Sunday, August 06, 2006

Is today's warming man-made?

As George Bush said at a recent press conference: “the globe is warming. The fundamental debate: Is it manmade or natural.”

Why does the scientific community think that humans are significantly contributing to today’s warming? To understand why, first recognize that whenever the climate shifts, there’s a reason for it. It does not wander around like a drunken sailor.

Based on decades of research, we can write down the factors that have influenced climate in the past:
  1. Tectonic activity: The arrangement of continents plays an important role in determining the climate, and if the continents move, the climate might very well change.
  2. Orbital variations: The ice age cycles of the past few million years are driven by changes in the orbit of the Earth about the Sun. The Earth’s orbit has important variations with time periods of approximately 25,000, 40,000, and 100,000 years.
  3. Solar variations: The sun is the primary energy source for our climate. As the output of the sun changes, so does the climate.
  4. Volcanoes: They inject ash and aerosols into the atmosphere, which reflect incoming sunlight. A strong eruption can cool the Earth for several years.
  5. Internal variability: The climate system is complicated, and internal modes of variability exist. The most well known one is the El Nino/Southern Oscillation (ENSO). During the El Nino phase, the Earth is much warmer than during the opposite phase, the La Nina.
Finally, there is a new player in the climate game: human-emitted greenhouse gases. These gases trap upwelling infrared radiation, thereby causing increases in the temperature of the surface.

If we look at the warming of the last few decades, we can immediately rule out tectonic activity and orbital variations because they are much much too slow to account for the warming over a few decades. We can rule out volcanic eruptions for a similar reason --- they affect the climate for only a few years. Thus, volcanic eruptions are also likely unrelated to the several-decades long temperature increase we are experiencing.

We can rule out solar variability because we have high-accuracy measurements of the output of the Sun from satellites since the mid-1970s, and we have not seen the increase in solar output necessary to explain the temperature increase. This is not to say that solar is playing no role, just that it cannot explain the majority of the observed warming.

Internal variability is the hardest to evaluate. We know that ENSO significantly changes the Earth’s temperature, and so long-term ENSO-like variation is something that we have to consider. However, nobody has yet put forth a viable mechanism or shown data that such a long-term cycle exists. In the absence of any evidence supporting it, we conclude that it’s likely that internal variability is playing a minor role in today’s warming. Clearly, future research might cause us to re-examine this conclusion.

Finally, we have greenhouse gases. In this case, things work out well. Both the timing and magnitude of today’s warming are well explained by greenhouse gases.

This is why scientists conclude that humans are likely responsible for most of the warming of the last few decades. Greenhouse gases provide a reasonable explanation for the warming, while no other factor can explain the entire warming (other factors, such as solar, might be playing a minor role, however). In the IPCC report, they attach the word “likely” to the statement about the importance of greenhouse gases, which denotes about 75% confidence that the statement is true. This takes into account our imperfect knowledge of the atmosphere, in particular with regards to internal variability, and that future work might lead to revisions of our views of this.

Finally, note that this conclusion does not come solely from GCMs. Instead, it sits on a much firmer foundation of a range of peer-reviewed studies that use a wide range of techniques. One of the things that gives us confidence is that the studies all paint a consistent picture of today's warming.


Anonymous said...

Superb article, Dr. Dessler, this is just the sort of succinct summary the layman needs to see.

Anonymous said...

Dr. Dessler, recent research at Duke shows the solar variations account for 30% of the observed warming over the last 2 decades, and water vapor likely (60-90% chance) explain about 40% more, so anthropogenic CO2 is only about 5-10% by some estimates, as natural variations are just as large as anthropogenic CO2 impacts. Indeed Dr. Lindzen has some very convincing arguments of the earth's adaptive iris effects of clouds and water vapour. I know you think Dr. Lindzen is a Luddite or oil company shill, but I assure you he isn't, and has very good reasons to be a skeptic, that are like mine, scientifically based, not based on profit motive. Why do you discount water vapor and solar variances so greatly? Why don't you think natural variations are material, the earth's history is full of them. I have never seen any studies that even try to explain water vapour, except Dr. Lindzen's of course.

