Clive Mitchell, SNH (Strategy Development), takes a look at the role of the humble leaf and its role in climate change.

Kicking piles of leaves. ©beckyduncanphotographyltd/SNH

As you kick your way through the leaves this autumn, spare a thought for the way that they have totally transformed the Earth.  It is only when leaves evolved a little under 400 million years ago that we got soils.  And with soils we get a modern water cycle, a modern carbon cycle and a modern climate system.

The modern carbon cycle transfers carbon from the atmosphere and vegetation, through rivers, into the sea.  Carbon is churned around in the sea, and some forms shells which eventually form rocks.  Rocks, especially limestones and coal, oil and gas make up the third part of the carbon cycle.

The amount of carbon stored in rocks is vastly more than in the oceans and vegetation, soils and the atmosphere.  Normally the carbon in rocks is returned to these other reservoirs very slowly, over tens or hundreds of millions of years.  Burning coal, oil and gas returns this carbon to the atmosphere as quickly as we can burn those fuels – very rapidly over the last 150 years.  This short-circuiting of the carbon cycle is the main driver of climate change and acidification of the oceans.

The Earth’s climate has always changed, but never before at the rate it has over the last 150 years.  For climate change what matters is the size of the slug of carbon entering the atmosphere, and the rate at which that occurs.

But we also know from the rock record that the Earth’s temperature goes up and down – so something must remove carbon from the atmosphere.

Tree growing out of rock in Coire Earb. © Greg Morss Creative Commons

Leaves.  Because leaves make soils, and soils eat rocks.  When rainwater mixes with carbon dioxide in the atmosphere it forms a weak acid, which is strong enough to dissolve rock dust.  The carbon in these dissolved rocks is then transported by rivers to the sea.  There, some of the carbon is used to grow shells (from plankton to clams) and some of these eventually form sediments and rocks.  This weathering of silicate rocks involves a net removal of carbon from the atmosphere into rocks.  And this is the mechanism by which the Earth has adjusted to releases of carbon into the atmosphere, and why the Earth’s temperature doesn’t keep rising and rising.

This process happens most efficiently in soils in warm humid conditions.  But it takes a long time – typically several hundreds of thousands of years.

As a result of the failure by the global community to act on climate change, there is increasing interest in enhancing the rate of rock weathering to remove carbon from the atmosphere.  But these geo-engineering solutions don’t come without their problems.  The problem of climate change results from short-circuiting one part of the carbon cycle by burning fossil fuels.  The best thing to do is to stop burning fossil fuels, rather than trying to speed up another part of the carbon cycle.  Especially when we don’t know what the side effects of that might be.  And there are some ethical questions, about using large amounts of land in tropical areas to mop up the emissions that we produce mainly in developed countries.  And, for all geo-engineering solutions, who has their hand on the global temperature tiller – who would you trust to control the immense power that resides in global temperature?

Such big issues from such a small thing as a leaf.