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What is a first-order climate forcing?

Filed under: — gavin @ 22 September 2005

Roger Pielke Sr. (Colorado State) has a blog (Climate Science) that gives his personal perspective on climate change issues. In it, he has made clear that he feels that apart from greenhouse gases, other climate forcings (the changes that affect the energy balance of the planet) are being neglected in the scientific discussion. Specifically, he feels that many of these other forcings have sufficient ‘first-order’ effects to prevent a clear attribution of recent climate change to greenhouse gases.

In general, I heartily agree – other forcings are important, even essential, for understanding observed climate variability and, as a community, we are only just starting to get to grips with some of the more complicated effects. Obviously, though, not all forcings are of the same magnitude (either globally or regionally) and so it is useful to separate the ‘first-order’ forcings from those that are relatively minor. But what exactly is ‘first-order’ and what is not?

It is helpful to distinguish forcings that are important in the global mean, from those which might be important locally but not have much impact for ‘global warming’. A good metric for the importance of a global forcing is the radiative forcing (i.e. the global mean radiation impact at the tropopause for an instantaneous change – the so-called “instantaneous forcing”). There are other definitions (e.g. the “adjusted forcing” where stratospheric temperatures are allowed to adjust), but for the purpose of this article these differences are not that important (for those who are interested there is a good discussion of the different forcing definitions in Hansen et al (2005, JGR)).

While the definition of a forcing may appear a little arbitrary, the reason why radiative forcing is used is because it (conveniently) gives quite good predictions of what happens in models to the global mean temperature once the climate system has fully responded to the change. Thus the forcing can be used as a shorthand for the climate response without having to do the experiment.

Of course, the global mean temperature isn’t the only useful metric of climate change – regional temperatures and precipitation are arguably much more societally relevant – but it does have a good signal-to-noise ratio. That is, changes to the system are more clearly discerned in the global mean temperature than at a regional level, mainly because the noisy ‘weather’ component increases as you go to smaller scales.

Global Mean Forcings 1750-2000
Figure 1: Two breakdowns of the global mean forcings since the pre-industrial. Factors causing warming are red, cooling factors, blue. The top panel shows the direct effects of the individual components, while the second panel attributes various indirect factors (associated with atmospheric chemistry, aerosol cloud interactions and albedo effects) and includes a model estimate of the ‘efficacy’ of the forcing that depends on its spatial distribution. (From Hansen et al (2005, JGR)). Subjective uncertainties associated with the well-mixed GHGs are around ~0.15 W/m2, for ozone ~0.07 W/m2, tropospheric aerosols around ~1 W/m2 and solar ~0.3 W/m2.

The figure here gives one estimate for how many of those forcings have changed over the industrial period (1750-2000). This assessment is from my own lab and so I may be a little biased, but although there are significant uncertainties (particular for the aerosol indirect effects), it probably gives a reasonable idea of the current thinking. The forcings illustrated here are from the well mixed greenhouse gases (GHGs) (CO2, CH4, N2O, CFCs), tropospheric and stratospheric O3, direct aerosol effects (from sulphates, nitrates, organic and black carbon), land use change, solar irradiance, volcanic aerosols, and various indirect effects (on clouds, stratospheric water vapour, snow albedo etc.). Reasonable estimates of these 16 different effects (and counting…) were included in the GISS simulations for the upcoming IPCC assessment.

The land use change used in the figure is related to the deforestation dataset of Ramankutty and Foley (1999) and includes the effects of albedo and vegetation type change, but not the impacts of increased irrigation or the ‘greening’ of the high latitudes (due to climate changes and possible CO2 fertilisation effects) . These latter two effects are expected to lead to slight warming, but the overall impact of land use changes is expected to be negative (i.e. a cooling) (Myhre and Myhre, 2003), although the uncertainty is still significant (maybe 0.5 W/m2 either way).

To my mind, the ‘first-order’ forcings would be the ones without which you can’t really do without in assessing global climate change. I would therefore argue that for the global mean the well-mixed GHGs and the counterbalancing reflecitve aerosol effects are ‘first-order’ – without GHGs there is no appreciable warming signal, and without the aerosols, the warming from GHGs is excessive and important changes in the diurnal cycle and cloudiness are not captured. Everything else (apart from volcanos, which are a special case) is in the noise. If we were to break it down even further, I would argue that CO2, CH4 and sulphates (the main non-soot aerosol) were the only ‘first order’ forcings. It is curious to note that this is the combination of forcings that were predominantly used in the simulations discussed in IPCC (1995) where the conclusion was made that the ‘balance of evidence’ supported the notion of ongoing human-caused climate change.

Before the emails come streaming in, let me make it clear that this isn’t to say that ‘second order’ forcings are unimportant. On the contrary, many of these effects have very specific signatures in the climate system (in the stratosphere, in the Arctic and in the tropics) that need to be understood much better – however they are unlikely to have a big impact on the global mean temperature. Thus when it comes to global warming, neither land use change/vegetation type, nor for instance, the biogeochemical effect of increased CO2 are ‘first order’. The first example is clearly important locally (impacts of deforestation, urban development etc.) while the second effect is as yet inadequately unquantified but there doesn’t appear to be any a priori reason to think it is globally important. It doesn’t therefore make much sense to claim that some of the smaller forcings are ‘first-order’ despite their importance, and conceivably dominance, at smaller scales.

