Summary: After a quarter-century of the climate wars, the chaff thrown up by political activists on both sides has largely obscured the key questions which we must answer in order to deal with this, perhaps the most important of the many shockwaves facing us. Today we look at the most important question of logic in the decision-making process.
“This impatience with ambiguity can be criticized in the phase: the absence of evidence is not the evidence of absence.”
— Carl Sagan in The Demon-haunted World – Science as a Candle in the Dark (1995), using the phrase attributed to British astrophysicist Martin Rees
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Content
- The very heart of the climate debate
- Absence of evidence is not evidence of absence
- Isn’t the climate signal from humanity obvious?
- For More Information
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(1) The very heart of the climate debate
Judith Curry (Prof Atmospheric Science, GA Inst Tech) carved to the heart of the climate wars — the debate over the public policy response to climate change (running on a different track than the policy debate in the pubic arena) comes down to questions of epistemology and logic. How do we leap from a body of evidence to knowing enough to rely on theories? Especially when the theories are compelling, but there is as yet minimal evidence to validate or prove them.
Curry starts with the basics of scientific reasoning, with two methodologies — both equally legitimate. Which to use depends on one’s professional judgement.
Recall the dueling papers on Climate Null Hypotheses by myself and Kevin Trenberth. Depending on which null hypothesis you select as a default position when conducting research you approach the problem in a different way.
- Humans have no influence on extreme weather events
- Humans are influencing extreme weather events
For #1, the null would be rejected if you find evidence of a human influence. In the absence of such evidence, #1 is not rejected. This is what Roger Pielke Jr argued.
For #2, the null would be rejected if there is evidence of no influence. RealClimate and Kerry Emanuel {Prof Meteorology, MIT} essentially conclude that the data is insufficient, so they argue from ‘physics’ and state that there is no evidence of absence.
To me, the ‘no evidence of absence’ argument is rather fatuous given that simple thermodynamical reasoning is not really useful in elucidating the impacts of AGW on extreme weather events.
This grounds the debate in science and logic, not a matter of certainty as implied by activists. For those of us who are not scientists, let’s look at this logical tool: “Absence of evidence is not evidence of absence.” What does it mean?
(2) Absence of evidence is not evidence of absence
Its first known use is by the British astrophysicist Martin Rees, discussing the search for extraterrestrial intelligence (SETI):
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“Some brains may package reality in a fashion that we can’t conceive. Others could be uncommunicative: living contemplative lives, perhaps deep under some planetary ocean, doing nothing to reveal their presence. There may be a lot more life out there than we could ever detect. Absence of evidence is not evidence of absence.”
— Our Cosmic Habitat, Martin Rees (2001). The phase was attributed to him long before this book. See his Wikipedia entry.
It’s a powerful by limited tool, as Rees explains in an interview of Martin Rees by Errol Morris in “The Certainty of Donald Rumsfeld“, New York Times, 28 March 2014 — Excerpt:
MORRIS: The absence of evidence is not evidence of absence — every time I’ve tried to track down its origins, it leads back to you.
REES: I’ve used it, but I’m sure I wasn’t the first to use it. …
MORRIS: Even if you didn’t originate it, I was hoping you would explain to me what you meant by it. … it appeared initially in the context of, “Is there life elsewhere in the universe?” And it’s not just an absence of evidence, because we have all kinds of statistical evidence that would suggest that there is.
REES: No, we don’t. We don’t have any evidence.
MORRIS: Yes and no. We have statistical evidence. We have evidence of how many planets that might be like ours. For example, the Drake equation [the equation that estimates the likelihood of life elsewhere in the universe] …
REES: Yes. But we don’t know if life started in these places.
Let’s look at use of this in a different context than climate science. As in this interview of Errol Morris by Calum Marsh, “On Donald Rumsfeld’s Inability to Separate Fact from Fantasy“, Esquire, 3 April 2014
EM: He {Rumsfeld} wrote this to the president of the United States: “The absence of evidence isn’t the evidence of absence.” What’s he doing? He’s taking a phrase that was popularized by Martin Rees, the British Astronomer Royal and former president of the Royal Society, and Carl Sagan. They’re the ones who used this expression, but they used it in a very specific context. They used it in the context of searching for extraterrestrial life and extraterrestrial intelligence, saying that the universe is a very big place, and that just because we haven’t had evidence of life doesn’t mean it isn’t there. Then all of a sudden the ballistic missile commission picks it up, and Rumsfeld runs with it, and it’s trucked out during the run-up to the Iraq war.
