UP TO half the global warming we have experienced over the past 130 years may
have been caused by an increase in the Sun鈥檚 output of energy.
This finding is sure to be seized on by those say that our emissions of
greenhouse gases are not the dominant cause of global warming. But Mike Lockwood
of the Rutherford Appleton Laboratory in Didcot, Oxfordshire, whose team made
the discovery, says the true message is much more disturbing. Taking away the
effects of the Sun鈥檚 increasing output reveals that, since 1970, greenhouse
emissions have had an even more dramatic effect than was thought. 鈥淚t is a
question of balance, and the balance is changing,鈥� he says. 鈥淲hatever happened
in the past, the greenhouse effect is now the dominant cause of warming.鈥�
Lockwood and his colleagues obtained their data from space probes that have
been measuring the solar magnetic field, which varies in strength with the
amount of energy the Sun emits. By analysing how these variations in solar
magnetism affect the Earth鈥檚 magnetic field鈥攚hich has been monitored by
scientists in Britain and Australia since 1868鈥攖he researchers were able
to use historical data to calculate the Sun鈥檚 energy output over the entire
period.
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Since the mid-19th century, average global temperatures have risen by around
0.6 掳C. Lockwood鈥檚 team calculates that solar changes account for about half
of this鈥攖wice the amount previously accepted by most climatologists. Solar
changes may have caused virtually all the warming that occurred between 1860 and
1930, says Lockwood. But since 1970, when the pace of climate change began to
accelerate, the Sun has been the source of less than a third of the warming. The
build-up of greenhouse gases is to blame for the rest (Nature, vol 399,
p 437).
One of the prime architects of the scientific consensus on global warming,
Tom Wigley of the National Center for Atmospheric Research in Boulder, Colorado,
says that Lockwood鈥檚 picture of events before 1970 is 鈥渜uite different鈥� from the
view until now. He now intends to look closely at Lockwood鈥檚 paper to see
whether the consensus needs to shift.
Past efforts to estimate historical changes in solar energy have mostly used
measures of sunspot activity and the length of the solar cycle, which are
thought to correlate with the Sun鈥檚 output. 鈥淏ut there isn鈥檛 really a strong
physical understanding of why the length of the solar cycle should be relevant,鈥�
says Lockwood. He argues that the magnetic field method is 鈥渕ore direct and
补肠肠耻谤补迟别鈥�.
But no method that relies on calculating historical events that cannot be
measured directly is 100 per cent reliable. 鈥淭here are still uncertainties,鈥�
Lockwood admits. 鈥淭here may be amplifying effects that we have yet to discover,
which could change the calculations again.鈥