Imagine rolling a ball up a hill. As you walk up the hill, if you push the ball up a bit, it will simply roll back into your hands.
However, once you roll the ball over the top of the hill it will keep on rolling away and you’ll lose it. Here, the top of the hill is the ‘tipping point’; at that point if you give the ball the tiniest extra push it will roll away from you and won’t stop!
Normally, our Earth’s systems behave in a similar way to the ball at the bottom of the hill – if we give them a little push by changing them slightly, they will eventually return to their natural state . However, if we cause our Earth system to change a lot, by increasing the amount of CO2 in the atmosphere for example , the ball moves to the top of the hill, so any little extra push will cause a huge and irreversible change . This is called a ‘tipping point’ .
Crossing a tipping point is irreversible because you can’t simply return to the original state of the climate system by undoing the small change . It triggers an unavoidable change which can be sudden and immediate , or may be delayed and happen years into the future .
 https://www.uu.nl/sites/default/files/lenton_2011.pdf (Page 202 Predictability and early warning signs Paragraphs 1& 2 with Figure 2)
 https://www.pnas.org/content/pnas/105/6/1786.full.pdf (Abstract)
 https://www.ipcc.ch/sr15/chapter/glossary/ (Tipping Points and Irreversibility)
 https://www.pnas.org/content/pnas/105/6/1786.full.pdf (Defining a Tipping element and its tipping point paragraph 1 – see option (iii). Also we know it can be on the scale of year because ‘T’ – the time for the result of the tipping point to be observed (see equation 1) can be of the order of 100-1000 years (see page 1787 paragraphs 6 (starting with Conditions 2-4) and paragraph 7 (starting with ‘One useful way’))
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