4.5 billion years of climate change

Increased atmospheric CO₂ catalysed the development of agriculture and civilisation as we know it. So why is rising CO₂ now a problem? By Anna Mouton.

“The evolution of modern human society was triggered by rising CO₂, and a warming, wetting and more productive climate,” says Prof. Guy Midgley, Director of the School for Climate Studies at Stellenbosch University. “Today, human society depends on a narrow range of CO₂ concentrations, probably 280–350 ppm.”

Recent atmospheric CO₂ measurements show the atmosphere approaching 430 ppm. “We’re out of equilibrium due to the sharp increase. We’re acidifying the oceans and we’re destabilising the climate,” says Midgley. “That’s very, very straightforward.”

When CO₂ was a 1000x higher

The Earth formed 4.5 billion years ago as a ball of molten rock enveloped in hydrogen, water vapour, methane, and ammonia. Once things cooled down, the water condensed to cover nearly the entire planetary surface. Meanwhile, violent volcanic activity and repeated asteroid bombardment added atmospheric gases, including CO₂.

The young Sun was only about 70% as powerful as the middle-aged Sun is today. If the young Earth weren’t well-insulated by greenhouse gases, the oceans would have been frozen. So, high CO₂ levels (about 1,000 times higher than present levels) were key to the emergence of the first life, 3.5–4.0 billion years ago.

Given that the Earth receives abundant solar energy, it’s not surprising that photosynthesis was an early innovation. It’s thought that the first forms of photosynthesis didn’t produce oxygen, but then the cyanobacteria came along and brought about the Great Oxidation Event, also known as the Oxygen Catastrophe.

“We went through hundreds of millions of years where cyanobacteria gradually fixed CO₂ and produced oxygen,” says Midgley. “That process fluctuated, and the atmosphere flipped between oxygenated and deoxygenated.”

Nonetheless, by photosynthesising for more than a billion years, cyanobacteria eventually pumped so much oxygen into the atmosphere that they killed most early life, which was anaerobic and unable to cope with the oxidative stress. Simultaneously, CO₂ levels dropped to a few thousand ppm.

Other life forms took advantage of the plentiful oxygen to develop aerobic respiration, and some of these cells were the ancestors of mitochondria, the energy generators of fungal, plant, and animal cells.

Trees and grasses fight it out

Modern vascular plants evolved approximately 500 million years ago. The subsequent explosion of terrestrial plants so depleted atmospheric CO₂ that the Earth entered an ice age.

About 60 million years ago, forests began to dominate the planet. There have been times during the past 2 million years when these plants drove atmospheric CO₂ levels as low as 180 ppm, and the Earth became cold, dry, and unproductive, with the CO₂ scarcity limiting photosynthesis.

Plants fix carbon through either C₃ or C₄ photosynthesis. Most plants, including all trees, use C₃ photosynthesis. Approximately 15% of plant species have evolved leaf structures and biochemical pathways that enable C₄ photosynthesis. At low CO₂ levels, C₄ plants are more productive and use less water than C₃ plants.

“If you put C₄ plants in a closed container with a tree, they could eventually use enough of the CO₂ to kill the tree,” says Midgley. “If atmospheric CO₂ dropped much lower than 180 ppm globally, trees could fall out of the system, and you would just have shrubs and grasses. And finally, grasses would win.”

By 15–20 million years ago, low atmospheric CO₂ levels were enabling grasses to displace trees. “Grasslands are very recent ecosystems,” says Midgley. “They owe their existence at least partly to low CO₂. Now that CO₂ is rising again, we’re seeing woodland encroachment across the African landscape.”

Given our current CO₂ levels, he thinks there’s little chance that plants will capture enough carbon to precipitate another ice age in the next several thousand years. “You didn’t have regular ice ages back when cyanobacteria first started photosynthesising, because CO₂ levels were just too high,” observes Midgley.

Global warming and the rise of civilisation

Early modern humans evolved in Africa around 300 000 years ago. They began migrating out of Africa about 180 000 years ago, and by 70 000–50 000 years ago, humans had spread as far as Australia.

Until about 12 000 years ago, humans were hunter-gatherers. Then we developed agriculture, and the rest, as they say, is history. But what motivated our species to start farming? According to Midgley, the stimulus was higher CO₂.

