A friend of mine from college emailed me the other day and expressed some skepticism about the connection between carbon dioxide emissions and global warming. It occurred to me that it would make a good topic for my next post.
So here is the reasoning that has led me to conclude that business-as-usual carbon dioxide emissions will lead to temperature increases over the next century of around 3 degrees C.
First, it has been known for over 150 years that adding carbon dioxide to the atmosphere will increase the temperature of the planet. In fact, the very small number of credible skeptics out there, such as Dick Lindzen and Pat Michaels, are on record agreeing that adding greenhouse gases to the atmosphere will warm the planet. What they argue is that the warming will be very small. More on that later.
The conclusion that emitting greenhouse gases will result in warming does not rest on the output of climate models, but is a simple physical argument that predates the invention of the computer. And if you don’t believe in physics, take a look at Venus. That planet features a thick carbon dioxide atmosphere and consequently a surface temperature hot enough to melt lead.
So we know that adding carbon dioxide is going to warm the planet. This leads us to the real question: How much warming are we going to get?
Carbon dioxide by itself will only provide somewhere around 1 degree C warming over the next century. In order to get really large warnings over the 21st century, there needs to be strong positive feedbacks to amplify the initial warming from carbon dioxide.
The strongest positive feedback is due to water vapor. The so-called water-vapor feedback refers to the process whereby an initial warming of the planet, caused for example by an increase in atmospheric carbon dioxide, causes an increase in the specific humidity of the atmosphere. Because water vapor is itself a greenhouse gas, the increase in specific humidity causes additional warming.
The water vapor feedback has long been expected to exert a powerful warming effect because of the belief that the atmosphere’s relative humidity would remain roughly constant—meaning that specific humidity would increase rapidly with surface temperature. Models reproduce this, and the water vapor feedback is the most important reason for the models’ large predicted warming for increases in greenhouse gases over the 21st century.
If you read the blogs, you’ll often see the argument made that no data exist to support the models’ strong positive feedback and that they are simply “computer-generated.” This is incorrect. I recently published a paper [PDF] estimating the magnitude of the water-vapor feedback exclusively from data. No climate model involved.
What we found was evidence that the water vapor feedback is indeed strong and positive. And there are several other observation-based analyses that agree with this conclusion (see the references in my paper).
Overall, the water vapor feedback about doubles the Earth’s response to carbon dioxide alone. If you throw in the other feedbacks (albedo, lapse rate, etc.), you’ll get about a 3 degrees C warming for doubled carbon dioxide (compared to 1 degree C for carbon dioxide alone).
While there is significant uncertainty in this number, it’s hard to believe that it will be fall outside a factor of 2 of this number.
The credible skeptics agree that the Earth will warm over the 21st century, but that the warming will be very small (less than 1 degree C). Could they be right? Possibly. The one way that climate change may not be large is if there exists, somewhere in the climate system, a large negative feedback that compensates for the positive feedback provided by water vapor. If such a negative feedback exists, it seems highly likely that it would somehow revolve around clouds.
To their credit, the few credible climate skeptics out there (Lindzen, Spencer) are indeed searching for negative feedbacks. So far, though, their arguments have been pretty weak and not convinced anyone in the scientific community.
One interpretation of the IPCC’s statement that humans are “very likely” responsible for most of the observed recent warming is that there is about a 10 percent chance that a significant negative feedback (or something equivalent) does exist in the climate system and will preclude significant warming over the century.
My personal opinion is that this is conservative, and that the actual chance we will discover a big negative feedback is actually far smaller. There is simply too much evidence in the paleoclimate record showing large swings of the climate, which tends to preclude the existence of stabilizing negative feedbacks.
This is how I reach the conclusion that warming of a few degrees Celsius is very likely unless we do something about greenhouse-gas emissions. Note that this conclusion is not derived from climate models. Rather, it is derived from fundamental physics combined with observations.
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Comments
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Colin Wright Posted 7:45 am
05 Jan 2009
Questions...
Andrew, this looks like a super paper and you have closed another loophole in denialist thinking ("computer models are only models").
Not being a climate scientist, let me pick your brain while I have the chance.
(1) Many of us are worried about the recent observation of methane release from the Siberian continental shelf. Do you any scientific opinion on the likelihood of increased warming above 3 degrees by methane release, and the decreasing albedo due to the loss of Arctic summer ice?
(2)Ken Caldeira has recently modeled the effect of spraying water into the air from ships and finds: "The simulated evaporation of about one-half inch of additional water everywhere in the world produced immediate planetary cooling effects that were projected to reach nearly 1 degree Fahrenheit within 20 or 30 years" here. I'm wondering if you have any opinion on the dangers of adding more water vapor into the air.