Anonymous said...

Don't forget land-use change: altered albedo, latent/sensible heat flux, moisture fluxes, etc. Your assessment?
(cf. Marland et al. 2003. Climate Policy, 3:149-157.)

Anonymous said...

dr. j,
Very interesting. As I had noted in a previous comment, I am at least provisionally a "skeptic," but in my case for the reason that I have not extensively studied climate science and of course would not want to come to any conclusions without such a background.

I have questions regarding the accuracy of temp measurements from the pre-"hi tech" past as well as the co2 record, but I have not reached a point in my personal studies of climate science where I should be seriously tackling those questions.

While I realize that there is much contention in public, I also recognize that climate science has a long and complex history and is well grounded on a vast knowledge and research base.

Or to put it another way, there is vastly more to climate science as a discipline and as a research and discovery enterprise than the public debate over global warming we are seeing in the media.

Andrew Dessler said...

Dr. J-

First, I agree that solar could be 30% of the warming. Remember that the IPCC said that humans were likely responsible for "most", meaning > 50%, of the warming. That leaves as much as 49.9% of the warming to be explained by other means. And solar is certainly one option.

As far as water vapor goes, let me assure you that I think about it a lot. In fact, water vapor is my specialty (see my web page for a long list of publications on water vapor and its feedbacks).

The reason I don't include it in this discussion is because water vapor is a *feedback*, not a forcing. If you're unsure what I mean, see the excellent post by Brian Soden (and the comments) here.

As far as the sign of the water vapor feedback goes, Dick Lindzen's 2001 paper has been tested by other scientists and has been roundly rejected every time. If you want to lose all confidence in the theory, read Dennis Hartmann's critique of the theory in BAMS: Hartmann, D.L., and M.L. Michelsen (2002), No evidence for iris, Bulletin of the American Meteorological Association, 83, 249-254.

Finally, the historical climate shifts you allude to can almost always be explained by the reasons I listed in my post (orbital, tectonic, etc.). These same explanations cannot explain today's warming. Thus, the existence of prior climate shifts does not mean that today's warming is not caused by GHGs.

Daniel Collins-

I'm not an expert in land-use changes, but I fully believe that they are causing some warming. However, I have trouble believing that they are more important than GHGs. I have an open mind, however, and continue to think about the partitioning of the warming between GHGs and land use.


Anonymous said...

Dr. Dessler, perhaps you disagree with the AGU, who said : "There are many atmospheric greenhouse gases, some naturally occurring and some resulting from industrial activities, but probably the most important greenhouse gas is water vapor. Water vapor is involved in an important climate feedback loop. As the temperature of the Earth's surface and atmosphere increases, the atmosphere is able to hold more water vapor. The additional water vapor, acting as a greenhouse gas, absorbs energy that would otherwise escape to space and so causes further warming. This basic picture is complicated by important interactions between water vapor, clouds, atmospheric motion, and radiation from both the Sun and the Earth. There are some aspects of the role of water vapor as a greenhouse gas that are not well understood, again mainly because we lack the necessary observations to test theoretical models."

So you claim there is no GHG effect by water vapour? I believe it is the missing link and the largest missing bit of rigour necessary to complete study of climate change, since it is ignored or treated as a feedback with no direct forcing for peopple who believe as you do. But that aside, if you think human produced CO2 is causing even half the warming, then .5 degrees F in the last century, how pray tell will you reverse this small effect without draconian actions to shut down most CO2 emissions? I would think .5 degree is far too small to worry about, considering the 5-10 degree changes witnessed over the last 15,000 years or so that is all natural and well documented.

tyler5, in my research on ice cores and stable isotope paleo-thermometry, I think I have some appreciation as to how coarse and gross the temperature reading are from that, now combine it for ancient times with erratic and non-reproduceable alcohol, mercury, and water based thermometers around in the 1800s and most of the last century and you have huge sources of error for any temperature studies. Along with the heat island effect and temp stations moving all over the map over time, it is a dog's breakfast of data only the most faithful would want to use to prove anything accurate within 1 degree F. This whole line of evidence in the IPCC document was met by many with concern and questions, which of course, didn't show up in the "Summary for Policymakers" section, all complaints and concerns were conveniently filtered out and ignored by the powers that be, so as not to confuse the public in the push for policy action.

