Deforestation and climate change
Deforestation refers to the conversion of forest to other land uses or the long-term reduction of tree canopy cover below a 10 percent threshold, driven primarily by agricultural expansion for crops and livestock, commercial logging, and infrastructure development.[1][2] This process contributes to climate change by releasing approximately 11 percent of global anthropogenic greenhouse gas emissions through the oxidation of biomass and soil carbon, while reducing forests' role as net carbon sinks.[3] Biophysical effects, including decreased evapotranspiration and altered surface albedo, further influence regional temperatures, with net warming predominant in tropical latitudes where most deforestation occurs, though high-latitude forest loss can yield local cooling via increased reflectivity.[4][5] Climate change, in turn, accelerates deforestation indirectly by intensifying wildfires, droughts, and biotic disturbances, forming reinforcing feedbacks that challenge global mitigation efforts.[6] Key controversies surround the quantification of these contributions, as models often emphasize carbon fluxes while underweighting biophysical dynamics, and initiatives like REDD+ have yielded mixed results in curbing net forest loss amid persistent economic pressures in developing regions.[7]Definitions and Background
Defining Deforestation and Forest Cover
Deforestation refers to the conversion of forest land to another land use or the long-term reduction of tree canopy cover below the 10 percent threshold, independent of whether the process is human-induced or occurs naturally.[1][8] This definition, established by the Food and Agriculture Organization of the United Nations (FAO) in its Global Forest Resources Assessments (FRA), emphasizes permanent or semi-permanent change in land use, such as conversion to agriculture, pasture, or urban development, distinguishing it from temporary disturbances like selective logging or natural dieback.[1] The World Bank aligns with this by specifying deforestation as the removal of forest stands followed by conversion to non-forest uses like farms or settlements.[9] Forest cover, in contrast, denotes the spatial extent of land classified as forest under standardized criteria. The FAO defines forest as land spanning more than 0.5 hectares with trees higher than 5 meters in situ and a canopy cover exceeding 10 percent, or areas capable of reaching these thresholds, excluding predominantly agricultural or urban lands.[10][11] This threshold-based approach facilitates global monitoring but has drawn criticism for including tree plantations and excluding certain wooded areas below the height or coverage minima, potentially inflating estimates of productive natural forests.[12] Globally, forests under this definition covered approximately 4.06 billion hectares in 2020, representing 31 percent of total land area.[13] Key distinctions exist between deforestation and related processes like forest degradation, which involves a reduction in canopy density or biomass without full land-use conversion, often retaining forest classification.[14] These definitions underpin international reporting, such as the FRA, but variations in national implementations—e.g., differing minimum sizes or crown cover percentages—can lead to inconsistencies in cross-country comparisons.[15] For climate-related analyses, precise delineation is critical, as only deforestation involving permanent conversion directly alters long-term carbon storage potential, whereas degradation may allow recovery.[16]Forests' Role in the Carbon Cycle and Climate Regulation
![Biophysical effects of deforestation on global temperature by latitude band][float-right] Forests play a central role in the global carbon cycle by absorbing atmospheric carbon dioxide through photosynthesis, storing it in biomass, soils, and dead organic matter. Globally, forests currently hold approximately 861 gigatonnes of carbon (GtC), representing a significant portion of terrestrial carbon stocks. [17] This storage occurs primarily in living biomass, with tropical forests accounting for about 55% of the total, or roughly 474 GtC. [18] Annually, forests act as a net carbon sink, sequestering an estimated 7.6 billion metric tonnes of CO2 equivalent, equivalent to about 2.1 GtC, though this sink has diminished due to disturbances like fires and logging. [19] Since 1990, global forests have sequestered 107 GtC, offsetting 46% of anthropogenic CO2 emissions over that period. [20] Beyond carbon sequestration, forests regulate climate through biophysical mechanisms that influence surface energy balance and atmospheric circulation. Evapotranspiration from forest canopies transfers water vapor to the atmosphere, providing a strong cooling effect by latent heat flux, which can lower local temperatures by several degrees compared to non-forested areas. [21] This process is particularly pronounced in tropical forests, where high evapotranspiration rates contribute to cloud formation and regional precipitation patterns. [22] Forests also affect albedo, the reflectivity of Earth's surface; their darker canopies absorb more solar radiation than lighter