For the last 2.5 million years, the Earth has experienced repeated short warmer periods followed by extended colder periods, called Milankovitch cycles. These are driven by regular but slow changes in the direction and angle of the Earth’s axis and in the shape of its orbit around the Sun.

When the Earth cools, more CO₂ dissolves in the oceans, and the atmosphere holds less water vapour. Lower atmospheric greenhouse gas concentrations lead to further cooling, which continues until orbital changes bring about a rewarming.

“The world our species grew up in was carbon-constrained. It was cold, dry, and unproductive, without much food or wood,” says Midgley.

Then, 18 000 years ago, Milankovitch-driven warming released CO₂ from the oceans, and atmospheric CO₂ started rising. By 10 000 years ago, atmospheric CO₂ reached 280 ppm, the world was warmer and wetter, and most plants became much more productive.

Atmospheric CO₂ levels profoundly affect plant growth. For example, wheat yields increase more than threefold when CO₂ levels rise from roughly 150 ppm to 250 ppm. Although C₄ plants have a greater edge over C₃ plants at lower CO₂ concentrations, C₄ plants also benefit from higher CO₂ concentrations, for example, through increased drought resistance.

There is growing evidence that low CO₂ levels prevented successful agriculture for most of our species’ history. Once CO₂ levels increased, agriculture developed relatively quickly in several different places. Stone tools, including stone scythes for cutting grasses, first appeared, and humans switched to a carbohydrate-based diet.

Fossil fools

Scientists estimate that living organisms have fixed about 20¹¹ gigatonnes of carbon through photosynthesis since life evolved (a gigatonne is 1 000 million tonnes). For much of Earth’s history, cyanobacteria did the heavy lifting, but nowadays terrestrial plants fix most of the carbon.

“Humans add ten gigatonnes of carbon to the atmosphere every year,” says Midgley. “About four gigatonnes stay there, while about three gigatonnes dissolve in the ocean, and the rest goes into plants, probably in the tropics.”

In ancient times, when organisms died, the carbon in their bodies effectively died with them, eventually becoming either oil or coal. Most oil is derived from cyanobacteria and marine algae, while most coal is derived from terrestrial plants. Once decomposition evolved as a lifestyle, dead organisms were recycled instead of being converted into fossil fuels.

“Although, in certain scientists’ offices, in big piles of paper, coal is still being formed,” jokes Midgley. “It takes time, and pressure.”

Fossil fuels are really a form of solar power. “You can think of gas, oil, and coal as a biological battery that was powered up hundreds of millions of years ago,” says Midgley. “We’re running that battery down, and it will never be recharged again.”

Consider a litre of petrol in an internal combustion engine: it will release about 34 million joules, which is equivalent to the work that 20 healthy adult male manual labourers can do when they all work for eight hours.

Midgley thinks humans are addicted to this super-concentrated energy. Could it be that fossil fuels are turning us into fossil fools, as we keep pumping greenhouse gases into the atmosphere?

Too much of a good thing

Despite the good things that have come from atmospheric CO₂, recent man-made increases are not a positive development. “We have to go back about 16 million years before we see the kinds of CO₂ levels we have now,” says Midgley. “The rate of CO₂ increase is now greater than it has been for the past 60 million years.”

Elevated atmospheric CO₂ is warming the planet and its atmosphere, making the climate more unstable and unpredictable, and undermining agriculture.

Global warming also causes sea-level rise, partly due to melting ice sheets and glaciers, and partly because water expands as it heats up. The average global sea level has risen approximately 25 centimetres in the last century, and the rate of increase is accelerating. Sea-level rise is already causing more tidal flooding. With 10% of the world’s population living in coastal areas, sea-level rise is projected to cause mass migrations even as early as 2050.

Then there’s the effect of heat. For the past 6 000 years, human populations have clustered in areas representing a narrow temperature range. Scientists estimate that 1–3 billion people live in the regions that will become too warm for human survival within the next 50 years.

“It’s very clear that you can’t keep warming the world without consequences,” says Midgley. “Nature will survive, but modern human society is at risk because so much of it depends on a stable climate.”

He underscores that we need to find ways to reduce atmospheric CO₂ levels if we want to ensure civilisation endures. “Nature will pull carbon out of the atmosphere, given enough time, but that’s not quick enough for us,” he says. “It’s crucial that we stop emitting.”