To be sure, some of these effects (such as the impact of irrigation on surface water vapour, or land use changes on evapotranspiration) are not easily dealt with in terms of the tropospheric radiative forcing – a point that was well made in the National Academies report on radiative forcing (on which Dr. Pielke was an author). However, the dominance of well-mixed greenhouse gases on the anthropogenic forcing over the last few decades is robust to almost any estimate of the uncertainty in the other forcings. This is clearly a different opinion to that held by Dr. Pielke. However, this is probably due to our different perspectives in what we feel are important questions (local vs. global), rather than a disagreement over fundamentals.


Hansen, J., Mki. Sato, R. Ruedy, L. Nazarenko, A. Lacis, G.A. Schmidt, G. Russell, I. Aleinov, M. Bauer, S. Bauer, N. Bell, B. Cairns, V. Canuto, M. Chandler, Y. Cheng, A. Del Genio, G. Faluvegi, E. Fleming, A. Friend, T. Hall, C. Jackman, M. Kelley, N. Kiang, D. Koch, J. Lean, J. Lerner, K. Lo, S. Menon, R. Miller, P. Minnis, T. Novakov, V. Oinas, Ja. Perlwitz, Ju. Perlwitz, D. Rind, A. Romanou, D. Shindell, P. Stone, S. Sun, N. Tausnev, D. Thresher, B. Wielicki, T. Wong, M. Yao, and S. Zhang 2005. Efficacy of climate forcings. J. Geophys. Res., in press.

Myhre, G. and A. Myhre, Uncertainties in radiative forcing due to surface albedo changes caused by land use changes, J. Climate, 16, 1511-1524, 2003.

Ramankutty, N., and J.A. Foley (1999). Estimating historical changes in global land cover: croplands from 1700 to 1992. Global Biogeochemical Cycles 13(4), 997-1027.

47 Responses to “What is a first-order climate forcing?”

  1. 1
    Lynn Vincentnathan says:

    Maybe I’m wrong, but my impression is that both sides agree that GW is happening, one suggesting faster, one slower. I’m not saying this is like arranging deck chairs on the Titanic as it either A) more quickly or B) more slowly heads toward the ice berg, but I think we (as people, if not as scientists) should now start being concerned about reaching milestones in the warming (whether we reach them faster or slower) at which positive feedback loops kick in – even if this is difficult scientifically to quantify or prove.

    For example:
    1) plants giving off net CO2 in hot conditions (r/t aborbing) – see:
    2) plants dying out due to heat & drought & wild fires enhanced by GW (reducing or cutting short their uptake of CO2 & releasing CO2 in the process)
    3) ocean methane clathrates melting, giving off methane
    4) permafrost melting & giving off methane & CO2
    5) ice & snow melting, uncovering dark surfaces that absorb more heat
    6) the warming slowing the thermohaline ocean conveyor & its up-churning of nutrients –reducing marine plant life & that carbon sink.

    Am I missing some positive feedback points in which the warming leads to conditions that lead to further warming in a spiral fashion?

    I do understand even in the worse case scenario of such “runaway warming” or “Venus effect,” that Earth would stabilize at a new level, then start cooling down again, and would not become like Venus. And maybe this wouldn’t happen, if at all it does, until much later, say, after 2100. But still I’d like to see more on this topic (not that we don’t already have plenty to worry about with regular “linear” warming).

  2. 2
    Jon Foley says:

    This is a good summary.

    But I would offer another perspective on the “global” versus “local”. A local effect, such as from deforestation or urban heat islands, which is seen throughout the world, is still an important global change phenomenon. For example, urban heat islands may be affecting a large fraction of the world’s population (perhaps as much as 50%, the reported fraction of the population living in cities today) — so is that a global effect, or a local one?

    Another point is that land use / land cover change is likely to change far more than just albedo. In fact, the albedo change may be completely overwhelmed by changes in evapotranspiration, sensible heat flux, etc. And locally, and perhaps regionally, this can be larger than the effects of CO2. For example, we have done many GCM simulations showing that tropical deforestation — at least averaged over the tropical land masses — induces warming that is greater than the effects of CO2. Again, maybe it’s not “global forcing” but it’s still very, very important in the global scheme of things.

    A final point. I’m glad that people are beginning to move away from global mean temperature as the primary indicator of climate change. It’s good to look at, to be sure, but how is it relevant to society? Who actually lives in a globally-average place? Where is that? Wisconsin? Bangok? Tokyo? Australia? Climate change happens in real *local* places, where it affects real *local* people. Let’s make sure we don’t forget that.

    Anyway, this is a nice post, and I like where this discussion is going. It’s important that we broaden the definition of climate forcing, and begin to recognize that “local” forces are sometimes globally relevant.

    Jon Foley
    University of Wisconsin

  3. 3
    JN says:

    Re the comment on feedback mechanisms. A potentially significant but manmade greenhouse accelerator could be the use of replacement fuels from coal, shale and tar sands. In addition to burning the end product there can be substantial CO2 generated in the production process. There is increasing clamour for such fuels as oil reserves deplete.

