Climate change? It was planetary history and plants so far – now mankind does it!

Mankind is soon likely to achieve the distinction of being the first animal species that has affected climate. Before, it was the planet itself, and plants. This is the short summary of David BEERLING’s fascinating book[1]. For the most part of its history the earth’s climate resulted from planetary history – that is happenstance: the planet’s place in the solar system, its size and inclination, the stabilizing moon, the presence of water, and of course the many large and small meteorites that impacted on the surface. Lesser known, but probably no less important, are the inner convulsions of the earth’s core – eruptions, continental drift, and the like[2]. Much of the long-term climate on earth was determined by the CO₂ content in the air. Before life emerged it was a stately long-term cycle, where volcanoes spewed out the gas. In the air it dissolved in rainwater to a weak acid; the latter went on to weather silicate in the rocks to form bicarbonates, which were deposited on the ocean floors. Subduction of the ocean floor took the deposits into the hot mantle, were they were “cooked” and ejected back into the atmosphere as CO₂. Then came photosynthesis, and life began to impact on the climate – active feedback loops arose. Cyanobacteria and phytoplankton drove the system for the first couple of billion years. Around 540 million years ago the Phanerozoic age arrived, and with it terrestrial plant life, which constitutes over 90% of the world’s biomass. It took over 50 million years for plants then to develop leaves. Once the leaf system was worked out oxygen accumulated, and in the Carboniferous it shot up to probably 30-35%. Giant insects ensued. At the boundary between the Triassic and the Jurassic, 200 million years ago CO₂ levels soared – biodiversity plummeted. BEERLING conjectures that methane hydrates – the product of the lowly methanogens, which has been frozen on sea-floors – were released catastrophically. Whether the extinction of the dinosaurs resulted from an asteroid or the eruption of the Dekkan Traps is debated. The hypothesis that these animals did themselves in with their methane flatulence has been quantified and found wanting. During the early Eocene – 55 million years ago – a surge in CO₂, coupled with methane from marshlands, nitrous oxide emitted from tropical terrestrial ecosystems, and tropospheric ozone conspired together to give us the warmest climate in recent ages. The latest climatic gyration happened eight million years ago. Genetic change in mainly tropical “grasses” – maïs and sugar cane are examples – made them better photosynthetizers than the established ones. The “C₄”[3] plants began replacing the “C₃”[4]. They displaced forest – and changed the climate – through complex feedback mechanisms involving fire. Happenstance and plants drove the earth’s climate – and then came mankind. Given the strain that 7 billion people with the ever expanding “carbon footprint” are placing on the earth bio-system it is hard to argue that our actions on the climate are “minor”. How the bio-system will react is a different matter – or what we may sensibly do to mitigate the effects of our behavior. Here is bet on surprises and discontinuities. One thing is emerging – just as we are the first species to evolve culturally, rather than just genetically, we are the first animal species that is significantly affecting the earth’s climate. Not bad for this bipedal ape which came into its own 100’000 years ago or so.

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