Global warming is based on the fact that humans have been contributing greenhouse gases to the atmosphere, mainly CO2 and H2O through the burning of fossil fuels, but also through changes in land use. The levels of CO2 have been measured directly since the 1950s and a record of past levels exists in glacial ice and even older proxies.

It appears that current levels are significantly higher than at any time in the last million years. Current levels, however, are much lower than they were 100 million years ago. During the Jurassic, levels of CO2 were ten times as high as they are today. 600 mya, levels may have been 15-20 times as high as today.

Obviously the Earth didn't heat up and kill off the mammals or burst into flame 100 mya.

Most people don't realize that global warming is a good thing. Increased CO2 fertilizes the plant life on Earth. Earth is also due to return to an ice age (according to the oxygen isotope record) and enhanced warming may postpone its onset. Global warming extends growing seasons and the extent of arable land. It invigorates the hydrologic cycle and makes higher latitudes and altitudes more hospitable.

When the ice age does return, it will be much harder to grow things. People should be thankful for global warming. I know that it makes some people worry, but that is silly. The Earth isn't warming very much...and how do you take an average temperature of the Earth anyway?

it is easy to explain the idea. convincing them that the idea is true is a different matter. "believers" ignore the uncertainties that lie within the assumptions, and non-believers tend to concentrate on those uncertainties.

If you do not admit that uncertainties exist, you are no better than your opposition that fails to admit there is some solid science behind the issue.

One thing that is demonstrable, that is shown by fact, is that the climate is in a warming period that has existed, more or less constantly, for about 200 years.

But the main point that you need to make, for what little value it will have, is to remind the others that local weather is not the same as global climate. Just because it has been cool HERE does not mean that it has been cool everywhere else (and it hasn't, actually).

Just point out that locally seasons vary, and show a them warmer region. (Alaska or Australia) Global warming is measured by averaging the WHOLE EARTH, over several years.

Global warming is real! It happens every spring and summer!