Global Warming

102 Modellingthe climate

Modelling of tracers in the ocean

A test that assists in validating the ocean compo-
nent of the model is to compare the distribution of a
chemical tracer as observed and as simulated by the
model. In the 1950s radioactive tritium (an isotope
of hydrogen) released in the major atomic bomb
tests entered the oceans and was distributed by the
ocean circulation and by mixing. Figure 5.20 shows
good agreement between the observed distribution

of tritium (in tritium units) in a section of the west-

ern north Atlantic Ocean about a decade after the

major bomb tests and the distribution as simulated

by a twelve-level ocean model. Similar comparisons

have been made more recently of the measured up-

take of one of the freons CFC-11, whose emissions

into the atmosphere have increased rapidly since the

1950s, compared with the modelled uptake.

Figure 5.20The tritium distribution in a section of the western north Atlantic Ocean approximately one

decade after the major atomic bomb tests, as observed in the GEOSECS programme (a) and as

modelled (b).

In these three ways, which cover a range of timescales, confidence

has been built in the ability of models to predict climate change due to

human activities.

Comparison with observations

More than fifteen centres in the world located in ten countries are cur-

rently running climate models of the kind we have described in which the

circulations of the atmosphere and the ocean are fully coupled together.

Some of these models have been employed to simulate the climate of the

last 150 years allowing for variations in aspects of natural forcing (e.g.

solar variations and volcanoes) and the increases in the concentrations

of greenhouse gases and aerosols.