Trees certainly do capture carbon dioxide from the atmosphere, and so more trees can really help in the fight against global warming.  But  trees do much more. They cool us, give us shade and regulate climate. Forests of trees are the prime regulators of the water and energy and carbon cycles.  Tree cover affects climate locally, regionally and at the continental level. 

Forests are important regulators of global temperatures and fresh water flows.  Plants bring water from their roots and transpire it through their leaves during photosynthesis thus recharging atmospheric moisture and contributing to rainfall locally and in distant locations. 

This “biotic pump” hypothesis was initially proposed by two Russian atmospheric physicists in 2007 to explain how forested regions can generate very significant large flows in atmospheric water vapour due to evaporation and condensation generating atmospheric pressure differences and sucking in moist air from over the ocean. The forests of the Amazon function in this way, as did centuries ago the Tanami Wind. On the east coast of Australia, rain forest once ran from Cape York to Melbourne in an almost unbroken line.

At that time, a moisture-laden thermal, colloquially known as the Tanami Wind, carried this warm, moist air to the centre of Australia. In Mataranka, NT, the Dreamtime stories of the Mangarayi and Yungman tribes describe a rain-laden wind sweeping in from the east that created the area. In the two centuries since Europeans came to Australia, almost all of that original rainforest is gone, and with it the great Tanami Wind.

Forests cover only about one third of the surface of the Earth.  It is estimated that perhaps as much as one sixth of the current global warming trend is due to deforestation.  Even if you planted only one tree, you have made a step in the right direction in the battle against climate change.