How soil imprisons ancient carbon

Blogging on Peer-Reviewed Research

Deep underground, there lies a sleeping giant that we would do well to avoid waking. The giant is a massive, dormant amount of carbon, and it’s better for us that it remains trapped in the ground rather than circulating in the atmosphere as carbon dioxide.

Soil changes characteristics with depthMany of the most crucial debates of the 21st century will involve reducing the amount of carbon dioxide being pumped into the atmosphere. The many possible solutions include trapping carbon, either in the trunks of trees or in underground vaults. The irony is that a massive amount of carbon is already locked safely away underground.

The world’s soil acts as a carbon prison and it holds more of it than the earth’s atmosphere and all of its living things combined. Over three trillion tonnes of the element are incarcerated in soil and about 80% of this is found at depths of up to 3 metres. At these levels, carbon is very stable and plays no part in the carbon cycle, the process where the element is exchanged between the land, air and sea.

Now, Sebastien Fontaine and other scientists from the French National Institute for Agricultural Research have found that deep soil retains carbon so well because it lacks enough fuel for the microbes that decompose organic matter.

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Human nitrogen emissions indirectly capture carbon by fertilising forests

Human activity has greatly increased the levels of active nitrogen in the environment. By acting as a fertiliser and speeding the growth of forests, this extra nitrogen has indirectly locked up more carbon dioxide in the world’s trees.

There is no doubt that of all the elements in the entire periodic table, carbon is currently hogging the limelight. As it cycles through our environment, the policy decisions and economic futures of entire countries hang in the balance.

Industrial exhausts pump huge amounts of nitrogen into the atmosphere.For all its media-whoring, you might be forgiven for forgetting that carbon is not the only element we are belching into the environment. Over the last century, we have greatly overwhelmed the natural nitrogen cycle too.

Nitrogen – the neglected element

Through the manufacture of nitrogen-based fertilisers and the exhausts of our cars, power plants and factories, we have more than doubled the natural levels of active nitrogen in the atmosphere.

Nitrogen is a valuable commodity in many parts of the world, and restricts the growth of local plant life. As such, the recent man-made influx has led to large increases in plant growth. In some cases like algal blooms that choke rivers and lakes, it’s too much of a good thing. But there is a silver lining.

Federico Magnani from the University of Bologna, together with an international team of scientists, have found that the changes in the nitrogen cycle may have been inadvertently fertilising our forests.

Carbon and nitrogen

The world’s forests act as massive carbon sinks, delaying the global warming effects of carbon dioxide by trapping it in prisons of wood and leaves. And larger forests mean more trapped carbon. The temperate forests of the Northern Hemisphere alone could store a massive 600 megatonnes of carbon every year.

The carbon and nitrogen cycles dance around each other in complex ways. When nitrogen levels increase, forests respond by channelling growth from roots to leaves and trunks. These above-ground organs are more enduring than roots and retain sequestered carbon for a longer time. More leaves also means increased photosynthesis, which serves to draw more carbon dioxide in from the air.

The extra nitrogen also delays the decay of leaf litter, further halting the release of organic carbon into the atmosphere.

The world’s forests act as massive carbon stores.Magnani’s colleagues are not the first group to try and look at the interplay between nitrogen levels and carbon capture. But other studies have found it difficult or impossible to account for the effects of nitrogen alone.

Carbon balance

A forest’s carbon balance – the amount of carbon trapped versus the amount released – depends on a variety of factors, including its age, logging, fires, and more. Some of these are easy to account for at a small scale. For example, when logging or fires kill off patches of forest, they become net sources of carbon as they start to regrow.

But after a couple of decades or so, the mature forest turns into a carbon sink, and the amount it stores outweighs the amount it releases. Clearly, a forest’s carbon balance changes as it matures, but real forests consist of patches of vegetation are very different ages.

