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:
I don't think people really comprehend what 5 C of warming looks like:
A world 5°C warmer than today could expect falling harvests in developing and developed countries, sea level rise threatening many major cities, and significant water shortages.
More species would be facing extinction (10 percent of species are thought to be at risk for every 1°C of warming), there would be more (and more intense) extreme weather, and a growing risk of abrupt and major irreversible changes in the climate system.
Among the foreseen consequences are:
the inundation of coastal cities;
increasing risks for food production potentially leading to higher malnutrition rates; many dry regions becoming dryer and wet regions wetter;
unprecedented heat waves in many regions, especially in the tropics;
substantially exacerbated water scarcity in many regions;
increased frequency of high-intensity tropical cyclones;
irreversible loss of biodiversity, including coral reef systems
A global mean temperature difference of 4°C is close to that between the temperatures of the present day and those of the last ice age, when much of central Europe and the northern United States were covered with kilometers of ice, and the current change—human induced—is occurring over a century, not millennia.
2 degrees of warming is manageable, 3 degrees is bad, 4 is catastrophic, 5 is apocalyptic. FWIW I think we're heading for ~3 to 3.5 C of warming by 2100 - very bad, but probably not human extinction levels of bad.