Carbon dioxide (CO2) is one of the main gases in our atmosphere that traps heat, creating the greenhouse effect that provides the conditions that allow human life to exist on Earth. Without the natural greenhouse effect, the average temperature on Earth would be a frigid minus 18 degrees Celsius.
And we know from the geological record that natural changes in carbon dioxide levels have directly related to past increases and decreases in global temperatures. Scientists use ice core records from Antarctica to reconstruct prehistoric temperature records, and those same ice layers trap little bubbles of CO2, which means we can see how they relate over time.
Temperature and CO2 have fluctuated together through ice ages and warm periods called interglacials. Slow changes in Earth’s orbit around the sun initiated the ice ages—CO2 was a natural amplifier, causing the global climate to warm and cool through each cycle.
Here, [animation shows an arrow indicating point on the time line/graph of CO2 concentration] at about 200,000 to 300,000 years ago is where modern humans had just evolved.
And here [animation shows an arrow indicating point on the time line] at the last interglacial period, the sea level peaked at about six to nine metres higher than it is today.
At about 20,000 years ago we have the last ice age. At that time, kilometre thick ice sheets spread across Europe and North America, eating up water and driving sea levels down to 130 metres lower than where we are today. From that low cold point the CO2 rises naturally by as much as 35 parts per million each 1,000 years, stabilising to near modern levels at around 10,000 years ago.
Oh look, there’s the birth of modern agriculture! [animation shows picture of person tilling the ground, and also indicates relevant point on the timeline/graph of CO2 concentration].
And the wheel! [animation shows picture of a wheel and also indicates relevant point on the timeline/graph of CO2 concentration]
And the pyramids in Egypt! [animation shows picture of a pyramids and also indicates relevant point on the timeline/graph of CO2 concentration]
And now we hit the Common Era, the year zero CE. At this point, the world population is just 200 million people. [animation shows a series of images of significant moments in history, and also indicates relevant point on the timeline/graph of CO2 concentration: the construction of the Great Wall of China, the Industrial Revolution].
And here’s where scientists first demonstrate the greenhouse effect of carbon dioxide, way back in 1861. It would take until 1938 for scientists to show Earth was warming and proposed that CO2 was the cause.
Here’s where we are now. [animation shows timeline/graph of CO2 concentration, with CO2 concentrations significantly higher] When Earth warmed out of the last ice age, CO2 levels naturally increased by 35 parts per million over 1,000 years. Humans have caused CO2 to rise by this amount in just the past 17 years. And, our population is at 7.5 billion people, with many cities around the world rapidly developing, increasing their CO2 output year-on-year.
Our current temperature is around 1 degree Celsius above pre-industrial levels. This might now sound like much but the last time our climate was that warm was the last interglacial period, when sea levels were 6 to 9 metres higher: enough to creep into cities like New York, Manila and Brisbane, and flood Bangkok, Miami, Amsterdam and many more.
On current projections by 2100 our climate could warm by 4 degrees Celsius or even more, depending on our actions to cut CO2 levels.
So, does this look natural to you? [animation shows an arrow pointing to the most recent point on the timeline/graph of CO2 emissions showing the high levels of CO2]
[credits written on screen:
Narrated by Dr Benjamin Henley (University of Melbourne)
Written by Assoc Prof Nerilie Abram (Australian National University), Dr Benjamin Henley
Animated and edited by Wes Mountain
Music Keven Macleod – Faster Does It
Antarctic temperature from ice cores: Parrenin et al., 2013, Science
Global scaling for Antarctic temperature: Snyder 2016, Nature
Historical temperature data: HadCRUT4, UK Met Office
Climate model temperature projections: CMIP5 RCP2.6 and RCP8.5 projections
Ice core CO2 history: Bereiter et al. 2015, Geophysical Research Letters
Historical CO2 observations: Mauna Loa observatory, NOAA/ESRL]