It has been a hot year. The average temperature in Britain for 2006 was
higher than at any time since records began in 1659. Globally, it looks set to
be the sixth hottest year on record. The signs during the past 12 months have
been all around us. Little winter snow in the Alpine ski resorts, continuing
droughts in Africa, mountain glaciers melting faster than at any time in the
past 5,000 years, disappearing Arctic sea ice, Greenland's ice sheet sliding
into the sea. Oh, and a hosepipe ban in southern England.
You could be forgiven for thinking that you've heard it all before. You may
think it's time to turn the page and read something else. But you'd be wrong.
2006 will be remembered by climatologists as the year in which the potential
scale of global warming came into focus. And the problem can be summarised in
one word: feedback.
During the past year, scientific findings emerged that made even the most
doom-laden predictions about climate change seem a little on the optimistic
side. And at the heart of the issue is the idea of climate feedbacks - when the
effects of global warming begin to feed into the causes of global warming.
Feedbacks can either make things better, or they can make things worse. The
trouble is, everywhere scientists looked in 2006, they encountered feedbacks
that will make things worse - a lot worse.
Next year, the UN's Intergovernmental Panel on Climate Change (IPCC) will
publish its fourth assessment on the scale of the future problems facing
humanity. Its last assessment, published in 2001, had little to say on the
subject of climate feedbacks, partly because, at that time, they were such an
unknown quantity. This year, scientists came to learn a little more about them,
and they didn't like what they learnt.
During the past two decades, the IPCC has tended to regard the Earth's climate
as something that will change gradually and smoothly, as carbon dioxide and
global temperatures continue their lock-step rise. But there is a growing
consensus among many climate scientists that this may be giving a false sense of
security. They fear that feedback reactions may begin to kick in and suddenly
tip the climate beyond a critical threshold from which it cannot easily recover.
Climate feedbacks could turn the Earth into a very different planet over a
dramatically short period of time. It has happened in the past, scientists say,
and it could easily happen in the future given the unprecedented scale of the
environmental changes caused by man.
There are two types of feedback that can play a role in the future direction of
the Earth's climate. The first is a "negative" feedback, which is largely good
for us, because it works against things getting worse. The classic example of a
negative feedback is the fertilising effect of carbon dioxide. As concentrations
rise, then so does the amount of carbon absorbed by the higher growth rate of
plants. The result is a negative feedback that tends to check rising levels of
carbon dioxide.
A "positive" feedback makes things worse by adding to the existing problem. It
brings about a vicious circle, in which a rise in carbon dioxide or global
temperatures causes some change in the climate system which, in turn, leads to
further rises in carbon dioxide or temperatures.
A classic example of a positive feedback is the melting sea ice of the Arctic.
As temperatures rise, the ice floating on the Arctic sea melts, exposing dark
ocean where once there was white ice that reflected sunlight, and heat, back
into space. The newly revealed dark ocean absorbs more sunlight and heats up,
causing more ice to melt, and so reinforcing the positive-feedback cycle.
But even this simple description belies the true complexity of life on Earth. In
fact, there is a negative feedback at work as well with Arctic sea ice, which
insulates the underlying ocean and keeps it warmer during the cold, dark
northern winters. However, on balance, it is the positive feedback that
dominates here, as it does in several other instances investigated by scientists
in 2006.
"The main concern is that the more we look, the more positive feedbacks we
find," says Olivier Boucher, a climate scientist at the Met Office. "That's not
the case when it comes to negative feedbacks. There seems to be far fewer of
them." The sentiment is echoed by Chris Rapley, the director of the British
Antarctic Survey in Cambridge: "When we look at the list of all the feedbacks in
the climate, the list of positive feedbacks is worryingly long - a lot longer
than the negative feedbacks. To be honest, it's a wonder that the climate has
remained so stable."
Let's stick with Arctic sea ice a bit longer before looking at other issues that
emerged 2006. In March, Nasa satellites monitored a 28-year record low for
winter sea ice. Normally sea ice recovers during the long Arctic winter, but
this was the second consecutive year that the ice failed to re-form fully to is
previous winter extent.
This meant there was less ice at the start of the northern summer, with the
result that last September saw the second monthly minimum for summer sea ice -
almost hitting the record minimum set in September 2005.
