Carbon dioxide and climate

To make accurate predictions about the future of the Earth’s climate, we need to look to the past.

Observations on fossils and other samples of ice layers, strata or ice and sediment cores provide a picture of how Earth’s climate has changed over time. Understanding the influence of greenhouse gases in the past helps us understand the causes of modern climate change. It also helps scientists forecast changes to future climates and how these changes will affect the oceans, weather patterns, the land and how we use it.

Stories from the past

As a child, Professor Jane Francis sorted through rocks on her family farm, looking for plant fossils. Fast-forward, and her interests are still much the same – but with a very different purpose. Professor Francis is a paleoclimatologist and Director of the British Antarctic Survey. She uses fossil plants as tools to interpret past climates and ancient biodiversity. For most of its history, Antarctica had a warm climate – fossilised pollens and turtle bones are among the evidence of the subtropical conditions that once existed at the South Pole. At other times, Antarctica’s climate was more like New Zealand’s South Island – the fossil record shows southern beech forests similar to those of the South Island’s West Coast rather than warmth-loving plants.

It is clear that, over time, the Earth’s climate has moved from warm periods to cold periods and back again. It is also clear from ice core records that there is a strong correlation between atmospheric temperature and CO₂ levels.

Geological processes influence CO₂ levels

For most of the Earth’s existence, natural geological processes controlled atmospheric CO₂ levels. For example, 50 million years ago – when the polar regions were warm – fossil and rock evidence shows that the atmospheric CO₂ level was around four times the pre-industrial level. CO₂ had been building up slowly from volcanic outgassing (eruptions) over hundreds of millions of years.

In the last 50 million years, natural processes like mountain building and the associated weathering of feldspars to clay absorbed massive amounts of CO₂. As a consequence, the climate cooled, allowing the oceans to absorb more CO₂. Organic matter was buried and became coal, oil and gas. Ice formed on mountaintops, cooling the air, which led to more ice formation. Polar ice sheets reflected sunlight back into the atmosphere, locked up huge amounts of freshwater and began to drive deep ocean circulation.

Human influences on CO₂ levels

With the Industrial Revolution, humans began to use the carbon (coal, oil and gas) that had been stored underground. As a result, in the last 150 years or so, CO₂ levels have risen at least 300 times faster than the combined effect of natural processes in the last 10,000 years. For comparison, CO₂ from global volcanic activity is less than 1% of the global industrial CO₂ emissions.

Predicting the future

By using clues from the past, scientists can learn more about current CO₂ levels and rising temperatures and the way they influence the Earth’s interconnected systems. For example, melting ice sheets will cause sea level rise and impact ocean functions like the Global Ocean Conveyor. These changes will affect weather and climate and, in turn, where and how we live and how we use the land.

Computer models can generate scenarios showing what the future may look like as global temperature rises. If we continue to use fossil fuels at the current rate, atmospheric CO₂ will have risen 3–4 times the pre-industrial level. That’s the level Earth experienced 50 million years ago when sea levels were about 60 m higher, crocodiles roamed the Arctic and Antarctica was green.