The phrase "human activity" refers to anything that people do or the actions they take. It includes everything from work and play to eating and sleeping. Basically, it's about all the things humans do in their daily lives. Full definition
Most responses to today's environmental woes aim to limit the impact of human activity by minimizing pollution and waste. (mcdonough.com)
Some greenhouse gases occur naturally in the atmosphere, while others result from human activities such as burning of fossil fuels such as coal. (sciencedaily.com)
Otherwise we're simply monitoring the effects of human activity in our neighborhoods, without any reference to the overall picture, about which we presently know damn little. (wattsupwiththat.com)