  4. 4

    Gavin-thank you for commenting on my Climate Science weblog ( With respect to what is a first order climate forcing, the perspective of the 2005 National Research Council report ( includes the following

    “Regional variations in radiative forcing may have important regional and global climatic implications that are not resolved by the concept of global mean radiative forcing. Tropospheric aerosols and landscape changes have particularly heterogeneous forcings. To date, there have been only limited studies of regional radiative forcing and response. Indeed, it is not clear how best to diagnose a regional forcing and response in the observational record; regional forcings can lead to global climate responses, while global forcings can be associated with regional climate responses. Regional diabatic heating can also cause atmospheric teleconnections that influence regional climate thousands of kilometers away from the point of forcing. Improving societally relevant projections of regional climate impacts will require a better understanding of the magnitudes of regional forcings and the associated climate responses.” (


    “Several types of forcings – most notably aerosols, land-use and land-cover change, and modifications to biogeochemistry – impact the climate system in nonradiative ways, in particular by modifying the hydrological cycle and vegetation dynamics. Aerosols exert a forcing on the hydrological cycle by modifying cloud condensation nuclei, ice nuclei, precipitation efficiency, and the ratio between solar direct and diffuse radiation received. Other nonradiative forcings modify the biological components of the climate system by changing the fluxes of trace gases and heat between vegetation, soils, and the atmosphere and by modifying the amount and types of vegetation. No metrics for quantifying such nonradiative forcings have been accepted. Nonradiative forcings have eventual radiative impacts, so one option would be to quantify these radiative impacts. However, this approach may not convey appropriately the impacts of nonradiative forcings on societally relevant climate variables such as precipitation or ecosystem function. Any new metrics must also be able to characterize the regional structure in nonradiative forcing and climate response.” (

    Although these climate forcings may not alter the global mean surface temperture, they are first order climate forcings in terms of their substantial role in influencing the climate system including the planetary atmospheric circulation. We both agree that the radiative effect of carbon dioxide, methane and sulphates are first order climate forcings. What we need to do now is discuss what are the criteria that we use to apply the term “first order”. The title of the National Research Council report “Radiative Forcing of the Climate System: Expanding the Concept and Addressing Uncertainties” clearly indicates that we need to move beyond the current perspective of referring to global averaged temperature as the primary metric to assess human caused climate change, and of “CO2, CH4 and sulphates (the main non-soot aerosol)” as “the only ‘first order’ climate forcings”. I discuss this subject further on my web log, and welcome further discussion of this issue.

    On “the conclusion was made that the ‘balance of evidence’ supported the notion of ongoing human-caused climate change.”, the evidence of a human fingerprint on the global and regional climate is incontrovertible as clearly illustrated in the National Research Council report and in our research papers (e.g. see

    [Response: Roger, thanks for your comment. I think the main point I was trying to get across was that a ‘first order forcing’ depends on what metric you are choosing to consider. For instance, stratospheric ozone is clearly first order for the southern hemisphere polar vortex strength, but second order (at least) for the global mean temperature. I fully understand your desire to move on from the global mean temperature as the sole metric being considered (and I think that most of the community is moving in a similar direction), but for better or for worse, it is still, and will likely remain, an important measure. This is partly for the reason described by Isaac below, but also because of the length and quality of the observational data and the favorable signal-to-noise ratio. The more local the metric, the more difficult it is to assess any potential forcd component, and the least useful any potential prediction. But since we seem to have determined that global mean temperatures do tend to track global mean forcings, the interesting science is now in determining the regional scale at which we can still make useful statements – and whether a forcing is ‘first order’ or not will depend quite crucially on what the scale is. -gavin]

  5. 5
    Pat Neuman says:

    I am concerned about the “vast expanse of western Sibera is undergoing an unprecedented thaw that could dramatically increase the rate of global warming, climate scientists warn”.

    I’m interested in the GSA meeting on the “Causes and Effects of the Paleocene-Eocene Thermal Maximum (approx. 55 myrs ago) and Other Paleogene Hyperthermal Events” 2005 Salt Lake City Annual Meeting (October 16 – 19, 2005)

  6. 6
    Isaac Held says:

    In response to John’s comment (#2) concerning the overemphasis on global mean temperature:

    Define some measure that you are happy with of the severity of local climate change. Take some representative set of climate models predicting the climate at the end of the 21st century given some scenario of emissions (for example the 20 models in the archive established by IPCC for the 4th assessment) and compute this global measure of local impact. Then correlate it with the change in global mean temperature across the different models. My guess is that it will correlate pretty well, with the model’s predicting larger global mean temperature responses also predicting larger local impacts.

    If your impact measure is very regional — for example, if all you care about is rainfall in the Sahel — then it likely will not correlate well with global mean temperature change (I’ve checked this one). But if your metric is an average over the Earth of the magnitude of regional climate change, then my guess is that you will see a strong positive correlation. This is the limited sense in which I think it is meaningful to use global mean temperature change as a surrogate for the local effects that, of course, we all care about — in the context of considering the implications of differing model predictions.

    [Response: Isaac, this might well hold true for the IPCC scenarios because they all have substantial GHG forcings that will dominate the other changes. If however you took a more diverse set of experiments (such as ‘single forcing’ simulations over the 20th Century), I think it would be less true. Of course, I could always design a synthetic forcing that didn’t affect the global mean T at all (i.e. by hemispherically compensating), but that would still have clear regional effects. However, I agree with your concluding comment. – gavin]

  7. 7
    kyan gadac says:

    I support Lynn Vincentnathan’s comment at #1 above. Ignoring the feedback loops and concentrating on linear change seems to be foolishness.

    I have a problem with this analysis with forcing. Consider the ice ages, the alleged cause in this case is changes in the earths orbit BUT the ice ages are characterized by significant changes in CO2 levels that would appear to be the direct driver (i.e.’forcing’) of the consequent climate change. The problem is – where did the CO2 come from/go to during the ice age climate shifts? The answer must be biomass changes.(where else? – clathrates in permafrost?). So if biomass changes are responsible for changing the CO2 level then shouldn’t they be considered a forcing albeit of the non immediate kind.

    It’s probably extremely naive of me but perhaps somebody could explain what is the current theory regarding the changing CO2 levels during the ice ages.

