By Matt Owens December 29, 2013
When it comes to global warming, one of my own pet peeves is how so many news articles imply or actually say that sea level rise cannot exceed some rate, and usually a relatively low one too. An article will often read something like: 'scientists think that in a worst-case scenario, sea levels might rise by as much as 2 feet by the end of the century.' But that's just not correct. Sea level rise researchers have been intentionally vague about how bad a “worst-case” scenario might be, because they don't rightly know with precision.
Apparently others share this pet peeve of mine.
In fact, a letter published in this week's issue of Science by the very authors themselves of the latest IPCC draft report on sea level rise says “[t]he upper boundary of [this report's] “likely” range should not be misconstrued as a worst-case upper limit, as was done in Kerr's story as well as elsewhere in the media and blogosphere.”
The story they're referring to is an October article by veteran Science writer Richard Kerr who incorrectly wrote that the authors projected a “worst case of 1 meter by 2100.” It's frustrating, but so telling that a journalist like Kerr of all people would stumble on this topic.
Kerr's fault is relatively minor on the face of it, but it's a common one I've seen so many times and it becomes a self-perpetuating myth that enables the self-lulling self-deception that prevails on this topic. Or maybe it's the other way around - it's hard to tell with these chicken and egg questions. And it's hard to fault Kerr - or it's very easy, depending on your level of frustration at the moment.
Aside from the prevailing self-deception, the problem is that climate science has been advancing faster and faster while assumptions about policy action have simultaneously been changing - but in a direction that makes climate change more likely to happen faster.
Just a very-few years ago, the most extreme of the core emissions scenarios (formerly known as A1FI) was considered unlikely by many. After all, the prevailing thinking went, 'who would be so stupid and insane to allow such a dangerous scenario to unfold?'
Now however, since deadlines to curb emissions have come and gone, the general sentiment has turned quite pessimistic. Today's thinking more or less now goes like this: 'how will these reckless lunatics do anything besides take our climate on a suicidal joy-ride down an ice luge to hell?'
So, to echo the authors of the IPCC sea level rise report, here are some key points that are often misunderstood about sea level rise estimates:
(1) The scenario labeled “RCP8.5” (generally equivalent to the former A1FI) is an emissions scenario from which climate projections are derived. The RCP8.5 emissions scenario is the highest emissions scenario considered by the IPCC consensus reporting process, but it is not the “worst-case” possible. Emissions from both humans and from feedbacks (such as thawing permafrost) could cause actual emissions to significantly exceed those in the RCP8.5 scenario.
(2) The RCP8.5 scenario is now considered a likely emissions scenario by many climate researchers because of (a) feedbacks that include thawing permafrost and arctic sea ice loss, and (b) the general ambivalence in words and deeds from the public and policy-makers on making emissions reductions.
(3) In the RCP8.5 scenario, sea level is modeled to “likely” rise by between 0.52 and 0.98 meters by 2100, with, and I quote from the letter, “a one-third probability that sea-level rise by 2100 may lie outside the “likely” range. That is, the [report] did not exclude the possibility of higher sea levels.”
(4) And finally, in the view of the report's authors, if sea level does rise much beyond 0.98 meters by 2100, it will be several 10ths of a meter, and it will be from a collapse of below-sea-level-grounded ice (for instance, the collapse of the West Antarctic Ice Shelf). However, they only have “medium” confidence in this statement, meaning that maybe sea level could rise even further from other processes that they don't know enough about and/or haven't anticipated.
At current rates, there are about two new processes discovered or observed for the first time each year.
One thing is certain: warming temperatures will not produce enough extra snow to offset the ice losses from warming. In fact, where snowfall increases, the slope of the ice sheets steepens, and the ice flows faster towards the ocean as a result.
What's more, unless one of the new discoveries in the upcoming years is an army of Santa's elves installing magical rebar to hold the ice sheets back, it's a good bet that any of the new processes are going to either mean sea level rise estimates stay the same or get worse. That's because the current estimates are based on conservative rates of bed lubrication and basal melt at the floating ice shelves.
A fair way to look at the situation now would be to consider these four points:
(1) Expect half a meter rise by 2100.
(2) Be prepared for a meter rise by 2100.
(3) Don't be too surprised if there's a two meter rise by 2100.
(4) And finally, know that if business as usual emissions continue, then the seas won't stop rising for a long time; sea level will keep rising by rates of more than one meter per century if CO2 levels reach and stay above something in the neighborhood of 600 ppm.