The condition is met if we ever cross the threshold, even if global temperatures are later brought below it. The resolution criterion is met if world population ever dips below 1 billion for any length of time before 2100.
Similar markets for various amounts of warming:
It is scary to consider the possible repercussions if global temperatures rise by 2 degrees Celsius over pre-industrial levels, as per the conclusions of the NASA-led study. According to the study, compared to the mid-20th century, over 25% of the world's population may endure one more month of extreme heat stress annually, and areas like the American West and the Amazon may see exacerbating impacts like a rise in the danger of wildfires. The downscaled predictions highlight the rise in climate extremes, such as heat stress and fire weather, and offer comprehensive climate data through the year 2100. Communities and businesses around the world are seriously threatened by the concomitant escalation of many climate variables.
Personally I’m ambivalent on this question objectively speaking. If we can demonstrate that this level of climate change will cascade into a series of events that will lead to a massive die-off, or a massive kill-off (by means of nuclear war, genocidal ideologies, weaponizing AI, etc.) then sure, we can expect the world population to drastically increase by then.
As they say, I pray for the best, but plan for the worst. I think our odds are much better than it seems, and that the better angels of our nature will prevail. Or barring that, there are plausible game-theoretic solutions which make it extremely unlikely that we’ll ever let things get that bad…