To look at the overall picture, Magnani’s group took direct measurements of the carbon balance over a long period of time, from a network of forest sites in Western Europe and the USA. This allowed them to account for short-term sources of variation. And by using direct measurements, they have surpassed the models and simulations of previous studies.

The group found that carbon balance corresponds well with nitrogen levels in the area. In fact, the prowess of some forests at carbon capture seem to be overwhelmingly driven by their extra nitrogen boost. Our effects on the nitrogen cycle may have been acting like an unexpected carbon offset scheme.


By fertilising forests, nitrogen emissions could offset carbon emissions.So should we start pumping nitrogen in our forests to trap more carbon dioxide? Certainly, Magnani’s results suggest that small extra amounts of nitrogen can cause unexpectedly large levels of carbon capture. But his view and those of other commentators is a resounding “Not yet”.

There are still many questions left to be answered, particularly about the exact relationship between nitrogen addition and carbon levels. There is some evidence that some temperate forests are suffering from nitrogen saturation. Could adding more nitrogen damage them, or prevent them from returning to a situation where nitrogen is limited and not free-flowing?

And what of the other risks and benefits? The extra wood from the faster-growing trees could find a use as a replacement for concrete, a notoriously eco-unfriendly building material. But additional nitrogen could affect other animals and plants in the local environment. Would biodiversity suffer if certain species monopolise the newfound nitrogen bonuses?

As future research addresses these questions, those involved in forest management would do well to heed the importance of the world’s forests in sequestering carbon dioxide. There are other ways of increasing forest coverage besides mass-fertilisation, and the most obvious one is safeguarding the forests that we already have!

Reference: Magnani, Mencuccini, Borghetti, Berbigier, Berninger, Delzon, Grelle, Hari, Jarvis, Kolari, Kowalski, Lankreijer, Law, Lindroth, Loustau, Manca, Moncrieff, Rayment, Tedeschi, Valentini & Grace. 2007. The human footprint in the carbon cycle of temperate and boreal forests. Nature 447: 848-850.

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Carbon offset schemes worsen global warming if trees are planted in the wrong places

Many carbon offset schemes rely on planting new trees to counteract rising carbon dioxide levels and the climate change they cause. But new research shows that these schemes only work if trees are planted in the tropics. Plant elsewhere, and you’ll only be adding to global warming.

Plant a tree and save the planet. If it sounds too good to be true, it’s usually because it is.

Temperate forests actually keep the world warmer by absorbing solar radiation into their shaded undergrowthIt is now crystal clear that modern global warming is a man-made phenomenon. But with this acceptance comes guilt, and the quest to find ways of mitigating our energy-hungry lifestyles

On the surface, carbon offset schemes appear to offer a win-win solution. People can assuage their guilt over yet another business flight, or drive to the shops, by paying for trees to be planted or investing in renewable technology.

Trees act as carbon sinks, sucking the gas in from the air and shunting the carbon atoms across into the plants’ own molecules. So plant enough trees, and the emissions you are responsible for will effectively be negated. You can whistle a jaunty tune and slap a carbon-neutral sticker on your car.

That’s the theory anyway. But Govindasamy Bala and colleagues form the Lawrence Livermore National Laboratory have found that it’s not just what you plant that matters, it’s where.

They ran complex simulations of how the planet’s climate would change if trees in different parts of the world were removed or restored. Unexpectedly, they found that overall, deforestation cools the planet down, and adding new trees in some regions may actually fuel global warming.

Carbon offset schemes are a poor alternative to not flying at all.In a simulation where all the world’s trees were removed, the global temperature fell by about 0.3 degrees Celsius.

Why should this be? After all, trees soak up carbon dioxide and store carbon in their bodies – this keeps the planet cool. They release water vapour into the air, which forms clouds that reflect solar radiation away form the earth, again resulting in cooling.

But forests are also dark and by absorbing the energy from sunlight, they heat the planet too. According to Bala’s simulation, this heating effect outweighs the cooling ones.

