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:
1,000
3.00Earth's temperature has not reached 15 C over the 1960-1990 baseline in...well, no one knows an exact date, but never in the past 500 million years. Going from "roughly typical for the Holocene" to "hotter than it's been for hundreds of millions of years" in the span of a century almost certainly leads to the extinction of almost all multicellular life on Earth. Even if humanity could survive 15 C of warming, we would wish we hadn't.
@Boklam This is absolutely correct. There a lots of things that could rapidly reverse the warming trend. Large volcanic eruption for instance. We are long overdue for a supervolcano eruption.
@BTE A large supervolcano eruption would change global temperatures for about one or two decades according to the latest research; after that, temperatures return to the baseline determined by the atmospheric concentration of greenhouse gases.
Quick edit: It's also worth noting that in the event of, say, 3 degrees of warming by 2080 followed by a supervolcano eruption strong enough to still be disrupting the climate by 2100, overall warming would still be ~2 degrees. Short of incredible advances in carbon capture technology, Earth will almost certainly be warmer in 2100 than it currently is.