Reforestation in the 20th century probably caused a “warming hole” in the eastern United States. Since the 1930s, there has been a lot of reforestation, and around 15 million hectares of forest have been created since then.
At the end of the 20ths century, weather stations in forests showed temperatures on average up to one degree Celsius cooler than in areas that had not been reforested. The effect is strongest at midday in summer: a cooling effect of two to five degrees Celsius is then possible.