Trees remove Carbon Dioxide from the atmosphere as they grow. They do this by the process of photosynthesis.

Photosynthesis is the process by which plants … use the energy from sunlight to produce sugar, which cellular respiration converts into ATP, the "fuel" used by all living things. The conversion of unusable sunlight energy into usable chemical energy is associated with the actions of the green pigment chlorophyll. Most of the time, the photosynthetic process uses water and releases the oxygen that we absolutely must have to stay alive.

We can write the overall reaction of this process as:

6H₂O + 6CO₂ ==>> C₆H₁₂O₆+ 6O₂

Source: http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookPS.html

Trees use the sequestered carbon absorbed through photosynthesis to grow leaves, stems, bark and roots. The amount of carbon in forest soils can also increase over time. While the forest is actively growing and sequestering carbon, the system is termed a carbon sink.

The rate at which forests sequester carbon is influenced by site productivity characteristics such as climate, topography and soils, as well as tree characteristics and management actions. Following land clearing, timber harvesting, bushfire or disease, carbon is released back into the atmosphere through burning or decay. Source: http://www.climatechange.gov.au/land/forest-sinks.html

In 2006 hype was generated over the 'trees as methane emitters' issue following an often misrepresented study published in the journal Nature from the Max Planck Institute in Germany. Following intense media interest the team issued a statement on January 18 (2006) declaring plants are not to blame for greenhouse gas emissions and 'the most frequent misinterpretation we find in the media is that emissions of methane from plants are responsible for global warming'. Frank Kepplar at the Institute stated "Trees absorb carbon dioxide, the most important greenhouse gas, so planting them is still beneficial." http://news.mongabay.com/2006/0118-mpg.html

A team from the CSIRO has studied the issue. The following is from http://www.csiro.au/resources/PlantationsValidCarbonSinks.html: A team of CSIRO scientists led by Dr Philip Polglase tested this assumption by comparing estimates of methane emissions for seven different regional case studies of afforestation in Australia with the amounts of carbon stored to determine the overall effect. They calculated that the average amount of methane emitted would off-set less than 5 per cent of the amount of carbon dioxide stored in terms of its effect in contributing to global warming. They conclude that the use of new forests as carbon sinks remains valid. The Department of Climate Change notes that: ' Trees and other plants take up (sequester) carbon dioxide from the atmosphere as they grow, through the process of photosynthesis. This decreases the concentration of carbon dioxide in the atmosphere and helps reduce the greenhouse effect. Trees use the sequestered carbon to grow leaves, stems, bark and roots. The amount of carbon in forest soils can also increase over time. While the forest is actively growing and sequestering carbon, the system is termed a carbon sink' http://www.climatechange.gov.au/land/forest-sinks.html

There is no real scientific dispute that trees and vegetation take carbon dioxide from that atmosphere as they grow, and store the carbon, releasing oxygen back out at a rate far greater then any emissions they might be responsible for. They have an important role in addressing climate change by helping to reduce atmospheric carbon dioxide levels.

There are two main natural mechanisms by which CO2 is removed from the atmosphere: the oceans and vegetation. The impact of vegetation can be identified seasonally as there is an uptake of CO2 by plants in spring/summer and a release of CO2 by the decay of dead plant material in fall/winter. This trend is most clear in the norther hemisphere, where most of vegetation on land is situated.

See below for the CO2 concentrations between 2004- 2009

The above graph clearly demonstrates the seasonal capacity of the earth to absorb carbon dioxide annually.

At current the world is covered by 30% forests or less than 4 billion hectares. This is at least one third less than before the dawn of agriculture. Global deforestation continues at 13 million hectares per year. We need to replant the earth to make sure the 'breaths in' are bigger than the 'breaths out'. Source: http://www.unep.org/billiontreecampaign/FactsFigures/QandA/index.asp