Temperature

Assuming my modeling of population and consumption is correct, then the famous 2° Celsius limit for global warming by century's end is twice what it should be. According to my first attempt to incorporate global warming into the model, if we are successful and the warming is already self-sustaining then we need to immediately start reducing our per-capita ecological footprint by at least 0.7% per year to avoid casualties between now and the year 2200.

A decline in total ecological resources due to degradation will have the same effect as consuming too much, eventually making it impossible for people to survive and our population will crash. Whatever causes it (global warming as an example) must be stopped before that critical threshold is reached, otherwise all we can do is delay the end date.

If, as I expect, humanity will soon be forced to consume less overall (through personally cutting back, losing population, or both), then our slowing rate of pollution will enable natural systems to process the lesser amounts resulting in the approximation of no net increase in the amount, and eventually a decline. In the case of greenhouse gases, I've assumed no decline in the next two centuries, which means that temperature (their effect on the environment) will not decrease either. As far as I can tell from my data, that effect has been masked by our overall consumption, so it hasn't yet resulted in a decrease in total resources; but with us now pushing against the envelope of those resources, there won't be enough left to both process our waste and provide for the survival of the species we directly depend on.

Perhaps by coincidence, my projected temperature will match with the historical trend in 2019, and others who are planning for future emissions seem to be targeting 2020 as their starting point. Also, I projected that direct emissions will decrease around the same time, except for short pulses corresponding to attempts to reach the resource limit after drops in population. For these reasons I chose 2019 as the starting time for a hypothetical decrease in total resources responding to global warming, and for attributing the difference in emissions to other factors that make it self-sustaining so that the temperature trend continues into the future.

The result, which is as close as I can currently come to a representation of future global warming, has consequences much worse than the case I first presented above, which is the best my model can achieve in terms of avoiding casualties with declining resources. Whereas my default case with no resource decline projects a world population of 5.8 billion people by 2200 (a "loss" of at least 1.5 billion), the global warming case projects that everyone will be dead by 2165. Adaptation in the form of limiting population and consumption growth adds only four years to that end date. For reference, in most scenarios I've looked at, the temperature above preindustrial times when the population crashes is about 2.5° C (it is currently 0.7° C, and would be 1.7° in 2100).

Climate Threshold

The news about global warming has been getting progressively worse, as detailed in a number of places, especially Climate Progress. It appears that severe climate change may be unstoppable, and could get much worse if we don't take immediate and drastic action to reduce our carbon emissions. This is largely due to the growing threat that permafrost will melt that contains the potent greenhouse gas methane, releasing it into the atmosphere. Within the next few decades, adaptation may be all but impossible for us and many other species; and for those species who can't adapt, entire ecosystems could collapse due to their absence. The Earth, for them and us, will become uninhabitable.

Both what we use and what we waste comprise our consumption of resources, and particularly our ecological impact (which I've been calling “consumption of ecological resources”; or, more recently, just “consumption”). Based on recent data, carbon dioxide emissions appear to be proportional to the square root of the cube of consumption (consumption to the 1.5 power), and the range of projected temperature rise due to global warming appears to be roughly proportional to the cube of consumption (the square of emissions).

Interestingly, the assessment by climate scientists of how much climate change we and other species can adapt to coincides with my estimate of our consumption of all global ecological resources (see “Habitability Limit”). If we somehow are able to keep our consumption growing, despite loss of ecological services provided by other species, we will drive them – and us – toward extinction, with the rapid increase in temperature acting as a proximate if not ultimate cause.

What I consider more likely is that our consumption and population will peak just before we can compromise other species' ability to maintain habitability. When our consumption drops to preindustrial levels, temperature may still be high due to the persistence of our previous emissions, the trapping of heat by the atmosphere, and the feedback effects we have unleashed. The populations of other species may not recover as expected for similar reasons: artificially modified areas take time to revert to natural habitat; devastated populations must breed and adapt to new conditions through evolution and learned behavior; and broken relationships between species due to death, migration, or ecosystem collapse may cause further devastation. Even if we manage to reduce our consumption without killing ourselves off, what to me is our most urgent priority as a species, the Earth will likely take a long time to heal.