- Global average temperature has risen over the last the century, and is still rising. This is a trickier claim than it looks. "Global average temperature" is not a directly measurable thing, it's a constructed aggregate with lots of potential pitfalls. And "rising" too is only a trend in a far more complicated time series. But over the last 20 to 30 years, we have learned enough to make this claim with high certainty anyway.
Global temperatures have risen. At any given time, they'll either be rising or falling. Again, a very big part of my objection...is we have no way of knowing if the current rise is unusual or not...and quite to the contrary, we've seen similar rises and drops just in the past 2000 years...suggesting that the current one is NOT unusual.
- Greenhouse gases exist, and are present in our atmosphere. That is, there are gases that are observed to cause heat trapping under laboratory conditions.
And they've always been present. Sometimes at higher levels, sometimes at lower ones. We do not know how the actual global ecosystem responds to rising levels...we do know there do exist mechanisms that are capable of reducing it.
- The presence of these gases has indeed a significant impact on the temperatures on the surface of the planet. This is a harder claim than the previous, since there is no direct counterfactual to compare it with in a laboratory. But any attempt to model temepratures on earth, from simple and rough to complicated, leads to far too low temperatures if they do not take greenhouse effects into account. And we have other planets and moons to verify such modelling attempts with. But simple models are not accurate enough to say something about the effects of changes in the greenhouse gas levels.
This again suffers from the similar issues. The Earth 'Reacts' to higher gas levels because of water and Life, in ways that Venus cannot. We cannot do a lab test that simulates or predicts the earth's capacity to react to higher levels. But we know historically it DOES react and absorb.
- Concentrations of several greenhouse gases are rising, in particular CO2. We can accurately measure their concentration in several ways, such as direct air samples or by observations of the optic properties of a column of the the atmosphere. This is a complicated technical issue, but pretty uncontroversial in its execution.
We're definitely putting CO2 up there, A lot of scientists point out that the current levels should be a lot higher, given how much we're putting up there...something must be starting to react and absorb it.
- The increase in greenhouse gases is caused by human activities. That's what our (imperfect) understanding of their spread and accumulation predicts, but it is also verified by isotopic measurements. The ratio of carbon-12 to carbon-13 in the atmosphere changes the way you would expect, if carbon levels are increasing as the result of fossil fuel burning.
And again, despite that, something's happening...automatic compensation seems to be occurring.
- The observed increases in concentration, combined with the laboratory properties, are more than enough to explain the observed temperature rise in a naive simple model without much feedback mechanisms. This is called "forcing" and is the input for more realistic models.
In a static world with no feedback or realism, CO2 rising would increase temperatures...not as much as they say however, since there are points in the past with higher Greenhouse gas levels but colder temperatures. There are other mitigating variables we are unaware of and cannot mimic.
The capstone of the evidence, which is perhaps more controversial, would be our detailed understaning and modelling of the relevant feedback mechanisms. The big weaknesses here are mostly forward-looking, in predicting which mechanisms will be relavant in the future, beyond the observed and studied range of parameters. But I'd say it is indeed reasonable to have doubts about the accuracy of our current understanding, even backwards-looking to already observed phenomena. This understanding is not as 'hard' as all the stuff above, even if it is the best we have. It is about combining imperfectly understood phenomena in complicated relations, which does have lots of room for error.
This is very much the area where the dispute truly does occur. Most scientific objection lies right here...in the 'your model does not have predictive ability' or 'your model is basically wrong' area. We don't know what the Earth's reaction is to this stuff. We're just assuming the worst case scenario.
Still, that doesn't somehow make all the other stuff "just assuming". If you want to doubt the causal link between human activity to the observed tmeperature increases, you have to assume both that
A. Our detailed understanding of feedback mechanisms is completely wrong, in such a way that the (observed!) increased forcing from greenhouse effects is nearly fully counteracted by those feedback mechanisms,
and B. There are also other, as yet unobserved and unknown mechanisms that do produce the observed temperature increase, but that escape from those feedback mechanisms that negate the effects from greenhouse gases.
A is definitely the case, we do not in anyway understand the chain of causality in the ecosystem in any way.
B is certainly true to a part...there are definitely other mechanisms we have no clue about (or in many cases that we DO know about, but are left out of models because it's less 'scary' if you include them)
I'd say that either A or B separately would be a plausible doubt. But both at the same time is asking a lot. At the very least, people would have to describe the alternative mechanisms, and show their active presence at the same level of detail that the greenhouse gas mechanism has been observed. And yes, people have studied sunspots.
Again, you want to compare a static thing to a system we know absorbs that thing...but without giving it time to do so. That's the big recency biased and irrational problem here...people have this mentality that these changes are immediate, perceptible and irreversible. There's simply no foundation for that in anything truly scientific.
There's some science stuff that others disagree with that say that like, in 100,000 years this could be an issue.
There is no immediacy here...there is no danger, there is no problem.
Natural economic forces are going to lead to us reduces CO2 emissions...literally thousands of years prior to it hitting any point where it'd be a problem for temperature.
There is no big deadline we have to stop doing something by. There is a complete and utter scare tactic.
The reality is we don't know if this is occurring...it's probably NOT occurring at all...and even if it was, we've got tons of time to 'fix it' even though its not even frigging harmful in the first place because we just arbitrarily picked a temperature and decided we like it.
There is a great deal of doubt as to what will happen next - but let's be clear about what we mean by that: the low end, most optimistic forecast is a 2C rise; Scientists agree that level of rise is now unavoidable due to past missed targets. The high end, pessimistic forecast is 8C+. 2 degrees is already enough to be mighty expensive economically, globally.
This as an example like I tend to mean...people seriously believe this sort of nonsense, and they'll tell you it's gonna happen in like '30 years man!' The Earth's temperature hasn't changed by 8 degrees in the past 30 Million years, let alone the next 30. People are being told this nonsense to SCARE THEM.
There's being told water levels could 'Rise 200 feet!' ...do a quick volume calculation on the Earth and you'll see how they got that number...they took 100% of the ice on Earth, ignored displacement (because much is already underwater), then added that volume to the current sphere of Earth and the radius rose 200 feet. Nevermind that temperatures would have to increase about 30 degrees for that to occur.
That's the type of propaganda you're dealing with from the 'Green' side.
If someone told the truth...'this might begin to be an issue in 800 to 1400 years, lets keep an eye on it' no one would care.