Geology Reference
In-Depth Information
So far we have not yet considered any significant negative feedbacks that might oppose
the warming trend. There is one important effect that we haven't mentioned yet—global
dimming. It turns out that the amount of sunlight reaching the planet's surface has been
going down by about 3% a decade for the last 50 years. Given the fact that we are in a
warming world, this result doesn't seem to make sense, and it took a while before sci-
entists began to work out what was behind this peculiar phenomenon. Now we know,
and the results give us good reason to be even more alarmed. It turns out that we don't
just emit greenhouse gases when we burn fossil fuels; we also release vast quantities
of aerosols, such as sulphates that seed dense white planet-cooling clouds and hazes. It
seems that the sulphates we emit can diminish rainfall over large areas, for they seed
very small water droplets that are less likely to tumble out of the sky as rain. Clouds that
don't produce rain have increased cooling effects simply because they stay in the sky
longer. This extra cooling could act to prevent evaporation from large water bodies, as
might well have happened over Lake Tanganyika, where aerosols could have decreased
the amount of sunshine reaching the lake by 10%, leading to a 15% decrease in rainfall.
Overall, then, our atmospheric pollution is having two opposite effects: one warming
and the other cooling, with warming the stronger of the two. The cooling effect of aer-
osols in the northern hemisphere has been so large that it may have changed the pattern
of the African monsoon, bringing the droughts and famines that recently killed so many
thousands of people in the Sahel region of North Africa. If the effect intensifies, there
is a danger that the Indian monsoon could also be affected, with the potential loss of
not thousands but hundreds of millions of lives. The good news is that we can easily do
something about global dimming, for it is easy enough to scrub the sulphur out of our
fuels and chimneystacks. We've been doing just this over the past few years, and global
dimming is indeed diminishing. But there is a tragic irony, for as we reduce sulphates
and other aerosols we lose the cooling effects of the clouds and hazes they seed, and the
warming increases. This means that models that the IPCC used for the TAR may well
be underestimating the extent of the warming to come by not incorporating the effects
of dimming or its removal. It could well be that temperatures in the cleaner air will rise
twice as fast as was previously thought, and that some of the critical tipping points will
be surpassed as soon as 25 years from now. This would imply global warming in excess
of 2°C, at which point the Greenland ice sheet would begin its irreversible melting. By
2040 the planet could well have warmed by 4°C, triggering the irreversible dieback of
the Amazon forest. This would release even more carbon into the atmosphere by the end
of this century, warming the world by 10°C, at a pace more rapid than any other previ-
ous episode of natural warming. With this amount of warming, methane hydrates would
begin to break down, releasing their vast store of methane into the atmosphere. Even-
