Other factorsthat might influenceclimate change 1372220181614
1850Circulation strength (Sv)1900No change
SRES A1FI
SRES A2
SRES B1
SRES B21950 2000 2050 2100(a)
1 20180 ° 90 ° W 0 ° 90 ° E 180 °90 ° S45 ° S0 °45 ° N90 ° N+ 1 0 − 1 − 2 − 3 − 4 − 5
Temperature change (°C)(b)
Figure 6.11(a) Change
in the strength of the
thermohaline circulation
(THC) in the north Atlantic
as simulated by the Hadley
Centre climate model for
different SRES scenarios.
The unit of circulation is
the Sverdrup, 10^6 m^3 s−^1.
(b) Changes in the surface
air temperature, relative to
the present day, twenty
years after the hypothetical
collapse of the
thermohaline circulation as
simulated by the Hadley
Centre climate model. To
obtain the temperature
distribution for a real
situation, the changes due
to increased greenhouse
gases at the time of
collapse would have to be
added to these changes
resulting from THC
collapse.Other factors that might influence climate change
So far climate change due to human activities has been considered. Are
there other factors, external to the climate system, which might induce
change? Chapter 4 showed that it was variations in the incoming solar
energy as a result of changes in the Earth’s orbit which triggered the ice
ages and the major climate changes of the past. These variations are, of
course, still going on; what influence are they having now?
Over the past 10 000 years, because of these orbital changes, the solar
radiation incident at 60◦N in July has decreased by about 35 W m−^2 ,
which is quite a large amount. But over one hundred years the change is
only at most a few tenths of a watt per square metre, which is much less
than the changes due to the increases in greenhouse gases (remember
that doubling carbon dioxide alters the thermal radiation, globally
averaged, by about 4 W m−^2 – see Chapter 2). Looking to the future
and the effect of the Earth’s orbital variations, over at least the next