To what year do we need to return to prevent climate change?
As Gemechu Fanta Garuma commented, the IPCC Assessment Reports (at time of writing, AR5 is the latest) and its special reports are the best place to go for a comprehensive summary of the science as it was a small number of years ago.

In terms of greenhouse-gas concentrations that are stable and civilisation-friendly, the range 300-350ppm CO2 is as useful a range as we're likely to get for now - 350 is when coral bleaching starts to become a serious problem, and trashing the ocean ecosystem is a shortcut to threatening human civilisation, so we should probably avoid that. And for methane, say around 1000-1300ppb. 

It's not a question of "returning to a year". We don't need a time machine, and we don't need to wind back everything that's happened. I've no wish to return to 1980, for example, and I'm not sure why anyone else would (I was there and it was largely rubbish). This is not about undoing decades of human progress. What we do need to do is to keep the concentration of greenhouse gases in the atmosphere at levels that keep the climate fairly stable at the timescales of human lifetimes, so that we avoid massive disruptions and a potential threat to human civilisation. Had we started doing that when this was first flagged up as a major issue by scientists in the 1970s, we would have done it much cheaper, much sooner, and with less collateral damage. (like in the old proverbs: an ounce of prevention is worth a pound of cure; and a stitch in time saves nine)

We have already caused some climate change. It will take 30 years for the full effects of what we've done to date, to show up. We may already have caused trillions of pounds of damage to our natural capital - but we'll only know the full scale of the damage when it's far too late. Theoretically, we could avoid some of that damage in the next 30 years by putting the global economy to work to tackle the problem seriously. However, the political will just isn't there. Most of the people who'll suffer the worst of the damage are too young to vote, or haven't been born yet. Most of the people with the ability to do something about it (voters, politicians, business leaders) will be dead before the worst of the damage hits.

The Keeling Curve shows how atmospheric CO2 concentrations have changed over time:



(chart Scrippsnews - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=69539214)

As for the relationship between atmospheric concentrations of greenhouse gases, the change in average global temperatures, and actual changes in climate, that's a set of very complex relationships wrapped in uncertainties and unknowns. And these things are generally nowhere near as simple as a neat, reversible one-to-one function: even if we were to restore greenhouse gas concentrations to where they were in 1980, does not mean that the climate will return to just as it was then.  And that's why mitigation action is so very urgent: we are driving in thick fog very close to the edge of a cliff. The smart thing to do is to stop driving and work out what's going on. Which means getting to zero net emissions as fast as we can without destroying everything else in the process.