Global warming, which ought more accurately to be called ‘anthropogenic (ie. man-made) global warming’ to distinguish it from natural temperature trends – is one of the more serious threats that currently face the human race, along with epidemic viral disease and nuclear war. It is without doubt the most controversial of these threats though, and for several reasons. Firstly the science involved in predicting the effects of global warming is so difficult and the data so sparse, that until very recently climate scientists themselves have lacked agreement about its extent, or even its reality.
Secondly, many of the debates over global warming
are dominated by partisan ideological positions rather than real science.
If the phenomenon is real and its effects really will be catastrophic,
then it represents a natural obstacle to any further expansion of our
technology-based civilisation – similar to the population explosion
and mass-starvation predicted by Malthus in the early 19th century (which
proved to be a chimera, or at any rate was surmounted by new technologies).
For this reason those who oppose any further growth – whether on
conservationist, moral or anti-capitalist political grounds – are
prone to exaggerate the evidence, while on the other side many economists,
techno-optimists, free-market capitalists, and the oil industry seek to
minimise or ignore it. Trying to steer a rational and prudent middle course
is far from easy.
Unravelling any warming trend from the ‘noise’
produced by these overlapped variations is extremely hard (see graph).
Nevertheless there are experimental data that suggest there’s an
accelerating warming trend over and above these ‘natural’
effects, and there are sound theoretical reasons for why it could be happening,
namely the ‘Green House Effect’, so-called by analogy with
the ordinary garden greenhouse.
For most of the year a greenhouse can maintain its inside hotter than its outside, even without any internal source of heat, thanks to a property of the glass in its windows which lets through infra-red (that is, heat) rays from the sun of different wavelengths with different ease. Short wavelength rays pass easily through the glass into the interior, where they are absorbed by the plants and soil – the warm plants re-radiate the heat at longer wavelengths which cannot easily pass through glass, and so they’re reflected back inside and raise the air temperature.
The earth’s atmosphere acts just like a greenhouse, by trapping infra-red radiation of longer wavelengths than those it receives from the sun, but in this case it’s certain gasses in the air, rather than glass, that cause the differential transmission. Life on earth would be impossible without some greenhouse effect – all solar radiation would be reflected or radiated back into space and our planet would resemble the moon, with temperatures of 126ºC (hot enough to boil water) on the daylight side and -175ºC on the night side. Our atmosphere and oceans moderate solar heating by many different mechanisms – reflecting some from clouds, trapping some via the greenhouse effect, storing some in ocean water – with the effect of evening out the daily fluctuations and maintaining the average temperature within the range at which life can flourish (roughly speaking between the freezing and boiling points of water).
It’s not oxygen and nitrogen, which make up most of the air, that cause the greenhouse effect but other gasses present in much smaller amounts – the so-called ‘greenhouse gasses’ – by far the most important of which are carbon dioxide (CO2) and methane (CH4). These gasses have the property, like glass, of absorbing long-wave infra-red rays more strongly than short-wave and so retain heat in the atmosphere. The levels of carbon dioxide and methane in the atmosphere therefore act as yet another factor that controls the surface temperature of the earth. Both the gasses are produced naturally: all animals exhale carbon dioxide as they metabolise food; plants alternately absorb carbon dioxide during the day and exhale it at night; animals also generate methane during digestion, and dead plants generate it when they rot. Now human activity now appears to be causing them to rise well beyond natural variation.
Since 1750 – roughly the start of the Industrial Revolution – atmospheric concentrations of carbon dioxide and methane have increased by 31% and 149%, to levels higher than at any time during the last 650,000 years. The cause is largely the burning of so-called ‘fossil’ fuels, and de-forestation. Plants remove carbon dioxide from the atmosphere, using it to build their own tissues with the aid of sunlight (photosynthesis). When we cut down a living tree and burn it, we merely return to the atmosphere the carbon that it had absorbed during its lifetime, so it doesn’t in itself change the long-term carbon dioxide level. However the earth’s crust contains vast quantities of buried, fossilised plant material deposited millions of years ago, in form of coal and oil. When we dig these up and burn them, we do add net carbon dioxide to the atmosphere because the carbon they contain had been removed from circulation. Also, that tree we just cut down is no longer alive to absorb any of the resulting carbon dioxide.
Recently it has been suggested that another effect is at work, namely the production of excess methane by human agricultural activities, including the digestion of vegetation by ruminants like cows and sheep (over 90% of the mammals alive today are domesticated ones) and the rotting of vegetation in flooded fields, for example during rice cultivation. If this is so, anthropogenic global warming actually started with the invention of agriculture rather than the invention of heavy industry, and if confirmed this would make the task of combating global warming even more intractable, since it would imply a return to hunter-gathering!
