There are various (non-mutually exclusive) ways to deny science. You can cherry pick the data and sources you like – there is always someone on the fringe with an opinion counter to the mainstream. You can use the all-purpose method of invoking a grand conspiracy theory. You can replace scientific opinion with pseudoscience. Or, if you have sufficient background scientific knowledge, you can magnify the uncertainty and disagreements that exist in all theories to make it seem like the core claims are in doubt, when they aren’t.

One of the primary targets for those who deny climate change and rely on the latter method is to focus on the notion of climate sensitivity. It is clear that CO2 is a greenhouse gas, only the most hard-core conspiracy theorist climate deniers deny that basic fact. Therefore, increasing the amount of CO2 in the atmosphere should increase this warming effect, and warm the planet. There are obviously many more technical layers to the process, including positive and negative feedback loops, other greenhouse gases, reactive gases that amplify the effect, and other factors that influence the climate. But the simple fact that increasing CO2 increases warming is essentially correct and non-controversial among experts.

So if you want to claim that CO2 release from human activity is no problem, you need to argue that the magnitude of the effect is too small to worry about. This is what climate sensitivity is – specifically defined as the amount of warming from pre-industrial levels that would ultimately result from a doubling of CO2 in the atmosphere. If we look at the last million years CO2 level fluctuated from around 200-280 ppm (parts per million). Over the last 100 years this amount has increased steadily, and is now over 410 ppm. How long it will take to double the CO2 depends on what we do, but unless there is a major change to our energy and industry infrastructure, we will get there by the end of the century.

What is, then, the climate sensitivity? In the 1970s climate scientists estimated that climate sensitivity is between 1.5 and 4.5 degrees C. Right now we are about 1 C above pre industrial levels. One way to deny at least the seriousness of climate change is to argue that climate sensitivity is at the low end of the range, and may even be lower. And of course you can cite fringe sources to back this up.

Interestingly, this range has not changed much in the last five decades. There have been many individual studies with varying ranges and different percent probabilities of where the climate will land, but they have mostly been within this range, and this is still the range used by many scientists. However, a recent review of all available evidence has narrowed this range (which is really a way of saying that the review increased our confidence):

An S value lower than 2 K is difficult to reconcile with any of the three lines of evidence. The amount of cooling during the Last Glacial Maximum provides strong evidence against values of S greater than 4.5 K. Other lines of evidence in combination also show that this is relatively unlikely. We use a Bayesian approach to produce a probability density (PDF) for S given all the evidence, including tests of robustness to difficult‐to‐quantify uncertainties and different priors. The 66% range is 2.6‐3.9 K for our Baseline calculation, and remains within 2.3‐4.5 K under the robustness tests; corresponding 5‐95% ranges are 2.3‐4.7 K, bounded by 2.0‐5.7 K (although such high‐confidence ranges should be regarded more cautiously).

What this means is that this new comprehensive analysis finds that climate sensitivity essentially cannot be lower than 2 K or higher than 5.7 K (K stands for Kelvin, which uses absolute zero as zero, but has the same interval as Celsius, so one K is equal to one degree C). But the 5-95% confidence range is 2.3-4.7 K. The probability of being outside that range is not zero, but it is very low. This range is a probability curve, so the highest probability is being in the middle of the range, around 3.5 K, with increasingly lower probabilities as you go further to either side.

This review pretty much destroys any claim that climate sensitivity is going to be at the low end of the prior range, at 1.5 K, or even lower. They are concluding that a climate sensitivity less than 2 K is incompatible with the evidence. What will happen if we double CO2 in 80 years or so, even if climate sensitivity ends up being at this lower boundary of 2 K? That is a great question. This, of course, is another way to deny the seriousness of climate change, to argue that even if average temperatures do increase to this degree, that won’t be a problem for the Earth or humanity.

What is likely to happen is a complex topic for another time, but the quick version is that most models predict there will be an increase in the number and/or severity of storms, droughts, and forest fires. Increasing parts of the world will experience increasing heat waves incompatible with human life. And sea levels will rise. How much and how fast sea levels rise is a matter of intense debate. A lot has to do with tipping points – what happens when temperature rise gets above a certain level, and will that trigger events that cause further warming, in a domino effect? Where will the new equilibrium climate wind up? Projections go from bad to worse, over a time frame of 100-1000 years. That may seem like a long time, but the primary concern is that some tipping points are irreversible on human timescales. If the antarctic ice sheet fails and collapses, that could take millions of years to replace.

In any case, it’s best not to do a natural experiment and see what happens.