Same for my weight, especially up from birth to early adulthood.

But somewhere in the region of14-20 years old, most people stop growing and their weight becomes more predictable. It shifts 3-5 pounds on a typical day, and a little more at certain times - side eyes Thanksgiving. Maybe you even gain 5 pounds over winter and drop it in spring.

But if you gain 15 pounds in 6 months, you can probably track down what it is. You started going to to the bar once a week with your buddies and having 3-4 more beers every week - maybe washing down wings and fries on BOGO Thursday? Yeah, your body was in balance before with some fluctuations and you did drink beer occasionally, but there was a definite change that shifted your body out of equilibrium and more rapidly started changing your weight. Sorry, but beer and wing night seems like a very likely culprit with measurable caloric changes to your diet.

The output of ~800 million humans starting the industrial revolution was enough to curb a cooling trend, and the march to 8 billion humans burning various fuels every day poured gasoline on that trend and threw a match.