Global Warming

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Global climate change is probably the greatest threat we face today (IPCC, 2007) (Figure 2.1). A wide range of greenhouse gases must be considered (Table 2.4). Carbon dioxide comprises over half of all greenhouse gas emissions.

The three principal anthropogenic sources of these climate gases are energy production, chemical industry and waste cycles. Of these, the energy related sources dominate. They stem mainly from fossil fuel combustion in power plants and the transport sector.

Table 2.3 Natural occurrence of the elements in the accessible part of the Earth's crust

Source Hagg, 1984. See also the Periodic Table in Figure 4.1.

1-

Amount (g/ton)

Elements

Û.

Greater than 100 000

O, Si

100 000-10 000

AI, Fe, Ca, Na, K, Mg

10 000-1000

H, Ti, P

1000-100

Mn, F, Ba, Sr, S, C, Zr, V, CI, Cr

100-10

Rb, Ni, Zn, Ce, Cu, Y, La, Nd, Co, Sc, Li, N, Nb, Ga, Pb

10-1

B, Pr, Th, Sm, Gd, Yb, Cs, Dy, Hf, Be, Er, Br, Sn, Ta, As, U,Ge, Mo, W,Eu, Ho

1-0.1

Tb, I, Tm, Lu, TI, Cd, Sb, Bi, In

0.1-0.01

Hg, Ag, Se, Ru, Pd, Te, Pt

0.01-0.001

Rh, Os, Au, Re, Ir

Source Hagg, 1984. See also the Periodic Table In Figure 4.1.

Source Hagg, 1984. See also the Periodic Table In Figure 4.1.

2.3.1 Climate emissions from the building sector

The construction sector is responsible for a large part of the total global emissions of climate gases. This has been estimated to about 30-40% (United Nations Environment Programme, http://www.unep.org) and relates to operational emissions (heating, lighting, etc) on the one hand, and on the other hand emissions related to production, maintenance and demolition.

Impacts related to the production of materials correspond closely to the embodied energy in the materials (Chapter 1, page 19) though chemical emissions from the products can also play a role. An important example is calcination of lime during cement production, where large amounts of CO2 are released. Other significant contributions come from volatile organic compounds (VOCs) in the paints industry

Projected surface temperature changes for the late 21st century (2090-2099). Temperatures are relative to the period 1980-1999 (IPCC, 2007).

Projected surface temperature changes for the late 21st century (2090-2099). Temperatures are relative to the period 1980-1999 (IPCC, 2007).

Table 2.4 Important greenhouse gases related to the production, use and waste management of building materials

Substance

CAS No

GWP [kg COz-ekv./kg]

Possible occurrence

Carbon dioxide

124-38-9

1

Processes based on fossil fuels, cement and lime production, waste treatment (incineration)

Chloromethane

74-87-3

16

Plastics, synthetic rubbers, insulation foams

Dichloromethane

75-09-2

15

Paints, insulation foams

Hydrochlorofluorocarbons

Insulation foams

- HCFC 22

75-45-6

1700

-HCFC 141b

1717-00-6

630

- HCFC 142b

75-68-3

2000

Hydrofluorocarbons

Insulation foams

- HFC 134a

811-97-2

1300

-HFC 152 a

75-37-6

140

- HFC 245

460-58-6

950

- HFC 365

406-58-6

890

Methane

74-82-8

21

Animal materials (ruminants), steel and cement production (coal mining), waste treatment (landfills, incineration)

Nitrous oxide

10024-97-2

310

Plant materials (artificial fertilizers), waste treatment (incineration)

Pentane

109-66-0

11

Insulation foams

Sulphur hexafluoride

2551-62-4

23900

Double glazing

Perfluorcarbons PFCs

Aluminium production

- Perfluoromethane

75-73-0

6500

- Perfluoroethane

76-16-4

9200

The Global Warming Potential (GWP) of a gas is its relative potential contribution to climate change over a 100 year period where carbon dioxide CO2 = 1. Emissions of organic solvents used e.g. in paints, adhesives and plastics, will have a GWP of approximately 3 by reacting to CO2 in the atmosphere. These gases are however not considered as ordinary greenhouse gases and are therefore not mentioned in the table. Greenhouse gases presented in this table are given a grey colou when they appear in the main text.

The Global Warming Potential (GWP) of a gas is its relative potential contribution to climate change over a 100 year period where carbon dioxide CO2 = 1. Emissions of organic solvents used e.g. in paints, adhesives and plastics, will have a GWP of approximately 3 by reacting to CO2 in the atmosphere. These gases are however not considered as ordinary greenhouse gases and are therefore not mentioned in the table. Greenhouse gases presented in this table are given a grey colou when they appear in the main text.

and perfluorocarbons (PFCs) from aluminium production. Hydrofluoro-carbons (HFCs), often used as foaming agents in insulation materials, are also potent greenhouse gases. These will continue to be emitted throughout the product's lifetime.

In a 50-year lifecycle for conventional buildings, about 10 to 20% of the total greenhouse emissions will be associated with the materials used. However, in the case of low energy buildings, where the heating-related load is less and the materials use is higher, this proportion can exceed 50% (Gielen, 1997). It has been estimated that the production and transport of building materials accounts for 7 to 9% of all climate emissions in Western Europe; the main contributors being steel, cements and plastics (Kram, 2001).