When Bala looked at the effect of deforestation in specific areas, a clearer picture emerged. The tropical rainforests are doing their bit in fighting global warming by forming clouds and absorbing carbon dioxide. Their loss led to a rise in global temperature.

In contrast, the temperate and polar forests aren’t pulling their weight. These verdant slackers heat the planet themselves by absorbing solar radiation. Without them, the underlying snow would reflect more of the sun’s energy into space and we’d get a cooler planet.

Carbon offset schemes only fight global warming if trees are planted in the tropics.These experiments suggest that tree-planting will only help to restrain global warming as planned if it happens in the tropics. In other parts of the world, it could even do more harm than good. When it comes to carbon offset schemes, the devil’s in the details.

Bala and his co-workers are modest on their work and are quick to point out that it is based on a single simulation. And they are careful to quickly stem the inevitable backlash from anti-environmental groups, who may well perversely suggest that this data warrants declaring war on trees.

Forests clearly have value beyond their influence on temperature. They harbour a great richness of life, keep the soil together and stop the oceans from acidifying by storing carbon dioxide – the list goes on. Deforestation is clearly not a solution to global warming, but wanton re-forestation won’t do any good idea.

Bala’s study gives pause for thought to those of us who seek to placate our environmental consciences by paying into carbon offset schemes.

At the very least, the details of any schemes should be checked carefully. Even better, serious thought should be given to preventive measures, like reducing car or plane use, rather than cures.


Reference: Bala, Caldeira, Wickett, Phillips, Lobell, Delire & Mirin. 2007. Combined climate and carbon-cycle effects of large-scale deforestation. PNAS 104: 6550-6555.

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Opinion: How biofuels could cut carbon emissions, produce energy and restore dead land

A new way of producing biofuels could not only curb carbon emissions and produce renewable energy, but also restore unusable agricultural land and improve biodiversity. But only if this winning breakthrough find its way onto the political agenda.

Grasses have great advantages as biofuels over monocultures of soy or corn. The twenty-first century is having a troubled infancy. Six years in and it is facing the twin perils of climate change and a looming energy crisis. Solutions to both are in high demand and many research dollars and pounds are being channelled into developing environmentally-friendly, renewable resources.

Biofuels – the product of living things – certainly fit the bill, being both renewable and biodegradable. But there is always a catch. Currently, biofuels are a matter of harvesting single crops grown on fertile soils such as corn or sugarcane or waste products such as straw.

In George Bush’s State of the Union address of January 2007, corn-based biofuels played a major role in reducing the USA’s dependence on oil. But it is highly unlikely that these fuels will make a large dent in America’s energy demands.

The fuel-bearing plants need land to grow on, and the choice becomes either using up current agricultural land that provides much-needed food for growing populations, or to clear natural land and damage the ecosystems they nourish.

Any new crops must also be irrigated and treated with potentially polluting fertilisers and pesticides. And the water, chemicals and eventual crops must be transported with fossil-fuel-burning vehicles.

At first glance, biofuels seem to create more problems than they solve. In an ideal world, we would source biofuels from crops grown on used land with no other agricultural value, with a minimum of chemical help.

But such a world may be just round the corner, thanks to scientists from the University of Minnesota. David Tilman, Jason Hill and Clarence Lehman have discovered that the key to low maintenance biofuels is diversity.

Tilman's biofuels experimentThe trio cultivated plants in 152 plots on agriculturally degraded soil (see left; photo taken by David Tilman), with low levels of the nitrogen that crop plants need to thrive on. They were irrigated once when the crops were planted, and left untouched by fertilisers.

They found that plots which cultivated a variety of plants produced far more energy than those with a single species, with the most productive ones containing 16 different species.

These so-called ‘low-input, high-diversity’ or LIHD plots contained a mix of humble woody plants, legumes and grasses, such as wild lupine, goldenrod, and switchgrass.. They produced over three times as much energy as monocultures of single species.

