 Carbon dioxide 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 the 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, at about 200 to 300,000 years ago, is where modern humans had just evolved. And here, at the last interglacial period, the sea level peaked at about 6 to 9 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. And the wheel. And the pyramids in Egypt. And now, we hit the common era, the year 0 CE. At this point, the world population is just 200 million people. 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 the Earth was warming and propose that CO2 was the cause. Here's where we are now. When the 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 last 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 about 1 degree Celsius above pre-industrial levels. This might not 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, 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?