A modern 1877 scale event would probably immediately fuel (pun intended) a furious climate change debate, even though attributing the existence of the El Niño itself to fossil fuels would not be scientifically justified... or maybe was the 1877 El Niño caused by fossil fuels (so many Wars happened in those years) ?
Nobody in planet modelling has ever suggested that the 3 phase El Niño–Southern Oscillation (ENSO) exists because of fossil fuel usage.
An extreme swing, here and there, has always been part of its nature.
The observed change is more and more extreme swings, greater amplitude, higher frequency.
in one hand i wanted to highlight that if it was today, we - as civilization - would definitely create a link between the events, based on our "feeling"
in another hand, i question if we really could rule out, since back in the days, everything that we have was coal.
If you care to get analytic, it's related to the insulation factor of the atmosphere, fairly consistent amounts of sunlight in (subject to know solar variation and orbit) that heats land and sea surface radiating thermal energy out to space with a "small" amount trapped to maintain a globally mean amount of energy that powers climate cells and cascades into other forms.
The global amount of coal use in the 1880s was non zero, sure, but overall the CO2 emissions from those tonnages were minuscule compared to modern emissions within the past 50 years .. and we have direct libraries of atmosphere created since the early years of the Cold War and many pocket captures for the past 200 years, with proxies (such as ice core bubbles) going back far longer.
ENSO predates heavy coal use - so it's not something created by human activity, and as noted the first uses of coal were barely enough to move the needle (recall that collieries kept records so we have a fair handle on tonnages extracted).