During the past four or five years, there has been an acceleration in the rate
at which sea ice is melting, a change that some scientists put down to a
positive feedback. "Our hypothesis is that we've reached the tipping point,"
says Ron Lindsay of the University of Washington in Seattle. "For sea ice, the
positive feedback is that increased summer melt means decreased winter growth
and then even more melting the next summer, and so on."
Professor Lindsay likens the positive feedback in the Arctic to a ball that has
begun to roll down a slope, gathering momentum and speed as it goes. In order to
reverse the direction of movement, the ball has to be pushed back up the slope.
But how? "Perhaps a cooling period could reverse the situation," he says. "But
with global warming, temperatures are only bound to rise."
While we are in the northern hemisphere, take a look at another positive
feedback that scientists investigated in 2006. This is connected to the frozen
permafrost of Siberia and northern Canada, which lock up vast stores of carbon
in the form of methane, a gas formed by the decomposition of organic matter. For
more than 12,000 years, this methane - a greenhouse gas 20 times more potent
than carbon dioxide - has been safely stored under the permanently frozen
ground. But now the permafrost is melting and the gas is bubbling free into the
atmosphere.
Sergei Kirpotin, a botanist at Tomsk State University in Russia, has been
studying the extent of the melting permafrost of Western Siberia, the site of
the world's biggest frozen peat bog. During the past few years, he has watched
lakes getting bigger and bigger as the solid permafrost underneath liquifies.
Normally, patches on white lichen on high Siberian ground reflect the sun's rays
and help to keep the ground underneath cold. But as the dark lakes expand, more
heat is absorbed and more permafrost melts. "As we predicted in the early 1990s,
there's a critical barrier," says Professor Kirpotin. "Once global warming
pushes the melting process past that line, it begins to perpetuate itself."
The once-frozen peat bogs of Siberia - bigger than France and Germany combined -
began to "boil" furiously in the summer of 2006 as methane bubbled to the
surface. Exactly how much is being released into the atmosphere is unknown,
although some estimates put it as high as 100,000 tons a day - which means a
warming effect greater than America's man-made emissions of carbon dioxide.
But Katey Walter of the University of Alaska believes even this could be
seriously underestimated. In a study published in Nature in September, Walter
and her colleagues calculated that the level of methane emissions from Siberia
could be anywhere between 10 per cent and 63 per cent higher than anyone had
hitherto suspected. "We have shown that the North Siberian lakes are a
significantly larger source of atmospheric methane than previously recognised,"
she says.
So the message is clear: frozen peat bogs that turn into heat-absorbing lakes
release methane, which means a stronger greenhouse effect and higher
temperatures, leading to more permafrost melting. The cycle was clearly
documented in 2006 but just how strong this positive feedback turns out to be
has yet to be fully determined.
Another study in 2006 looked at perhaps the most important climate feedback
there is. Yet it went unreported - so listen up. The Earth has been a very
accommodating planet. During the past 200 years, it has absorbed more than half
of all man-made emissions of carbon dioxide through natural carbon "sinks",
mostly in the ocean but also on land. The rest of the emissions have been left
in the air to aggravate the Earth's natural greenhouse effect, so raising global
average temperatures.
But what if something were to interfere with these very useful carbon "sinks"?
Can we forever rely on them to remain sinks, or could they turn into dangerous
sources of atmospheric carbon? A huge international team of climatologists asked
these questions in a little-known study published in the July issue of the
Journal of Climate. The conclusion makes depressing reading.
The scientists investigated what would happen if they tinkered with 11 of the
world's biggest computer models of the complex climate-carbon cycle. They wanted
to simulate what would happen to the carbon sinks on the land and the ocean for
each model as the world gets warmer. All the models agreed that as the world
heated up, the ability of the land and the oceans to keep on absorbing carbon as
efficiently as they have in the past 200 years gets appreciably worse.
In other words, we cannot rely on planet Earth to be so accommodating in terms
of mopping up half of our carbon pollution. But could something even worse
happen? Could these carbon sinks turn into carbon sources? The answer is yes.
Many models suggest that the terrestrial biosphere could become a net carbon
producer by the mid 21st century. Signs are that it is already happening in some
parts of the world.