  8. 8
    lee says:

    kyan— glad you wrote that! many climate modellers etc. will say that its Milankovitch which causes an Ice Age… ask any paleoclimatologist and they will probably laugh at you. Also at the end of an intergalcial period– after 12,000 years of leeching and erosion a lot of nutrients are missing from the soil (especially micronutrient). That being the case, most likeley at the end of the last interglacial there were weakened forest systems, weakened root mass, and less moisture being held by the forests. so that would likeley lead to forest fires…. without anyone to cut fire rows and fly overhead dumping water, these fires could just keep on going, right? So how much CO2 is released into the atmosphere from that? (considering a lot of our wood is now stored in landfills, old newspapers, libraries and buildings… it seems that we have found new and interesting ways to release CO2 (just my opinion)

    the only thing that scares me with those climate forcing charts is if policy makers look at the negative effect from particulates and aerosols… will this make some want to release more aerosols to abate the changes?

  9. 9
    Stephen Berg says:

    “Impact Of Global Warming On Weather Patterns Underestimated”:

  10. 10
    Dano says:

    Re #5:

    Pat, what is the concern? Do you not expect speciation rates to advance in such events, or is there some concern about speciation in the present case?

    Thank you,


  11. 11
    Pat Neuman says:


    Much of Earth’s vegetation will not adapt or be able to advance to new areas, particularly during the expected longer hotter and drier summers. Insects and animals that depend on vegetation will not reproduce as usual, or their young will perish. Humans will have a tough time, especially with limited air conditioning available only to those who can afford it, with limited fuel to run the power plants and work-home backup generators. What concerns me most is the environment that young people now will have later. I’ve advised my young adult daughters to think about what kind of environment will be here 50 or more years from now – before deciding whether or not to start their families. Think about the life that the yet to be born will likely have when they reach their senior years, then make the decision to not have children.

  12. 12
    Dan Pawlak says:

    Why is the hydrological cycle always omitted from the lists of first order forcings? Water vapor is clearly the most abundant greenhouse gas, and clouds most certainly have a first order impact on the global mean temperature.

    The hydrological cycle reacts to each of the other effects very rapidly. It has such a significant role and we cannot accurately quantify its impact on global climate change. I’m afraid the error bars on H2O would dwarf the other greenhouse gases.


    [Response: That’s easy. Water vapour is a feedback, not a forcing (at least most of the time). – gavin]

  13. 13
    Dano says:

    Got it Pat. I thought you were on about speciation or selection pressure or something.

    I recently told my sophmore-in-college niece pretty much the same thing you just wrote, only not so direct. We will soon learn as a society how far we expanded our range due to cheap energy availability, and when we contract our range we will have limited arable land on which to support ourselves, in addition to issues of water accessibility.

    Hopefully soon our societies can have a dialogue about what to do with our environmental refugees.



  14. 14
    antonio says:

    Where are we heading, this is very sorry state of affairs. I wiah all of us could do something for this.

  15. 15
    PHEaston says:

    Please cheer up! – Pat (11), Dano (13) and Antonio (14). Pat says:

    “I’ve advised my young adult daughters to think about what kind of environment will be here 50 or more years from now – before deciding whether or not to start their families.”

    As a start, they should perhaps be pleased they aren’t living 60 yrs ago, 100 yrs ago, 200 yrs ago or more. The standard of living, quality of life, life expectancy (in developed countries and many developing countries) is better than at any time in human history. While its easy to idealise the past, my ancestors had to live through world wars (don’t forget the Somme and Hiroshima), high infant death rates, no cures for many common (and now forgotten) illnesses, much higher levels of air pollution, no disaster relief schemes, etc, etc. While many believe global warming is a serious threat, we need to keep it in persepective. The predictions of armagedon are at best worse case and unlikely scenarios, and at worst just pessimistic speculation bearing no relation to rational scientific assessment. To scare you children into not having children of their own is, in my view, very sad and highly mistaken.

    If anything, the current article indicates the threat is less likely to be in line with worse case scenarios which assume the simplistic model of: more CO2 = higher temperatures. It also supports the view that there is a lot more research to be done to properly understand our climate and man’s capacity to influence it.

  16. 16
    Pat Neuman says:

    Although global warming will certainly be bad in itself, the likely prospect of beyond peak oil will compound the situation drastically, with little help to reducing forcings and the rate of global warming. With the supply and price of oil skyrocketing in the years aheads, farmers will not be able to afford to continue heavy fertization of the crops that the world needs to survive. On top of that, increased terrorism and widespread crime is another certainty. I’m pretty sure that I’m right.

  17. 17

    Hi Gavin- Regarding comment #12 and the reply, the alteration of the surface fluxes of sensible and latent heating by land use/land cover variabilty and change is a climate forcing. This aspect of the hydrologic cycle was recognized by the 2005 National Research Council report (see pages 93-98 in as a non-radiative climate forcing. Similarly, the influence of aerosols on precipitation processes is another example of a non-radiative climate forcing (see pages 6, and 42-44, for example, in the NRC report).

    On the reply to my comment #4, I agree with you that the term “first order” depends on the selected metric chosen. In the National Research Council report, we emphasized that we need to move beyond the global mean surface temperature. That climate metric very inadequately tracks with actual atmospheric and ocean patterns, which, afterall, is what detertmines the weather we experience. With respect to global warming, I have been urging we track ocean heat content changes, which, from our analyses, is a much more robust metric of climate system heat changes, as I discussed in 2003 in Heat storage within the Earth system (

  18. 18
    nanny_govt_sucks says:

    “Although global warming will certainly be bad in itself,”

    I just don’t see this at all. How will a warmer, wetter, CO2 fertilized planet be a bad thing? Other than possibly slightly higher sea levels, I see a lot of good coming from increased vegetative growth, and more arable land for vegetation to grow in. More vegetation means more food and a proliferation of animal life. It means more life in general, and I fail to see the downside in that.

