There is lots of evidence that I have seen showing correlation between human activities and climate change but what evidence is there to support causation?
Humans affect the weather in mainly the following ways:
Direct emissions of various gasses
Typically CO2 is considered, but also other greenhouse gasses. The greenhouse effect of carbon dioxide was first measured in 1859.
Plants "fix" carbon (a phenomena called "Carbon sequestration"), the less plants, the less fixing (and the more carbon released by fires).
All four of these effect can be shown in a laboratory and no model is required to do so, but we have very very good models to explain the lab experiments.
Differently from the lab, the whole climate system is much less understood. And, yes, the model are not as reliable as we would like.
However — due to our knowledge of chemistry — it is undeniable that we are affecting climate. Note that nobody has asserted that human intervention is the only cause of climate change, but it can be said, with a straight face, that humans are changing climate. A very simple example, the rise in temperature melts ice at the pole - which is not only responsible for reflecting some light out of the atmosphere, but also contains methane, which is then released.
The debate can only be on "how much" and "how well can we reverse the trend (even beyond our contribution)".
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Yes, humans cause climate change (each doubling of C02 causes about 1C increase). It's really a meaningless question. Any input to any chaos system will cause some effect on that system. Do we know what effect we are having? Can we measure/predict it? Do we have any idea how to alter/change/control that change? And really, what the hubbub is about is not "will the client change", but "will it change in a really bad way"
According to Peter Stott models failed to predict current temperatures (though he echoes the recurring claim that they'll be correct in the future), which means that no existing model has predicted, correctly, any significant amount of future climate change, and new research is steadily revealing flaws in existing catastrophic prediction models, so the answer to those questions should be no.
A model which has yet to make an accurate prediction cannot be said to be an accurate model. Therefore, we don't know what effect we're having, we can't predict it, and as a result of those two, we do not know how to alter or control that affect.
Causation on a chaos system is nigh impossible to prove with our current abilities, so we rely on modeling. Unfortunately, instead of insisting that a model make a prediction and have it come true before accepting it, we accept models as true if they accurately predict past events (not kidding), which is trivially easy.
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I can’t answer the question directly.
However, there has been at least one large-scale review on the scientific consensus. And it can safely be said that the scientific consensus is overwhelmingly that the current trend in global warming is caused by mankind. It would be weird if this consensus came to be without good evidence.
The review did a literature mining for peer-reviewed literature published between 1993 and 2003 with the words “global climate change” in their abstracts. They found 928 abstracts. Of those, 75% explicitly or implicitly endorsed AGW. 0% rejected it. 25% did not take a position.
As Russell has noted in the comment, these also include mitigation proposals which shouldn’t be counted towards the consensus (since they merely refer to other papers) but were. Furthermore, the review only used one key phrase for their search, excluding parts of the available literature.
So the review contains one systematic error (inclusion of mitigation proposals) and one unsystematic error. Nevertheless, because of the large number of papers it is still safe to assume that these will not change the reported consensus significantly.
Note that this does not mean that there are no dissenting opinions in the scientific community – there are – merely that the overwhelming majority of experts accepts AGW and that they probably have good reasons to do so.
(Still, this “answer’ is more of an FYI than an actual answer since, I want to stress again, it does not provide any of the evidence asked for.)
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The IPCC report gives the following probabilities:
What do they mean when they say very likely? They mean 0.95 < p < 0.99. When someone says that the evidence for climate change is comparable to the evidence for evolution they are either advocating that the IPCC is wrong by an order of magnitude or they are gravely insulting academic biology.
255 members of the US National Academy of Sciences including 11 Nobel Price winners issued a letter that claims:
In an attempt to defend orthodox wisdom mainstream scientists seem to be willing to pretend that the evidence is for climate change is a lot better than it actually is.
Other people who see themselves in defense of climate change think that the IPCC is a bit overconfident.
There are a lot of reasons why that might be the case:
That doesn't mean that we should assume p=0 but it might be reasonable to use a p value that's a bit lower than the IPCC value. If we go from 0.95 < p < 0.99 to 0.80 < p < 0.90 the p value for the climate change claims isn't statistically significant anymore. Even if we just go to 0.90 < p < 0.95 it's not significant anymore.
Why does that matter? Isn't p=0.80 enough for starting to reduce CO2 emissions? That might be true. If we however start geoengieering the confidence in our models matter a great deal. Starting geoengieering on the assumptions that our models are magnitudes better than they really are is dangerous.
Part of being a good skeptic should be to avoid being more confident in your beliefs than the data warrants. We should move past binary classification. Instead of showing tribal loyalty we should call out our friends when they overstate the evidence.
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Trends in solar radiation don't match up with trends in temperature. One of the arguments from skeptics of climate change is that rising global temperatures are a natural phenomenon caused by the Sun. However, most measures of total solar irradiance (also known as solar radiation, the electromagnetic energy incident on Earth's surface) show that, on the whole, it is falling. (This, of course, necessitates taking a step back to see larger TSI trends, beyond the valleys and peaks caused by the solar cycle.)
In short, it looks like the Sun is actually cooling. Not dramatically, but it's certainly not becoming hotter, and certainly not enough to account for rising global temperatures. In fact, when we juxtapose climate temperature with solar irradiance, as shown below, we find that they have little to do with one another. This is a basic, common sense approach, but if you require mathematical proof, then Skeptical Science has put together a digestible calculation and analysis. Anyhow, just a graph:
So you may not agree that global warming is anthropogenic. But as scientists look at solar irradiance as just one piece of evidence that correlates with various others that fellow commenters have left, it's becoming increasingly clear that it's not caused by the Sun. What does that leave?