Tilman found that every hectare of the LIHD plots yielded 68 gigajoules of energy a year but because they were so low maintenance, they only needed 4 gigajoules to pay off the energy debt of production, harvesting and transport. At processing plants, the fuels can be converted into gasoline, diesel and electricity.

In this way, each hectare of LIHD plots produce over 50% more usable energy on abandoned soil than other crops do with fertile soils.

Part of the LIHD crops’ success lay in the fact that legumes can seed impoverished soils with valuable nitrogen. Over the decade the experiment ran for, nitrogen levels in the LIHD plots increased by a quarter.

The biological diversity in each plot also warded against diseases and marauding species, never allowing a single invader to gain a proper foothold. This greatly reduced the need for pesticides and chemical protections.

Providing an alternative to fossil fuels is just one way in which LIHD biofuels could help to curb carbon emissions – they also act as carbon sinks. Monocultured crops such as corn and soybean produce less greenhouse gases than petroleum-based petrol and diesel, but they are still carbon-positive – their production leads to a net increase in carbon dioxide.

In contast, LIHD biofuels are carbon-negative, removing carbon dioxide from the atmosphere and storing it in both the soil and the growing roots of the plants themselves.

This stored CO2 outweighs the total amount emitted during production and transportation by more than ten times and every hectare of crop captures about 4 tonnes of carbon dioxide every year. Compared to corn-based biofuels, the greenhouse gas reductions achieved by LIHD fuels were 6-16 times greater.

The world currently has at least 500 million hectares of agriculturally abandoned land that serves no fruitful purpose, and could be used to sow LIHD crops. The resulting biofuel harvest could replace 13% of the world’s petroleum consumption and 19% of its electricity needs.

LIHD biofuels are an environmentalist’s dream, and could provide a very rare win-win situation for the world’s energy providers. They represent a way of providing renewable energy while reducing carbon emissions, conserving biodiversity and both using and renewing otherwise degraded land.

It is an opportunity that scientists need to explore further and the world’s policy-makers need to start taking seriously.


Reference: Tilman, Hill & Lehman. 2006. Science 314: 1598-1600

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Climate change – one degree till the point of no return

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Climate change – one degree till the point of no return

The world is beginning to take the problem of climate change very seriously, and it should. New data from NASA shows that the world is within a degree of the hottest temperature in the last million years. Hit this target, and the disastrous effects of global warming will become irreversible. We are nearing the point of no return.

When it comes to temperature, one degree Celsius seems like nothing. If the temperature outside changed by a single degree, almost none of us would notice and if it rose by this much, almost none of us would complain.

But according to new research, this single degree now separates us form the point of no return, when the threat and problems of climate change become irreversible.

Around the world, scientists, politicians and the public alike are starting to realise that climate change is a very real and very serious problem. Reports of record temperature levels seem to be an increasingly common fixture in the British press.

But the world’s entire climate is connected. To get a proper picture of the impact of climate change, it is useful to look at the global situation.

Global temperature has risen since 1880This is what James Hansen and colleagues at NASA’s Goddard Institute for Space Studies did in September of this year. They looked at global changes in the world’s temperature over the last century, using a wealth of measurements from land stations, ships and satellites.

Their results show an unmistakeable pattern of increasing temperatures, particularly during the turn of the century.

Compared to climate in 1951-1980, the world in the first five years of the 21st century is warmer almost everywhere, although more so over land than sea, and at high northern latitudes in particular.

Over the last century, the global temperature has risen by 0.8C in the last century, and 0.6C of that was in the last three decades. On average, 2005 was the warmest year on record, largely because of aberrantly high temperatures in the Arctic.

Wasting time

As ever, climate sceptics are not convinced. Some have suggested that these higher average temperatures are invalid and biased because of measurements taken in typically hotter urban centres. But studies in some of the world’s remotest regions are clearly saying otherwise.

