BuildingGreenwithWoodMODULE 9Climate Change
Greenhouse Gases,Carbon, and ForestsThe Greenhouse EffectThe glass panels of a greenhouse let in light and keepheat from escaping, providing warmth for the plantsgrowing in them. A similar process occurs when thesun’s energy reaches the Earth – some is absorbed bythe Earth’s surface, some radiates back into space,and some is trapped in the Earth’s atmosphere, whichkeeps the planet warm enough for life to flourish. Thisis called the greenhouse effect.The carbon cycle affects the amount of energytrapped in the atmosphere. Plants absorb carbondioxide and emit oxygen during photosynthesis;oceans also absorb carbon dioxide. Humans andother animals inhale oxygen and exhale carbondioxide. Carbon dioxide is emitted when substancesdecompose or burn.Scientists agree this natural balance has been upset.The biggest human cause is the amount of carbondioxide being released into the atmosphere throughthe burning of non-renewable fossil fuels, such as oil,natural gas or coal. Carbon dioxide accounts for morethan 75 per cent of total greenhouse gas emissions.Close to eight billion tonnes of carbon dioxide areemitted every year – most of this through fossil fuelcombustion and deforestation in tropical regions.Some is absorbed by water bodies, some is absorbedby forests – and some is emitted into the atmosphere.If too much carbon is emitted, it causes theatmosphere to trap more heat, warming the planet.Rising temperatures may, in turn, produce changes inweather, sea levels, and land use patterns, commonlyreferred to as climate change.Forests and the Carbon CycleQuantifying the substantial role of forests as carbonstores, as sources of carbon emissions and as carbonsinks has become one of the keys to understandingand modifying the global carbon cycle.In its Global Forest Resources Assessment 2005 4 ,the United Nations Food and Agriculture Organizationsays the total carbon content of forest ecosystemsfor the year 2005 is more than the amount of carbonin the entire atmosphere. Roughly half of total carbonis found in forest biomass and dead wood combined,and half in soils and forest debris combined.4Global Forest Resources Assessment 2005 (FRA 2005). Food and AgricultureOrganization of the United Nations. www.fao.org/forestry/fra2005.
Carbon sequestered in a typical 2,400-square-foot North American home is theequivalent of offsetting the greenhouse gas emissions produced by driving apassenger car over five years (about 12,500 litres of gasoline).Solid Wood and ClimateChangeUsing wood products that store carbon insteadof building materials that require large amounts offossil fuel energy to manufacture can help to reducegreenhouse gases in the atmosphere. Trees grownaturally, and the little waste generated duringprocessing is often used to meet the energy needs ofthe mill. At the end of their first life, forest productscan be easily reused, recycled or used as a carbonneutralsource of energy.A typical 2,400-square-foot woodframehouse contains 29 metric tonnesof carbon, which is the equivalentof offsetting the greenhouse gasemissions produced by driving apassenger car for five years (about12,500 litres of gasoline). No othermaterial offers this kind of carboncredit.wood or wood-based products be considered asthe main structural materials for new governmentfundedbuildings up to four floors. In Canada, thegovernments of British Columbia and Quebec havemoved to policies that encourage the use of wood inpublic buildings.Greenhouse gas emissions due to manufacturing30,00020,00010,00031% more greenhousegas emissionsAround the world, government andbusiness leaders are developingpolicies and procurement processesthat encourage the use of more forestproducts from well-managed forests.As part of its promotion of a carbonneutralpublic service, the Governmentof New Zealand is requiring that0Wood frame houseConcrete block houseLife cycle assessment is the appropriate tool for examining the carbon footprint of buildingmaterials because it considers the greenhouse gas emissions associated with their production,transportation, construction, use and eventual disposal.• In this graph, the embodied effects are shown for two typical, identical homes, one madewith wood and one with concrete. (Embodied effects are the environmental impactsassociated with manufacturing, transporting and constructing the houses – heating andcooling the houses are not included);• It shows that the concrete-block house resulted in 31 per cent more greenhouse gasemissions than the wood-frame house.
On the cover:The Craig Thomas Discovery and Visitor Center,Grand Teton National Park, WyomingArchitect: Bohlin Cywinski JacksonCanada has most of its original forest area. More than half ofCanada’s forest are naturally reforested, and this is supplementedby the planting of 600 million seedlings per year.Green buildings• Mitigate climate change• Use less energy and water• User fewer materials• Reduce waste• Are healthy for peopleand the planet