29 April 2010

Earth's Nine Life Support Systems

Note that the big two are not climate change - but species loss and the nitrogen cycle - both intimately connected with human appropriation of land, for food in particular...

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Excerpt from the New Scientist, 24 February 2010

'Up to now, the Earth has been very kind to us. Most of our achievements in the past 10,000 years - farming, culture, cities, industrialisation and the raising of our numbers from a million or so to almost 7 billion - happened during an unusually benign period when Earth's natural regulatory systems kept everything from the climate to the supply of fresh water inside narrow, comfortable boundaries.

This balmy springtime for humanity is known as the Holocene. But we are now in a new era, the Anthropocene, defined by human domination of the key systems that maintain the conditions of the planet. We have grabbed the controls of spaceship Earth, but in our reckless desire to "boldly go", we may have forgotten the importance of maintaining its life-support systems.

The demands of nearly 7 billion humans are stretching Earth to breaking point. We know about climate change, but what about other threats? To what extent do pollution, acidifying oceans, mass extinctions, dead zones in the sea and other environmental problems really matter? We can't keep stressing these systems indefinitely, but at what point will they bite back?

Last year, Johan Rockström, director of the Stockholm Environment Institute in Sweden, sat down with a team of 28 luminaries from environmental and earth-systems science to answer those questions. The team included Nobel laureate Paul Crutzen, NASA climate scientist James Hansen, Gaia researcher and "tipping point" specialist Tim Lenton, and the German chancellor's chief climate adviser Hans Joachim Schellnhuber.

They identified nine "planetary life-support systems" that are vital for human survival. They then quantified how far we have pushed them already, and estimated how much further we can go without threatening our own survival. Beyond certain boundaries, they warned, we risk causing "irreversible and abrupt environmental change" that could make the Earth a much less hospitable place (Ecology and Society, vol 14, p 32).

The boundaries, Rockström stresses, are "rough, first estimates only, surrounded by large uncertainties and knowledge gaps". They also interact with one another in complex and poorly understood ways. But he says the concept of boundaries is an advance on the usual approach taken by environmentalists, who simply aim to minimise all human impacts on the planet. Instead, he says, boundaries give us some breathing space. They define a "safe space for human development"....


Boundary: Annual species extinction rate no more than 10 per million per year
Current level: At least 100 per million per year
Diagnosis: Boundary far exceeded

Humans are driving species to extinction by ploughing up or paving over their habitats, by introducing alien species like rats and weeds, by poisoning them with pollution, by hunting them for food and, increasingly, by changing the climate. Individual species may not matter much on their own, but collectively they form ecosystems that provide a range of vital "ecosystem services", such as recycling waste, cleaning water, absorbing carbon and maintaining the chemistry of the oceans.

Although we know that high levels of biodiversity are essential to healthy ecosystems, it is not yet clear how much can be lost before ecosystems collapse, nor which species are the key players in a given ecosystem. So Rockström's team settled on crude extinction rates as the best "interim indicator" of the state of ecosystems. They put the current extinction rate at more than 100 extinctions per million species per year, and rising. That compares with a natural "background" extinction rate of around 0.3. Up to 30 per cent of all mammal, bird and amphibian species will be threatened with extinction this century.

This cannot go on safely. Current rates may even mirror those of the "big five" mass extinctions of the past half-billion years, including the meteorite strike that did for the dinosaurs. While the world carried on after those events, it was massively transformed. To avoid a repeat, they suggest a safe long-term annual extinction rate of no more than 10 per million species per year. By that measure, they say, "humanity has already entered deep into a danger zone... if the current extinction rate is sustained"


Boundary 1: No more than 35 million tonnes of nitrogen fixed from the atmosphere per year
Current level: 121 million tonnes per year
Diagnosis: Boundary far exceeded and effects worsening...

Nitrogen is an essential component of all living things, yet only a small amount of the planet's stock of nitrogen is in a form that living things can absorb. This is "fixed" out of the air by bacteria in a range of leguminous plants. But you can have too much of a good thing. So other microbes "denitrify" ecosystems, converting the element back into forms not available for living things. This is the nitrogen cycle.

Farmers have always interfered with the cycle, because nitrogen availability often limits the fertility of soils. They have boosted production by planting more leguminous crops, like clover.

Then, a century ago, the nitrogen cycle changed forever when Fritz Haber, a German chemist, invented an industrial process for fixing nitrogen from the atmosphere to make chemical fertiliser. Today, 80 million tonnes of nitrogen is fixed from the atmosphere in this way each year and poured onto the world's fields.

But farming inefficiencies mean that most of this nitrogen runs off the land into rivers and oceans. Much of the nitrogen that does get into crops is later excreted by humans into sewers. We further fix nitrogen by cultivating legumes and burning fossil fuels, timber and crops. Put all that together, and we fix around 121 million tonnes of nitrogen a year, far more than nature does - and nature cannot cope.

The excess nitrogen is acidifying soils, killing vulnerable species and saturating ecosystems so that they lose the ability to recycle the nitrogen back into the air. Meanwhile, some over-fertilised lakes and seas in heavily farmed regions fill with "blooms" of aquatic life which then die and decompose, sucking all the oxygen out of the water in the process. The legacy of such blooms is anoxic "dead zones". At the last count there were more than 400 such zones in the oceans, covering 250,000 square kilometres, including parts of the Gulf of Mexico, the Baltic Sea and waters between Japan and Korea.

Rockström tentatively sets the safe level for human additions to the nitrogen cycle at about 35 million tonnes a year, one-quarter of the current total. Reaching that figure while continuing to feed the world is, to say the least, a tough ask...'

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