The biggest difficulty in establishing the reality
of anthropogenic global warming is that the earth’s temperature
varies naturally on a roughly 100,000 year cycle which caused the series
of Ice Ages (see graph above). We live during an ‘interglacial’,
that is a period between Ice Ages – it appears that the evolutionary
transition from earlier Homo species to modern Homo sapiens occurred during
the last Ice Age – but we don’t know how long this interglacial
will last. Hence we can’t be sure whether we’re still on an
upswing or on the downslope to the next Ice Age: in other words, whether
the climate is naturally warming or cooling. This allows global warming
sceptics to argue that the warming we see is real, but natural rather
than man-made. To complicate the issue further still, US palaeoclimatologist
William Ruddiman  has argued that human influence on the climate actually
began 8000 years ago with the discovery of agriculture, rather than with
the 18th century Industrial Revolution. He contends that this influence
in fact prevented carbon dioxide (and later methane) levels from falling
as rapidly as they would otherwise have done, and that we would otherwise
be entering (perhaps would already have entered) the next Ice Age. In
other words, even if we could do something to stop global warming, maybe
Sources of such evidence are found in fossils; ice cores drilled out of the polar caps; sediment samples from lake beds; marks left in the geological record by sea level changes; by counting tree rings (dendrochronology); from the geological evidence left by retreating glaciers; and for the last few thousand years, from human historical records, including cave paintings and grave depths as well as written descriptions. Many of these studies employ powerful dating techniques that depend on the constant decay rate of naturally-occurring radioactive isotopes like carbon-14 (the famous ‘radio-carbon dating’), potassium-argon and rubidium-strontium.
Results to date – published by The Intergovernmental
Panel on Climate Change in a 2001 report – suggest that there is
a real, if modest, warming going on. Average temperatures have increased
over both land and sea by between 0.4 and 0.8ºC over the last century,
and are predicted to rise further by 1 to 5ºC over the coming century,
while temperatures in the lower troposphere have been increasing by between
0.12 and 0.22ºC per decade since 1979. However there’s still
some uncertainty over what proportion of these rises is due to human activity,
over the accuracy of the underlying climate models, and over the accuracy
of estimated future carbon dioxide emissions.
Given the uncertainty of the data it’s not
surprising that neither side has yet conceded defeat, and it’s terribly
difficult for lay persons to judge the evidence effectively. It would
be fair to say though that a majority of climate scientists have quite
recently swung around to believing in global warming, thanks to analyses
of ice-core data collected in the late 1990s coming on stream.
The water released by melting ice caps will certainly raise average sea levels, though predictions of how much and how fast still vary enormously. The present official estimate by the IPCC predicts a rise of just under 1 meter by the year 2100, but the panel also warns that this is based on current rates of melting: if the rate increases over that period it may cause irreversible changes in the earth's glacial system that will raise the sea level by many meters over the longer term. Many of the world’s cities are for obvious historical reasons located on coasts and estuaries and even quite small rises in sea level could flood many densely-populated areas, rendering them uninhabitable and destroying their agricultural value, so causing mass starvation and creating massive movements of refugees. Around 200 million people are vulnerable, especially in Egypt, low-lying Pacific islands, the Philippines, Bangladesh, China, India, Vietnam, Thailand, and Indonesia. A sea level rise of more than 4 metres would inundate almost every coastal city in the world, causing serious disruption to trade and manufacture.
A complete switching off of the Gulf Stream, which has happened before in geological times (for example during the great Ice Ages) could make Britain and Northern Europe as cold as Newfoundland. Indeed it’s a grim climatological irony that the phenomenon labelled ‘warming’ may in fact bring cold, dry weather – disastrous to agriculture – to many parts of the world, particularly the interiors of large continental landmasses . Other effects of decreased ocean circulation might include increased severity of typhoons and hurricanes, prolonged droughts and failure of the Monsoons, all of which would have desperate economic consequences.
These are the more moderate catastrophe predictions.
The ultimate worst case scenario – fit for a sci-fi blockbuster
– would be a runaway heating effect that transforms the earth’s
climate so completely that it obliterates all life and turns the planet
into something resembling Venus. Disturbingly, there is a scientifically
plausible (which is not to say probable) mechanism for such an event,
which depends on the fact that in addition to the fossil fuels we burn,
there are enormous quantities of carbon stored under the earth and in
solid ice crystals that chemists call ‘methane clathrates’,
formed when methane dissolves in cold water. If the ice melts, these clathrates
release methane gas like bubbles released from champagne. Vast deposits
of methane clathrates exist on the sea-bed in the deepest parts of the
oceans, and more a few metres below the ground in the permafrost soils
of the Arctic tundras (where they were formed by vegetation rotting in
swamps). Were global temperatures to rise to a point where the permafrost
melts, or the deep ocean warms sufficiently, all this methane could be
released and lead to a runaway greenhouse effect. Some geologists believe
that the earth has experienced these sudden heatings due to clathrate
decomposition in the past (for example the Permian-Triassic extinctions
and the Paleocene-Eocene Thermal Maximum). Unlike the slow rise of sea
levels, such an event could happen quite quickly once some temperature
‘tipping point’ was exceeded, making it effectively impossible
to predict like the timings of earthquakes or volcanic eruptions. The
consensus at present is that it is extremely unlikely.
