Personally, I think refining climate sensitivity estimates might be one of the most important things climate scientists can do to help to establish a consensus on the policy and political implications of climate change. Admittedly my opinion is not universally accepted. One of my regular commenters, Dr. Michael Tobis at his blog Planet 3.0, takes an entirely opposite view on the topic. Other blogs present similar views to Dr. Tobis, most of them are based on the argument that sensitivity is reported as a range and as long as a risk of a higher end sensitivity exists then we have to take action now. My response to this will be detailed below but first, for my non-climate scientist readers out there, let's go back to the basics (and in this discussion I mean basics I'm not looking at transient climate sensitivity etc...).
In plain English climate sensitivity represents the anticipated heating expected based on changes in CO2 concentrations in the atmosphere. Climate sensitivity is formally defined as a measure of the anticipated equilibrium temperature change in response to changes of the radiative forcing associated with Tyndall gases. Radiative forcing is the measurement of the capacity of a gas or other forcing agent to affect that energy balance, thereby contributing to climate change. Radiative forcing expresses the change in energy in the atmosphere due to Tyndall gas emissions (reference).
What many of my readers may not know/understand is that due to the chemistry, increases in CO2 concentrations do not have a linear relationship with anticipated heating. Or put another way, 1 unit of CO2 does not result in 1 unit of anticipated heating. Rather, each molecule of CO2 added to the atmosphere has a tiny bit less heating effect than the molecule added before it. Specifically, rather than a linear relationship the relationship is logarithmic. As detailed at the Skeptical Science web site (yes, many of my readers dislike that web site but while it has a deeply political bent it does serve as a useful information source) “this logarithmic relationship means that each doubling of atmospheric CO2 will cause the same amount of warming at the Earth's surface. Thus, it takes as long to increase atmospheric CO2 from 560 to 1120 parts per million by volume (ppmv) as it did to rise from 280 to 560 ppmv".
So why do I care about climate sensitivity? Well it becomes a bit of a math game. If climate sensitivity is determined to be 1.5oC then each doubling in atmospheric CO2 concentrations would result in an increase in 1.5oC. Using the Skeptical Science numbers, an increase in CO2 from 280ppmv to 1120 ppmv would represent two doublings, so with a sensitivity estimate of 1.5oC you would expect an increase of 3oC (two times 1.5oC). If your sensitivity estimate is 4.5oC then the same 3oC would occur below 500 ppmv. So you ask why sensitivity is important? Well the math is pretty easy here. If we only have until 500 ppmv to avoid 3oC (which in this thought experiment we will define as a less than scientific "point of disaster") then we need to act immediately. If we have until 1120 ppmv then we can wean our society off fossil fuels more gradually and decrease the pain.
Now that we understand the concept, it becomes clear how fundamental differences with respect to climate sensitivity estimates will affect political and policy implications. In one corner we have extreme Lukewarmers who think that climate sensitivity is around 1oC (note I am not from that group). For them getting to the 3oC (which for our thought experiment is really bad) would entail three doublings, or atmospheric CO2 concentrations of 2240 ppmv. Even at our current rate of atmospheric CO2 deposition that is not happening anytime soon. So trying to convince them that we should divert trillions of dollars to completely re-tool our planetary energy/power systems would be a pretty hard sell. At the other extreme are people who believe climate sensitivity might be closer to 6oC. For them we are not merely approaching the cliff but have already crossed over it and are starting the dive to collapse. For them immediate action is essential to avoid a hard landing at the bottom of the chasm. In both cases we have good, honest people who have a genuine disagreement about the science but their opinion on the science leads to dramatically differing policy positions. One side says that as long as we get this done in the next 50 years or so we are fine and the other thinks that every day's delay will result in more pain and hardship. The first group will deeply resent being asked to make serious sacrifices to address climate change and the second group will think that actions are so necessary that maybe governments should be forcing people to make changes involuntarily.
To add complexity to the discussion is the point made above by the colleagues of Dr. Tobis. There are lots of things we still do not understand about our climate. Right now we have a pretty good thing going. We live on a planet in an interglacial period with a relatively mild climate with storms and disasters that come at infrequent intervals. Any change could tip that balance towards a world with less good days and more bad days (or maybe even the other direction). Since our civilization has thrived under current conditions isn't it thus a good idea to try and keep it in this current place? Also for the gamblers out there we are reminded that sensitivity represents a range and while there may be 90% (this is a made up number as no one actually has a clue what the real numbers are) chance of a small sensitivity that means there is still a 10% chance of a bigger number and given any non-trivial uncertainty, isn't it better to not role the dice?
The most important thing to recognize in all this that any uncertainty in the science is going to be magnified by people at both ends of the spectrum who have political and/or financial stakes in either maintaining the status quo or eliminating the status quo. The best way to get these people off message is to reduce the uncertainty. From a policy perspective uncertainty also makes for bad policy as the more contingencies you have to consider the more complex (and thus the less practical) the policy will have to be. Finally, as we work to reduce the uncertainty it would be nice if scientists stopped feeding denialists and catatrophists all that red meat. As a knitter myself, I can say this: stick to your knitting and to mix my metaphors horrendously, stop running around stamping on each other's toes.
Author's note: at this point in this posting at least a couple readers are going to be apoplexic over my use of 3oC in my thought experiment. Please accept that the choice was simply to make the math easier for the discussion. I know that the IPCC has a 2oC target but that number is very much a topic for a future posting.