Final answer:
Since the mid-1970s, global warming has been characterized by a rise in Earth's average surface temperature, mainly caused by human activity that enhanced the greenhouse effect, leading to an overall increase in temperature.
Step-by-step explanation:
Global warming since the mid-1970s has been about a steady rise in the Earth's average surface temperature, generally attributed to an enhanced greenhouse effect caused mainly by human actions, particularly the increase in the use of fossil fuels and deforestation.
The resultant rise in greenhouse gas levels, such as carbon dioxide, traps more heat within the Earth's atmosphere and contributes to changes in climate, weather patterns, sea levels, and other environmental impacts. The overall increase in global temperature since the mid-20th century is approximately 0.75 degrees Celsius, and due to the inertia in the climate system, there is an expected additional increase in temperature even if greenhouse gas emissions were to stop immediately.