Jungle
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The contributions of tropical forests to earth systems also reach far into our skies. Their large leaves and extensive canopies provide the perfect transfer platforms for water back into the atmosphere. Tropical forests are responsible for one-third of the evapotranspiration (evaporation of water from leaves) around the globe at any one time. Once in the air, the pollen, fungal spores, and compounds unleashed by tropical forests bind this moisture together to form clouds and, thus, life-giving rain. Climate scientists, using a sandbox technique similar to that of Tim Lenton, whom we met when exploring the first planetary impacts of plants in Chapter 1, have shown that this evapotranspiration is incredibly important to the water cycle (or hydrosphere) on both local and regional scales. If one were to completely deforest the Amazon Basin, precipitation could decrease by an average of 324 millimeters per year locally (or as much as 640 millimeters in some estimations, which would be more than the entire average annual rainfall in and around Newcastle-upon-Tyne), while also leading to reductions more widely across South America. Similarly, deforestation of the Congo River basin in Central Africa would not only reduce rainfall by approximately 50 percent in the immediate region but could also lead to less precipitation in West Africa along the Guinea Coast. Modeled impacts of tropical deforestation even extend beyond the tropics, thanks to global climate circulation systems. Complete deforestation of the Amazon Basin could potentially lead to declines in rainfall in midwestern and northwestern portions of the United States. Meanwhile, models of deforestation in the Congo Basin simulate reduced rainfall in Mexico, the United States, the Arabian Peninsula, southern Africa, and southern and eastern Europe.6
The cloud-forming canopies of tropical forests are not just significant for their role in the water cycle; they can also quite literally act as parasols in the regulation of local, regional, and global temperatures. Due to the fact that the tropics are exposed to energy from the Sun more prominently than any other parts of the planet, the shade and moisture provided by the water-filled air skirting above these forests not only cools the ground beneath them but actually impacts the distribution of temperature across the Earth more widely. Add to this that the forests of the tropics store nearly one-quarter of the world’s land-based carbon, with peat swamp forests acting as particularly bulky carbon “sinks.” As tropical forests perform a staggering 34 percent of the planet’s photosynthesis, if they disappeared, not only would this trapped carbon be released into the atmosphere, but less CO2 would be absorbed into the biosphere in their absence. Together, these greatly increased emissions would trap more of the Sun’s energy and exacerbate global warming still further. Although interactions between changes in landcover, “albedo” (how much of the Sun’s rays are reflected back into the sky, determining how much the surface warms), and CO2 shifts are highly complicated, a series of modeling studies, as well as direct observations, have highlighted the potential impacts of deforestation on regional and global temperatures. Complete deforestation of the Amazon Basin, Central Africa, and Southeast Asia is expected to produce regional warming of anywhere between 0.1°C and 3.8°C, 0.5°C and 2.5°C, and approximately 0.2°C, respectively. Meanwhile, pantropical deforestation would also lead to an increase in average temperatures globally by as much as 0.7°C, a total that would double the warming observed since the mid-nineteenth century and the intensified industrial burning of fossil fuels. This would undoubtedly lead to further polar ice melt, climatic instability, and the demise of various species of plants and animals within and beyond the tropics. This is not to mention the fact that most of the coal already burned since the start of industrialization, as we saw on our search for some of the first trees on Earth in Chapter 1, originated from the earliest tropical forests of the Carboniferous period in the first place.7
Tropical forests are also often described as the “lungs of the Earth.” Although they do not actually supply 20 percent of the planet’s oxygen, they remain entwined in regional and global air quality. Without forests, landscapes become dustier. Meanwhile, the loss of their contributions to evapotranspiration increases the likelihood of fires. These impacts would only be exacerbated should their deforestation, or the draining and removal of tropical peat, occur as a result of deliberate burning to begin with. In 2019, Southeast Asian forest fires showed how widespread burning can have drastic impacts on the air quality across a sizable area, with blazes in Indonesia argued to be behind smog descending on Malaysia, Singapore, and Vietnam and even forcing school closures in these areas—although, it should be noted, Indonesia disputed that it was the only country at fault. A lack of air filtering from forest cover, increasing airborne dust, and rising fire frequency, alongside