The date it started. Obviously, the rate of GHG emissions was low at first, but locomotives and ships were productive polluters.Prior to the Industrial Revolution, the Earth had been cooling for 5,000 years, apparently moving towards the glacial phase of our cycle.
When we started burning significant amounts of fossil fuels, temperatures began to rise and the increases in CO2 and temperatures have both accelerated since then.
What start date do you use for the Industrial Revolution?
This graph shows warming beginning about 1800. The steam engine was invented in 1712.