Why is a 2˚ Celsius increase the threshold that the Earth can handle?

What is so special about 2˚ Celsius? G20 world leaders, scientists and policymakers met in Paris for the COP21 this past December to discuss specific areas of special relevance, most importantly, efforts to combat climate change. This climate change conference, is just one of many crucial meetings for the implementation of plans to reach targets for sustainable development by 2030. World leaders have reached a binding agreement that would stop the average global temperature from rising 2˚ Celsius, which is the equivalent of 3.6˚ Fahrenheit. But why is 2˚ the magic number? The global average temperature during the industrial revolution in the late 1800s was 13.7˚ Celsius. This is when our use of fossil fuels started to accelerate. The amount of CO2 emissions was constant for at least a century after that. The burning of coal gas and oil began building in the atmosphere and oceans, which started to raise the temperature of the planet. Since then the Earth’s global temperature has increased about 1˚ Celsius. In the 70s this trend spiked upward. In the 70s, an economist named William Nordhaus first suggested the significance of 2˚ Celsius. In the 90s, scientists and policymakers put forth 2˚ has the amount of temperature increase that the Earth could handle without experiencing social and economic consequences. The United Nations Climate Change Group formally recognized this number in 2010. At the current rate of emissions the Earth’s average temperature will rise 4˚ Celsius. To limit this increase to 2˚ we have to stop the use of fossil fuels and move the energy supply to renewable sources and this transition must take place very quickly to avoid devastating consequences.