How do they know we've reached a tipping point? From their climate models, of course. You know them; they're the ones that have never predicted past climate remotely accurately, have not predicted future climate on a 5 or 10 year scale but, apparently, are accepted as providing accurate forecasts for 100 years into the future.
The following shows the temperature record from Canadian Arctic ice core data.
Current temperature trends show that over the past 50 years Nunavut has experienced both strong warming and cooling. The warming trend occurs primarily in the west, where as the east has undergone a cooling trend. Of course, we never hear about the bits that have cooled.
So what is the variability over the last 60 years?
If you believe that man made CO2 emissions have created a 'climate crisis' and pushed us to a 'tipping point' then, to be blunt, you're profoundly ignorant of:
- the failure of climate scientists to follow the Scientific Method;
- the failure of those climate scientists whose work is relied upon the most to make available to the public their data - even when it's publicly funded research;
- the demolition of the validity of the iconic Hockey Stick, which has seen even the IPCC derogate its position in its recent Fourth Assessment Report; and
- the failure of climate models to hindcast or forecast with any accuracy. The back-fitting that goes on is unbelievable. If we relied on financial models that followed the same method then it'd take about a month to bankrupt the planet.