

Why does climate change? Causes and Timescales


Whenever the topic of global warming comes up at a dinner table conversation, someone will always suggest something along the lines of “well how do we know global warming isn’t natural?” or will mutter a thing or two about “natural cycles.” It is a reasonable question but saying something is “natural” is not itself an explanation.  Whether climate changes are due to human or non-human factors, there should always be a more specific cause that we can identify, and we are bound by the laws of physics when attempting to make any sense of those causes.   I won’t be talking about the evidence behind man-made global warming in this post, but I want to visit some of the key mechanisms that cause climate to change in the past.

The term “climate” itself requires us to think outside the timescale of a few days or even a few years, and consider the long-term statistical nature of how a place behaves.  We are all familiar with variations between day and night, summer and winter, but these are all “cyclical” changes that are highly predictable and are part of the equilibrium climate we have grown used to in our lives (see also the difference between weather and climate).  In contrast, the bona fide “climate changes” in this post imply changes over many decades, centuries, or even millions of years.  They correspond to a shift in the long-term characteristics of temperature, precipitation, or other things—for example entering an ice age, or making New York, on average, as hot as Florida over the next 100 years.  The changes here tend to be much less “cyclical” than people seem to assume, although not always.

The climate changes due to causes that operate over a variety of different timescales, which I will discuss going from the longest to some of the shortest.

The New-Born Earth

Most of the first several hundred million years of Earth’s history are clouded in mystery, with the lack of preserved rock records. However, we know that two “planets” collided, one of which became our world, and one became the moon. The aftermath of the moon-forming impact would leave the Earth as a hot, CO2-rich, steam atmosphere that took tens of millions to a hundred million years to cool from temperatures in excess of 400 degrees Fahrenheit.

It turns out that planets with a lot of liquid water, like Earth, have a theoretical limit on how much heat they can lose to space. Currently Earth loses about 240 Watts per square meter to space, but it would need to lose more if the sun got brighter and the temperature hotter.  The maximum allowed heat loss rate for Earth is about 310 Watts per square meter. If Earth were to receive energy from the sun faster than this, it would continue to heat up until the oceans boiled away. As discussed in the next section, the sun was actually dimmer than present over most of geologic time, so receiving too much sunlight was never an issue— but it is possible that more frequent impacts and internal heating from the formation of the early Earth provided enough energy to make up for the deficit in solar energy, and forced the Earth into a temporary runaway greenhouse in the first several million years of its history. The runaway greenhouse may also have occurred as a more permanent feature on Venus, although this is still up for debate, but it is evident that Venus used to have much more water than present.

Billions of Years

Jumping to the very longest timescale (billion of years), we need to consider that our own sun is getting brighter over geologic time.  The reason for this is due to the way the sun adjusts its internal temperature as it goes through nuclear fusion and converts its hydrogen into helium.  When the Earth first formed, the sun was about 30% fainter than today’s value.  The faint young Sun imposes a stringent constraint on the climate of the young Earth; all other things being equal, the dim sum means the planet would be completely frozen over, yet geological evidence indicates that liquid water was present on the early Earth.  This conundrum became coined the ‘Faint Young Sun Paradox’ by the great scientist and science populizer, Carl Sagan.

One way to get around this paradox is to have a much stronger greenhouse effect (discussed here) than the modern day.  The best proposed way to do this is to have a lot more carbon dioxide in the atmosphere, supplemented with methane or other gases. In 1981, it was proposed that planets with both a) liquid water and b) working plate tectonics have a way of adjusting their CO2 concentrations to maintain reasonably moderate surface temperatures.

The idea is that volcanoes are constantly releasing CO2 into the atmosphere, while at the same time CO2 is removed from the atmosphere by dissolving into rain water and beating down on silicate rocks (i.e., weathering), and whose byproducts wash into the ocean and settle onto the seafloor.  Rather than CO2 building up in the atmosphere forever, it only builds up to the point where the rate of outgassing from the Earth equals the rate at which the formation of carbonates by reaction with silicates removes it.  Eventually, CO2 returns to the atmosphere through volcanism at locations where carbonate-rich oceanic crust is being subducted beneath moving continental plates. Volcanic eruptions are rather random and don’t listen to the climate, but the rate of weathering goes up with higher temperatures and precipitation, which means that if it gets too hot, CO2 can be drawn out of the atmosphere faster.  We’ve now constructed a thermostat. If it gets too cold, weathering goes down, and CO2 builds up in the atmosphere making it warmer.  This acts as a climate stabilizer, but only over long geologic timescales.

Still on the billion-year-timescale, we must also consider the evolution of life and its influence on the atmosphere.   In particular, the evolution of photosynthetic organisms like plants replaced the primitive atmosphere with an oxygen dominated one.  The last ~3.8 billion years of Earth’s history can be divided into several stages: from the beginning to about 2.45 billion year ago, there was virtually no atmospheric oxygen, and the oceans were anoxic.  At the second stage, up until about 1.85 billion years ago, there was a small amount of O2 in the atmosphere with some oxygenation of the shallow ocean; Stage 3 from 1.88 to 0.85 billion years ago is called the “boring billion” when O2 levels did not change significantly; up until a little over 500 million years ago, O2 would climb to about 20% of the atmosphere, and eventually reached modern values.

Methane, a greenhouse gas, cannot survive in high concentrations once a lot of oxygen is present.  The decrease in methane as O2 rose may have helped trigger so-called “Snowball Earth” events, periods when the Earth was covered over entirely with ice.  The last of these episodes was the Sturtian at 720 million years ago, and the Marinoan at 635 million years ago. These correspond to some of the most dramatic climate changes experienced on Earth, and it was not until after this that complex life began to arise.

