Climate and Climate-System Modelling - Environmental Modelling: Finding Simplicity in Complexity - page 154

continuously changing, then the climate and terrestrial
biosphere will never get a chance to reach an equilibrium.
The following outlines the equilibrium and transient
calculations for both climate and the terrestrial biosphere
that are performed in the online Excel package. A much
more detailed explanation of and justification for the cal-
culations is contained in the online explanatory material
that accompanies the Excel package.

detritus on the soil, and carbon in the soil. Each box
is globally aggregated; that is, it represents that par-
ticular type of carbon worldwide. Similar box models
have been applied to each land grid cell in the global
grid of AOCGMs. The key parameters in Worksheet 7
govern how the rate of net primary production (gross
photosynthesis minus plant maintenance respiration)
varies with temperature (first increasing, then decreas-
ing as temperature increases), how respiration of detritus
and soil carbon increase with temperature, and how
net primary production and the steady-state increase in
above-ground equilibrium biomass vary with the atmo-
spheric CO 2 concentration. The control parameters can
be adjusted to replicate the aggregate global behaviour of
complex, spatially resolved models, and the interaction
between temperature- and CO 2 -driven changes in net
primary production and in temperature-driven increases
in respiration can be explored.

9.4.1 Equilibriumandtransient climate response

The first two worksheets use a zero-dimensional model
(in which a single point, with a single temperature, rep-
resents the entire global average) to illustrate how the
change in global mean temperature in response to a
radiative forcing depends on the rate of change of net
radiation with temperature, dN/dT, and how dN/dT can
be decomposed into terms involving individual feedback
processes. Worksheet 3 introduces linear feedback anal-
ysis for fast-feedback processes, shows how to quantify
the inherent strength of a given feedback, and shows
that the impact on temperature of adding a feedback
of given strength depends on the overall strength of the
pre-existing feedbacks. Worksheet 4 extends the analysis
to take into account climate-carbon cycle feedback.
The next two worksheets explore the transient (time-
varying) approach to the final equilibrium temperature
change. Worksheet 5 presents the transient response for
the simplest possible case, in which only a single tempera-
ture is computed, which can be thought of as representing
a single box. This box represents some combination of the
atmosphere, land surface and upper layer of the ocean.
Worksheet 6 presents the transient response for a two-box
model, where the second box represents the deep ocean
as a single, well-mixed thermal reservoir (that is, having
only a single temperature). Although this representation
is highly simplified, it does permit the elucidation of a
number of important conceptual points, including: the
fact that the first two thirds or so of the transient response
is relatively rapid, being governed by the heat capacity of
the ocean surface layer, while the final approach to the
equilibrium change is much slower.

9.4.3 Futuregreenhousegasemissions, climatic
change, andclimate-carboncycle feedbacks

Worksheets 8 to 10 present a simple framework for pro-
jecting future global energy demand and themix of energy
supplies into the future, and from that, generating a sce-
nario of future fossil fuel CO 2 emissions. In Worksheet 8
the world is divided into two regions (roughly, devel-
oped and developing countries), and in each region total
primary energy demand is computed as the product of
the following factors: population × GDP (gross domestic

product) per year per capita × primary energy per unit

GDP or, in terms of units,

Energy demand (MJ) per year = P × ($ / yr) / P × MJ / $
(9.1)

where MJ/$ (primary energy use per unit of GDP) is
referred to as the energy intensity of the economy. Work-
sheet 9 contains data on the age distribution of existing
nuclear power plants as of 2009; these plants are assumed
to continue operating until the end of a 40-year lifespan
and are not replaced as they retire, leading to a grad-
ual phaseout of nuclear energy. Worksheet 10 contains
information for specifying the timing of peaks in the
global supply oil and gas and in the subsequent decline
using logistic functions. Logistic functions are also used
to specify the growth in power supply from C-free energy
sources (biomass, hydro-electric, and other). The differ-
ence between energy supply from oil, natural gas, nuclear
power and the C-free power supplies is assumed to be
met by coal. The annual amounts of oil, natural gas and

9.4.2 Globallyaggregatedterrestrial biosphere
model

Worksheet 7 presents a three-box representation of the
global terrestrial biosphere that is used to represent basic
modelling principles and to illustrate some features of how
simple models respond to temperature and CO 2 pertur-
bations. The three boxes are: above-ground vegetation,