It should be noted that the point in time when these climate emissions are addressed is of great importance. If we wait for 20 years before reducing emissions then the reductions needed will have to be 3 to 7 times more in order to achieve the same effect (Kallbekken and Rive, 2005). This means that material choices are even more important, since their climate emissions mainly occur during the production phase; in other words in the present and immediate future - as compared to reduced heating-related emissions in, say, 40 years' time.

2.3.2 Carbon processes in building materials

During growth, plants absorb and bind large quantities of CO2 from the air, as well as transferring a quantity to the surrounding soil. Each kilogram of dry plant matter contains about 0.5 kilograms of carbon. This corresponds to sequestration of 1.8 kilograms of CO2 from the atmosphere. This carbon will remain intact until the material is combusted or decays.

Buildings in timber and other vegetal products, therefore, store carbon for as long as they stand. Assuming that the timber extracted is also being replanted, the overall stock of plant products in the system will be increasing. Carbon storage in building products can thus contribute to reducing the atmospheric CO2-concentration over a long period and is therefore considered to be a significant contributor to reducing global warming. It 'buys time', since this carbon will not be released back into the atmosphere before the buildings decay in 50 or 100 years' time.

The Kyoto protocol considers reduction options of CO2 in a 100 year perspective. The lifetime of plant materials storing carbon will thus be critical; if buildings last for only 50 years, their climate effect will be estimated at 50% of that. However, timber constructions can easily have a far longer lifetime - and this can be further increased through more re-use in up to several cycles for some components.

There are other materials that can store carbon. Alkaline earth compounds such as magnesium oxide (MgO) and calcium oxide (CaO) are present in naturally occurring silicate rocks such as serpentine and olivine. When these react with CO2 they form stable carbonates that can be used both as building blocks and as aggregates in concrete. In theory, there are sufficient of these minerals in the earth's crust to bind all anthropogenic emissions of carbon (Metz, 2006). However, the effects of mining, associated energy use and costs render this an unlikely scenario for the near future.

It should be noted that, whereas the production of cement and other calcium-based building products causes emissions of very large quantities of CO2 during the calcination of limestone (constituting approximately half of the emissions from the production, the rest resulting from energy use), part of this is later reabsorbed into the materials by carbonation. This requires the presence of air and water and takes place over many decades. In the course of a building's lifetime of say 50 years, of the order of 25 to 50% of the carbon originally emitted during calcination may be recaptured. This is significant, but does not greatly reduce the large emission impact of using these materials.

CLIMATE NEUTRAL BUILDINGS

It is possible to construct buildings that are climate neutral throughout their entire life -cycle.This requires that we take into consideration both the materials' aspects as well as operational energy use. Such buildings will have both a climatic 'debit' and 'credit' account, and must be based on thefollowing principles.

FIRST PRINCIPLE: CHOOSE LOW IMPACT MATERIALS AND CONSTRUCTIONS

All materials chosen must have minimal fossil energy demand in production and transportation. Products with chemical emissions of greenhouse gases should be omitted.

Using timber instead of concrete or bricks, for example, reduces emissions from the materials production by approximately 1 kg of CO2 per kilogram of timber used (Kram, 2001). Even with fairly moderate substitution, one may reduce the climate emissions by some 20 to 30% (Nemry etal., 2001; Pingoud etal., 2003;Thormark, 2007). Over 50% is possible given less conventional materials and solutions (Goverse etal., 2001).This also requires choosing materials that are easy to maintain and to modify and recycle. One should also ensure that the combustion value of biological waste materials is energy-recovered soasto replacefossilfuels. In the caseof materials based on fossil resources, in particular plastics, controlled dumping may be the best solution from the climate point of view, since energy recovery from these results in emissions of greenhouse gases corresponding to burning the same amount of fossil fuel.

Use of lightweight materials will reduce transport related emissions. It is estimated that 1 kg of wood can replace 3.6 kg of concrete or brick (Pingoud et al, 2001). On the other hand, the thermal capacity provided by the heavy materials will reduce this advantage somewhat. This depends to a large extent on factors related to construction methods, local climate and building type.

SECOND PRINCIPLE: REDUCE ALL OPERATIONAL ENERGY, IN PARTICULAR THAT BASED ON FOSSIL FUELS

Operational energy includes space-heating, electricity and hot water.

It is important to remember that choice of the best strategies will often depend on very local climaticfactors. Forexample, in windy coastal regions improved air tightness measures will have far more effect than extra insulation.

THIRD PRINCIPLE: MAXIMIZE STORAGE OF CARBON

Use as much construction material as possiblethat is of plantorigin - in practice mainly timber - and in ways that ensure long life as well as reusability (Figure 2.2). Even in Finland, where timber construction is already dominant, it has been estimated that use of timber in construction could well be increased by 70% (Pingoud etal., 2003). A potential of up to 550 kilograms of timber products per square metre of floor area is achievable in small houses.This is based on the use of massive timber constructions in walls, floors and roofs. For larger building types, 300 to 400 kg/m2 maybe realistic (Berge, 2004).

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