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SHIMMERING waters in remote mountain reservoirs providing hydroelectricity
seem a world away from the grimy coal-burning power stations of industrial
landscapes. Hydroelectric plants burn nothing and apparently cause no air
pollution鈥攏o acid rain, no grimy particles. They are, surely, the ultimate
clean sources of energy. Most countries categorise hydropower as a
鈥渮ero-emission technology鈥�. And governments around the world are committed to
cutting their future output of greenhouse gases by building hydroelectric dams
instead of fossil-fuel power stations.

But their efforts may be misguided. Studies on reservoirs from the Canadian
Arctic and the jungles of the Brazilian Amazon are starting to show something
odd. Many of these large bodies of water鈥攊n effect pent-up energy waiting
to pour through turbines鈥攖hemselves produce greenhouse gases. They give
off large amounts of both carbon dioxide and methane, which accumulate in the
atmosphere, trapping solar heat and so contributing to the greenhouse effect. As
a result some reservoirs cause more global warming than coal- or oil-fired power
stations producing the same amount of energy.

One of the worst offenders is the Balbina reservoir on the River
Uatum茫, a tributary of the Amazon deep in the Brazilian rainforest. The
dam supplies most of the electricity for Manaus, the capital of the Amazon
region. It is a modest 50 metres high but floods 310 000 hectares of river
valley, an area the size of Lancashire or Luxembourg. From the air, the great
expanse of water looks more like a temporary flood than a permanent reservoir. A
third of the water is less than 4 metres deep and the skeletons of dead and
dying trees break the surface almost everywhere. Water flow is sluggish and much
of the reservoir is stagnant and covered in weeds.

In common with many other dam builders, the engineers who built the Balbina
dam in the mid-1980s made no effort to remove the trees before flooding the
site. With no accessible market for the timber, felling was not economically
viable, so the reservoir was allowed to inundate more than 100 million tonnes of
vegetation. As it decays most of the carbon it contains will eventually be
released into the air.

Reservoir bogs

Philip Fearnside, an ecologist at Brazil鈥檚 National Research Institute for
Amazonia, has studied the gases emitted from the reservoir. Decaying plants
breaking the surface water produce CO₂, as does rotting vegetation in
the oxygenated waters within a metre or so of the surface. But vegetation in the
anaerobic waters produces methane. In shallow areas of the Balbina reservoir,
says Fearnside, 鈥渕ethane bubbling can be seen everywhere鈥�. And more is lurking
in the depths. It is sometimes released suddenly, as happened in 1993 at
Balbina, killing thousands of fish. Otherwise, it may leak into the atmosphere
as water passes through the turbines.

Fearnside calculates that in the nine years since the Balbina reservoir
formed behind its dam, there has been a dramatic release of CO₂. In
1988, the first year after flooding, the reservoir emitted more than 10 million
tonnes. Today, the annual figure has fallen to about a quarter of that. Methane
is produced much more slowly. In the first year, some 150 000 tonnes emerged
from the reservoir. But that figure will be maintained more or less
indefinitely. In the Amazon, says Fearnside, it takes 500 years for a tree trunk
to decay in anoxic water.

Assessing the damage resulting from the release of CO₂ and methane
is complicated. CO₂ persists much longer in the atmosphere鈥攁n
average molecule will stick around for a century, compared to a decade or so for
methane. But while it is there, methane is a much more potent greenhouse gas. So
what time frame should be used for any damage assessment? Fearnside adopts a
formula from the UN鈥檚 Intergovernmental Panel on Climate Change that, for the
first century after its release, a molecule of methane will have 11 times as
powerful a greenhouse effect as a molecule of CO₂. A longer time frame
produces a lower figure. A shorter period produces a higher figure. Over the
first 20 years, for instance, methane is 60 times as potent.

