“It’s all about figuring out how much forests can cool down our environment and the extent of the effect,” said Mallory Barnes, lead author of the study and an environmental scientist at Indiana University. “This knowledge is key not only for large-scale reforestation projections aimed at climate mitigation, but also for initiatives like urban tree planting.”

The study was published in the AGU journal Earth’s Future, which publishes interdisciplinary research on the past, present and future of our planet and its inhabitants.

Return of the trees

Before European colonization, the eastern United States was almost entirely covered in temperate forests. From the late 18th to early 20th centuries, timber harvests and clearing for agriculture led to forest losses exceeding 90% in some areas. In the 1930s, efforts to revive the forests, coupled with the abandonment and subsequent reforestation of subpar agricultural fields, kicked off an almost century-long comeback for eastern forests. About 15 million hectares of forest have since grown in these areas.

“The extent of the deforestation that happened in the eastern United States is remarkable, and the consequences were grave,” said Kim Novick, an environmental scientist at Indiana University and co-author of the new study. “It was a dramatic land cover change, and not that long ago.”

During the period of regrowth, global warming was well underway, with temperatures across North America rising 0.7 degrees Celsius (1.23 degrees Fahrenheit) on average. In contrast, from 1900 to 2000, the East Coast and Southeast cooled by about 0.3 degrees Celsius (0.5 degrees Fahrenheit), with the strongest cooling in the southeast.

Previous studies suggested the cooling could be caused by aerosols, agricultural activity or increased precipitation, but many of these factors would only explain highly localized cooling. Despite known relationships between forests and cooling, studies had not considered forests as a possible explanation for the anomalous, widespread cooling.

“This widespread history of reforestation, a huge shift in land cover, hasn’t been widely studied for how it could’ve contributed to the anomalous lack of warming in the eastern U.S., which climate scientists call a ‘warming hole,'” Barnes said. “That’s why we initially set out to do this work.”

Trees are cool

Barnes, Novick and their team used a combination of data from satellites and 58 meteorological towers to compare forests to nearby grasslands and croplands, allowing an examination of how changes in forest cover can influence ground surface temperatures and in the few meters of air right above the surface.

The researchers found that forests in the eastern U.S. today cool the land’s surface by 1 to 2 degrees Celsius (1.8 to 3.6 degrees Fahrenheit) annually. The strongest cooling effect occurs at midday in the summer, when trees lower temperatures by 2 to 5 degrees Celsius (3.6 to 9 degrees Fahrenheit) — providing relief when it’s needed most.

Using data from a network of gas-measuring towers, the team showed that this cooling effect also extends to the air, with forests lowering the near-surface air temperature by up to 1 degree Celsius (1.8 degrees Fahrenheit) during midday. (Previous work on trees’ cooling effect has focused on land, not air, temperatures.)

The team then used historic land cover and daily weather data from 398 weather stations to track the relationship between forest cover and land and near-surface air temperatures from 1900 to 2010. They found that by the end of the 20^th century, weather stations surrounded by forests were up to 1 degree Celsius (1.8 degrees Fahrenheit) cooler than locations that did not undergo reforestation. Spots up to 300 meters (984 feet) away were also cooled, suggesting the cooling effect of reforestation could have extended even to unforested parts of the landscape.

Other factors, such as changes in agricultural irrigation, may have also had a cooling effect on the study region. The reforestation of the eastern United States in the 20^th century likely contributed to, but cannot fully explain, the cooling anomaly, the authors said.

“It’s exciting to be able to contribute additional information to the long-standing and perplexing question of, ‘Why hasn’t the eastern United States warmed at a rate commensurate with the rest of the world?'” Barnes said. “We can’t explain all of the cooling, but we propose that reforestation is an important part of the story.”

Reforestation in the eastern United States is generally regarded as a viable strategy for climate mitigation due to the capacity of these forests to sequester and store carbon. The authors note that their work suggests that eastern United States reforestation also represents an important tool for climate adaptation. However, in different environments, such as snow-covered boreal regions, adding trees could have a warming effect. In some locations, reforestation can also affect precipitation, cloud cover, and other regional scale processes in ways that may or may not be beneficial. Land managers must therefore consider other environmental factors when evaluating the utility of forests as a climate adaptation tool.

Change is hard. Change takes courage. And change starts with turning to face the challenge. Author James Baldwin is reported to have once said “Not everything that is faced can be changed, but nothing can be changed until it is faced.” What is a change you want to make? Face it and start that change today, the future will thank you.

Post and share Earth911 posters to help people think of the planet first, every day. Click the poster to get a larger image.

