The global population explosion of the 20th century continues to have a profound effect on the Earth and its environment. The United Nations forecasts that the world population will reach 8 billion on November 15 2022, and will continue rising to peak at 10.4 billion in the 2080s.¹ However, the rate of growth has slowed dramatically compared to the mid-20th century. The “demographic transition” — the shift from the high birth rates and high childhood death rates characteristic of premodern societies to a regime of low birth and death rates — is nearly complete. Only a few areas of the world, including parts of South Asia, some Middle Eastern countries and most of Africa south of the Sahara Desert, still exhibit high rates of population growth.

The people responsible for most of the historical emissions leading to climate change — and certainly for the bulk of the per capita emissions — are the 1 billion or so people who live in the wealthy, developed countries. The United States alone, with less than 5 percent of the world population, is responsible for 20 percent of historical emissions, more than any other country.² Catching up fast on emissions are the residents of middle-income countries like China and India, where fossil-fuel use is booming as those countries climb out of poverty. India will shortly surpass China as the world’s most populous country, but it appears to have both the ability and willingness to bring fertility down over the next decade or so in the country’s remaining hotspots. Whether runaway global warming can be averted depends more on whether these countries can leapfrog to clean energy than it does on their population trajectories.

The population explosion has had its most profound influence on the course of climate change through the increase in the global demand for food. In the 19th and early 20th centuries, forests were chopped down across the Northern Hemisphere so that land could be converted to farming. This trend long ago ran its course as northern agriculture grew more intensive and efficient, and many of those northern forests are now regrowing, helping to soak up a significant fraction of humanity’s carbon emissions.

The cycle is now playing out in the tropics, where deforestation is an acute problem, as Figure 23 shows. Three basins account for most of the world’s tropical forests: the Amazon in Brazil and neighbouring countries, the Congo basin in Africa, and the islands of the Indonesian archipelago. Under previous governments, Brazil succeeded in cutting the deforestation rate in the Amazon, but the gains have been partially reversed under the far-right government of Jair Bolsonaro. Deforestation in Indonesia has been occurring at very high levels, with entire landscapes being denuded in order to plant palm-oil farms to supply the global market. Outside pressure has helped to bring it down somewhat. Much of the deforestation in the Congo basin was historically done by families gathering wood for subsistence cooking, but commercial deforestation there is rising. Among the coming stresses on the forest are new concessions for the production of oil and gas.³

Overall, global deforestation has been declining for the past several decades, with ups and downs.⁴ It needs to stop immediately, not only to permit continued uptake of greenhouse gases, but to salvage what is left of the biological richness of life on Earth. (See “The Sixth Extinction” below for further discussion.)

Among the most promising opportunities of coming decades is to turn nature into a greater ally in the fight against global overheating. Trees and other plants already absorb some 30 percent of humanity’s emissions of carbon dioxide.¹¹ Planting more trees across degraded lands and on the margins of agricultural land could have enormous benefits, though doing it at scale will require large amounts of money. Other potential interventions include managing soils in a way that they accumulate carbon, turning the land surface into a giant carbon sponge. That can be done with methods like planting cover crops, but it costs farmers more money than they see in immediate economic benefits, so it will likely be necessary to pay the farmers to manage their land differently.

Such nature-based solutions are already being turned into projects that create carbon “offsets”, which are essentially certificates that are sold into a voluntary market of companies and people wanting to reduce their emissions. However, some of these offsetting programmes have received intensive scrutiny and criticism on the grounds that they promise more than they deliver, such as “saving” forests that would not have been cut down anyway.¹² One of the great challenges of our time is finding ways to steer more private money into these kinds of projects, while at the same time setting strong rules to guarantee their environmental integrity.