Glaciers are retreating and the ice on rivers and lakes is breaking up earlier. Even the open ocean is heating up, and if that were not enough, the largest temperature rises have been found in remote locations in the northern hemisphere. Global climate change is very real and is happening faster than ever.

This is not the first time that Hansen has sounded the alarm. In 1988, he published a model for global temperature change and presented it to the US Congress.

Unfortunately, he was heavily criticised in some quarters, most notably by Michael Crichton in his lamentable novel, State of Fear, which asserted that Hansen’s data was out by 300%.

Greenhouse gases are causing record temperature rises. The result was unproductive time-wasting at a critical juncture. As the window of opportunity shrank, Crichton, firmly leaning towards fiction over science, was invited to provide testimony to the US Senate and was even granted an audience with the President.

Hansen’s new analysis soundly trashes Crichton’s criticisms and shows that his earlier model of the ‘most plausible’ warming scenario has come to pass some 20 years later.

Looking into the past; staring into the future

The recent data all well and good, but variations over a short time could be one-off incidents, becoming mere blips when longer time-scales are considered. To account for this, Hansen compared our current temperature with that of prehistoric times, by looking at fossil shells.

Shelled animals deposit different amounts of minerals into their shells depending on the surrounding temperatures, so fossilised marine shell-wearers can tell us how hot it was in prehistoric times to within a degree’s accuracy. .

The results are astonishing. Because of our unremitting greenhouse gas emissions, the Earth is now within one degree Celsius of the hottest temperature it has experienced in the last millions years.

The situation is especially stark in the Western Equatorial Pacific region, an crucial area that regulates much of the world’s atmosphere and oceanic weather. Drastic changes here will not go unnoticed elsewhere.

They can even affect the rate at which the polar ice caps melt, as sub-tropical Pacific waters intermingle with Antarctic currents. Temperature rises in this region have not been matched by rises in the Eastern Pacific, and this difference may be driving more frequent El Nino events, like those in 1993 and 1998.

At the current rate, global temperatures are increasing by 0.2C every ten years. By 2056, the world will be a degree higher, and Hansen’s analysis shows that this is the turning points where things go from bad to irreversibly catastrophic.

If we halt climate change so that future warming occurs at under 0.1C per decade, things still don’t look rosy. Sea levels will still rise by about a metre every century, spelling problems for the world’s substantial coastal populations, such as Bangladesh and many island nations.

But these problems are completely dwarfed by the terrifying potential of what could happen if we let greenhouse gas emissions continue unabated.

What happens if we do nothing

One degree warmer and polar melting will be irreversible. In this worst-case scenario, CO2 emissions continue to grow at 2% a year and other greenhouse gases such as methane and nitrogen oxide continue to rise.

As a result, Earth becomes 2-3 degrees hotter by the turn of the next century, reaching a level not seen for 3 million years.

Much of the polar ice caps will melt. As they do, less sunlight will be reflected, ice streams will flow faster, and the structural integrity of ice shelves will collapse, accelerating the process in what scientists call ‘positive feedback loops’.

The influx of meltwater will raise sea levels by several metres each century, driving coastlines 25-35 metres higher than they are today, and completely altering the face of the globe.

The world’s animals and plants will undergo mass migrations in range, habitats will fragment and the carefully balanced ecosystems of today will be sundered. Extinction will claim some 60% of today’s species, and if the planet’s history is to be believed, a rise of 5C could kill 90%.

The message to our generation is clear – global warming is real and is following a path that we can accurately model. For anyone under the age of 40, the point of no return will probably happen within our lifetimes if nothing is done.

In as little as another decade, a lack of action could render the challenge of climate change insurmountable. Everyone from Governments to individuals must do their part and they must do it now.

Hansen, Sato, Ruedy, Lo, Lea & Medina-Elizade. 2006. PNAS 103: 14288-14293 .