Within four years of the dramatic
discovery of a huge “hole” in the ozone layer over the Antarctic, these
gases were phased out, in a rare act of international agreement, by the
1989 Montreal Protocol.
But CFCs had a second unexpected property.
They were greenhouse gases of unusual potency: molecule for molecule,
one of them was rated at more than 17,000 times more effective at
trapping infra-red radiation than carbon dioxide. CFCs were released
only in comparatively tiny quantities, but they were calculated to
account for up to 24% of global warming.
Francisco Estrada of the National
Autonomous University of Mexico and colleagues report in Nature
Geoscience [link available after publication] that they spotted the
signal of the missing CFCs when they used statistical analysis to
examine average temperature records in the two hemispheres from 1880 to
Tentative results, clear
Exercises such as these are not simple:
the scientists had to find a way of eliminating natural cycles that keep
the weather in a permanent pattern of change, and identify long-term
trends that could be identified as evidence of human activity.
In the first place, for a mix of reasons,
rates of change in the two hemispheres are out of step; in the second
place there are natural cycles linked to ocean and atmospheric
circulation such as the Atlantic Multidecadal Oscillation that are
associated with extended periods of warming and cooling; in the third
place things like greenhouse gas emissions are tied into cycles of
economic growth, and affected again in many cases by legislation,
perhaps to reduce the aerosols that make citizens cough and splutter,
but that also block sunlight and have a mild cooling effect.
And to get the results that they did, the
researchers had to use advanced statistical methods (for example, they
say, the “Perron-Yabu testing procedure, valid with integrated or
stationary noise”) that would baffle most of the human race.
So like all research results, the findings
are tentative and published to provoke more research. But even so, the
graphs in Nature Geoscience show a pattern of human influence in global
In the last century, the planet warmed
overall by 0.8°C. In the trend discernible in the sometimes dramatic
oscillations of temperature, there is evidence of a slowdown in this
warming during two world wars, and the Great Depression of the 1930s,
when world trade came to a standstill, factories closed everywhere, and
chimneys stopped smoking.
The authors see a pronounced rise in both
greenhouse gas emissions and in global temperatures in the 1960s - the
start of sustained global warming. But they also see a distinct slowdown
that begins in the 1990s in response to the Montreal Protocol, which
began the phase-out of the CFCs and the slow restoration of the ozone
Slowdown caused by
The Montreal Protocol is not the only
possible explanation for the apparent slowdown. There is also evidence
that at least one sector of deep ocean is warming 15 times faster than
at any time in the last 10,000 years, which suggests that ocean
circulation may be carrying away the heat that meteorologists expected
to record in the atmosphere.
Two scientists in October proposed that
the slowdown in the rate of global warming could possibly be explained
by a much longer-term natural cycle that nobody had yet identified: a
climate signal could propagate across the northern hemisphere in a
self-organising way, just as the so-called “stadium wave” propagates
around a sports arena as excited spectators stand and sit down again.
They say in Climate Dynamics that such a
phenomenon could be linked to a brake-and-accelerator pattern of
influence of northern sea ice on atmosphere and ocean circulation.
If this pattern exists, it would explain
why climate modellers had not predicted the present lull or slowdown in
global warming. But the wave itself would have nothing to do with global
warming, the two authors say. It would just offer a new perspective for
The Nature Geoscience paper, however, does
once again confirm the link between average temperatures and greenhouse
gas emissions. “Paradoxically the recent decrease in warming, presented
by global warming sceptics as proof that humankind cannot affect the
climate system, is shown to have a direct human origin,” the authors
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