When we talk about why scientists disagree about different aspects and approaches to climate science, I think it’s helpful to break it down into general areas. We’ll see how this works when we move on to scientific disagreements of various kinds. So let’s start at the 30,000 foot level.
Generally Agreed Upon:
(1) Over time, climate has influenced humans, and humans have influenced climate. I think here of overgrazing, and deforestation in the historic past. Humans have adapted to changing climates with more or less difficulty, including migration from one place to another.
(2) More recently (last 50 years or so?), scientific knowledge has produced a) evidence of greenhouse gases having a role in changing climate as well as b) other land use practices, e.g. irrigation, urbanization.
Where Scientists Diverge:
(3) Teasing out the role of different anthropogenic factors, including greenhouse gases, in specific measures of climate change is difficult for a number of reasons. They include:
a) many other changes have occurred through the same timeframe as increasing greenhouse gases, including land use practices. Correlation is not causation, so different techniques try to remove other causes of variation. Scientists may disagree about these techniques.
b) we don’t have good climate data very far back in time, so have to use proxies, and they may not all be in agreement.
c) records from historic observations by people may be hard to blend with proxies.
d) measuring techniques have changed through time and different scientists have different methods of accounting for this.
e) there just hasn’t been enough time elapsed to detect changes (say something happens once every hundred years to twice every hundred years).
(4) Scientists go ahead and make estimates about 3) and predict the future based on those estimates. But even if we know what proportion of change is due to say, carbon, we don’t know how much carbon will be in the atmosphere in the future. So we model what might happen based on different factors, including economics and technology, that have been difficult or impossible to predict in the past. Given that, how lightly to hold these predictions is an area of disagreement. In general, disciplines with a history of prediction successes and failures tend to have more GDH (generic disciplinary humility).
(5) When it comes to predicting potentially bad impacts of climate change (floods, fires, drought, and so on), though, sometimes the study does not include the work or experience of people in the business of say, managing floods, farming, running dams, or fighting fires. So there is sometimes a disconnect between (the published and believed literature) and those communities and the scientific disciplines that support them (civil engineering, agriculture, wildfire management).
(6) Even if scientists agreed on everything else, no one knows what will happen depending on the rate of slowing of carbon to the atmosphere. Would the climate ultimately “change back”? But what about all those other natural variation and human factors? Since we don’t know, is it best to assume that adaptation will need to be ongoing, and maybe climate science funds should be directed less at exploring all the things that might go wrong, to adaptation and low-carbon technologies. (Where, or does, utility of results factor in to science funding processes?)
(6) Even if scientists agreed on 1-5, they still wouldn’t agree on policy solutions. Even if we just use the example of a carbon tax versus cap and trade, reasonable people, scientist and non-scientist could disagree. If we think of scientific studies as information packets designed to inform policy makers, collectors of information such as the IPCC are very helpful in giving policy makers a summarized packet. But they can still use scientific information outside the packet, or other information entirely. At its best (or best organized), information from various scientific disciplines could give us a target (perhaps measured in global average temperatures) and compare the feasibility of various solutions. When we get to solutions, though, it’s not difficult to think scientists might disagree- for example, bat specialists might not want the landscape covered with wind turbines. And so on.
7) Finally, some scientists say “unpredictable things could happen and be even worse than models project”. This is the idea of tipping points. I’m not sure about the utility of trying to predict the unpredictable. If we take any intervention by humans (say legalizing marijuana) certainly unpredictable things happen and some are worse than we expect. We note them and try to work on them. I think the difference with climate scientists is “because it is the climate of our planet we are thinking of, we should be super-precautionary.” I tend to think these are philosophical ideas “really unpredictable things could happen” and “we need to be careful,” and not actually “science”, although some scientists write papers about it.
I don’t think that this list is exhaustive, so others are welcome to contribute.