But what the chorus of shrieking deniers does is to keep the focus on the lower end of the IPCC projections. There is little discussion devoted to the high end. In particular, there is very little discussion of (1) What is the worst case? and (2) How likely is the worst case? If for example, the worst case is human extinction, it would be critical to know how likely that is. It's one thing to ask: what percentage risk should we accept of displacing a million people who live on coastlines? But something entirely different to ask: how much risk should we accept of killing off the human race? I think the first number would be quite a bit higher than the second.
I'd like to talk about the worst case and how likely it is. I'd also like to talk about the notion that climate scientists exaggerate the danger. There are some very good reasons to believe they do exactly the opposite. In particular, I'd like to focus on major feedbacks in the climate system that we know will raise the significantly raise the temperature, but that are not included in the present forecasts.
I intend this thread to be focussed narrowly on those topics. I'm simply going to be laying out a case that is not typically discussed because climate deniers drown it out. For purposes of this thread only, attacks on the climate science, as found in the literature and summarized in the AR 5 WG 1 are off topic. There are at least three other threads in which Water Dog has been cutting and pasting from denier websites. This thread is about the risks of the worst case, not about Argo floats or the global warming non-pause.
I want to start with where we are in terms of the risks of the worst case. This graph is from the most recent National Climate Assessment published by the U.S. government:
https://science2017.globalchange.gov/do ... Report.pdf
The RCPs are "Representative Concentration Pathways." They are represent plausible future scenarios given different assumptions about future changes in human contribution to climate change. The numbers represent the change in radiative forcing in 2100 relative to the preindustrial baseline, expressed in watts/meter squared. RCP 8.5 is commonly referred to a the "business as usual" scenario. RCP 2.6 is the most optimistic of the scenarios -- the only one that results in a leveling off of temperature by 2100. It's important to note that these temperatures are in Fahrenheit, cuz it's an American report.
Each scenario has a range of value for any given year. That represents the range of uncertainty in the projection. (To convert changes in temperature to C, multiply by 5 and divide by 9.)
The black line represents observed temperature. This report was released in 2017. It shows that global temperature was with the predicted range of all three RCPs, and slightly above the mid range of each. Just as important, the predictions based on the three scenarios are almost indistinguishable at this scale in 2017. To distinguish among them, we really have to zoom in to a much finer scale.
If we use business as usual as the worst case, the median projection for 2100 is an increase of 8F. We've warmed 2F, which means another 6F to go. The top end of range exceeds 10F and the bottom is 5F
So, we have a projection based on just doing what we've been doing. That gives us a worst case projection for temperatures in 2100. And they keep increasing after that. So the next question to look at is how likely we are to end up in that RCP 8.5 range in 2100? And for that, we'll take a little dive into the AR 5 WG 1 report.
ETA: To avoid any confusion, I’m referring to RCP 8.5 as “business as usual” because it is the business as usual RCP included in the IPCC reports. It’s in the high range of the universe of scenarios that would qualify as business as usual, which makes it a good basis for discussing worst case.
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