Is climate change caused by human activity?
Climate researchers are always evaluating new evidence to see if there are any other possible causes for climate change.
Is the extra CO2 from natural causes?
It's true that there's a natural carbon cycle. This involves CO2 getting into the atmosphere from a variety of sources including volcanoes and animals breathing, it is then absorbed by trees, plants, rocks and the oceans. This cycle has been delicately balanced for thousands of years, but human influence has offset that balance to send the levels of CO2 rising.
What about volcanoes?
Erupting volcanoes can have a significant effect on our climate. They emit large amounts of 'aerosols' (suspended dust particles, which reflect the sun's energy back into space) but even they haven't put out nearly as much CO2 as humans over the past century.
Isn't it just a natural temperature cycle?
There is natural variability in Earth's climate but the current climate change is very unusual as it is not exclusively part of a natural cycle.
Natural factors include aerosols and phenomena such as El Niño and La Niña (which cause warming and cooling of the Pacific Ocean surface). Natural climate variations can lead to periods with little or no warming, both globally and regionally, and other periods with very rapid warming. However, there is an underlying trend of warming that is almost certainly caused by man's activities.