grasslands or croplands, increasing shortwave absorption but often resulting in net cooling when combined with evapotranspiration, especially at low latitudes. [23] The combined biogeochemical (carbon) and biophysical effects of forests yield a net radiative cooling globally, estimated at about 0.5°C when accounting for both CO2 sequestration and non-carbon processes. [24] In tropical regions, deforestation induces warming through both CO2 release and reduced biophysical cooling, with biophysical effects amplifying carbon-related forcing. [22] Conversely, in boreal zones, albedo increases from deforestation can produce short-term cooling, though long-term carbon release dominates. [25] These mechanisms underscore forests' multifaceted role in stabilizing climate, distinct from purely biochemical carbon accounting.[23]Causes of Deforestation
Dominant Anthropogenic Drivers
Agricultural expansion constitutes the predominant anthropogenic driver of global deforestation, accounting for approximately 75% to 90% of tree cover loss in tropical regions, where 95% of worldwide forest loss occurs.[26][27] Between 2001 and 2022, 86% of global deforestation was linked to agriculture, encompassing both commercial commodity production and subsistence farming practices such as slash-and-burn.[28] This driver has been consistently identified in assessments by organizations like the Food and Agriculture Organization (FAO) and peer-reviewed analyses, surpassing other factors in scale despite varying regional emphases.[29] Within agriculture, the production of beef, soy, and palm oil emerges as the most significant contributors, responsible for around 60% of tropical deforestation.[26] In the Brazilian Amazon, cattle ranching alone drives about 80% of current deforestation, converting vast tracts of forest into pastureland to meet domestic and export demands.[30] Soy cultivation, often for animal feed and biofuels, has similarly accelerated forest clearance in the Amazon and Cerrado regions, with deforestation rates tied to expanding farmland peaking in periods of high global commodity prices.[26] In Southeast Asia, particularly Indonesia and Malaysia, palm oil plantations have led to substantial losses, with oil palm expansion accounting for 7% of global deforestation from 2000 to 2018.[31] These commodities are frequently embedded in international trade, where 29-39% of related emissions stem from exported goods.[32] Commercial logging ranks as the second major driver, particularly in tropical hardwoods, where selective harvesting often precedes full conversion to agriculture by creating access roads and fragmenting forests.[29] In many cases, logging contributes indirectly by facilitating subsequent agricultural encroachment, though direct wood extraction accounts for a smaller share of gross forest loss compared to agriculture.[33] Infrastructure development, including roads for logging and mining, and urbanization further enable these processes, but their direct impact remains limited to under 10% globally.[34] Mining, while localized, has induced notable deforestation in regions like the Colombian Amazon and parts of Africa, with industrial operations clearing forests for extraction sites across 26 tropical countries.[35] Overall, these drivers reflect economic pressures from population growth, demand for resources, and policy incentives favoring short-term land conversion over sustained forest management.[26]Secondary Climate-Related Factors
Secondary climate-related factors in deforestation encompass disturbances amplified by global warming, including intensified wildfires, prolonged droughts, and expanded ranges of pests and pathogens, which collectively contribute to tree mortality and reduced forest cover beyond direct human activities. These mechanisms operate through altered temperature and precipitation regimes, weakening forest resilience and facilitating loss that may preclude natural regeneration.[36] Wildfires represent a primary disturbance vector, accounting for 38% ± 9% of global forest loss from 2003 to 2018, with an annual average of 91,000 ± 22,000 km² affected.[37] Warmer temperatures and drier fuels, driven by climate change, have extended fire seasons and increased burned areas; in the western United States, anthropogenic warming doubled the occurrence of large fires between 1984 and 2015.[38] Projections indicate that a 1°C temperature rise could expand median burned areas by up to 600% in certain western U.S. forests, while events like El Niño-induced droughts have spiked fire-related losses in regions such as the Amazon.[38][37] Droughts induce hydraulic failure and carbon starvation in trees, leading to widespread die-off; global observations link hotter droughts to elevated tree mortality rates across biomes, with intensified evaporative demand exacerbating water deficits.[39] For instance, drought stress has triggered pulses of mortality requiring extended drought durations in some ecosystems, compounding vulnerability in already stressed forests.[40] These events often interact with fires, as desiccated vegetation heightens flammability, creating feedback loops that hinder recovery.[41] Pest outbreaks, particularly bark beetles, thrive under milder winters and drought-weakened hosts, causing extensive mortality; climate-driven expansions have enabled species like the mountain pine beetle to infest new territories, building fuel loads for subsequent fires.[38] In Europe, post-2018 droughts precipitated the largest recorded bark beetle outbreak in Norway spruce forests, reducing growing stock significantly.[42] Pathogens similarly exploit stressed conditions, with warmer climates enhancing their survival and spread, further diminishing forest productivity and cover.[36]
