LONG TERM: The Earth’s orbit around the sun changes from circular to elliptical and back over about 100,000 years. <ul><li>When the Earth is closer to the sun, temperatures are higher ..when further away, the Earth is cooler. </li></ul><ul><li>Also…. </li></ul>
The Earth is tilted on its axis – the angle of tilt changes (41,000 year cycle) – and it also wobbles about (26,000 year cycle)! <ul><li>These changes also affect the amount of sunlight received. </li></ul><ul><li>NB These 3 cycles are known as ‘Milankovitch Cycles’ </li></ul>
MEDIUM TERM: Cycles of sunspot activity cause variable solar output over a few hyndred years <ul><li>Sunspots – black areas on surface of sun – areas of greater activity – vary in number – more spots, more energy towards Earth. Changes over periods of a few hundred years may link to cycles of sunspot activity. </li></ul>
This graph suggests that sunspot activity might be a factor in temperature change (– but be careful – graph only covers a short period of time)
SHORT TERM: Volcanic eruptions can send ash particles & gases many kms into the atmosphere – blocking out the sun’s rays. Mount Pinatubo, Philippines, eruption June 1991, led to a fall in average global temperatures of 0.5 degree C. However, the effect is likely to be for a few months or years only. <ul><li>, </li></ul>
Tambora, Indonesia, 1815 – the biggest known eruption in human history - <ul><li>So much ash was shot into the upper atmosphere that 1816 was called ‘the year without a summer’ – crops failed, famine was widespread and 200,000 people died. The effects lasted 5 years. </li></ul><ul><li>(but it did lead to spectacular sunsets, painted by artists like Turner: </li></ul>
Since the 1850s, there has been a rapid rise in temperatures – and human activities are being blamed for this… but that’s the next instalment!