  19. 19
    Stephen Berg says:

    “Coming To The Arctic Near You: The Longer, Hotter Summer”:

    Re: #18,

    This is factually incorrect. What will likely happen as a result of global warming is a migration of arable land northwards, but also mass desertification. There will not be an increase in vegetation, but likely a decrease due to the increases in frequency and intensity of extreme climate and weather events (i.e. droughts, flooding rains, hailstorms which wipe out crops, etc.).

    You’ve been reading too much of the Greening Earth Society’s propaganda. It is only there to obfuscate the public.

  20. 20
    Lynn Vincentnathan says:

    RE #18, it’s all a matter of perspective, depending on whether or not you require food to survive. Those who do will surely suffer, since GW is predicted to reduce world net food crop output in several ways: droughts, floods, heatwaves that kill plants; land loss from sea rise; no irrigation in glacier-fed rivers after glaciers melt in a few decades – putting 40% of India & China at starvation risk; crop loss due to increased pests (weeds & bug); fish decline from several GW factors; there’s probably more. World food productions is already declining, a small preview of the future. Then, of course, there are the other GW problems besides the boring subject of food….

  21. 21
    Pat Neuman says:

    Editorial: Arctic ice/Shrinking, with consequences
    Minneapolis Startribune
    Saturday September 24, 2005

    The Startribune article focuses on global warming feedbacks (not climate forcing).

    Relatd question… When permafrost thaws, is the methane and CO2 that’s released considered 2nd order forcing? Also… Not discussed in the article … As polar ice becomes greatly reduced, oceans will likely warm much more rapidly (similar to what happens when ice in a glass of water becomes minimal).

    Other questions… Has anyone seen estimates of the magnitude of ocean temperature change due to loss of sea ice? Is the effect significant? How so?

  22. 22
    Pat Neuman says:

    Recent comments by arctic explorer Mr. Will Steger

    “I had been living a quiet, private life, not in the public eye at all.” … “I was feeling politically isolated, and I had a growing concern about the state of the environment and the need for the United States to take more of a leadership role on issues such as global warming.” … “I started looking at maps of Canada, and the next day I knew I’d do the Arctic Transect.” …

    “Global warming is a reality for the Inuit. They see major changes affecting their lifestyle, with earlier springs, warmer summers and later falls. They used to dry their meat and fish in the summer, but now it gets so warm that the meat rots. There also is the migration of southern species â?? animals, fish, even insects. Hunting patterns change. Sea ice isn’t as thick.” …

    “I always have had big dreams. But I don’t see barriers, and if they are there I don’t let them get in my way. I can overcome barriers because I believe in myself and I’m practical. I’ll go over or under or around them.” …

    “I want to live in the wilderness, be active every day outside, be close to nature, raise my own food and live as simple a life as I can. That’s not a goal, though. It’s a journey.”

    Excerpts: Comments by Will Steger from “A Dreamer and a Doer”
    by Doug Hennes
    Winter Magazine, 2005.
    The University of St. Thomas
    Twin Cities, Minnesota

  23. 23
    TCO says:

    1. From a semantic point of view, your use of first order to mean global as opposed to (I guess) second order meaning local seems odd. That is not how I would see the term used in a complicated phenomena in business or engineering or industry or sociology. I would assume that the term would most likely mean “direct” as opposed to “indirect” or maybe “big” as opposed to “small”.

    2. Also, do you consider the H2O feedback of CO2 (as opposed to the CO2 itself) to be first or second order? Just curious…and in your chart, not clear if H2O feedback of CO2 is included? If so, why is “H2O from CH4” broken out seperately?

    3. Using your definition of “global” as opposed to “local” would think that all the aerosols would be included as they change the global temp. Or are you saying that things which change the local (even though significantly impacting global) but are not actually acting all over the globe are second order? Wouldn’t the impact of CO2 also vary by location (for instance dependant on sunlight, temp surface, clouds)?

    4. Could you blow up the figure? It’s fascinating, but impossible to read. There is room to do so.

    [Response: H2O from CH4 refers to the direct effect of stratospheric oxidation of CH4 which provides around half of stratospheric water vapour. Since the stratosphere is so dry, this turns out to be significant. This is completely separate from any water vapour feeedbacks to warmer temperatures. Aerosols are more spatially hetrogeneous than GHGs, but the largest component (sulphates) is large enough to effect the global temperature – other aerosols do also, but they have smaller effects. CO2 has a much more even effect due to balances between variation in outgoing long wave (low at the poles, high in the tropics) and water vapour overlaps (high in the tropics, lower at the poles) etc. The figure can be seen in more detail in the referenced paper (available in pdf). – gavin]

  24. 24
    Dan Pawlak says:

    Certainly the amount of H2O in the atmosphere may be partially considered to be a feedback response to other forcings. That’s thermodynamics. But the first order effect is the absorption and emission of radiation by water vapor, i.e., the greenhouse effect.
    Furthermore, the hydrological cycle is not confined to water vapor. Clouds play a very complicated first order role in scattering solar radiation as well as absorbing and emitting terrestrial radiation, again, the greenhouse effect.
    Add similar effects for the role played by ice and we have a very complex first order forcing that is not well-quantified.


