If you are wondering what catastrophes could be caused by 
global warming, just look at the below picture. All red zones will submerge if the 
sea level raises with 20 meters as result of the 
increase in precipitation.
David Archer started 
his PI lecture with the statement that satellites and weather stations agree that Earth is warmer now with 0.5 degree Celsius since 1960 and the trend is steady upward. The difference is more evident (of around 1 degree Celsius) if we compare today's Earth temperature with that at the beginning of last century.
Here there are some other ideas that I've picked up during the lecture:
Even though the current warming looks like the 
Medieval warming,
by 
analysing historic climate change patterns it is estimated that the Earth in the year 2100 will be its 
warmest in millions of years. As a consequence by year 2300 
tundra is lost.
The danger is that once engaged in the warming trend there is no easy way back since some percentage of the 
CO2 released into atmosphere 
stays for thousands of years before it is absorbed.
If the current 
fossil fuel CO2 pollution continues it could look similar with the sudden natural CO2 release in 
paleocene/
eocene. As you can see 
in this picture the warming recovery took 100,000 years.
The 
raise of sea level could be caused by the debris rafted by the 
flow of big ice sheets into the ocean. Since it is not well known 
how water gets through the ice such events could happen much sooner than expected. Unfortunately we have a big proof of 
water flowing into the ice in Greenland 
moulins. The question is if we are today at the beginning of a 
dejavu scenario of sea level increase.
Then there are 
methane hydrates that are 
worse polluters than coal. Unfortunately there is lots of methane hydrates (much more than world's coal) below the sea bed that 
could be released if the ocean temperature increases.
It looks like the Earth is unforgivable if we are messing up with it.