Anonymous said...

Cheers. I don't know how much to attribute to land-use - it is one of those things that are yet to be settled. I do find it interesting and unfortunate, though, that in public discourse of climate change the ratio of GWG to land-use is far greater (infinitely?) than it is in sci lit. It's as if we can only grapple with one scape goat at a time.

Andrew Dessler said...

Dr. J-

I fully agree that water vapor is the atmosphere's most important greenhouse gas. That said, water vapor reacts to other forcings (orbital, tectonic, solar, etc.), it is not a forcing agent itself. You cannot look at any climate variations and say "water vapor caused that." Rather, some other forcing agent caused it, and water vapor (and other feedbacks) reinforced it --- this is, in fact, why feedbacks are different from forcing. In a doubled CO2 world, only about 1 K of warming is directly due to CO2. The water vapor feedback and other feedbacks are responsible for about 2 K.

As to your other questions: how are we going to stop the warming and how important are the warmings, those are good questions that I hope to blog about soon.


Anonymous said...

Dr. Dessler,
You said "I fully agree that water vapor is the atmosphere's most important greenhouse gas. That said, water vapor reacts to other forcings (orbital, tectonic, solar, etc.), it is not a forcing agent itself. You cannot look at any climate variations and say "water vapor caused that." Rather, some other forcing agent caused it, and water vapor (and other feedbacks) reinforced it --- this is, in fact, why feedbacks are different from forcing. In a doubled CO2 world, only about 1 K of warming is directly due to CO2. The water vapor feedback and other feedbacks are responsible for about 2 K."

I fail to understand, since burning of hydrocarbons produces both CO2 and water, that only CO2 is considered as forcing. Is it because the water forms clouds and rain while CO2 doesn't? Or is "forcing" just a modeler's convention.

Andrew Dessler said...


Atmospheric water vapor is set by the global balance between evaporation from the ocean and precipitation. Water from any human activities is completely and utterly negligible compared to evaporation from the ocean. Contrast this to CO2, where humans have dramatically increased its atmospheric abundance by direct emission of CO2.

Here's another blog post to check out on this issue: here


Anonymous said...


You wrote: 'Rather, some other forcing agent caused it, and water vapor (and other feedbacks) reinforced it --- this is, in fact, why feedbacks are different from forcing.'

There is an intuitive problem with forcings and feedbacks as handled in the AGW theory. I would have to assume that a positive water vapor feedback is a response to increasing atmospheric temperatures, regardless of what caused the increase in temperatures. There is no way that water vapor can be a positive feedback for CO2 induced warming and not a positive feedback for an identical amount of solar induced warming.

Like wise, the positive water vapor feedback must operate the same way for an identical amount of warming due to water vapor, and there's the rub.

Nature must respond to a feedback warming the same way it responds to a forcing warming of equal value, since these feedbacks are all said to be strictly temperature dependant. According to AGW theory, any warming should produce at least twice as much warming, even if the warming is from a feedback, which obviously leads to a runaway greenhouse effect.

It seems likely to me, that all the feedbacks, including water vapor, likely sum up to a neutral or even dampening factor for external forcings? Otherwise our climate would not be as stable as it is, and something as common as a strong El Nino would flip the climate into chaos.

If the 1998 El Nino warmed the planet by .5 degree C, why didn't the positive feedbacks warm the planet another 1 to 1.5 degrees C. which would then trigger a warming of an additional 3 to 5 degrees? Put another way, how can half a degree warming triggered by an El Nino NOT trigger the same positive feedbacks as a half a degree warming caused by CO2?

