well, the problem is, in general, not the temperature rise
It's a rate of change that is too fast for things like forests to migrate, etc.
a few hundreds or thousands of years? the long the transition time, the better.
Decades? we have a problem, because things will not adapt quickly enough or be able to migrate fast enough, and you will wind up with massive die-offs
Unless that is bullshit too.
we know that CO2 is rising.
This has been measured since the late 1950s, and college students replicate this around the world every year.
And we can measure CO2 content in air bubbles in Ice cores.
We can calibrate the ice cores with known events like volcanic eruptions, because the dust from each volcano is pretty unique.
and we can calibrate this with weather reports from around the world.
for example, dates when ponds thaw in spring, etc.
Climatic variations in Japan reconstructed from historical documents
all of which demonstrate a general increase in temperature since the start of the Industrial revolution.
Damn those ancient Japanese monks for being part of the grand global conspiracy for global warming.
BTW, warmer is good for life. History proves this.