Guy Kirk of the National Soil Resources Institute at Cranfield University found
that the soil of Britain is releasing more carbon dioxide into the atmosphere
than a quarter of a century ago because increasing temperatures are speeding up
the rate of organic decay. "It's a feedback loop," says Professor Kirk. "The
warmer it gets, the faster it is happening." In fact, he estimates that since
1978, Britain's soil has released on average an extra 13 million tons of carbon
dioxide a year, which is more than the 12.7 million tons a year Britain saved by
cleaning up its industrial emissions.
The outlook does not look any better out at sea. The important carbon sinks of
the ocean are also suffering from feedback. As more carbon dioxide dissolves in
seawater to form carbonic acid, the acidity of the ocean increases - the rate is
100 times faster than at any time for millions of years.
There is a physical feedback - it is just harder for more carbon dioxide to
dissolve in acid water - as well as a biological feedback. Tiny organisms called
coccolithophores use dissolved carbon to make their shells, but acidic seas make
this more difficult. This blocks an important biological pump that pushes carbon
to a long-term store on the seabed - which is what happens when billions of tiny
shells sink to the depths as coccolithophores die.
Yet another ocean feedback was monitored in 2006, this time involving
phytoplankton, the tiny microscopic plants of the sea that form the basis of the
entire marine food chain. Nasa satellites showed earlier this month that
phytoplankton - which absorb carbon dioxide - are finding it harder to live in
the more stratified layers of the warmer ocean, which restrict the mixing of
vital nutrients. Since 2000, when the sea surface temperatures began to rise
more noticeably, the photosynthetic productivity of phytoplankton have decreased
in some ocean regions by 30 per cent.
"As climate warms, phytoplankton production goes down, but this also means that
carbon dioxide uptake by ocean plants will decrease," says Michael Behrenfeld of
Oregon State University. "That would allow carbon dioxide to accumulate more
rapidly in the atmosphere, making the problem worse." Some climate scientists
believe that the risk of dangerous feedbacks tipping the Earth's climate system
beyond a threshold is so great that there should be wider recognition of what
they term "abrupt changes". The point is, they say, it has happened repeatedly
in the past. It happened 55 million years ago when a trillion tons of methane
were suddenly and mysteriously released from frozen stores on the seabed,
causing global temperatures to soar 10C, and a mass extinction of species.
It happened 14,500 years ago when ice sheets catastrophically collapsed into the
ocean causing sea levels to rise by 20 metres in just 400 years. And it happened
6,500 years ago when the Sahara was suddenly turned from lush vegetation to dry
desert.
Scientists say that what is happening now to the planet in terms of carbon
dioxide levels and global temperatures is just as abrupt as anything that has
occurred in the past. "What we are doing now to the Earth is unprecedented,"
says Professor Rapley of the British Antarctic Survey, "so we cannot rule out
the possibility that we are doing something that will create a strong positive
feedback, which will push the Earth into a domain where things will happen that
have never happened before."
It is a sobering thought as 2006 draws to a close, and one that must be in the
minds of all the IPCC scientists preparing next year's Fourth Assessment Report
on climate change.
A VISION OF THE FUTURE
The single most momentous environmental image of 2006 was a holiday snap. Of
sorts. It showed typical European package tourists on a nice sandy beach in
Tenerife. Until a few minutes before the picture was taken, on August 3 on
Tejita beach in Granadilla, it had been a day of utter normality for these
tourists. Then something very different erupted on to the scene.
From the sea came a boat. Out of it fell pitiful figures - exhausted, terrified,
dehydrated, starving. They were African migrants who, out of desperation, had
risked the long voyage from the African coast to the Canaries; for the Canaries
are part of Europe, a place of hope and opportunity. What did the tourists do?
They did the decent thing. They rushed to the aid of fellow men and women.
But will they offer such a welcome when the boat people are not just a boatload,
but a whole country- or region-load? For that is coming. As climate change takes
hold this century, agriculture may fail in some of the poorest and most densely
populated parts of the world.
Sir Crispin Tickell, Britain's former Ambassador to the UN, who is one of the
most far-sighted of environmental commentators, pointed out as long ago as 1990
that global warming is likely to create environmental refugees in the hundreds
of millions. We have paid little attention to his warning.
But if you look at the picture taken on Tejita beach, you can see something even
more dramatic than the fact that the ordinary European holidaymaker has a
lifestyle most Africans can only dream of. You can see the future, starting to
happen.