, their ability to store carbon.

Forests on the move

As atmospheric warming frees trees from the icy grip of cold temperatures, adult trees can respond by growing faster. Milder temperatures also allow young seedling trees in the most northern boreal forests to gain a foothold where previous conditions were too harsh for them to become established.

In the warmer, southern boreal forests, the situation is quite different. Here, conditions have become too warm for cold-adapted boreal trees, slowing their growth and even leading to their death. With warming comes dryness, and water stress leaves trees more susceptible to insect infestation and fires, as Canada has experienced in 2023 and Siberia in 2019 and 2020.

If this happens at a larger scale, southern boreal forest boundaries will thin and degrade, thereby retreating farther north, where temperatures are still suitable.

If boreal forests expand northward and retreat in the south at the same rates, they could slowly follow warming temperatures. However, our combined research using satellite and field data shows that the story is more complex.

Tracking forests from space

Satellites are invaluable for tracking how boreal forests have changed in recent decades and whether these changes are consistent with an overall northward shift. Researchers can use satellites to monitor year-to-year changes in forest characteristics, such as annual tree growth and tree cover.

Our recent studies using satellite data showed that tree growth and tree cover increased from 2000 to 2019 throughout much of the boreal forest. These changes occurred mainly in the coldest northern areas. However, there was limited evidence to indicate that forests were expanding past current tree lines.

Our studies also revealed that tree growth and tree cover often decreased from 2000 to 2019 in warmer southern areas of the boreal forests. In these regions, hotter and drier conditions frequently reduced tree growth or killed individual trees, while wildfires and logging contributed to tree cover loss.

Satellite data makes it clear that climate change is affecting both the northern and southern margins of the boreal forest. However, if tree cover loss in the south occurs more rapidly than gains in the north, then the boreal forest will likely contract, rather than simply shifting northward.

Zooming in to understand forest change

Forests advance when individual tree seeds germinate and grow, but boreal trees grow slowly and require decades to reach a size that’s visible from space. Finding young trees whose presence would signal tree-line movement requires data from the ground.

In the late 1970s, one of us (David Cooper) documented that young spruce trees were growing at altitudes hundreds of yards higher and locations miles north of the highest-elevation cone-bearing trees in Alaska’s Brooks Range. Returning in 2021, we found those little trees had grown to be several yards tall and were producing cones. More importantly, 10 times the number of young spruces now grow above and beyond the tree line than during our first field forays.

Crisscrossing the boundary between Alaska’s boreal forest and its Arctic tundra on foot, we have found thousands of young boreal trees growing up to 25 miles north of established tree lines. Most grow where deeper snows fall, due to an Arctic Ocean version of the “lake effect”: Cold air moves across open water, picking up warmth and moisture, which then falls as snow downwind.

Retreating sea ice leaves more open water. This generates stronger winds that propel tree seeds farther and more snowfall that insulates seedlings from harsh winter conditions. The result is that trees in Alaska’s Brooks Range are rapidly moving into the treeless tundra. However, these rapid expansions are localized and do not yet happen everywhere along the northern tree line.

The future face of boreal forests

Our combined research shows that boreal forests are, in fact, responding to rising temperatures. But rapid rates of climatic change mean that trees likely can’t move northward at a pace that keeps up with their loss in the south.

Will trees in the far north ever catch up with climate and prevent forest contraction? At this point, scientists simply don’t know. Perhaps the newly established trees in the Brooks Range herald such an expansion. It’s also unclear whether the northern parts of boreal forests can accumulate enough carbon through increased growth to compensate for carbon losses in the south.

If boreal forests are indeed on the verge of contracting, they will eventually disappear from their current southern edge. This would harm many native and migratory animals, especially birds, by reducing their boreal habitat. The forests also are culturally important to several million people who call them home, such as Canada’s aboriginal communities.

Monitoring boreal forests around the world more closely, using both satellite data and on-the-ground measurements, will help fill out this picture. Only then can researchers hope to glimpse the future of one of the Earth’s last wildernesses.

Ronny Rotbarth receives funding from the Dutch Ministry of Education, Culture and Science.

David J. Cooper receives funding from the National Science Foundation, the US National Park Service, and National Forest Service.

Logan Berner receives funding from the National Science Foundation and National Aeronautics and Space Administration.

Roman Dial receives funding from the US National Science Foundation and NASA Alaska Space Grant.