Effects of Deforestation on Climate
Direct Greenhouse Gas Emissions
Deforestation directly emits greenhouse gases primarily through the oxidation of stored carbon in forest biomass and soils, releasing carbon dioxide (CO₂) when trees are felled, burned, or left to decay.[43] Burning, a common clearing method, accounts for immediate CO₂ release alongside methane (CH₄) from incomplete combustion and nitrous oxide (N₂O) from nitrogen-rich soils, though CO₂ constitutes approximately 87% of total forest-related GHG emissions.[44] Soil carbon stocks, which can exceed 50% of total forest carbon in some ecosystems, contribute additional emissions via disturbance and erosion post-clearing.[45] Global gross emissions from deforestation have been estimated at 5.7 Gt CO₂-equivalent per year on average for tropical forests, the primary source, though net land-use change emissions (accounting for some regrowth) stood at 4.1 Gt CO₂ annually around 2011–2020, representing about 10% of total anthropogenic CO₂ emissions.[44][1] Recent FAO data indicate a decline in emissions from net forest conversion to below 3 Gt CO₂ per year by the late 2010s, with projections holding steady or slightly reversing through 2025 amid varying regional enforcement of conservation policies.[46] These figures vary due to methodological differences, such as remote sensing versus ground inventories, and exclude degradation emissions, which add roughly half as much again in selective logging scenarios.[47] Tropical regions dominate, with Amazonian and Southeast Asian deforestation driving over 80% of emissions; for instance, Brazil's forest loss contributed around 0.4–0.5 Gt CO₂ annually in peak years like 2019–2022 before policy shifts reduced rates.[48] Updated assessments place deforestation's share of global anthropogenic GHGs at 6–10% in recent years, lower than earlier 20–30% claims that aggregated degradation and overlooked reforestation offsets.[49][48] This direct flux underscores deforestation's role as a pulsed source, contrasting fossil fuel emissions' steady release, though uncertainties in belowground carbon accounting persist across datasets.[50]Biophysical Mechanisms Beyond Carbon
Biophysical mechanisms of deforestation on climate encompass alterations to surface properties that influence energy, water, and momentum fluxes between land and atmosphere, distinct from biogeochemical effects like carbon dioxide emissions. These include changes in albedo, evapotranspiration, and aerodynamic roughness, which modify local and regional temperature, humidity, and precipitation patterns.[51] Models and observations indicate these processes often dominate locally, with forests providing cooling through enhanced latent heat fluxes and reduced temperature variability.[4] Surface albedo, the fraction of incoming solar radiation reflected, decreases under dense forest canopies due to dark foliage, promoting greater absorption of heat. Deforestation replaces forests with higher-albedo surfaces like grasslands or croplands, increasing reflection and inducing cooling, particularly in extratropical regions where snow-albedo feedbacks amplify the effect during winter. In boreal zones, this albedo increase can cool local temperatures by up to 1.8°C, outweighing any warming from reduced evapotranspiration. In contrast, tropical albedo changes are minimal, as cleared lands do not brighten substantially relative to moist forest understories. Historical global land cover changes, including deforestation, have yielded a net biophysical cooling of -0.10 ± 0.14°C via albedo effects.[7][51] Evapotranspiration represents a primary cooling pathway, as forests transpire vast quantities of water vapor, partitioning surface energy toward latent heat rather than sensible heat that warms the air directly. Tropical forests, with high moisture availability, sustain year-round evapotranspiration that cools surfaces by 0.2–2.4°C annually and humidifies the atmosphere, fostering cloud formation and further radiative cooling. Deforestation curtails this process, shifting energy to sensible heat and elevating local temperatures, with studies estimating 1–3°C warming in deforested tropical areas. This reduction also diminishes regional precipitation by 10–30% through decreased atmospheric moisture, creating drier conditions that exacerbate heat extremes. In water-limited extratropical regions, the evapotranspiration effect is seasonally constrained, yielding summer warming but less overall impact than albedo changes.