Forests are among nature’s best defenders against climate change – if we could just leave them alone long enough that they can fulfill their destiny as avid carbon consumers.

Trees naturally suck carbon dioxide out of the air, a function that helps counteract human emissions into it each year. In fact, by some estimates, the world’s forests absorb around one-third of human-caused CO2 emissions.

And tropical trees tend to be even thirstier for CO2 than their counterparts in temperate regions, thanks at least in part to longer growing seasons.

Human activity, however, is undercutting that natural potential.

According to new University of Maryland and World Resources Institute (WRI) analysis, 2017 marked the world’s second worst year of tropical deforestation, with 39.8 million acres of tropical forests lost – about the size of Florida. This was a slight improvement from 2016 with its 41.7 million acres lost, making it the worst year on record since satellite imagery became available in 2001.

Forest loss is not exactly a new problem – more than half of the world’s tropical forests have been destroyed since the 1960s. But new information is piling-up on possible implications. Consider, for example, the recent study suggesting that deforestation, degradation, and general disturbance have already combined to make tropical forests a net carbon source rather than a sink, meaning they’re losing more carbon than they can absorb. Consider also that losses continue to pile-up despite major efforts, across many nations, to counteract them.

So what exactly is happening to the world’s tropical forests, and why?

Roots of problem: Social, economic, political, climatic

Most of the responsibility can be laid at the feet of humans, whether because we’re cutting down forests – or causing the climate change that contributes to their destruction.

In the Democratic Republic of Congo (DRC), for example, the WRI report shows record high forest loss, with deforestation up 6 percent from 2016, primarily as a result of farming and artisanal logging. Considering this African nation ranks among the poorest countries globally, 176th out of 187 countries on the UN’s most recent Human Development Index, it’s understandable that people may be willing to give up some shade if they think it can help them eke out a living.

But sometimes even positive social change can feed forest loss. In Columbia, for instance, hard-won peace was recently brokered between the government and a major rebel group that had been occupying remote, forested areas. That rebel group’s presence is thought to have helped protect the land from development for years. But with that deterrent out of the way, there’s been a race to log, mine, and clear-cut for coca cultivation. The government has created new laws to protect these areas, but it’s unclear if implementation and enforcement will be effective.

Climate change is also contributing to the loss, bringing forests more severe tropical storms and perhaps also making them more frequent. In 2017, hurricanes destroyed 32 percent of Caribbean island Dominica’s forests, according to WRI. Puerto Rico offers another grim preview of what’s to come, with storms like Hurricane Maria destroying 50 percent of the canopy last year alone – compared with 1 percent in a typical year.

Then there’s the gray area in between, where people take direct action to remove trees, and then climate change expands the path of destruction.

Tropical forest fires, often started by humans to clear land for farming or mining, get worse with climate change impacts like drought and severe heat. In 2017, record-breaking fires ripped through the Brazilian Amazon, the most since monitoring began in 1999. These fires contributed to Brazil’s posting its second highest losses – 31 percent lost – despite otherwise marked successes in deforestation efforts.

Tropical forest loss and climate change

When a tree falls in a tropical forest, it doesn’t matter if anyone is there to hear it; its ability to actively sequester carbon ends. These days, tropical forest loss is projected to account for about 10 percent of human-caused greenhouse gases.

Tropical forest loss is a bigger contributor to global emissions than loss of forests in temperate climes, in part because tropical trees hold more carbon in their biomass than in soil. Plus, some research has shown that – unlike in temperate forests where timber may continue to store at least some portion of carbon, as with the wooden floor of a house – most tropical forest wood is either turned into paper, used for fuel, or simply burned, none of which sequesters much, if any, carbon.

And while many tropical forests have had their fair share of extreme weather, the increasing frequency of severe storms associated with climate change could be a game-changer. Far more study is needed in this realm, but early-stage investigation points to more or more severe hurricanes translating to shorter, smaller forests, which means less carbon absorption, too.

