**Core Concept**
Global warming refers to the long-term rise in Earth's average surface temperature, primarily caused by human activities that release greenhouse gases, such as carbon dioxide (CO2) and methane (CH4), into the atmosphere. This increase in atmospheric CO2 concentrations enhances the natural greenhouse effect, leading to a warming of the planet.

**Why the Correct Answer is Right**
Global warming is primarily caused by the increasing levels of CO2 in the atmosphere, which is a result of fossil fuel burning, deforestation, and other human activities. The enhanced greenhouse effect leads to a trapping of heat, causing the Earth's average surface temperature to rise. This warming is observed in various parts of the world, including the Arctic, which is warming at a faster rate than the global average. The increase in CO2 levels also leads to ocean acidification, as the absorbed CO2 reacts with water to form carbonic acid.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because while deforestation contributes to global warming, it is not the primary cause.
**Option B:** Incorrect because the ozone layer depletion is a separate environmental issue, primarily caused by the release of chlorofluorocarbons (CFCs) and other halogenated gases.
**Option C:** Incorrect because the increase in solar radiation is a natural phenomenon, but it is not the primary cause of global warming. Human activities have significantly contributed to the increase in greenhouse gases.

**Clinical Pearl / High-Yield Fact**
The increase in CO2 levels is directly linked to the burning of fossil fuels, such as coal, oil, and natural gas. This highlights the importance of transitioning to renewable energy sources to mitigate global warming.

**Correct Answer: D.