'Solid air': how our emissions are altering the carbon cycle

I doubt that John Martyn was thinking about the conversion of carbon dioxide and water to wood when he wrote his 1970s classic, Solid Air. His lyrics focus on the difficulty of moving forward through life as though through solid air. But, as metaphors go, it could hardly be better suited to the springtime reawakening of the plant world.

This photosynthesis reboot is a familiar but momentous occasion – the absorbing power is so great that it produces a wobble in the world’s foremost carbon dioxide record in the middle of the Pacific Ocean. This spring is especially important to the Birmingham Institute of Forest Research (BIFoR), because it marks one year until a major new experiment is switched on.

Carbon dioxide plus water plus light gives sugar and oxygen. That’s photosynthesis, and all carbon-based life proceeds from this sugar, as long as nitrogen, phosphorus and other nutrients are available in the right amounts. Like a green carpet of photovoltaic cells, the plant world captures energy, uses that energy to build itself, and sacrifices large parts of itself to every respiration-only organism – including you and me – that hitches a ride on this air solidification trick.

Respiration, as every school-kid biologist knows, is essentially the reverse reaction to photosynthesis, completing a circle that uses the energy of sunlight to sustain an immense and balanced complexity. We used to call all this ‘life’ and ‘nature’, and enjoyed the delicate tracery left by human activity. Now we must speak of Natural Capital and Ecosystem Services and translate the various stocks and flows of global metabolism into a single unit of exchange, like carbon or money. Why? Because we have pushed the living planet beyond the boundaries of its resilience by intervening in the natural circular flow of carbon and nutrients, resulting in a pressing need to remember, to quantify and to value the work done by the non-human inhabitants of Earth.

Don’t we know all we need to know? The photosynthesis-respiration story has been researched in minute detail, but, surprisingly, we still do not know where the carbon goes in a mature forest. When human activity, particularly emissions of carbon, nitrogen and phosphorus, move the ecological goalposts, the sun will no longer support a steady, circular flow of elements through a forest ecosystem. Instead, the system will respond by moving material from one reservoir to another.

So far, the response of the world’s land surface (especially its forests) to the carbon dioxide we have put into the air by burning fossil fuels has been to absorb about a third of the emissions and store the carbon somewhere. It is not entirely clear where all the carbon is stored, for how long the carbon will be stored, and whether this storing process will keep pace with our carbon emissions.

These are the uncertainties to be clarified in a major new experiment: the BIFoR Free-Air Carbon Dioxide Enrichment (FACE) experiment at Mill Haft woodland in Staffordshire. The FACE technique isolates the effect of carbon dioxide changes while retaining the complexity of the oak woodland ecosystem in which it is established. The experiment, including preparatory measurements, is designed to run for more than a decade – much, much longer than conventional lab and chamber experiments and therefore much better suited to establishing by just how much we are moving the carbon, nitrogen and phosphorus goalposts.

Each dawn and each spring, as photosynthesis begins, extra carbon dioxide will be made available to patches of oak-and-hazel woodland. Each dusk and each autumn, respiration will dominate, and the carbon dioxide be returned in part to the atmosphere. As the days and seasons pass, each part of the ecosystem – above and below ground; plant, invertebrate and microbe – will be monitored for change and to see by how much, and in what ways, the circular cycling of carbon and other nutrients has changed.

The more ‘solid air’ we find as the years go by, the more confident we can be that the trees are continuing to protect us from the worst excesses of our addiction to fossil fuels.

Professor Rob MacKenzie
Professor of Atmospheric Science and Director, Birmingham Institute of Forest Research, University of Birmingham