Peer-reviewed study: Without tropical forests, global temperatures would be 1°c warmer

Lausanne, Switzerland (24 March)—New research released today offers the most comprehensive and detailed evidence to date that forests are more important to the climate (globally and locally) than we think due to the way in which they physically transform the atmosphere. The first-ever research to pinpoint the local, regional and global non-carbon dioxide benefits of specific forest zones worldwide finds that the entire world gains the most benefits from the band of tropical rainforests spanning Latin America, Central Africa and Southeast Asia.  

It finds that, together, forests keep the planet at least half of a degree Celsius cooler when we account for the understudied biophysical effects—from chemical compounds to turbulence and the reflection of light. These effects in the tropics alone deliver planetary cooling of one-third of a degree Celsius; when combined with the carbon dioxide, the cooling effect is over 1 degree Celsius. 

“All forests are precious. Increasingly, we are discovering they also keep the air near and far cool and moist,” said Deborah Lawrence, a professor at the University of Virginia and the lead author of the study, The Unseen Effects of Deforestation: Biophysical Effects on Climate. “The heart of the tropics is at the heart of the planet and these forests are critical for our survival.”  

According to the study, “Locally at all latitudes, forest biophysical impacts far outweigh carbon effects, promoting local climate stability by reducing extreme temperatures in all seasons and times of day. The importance of forests for both global climate change mitigation and local adaptation by human and non-human species is not adequately captured by current carbon-centric metrics, particularly in the context of future climate warming.”    

Scientists already have a well-established understanding of how tropical deforestation contributes to global climate change through emitting carbon and reducing the ability of the world’s forests to take more carbon pollution out of the atmosphere. This is the latest and most comprehensive study in a body of emerging evidence showing how tropical deforestation has climate impacts beyond carbon: Deforestation immediately increases heat and extreme heat locally and decreases regional and local rainfall. Forest loss also disrupts the climate in faraway places. Because of this, forests are even more valuable to climate efforts than previously accounted for in international climate plans and projections.  

The study reviewed the available literature on this emerging science to determine that forests up to 50 degrees north latitude deliver benefits at a global scale that cumulatively keep the entire planet cooler by 1 degree Celsius. This means that any forest protection or restoration efforts taking place between 40 degrees south latitude and—50 degrees north latitude help at the local level as well as the global level. For example, destroying rainforests in the 10 degree band just south of the equator could warm the planet by half of a degree. And restoring forests in the 10 degree band just north of the equator would deliver 25% more global cooling than expected based on CO2 sequestration alone. But the study shows that even those forests outside of this band deliver a host of benefits warranting their protection.  

“A recent major UN climate report showed we must urgently act now to avoid the worst case scenarios for our planet,” Lawrence said. “If we lose these forests, we will get there 10 years faster. If we protect these forests, they will shield us from extreme climate disasters, droughts and impacts on our food and agriculture. We are benefiting now from the tropics keeping us cooler; they are keeping us from feeling these extremes already.”   

The study notes that deforestation, for example, is responsible for one-third of the increase in intensity of hotter days; forest loss is also behind the increase in hot, dry summers. Our loss in tree cover has also led to local increases in extreme temperatures comparable in magnitude to changes caused by 0.5 degrees Celsius of global warming.  

“Put another way, deforestation pushes people today into an experience we are trying to avoid by hitting 2 degrees rather than 1.5 degrees of warming,” Lawrence said. “People living with deforestation are already suffering the effects of that warmer, more extreme world. Forest restoration would bring them back to a more livable climate.”    

Forest cooling is due to a range of biophysical effects. The study reveals that all forests emit chemicals called Biogenic Volatile Organic Compounds (BVOCs). On the one hand, BVOCs create aerosols that reflect incoming energy and form clouds; both are cooling effects. On the other hand, they lead to a build-up of ozone and methane, both greenhouse gases. This is a warming effect. On balance, the cooling outweighs the warming. These complex chemical compounds emitted by forests represent a new frontier in our understanding of how forests keep the planet cool near and far. 

Other aspects of forests that enable them to minimize drought associated with extreme heat include their deep roots, high water use efficiency and high surface “roughness.” These qualities allow trees to dissipate heat and move moisture higher into the atmosphere, which directly cools the local area and influences cloud formation and rainfall—which has ramifications far away.  

“Research is making it increasingly clear that forests are even more complex than previously understood. When we cut them down, we see devastating impacts on our climate, food supplies and everyday life. The benefits of keeping forests intact are clear; it’s imperative that we prioritize their protection,” said Wayne Walker, carbon program director at the Woodwell Climate Research Center and one of the study co-authors.  

The recently released Intergovernmental Panel on Climate Change (IPCC) report warned about the impacts, adaptation and vulnerability humans face with rising temperatures. This new study suggests that forest protection, important to both mitigation and adaptation, protects us from some of the worst climate disasters. And it shows that forests provide local cooling during the hottest times of the year everywhere on the planet, improving the resilience of cities, croplands and conservation areas. In the tropics, where forest carbon stocks and sequestration rates are highest, the biophysical effects of forests amplify the carbon benefits.  

“Protecting primary forests throughout the world should be one of our greatest priorities. These forests are critical for adapting to a warmer world,” said Michael Coe, tropics program director at the Woodwell Climate Research Center and a study co-author. “Without the forest cover we have now, the planet would be hotter and the weather more extreme. Forests provide us defense against the worst-case global warming scenarios.”  

This study adds a more sweeping global perspective to research on the local non-carbon climate impacts of deforestation: 

Researchers recently found that the destruction of forests and other ecosystems in Brazil’s Amazon and Cerrado regions endangers local soy agriculture, calculating that extreme heat costs $3.55 billion annually on top of $1 billion annually for drier conditions.  

Another study showed that rising temperatures and humidity tied to tree loss has already reduced the number of hours in the day people can safely work outside—and will only get worse if more forests are destroyed.  

A third study showed that in the case of Brazil, by 2100, roughly 12 million people could be exposed to extreme risk of heat stress, with vulnerable populations, including Indigenous Peoples, set to be the most severely impacted.  

“Despite the mounting evidence that forests deliver myriad climate benefits, trees are still viewed just as sticks of carbon by many policymakers in the climate change arena,” said Louis Verchot, a principal scientist at the International Center for Tropical Agriculture (CIAT) and one of the study co-authors. “It’s time for policymakers at the local and global levels to realize that forests have even greater value to people and economies, now and in the future, due to their non-carbon benefits. Forests are key to mitigation, but also adaptation.” 

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