A fundamental tenet during the forest-climate debate was that evapotranspiration cools the surface climate and provides water vapor that condenses and falls back onto the land in a recycling of precipitation. It is this process by which tropical forests lessen the heat and promote rainfall through a positive feedback between forest cover and rain. The premise, too, of irreversible climate change, voiced in the decline of ancient civilizations due to deforestation, was common. In today’s climate science, the Amazonian rainforest is a case study for forest-climate coupling, and one in which deforestation is a possible tipping point that irreversibly changes the climate. Many participants in the debate speculated that planting trees in the Sahara would increase rainfall. A green Sahara has, in fact, occurred in the past and increased rainfall at the time. This, too, provides another climate tipping point. The boreal forest presents a further example of positive feedback by which the northward advancement of the treeline in a warmer climate decreases albedo and reinforces the warming; the southward retreat of the treeline with a cooler climate conversely exacerbates the cooling.