Climate Sense

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Modelling in earth system science

Michio Kawamiya and Tatsushi Tokioka, Research Institute for Global Change,

Japan Agency for Marine-earth Science and Technology

hanges in the natural environment due to human activi-

ties are becoming visible. While one of the most evident

examples is global warming, the problem goes well

beyond the single issue and includes ocean acidification, pertur-

bations of the global nitrogen cycle due to industrial fixation

and others. These issues are mutually related, and none of them

can be solved without interdisciplinary collaboration among

scientific fields such as meteorology, oceanography, geochemis-

try, biology and even social sciences. Recognizing the situation,

scientists have been arguing the necessity of an ‘earth system

science’ (ESS), where the global environment is recognized as

a system composed of its interacting subsystems: atmosphere,

ocean, biosphere, cryosphere and society.

Among early advocates of the concept of ESS were the Russian scien-

tist V. Vernadskii, who established the biosphere concept in the

1920s, and J. Lovelock and L. Margulis, the founders of the Gaia

hypothesis, who provoked a lot of dispute in the 1970s by claiming

that the biosphere plays an active role in maintaining the global

environment. Based on these preceding ideas, a report by NASA

Advisory Council in 1988 first used the term ‘earth system science’

explicitly and provided a clear definition of ESS as used today. It sets

the goal of ESS as: ‘to obtain a scientific understanding of the entire

earth system on a global scale by describing how its component

parts and their interactions have evolved, how they function, and

how they may be expected to continue to evolve on all

timescales’. The Advisory Council also points out that

accomplishing this goal would require various research

schemes such as numerical modelling, global observing

systems and information networks that enable efficient

dissemination of observed data and research outputs.

The predictive statement was surprisingly accurate

taking into consideration that, around that time, inter-

net usage was limited to certain advanced institutes and

there were very few attempts to incorporate biogeo-

chemical processes into general circulation models.

Models used in ESS, and conceptual models

As the NASA report emphasized, modelling can be a

powerful tool for investigating the dynamics of the earth

system. Models that have been developed and applied for

this purpose can be categorized according to the degree

of complexity and integration. Models in the category

‘conceptual models’ with the lowest level of complexity

consist of several simple equations mimicking certain

aspects of the complicated behaviours of the earth system.

The simplest example is the model for calculating the

radiative equilibrium temperature of the earth, which a

meteorology student will find in the first chapter of their

textbook. During the late 1960s and the 1970s, authors

including M. I. Budyko and W. Sellers proposed models

of this category for discussing multiple equilibriums inher-

ently possessed by the earth system due to interactions

between the cryosphere and atmosphere. These concep-

tual models can often be analytically manipulated yet

remain instructive, allowing us to grasp the mechanism

of interactions and feedback operating in the earth system.

The degree of abstraction is, however, extremely high for

this type of model and the correspondence between their

equations and processes in nature is not readily conceiv-

able, which leads to a fundamental difficulty in estimating

model parameter values. Models of this category are, there-

fore, mainly applied as educational tools or as supporting

material to construct a theoretical framework, and rarely

used for a projective purpose.

Earth system models with intermediate complexity

At the other extreme, atmospheric and oceanic general

circulation models (GCMs) have been applied to

projections of El Niño-Southern Oscillation events and

global warming. GCM-based earth system models have

the drawback that they are computationally expensive.

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Categorization of earth system models

Model types categorized according to the degree of complexity and integration.

Models in the category ‘conceptual models’ have the lowest level of complexity

Source: Claussen et al. (2002), Clim. Dyn. 18, 579-586