Very sure. Humans contribute very little (Which I can show you if you wish).
TM
Please do... that the Earth's climate has been radically different in the past does not mean that anything that humanity can possibly do is insignificant in comparison.
The simple answer to that would be; We don't know
You cannot be 'very sure' and then say that 'we don't know'. As Famine says, this is every bit as bad as an environmentalist claiming that they know for certain exactly how the climate is changing. There are uncertainties associated with every process that we know about, and there are also processes that we probably don't even know about to add to that. But, this does not equate to being able to say that the net effect of all human activity has been totally benign. Far from it infact.
Sam48
So to suddenly blame (a completely natural event) on Co2 alone is very ignorant.
To put the blame solely on CO2 would be quite wrong, but find me a single professional climate scientist who has
ever done that. CO2 and other long lived greenhouse gases are just a few of many influences on the climate. Ironically, CO2 levels are one indicator that is relatively easy to measure (both current and historic levels) as well as being one that can be attributed to 'non-natural' sources as well.
By quoting those (inaccurate) figures regarding CO2 levels in the atmosphere, you give me the impression that you simply believe that these small numbers somehow constitute proof that CO2 (or any other anthropogenic influence) must be benign. Yes, CO2 levels - relative to other gases - is very low. But what of it? CO2 levels have not strayed outside the range of 180-300 ppm for
several hundreds of thousands of years. Yet, in just over a century or so, we've already reached 391 ppm. Furthermore, CO2 levels are projected to keep rising - even climate change skeptics
have projected levels of 570 ppm by the end of this century (that is three times the historical low, and double the mean historical high). Others project far higher rises are expected, up to ~1000 ppm (or roughly four times the historical average) by the end of this century alone, and that CO2 levels will continue to rise beyond that.
The point is, 0.018-0.030% sound like very low numbers - 0.0091% is an even lower number (which is the excess CO2 currently around that cannot be adequately explained by natural events alone), but the question is, are these numbers significant? The short answer is a
very likely yes - and not, as you have suggested, a
definite no. It is the relative change in CO2 and other long-lived GHG's that is of concern, because of the potential for warming that increasing levels of these gases hold. Of course, other processes that we don't know a great deal about may act to mitigate the warming influence of these gases in ways we still don't fully understand - but this would still be an effect that has been precipitated by something that we have done - the cause may be manmade, but the effect will be 'natural'. The key question for me is, how can we be so sure that our influence of the climate does not exist, or is entirely benign? It would really help to have a firm answer to this
before we quadruple the amount of CO2 and other GHGs in the atmosphere...