[4][7][51] Aerodynamic roughness from forest canopies enhances turbulence and vertical mixing, facilitating heat and moisture transport; its loss post-deforestation stabilizes the boundary layer, potentially trapping heat near the surface. Combined with albedo and evapotranspiration shifts, these mechanisms yield divergent regional outcomes: tropical deforestation drives net local warming of ~0.1°C per 10° latitude band via evapotranspiration dominance, while high-latitude (>50°N) clearing produces cooling 3–6 times greater than offsetting CO2 warming due to albedo. Globally, biophysical effects from deforestation partially counteract biogeochemical warming, but local stabilization by intact forests—reducing diurnal ranges and extremes—remains critical, with tropical losses immediately intensifying heat by diminishing non-carbon cooling equivalent to one-third of a degree Celsius. Empirical trends confirm these patterns, with models showing tropical biophysical warming amplifying extremes more than carbon effects locally.[4][7][51]Long-Term Feedbacks and Measurement Challenges
Deforestation initiates long-term feedbacks that can amplify climate warming and alter regional hydrology, primarily through biophysical mechanisms such as reduced evapotranspiration and changes in surface albedo. In tropical regions, these feedbacks lead to decreased precipitation—by up to 14.7 mm/year globally from large-scale clearing—and lower potential evapotranspiration, resulting in net runoff reductions despite initial increases from bare soil exposure.[52] Such drying exacerbates drought stress on remaining forests, increasing wildfire susceptibility and promoting further degradation, as observed in the Amazon where biophysical warming and precipitation declines reduce aboveground biomass by 5.1% in adjacent stands.[53] These processes contribute to potential tipping points, where exceeding 20-40% deforestation in the Amazon could trigger widespread dieback via self-reinforcing savannization, releasing additional carbon and altering atmospheric circulation patterns over decades.[54] [55] Biophysical effects from tropical deforestation cause local surface warming of 0.2–2.4°C annually (mean 0.96°C), equivalent to 35-60% of the warming from CO2 emissions depending on latitude, through lost cooling from evapotranspiration and canopy roughness despite minor albedo gains.[4] Over longer timescales, these interact with biogeochemical cycles, as reduced moisture recycling diminishes cloud formation and biogenic volatile organic compounds that provide net cooling, potentially sustaining elevated temperatures and hindering forest regrowth.[4] In extratropical zones, feedbacks may differ, with increased albedo sometimes yielding transient cooling, but global net effects favor warming due to dominant tropical influences.[4] Quantifying these feedbacks poses significant challenges, as Earth system models exhibit variability in simulating precipitation-temperature responses, with discrepancies arising from nonlinear dependencies on deforestation extent and coarse resolution failing to capture cloud-aerosol interactions.[53] Biophysical impacts are often omitted from carbon accounting frameworks, underestimating total climate forcing, while satellite-derived aboveground biomass data underestimate losses in humid tropics due to cloud obscuration and lack of ground validation.[56] Distinguishing local from nonlocal effects remains difficult, as deforestation alters remote atmospheric circulation, complicating attribution in observational records.[4] Long-term monitoring is further hindered by model biases in energy partitioning and indirect feedbacks, leading to uncertainties in projecting tipping point thresholds beyond current data spans of decades.[4][53]Effects of Climate Change on Forests and Deforestation
Enhanced Disturbances from Warming
Rising global temperatures have intensified forest disturbances, including wildfires, insect infestations, and drought-induced dieback, leading to accelerated tree mortality and altered forest dynamics. Empirical observations indicate that warmer conditions extend fire seasons, reduce fuel moisture, and increase ignition risks, resulting in larger and more severe burns. For instance, in the western United States, fire weather conditions suitable for large wildfires have increased by up to 400% since the mid-20th century, correlating with a fourfold rise in forest fire extent.[57] Similarly, boreal forests in North America and Eurasia have experienced heightened fire activity, with burned areas expanding due to drier summers and reduced snowpack.[58] Insect outbreaks, particularly bark beetles, have surged in response to milder winters and prolonged growing seasons, diminishing natural population controls. The mountain pine beetle (Dendroctonus ponderosae), for example, has devastated over 18 million hectares of lodgepole pine forest in British Columbia since the 1990s, with warmer temperatures enabling one-year life cycles and northward range expansion into previously unsuitable habitats.[59] In Europe, the spruce bark beetle (Ips typographus) outbreak in 2018-2020 affected millions of cubic meters of timber, exacerbated by drought-stressed trees and temperatures exceeding historical thresholds for beetle survival.