    [Response: You appear to be confusing what are the most important factors in the current climate, with the most important factors that are causing cliamte to change. See my previous post “Water Vapour: feedback or forcing?” for a clearer discussion. All of the effects you mention are feedbacks, not forcings – though I agree that some of those feedbacks are not well quantified. -gavin]

  25. 25
    Mark Bahner says:

    #18: “I see a lot of good coming from increased vegetative growth, and more arable land for vegetation to grow in.”

    #19: “This is factually incorrect. What will likely happen as a result of global warming is a migration of arable land northwards, but also mass desertification.”

    Says who? And more importantly, based on what evidence?

    I challenge anyone to find a single published scientist who will make the claim that the total biomass on earth will actually go DOWN with global warming (of 5.8 degrees Celsius or less).

    [Response: This is a strawman argument. Total biomass may still go up even though deserts expand. -gavin]

  26. 26
    Michael Jankowski says:


    Dead on. If people have such gloom-and-doom outlooks today, I can’t imagine what they thought at so many other periods in history.

  27. 27
    Pat Neuman says:

    Re 11

    I forgot to mention spreading diseases as another factor that young women and men need to think about before they decide to have offspring, who may be here 80 or more years from now. For example:

    “Variations in climate have had a very noticeable impact,” said
    Elisabeth Lindgren of the University of Stockholm’s department of
    systems ecology. “We’re seeing disease in areas where we’ve never had
    it before, as well as more cases in areas where it previously

    Also, the problem with vegetation adapting or migrating to high rates of climate change is time. I see a rapid fall in total biomass for this century, and beyond. Significant evclution would need tens or hundreds of thousands of years, or more.

  28. 28
    Lynn Vincentnathan says:

    RE #25 & #18, I forgot to mention in #20 that the crop loss from increased pests (weeds & bugs), according to the scientific study I read (don’t have the source with me right now), was not due to GW, but due to the excess atmospheric CO2 – which the scientists had predicted would enhance crop growth. Yes, the crops grew better, but the weeds grew much better, choking out the crops. And the crops were less nutritious, which led to insects eating a lot more. Net result, crop decline. Now if we add to that the net crop & fish loss from various aspects of GW, then we’re facing very serious problems.

    That the global “North” may possibly have slight net benefit from GW (which I sort of doubt, based on other studies I’ve seen), at the expense of tremendous suffering in the global “South” (where the bulk of humanity lives), especially when it is the North that has reaped the most benefits associated with producing GHGs, is so unethical that even the most hard-hearted persons should quiver.

    BACK TO TOPIC: If CO2 & CH4 are important forcings in a linear GW scenario, then in a “runaway” GW scenario of the warming triggering further mechanisms of warming, triggering further mechanisms, our anthropogenic GHG emissions have even more ultimate impact. And their reduction becomes even more urgent. And every tiny bit of GHGs we can reduce — assuming everyone’s combined little efforts together can keep us from laying that lethal straw on the camel’s back (say, prevent us from getting us up to 6 degree C warming) — will really count, and not just be a drop in the bucket.

  29. 29
    Mark Bahner says:

    #25: Response: This is a strawman argument. Total biomass may still go up even though deserts expand. -gavin

    Who says deserts will expand…let alone that there will be “mass desertification,” as claimed in comment #19?

    Even more important, can you name even one one scientist who has ever claimed that, in a world globally warmed by 5.8 degrees Celsius or less, “There will not be an increase in vegetation, but likely a decrease…” as claimed in comment #19?

    The claim that vegetation will not increase, but will likely decrease, is absolute nonsense. I challenge anyone to provide the name of any competent scientist who has made the claim that vegetation will not increase, but will likely decrease.

    [Response: I don’t claim any particular special competence in the vegetation response to changing climate, but it will clearly depend on region, and it will depend crucially on changes to precipitation patterns as well as temperature or CO2 fertilization. Some models predict a significant die-back in Amazonia (Cox et al, Theoretical and Applied Climatology, 2004), which could easily outweigh gains in the Arctic. I don’t necessarily believe this is probable, but overly dramatic claims to the contrary are unwise. – gavin]

  30. 30
    nanny_govt_sucks says:

    #19 “There will not be an increase in vegetation, ”

    There is and has been:

    Earth is becoming a greener greenhouse
    “Our results … indicate that the April to October average greenness level increased by about 8% in North America and 12% in Eurasia during the period 1981 to 1999.”
    “the growing season is now about 12 ± 5 days longer in North America and 18 ± 4 days in Eurasia”

    Climate driven increases in global terrestrial net primary production from 1982 to 1999
    “Most of the observed climatic changes have been in the direction of reducing climatic constraints to plant growth.”

    Greening of arctic Alaska, 1981-2001
    “Here we analyzed a time series of 21-yr satellite data for three bioclimate subzones in northern Alaska and confirmed a long-term trend of increase in vegetation greenness for the Alaskan tundra that has been detected globally for the northern latitudes.”

    [Response: See response above. -gavin]

  31. 31
    Pat Neuman says:

    Re: 30

    A longer growing season does not mean that “Earth is becoming a greener greenhouse”.

    The report below by French researchers is an indication of what we may expect over larger areas of the world in future years and decades.

    2003 heatwave ‘may have sped up global warming’
    Wednesday, 21st September 2005, 17:47
    UK News

    Now French researchers have calculated that plant growth across the
    continent was reduced by about 30 per cent.

    Climate models have generally tended to predict global warming will
    enhance plants and prolong the growing season – raising the amount of
    carbon fixed in plant tissues. But the 2003 heatwave had the opposite
    effect, according to the findings published in Nature.

