What we want out of life comes at the expense of Nature. This fact drives our relationships with each other and the other species we share the planet with, as well as how long and how many of us can survive.
No matter what group or groups we might be part of, we are all part of one group: the human species. The number of people in that largest group, its population, has been growing since the beginning of civilization, reflecting the value we collectively place on people. Like other animals, each of us requires a minimum amount of resources to survive, so the amount of resources we consume increases with how many of us there are. Since the populations of other species use or embody much of those same resources, the resources must be shared over time for the system of life to last as long as possible.
Most of us would prefer to live long and healthy lives. Because sharing resources is so critical, and we embody resources that other species can use for food, extending our lifetimes reduces the amount of resources available to other life during the time they are needed by those other species. Thanks in large part to our ability to develop and share knowledge and skills, and translate those things into material objects that can accelerate the process, we have enabled more people to live past birth and to thwart more of other species' attempts to consume us. As a result, we have increased consumption and more than tripled the average amount of time people can expect to live from birth (life expectancy) since the beginning of civilization.
We would all like to have satisfying lives. Happiness means different things to different people, but statistically it tends to increase with consumption in much the same way as life expectancy. This can be interpreted to mean that it is an expression of how well people are able to match their lifestyles to their personal preferences, which is enabled by economies that distribute resources among people through trade. Economic activity, which is easier to measure than happiness and tied more directly to consumption, is therefore often used as a proxy for happiness.
The Timelines model projects that, if we live like people in the simulated scenario Timeline 2, then global population, life expectancy, and happiness will reach their maximum average values between 2020 and 2022. The good news is that most people will get to see the pinnacle of human achievement, even though it won't feel good for half of them. The bad news is that life expectancy and happiness will reach zero for everyone between 2029 and 2030, and no one will be left alive by 2038.
Since Timeline 2 has 82% of its recent history in common with ours, it makes sense to use it as a baseline for discussion of the future. The values of population, life expectancy, and happiness are all dependent on what fraction of remaining resources people minimally consume: as the fraction grows, the values grow until the fraction reaches about 57%, falling to zero for fractions larger than that.
If we don't want the values to drop (especially for population), then it makes sense to keep the population below its peak (affecting minimal consumption) and our additional consumption from growing (affecting the remaining resources). This is the logic behind the Fix timeline. We could alternatively let events unfold without intervention and hope that people will not overshoot the peaks; but if they do, that the apparent desire to consume more will be overcome by a desire to seek out the peaks, and they will voluntarily reduce their consumption to reach the peaks again.
The Fix and the "wait-and-seek" strategies include an implicit assumption that depletion of remaining resources is totally within people's control. The depletion beyond the economic equivalent of renewable products and services provided by other species is largely due to harm and killing of the species that provide them, and harm to species that those species depend on for survival. Those effects are caused by the same drivers as extinction: habitat loss, alteration of ecosystems by invasive species, pollution, use of common resources by the human population, and direct killing due to hunting and over-harvesting. Pollution in particular is having a greater role by changing the climate, whose predictability all species (including us) depend on for a variety of deep biological reasons, not the least of which being growth of plant life needed for food and oxygen production. When changes people make to the environment lead to cascades of changes that are self-reinforcing (positive feedback), then depletion multiplies beyond control.
In Timeline 2, the "waste" component of consumption (consumption in addition to basic needs and wants) accelerates its increase while population drops after the population peak. This may indicate that during that period it will be self-sustaining. If so, perhaps the significance of the population peak – and maybe the others – is that humans in Timeline 2 have a biological self-destruct trigger that activates when their actions initiate uncontrolled collapse of the biosphere they depend on for survival. The Fix timeline would therefore be impossible without some extremely powerful, and as-yet nonexistent, technology that could repair the damage as it occurs.
Another timeline diverged from ours and Timeline 2 in 1939. World War II was averted there, resulting in slower development of technology and science. The same desires and constraints existed, however, and the population peak was only delayed another 60 years...