[60] These epidemics compromise forest regeneration, as dead trees provide fuel for subsequent fires, creating compounding disturbance cycles.[61] Droughts compounded by warming—termed "hotter droughts"—have triggered widespread forest dieback, where hydraulic failure and carbon starvation lead to mass tree mortality. Global field data from 2000-2020 document die-off events across biomes, with vapor pressure deficit (a measure of atmospheric dryness) rising 25% since 1990, surpassing precipitation declines in stressing trees.[39] In southwestern North America, piñon pine and juniper dieback affected 12% of woodlands during the 2002-2003 drought, while Amazonian forests showed increased mortality from prolonged dry seasons.[62] Such events reduce forest cover and carbon stocks, potentially transitioning ecosystems to non-forested states if recovery lags, thereby amplifying deforestation pressures through salvage logging or land conversion.[63] These enhanced disturbances interact synergistically; for example, drought-weakened trees succumb faster to insects, and beetle-killed stands ignite more readily, as observed in Rocky Mountain lodgepole pine forests where post-outbreak fire severity doubled.[64] While some regions exhibit resilience through refugia or adaptive management, projections under continued warming forecast 2-4 times higher disturbance rates by mid-century, challenging forest persistence without intervention.[65] Peer-reviewed syntheses emphasize that these trends stem from thermodynamic principles—warmer air holds more moisture, intensifying evapotranspiration and desiccation—rather than isolated anomalies.[66]Alterations in Forest Productivity and Dieback
Climate change alters forest productivity through competing mechanisms, including CO2 fertilization that enhances photosynthesis and water-use efficiency, potentially boosting growth in moist environments, contrasted by warming-induced drought stress, increased evapotranspiration, and heightened vulnerability to pests and fires that diminish biomass accumulation.[67] Empirical data from satellite observations reveal global increases in vegetation greenness and gross primary productivity (GPP) attributable partly to elevated CO2, yet regional declines predominate in arid and semi-arid zones where drought overrides fertilization effects.[68] For example, in the western United States, forest productivity trends have shifted negative since the 1980s, driven by warmer temperatures and reduced soil moisture rather than harvesting or mortality alone.[69] Tree dieback, defined as widespread mortality leading to canopy thinning and stand replacement, has intensified in response to hotter droughts, with events documented across all forested continents since the early 2000s. These episodes exhibit a "hotter-drought fingerprint," where elevated vapor pressure deficits—amplified by warming—exceed physiological tolerances, causing hydraulic failure and carbon starvation in species like pines, oaks, and eucalypts.[39] In Europe, the 2018–2022 drought-heatwave sequence damaged millions of hectares, with mortality rates exceeding 20% in affected stands of beech and spruce, as quantified by ground inventories and remote sensing.[70] Productivity alterations manifest unevenly by biome and latitude: boreal forests face growth reductions from thawing permafrost and extreme winters, while tropical regions experience suppressed GPP under prolonged dry seasons, though some models project CO2-driven resilience until tipping points like Amazon dieback, which remains hypothetical without full empirical confirmation.[71] Long-term studies indicate that while global drought sensitivity of vegetation has marginally declined due to acclimation or land-use shifts, warming has accelerated drought severity by 40% on average, eroding productivity gains in water-limited ecosystems.[72] Feedbacks from dieback, such as reduced transpiration cooling, further amplify local warming, potentially hastening succession to less productive states.[73] These dynamics underscore causal linkages from anthropogenic warming to forest decline, though baseline variability and management practices modulate outcomes.[74]Empirical Trends and Net Impacts
Global Deforestation Rates and Declines
According to the Food and Agriculture Organization of the United Nations (FAO) Global Forest Resources Assessment 2025, global deforestation rates have declined significantly over recent decades, with the annual rate of forest conversion to other land uses dropping to 10.9 million hectares in the 2015–2025 period from 17.6 million hectares in 1990–2000, representing a 38% reduction.[75] [76] This slowdown occurred across all world regions, attributed partly to increased forest expansion through afforestation and natural regrowth, which offset some losses.[75] The net annual forest area loss further decreased to 4.12 million hectares in 2015–2025 from 10.7 million hectares in the 1990s, reflecting a net global forest cover of 4.14 billion hectares, or 32% of total land area as of 2025.[75] [77]| Period | Annual Deforestation (Mha/yr) | Annual Net Forest Loss (Mha/yr) |
|---|---|---|
| 1990–2000 | 17.6 | 10.7 |
| 2015–2025 | 10.9 | 4.12 |