    Low rainfall in eastern Europe and extreme temperatures in western
    Europe, which topped 40 degrees centigrade in France, for example,
    combined to hinder plant growth in a way that was unprecedented over
    the past century.

    The researchers at the Laboratory for Climate Sciences and the
    Environment at Gif sur Yvette analysed computer models of the
    interactions between climate and the biosphere, and combined them
    with observations of carbon dioxide uptake from ecosystems, and
    records of crop yields.

    Dr Philippe Ciais, who led the research, warned the findings may be
    indicative of a future pattern in which droughts transform ecosystems
    from carbon sinks to carbon sources – accelerating climate change.

  32. 32
    Dano says:

    Please cheer up…Dano…

    ‘…one of the penalties of an ecological education is that one lives alone in a world of wounds.’ — Aldo Leopold.

    Some go on blithely ignoring what’s around them, some don’t. But that doesn’t make me unhappy because I make certain statements.


    Regarding desertification and greening (and ignoring the specious requirement for ‘competent scientists’), one must remember that human-ecological systems are coupled:

    – desertification and climate change:

    Geist HJ, Lambin EF 2004. Dynamic causal patterns of desertification. Biosci 54:9

    Le Houerou HN 2002. Man-made deserts: Desertization processes and threats. Arid Land Res and Mgmt 16:1

    Asner et al 2004. Grazing systems, ecosystem responses, and global change. Ann Rev Envr Res 29

    Zhang et al 2004. Sources of Asian dust and role of climate change versus desertification in Asian dust emission. GRL 30:24

    Du et al 2004. Mutual influence between human activities and climate change in the Tibetan Plateau during recent years. Global and Planetary Change 41:3-4

    United Nations Convention to Combat Desertification report section

    – Greening:

    There is a lot of work to be done with NDVI and whether increased terrestrial greening is a greening of existing plants, or new plants on the move [such as non-woody spp such as grasses], and what that means. Touting a greening and implying that’s good smacks of GES and mendacicization, as this omits valuation and mentioning whether the extant plants are outside of their temp tolerance and thus are susceptible to mortality. Many graminaceous crops, e.g., are at the upper end of their temp tolerance, meaning cropping areas may have to move, and perhaps entire cultures as well; tens of millions are spent in the Philippines to increase the heat tolerance of rice. Plus, we have recently seen what happens to environmental refugees in Louisiana.

    Bergengren et al 2001. Modeling global climate-vegetation interactions in a doubled CO2 world. Clim Change 50:1-2



  33. 33
    Joseph O'Sullivan says:

    This was an informative summary. I knew the NAS had put out a report but I did not have a chance to read it. If I am getting this right it is about the scale. Global averages can allow us to determine anthropogenic forcings, but the regional scale does have the interesting scientific questions.

    Personally I think the effect of climate change on ecosystems is the most interesting part of the science, and the local secondary forcings are what is important in determining exactly what ecosystem disruption will occur.

    Re # 30, 29, 25 et al
    There have been climate changes in the past that have affected ecosystems and have been extensively studied. Long term and large-scale changes like ice age/interglacials and small-scale and short-term changes like El Nino show what happens to ecosystems in a changing climate. Climate is the primary factor in determining type of ecosystem (e.g. tundra, coral reef or savannah), and climate changes cause ecosystem changes. Climate changes, like warming, do not lead to simple ecosystem changes. Ecosystems are generally considered to be chaotic systems.

    Warmer and wetter does not simply mean more growth. Climate changes result in ecosystem disruptions. Past climate changes have resulted in shifts in the geographical distributions of individual species and entire ecosystems, and when movement was not possible entire ecosystems disappeared and individual species became locally and at times globally extinct. Important factors in the current anthropogenic warming episode is that the warming in ecological terms is substantial faster then other known episodes and temperatures could be higher then they have been in millions of years. These factors make adaptation less likely. Other anthropogenic changes like habitat destruction and fragmentation also make it less likely that ecosystems can cope with climate change by shifting.

    On CO2 as a fertilizer, in the oceans CO2 is acting as a pollutant and is altering the chemistry (the ph) and the emerging data shows a harmful effect on organisms. See the acid ocean post here on RealClimate, and this press release
    On terrestrial ecosystems CO2 has shown in recent studies only to have a minor effect on plant growth. See here

    For a review of the scientific literature on the effects of climate change on ecosystems

  34. 34
    Steve Bloom says:

    In the news in just the last few weeks was a study showing serious problems (pests and fire) in Canadian boreal forests due to warming. Presumably this would involve a reduction in biomass through the reduction of these forests. I couldn’t quickly locate this particular study, but found various related items including this (which includes maps showing the potential changes in the Canadian prairie provinces): . CA is another place that may see a biomass reduction, with more forests suffering the same fate those in Southern CA are already encountering (e.g., pest and fire problems in the Lake Arrowhead area). Also, I rather doubt that all else being equal a longer growing season will lead to more biomass here.

  35. 35
    JN says:

    Rainfall intensification creates problems for hoofed animal grazing. If heavy rain is punctuated by severe hot dry weather this could loosen the soil to be washed away in the next storm. Compacted or thinner soil may favour woody weeds rather than grass and give less animal protein per acre. This could entrain a costly cycle of increased weed spraying and tractor tillage to compensate. Yet another sign that says eat less meat more veg.

