Trees do need carbon dioxide, but an excess of carbon dioxide thickens the layer of greenhouse gases in the atmosphere, meaning less of the heat radiating from the earth can escape into space. It works like a car parked in the sun; the sun's energy streams through the glass windows, heating up all the mass of the car's interior (seats, dash, etc.), and then the heat radiating from all this mass warms the air in the car. Very little of the heat can escape because the windows and doors are closed. If you left your windows open, the car would be much cooler when you returned. The upper atmosphere acts like a permeable tinted window. It deflects a lot of the sun's energy into space, which prevents the earth from overheating. It also allows radiant heat to escape, thereby regulating the earth's temperature within a viable range.
More trees would actually help, but because of other limiting factors, such as soil nutrients, planting more trees would require returning some cleared land--maybe a lot--to forest. Much of the world has been deforested, and deforestation continues at a tremendous pace, partly because of greed and partly because of increased population pressure. It's unlikely we can control either of these any time soon.
actually the scientists involved admitted that all the leaked emails were authentic.