For at least the last 800,000 years, atmospheric CO2 concentrations fluctuated between about 180 ppm during the ice ages and about 280 ppm during warmer periods as carbon moved between oceans, forests, soils, and the atmosphere. These changes have been in step with global temperatures and are a major reason why the entire planet warmed and cooled during glacial cycles, not just the frozen poles.
Today, however, the CO2 level has risen to 420 ppm – the highest level in at least three million years. The CO2 concentration also rises about 100 times faster than at the end of the last ice age. This suggests something else is going on, and we know what it is: since the industrial revolution, humans have been burning fossil fuels and releasing greenhouse gases that are now heating the planet (see question 5 for more details on how we do this know, and questions 4 and 8, how do we know that other natural forces are not to blame).
In the next century or two, societies and ecosystems will experience the consequences of this climate change. However, our emissions will have an even more sustainable geological impact: According to some studies, greenhouse gas levels may have already warmed the planet enough to delay the start of the next glacier cycle by at least another 50,000 years.
How do we know global warming is not due to the sun or volcanoes?
The sun is the ultimate source of energy in the earth’s climate system and therefore a natural candidate for causing climate change. And solar activity has certainly changed over time. We know from satellite measurements and other astronomical observations that solar power changes in 11-year cycles. Geological records and sunspot numbers that astronomers have tracked for centuries also show long-term variations in solar activity, including some exceptionally quiet periods in the late 17th and early 19th centuries.
We know that solar radiation increased from 1900 to the 1950s. And studies suggest that this had a modest impact on the early 20th century climate, explaining up to 10 percent of the warming that has occurred since the late 19th century. However, in the second half of the century, when the greatest warming occurred, solar activity actually decreased. This inequality is one of the main reasons we know the sun isn’t the driving force behind climate change.
Another reason we know that solar activity has not caused warming recently is that all layers of the atmosphere should warm up if they did. Instead, data shows that the upper atmosphere has actually cooled in the past few decades – a sign of the warming of the greenhouse.