The researchers found that nitrate radicals (NO3) in the air degrade the scent chemicals released by a common wildflower, drastically reducing the scent-based cues that nighttime pollinators rely on to locate the flower. In the atmosphere, NO3 is produced by chemical reactions among other nitrogen oxides, which are themselves released by the combustion of gas and coal from cars, power plants and other sources. The findings, published Feb. 9 in the journal Science, are the first to show how nighttime pollution creates a chain of chemical reactions that degrades scent cues, leaving flowers undetectable by smell. The researchers also determined that pollution likely has worldwide impacts on pollination.

The team — co-led by Jeff Riffell, a UW professor of biology, and Joel Thornton, a UW professor of atmospheric sciences — studied the pale evening primrose (Oenothera pallida). This wildflower grows in arid environments across the western U.S. They chose this species because its white flowers emit a scent that attracts a diverse group of pollinators, including nocturnal moths, which are one of its most important pollinators.

At field sites in eastern Washington, the researchers collected scent samples from pale evening primrose flowers. Back in the laboratory, they used chemical analysis techniques to identify the dozens of individual chemicals that make up the wildflower’s scent.

“When you smell a rose, you’re smelling a diverse bouquet composed of different types of chemicals,” said Riffell. “The same is true for almost any flower. Each has its own scent made up of a specific chemical recipe.”

Once they had identified the individual chemicals that make up the wildflower’s scent, the team used a more advanced technique called mass spectrometry to observe how each chemical within the scent reacted to NO3. They found that reacting with NO3 nearly eliminated certain scent chemicals. In particular, the pollutant decimated levels of monoterpene scent compounds, which in separate experiments moths found most attractive.

Moths, which smell through their antennae, have a scent-detection ability that is roughly equivalent to dogs — and several thousand times more sensitive than the human sense of smell. Research suggests that several moth species can detect scents from miles away, according to Riffell.

Using a wind tunnel and computer-controlled odor-stimulus system, the team investigated how well two moth species — the white-lined sphinx (Hyles lineata) and the tobacco hawkmoth (Manduca sexta) — could locate and fly toward scents. When the researchers introduced the pale evening primrose’s normal scent, both species would readily fly toward the scent source. But when the researchers introduced the scent and NO3 at levels typical for a nighttime urban setting, Manduca‘s accuracy dropped by 50% and Hyles — one of the chief nocturnal pollinators of this flower — could not locate the source at all.

Experiments in a natural setting backed up these findings. In field experiments, the team showed that moths visited a fake flower emitting unaltered scent as often as they visited a real one. But, if they treated the scent first with NO3, moth visitation levels dropped by as much as 70%.

“The NO3 is really reducing a flower’s ‘reach’ — how far its scent can travel and attract a pollinator before it gets broken down and is undetectable,” said Riffell.

The team also compared how daytime and nighttime pollution conditions impacted the wildflower’s scent chemicals. Nighttime pollution had a much more destructive effect on the scent’s chemical makeup than daytime pollution. The researchers believe this is largely due to sunlight degrading NO3.

The team used a computer model that simulates both global weather patterns and atmospheric chemistry to locate areas most likely to have significant problems with plant-pollinator communication. The areas identified include western North America, much of Europe, the Middle East, Central and South Asia, and southern Africa.

“Outside of human activity, some regions accumulate more NO3 because of natural sources, geography and atmospheric circulation,” said Thornton, who added that natural sources of NO3 include wildfires and lightning. “But human activity is producing more NO3 everywhere. We wanted to understand how those two sources — natural and human — combine and where levels could be so high that they could interfere with the ability of pollinators to find flowers.”

The researchers hope their study is just the first of many to help uncover the full scope of pollinator failure.

“Our approach could serve as a roadmap for others to investigate how pollutants impact plant-pollinator interactions, and to really get at the underlying mechanisms,” said Thornton. “You need this kind of holistic approach, especially if you want to understand how widespread the breakdown in plant-pollinator interactions is and what the consequences will be.”

The study highlights the dangers of human-fueled pollution and its implications for all pollinators as well as the future of agriculture.

“Pollution from human activity is altering the chemical composition of critical scent cues, and altering it to such an extent that the pollinators can no longer recognize it and respond to it,” said Riffell.

Approximately three-quarters of the more than 240,000 species of flowering plants rely on pollinators, Riffell said. And more than 70 species of pollinators are endangered or threatened.

Lead author on the paper is Jeremy Chan, a postdoctoral researcher at the University of Copenhagen who conducted this study as a UW doctoral student in biology. Co-authors are Sriram Parasurama in the UW Department of Biology; Rachel Atlas, a postdoctoral researcher at the Pierre Simon Laplace Institute in France who participated in this study as a UW doctoral student in atmospheric sciences; Ursula Jongebloed, a UW doctoral students in atmospheric sciences; Ruochong Xu, a doctoral student at Tsinghua University in China; Becky Alexander, a UW professor of atmospheric sciences; and Joseph Langenhan, a professor of chemistry at Seattle University. The research was funded by the Air Force Office of Scientific Research, the National Science Foundation, the National Institutes of Health, the Human Frontiers in Science Program, and the University of Washington.

