Carbon Dioxide (CO2) is a potent greenhouse gas. Once it reaches the earth's upper atmosphere, each molecule remains active for at least a century.
What this means is that the heating effect of today and tomorrow's CO2 emissions are added to all the emissions since around 1900 that have made it into the atmosphere.
The problem therefore is cumulative. Today's temperature increases are the legacy of extra carbon in the atmosphere from up to 30 years ago. The full impact of all the CO2 we've already pumped into the skies hasn't yet even been felt – while the tens of billions more tons we will certainly add in the coming years pushes us into entirely unchartered climatic territory.
To get an idea of just how massive human influence is on the planet's atmosphere, have a look at the calculator below. Every second, another 761 tons of C02 is pumped out worldwide. It is difficult to countenance how we can continue to add this kind of heating into a 'closed' atmospheric system without seeing dramatic consequences.