Anonymous said...

dr. j,

Thanks for your reply, and the insight on the historical record. I am thrilled to receive this sort of help in my undertaking.

As I noted I have not yet reached a point in my studies of this great science where I ought to be forming independent opinions. I'll just say here that I have been intuitively aware that the accuracy of the temp records as well as the co2 records would be problematic since I first began studying the authentic lit on global warming.

[While I have often beat the table with the heels of those shoes for fun and upmanship during popular-level debates over at Eric Berger's SciGuy blog at the HouChron, as an amatuer I have far too much respect for this vast science and the professionals who have built it to undertake a study of their work with pre-conceived notions. Climate science is a tremendous achievement and I am in awe. ]

Concerning the politics side of it, you mention the IPCC summary. I read that the policymaker summary was written by the bureaucrats and politicos, and not the scientists.

Dr j, I wouldn't trust a UN or EU bureaucrat or policy wonk to fill out my name on a form, let alone summarize the available climate change policy options for the human race.

[In fact, if it was my call, I'd have the guards at the UN building check the silverware in the dining hall before they let anyone out of the doors at 5. :^D]

As I have noted in other comments, the media, the bureaucrats, the politicians, and the activists seem each in their own way to have attempted a coup d'etat here against both the estate of the scholars and the estate of the peasants. :^D

And frankly Dr J from what I have seen of the political antics surrounding this issue, it's the scholars, whether convinced or skeptical, who seem to be most concerned with the interests of the people in these matters and not our supposed public watchdogs, servants and guardians.


Anonymous said...

Dr. Dessler, what Jim Clarke says is what I am also talking about, you cannot say water vapour is but a feedback with no forcing of its' own, it is the most important, and most ubiquitous GHG, to just assume it varies as other much smaller GHG forcing do and ignore it's internal variability and causes lack scientific rigour to examine the AGW hypothesis for validity, or modifications. And since you mentioned human CO2, you do know, and all should understand, that human CO2 is tiny compared to natural CO2 from plant aspiration and ocean aspiration. It is a miniscule, 2% to total CO2 sources, which you seem to think huge, yet you ignore water vapour which is hundreds of times larger in volume and forcing. I think scientifically, there is a need to examine the largest and most common sources of warming first to sort out what they cause, then look at smaller human impacts signals on top of this noise, that has not been done and is a huge flaw in AGW assessments.

Also, I have to admit your sources on realclimate are suspect. realclimate is a decidedly left leaning political blog, Gavin makes sure no posts get through that would scientifically question AGW, I find that oppressive and do not regard it as an objective, uncensored source of science info. And, if you check Dr. Soden's resume, he says his :"research is motivated by the need to better understand how human activities are altering Earth's climate". This is not an objective way to start any hypothesis or research study, since he is already emotionally invested in the pre-conceived outcome. I have to be very skeptical of anyone with a frozen mindset on the issue, they have no objectivity and are prone to find what they are looking for.

tyler5, many have criticized the IPCC process and reports. Most of the scientists involved were general government scientists, few were independent university profs with specific education in the specific fields needed. The UN could hardly stage an objective dog fight, much less the massive undertaking of this topic. The sorry record of the inaccurate temperatures is best captured in the famous Mann hockeystick graph from IPCC, which was seized upon by the pro-AGW forces to shock and awe the public. There we see numerous problems, but the most dishonest scientifically is throwing together coarse tree ring, coral, stable isotope and other proxies with somewhat accurate thermo records with highly accurate and repeatable digital satellite records. I am still astonished that any scientist could get away with such shoddy, misleading data. However, the IPCC used it as a centerpiece of their reports.

Andrew Dessler said...

Jim Clarke-

You're absolutely right. The water vapor feedback acts on *any* forcing. So if we determine that solar variability has increased surface temperature by X or an El Nino caused an increase of Y, then those values *include* the effects of the water vapor feedback.

