If all the peoples of the world got together tomorrow and agreed to never build another fossil fuel power plant or gasoline-powered automobile, all the carbon-emitting structures already built would still produce 496 gigatonnes of CO2 between now and 2060. 

what if we stop using fossil fuels

According to Steven Davis and his team at the Carnegie Institution, our energy infrastructure is already so hugely built up that producing another 496 gigatons of CO2 is almost a certainty, even if we had zero growth of CO2-emitting devices. Here’s the (slightly) good news – if only that amount of carbon is emitted, then carbon dioxide levels would stabilize at about 430 parts per million in the atmosphere, and that would only raise temperature levels to about 1.3 degrees above pre-industrial times.

That figure is somewhat moving, because most climatologists agree we will start to see the more dire effects of global warming once temperatures rise two degrees. That means we have to figure out a way to utilize lots more alternative energy sources over the next fifty years without using that final 0.7 degrees worth of carbon emissions. Davis estimates we will need to produce 30 terawatts of power from alternative energy sources if we want to maintain current growth while avoiding the worst effects of climate change. While 30 terawatts is about double the world’s total energy output in 2006, and fifteen times the amount of renewable energy we managed to produce.
To address this issue the UN’s Intergovernmental Panel on Climate Change (IPCC) said to avoid “severe, pervasive and irreversible” consequences, the majority of the world’s electricity should produced by renewable energy by 2050, and the use of fossil fuels must be eliminated by 2100.
The scientific consensus is that keeping global temperatures from rising above 2 °C is the threshold for acceptable levels of global warming. The IPCC outlined various approaches to meet this target—but they all end with the elimination of coal, oil, and gas by the end of the century, as mentioned in the report by IPCC.
The IPCC estimates a cost of only 0.06% of global GDP every year to keep temperatures from rising more than 2 °C in 2100, while global GDP in that same period will grow by 300%. In other words, it is affordable to fight climate change.[1]
Environmentalist George Marshall argues that all the dithering since 1992 may mean it’s already too late. Instead of containing rising temperatures to 2 °C—which is dangerous enough—the Earth is heading for a rise of 4 °C. The influential climate scientist John Schellnhuber (paywall) tells Marshall that “the difference between two and four degrees is human civilization.” [2] Among the consequences of a 4 °C rise:
  • Temperatures the Earth has not experienced in at least 5 million years, leading to heat waves that the human race may not be able to cope with
  • Sea levels rising by 30 feet, submerging two-thirds of the world’s major cities, through the complete melting of the Greenland ice sheet.
  • 40% of plant and animals would be at risk of extinction.
  • The loss of most of the Amazon through fire, as well as a third of Asian rainforests.
And this is not a problem that most of us can leave to our children, even if we wanted to. Marshall warns that we may see that rise of 4 °C in the next 60 years. (None of this is new; the World Bank warned we are on track for “4°C world” in 2012.) [3] And it’s going to get even harder to prioritize anything over growth—the Earth’s population could reach 11 billion by 2100[4], when the IPCC wants us to be fossil fuel-free.
 “The challenge that remains is that the scale of action is incremental and we need transformational approaches.”
 [1] http://qz.com/266329/countries-can-rescue-the-planet-from-meltdown-and-save-money-doing-it/