Assessing Temporary Carbon Storage in Life Cycle Assessment and ...

Explicit consideration of impacts and relevant timing also changes the relative importance of different
greenhouse gases. It would reduce the relative importance of methane and increase focus on the
longer-lived greenhouse gases, instead.
7.4.1 Introduction
Climate change is now recognised as a global problem, and the global community is seeking costeffective
means of reducing the net emission of greenhouse gases to the atmosphere. The use of trees,
or more correctly the use of the biosphere for carbon storage, is often considered as a cost-effective
means of reducing net emissions to the atmosphere. Trees are used as a means to ‘buy time’. The
question is addressed here whether trees, or the biosphere more generally, can make a useful
contribution to reducing net emissions to the atmosphere. This discussion is based on previous work
(Kirschbaum 2003a, 2003b, 2006) and is developed further from those publications in the following.
It essentially combines considerations of feedbacks via natural carbon reservoirs, especially the
world’s oceans, with an explicit definition of the key climate-change impacts we are concerned about.
It leads to important consequences for the choice of timing of emissions and emission reduction
activities.
Importantly, it allows a distinction to be made between net emissions and the atmospheric
concentration of greenhouse gases. Net emissions are not important on their own, but it is the ultimate
atmospheric concentration that leads to radiative forcing and thus climatic impacts. In the case of fossil
fuel emissions, there tends to be a tight linkage between emissions and resultant atmospheric
concentrations. There is little choice in the timing of emissions. Fossil fuels are emitted when they are
needed to produce a service, such as generating electricity or driving a car. The timing of emissions is
therefore determined by the demand for the ultimate service.
In the case of biospheric carbon stocks, however, there is more choice in affecting the timing of
emissions, and more choice in affecting carbon-stock changes that are not associated with any end-use.
Land can be converted between non-productive grassland and forests, with large carbon implications,
or productive land can be managed in different ways to achieve lower or higher carbon stocks without
necessarily affecting the service the products are put to. Since these carbon stock changes can be
undertaken at different points in time, the importance of timing needs to be explicitly addressed.
7.4.2 The Carbon Cycle
The World contains massive carbon pools that naturally interchange carbon between each other
(Kirschbaum 2003b). The principal ones are the atmosphere, the oceans and the land area. The
interchange between the atmosphere and these other carbon pools is diagrammatically represented in
Figure 7.4.1.
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