At the start of the century, we had 6 billion people. 6 years ago, we hit 7 billion people. As of last year, we have 7.4 billion people living here, mostly from poor countries. By 2030, we will have an extra 2 billion people considering the rule of 40+ million people added each year. With more people, we will have a water crisis, which means a food crisis, killing people from starvation. Will we have a future with an overpopulated world of which we will have to conquer the entire solar system to sustain ourselves, or will we have a fine future?
To understand the population crisis, we have to understand the demographic transition.
Let's go back to the 1750s, where everywhere (including Britain) was in stage 1.
Stage 1 explained:
Then the industrial revolution happened and bought the greatest living condition changes since the agricultural revolution; which had happened thousands of years earlier. People went from being peasants to workers. Items were mass-produced and caused advancements in medicine, food, and sanitation. Britain essentially stepped up the living game with this. Stage 2 went on, with high birth rates and low death rates causing a population explosion. Stage 3, birth rates slow down, stage 4 balance of birth/death rates.
"But with all countries in 4 stages everything will be fine right?"
No. You see, let's take the country Nintendo was made in as an example. Japan is actually in a reverse population crisis (falling population). Since the average lifespan for the Japanese is over 80 due to their fish-rich diet, the fertility rates are falling because the females are working for money. As of 2017, Japan has 126,045,211 people and the number is actually lower than 2015 (126,573,481) and by 2100 it will only be 83 million. So saying that the demographic transition prevents crisis is like trying to make a good piece of music with no software. Surely, you could do it but it wouldn't end well.
Another factor is child mortality. We had quite a fall in child mortality; global child mortality fell from 18.2% in 1960 to 4.3% in 2015. While that may not sound quite a lot, it's still quite a deal with the explosion. If you think that's high, in 18th century Sweden every third child died, and in 19th century Germany every second child died. This is why programs that help advance education and lower child mortality are very important. So for now we shouldn't worry about population crisis.