I disagree with the general idea here that changes in technology, changes initially available only to the few (especially on a global scale), is the definition of “dramatic global change”. Yes, for a hypothetical man born in 1830s England, one able to afford the new gadgets produced in his lifetime, his “world” changed in ways incomprehensible to someone born 2,000 years earlier. However, for a woman born in Calcutta on the same day, her life was unaffected and the traditional rhythms of life continued largely unaffected by these inventions and technologies.
Except for things involving the British. And they had already been there for 200 years, which, for her… and billions like her worldwide… was the first harbinger of the Shape of Things to Come.
The overall trend of human history, since 1 Million BC, was towards a world of differing civilizations so that by, say, 1500 CE, there were, at least, 7 distinct centers of civilization in the world where the ways people organized their lives differed dramatically - from their religions to the shapes of vehicles used to carry goods (cart? Llama? Horse?) to how they organized their lives governmentally and more. (Off the top of my head, in 1492 there were the civilizations of the Inca, West Europe, Chinese, Indian, Japanese, Islam, North American Nations… who am I forgetting?)
Today that is no longer the case. The arc of human history shifted, for the first time ever, to a world dominated by the extraordinary success of one of those centers of civilization, a capitulation so total to the point where it, and it alone, remains as a true civilization, having replaced the others and reducing them to differing “cultures” within the gambit of what is no longer considered “western civilization” but “modern civilization”, the “developed world” as it is referred today.
This process of global integration began in 1350-1570, which is why that period received my vote. The integration of the entire globe into one civilization is the single biggest global historical development since the dawn of history. Bigger than literacy. Bigger than WW2. Bigger than industrialization. A lot of technological, social, organizational, and economic developments occurred which made this happen, yes, but the eventual result is greater than the sum of the pieces.
Think about China: A proud civilization for 2,000 years, finally having, for survival in the new “Western” world, to form a national government dedicated to the ideals of a German Jew who never went east of the Elbe river, spoke of Asia with contempt, and developed a philosophy/economic theory based upon his and a friends studies of the working class in Britain… then, because they saw America winning the Cold War, modified their economy and society to where they can follow the dictates of a Scottish moral philosopher and his intellectual descendents, integrating into a global economy which got its start in Italy with the use of double-entry accounting and the merchant fleets of Genoa and Venice.
And this cultural osmosis was exclusively one-sided - Western Civilization never adopted the ideals of the world except at the most superficial of levels, no, they made the world accept the ideals of Western Civilization.
And this pivot began in the 1350-1570 period, which is why it receives my vote.
However, I can see arguments for the last two periods (1980, 2001) as those are the periods when the benefits of Western, er, “modern” civilization began to be felt on the scale of billions achieving longer lifespans, greater literacy, more calories, industrialization (and post-industrialization), rising incomes, increased pollution, more material goods. And we may be coming upon a new age in Western/Modern Civilization, where the development of the civilization is no longer in the control of the ethnic group which did so much to spread it globally, but in the hands of the supposedly vanquished Chinese/Indian cultures.
But that last is another thread.