However the sums are done, it is clear that any increase in the amount of
vegetation rotting to methane rather than CO₂ will have a major
influence on the greenhouse effect of a reservoir. Flooded forests are often
major sources of methane and none more so than those of the Amazon basin. Here,
says Fearnside, 鈥渢he environment, devoid of oxygen, with relatively high
temperatures and high levels of nutrients, makes it ideal for methane-producing
decay processes鈥�. Even without artificial flooding, the Amazon is 鈥渙ne of the
world鈥檚 major sources of methane鈥�, says Fearnside. About 2 per cent of the
Amazon region鈥攁round 100 000 square kilometres鈥攆loods for part of
each year. And reservoirs add significantly to the region鈥檚 natural propensity
to generate methane.

Dam pollution

At Balbina, Fearnside has aggregated the greenhouse effect of methane and
CO₂ to give a 鈥淐O₂ equivalent鈥�, that he can compare directly
with emissions from fossil-fuel power stations. He calculates that Balbina
emitted the equivalent of more than 12 million tonnes of CO₂ in its
first year. That figure fell to some 7 million tonnes in 1990 and to around 2
million tonnes last year. It will slip below 1 million tonnes in about 10 years鈥�
time, and drop to 0.5 million tonnes in perhaps 50 years.

How do these figures compare with conventional power stations? Balbina鈥檚
average output of electricity in the first eight years of its operation was 112
megawatts鈥攏ot much for a reservoir the size of an English county. It
floods the equivalent of two soccer pitches to generate enough power to run a
1-kilowatt electric fire.

If the Balbina dam had not been built, the authorities in Manaus would
probably have constructed a conventional power station burning diesel and fuel
oil. Such a plant would have produced annual emissions鈥攁lmost all of it
carbon dioxide鈥攐f some 0.4 million tonnes, says Fearnside. So far, Balbina
has had something like 16 times as potent a greenhouse effect as an equivalent
fossil-fuel power station. And, says Fearnside, despite gradually reduced
emissions, it will continue to be more polluting 鈥渇or 50 years, and probably
颈苍诲别蹿颈苍颈迟别濒测鈥�.

Fearnside has looked at other reservoirs in Brazil. None is as bad as Balbina
but emissions are far from negligible. Take the giant Tucuru铆 dam on the
River Tocantins. Completed in 1984, it is one of the world鈥檚 ten most powerful
hydroelectric dams, with a capacity of 4000 megawatts. It flooded a little less
forest than Balbina, but generates more than 30 times as much power. Even so,
after six years of operation, it was still emitting the equivalent of almost 3
million tonnes of CO₂ per annum. This is almost half as much as a
fossil-fuel power station of a similar capacity, says Fearnside. He estimates
that in 1990, Brazil鈥檚 four rainforest reservoirs were producing the equivalent
of some 11 million tonnes of CO₂ annually.

Trends in the North

It is hard to imagine two environments more different than the Amazon
rainforest and the Canadian Arctic. Yet here too researchers have found high
emissions of greenhouse gases from reservoirs. Canada has hydroelectric
reservoirs covering about 20 000 square kilometres, an area nearly the size of
Wales. They produce around 60 per cent of the country鈥檚 electricity.

Three years ago, John Rudd and his colleagues at the Canadian government鈥檚
Freshwater Research Institute in Winnipeg investigated several hydroelectric
sites to produce the first detailed calculations of gas production from
reservoirs. Rudd concluded that upland forests and peat bogs鈥攖he two main
habitats flooded for reservoirs in Canada鈥斺�漚re sites of intense microbial
decomposition and greenhouse gas production when they become covered in
飞补迟别谤鈥�.

At the Notigi reservoir in northern Manitoba, Rudd found that two years after
the flooding of a forest there were massive amounts of methane dissolved in the
surface layers of the lake. He estimated that annual production at that time was
more than 7 grams of methane for every square metre of the reservoir鈥檚 surface.
In a second study Rudd鈥檚 team looked at methane and CO₂ emissions from
flooded peat bogs. They found still higher gas production鈥攗p to 30 grams
of methane and between 450 and 1800 grams of CO₂ per square metre.

