Overshooting global warming and overshooting fertility decline. Beyond the smooth stabilization paradigm

Our thinking about future trends in both population and climate change has traditionally been dominated by the view of smooth trajectories towards ultimate stabilization. But reality turns out to be different: climate warming will not stop at the Paris goal of 1.5 degrees Celsius, but is expected to overshoot, and will therefore need to be addressed through negative emissions (taking carbon out of the atmosphere) later in the century; similarly, fertility decline has not stopped at the replacement level, and instead seems to be falling to lower and lower levels, with nobody knowing when it will stop and whether there will be an upturn. In both cases, societies will need to adapt to rather extreme discontinuities, rather than being able to count on smooth stabilization. Furthermore, the reality that climate change overshooting will require serious mitigative action during the second half of this century brings alternative demographic trends back into the picture as potentially relevant factors in mitigation, since alternative fertility and education trends in the near future will take decades to be reflected in changing population sizes and structures, including in human capital.