"Human activity causes global warming" is not science.

In order for a hypothesis to be scientific, it must be testifiable.

In order to test that hypothesis, you would need 2 systems (globes), 1 with the forcing (human activity) and 1 without.

We do not have multiple planets, therefore this hypothesis can not be tested.

Therefore, the hypothesis that humans cause global warming is not science.

Scientific experiments must have controllability, observability, and be able to be reproduced. We have only observed 1 single experiment.