As far as your question about the stability of the climate goes, it *is* possible for a feedback to be strong enough that it would cause a runaway climate change. Much science has been done on that area, particularly in trying to understand Venus. However, the feedbacks on the Earth are not that strong. The important thing to realize is that feedbacks, as they are conventionally defined in the literature, do not operate on themselves. If they did, then you'd be correct.


Andrew Dessler said...

Dr. J-

First, let me reiterate that water vapor is a follower, not a leader, of the climate system. It reacts to other forcings, but is not a forcer itself. If the climate system were a mafia family, water vapor would be low-level muscle. It is powerful, but it takes its orders from above (solar, tectonic, GHG, etc.)

As far as your statement about CO2, I agree that the biogenic fluxes are much larger than the human fluxes. But they balance over a year. As a result, the build up in atmospheric CO2 is primarily from human activities. This is virtually never disputed, by even the most odious skeptics.

Finally, I would suggest you be more open minded about reading the science, even if the conclusions are disagreeable with your world view. Brian Soden is an extremely well respected scientist, and his knowledge of climate feedbacks is substantially superior to yours. If I adopted your position, I would never read anything that Dick Lindzen wrote. However, I read and carefully consider everything he publishes --- because I have to be open minded to be an effective scientist. If you read Brian's work and disagree with it, then that's fine, but don't reject it because it is your view that his web page makes him looks biased.


Anonymous said...

I didn't mean to impune Dr. Soder's character, but it seems he believes in AGW. I have never and would never start any of my research with a premise to show that AGW is wrong, for instance. I look at a segment of an issue and see what my research will bring to it, without a larger agenda of believing or not believing some hypothesis like AGW which is driving my research. But that's just me, scientists are human and prone to bias, but even if you look at Dr. Lindzen's website, he will not say his research is focused on disproving AGW, so why would Dr. Soder say his research is focused on human's altering the climate?

I think he truly believes it and has moved on in his mind and research. That's not wrong or bad, just biased in my opinion. I am extremely open minded on all scientific theories and articles, but I am not naive and think critically when I read something, using the context with which the author wrote it. Much of the basic research Dr. Soder presents is fine and I could agree with it, although I think clouds should also have negative feedbacks as other authors have suggested, and I am no expert on climate models so I have no idea if his computer models are accurate. But I do worry about scientists who have made their minds up on AGW, perhaps I am wrong and Dr. Soder has not, if so I apologize. I assure you I have not made mine up and am perfectly willing and open to all data and ideas, just that I have not yet seen convincing enough evidence to overcome my knowledge and research in the topic and the uncertainty of the AGW hypothesis.

I also do not think all the natural CO2 sources balance over a year, where is the proof of that? There are numerous studies showing large scale, cyclical and spikes of CO2 variances over time when humans didn't exist, so why didn't they balance then? It sounds like you think there is some steady state equilibrium of CO2 in the atmosphere that would operate if only humans weren't around, I see no scientific evidence of that at all, CO2 contents vary as does climate, naturally.

Andrew Dessler said...

Dr. J-

The most convincing evidence that the CO2 increase is manmade comes from isotopic analysis. Atmospheric measurements show that the increase in atmospheric CO2 is caused by a flux of "radiocarbon dead" CO2, meaning that it has no carbon 14 in it. The source of this CO2 must also be radiocarbon dead. Fossil fuels are indeed radiocarbon dead, while biomass is not. One can therefore conclude that the increase is due to fossil fuel combustion and not a biogenic source. I'm certain there are other reasons to conclude this, but to me the isotopic analysis is entirely conclusive.


Anonymous said...

