Ending greenhouse gas emissions may not stop global warming

Nature (With minor editing by PinP)

One of several steel power pylons toppled in an historic wind, snow and ice storm which swept through eastern Manitoba about a year ago. It left thousands without power in what was described as the worst power outage in the history of Manitoba Hydro. Damages are expected to exceed 100 million dollars.
A Manitoba Hydro photo. 

Even if human-induced greenhouse gas (GHG) emissions can be reduced to zero, global temperatures may continue to rise for centuries afterwards, according to a simulation of the global climate published in Scientific Reports.

Jorgen Randers and Ulrich Goluke modelled the effect of different greenhouse gas emission reductions on changes in the global climate from 1850 to 2500. They also created projections of global temperature and sea level rises.

What do they show? Under conditions where manmade greenhouse gas emissions peak during the 2030s, then decline to zero by 2100, global temperatures will be 3°C warmer and sea levels 3 metres higher by 2500 than they were in 1850. Where all such emissions are reduced to zero during the year 2020 here's the scenario the models portray. 

After an initial decline, global temperatures will still be around 3°C warmer and sea levels will rise by around 2.5 metres by 2500, compared to 1850. Global temperatures could continue to increase after emissions have reduced, as continued melting of Arctic ice and carbon-containing permafrost may increase the levels of water vapour, methane and carbon dioxide in the atmosphere. Melting of Arctic ice and permafrost would also reduce the area of ice reflecting heat and light from the sun.

To prevent the projected temperature and sea level rises, the authors suggest that all GHG emissions would have had to be reduced to zero between 1960 and 1970. To prevent global temperature and sea level rises after greenhouse gas emissions have ceased, and to limit the potentially catastrophic impacts of this on Earth’s ecosystems and human society, at least 33 gigatonnes of carbon dioxide would need to be removed from the atmosphere each year from 2020 onwards through carbon capture and storage methods.