Tropical forests are nature's great green engines, a lush green girdle across the earth's moist equatorial regions, occupying approximately 17 000 × 10³ km² in 1990. The turnover and response times of tropical forests are short, and hence the response to global perturbations may be rapid. The two prime candidates are tropical and temperate/boreal vegetation; analyses of CO₂ concentration gradients in the atmosphere suggest some form of equipartitioning between these possibilities. Long-term clearance of tropical rainforests was much less, with notable localized exceptions such as the areas occupied by Mayan civilization in the Americas and the Khmer civilization in Cambodia. Despite the gnawing threats of deforestation and degradation, there are still large areas of tropical forests that are among the world's great wilderness areas, with fairly light human impact. Tropical forests are a prime candidate for the CO₂ fertilization hypothesis, because of their intrinsic high productivity.