In fact, we're at a 250-million-year low, teetering on the brink, not of uncontrolled global warming, but of a return to ice-age conditions that the planet left only 12,000 years ago.

Take a look at the accompanying graph. It shows carbon dioxide levels (the black line) and temperature (grey line) over the past 600 million years, which is most of the time that life has been on the planet's surface.

Note that the average global temperature for tens of millions of years, when the dinosaurs and mammals evolved, was around 22°C -- 10 degrees higher than today. Note that carbon dioxide levels have been up to 10 times today's levels. And yet, somehow, the plants, mammals and dinosaurs managed to survive; it took an asteroid to drive the dinosaurs into "oblivion."

Note that, as the graph shows, carbon dioxide levels don't have that much correlation with temperature over this time period. CO2 levels have fallen steadily for more than 150 million years, while the planetary temperature stayed at 22°C until about 30 million years ago.

That's when the planet's temperature began its plunge into ice-age conditions colder than anything in the past 300 million years. Our planet now is unusually cold by paleo-climatic standards, not warm. Carbon dioxide levels are unusually low, not high.

Note, too, that in 600 million years the global temperature hasn't gone much above 22°C no matter how high the CO2 levels were.

That's because the relationship of CO2 to temperature is logarithmic: the more CO2 you put in, the less effect it has on temperature.

This means there's no need to fear "runaway" global warming.

Moreover, most of our planet's species (including us) would face oblivion should temperatures and/or carbon dioxide levels get too much lower. If CO2 dropped to, say, 125 ppm, there would be very little plant life and most forms of animal life would die.

On the other hand, the plant world would thrive if carbon dioxide levels were three or four times today's levels. That's why hothouse growers deliberately pump carbon dioxide into their greenhouses to levels of over 1,000 ppm -- the plants grow better. It's ironic that so many "greens" oppose global warming because a world with more carbon dioxide would be a greener world, with more biodiversity, not less.