Rudd calculated the likely impact of these gases on the atmosphere during the
expected 50-year productive lifetime of a hydroelectric reservoir. He estimated
that two-thirds of the vegetation and soil flooded by new reservoirs would decay
over that period, but much less of the woody material. And that up to a tenth of
the carbon in the vegetation would be released as methane, with the rest forming
CO₂. Averaged over the 50 years, he says, each square metre of a
typical reservoir in northern Canada will emit the equivalent of between 400 and
700 grams of CO₂ per year鈥攖he higher figures corresponding to
those reservoirs where peat bog dominates.

Is this worse than a fossil fuel plant? Again, it depends on how much land is
flooded and how much electricity is generated. In another study, Rudd looked in
detail at the Cedar Lake reservoir, part of the Grand Rapids hydroelectric
project in northern Manitoba. He estimates that, over its lifetime, the
greenhouse effect of the 1200 square kilometre lake will be almost identical to
that of an equivalent coal-fired power station.

Critics say that Cedar Lake, like Balbina in Brazil, is an extreme case. Luc
Gagnon of Hydro-Quebec, the company that runs the huge James Bay hydroelectric
complex in Quebec, points out that Cedar Lake floods six times as much land for
every megawatt of generating capacity as the James Bay reservoirs. And he says
Rudd exaggerated the effect of methane emissions by assigning it a greenhouse
effect 60 times that of CO₂, compared to Fearnside鈥檚 factor of 11.

There is no one right way of doing this calculation. Rudd is simply
reflecting the impact of methane over a shorter timescale. But Gagnon points out
that if Rudd had used Fearnside鈥檚 formula, it would nearly halve the calculated
greenhouse effect of Cedar Lake鈥檚 emissions.

Despite the different methodologies of Fearnside and Rudd, there is no
denying that hydroelectric reservoirs are contributing to the greenhouse effect.
Both scientists agree that the overall impact on global warming is too great to
be ignored. And yet, so far, that is just what is happening. More than 150
countries have signed the Climate Change Convention and are currently
negotiating national targets and timetables for reductions in emissions of
greenhouse gases. To aid their work, scientists are preparing national
inventories of current emissions. Reservoirs could prove to be a significant
part of many nations鈥� outputs of greenhouse gases. Yet, to date, no country has
included emissions from reservoirs in those inventories.

Flood plans

Balbina is not the only reservoir flooding huge areas to produce relatively
small amounts of electricity. The Akosombo dam in Ghana has inundated more than
11 000 square kilometres of lush tropical vegetation, flooding an area
equivalent to more than one soccer pitch for every kilowatt of capacity. In
Russia, the Kuibyshev dam on the Volga floods half a soccer pitch for each
kilowatt, and the Tsimlyansk on the Don floods two pitches for each
kilowatt.

If all Canada鈥檚 reservoirs emit as much greenhouse gases per square kilometre
of their surface area as Rudd estimates, then over the next 50 years they will,
on average, emit the equivalent of 13.8 million tonnes of CO₂. That
would add 12 per cent to Canada鈥檚 national emissions.

Brazil has plans for a massive reservoir-building programme in the Amazon
region that would increase the surface area of the country鈥檚 reservoirs
twentyfold. That, says Fearnside, would raise emissions of greenhouse gases from
Brazil鈥檚 reservoirs to the equivalent of more than 200 million tonnes of CO
₂ a year鈥攐ver three times the country鈥檚 current annual emissions from
burning fossil fuels.

Worldwide, the amount of land flooded by hydroelectric reservoirs is around
600 000 square kilometres鈥攁n expanse of water larger than the North Sea.
At Canadian rates of emissions that would produce more than 400 million tonnes
of CO₂ equivalent, or 7 per cent of total man-made emissions of CO
₂. And with many more reservoirs planned, Fearnside warns that they are 鈥渁
significant source of emissions of greenhouse gases that can no longer be
颈驳苍辞谤别诲鈥�.