There could be different weather patterns that affect agriculture, drinking water supplies, flooding, or sea level. It also could help some plant species, such as weeds, gain new ability to compete. The other problem is speed of change. Past climate changes have been slow, occurring over thousands of years, which gave natural systems and people time to adjust. This one could be far more rapid than anything in the historical record, raising concerns about whether natural or human systems can adequately respond.
Trees are an important part of how our planet regulates the carbon dioxide level in the atmosphere.
The world's forests gather in and store vast amounts of the carbon in carbon dioxide. This action helps to lower the greenhouse effect and moderate the climate. City trees also contribute by shading buildings and reducing energy consumption. The fundamental split is between projects that protect existing carbon reservoirs by stopping deforestation and those that create new forests specifically to be carbon reservoirs. In the former case, the assumption is that logging and manufacturing will release a significant proportion of the carbon held in a mature forest.