  36. 36
    Juola (Joe) A. Haga says:

    If there were a methane burp, as apparently has occurred in the past, that would be a climate forcing,–no? If just on the edge of the Black Sea, a polite belch,–regional–would not be a forcing. If major,–say along the San Juan da Fuca plate–would it be a forcing? Sorry, if I’m writing out of turn. Joe Haga

    [Response: Yes this would be a forcing. Whether it was important globally would depend on how big it was. -gavin]

  37. 37
    Johnny Jupiter says:

    Global warming is here to stay a while, and probably will get worse long before it gets better. Every day, whether on the tv news programs, the newspapers, or online reports, the main topic is pro and con statements and arguements about what can be done, or needs to be done by usually someone else. I took a pro-active step a few years ago, establishing an insect garden.
    Last year the chemical smoke from chlorine of the Bio-Lab fire in my area destroyed my organic insect garden. An area of about 20 ft diameter hosted over 100 varieties of insects with an increase of 5 to 10 varieties a year for the last seven years. Now I have only about 6 or 7 visiting. Hopefully they will replenish themselves, since on a global scale the damage was slight. My reasoning for the habitat for bugs is to furnish an oasis for insects. A place to find food, water, shelter, nesting sites, and a refuge from all the sprays, poisons, and other things all my neighbors use to “destroy the pests”. A lot of my friends maintain a “bug garden” also. Herbs seem to host the most insects. I think because herbs have been around for so long, more insects are familiar with them as food sources. Oregano, dill, mints, parsley, and cilantro seem to draw the most. Other herbs are added as I find them, like rosemary, and french and russian tarragon. Some vegetables and a few flowers added along with a goldfish pond under the rosemary shrub, for water, seems to work well.
    I have also set out some herbs in windowsill pots for apartment dwelling friends. They report daily visits by several insect varieties. So anyone can furnish a habitat for something, whether a humming bird feeder, or a bluebird box or martin gourd, to patio containers of blooms that are not ever sprayed with anything that would harm insects.
    I guess what I am saying is be pro-active and do something of benefit for something alive, besides humans…jj

  38. 38
    Pat Neuman says:

    Re: 37

    A few years ago, seeing that global warming was happening and likely to worsen, I knew I needed something to do something that would help me feel good about myself. I began spending much of my time working on improving habitat for wildlife and spreading the word about global warming.

  39. 39
    Stephen Berg says:

    “Air Pollution Linked To Heart Attack”:

    This article proves that excess consumption of fossil fuels is exceedingly dangerous to human health.

  40. 40
    Lynn Vincentnathan says:

    RE #39, Air pollution also aborts fetuses & causes fetal damage (as do pesticides – which also contribute to GW in their manufacture).

    So when we do things that emit GHGs, we also likely emit local pollution that kills, & becomes regional acid rain (destroying lakes, crops & forests), & acidifies the ocean, depletes the ozone layer (some pollutants), and maybe a hundre other harms, in addition to contributing to GW, and to runaway GW. Now what was the argument in favor remaining energy/resource inefficient and wasteful? I keep forgetting.

  41. 41
    Dano says:

    Re: 39 and 40-

    The SciDaily linky is a rehash of a paper that shows an association, not proof. And the first statement in 40 needs qualification as well – as

    Trasande L, Thurston GD 2005. The role of air pollution in asthma and other pediatric morbidities. Journ. All. Clin. Immun. 115:4 pp. 689-699 APR 2005 ISSN 0091-6749

    points out. There is a risk of increase, but it is spatial and not everywhere. This is an important environmental justice issue, and should be teased out, but teased out accurately.

    RC uses language to show that science is uncertain and probabalistic – science doesn’t prove anything. We would do well to frame our arguments as the authors do here.



  42. 42
    Pat Neuman says:

    Re 2

    I think global mean temperature data is an important indicator of global warming. I think numerical measures for global warming are important, than indicate how the rate of global warming may be changing. A graph of 10 year moving annual averages for above land temperatures is at:

    Please show how “local” forces are sometimes globally relevant.

  43. 43
    Stephen Berg says:

    More news:

    “Report: Ice-free Arctic summers possible by 2100”:

    “Warm climate transforms Alaska terrain”:

  44. 44
    Pat Neuman says:

    Warming trends are similar for the Arctic and Midwest.

    Declining Arctic Sea Ice:

    Earlier Midwest Spring Snowmelt Runoff

    The trend in declining Arctic Sea Ice (NSIDC) is similar to the trend for earlier Midwest Spring Snowmelt Runoff (

  45. 45
    ag says:

    Is there exists a model that run starting from now and going back for a 100+ (or even just 50+) years that agrees well (<5-10% temp., precip., wind patterns, particulates in the atm.,,, etc. differentials) with past reality?
    Without inclusion of available intermediate measurements and/or free parameters tweaking during the run, of course.
    I also would prefer a model without free “tweakable” parameters at all, but I do realise that it might not exist.

  46. 46
    Ken Caldeira says:

    The idea that land-surface change may be an important factor in climate change was suggested by Fourier as early as 1827, so this topic has a long and interesting pedigree.

    A translation of the paper can be found at:

    An extract from Connolley’s translation of Fourier’s words reads:

    The movements of the air and the waters, the extent of the seas, the elevation and the form of the surface, the effects of human industry and all the accidental changes to the terrestrial surface modify the temperatures in each climate. … The establishment and progress of human societies, the action of natural forces, can notably change, and in vast regions, the state of the surface, the distribution of water and the great movements of the air. Such effects are able to make to vary, in the course of many centuries, the average degree of heat; because the analytic expressions contain coefficients relating to the state of the surface and which greatly influence the temperature.

  47. 47

    […] of short-run climate changes, along with their estimated relative importance, you can look here: RealClimate For all intents and purposes, "first order" as used there means "large". It’s […]