Yes, CO2 naturally is made from trees and volcanoes.  The fact is though that these sources cause only minor variations in the level of CO2 over a period of 100 years.  CO2 has increased by over 50% in the last 100 years and is still rising.  Given that industry and transportation produce large levels of CO2 and that these started being substantial about 100 years ago, it's reasonable to conclude that the cause of the rise in CO2 is man-made.

Think of a ski hill.  Some snow falls naturally, and some is produced by machines.  If we turn on a machine to make snow on a certain run, and the next day we wake up and find that there is a lot more snow on that run, doesn't it make sense to think that the snow came from the snow-making machines?  Maybe some snow fell naturally from the sky, but the best explanation seems to be the snow-making machines to account for much of the snow.

re: how can scientists be so sure?

Look - noone can attribute any event that has happened so far to global warming.  The increase in CO2 made it more *likely* that temperatures would rise, but there are a lot of natural phenomena as well.

However, there is scientific evidence that the greenhouse effect exists.  For example, look at the planets.  Planet temperature is largely determined by procimity to the Sun and the atmosphere.  Venus is actually a little hotter than Mercury, even though Mercury is closer to the Sun.  What causes that - large concentrations of CO2 in the atmosphere.

By way of analogy.  Suppose the scientist says you can't make money playing roulette in Vegas.  The skeptic says "Oh yeah?  -  I just played 5 bets and I made money"  The truth is that was luck.  If you keep playing indefinitely, you'll lose.  There will be short term fluctuations, but the long term trend is a declining bank account.  Similarly, you can have bad luck at the roulette table.  If you make 5 bets and lose them all, it doesn't prove the scientists point.  But the fact that the odds are against you made it more likely that you'd lose money.

Science suggests that increasing the CO2 in the atmosphere is going to raise temperatures in the long-run.  Just like gambling at roulette, there may be short-term fluctuations, but an understanding of the science tells you temperatures will rise.

re: what's so bad?

There will be some positive effects from global warming but most will be negative because man and nature will have to adjust rather abruptly.  eg. Coniferous trees require a certain type of soil and a certain temperature to survive.  If their current environment become too hot, they won't be able to walk north.  And even if they did, the northern soils might not have enough nutrients to support them.  So if these trees die out, certain animals that depend on them will die out too.  You suggest more "ideal conditions" but this is not necessarily so.

Similarly man has a lot of infrastructure built in coastal areas.  Science suggests that we may have increased sea levels and more hurricanes in a warmed-up world.  New Orleans will end up under-water.  You can't just truck the buildings 40 miles north.  A mild economic hit now, might save a lot of economic pain in the future.

re: models

I like the roulette example.  A computer model of roulette would show you going up and down over time.  It won't accurately predict when you win or when you lose, but it will accurately predict that in the long-run you will lose.  Climate models are similar.  We can't predict what the weather on Dec 5, 2106 will be, but chances are it will be warmer than today.

You said that "we require proof that is irrefutable".

I said that that is an unreasonable standard, and I gave two examples where a reasonable person would take action without irrefutable proof.  Thus I'm suggesting that action on global warming can be supported in the absence of "irrefutable proof".  It is possible to argue by analogy.

re: petition

Look, obviously I can't go through a list of 17,000 people and discredit them one by one.  For one thing, I don't know them.  Some of the articles I've read criticizing these petitions say that a large percentage of people who signed are either:

a) paid to give the opinion

b) not academically qualified to give the opinion

or c) misled into signing

In any case, a petition isn't the best way to judge a scientific consensus.  The best way is to look at all peer-reviewed articles determine what they suggest in aggregate.

Time for argument by analogy ;)

You could find 17,000 people that think Elvis is alive.  I would still say there is a consensus that Elvis is dead.

You could find 17,000 people that think the Kansas City Royals were the best team in baseball last year.  I would still say there is a consensus that they were not.

Global warming (solar warming) the liberals pathway to global government.