Volcanic eruptions can have both cooling and warming effects on the Earth's climate. Initially, eruptions release large amounts of sulphur dioxide (SO₂), which when combined with water, form sulphate aerosols in the stratosphere. These aerosols reflect solar radiation, leading to a temporary cooling effect. However, volcanoes also release CO₂, a greenhouse gas. Over longer periods, this CO₂ can contribute to the greenhouse effect and, thus, global warming. The immediate cooling effect from the aerosols usually lasts a few years, while the warming effect from the CO₂ can last much longer, depending on its atmospheric concentration.

Absolutely, the natural greenhouse effect is crucial for life on Earth. Without it, the average temperature on Earth's surface would be about -18°C, which is far too cold to sustain the current diverse ecosystems and human civilizations. The greenhouse effect, in its natural state, keeps our planet warm enough to be habitable, maintaining it at an average of about 15°C. However, problems arise with the enhanced greenhouse effect caused by human activities, which leads to excessive warming, impacting climates and ecosystems.

Water vapour is indeed a significant greenhouse gas, often holding the most considerable concentration in the atmosphere. It contributes greatly to the natural greenhouse effect. However, its concentration in the atmosphere is not directly influenced by human activities. Instead, it acts as a feedback mechanism. As temperatures rise due to other greenhouse gases, more water evaporates, increasing the amount of water vapour in the atmosphere, which can amplify the greenhouse effect. But, since humans don't emit water vapour directly in significant quantities, the focus is often on gases we influence more directly, like CO₂ and methane.

Apart from burning fossil fuels, numerous human activities intensify the greenhouse effect. Deforestation is a primary factor; trees absorb CO₂ and release oxygen, so cutting them down not only reduces CO₂ absorption but also results in stored CO₂ release. Agriculture is another significant contributor. Livestock like cattle produce methane during digestion, and rice paddies emit methane too. Industrial processes can release various greenhouse gases, including nitrous oxide from chemical manufacturing and hydrofluorocarbons (HFCs) used in refrigeration. Urbanisation and waste management, where organic waste decomposes without oxygen in landfills, also produce methane.

The "greenhouse effect" is a natural process that warms the Earth's surface. It occurs when the sun's energy reaches the Earth's atmosphere; some of this energy is reflected back to space while the rest is absorbed and re-radiated by greenhouse gases. This trapped heat warms the Earth's surface, making life possible. On the other hand, "global warming" refers to the long-term increase in Earth's average surface temperature due to human activities, particularly the emission of increased levels of greenhouse gases. In essence, while the greenhouse effect is a natural occurrence, global warming is a human-induced enhancement of this effect.