Greenhouse gas emissions continued unabated in 2021, and there is no sign that countries are going to honor their commitments to lower them anytime soon. This is evident as the West scrambles for ways to replace Russian fossil fuel supplies as quickly as possible. This means that fossil fuel emissions will not abate, and that we will reach a global mean surface temperature 1.5 degrees Celsius higher than the preindustrial level decades sooner than had previously been predicted. Indeed, 1.5 degrees is the threshold that climate science tells us spells the point at which the effects of global warming will become irreversible and devastating, and will have profound consequences to global ecosystems and to our global civilization.
These impacts are already bad and rapidly worsening. More and hotter heat waves, more frequent and worsening storms, catastrophic flooding, prolonged droughts, wildfires of unprecedented frequency and scale, continuing acidification of the oceans and consequent destruction of marine ecosystems, and a species extinction rate 1,000 times higher than the normal background rate are already wreaking havoc with planetary ecosystems and will only get worse. Feedback loops reinforce these devastating trends, and tipping points threaten to make then suddenly and rapidly turn even more deadly.
All of this promises that, rather than a gradual effect on humankind, the impact of global warming on human population may be rapid and catastrophic. The main compounding factor will be human migration. As heat begins to become an existential threat to the populations in the tropic and subtropic zones, up to three billion people will migrate towards the temperate zones. This is 40% of the human population, and its movement will present an existential threat to the populations in the temperate zones. The inevitable consequence will be conflict on a scale that dwarfs anything in human history.
This conflict, which will manifest itself on national borders across the world, may unleash the use of nuclear weapons that will make the loss of life even more devastating. The deteriorating climate and this unparalleled level of conflict will reduce food production and distribution, and this will further exacerbate the loss of life. As the pace of climate change continues to accelerate, this scenario is likely to play out in a matter of years, not decades, and consequently, the decrease in the human population will be sudden rather than gradual. The following graph shows a potential scenario.
This graph shows two population scenarios. The blue line is the UN’s low population scenario. The orange line is a scenario that anticipates a draconian impact of climate change on the human population. It envisions global population peaking in 2030. During the subsequent decade, the scenario projects that population will decrease by one billion as the reproduction rate even in developing countries turns negative, and as the global mortality rate begins to climb. Between 2040 and 2050, this scenario envisions the global human population falling by over six billion people as the human mortality rate soars due to the catastrophic effects of global warming, the reduction of food supplies, global warfare, and the destruction of the infrastructure and communication on which our global civilization depends.
This is speculation. There is no certainty that this scenario will play itself out. But it is not unreasonable speculation. Without a rapid and dramatic change in human behavior on a planetary scale, there is no reason to assume that we are immune from such a horrendous fate.