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:
Earth has not hit 10 C over the 1960-1990 average since 40 million years ago, during the early Eocene. There was no ice on Earth, a very small temperature gradient between the equator and the poles, and sea levels were 150 m higher than current levels, or enough to submerge northern Europe, most of the eastern U.S., most of eastern South America including the Amazon, most of India, most of Australia, and every coastal city on the planet. Roughly one third of all humans on the planet live less than 100 m above sea level.