The Earth has always experienced major fluctuations in climate. What is the scale of these fluctuations in temperature and in time? Do we know what caused these changes? What is driving the present changes in the Earth's climate?

GEOLOGICAL FACTORS INFLUENCING CLIMATE CHANGE

Five main factors have influenced Earth's climate in the geological past:

• changes in solar activity

• variations in the Earth's orbit

• plate movements and mountain building

• volcanic eruptions

• greenhouse gas interactions.

An obvious trigger would be variations in the intensity of solar radiation. Perhaps it was less intense at various times, before returning to the more typical state. Alternatively, the Earth might have experienced major changes in its orbit, at times being much further from the Sun, so that solar heating would be lessened. Volcanic eruptions can fill the atmosphere with dust and aerosols, screening out solar radiation. Mountain building can thrust landmasses upwards to intersect the colder air higher in the atmosphere. Greenhouse gases, especially water vapour, carbon dioxide and methane, retain heat in the atmosphere; higher concentrations of these gases match warmer periods, and lower concentrations the glacial episodes in Earth history.

Changes in solar activity

Our current understanding of the Sun's nuclear processes predicts the Sun's output will gradually increase for the next 5 billion years, and that the Sun now is emitting some 25–30% greater radiation than it was when the Earth first formed 4.6 Ga ago.