Forests are often called the “lungs of the Earth,” as if they single-handedly supply the air humans breathe. In reality, NASA and other agencies find that the ocean, driven by microscopic plankton, produces roughly half of Earth’s oxygen, which means forests share that role rather than dominate it. Yet forests do far more than top up the atmosphere: as the Millennium Ecosystem Assessment has framed it, they underpin human well-being by regulating water, filtering air, moderating disease and supporting the basic life-support systems that keep societies functioning.
Beyond Carbon: Forests’ Oxygen Role in Context
The idea that forests make all our oxygen collapses under basic atmospheric accounting. According to the United States National Oceanic and Atmospheric Administration, the ocean produces about half of the planet’s breathable oxygen, with marine phytoplankton responsible for a large share of that output. A complementary explainer from NASA describes how these microscopic organisms drive global primary production and feed ocean food webs, reinforcing that any story about oxygen that ignores the sea is incomplete.
That does not sideline forests; it simply places their oxygen role in context. Peer-reviewed studies of global primary production show that land vegetation, including forests, accounts for a large fraction of photosynthesis, but they also highlight thin evidence on the exact percentage of atmospheric oxygen attributable to forests alone. Instead of treating forests as the sole “lungs of the Earth,” researchers and assessments urge a broader view in which forests are vital partners in a planetary life-support system that includes oceans, soils and other ecosystems.
Guardians of Water: How Forests Ensure Reliable Supplies
If oxygen is a shared output, water is where forests start to look indispensable. A pan-tropical observational analysis of forest loss between 2003 and 2017 found that deforestation measurably reduces rainfall across the tropics. At a spatial scale of about 200 km, the researchers reported that a 1 percentage point loss of forest cover reduced precipitation by roughly 0.25 ± 0.1 m millimeters per month, and they used those metrics to estimate how future forest clearance in the Congo Basin could further dry regional climates. The authors stressed that while the direction of change is clear, the magnitude of future shifts remains uncertain, especially as climate patterns evolve.
Far from the tropics, modern water systems already bank on intact, forested headwaters. An Open-access dataset has cataloged 641 interbasin water-transfer projects across the United States and Canada, detailing how cities move water from often forested source watersheds to distant taps. The authors argue that these engineered networks Supports the idea that drinking-water reliability depends on both natural catchments and built infrastructure, and they flag that future changes in forest cover could alter the security of those transfers.
Air Purifiers Saving Lives: Pollution Removal and Health Gains
While climate debates tend to focus on carbon, trees quietly clean up another invisible threat: air pollution. A Peer-reviewed modeling study on pollutant removal reports that forests and urban trees remove nationwide quantities of air contaminants on the order of tens of millions of tonnes each year through deposition on leaves and stomatal uptake. The Reports link these processes to reductions in fine particulates and ozone, two pollutants with well-documented health impacts.
Crucially, the same study translates cleaner air into human outcomes. By combining pollutant removal estimates with epidemiological data, the authors calculated avoided mortality, fewer respiratory symptoms and substantial economic value from reduced healthcare costs and productivity losses. They emphasized that while the mechanisms of deposition and uptake are well established, regional variations in tree species, pollution sources and meteorology mean that the size of the benefit can differ sharply from one community to another.
Disease Barriers: Forests as Shields Against Outbreaks
Forests also shape where and how infectious diseases spread. A municipality-scale geospatial analysis in the Brazilian Amazon examined the relationship between deforestation and malaria between 2003 and 2015. The Peer-reviewed study Finds that a 10 percent increase in deforestation within a municipality is associated with about a 3.3% rise in malaria incidence, after controlling for other factors.
The authors Provides a causal explanation rooted in ecology: clearing forest changes local hydrology and creates sunlit pools that favor malaria-carrying mosquitoes, while bringing human settlements closer to vector habitats. They also describe a bidirectional feedback in which outbreaks can slow economic activity and land clearing, complicating projections of future disease risk. While the mechanisms are well supported, the researchers caution that extending these findings beyond the Brazilian Amazon or to different diseases remains speculative without comparable data.
The Bigger Picture: Forests in Global Life-Support Systems
Viewed together, these functions fit into a broader scientific framing of how ecosystems keep societies alive. A Landmark global assessment of ecosystems and human well-being, often described as Useful for policy-makers, concluded that forests supply water regulation and purification, food and fuel, climate regulation and disease regulation that together support “a decent, healthy, and secure life.” That framing shifts the conversation from forests as carbon vaults to forests as infrastructure that underpins public health, food security and economic stability.
Scientists have also explored how forests interact with the atmosphere to move moisture across continents. A Peer-reviewed theoretical paper proposes the “biotic pump” hypothesis, arguing that intact, continent-scale forests help sustain inland moisture transport and precipitation. The authors describe how replacing natural forest with low-leaf-area vegetation could sharply reduce continental precipitation and runoff, and they present the idea as Useful for understanding why large-scale forest loss might destabilize regional hydrology, while acknowledging that the magnitude of these effects is still debated.
Threats and Uncertainties: What Happens If We Lose Them?
Despite this growing body of evidence, forests continue to be cleared or degraded. The same pan-tropical analysis that linked forest loss to rainfall decline documented widespread deforestation trends between 2003 and 2017, but it also highlighted gaps in how global models capture protective functions such as local cooling, soil stabilization and biodiversity. Researchers caution that current climate and hydrology models may underrepresent these roles, which makes it harder to quantify the full risk of large-scale forest loss, especially when projecting potential tipping points in coupled climate and water systems.
Urban forests illustrate both the potential and the blind spots. A Boston University study reported that city trees and soils are absorbing more carbon than many inventories assumed, suggesting that even fragmented urban canopies contribute meaningfully to climate regulation alongside their air-quality and heat-reduction benefits. At the same time, the latest global forest-resources assessment portal from the Food and Agriculture Organization compiles nationally reported figures on forest extent, change and designated protective functions, yet the agency notes that some ecosystem services remain underreported or poorly measured, especially in regions with limited monitoring capacity.
Why Forest Protection Is a Public Health Strategy
For public officials weighing budgets and priorities, these findings have practical implications. Protecting forests is not only about meeting climate pledges or conserving charismatic wildlife; it also functions as preventive investment in water security, air quality and disease control. The Millennium Ecosystem Assessment framed these services as integral to human well-being, and subsequent pollution and disease studies show that losing forest cover can translate into higher healthcare burdens and infrastructure costs.
At the same time, scientists stress that uncertainties remain, especially around future precipitation patterns and the scale at which mechanisms like the biotic pump operate. Conflicts between different climate and hydrology models, as flagged in the pan-tropical rainfall analysis and the moisture-transport paper, argue for careful language about global tipping points rather than sweeping claims. What is clear from the converging evidence is that forests are entwined with the systems that deliver air, water and disease regulation, so decisions about clearing or conserving them are, in effect, decisions about how safely humans can live on Earth.
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*This article was researched with the help of AI, with human editors creating the final content.
