Global warming is believed to have started in:

Global warming, the long-term heating of Earth's climate system, is widely believed to have significantly accelerated and become observable during the Post-industrial era. This period, generally considered to begin in the mid-18th century and intensify through the 19th and 20th centuries, saw a dramatic increase in human activities such as the burning of fossil fuels for industrialization, deforestation, and agriculture. These activities released vast amounts of greenhouse gases into the atmosphere, trapping heat and leading to the warming trends we observe today.

• The "Pre-industrial era" is incorrect because significant human-induced warming began after this period.
• "Modern industrial era" is too narrow; the start of the warming trend is rooted in the initial post-industrial expansion.
• "None" is false as there is a well-established historical context for the onset of global warming.