Yes, the C14 evidence is much better than the other theories about C12/C13 ratios traced to fossil fuel burning, since plant respirations in all rainforest and upper latitude areas have C12/C13 ratios that overlap fossil fuels CO2 ratios, and are thus inconclusive. However, I have some doubts about the C14 results since they are not as reproduceable and rigouros as the C12/C13 analyses are, due to the small amounts of C14 that have been analyzed and the lack of complete, worldwide collection practices and sample handling. I hope further research along these lines produce more reliable data, as the small ppm rises in the last century make even small sampling and analysis errors fatal.

Anonymous said...

Gentlemen this is a wonderful discussion to follow.

And Dr. Dessler, this is one of the the most enjoyable comparisons I think I've ever read at a science blog:

"If the climate system were a mafia family, water vapor would be low-level muscle. It is powerful, but it takes its orders from above (solar, tectonic, GHG, etc.)"

Now, you just can't beat that! :^D :^D :^D :^D

I guess your students must be watching that mafia show? :^D

Anonymous said...

My prior comments were specfic to paleo-climate science, and to the degree that detection and attribution studies and/or GCMs are (or are not) shaped by the same cultural attitudes they should be suspect.

Still, historical highs and lows are only indirectly related to future concerns. The key question is the rosbustness of GCM's. If they are correct, then earth warming will continue REGARDLESS of the causes of prior warm/cooling periods.

The only way to find out if the models are robust is to test them. Models can seem real BUT, as any positivist would tell you, the 'realism' of a model is NOT its test of confidence - it's utility is confirmed in prediction and confirmation.

If we know (for example) what the CO2 level of 1050, 1350, or 1750 was (and can figure out the solar or volcanic outputs) do these models pan out qualitatively and relative to each period - that is, would the models predict highs and lows?

Do they 'pan out' for the last 10 years or the early century cooling (assuming the models are not calibrating against those periods anyway)?

I was a provisional warming believer - now I'm a fence sitter. I'd like to hear about GCM's and how they are varified by more than arguments over internal assumptions and idealized process loops.

EliRabett said...

The issue of whether the additional CO2 in the atmosphere comes from burning fossil fuel or biomass is dealt with in Jan Schloerer's now ancient FAQ at

In short, there are multiple lines of strong evidence, and studies in the nine years since Jan wrote the FAQ strengthen the argument.

.brian said...

Mark H.,

There are a lot of GCMs, and the "best" ones do pan out pretty well. There will be a lot more talk about this in the next few months in the lead up and publication of the IPCC 4th Assessment Report (due around February 2007). When the models are only given natural levels of CO2, they show little or no warming, even when other forcings (e.g., volcanic emission) are included. However, when all the known forcings are imposed, including anthropogenic CO2, every GCM shows strong (statistically significant) global warming. Some of the models do a better job of simulating the observed record than others, but that is mostly due to how they treat various aspects of the climate system.

Actually, that the models all show such strong warming even when they are formulated in such different ways is a pretty convincing argument that we understand the gross effects of increasing atmospheric CO2. Understanding how and why the different GCMs get slightly different answers is the subject of much ongoing research that will help to improve the next generation of climate models.

EliRabett said...

One additional point. Forcing (in particular what is called top of the atmosphere (TOA) forcing is a useful concept which puts different forcings on a common basis (solar, GHG, etc.)

However, it is only a first order approximation, and all forcings are not quite equivalent, if nothing else because of their variation with altitude and latitude. There is a National Academy report on this, but for purposes of discussions at this level, TOA forcing is useful

Anonymous said...

Excellent article. But why do so many people get uptight about the probability that western lifestyles have a negative impact on the planet's climate? Whatever the cause, human activity IS NOT HELPING the situation! Current lifestyles are not sustainable and as a species, we need to take action to improve the situation instead of sitting around arguing the toss. Do the sceptics not care about future generations? Or are they more concerned about paying higher fuel costs to drive their Hummers and SUV's? The US is by far the largest polluter on the planet, so to use one of your better cliches, 'Wake up and smell the coffee'. The billions of dollars spent on war, armaments and political shenanigans could go a long way towards improving the health of the earth and it's inhabitants. Remember, it is the only home we've got.