Global warming is caused by a complex interaction of many factors.
Trees have been cut down at a rate that exceeds regrowth in many parts of the world. This not only means there are fewer trees to absorb the greenhouse gas carbon dioxide, but the carbon dioxide locked up in the trees is released as they are burnt for fuel, or decay. Lack of trees therefore increases the amount of carbon dioxide (CO2) in the atmosphere, which traps heat. (Trees are and were cut down for fuel, building materials and to clear land for farming).
There are many more issues though. CO2 is also released by burning fossil fuels. Coral absorbs CO2, but coral is dying due to environmental changes. The cold oceans can hold a lot of CO2, but they are warming up so hold less of it.
Also, there is the planetary albedo to consider. Albedo is a measure of white-ness. The whiter a planet, the more light and heat it reflects into space instead of absorbing it. (This is one reason it is hard to come out of an ice age - the planet is so white it reflects the very heat it needs to warm up). As the ice caps melt, we have a lower planetary albedo, meaning the planet is absorbing more heat from the Sun. Clouds can contribute to albedo though - so all those aircraft contrails are actually helping to increase the albedo.
A big part of the complexity is that one environmental effect feedbacks onto others. Melting icecaps feed cold freshwater into warm salty seas, which changes current flows. The currents help distribute heat across the world, but this can stop due to the cold/fresh water, causing Britain to get colder for example. What effect does this have on the albedo? Does it increase (due to more frost) or decrease (due to less cloud)? I don't know.
Planting trees is not the answer. It would slow down the rate of CO2 increase for a while. But once the trees need to be felled or die off, all the CO2 they absorbed would be released again. This is really the problem we are facing just now - all the CO2 from fossil fuels comes from millions of years of life absorbing CO2 from the atmosphere, then being buried in the ground. Really, we have just been living in a period of low-CO2 levels, with correspondingly cooler temperatures. The fossil record indicates that the Earth used to be hotter and had more CO2, we're just returning to this natural state because we've released the trapped CO2.
A better idea would be to stop felling the rainforests. They are natural ecosystems that absorb CO2 and harbour a diverse range of life. They won't solve the problem (too much CO2 from fossil fuels) but will slow it down (and the life there will continue). More radical ideas to scrub CO2 from the air are needed if we want pre-industrial revolution CO2 levels. Realisticly, we should just concentrate on adapting to the warmer environment. If there's one thing we humans should have learnt by now, it's that our actions are harmful to our species in ways we can't predict. For example, if we stop global warming, how do we know we won't trigger an ice age? Or a Snowball-Earth scenario?