The web-based tool, and the methods undergirding its creation, appears in the journal Science Advances.

The tool stems from a study involving nearly 250 researchers that drew more than 59,000 participants from 63 countries, including Algeria, China, Denmark, Germany, Israel, Japan, New Zealand, Peru, and the United States.

“We tested the effectiveness of different messages aimed at addressing climate change and created a tool that can be deployed by both lawmakers and practitioners to generate support for climate policy or to encourage action,” says Madalina Vlasceanu, an assistant professor in New York University’s Department of Psychology and the paper’s lead author.

The tool, which the researchers describe as a “Climate Intervention Webapp,” takes into account an array of targeted audiences in the studied countries, ranging from nationality and political ideology to age, gender, education, and income level.

“To maximize their impact, policymakers and advocates can assess which messaging is most promising for their publics,” adds paper author Kimberly Doell, a senior scientist at the University of Vienna who led the project with Vlasceanu.

Previous studies have examined the effectiveness of intervention strategies aimed at boosting sustainable intentions and behaviors, such as recycling, public transportation use, and household energy saving. But these have focused on singular private mitigation actions rather than on a broad array of climate-friendly activities and support for systemic solutions. In addition, earlier work has generally centered on western, industrialized nations, raising questions about the broader applicability of these findings.

Among the messages the authors of the new paper tested presented the consequences of climate change in a “doom and gloom” style (e.g., “Climate change poses a serious threat to humanity.”). Another featured examples of successful climate actions people took in the past. An additional intervention asked participants to write a letter to a future generation member outlining what climate actions they are undertaking today to make the planet livable in 2055. Others included emphasizing the scientific consensus on the facts and framing climate action as either a patriotic or a popular choice.

To gauge the effectiveness of these interventions, the paper’s authors tested participants’ support for several climate-related views, policies, and actions (e.g., “Climate change poses a serious threat to humanity,” “I support raising carbon taxes on gas/fossil fuels/coal,” participation in a tree-planting initiative). Finally, the paper’s authors gauged the desire of participants to share climate-mitigation information on social media: “Did you know that removing meat and dairy for only two out of three meals per day could decrease food-related carbon emissions by 60%?” The data were collected between July 2022 and May 2023.

Overall, while responses varied significantly among geographic location and participants’ demographics and beliefs, 86% recognized the dangers posed by climate change and more than 70% backed systemic/collective action addressing climate change.

“These responses reveal a global consensus regarding the dangers posed by climate change and the importance of enacting climate mitigation at the systemic level,” observes Jay Van Bavel, a professor of psychology at NYU and one of the paper’s authors. “It’s important that people realize that there is an overwhelming global consensus on this issue.”

However, there were notable differences among countries in response to the same messages or interventions. For example:

• Emphasizing scientific consensus on climate change (i.e., “Ninety-nine percent of expert climate change scientists agree that the Earth is warming, and climate change is happening, mainly because of human activity.”) increased support for climate-friendly policies by 9% in Romania, but decreased such support by 5% in Canada.
• Asking participants to write a letter to a socially close child, as a member of the future generation, had the following effects: * The intervention increased climate policy support in the following countries: the United States (10%), Brazil (10%), Ghana (8%), Russia (7%), and Nigeria (5%). * The intervention decreased policy support slightly in the UAE and Serbia (3%) as well as in India (2%).

Among participants who used social media, willingness to share climate change information on these platforms generally increased in response to all interventions tested. Notably, the largest gains occurred after participants read facts about the negative impacts of climate change — a “gloom and doom” style of messaging. After hearing these messages, participants were 12% more likely to share pro-environmental messages on social media.

Conversely, no intervention increased support for the tested action: a tree-planting initiative. In fact, some interventions decreased the likelihood of expressing willingness to undertake this individual-level action.

Taken together, the findings shed new light on the effectiveness of climate messaging. Some activists have argued in favor of a “doom-and-gloom” messaging style as a way to encourage action. Others, however, have said that such messaging may have no impact on behavior, or worse, that it may depress and demoralize the public into inaction.

The new study offers support for both approaches — depending on the objective. While “doom and gloom” messaging was effective at stimulating sharing on social media, which the researchers acknowledge is a low-effort activity, it decreased support for tree-planting — a more labor-intensive task. Moreover, this messaging decreased policy support among study participants who were climate-change skeptics.

“Our results illuminate the impact of messaging aimed at achieving specific objectives,” concludes Vlasceanu. “At the same time, these findings make clear that effective outreach depends on peoples’ pre-existing belief in climate change, showing that policymakers and advocates need to tailor their outreach to the characteristics of their audience.”