ESQ: Not exactly the same situation.
EM: But no one seems to notice that the context is different. This is not the universe at large, this is Iraq, and a very specific site in Iraq where it was suspected that a WMD could be found. A UN weapons inspector goes to Iraq and can’t find any evidence of a WMD — that’s not absence of evidence, that’s direct evidence that the suspected WMDs are simply not there. The way I describe it is that it’s like someone tells you there’s an elephant in the room. You open the door and you look in the room, you open the closets, you look under the bed, you go through the bureau drawers, and you don’t find an elephant. Is that absence of evidence or evidence of absence? I would submit it’s the latter.
So is the present state of climate science like looking for data in a room and not seeing it? Or looking for data in the vastness of space and not seeing it (yet)? The answer to this question determines how you see the debate.
(3) Why the debate? Isn’t the climate signal from humanity obvious?
Finding a signal of anthropogenic climate change is far more difficult than activists imply. The strongest consensus among climate scientists is, in the words of the new IPCC AR5 (Working Group I):
“It is extremely likely (95 – 100% certain) that human activities caused more than half of the observed increase in global mean surface temperature from 1951 to 2010.”
While a well-established finding, even that is not obvious in the data, as seen in this graph from the UK Met Office report “The recent pause in global warming: What do observations of the climate system tell us?“, July 2013.
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The anthropogenic climate signal is far more difficult to see in other forms of weather, and immensely more difficult to detect in the relatively rare instances of extreme weather since 1950. Hence the IPCC’s tentative statements about the likelihood of anthropogenic factors increasing extreme weather.
Other than temperature and (to a lesser degree, precipitation) the IPCC — reflecting the climate science literature — sees more absence of evidence than evidence. Hence many people’s reluctance to take large-scale measures to prepare for future extreme weather.
Yet consensus climate science theory (seen in the IPCC’s reports) tells us to expect severe increases in extreme weather during the rest of this century. Hence the perfectly legitimate warning that the absence of evidence is not evidence of absence.
Eventually the climate will answer all our questions, but unfortunately probably too late for us to prepare if the answer is bad news. So we need to make decisions now. We have the logical tools and experience to make sound decisions, but are hindered by chaff thrown into the air by both sides. But, in my opinion, mostly by the climate activists on the Left. The smears and exaggerations make rational debate difficult or impossible, so we do almost nothing.
This is not the way a great nation conducts its business, or provides leadership to the world. We can do better.
(4) For More Information
(a) Posts about epistemology and the progress of science:
- Magical theories of science, and how they influence us, 14 February 2014
- A key to understanding the climate wars (about one of our big weaknesses), 15 March 2014
(b) Reference Pages about climate on the FM sites:
(c) A few important things to remember about global warming
Please read this before commenting about my views about global warming and climate change. It also has links to the key posts on the FM website on this topic.
(d) Posts asking if we’re prepared for past weather:
- Have we prepared for normal climate change and non-extreme weather?, 11 February 2014
- Droughts are coming. Are we ready for the past to repeat?, 12 March 2014
(e) Posts about extreme weather:
- Ignorance and propaganda about extreme climate change, 10 July 2012
- A look behind the curtain at the news of extreme climate events in the US, 22 August 2012
- Hurricane Sandy asks when did weather become exceptional? (plus important info about US hurricanes), 28 October 2012
- Has global warming increased the frequency & virulence of extreme weather events?, 10 February 2013
- The Oklahoma tornadoes can teach us about our climate, and ourselves, 22 May 2013
- The IPCC gives us straight talk about Extreme Weather, 4 October 2013
- The IPCC rebukes the climate doomsters. Will we listen?, 15 October 2013
- A summary of the state of climate change and extreme weather, 12 December 2013
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