deforestation-linked emissions of CO2, nitrous oxide, and methane, will eventually impact the air we breathe, and perhaps not only in densely populated tropical areas. If not our own breathing directly, alterations to tropical forests will certainly interfere with our food supplies. Human populations in Africa, Asia, the Pacific, and the Neotropics are already facing challenges to food security as increasingly unpredictable climates impact their harvests. Furthermore, given that those of us in the Western world often rely on vast international networks, thanks to the colonial processes reviewed in Chapter 11, tropical and subtropical climate changes will also soon affect the coffee we import from Brazil and Indonesia and the rice we obtain from Asia. The potential global climate changes wrought by dramatic declines in tropical forests could even damage agricultural growth of staple crops across the United States, China, and India, with disastrous impacts for the large human populations of these regions. Tropical forests therefore provide perhaps the clearest case of twenty-first-century human entwinement in earth systems. We can see how we are all reliant on these environments and are all, in some way, linked to them. So when did this begin? And how have past interactions with tropical forests shaped the potentially perilous position we now find ourselves in, where the loss of these environments will be felt all over the world?8
CHAPTER 6 DEMONSTRATED how tropical forests were repeatedly occupied by late Pleistocene humans potentially as early as 200,000 to 100,000 years ago and certainly by 80,000 to 45,000 years ago. But could these early hunting-and-gathering groups have somehow left lasting impacts on these environments and thus earth systems? One way they might have done so is through removal of the megafaunal inhabitants of tropical forests. Conventionally classed as animals heavier than forty-four kilograms, many of these giants faced extinction around the world during the late Pleistocene, at a time when Homo sapiens was occupying new areas of the planet. In South America, including the Amazon Basin, a staggering array of weird and wonderful megafauna disappeared during this time, such as giant sloths, the elephant-like gomphothere Stegomastodon, and the rhino-sized Toxodon. Similarly, in Chapter 8, we saw how humans have been linked to later megafaunal extinctions in the Caribbean and Madagascar. Phylogenetic analysis of the late Pleistocene and early–middle Holocene extinctions of a series of unique, large mammalian lineages, notably within South America, has been argued to show that this represented the demise of 2 billion years’ worth of evolution. Not only that, but because megafauna are so crucial to tropical forest ecosystems, through the dispersal of large seeds and fruits and the disturbance of forests with the swinging of their bulk, some scientists have suggested that if, indeed, humans brought about their demise, this could be the first example of our species impacting not only the biosphere but also the carbon cycle, as their absence hindered the regrowth of large trees. Other cases of potential early human impacts on earth systems come in the form of deliberate burning and maintenance of forests in Borneo in Southeast Asia, the New Guinea Highlands in Near Oceania, and Queensland in Australia. Ethnographic study of Indigenous populations demonstrates how widespread and long-lasting such activities can be in altering and managing tropical ecosystems through the maintenance of forest boundaries, open patches, and clear forest floors. Nevertheless, it can be difficult to definitely and directly attribute human activity to these change
s, and in the three “megafaunal” cases mentioned, it seems unlikely that our ancestors were the sole factor in their tropical decline. Moreover, in general, the overall earth systems influences of these activities would, while visible, also seem to be relatively localized.9
More continental and global-scale human impacts on tropical forests and earth systems may have begun with the emergence of farming practices. We saw in Chapter 7 how a number of our favorite domesticated plants and animals have their origins in the tropics, including as part of some of the earliest examples of cultivation in the archeological record. Meanwhile, we witnessed how the entrance of agricultural systems from extratropical areas could have major environmental repercussions. One of these latter cases has actually even been implicated in possible globally recognizable preindustrial “greenhouse gas” emissions. We are used to thinking of the rear ends of cows as significant methane producers but perhaps less so rice. Nevertheless, where rice grows in flooded fields, or rice paddies, the water stops oxygen from entering the soil, leaving it a hotbed for methane-producing bacteria. When William Ruddiman became the first scientist to suggest that the beginnings of the anthropocene could actually be found with the origins of agriculture approximately 8,000 years ago, he suggested that, combined with CO2 emissions from rampant agricultural deforestation in Eurasia, methane emissions due to