I don't think there is any legitimate about the seriousness of climate change. We have already warmed 0.8C and even if we instantly stopped all fossil carbon output, we will see at least another 30-40 years of warming - that would bring us up to ~1.5C.
The only debate is about whether any/some/all the positive feedback forcings kick in.
Obviously, we cannot stop using fossil fuels instantly. So, every decade that we continue to
increase our carbon output means that in all likelihood - we will blow past the 2C temperature rise by 2100.
The most critical debate is around the threshold of a runaway warming - the generally accepted threshold is ~2C including the 0.8C we have already seen. If we reach the level high enough to tip us over into melting tundra and/or causing the ocean to start releasing the carbon dioxide it has been absorbing up until now, or any other forcing - then we are up a creek without a paddle.
The current climate change is *far* more rapid than anything we know about before ever before. And even if it did happen this quickly before, it has never happened while humans existed - and it would be a terrible thing. It is this rapidity and it's "momentum" that is the main reason for the uncertainty.
Here's a "non-sucky" information graphic laying out the possibilities:
A larger more readable version is available here, and additional information - click on the image to enlarge:
The rare non-sucky infographic on climate change | Grist
To review - we have warmed 0.8C and the ocean has risen about 8" in the past 100 years or so, and we have about 4-5% more evaporation. If we hit the 2C but stay under it, we have about 13 years if we do not cut our current fossil carbon emissions to exhaust our carbon budget of another 500 gigatons of carbon - and then we *would* have to stop all fossil carbon use. To put it another way, if we want to stay under the 2C rise is global average temperature, and if we want to continue to use some fossil fuels longer than ~13 years into the future - we have to greatly reduce our use of them *now*.
Here is the best estimate of what a 2C warmer planet looks like (from the graphic):
1.04m/3.4ft sea level rise from the 1990 level
The means Amsterdam and other cities like it are in knee high flooding
Coral (and plankton?) starts dissolving their shells
Summer heatwaves reaching 10C/18F higher than normal
20% reduction of corn and wheat crops
13% heavier rainfalls
~30% of all species at risk of extinction
Greenland ice sheet will start to melt, and while it will take about 50,000 years to completely melt - this will raise sea level by 6m/19.7ft