How can the fertility rate be below 2 but the number of births is greater than deaths (South Korea)? I read an article published in 2020 which claimed that the South Korean population had declined "for the first time", the number of deaths (~300,000) outpacing births (~275,000). However, this would seem to be mathematically impossible because the birth rate has been below 2 in South Korea for decades (see chart below from World Bank). How is it even remotely possible?

Update: Based on one answer there is the idea that the increase in life expectancy could be an explanation. This seemed improbable to me originally, so I didn't consider it. But in any case, let's consider it:

So we see the life expectancy has increased from say 80 to about 82 apparently between 2010 to the present time, while the birth rate has been well below 2 that whole time. So, I guess I don't know the math of this. How do you compute birth rate vs life expectancy?

If life expectancy rises quickly enough, the population can grow, with births outpacing deaths, even if the birth rate is below 2, and this can indeed continue for a long time (essentially, the length of time this can happen is not mathematically bounded, but only by a slowdown in the increase of life expectancy). Importantly, the "kind" of life expectancy which matters is not that at birth, but the probability of older people to die in an increment of time.
Another effect that could lead to growing populations in spite of low birth rates could be net immigration, but I would not expect that to be very important in South Korea.
