Is global warming caused by humans or is it caused naturally?

There is a natural greenhouse effect that keeps the planet warm. But that is not causing global warming. It is the enhanced greenhouse effect, caused by man, that is causing global warming.

Carbon dioxide is a naturally occurring greenhouse gas. At normal concentrations, it, along with other greenhouse gases, helps maintain global temperatures at a level that is comfortable for humans and other animals that have adapted to the present climate. This carbon cycle is the natural part of global warming.

By adding new carbon dioxide to the atmosphere, which we do by burning fossil fuels such as coal, oil and natural gas, and by cement manufacture, we interfere with the natural carbon cycle and create enhanced global warming. Throughout human history until the Industrial Age, atmospheric carbon dioxide levels were in the range 260-280 parts per million (ppm) and never higher than this. The increase in concentrations of carbon dioxide to the present level of over 400 ppm is accepted by research scientists as principally the result of human activity and so the resultant enhanced global warming is caused by human activity.

We have also destroyed huge forests all over the world (deforestation) that used to remove carbon dioxide from the atmosphere.