prehistoric rice expansion across Asia produced a greenhouse gas effect so large that it actually prevented the onset of a glacial cycle. Other scientists have linked the expansion of ruminating livestock into sub-Saharan Africa, monsoonal India, and central China, as well as the expansion of irrigated rice and water buffalo throughout Southeast Asia, to later, significant methane emissions between 6,000 and 3,000 years ago. These models do not yet even include detailed estimates of the potential CO2 emissions caused by prehistoric deforestation across the entire tropics, as a product of both locally developed and externally introduced farming innovations. Although precisely and directly linking human activities to global atmospheric records remains challenging, and reconstructing the resulting climatic feedbacks is even more so, these examples illustrate how prehistoric farming activities in the tropics may have already interacted with planetary systems.10
The impacts of past tropical food producers on regional biospheres and geospheres can be a little easier to discern than those of their planetary counterparts, particularly in island ecosystems. The introduction of the domesticated pig and the Pacific rat as humans expanded across Micronesia and Polynesia are cases in point, resulting in observable soil erosion and the mass extinction of thousands of bird species, respectively (Chapter 8). Scientists believe that the arrival of swidden farming on another Pacific island, Guam, led to a complete environmental transition from forest to “savannah.” Such changes did not only occur on islands, however. In the Amazon Basin, cultivation of Indigenously domesticated plants involved significant landscape modifications to produce raised fields. Furthermore, the active selection and promotion of economically useful species by past human societies has shaped the modern genetics and distribution of rainforest trees across this vast region to this day. Working at Lake Caranã within a forest reserve in the Brazilian Amazon, paleoecologist Dr. Yoshi Maezumi of the University of Amsterdam and her colleagues were able to demonstrate that 4,500 years of forest management not only showed an enrichment of edible plant species represented in the sediment record of the lake but had also left its mark on the composition of the surrounding forests. Nonetheless, as Yoshi puts it, “the widespread role of humans in ancient deforestation and burning associated with these early tropical farming systems, as well as the legacy of human land use on biodiversity, soils, rainfall, and temperature, remains to be better-resolved through the comparison of detailed, well-dated multiproxy records.” The same is true across Southeast Asia, southern China, much of tropical Africa, and most of the island environments we have visited on our tropical tour, where current investigations are often limited to local comparisons and the integration of archeological and past environmental data into regional earth systems models remains a pressing area for future research.11
The impacts of food-producing societies on tropical environments and earth systems were, however, only to increase and, as Chapter 9 showed us, between 2,500 and 500 years ago, a variety of populous urban settlements, states, and empires had emerged in many of these regions. LiDAR and other remote sensing technologies have proven particularly powerful for revealing the sheer scale of the past populations buzzing across the tropics. In the Neotropics, in North and Central America, the Classic and Postclassic Maya occupied extensive urban landscapes, planted crops, and manipulated forests. Recent assessments suggest as many as 11 million people may have been present in the region in AD 800. Estimates of the deforestation associated with these populations have now been factored into climate models to suggest that Mesoamerican urban societies reduced precipitation by 5 to 15 percent in the region, amplifying droughts and potentially contributing to their own issues of sustainability in dry, lowland regions. This is not to mention the vast hydrological and agricultural modifications made by the Triple Alliance (Aztec Empire) across the Valley of Mexico and out into their area of imperial control. Turning further south, the Inka Empire ruled over approximately 20 million people, building massive terracing and irrigation networks that deforested mountains and hillsides. Inka rulers were continuously combatting soil erosion through the planting of trees and regulating the use of wood. Meanwhile, according to recent demographic estimates, another 8 million to 20 million people among the “garden cities” of the Amazon Basin, through significant earthworks, occupation mounds, and networks of causeways, were by five hundred years ago permanently shaping forest composition, dynamics of regrowth and succession, and perhaps even overall forest cover aboveground, as they simultaneously changed the nutrient contents and fertility of soil below. Although evaluations of their population sizes based on census data, later colonial records, and archeological records remain hotly debated, these precolonial Neotropical societies, additionally including the precolonial populations of the Caribbean, left their mark not just on tropical forests but also on the regional and continental biosphere, geosphere, hydrosphere, and, perhaps, as we will see shortly, the Earth’s atmosphere. Notably, however, these populations, even in urban systems, were often dispersed across the landscape, practicing a combination of crop cultivation, tree management, and wild animal use on land and in rivers, reducing the overall per capita pressure on each parcel of land at any given time. As a result, despite some local and regional challenges, a population approaching that of Europe was broadly being successfully sustained in the Neotropics by the time of contact with Europeans.12
