TreeStewards.org

By Chelsea Harvey, E&E News

Tropical forests suffered some of their worst losses in history last year, according to a new report from the monitoring group Global Forest Watch.

About 39 million acres, or 61,000 square miles, of forest cover disappeared in 2017 — an area approximately the size of Bangladesh. That makes it the second-worst year on record, topped only by losses in 2016.

It’s discouraging news for global climate mitigation efforts. Healthy tropical forests store vast amounts of carbon, while deforestation can release that carbon back into the atmosphere.

And research suggests declines in tropical forest cover are taking their toll: Last year, a blockbuster study in Science concluded that tropical forests — because of their widespread destruction — are actually a net source of carbon to the atmosphere, rather than a carbon sink, as many experts had previously assumed.

The new data present “an alarming story of the situation for the world’s rainforests,” Andreas Dahl-Jørgensen, deputy director of Norway’s International Climate and Forest Initiative, said during a teleconference announcing the findings. “We simply won’t meet the climate targets that we agreed [to] in Paris without a drastic reduction in tropical deforestation and restoration of forests around the world.”

The findings were released Thursday, June 28, as representatives from around the world convened in Oslo, Norway, for an international forum on conserving tropical forests. A major focus of the conference includes the role of forests in global climate action.

Several recent estimates have underscored the significant contributions of deforestation to global carbon output — both the 2017 Science paper and a more recent estimate from the Global Carbon Projects may account for more than 10 percent of the world’s emissions.

But while the potential of forests to store or emit carbon remains their most substantial role in global climate efforts, some scientists note that forest losses may influence climate in other ways, as well. A new report from the World Resources Institute (WRI), also released this week to coincide with the Oslo forum, points out that deforestation can affect local temperatures and even alter the local water cycle. The report cites a range of recent studies on these effects.

Tree cover, for instance, has the potential to either warm or cool a local climate, depending on a combination of factors. On the one hand, trees tend to be darker in color than their surroundings, meaning they absorb more sunlight and more heat. On the other hand, they also release water into the air through their leaves, and they help to break up landscapes in ways that can disperse heat — both factors that may cool the local climate. Trees also release certain chemical compounds into the atmosphere that can have either cooling or warming effects.

But some recent research suggests that the cooling effect of trees may win out — meaning deforestation can drive local temperatures up and exacerbate the influence of ongoing climate change. A paper published in Nature Climate Change in April, for instance, links deforestation in the Northern Hemisphere to an increase in the intensity of hot days throughout the year.

Overall, the study suggests that deforestation probably accounted for more than half the warming that occurred over North America between 1920 and 1980. This effect has now been outstripped by the growing influence of human-caused climate change, but the researchers say deforestation may still account for nearly a third of the region’s warming (Climatewire, April 24).

A 2016 paper in Science had a similar message, suggesting forest losses around the world generally drive local temperatures higher. In fact, on a global average, it suggests the warming they produce may be the equivalent of about 18 percent of the influence from human-caused greenhouse gas emissions.

Other research suggests that deforestation could affect regional precipitation patterns. Trees lose water through their leaves, putting moisture back into the air — so tree cover losses can lead to drier local climates.

The effect may be particularly pronounced in tropical rainforests. One 2015 study found that deforestation in the Amazon basin reduces the region’s rainfall — and suggests that if the current deforestation rate continues, average rainfall throughout the Amazon basin could decline by more than 8 percent by 2050.

The point, the WRI report notes, is that “tropical forest loss is having a larger impact on the climate than has been commonly understood.”

Deforestation and degradation contribute substantially to global carbon emissions, thus helping fuel the progression of human-caused climate change. And at the same time, other non-carbon climate effects of deforestation may also be compounding the influence of global warming.

“When you add up these impacts of forest loss, one thing is clear: People living closest to deforested areas face a hotter, drier reality,” said Nancy Harris of WRI, who co-authored the report with Michael Wolosin of Forest Climate Analytics.

The new findings from the Global Forest Watch add renewed urgency to the global conversation on forest conservation and its role in international climate mitigation.

“A lot is hinging on our success in reversing these trendciences,” Dahl-Jørgensen said.