SINKS, ENERGY CROPS AND LAND USE: COHERENT CLIMATE
POLICY DEMANDS AN INTEGRATED ANALYSIS OF BIOMASS
An Editorial Comment
The large natural carbon fluxes between atmosphere and terrestrial biosphere
in combination with our substantial control over terrestrial biotic productivity
(Vitousek et al., 1986) grants us a powerful lever for manipulating atmospheric
CO2. Proposals to use this leverage to offset CO2 emitted from fossil fuels are
roughly as old as modern knowledge of the CO2-climate problem (NAS, 1977).
More recently, modern biomass energy, which was first advanced as a solution to
supposed shortages of fossil fuels, has emerged into the climate policy debate as
an (almost) CO2-neutral substitute for fossil fuels.
The use of terrestrial biotic productivity – either as a substitute for fossil fuels
or as a carbon sink to offset their CO2-emissions – is one of the few technical
options that promises to slow rising atmospheric CO2 concentrations at moderate
cost. But that promise comes with substantial uncertainties with respect to cost,
effectiveness, and environmental impact. These uncertainties have been well cata-
loged elsewhere (e.g. Smil, 1983; Giampietro, 1997); here, I focus on the problems
that emerge from the lack of an integrated understanding of the manifold ways in
which our leverage over the terrestrial carbon cycle may be exercised to mitigate
the growth of atmospheric CO2.
Integrated analysis is needed to account for the strong linkages between the use
of sinks and the use of biomass energy, linkages that are inadequately addressed
in most estimates of the cost of CO2 mitigation, and in most integrated assess-
ment models. Biomass energy and sinks are not, however, the only relevant ways
to manipulate the carbon cycle. There are two other possibilities: the engineered
remote sequestration of biomass and the use of biomass energy with capture and
sequestration of CO2. I first describe these four distinct methods for using biomass,
and then summarize the economics of biomass as a means to CO2 mitigation,
providing comparisons with other estimates of the cost of mitigation. I next de-
scribe the basis and need for integrated analysis of biomass using two examples,
one centered on spatially resolved economic modeling and a second drawn from
engineering analysis of lifecycle costs and impacts. Finally, I sketch the challenges
of integration and close with a cautionary note about the limitations of biomass for
CO2 mitigation.
Climatic Change 49: 1–10, 2001.

2 EDITORIAL COMMENT
1. The Fourfold Way
There are four ways in which terrestrial biotic productivity, which I will call
biomass, may be harnessed to retard the increase in atmospheric CO2.
1. Sinks. Carbon may be sequestered in situ in soil or standing biomass. Al-
though the distinction between the protection of existing carbon pools and
actions intended to increase carbon storage (e.g., forest protection versus re-
forestation) is vital for policy implementation, the tight biological coupling
between the protection and enhancement of sinks leads me to treat them
jointly.
2. Bioenergy. Biomass may be harvested and used as fuel so that CO2 emissions
from the fuel’s use are (roughly) balanced by CO2 captured in growing the
energy crops.
3. Remote sequestration. Biomass may be harvested and separately sequestered;
for example, by burying the trees.
4. Bioenergy with sequestration. Biomass may be harvested and used as fuel
with capture and sequestration of the resulting CO2; for example, we may
use biomass to make hydrogen and sequester the resulting CO2 in geologic
formations.
Sinks and bioenergy, the first two options, both figure prominently in contem-
porary climate policy analysis. While remote sequestration, the third option, was
described in early climate assessments such as the National Academy’s ‘Energy
and Climate’ report (NAS, 1977), it currently attracts little attention, the article
by Metzger and Benford (2001) being a notable exception. Unlike the first three
options which have been analyzed for decades (however inconsistently), bioenergy
with sequestration, the fourth option, is a newcomer. Though it was included in the
IPCC second assessment (Watson et al., 1996, Chapter 19), it has received little
subsequent analysis.
Climate policy assessments have generally treated biomass inconsistently, often
in ways that overstate its potential for mitigation. Inconsistency often arises when
the various ways of using biomass are treated separately, so that their essential
linkage via the competition for scarce arable land is obscured. The IPCC second
assessment, for example, treats options 2 and 4 in the chapter on ‘Energy supply
mitigation options’ and treats option 1 in separate chapters on agricultural and
forest management for the mitigation of greenhouse gas emissions.
Although there has been little explicit analysis of bioenergy with sequestra-
tion, there is nevertheless a significant body of relevant knowledge because it is
a hybrid of two technologies that have separately been the subject of substantial
analysis. First, the use of biomass for production of hydrogen or electricity using
gasification or direct combustion (e.g. Williams and Larson, 1996; Hughes and
Tillman, 1998). And, second, the use of fossil fuels without CO2-emissions via
the capture and sequestration CO2 (e.g. Parson and Keith, 1998; Eliasson et al.,