Global warming and rising carbon dioxide levels in the atmosphere may delay the usual autumn spectacle of changing colors and falling leaves in northern hardwood forests in the future, and increase forest productivity.[26] Specifically, higher autumn temperatures in the Northeastern United States is delaying the color change.[27] Experiments with poplar trees showed that they stayed greener longer with higher CO2 levels, independent of temperature changes.[26] However, the experiments over two years were too brief to indicate how mature forests may be affected over time.