Scientists are asking: What is the best way to fight climate change? A new study says: Plant many trees; one trillion trees, maybe more. Swiss scientists say there is enough room for that many trees. In their report in the journal Science, they say even with existing cities and farmland, there is enough space for new trees to cover nine million square kilometers. That is about the size of the United States. Trees take carbon dioxide out of the air, and in return, put oxygen back into the air. The study estimated that over a long period of time, the trees could take in almost 750 billion tons of carbon dioxide from the atmosphere. That is about as much carbon as humans have put into the atmosphere in the past 25 years.