Turning to Asia, the world’s largest preindustrial urban center had documented impacts on the dry tropical forests of Greater Angkor in Cambodia. Studies of lake cores and the urban infrastructure itself have shown that human deforestation across this vast artificially constructed drainage basin gradually led to soil erosion that contributed, alongside climatic variability, to rulers eventually abandoning the region as their capital. Limited paleoenvironmental investigation means that we have almost zero understanding of how the significant states and empires of West and Central Africa, southern China, elsewhere in Southeast Asia, and Sri Lanka and the Indian subcontinent impacted their surrounding tropical forest environments and thus earth systems, though current representations are almost certainly underestimates. In contrast to perspectives often put forward in conservation, ecology, and earth science circles, then, these environments were not a blank canvas for what came next. Instead, many millennia of increasingly intensive manipulation meant that human societies were already locked into earth systems feedback mechanisms with plant and animal communities, soils, and perhaps even local and regional climates. Some scientists have suggested that such examples are, to some extent, anecdotal and incomparable to what happened next, and they might be right—not necessarily only because these examples were on a smaller scale o
r because they were inherently less impactful when compared to the dominating impacts of humans on the tropics and earth systems today. Instead, while precolonial tropical societies modified their environments, often with far-reaching consequences, they clearly retained a certain flexibility to adapt. We have seen that, with some notable exceptions, elites, merchants, and, most particularly, farmers and horticulturalists in these societies could see what was happening around them, change where they planted and what they exploited, modify landscapes, shift their political centers and structures, or even simply move on. The viselike grip of proto-global and, later, global economic, political, and social structures increasingly swept these options from the board, particularly for marginalized, oppressed groups. This period set us on a course for the present day, where some of the people most impacted by anthropocene-related changes to tropical forests are also the least able to change their practices and protect their livelihoods.13
THE SIZE OF human populations and extent of their modifications across tropical forests by 1491 is dramatically illustrated by what happened as their worlds collided with Europe. We saw in Chapter 10 how Alexander Koch and his colleagues have used models of existing population estimates from historical documents, settlement areas, and other sources to estimate that within 150 years after European arrival, staggering numbers of Indigenous people across the Neotropics had died as a result of diseases arriving from across the Atlantic. However, he and his team did not stop their research there. Assuming a given area of land use by these Indigenous populations in different parts of the Neotropics, they sought to work out what would happen if these human societies were suddenly largely removed from tropical landscapes. They found that the estimated additional carbon uptake, as new forest growth rapidly occurred across the Americas in their absence, would have made up a significant amount of the seven-parts-per-million decline in global atmospheric CO2 concentration recorded in polar ice cores between the late sixteenth and early seventeenth centuries. This decline reduced global temperatures by 0.15°C. This might not seem like a lot, but it actually contributed to the coldest part of the Little Ice Age from approximately AD 1600 to 1650, when harvests failed across Europe and ice-skating on frozen lakes became a source of artistic inspiration. More detailed archeological-, paleoecological-, and historical-based models of changing land use are necessary to confirm the scale of this “reforestation.” However, for Alexander, the current balance of evidence suggests that “the ‘Great Dying’ of the Indigenous Peoples of the Americas, particularly within the Neotropics, initiated by European colonization led to a human-driven global impact on earth systems, with serious feedbacks occurring far away from the tropics, long before the Industrial Revolution.”14