Global Warming

104 Modellingthe climate

Temperature anomalies ( C) Temperature anomalies ( C)

(a) Natural (b) Anthropogenic

(c) All forcings

1850 1900 1950 2000

Year

−1.0

−0.5

0.0

0.5

1.0

Temperature anomalies ( C)

Simulated annual global mean surface temperatures

1850 1900 1950 2000

Year

−1.0

−0.5

0.0

0.5

1.0

−1.0

−0.5

0.0

0.5

1.0

1850 1900 1950 2000

Year

model

observations

model

observations

model

observations

Figure 5.22Annual global
mean surface temperatures
simulated by a climate model
compared with observations
for the period 1860–2000.
The simulations in (a) were
done with only natural
forcings – solar variation and
volcanic activity; in (b) with
only anthropogenic forcings –
greenhouse gases and
sulphate aerosols; and in (c)
with both natural and
anthropogenic forcings
combined. The simulations are
shown in a band that covers
the results from four runs with
the same model and therefore
illustrates the range of natural
variability within the model.

been observed in the climate record. In other words has the ‘signal’ that

can be attributed toglobal warming risen sufficiently above the ‘noise’

of natural variability? The Intergovernmental Panel on Climate Change

(IPCC) has been much involved in this debate.

The IPCC’s first Report published in 1990^16 made a carefully worded

statement to the effect that, although the size of the observed warming

is broadly consistent with the predictions of climate models, it is also of

similar magnitude to natural climate variability. An unequivocal state-

ment that anthropogenic climate change had been detected could not

therefore be made at that time. By 1995 more evidence was available

and the IPCC 1995 Report^17 therefore reached the cautious conclusion

as follows.