The most significant political initiative to combat global warming so far is the 1997 Kyoto Protocol, an amendment to the United Nations Framework Convention on Climate Change (UNFCCC) . All countries that ratify this protocol commit themselves to reducing their emissions of carbon dioxide and five other greenhouse gases, and a market-based system of permit is set up that enables countries that fail to reduce emissions to trade ‘permits to emit’ with those who bettered their quota. More than 150 countries have so far ratified the Protocol, but notable non-signers include the USA and Australia, both of which claim that the economic costs are too high for the benefit.
There are also political and technical obstacles
to each of the types of solution listed above:
All these objections though seem like nitpicking when compared to the real problem, which is that the Industrial Revolution is nowhere near complete yet. The phenomena we generally label as ‘globalisation’ and ‘outsourcing’ in fact represent the continuing spread of that industrialisation which started in 18th century Britain to the remaining parts of the world, most particularly to China, India and Indonesia. All of these nations want to achieve the levels of affluence that we have in Europe and the USA, and no amount of environmental pleading is likely to deflect them from this aim. And their unprecedentedly rapid industrialisation is almost totally dependent on increasing fossil fuel usage. It seems increasingly probable that they will not be able achieve their goal, but it will be shortages of oil, water, soil and possibly food that will stop them, not any amount of moral pressure from Westerners who are themselves loathe to give up their cars, fridges, and air-conditioners. Far the most likely scenario must be that industrialisation is forced into reverse gear by a slow, involuntary and painful series of famines, floods, droughts, wars over natural resources, financial collapses and mass migrations. Such a reversal may not involve a complete return to pre-industrial ways, but it might reduce the world’s population and living standards substantially.
If we have difficulty accepting this stark reality, that’s because we suffer a hangover from 20th century positivism. Almost all the great thinkers of the 19th and 20th centuries believed that technological advance would only continue, and that it was leading to a convergence on some kind of superior social organisation. To be sure they disagreed profoundly about what that organisation would be – from the techno-religion of August Comte, through Herbert Spencer’s laissez-faire capitalism to the egalitarian communism of Karl Marx. (Most of our current thinkers, from neo-cons to anti-globalizers, still cling to some derivative of one or other of those models). What none of them could imagine, because the facts were not then known, is that technological advance might eventually prove self-limiting due to processes like anthropogenic global warming. We are after all another animal species, subject like others to population pressures generated by the environment, and our belief that we could control every aspect of nature is being shown to be hubristic. Perhaps Malthus was right in principle (though wrong in details and timing) and the planet really does set a limit to the number of us it is prepared to support.
 Jared Diamond, Guns, Germs and Steel, W. W. Norton
1999 and Collapse: How Societies Choose to Fail or Survive, Allen Lane,
 IPCC Third Assessment Report published in 2001 by the Intergovernmental Panel on Climate Change – http://www.grida.no/climate/ipcc_tar/
 Richard A. Muller, ‘A Brief Introduction to History of Climate’, UC Berkeley – http://muller.lbl.gov/pages/IceAgeBook/history_of_climate.html
 Congressional Research Service (CRS) Reports regarding Climate change – http://digital.library.unt.edu/govdocs/crs/
 NAS: National Academy of Sciences: ‘Understanding and Responding to Climate change’ – http://dels.nas.edu/basc/climate-change_final.pdf
 Committee on the Science of Climate Change, National Research Council, ‘Climate Change Science: An Analysis of Some Key Questions’, National Academies Press, 2001 – http://books.nap.edu/books/0309075742/html/index.html
 Naomi Oreskes, ‘Beyond the Ivory Tower:
The Scientific Consensus on Climate Change’, 2004 – http://www.sciencemag.org/cgi/content/full/306/5702/1686
 UNFCCC: Kyoto Protocol, Status of Ratification – http://unfccc.int/files/essential_background/kyoto_protocol/application/pdf/kpstats.pdf
 Michael Grubb of the Carbon Trust and Imperial College London on implementing the Kyoto Protocol – http://www.opendemocracy.net/debates/article-6-129-2517.jsp
 The Byrd-Hagel Resolution (US refusal to sign Kyoto), ‘Expressing the sense of the Senate regarding the conditions for the United States becoming a signatory to any international agreement on greenhouse gas emissions under the United Nations’ – http://www.nationalcenter.org/KyotoSenate.html
 David M. Ackerman, ‘Global Climate Change: Selected Legal Questions About the Kyoto Protocol’ – http://www.opencrs.com/getfile.php?rid=33856
 Giulio De Leo, Luca Rizzi, Andrea Caizzi and
Marino Gatto, ‘Carbon emissions: The economic benefits of the Kyoto
Protocol’, Nature #413, 478-479 , 4th Oct 2001.