You write: it is "tropical q that primarily determines the size of the water vapor feedback". Does this mean that if water vapor were to be added to the atmosphere -- as a last resort -- it would be better to do it, say, over the Arctic Ocean?
Sorry to put you on the spot outside of your area of expertise...
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JMG Posted 8:00 am
05 Jan 2009
Weird poll
What is the purpose of that poll? Polls are for popularity, not for questions of science. What possible value is there in a poll of a non-random poll of a non-random group of Gristmill readers ... to see if your arguments have persuaded some?
I raise this point because I object to the idiots' petitions denying global warming (like the one by the father-son nutjobs in Oregon) as if the number of signatures on a petition has anything to do with reality. Equally so, a poll of gristmill readers, including the number of regular trolls, says nothing about reality. So why risk equating the two?
The 5% Project Let's live on the planet as if we intend to stay.
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Andrew Dessler Posted 8:44 am
05 Jan 2009
A few thoughts ...
Colin Wright-
My interpretation of the IPCC results is that there is about a 50% chance that warming will be greater than 3°C if we do nothing to rein in greenhouse gas emissions. It is very difficult to establish a probability for unlikely but high consequence events, such as release of methane from clathrates or tundra --- so the answer is I don't really have a good probability estimate.
The article about spraying water into the atmosphere is interesting. I am quite uncertain as to whether it would actually work, and I am extremely doubtful that it would be practical. In general, geoengineering solutions are really a last resort, if every other approach has failed and you're heading for the abyss.
For this kind of application, the best place to inject water (and minimize the greenhouse effect of the gas) would be in places where the atmosphere is basically isothermal, which is likely high latitudes.
JMG-
The poll is just for fun. As you correctly surmise, it is not a confident measure of anything.
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WWAGD?! Posted 9:15 am
05 Jan 2009
Queen of Outer Space
Now I see!
Venusians screwed up their planet by buying SUVs. Eventually SUVs became so huge that each on was three stories high and 20 feet long. This caused global warming and made Venus uninhabitable.
They sent a small pod to earth containing one male and one female.
This "Adam and Eve"...at first it was a paradise. But then Eve said to Adam, "I'm bored, I want to go to the mall".
This began a millenial long quest to use the Venusian primal mind to build a technological society capable of manufacturing SUVs....which has been a military code word for Surface Utilicar for VENUSIANS!!
An honest man is always in trouble. --Henry Fool
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stevenearlsalmony Posted 11:02 pm
05 Jan 2009
Taking the road to ruin......
............and getting there fast.
If it turns out to be true and real that the global challenges presented to the human family in our time are primarily the result of the colossal scale and fully expected unbridled growth of worldwide consumption, production and propagation activities by the human species, then it is plainly untrue to suggest that human beings can make no difference now with their efforts to ameliorate these human-induced and -driven conditions.
Unfortunately, we have 'experts' among us who have widely reported, of all things, that what is required of the human community now is "to have the courage to do nothing" in the face of the daunting challenges. This is purely music to the ears of the economic powerbrokers, their bought-and-paid-for politicians and the absurdly enriched 'talking heads' in the mainstream media who are intent on doing nothing more or less than protecting their wealth, power and privileges. Nothing else matters to them.
The family of humanity will soon enough stand up and speak out loudly and clearly to those who maintain the status quo because the very future of our children and life as we know it is in eminent danger, even in these early years of Century XXI.
Any problem or condition the human family can cause to exist is a situation over which human beings have at least a modicum of control.
For example, what is to keep people from consuming fewer resources.....and sharing them with those less fortunate? What keeps large-scale producers of stuff from "right-sizing" their organizations.... and making them sustainable? What prevents a human being from making a decision about bringing offspring into the world? These are distinctly human choices. Ours and ours alone to make, I suppose.
It is supremely ironic but the horrendous leadership provided by the self-proclaimed Masters of the Universe in my "Not So GREAT GREED GRAB Generation" may have inadvertently pushed the real issues of our time to the front of the world's stage. Were it not for the colossal mistakes of such woefully inadequate and remarkably selfish leaders, the dire circumstances of the current situation presented to the human family by the explosive growth of global human overgrowth activities would not be so easily seen or understood by all of us.
Steven Earl Salmony
AWAREness Campaign on The Human Population
established 2001
http://sustainabilityscience.org/content.html?contentid=1 ...
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