I've not read my skeptical colleagues answers just yet but I'll have a stab at guessing what argument they've put forward.
It probably goes something like ... 'CO2 levels were WAY higher than they are now. Like, they were 1000s of ppm. And during those times plant life was amazingly abundant,...
Best answer: I've not read my skeptical colleagues answers just yet but I'll have a stab at guessing what argument they've put forward.
It probably goes something like ... 'CO2 levels were WAY higher than they are now. Like, they were 1000s of ppm. And during those times plant life was amazingly abundant, ecosystems thrived. It was close to paradise! This piddling little 400 or 500 ppm level of CO2 we're experiencing today is tiny in comparison, nothing to worry about, and hence the implication I'm making is that we don't have to do anything about so-called "global warming"'.
Now let me read some of the answers. Ok, I'm not too far off the mark. Now let me paraphrase that argument in different terms ...
'There was an earthquake in California that was really, really big. But Californians thrived despite it. Look at them there with their toned bodies, high incomes, and a State economy equivalent to entire nations! A piddling little earthquake is nothing to worry about, and the implication I'm making is that we shouldn't spend any money on protecting buildings against smaller earthquakes'.
Now Solar Wind, my old mate my old pal, has kindly provided a graph showing you the last 600 million years with the aim of trying to convince you that there is no relationship between CO2 levels and temperature. Of course, no one, including Solar Wind argues that temperatures are ONLY dependent on CO2 - during that 600 million year timespan we've had changes in the output of the sun and changes in the earth's orbit. This is why you get those dips in temperature. What the graph also doesn't really show is the rate of change of CO2. These sorts of natural processes result in changes of 100 parts per million in CO2 concentrations over periods of many thousands or even tens of thousands of years. We've changed the CO2 concentration by 100 ppm in 120 years.
So it's pretty irrelevant because
a) we are living at a time when solar output and the orbital dynamics of the planet are going to be pretty stable over the next few hundred to a few thousand years
b) under these stable conditions (like the 50 million year bit in the graph between 250 and 200 million years ago) CO2 levels are well correlated with temperature changes
c) we've changed the CO2 concentration by 100 parts per million in 120 years, a rate which is nowhere reflected on this graph
So, at the present time, if we increase CO2 levels to 500 ppm the planet will get warmer and this warming will occur on century timescales not millions of years. What happened in the past isn't important in terms of global warming today because there weren't 7 billion people distributed across the planet.