Climate emissions tracked to major disasters serve as an apt illustration of the potential impact of tropical forest loss: Scientists used mapping and modeling data to estimate that the roughly 320 million large trees lost during Hurricane Katrina had been holding 105 tera-grams of carbon, representing 50-140 percent of the net annual U.S. forest tree carbon sink.

Meanwhile in Indonesia, a 2015 fire and haze crisis across the nation’s forest and peat lands pumped more CO2 into the atmosphere than came from the entire European Union during the same time.

And in addition to contributing to emissions, tropical deforestation also actively contributes to the vicious cycle of climate change. Scientific modeling, according to WRI, “strongly agrees” that continental-scale deforestation of tropical forests would make those areas warmer and drier.

Deforestation in areas like the Amazon, Southeast Asia, and the Congo can also affect the water cycle, with local and global implications. Fewer trees make for fewer to draw water from the soil and release it as vapor into the air, where it goes to form clouds and then becomes rain. Scientists say this effect could translate to drought in key agricultural areas from China and India to the U.S. bread basket states.

In a tropical forest, drought could affect which tree species prevail over time, perhaps creating more favorable conditions for trees with relatively harder wood and tougher leaves, which are less vulnerable to drought than the lusher species that contribute to canopy cover. That approach theoretically could result in less cover to store carbon and more open, hotter forests, contributing further to climate change.

Plus, trees provide important cooling and comfort. With fewer trees to cool things down with shade and transpiration, the warming effects of climate change will only become starker in terms of surface temperature. In Indonesia, for example, surface temperature differences between forested and clear-cut lands have climbed to as much as 18°F in some cases.

So, deforestation exacerbates climate change – and vice versa.

Where does that leave us? What now?

Reducing fossil fuel use therefore is vital to addressing tropical forest loss, because it can help stave off even worse impacts of increasing fire, drought, and severe heat. But addressing deforestation itself can also have an outsized impact. According to WRI, tropical forest loss is currently responsible for 8 percent of the problem, but that contribution could be key to 23 percent of the solution by 2030. (The explanation here? The 8 percent figure is based on subtracting carbon sequestered by regrowing trees from what’s being lost to deforestation now – which doesn’t account for the carbon-additive qualities of tropical trees over time. Because tropical forests and wetlands can store more carbon the longer they’re left to grow, the idea is that conserving and regrowing them could deliver nearly a quarter of the mitigation needed to keep warming below 2 degrees Celsius.)

Giving tropical forests what they need to help store more carbon requires deeper understanding of the problems they face. While great strides have been made in this field of study, it can still be difficult to precisely count tree loss. Remote sensing data and other technologies are emerging to help scientists estimate tree damage, but it’s an imperfect science, and still needs ground data. More research is also needed to capture detail on how well trees are growing back after storms, fires, or logging.

But as we learn more, we can develop more strategic and better informed ways of conserving and improving management of the world’s tropical forests, so they can live up to their carbon-storing potential.

Addressing deforestation will also likely take more money, considering forest conservation is dramatically underfunded, when compared with land development financing.

Continuing and expanding international cooperation is another way forward, so long as leaders look not solely at environmental and economic issues, but also at social ones.

For example, indigenous rights have often been overlooked or under-considered in forest policy. But people who have lived in and around these lands for generations could actually be meaningful allies in protecting them. By some estimates, tree cover loss is said to be less than half on indigenous or community-run land than in other areas, and the difference is even greater when their rights to the land are legally recognized.

On-the-ground success stories are out there, from Brazil to Indonesia, pointing to the power of combining local efforts, socially-minded policy, and economic incentive. Repeating those successes around the world could theoretically help buy humanity another 40-50 years to get off fossil fuels.

Plausible? Perhaps, but clearly not a slam-dunk. But such steps might help make the case for ramping-up efforts to fight tropical deforestation. And doing so sooner rather than later.