Once in a snowball planet, the planet is very good at reflecting incoming sunlight away (ice/snow are bright), cooling it down further, and it is very difficult to get out of an ice-covered state. The weathering “thermostat” described previously would have kicked in; volcanoes would continue to inject CO2 into the atmosphere. With cold temperatures and no liquid water present, weathering would shut off and there would be no way to take out CO2 from the atmosphere. Eventually CO2 would build up to a substantial fraction of the atmosphere, and help generate a strong enough greenhouse effect to get of a snowball.

Over Millions of Years

Over millions of years, plate tectonics causes changes in continental positions, forcing the uplift of mountains, and opening new ocean pathways, all of which impact the circulation of the atmosphere and ocean.  When continents were close together so that the Earth was covered in a “super-continent,” the interior of the land would become quite dry, as water vapor from the ocean was lost near the continental margins. 

However, simply changing the distribution of how heat is transferred around the planet is generally not ncredibly effective at causing large climate changes. Instead, forcing the planet “externally” through changes in sunlight or altering the atmospheric composition (changing the greenhouse gas content) affects the global energy balance. This can also be done by changing how reflective the planet is, for example by increasing cloud cover, and blocking sunlight.

However, tectonics can impact the global carbon cycle, thereby changing the CO2 composition of the atmosphere. If a mountain rises, freshly exposed rock can draw down large amounts of CO2 from the atmosphere. Similarly, the rates of weathering will change if most of the landmass is concentrated at the equator or at higher latitudes. As a case in point, between 60 and 50 million years ago, India moved north rapidly, eventually collided with Asia, and forced the uplift of the Himalayan Mountains and Tibetan Plateau.  CO2 was drawn down as a consequence, and a long-term cooling trend since that time began.  Eventually Antarctica became covered with ice by ~34 million years ago.

Timescales of Tens to Hundreds of Thousands of Years

We are now onto the timescales of a few hundred thousand years or shorter. The event described above gave rise to a relatively “cool” planet (i.e., one with ice cover at both the North and South pole), especially during the last 2.5 million years.  Over this period, changes in the Earth’s orbit dominate the pacing of climate change. In particular, variations related to how tilted the Earth is, the wobble of its axis, and how “oval-shaped” the Earth’s orbit is around the sun conspire to pace the coming and going of ice ages.  The timing of these orbital changes are 41,000 years for the increase and decrease in tilt, which determines whether high latitudes receive more or less sunlight (a less tilted planet is favorable for ice growth at the poles); there is a 23,000 year cycle for the wobble of the planet, which controls the timing of the seasons; and 100,000 and 400,000 years for changes in the circularity of Earth’s orbit.  These cycles are too slow to be responsible for global warming today, but 20,000 years ago the capital region was covered in perhaps a mile of ice! These long-term changes in the Earth’s orbit are causes by the gravitational effects of the other planets, especially Jupiter, as well as the pull of the sun and Moon are the Earth’s “bulge” at the equator. A more thorough description I wrote on this is here.

The following image shows estimates of deep-ocean global temperature over the last several tens of millions of years. Panel A goes back 60 million years. Panel B zooms in to the last 5.33 million years, and Panel C zooms in further to the last half million (500,000) years. Note that the more recent period is on the right, and dates get older as you go to the left.

Estimates of Deep-Ocean Global Temperature at different resolutions in the past. Figure from Hansen and Sato, 2011, in press

The orbit isn’t everything for causing ice ages.  To get the full extent of temperature change between glaciations and the in-between “warm” interglacial’s also requires, once again, changes in CO2.  CO2 fluctuates from roughly 180 parts per million in the atmosphere during cold periods to 280 ppm during warm periods.  Methane also plays a secondary role. It didn’t cause ice age swings, but the response of the oceans and terrestrial biology to changes in the orbit caused CO2 to fluctuate, which then amplified the amount of warming or cooling that occurred.

In addition to all of this, there are many processes that are incapable of causing climate change on their own, yet amplify or dampen the amount of change that occurs when you “force” the climate externally. The magnitude of change that occurs is augmented by various so-called feedback processes. For example, when the atmosphere gets warmer, it holds more water vapor; water vapor is a greenhouse gas and this amplifies temperature. When it gets colder, more ice is present, and ice reflects sunlight. Reducing the sunlight, by reflecting it away, helps amplify the cooling. Responses of this sort are critical for understanding past and modern climate, and the past offers some of the best insights into how sensitive the Earth is to change.

Timescales more familiar to us…

On still shorter timescales, the ice core records reveal that several abrupt climate changes (over decades) occurred during the last ice age.  These are thought to arise from changes in ocean salinity as ice expands or breaks off into the ocean and freshens the water.  This helps modify the density of seawater, and determines whether water moving north from the equator sinks and returns south, or freezes at high latitudes and forms sea ice.  The idea that such a “conveyor belt” of ocean water, which influences heat transport, might shut down was the basis for the “Day After Tomorrow” blockbuster hit.  Although a highly exaggerated movie, ocean changes have been responsible for differential warming/cooling between the low and high latitude regions before.

Climate can also change due to fluctuations in solar activity, although direct changes in sunlight are typically very small on timescales of decades or centuries. There is, for example, an 11-year cycle where the sun gets slightly hotter and dimmer, but it is a very small climate impact.  Volcanic eruptions can also cause the planet to cool for a few years by injecting large amounts of sulfate aerosols into the upper atmosphere, which reflect sunlight.  A relatively cool period called the ‘Little Ice Age’ between roughly 1400 to 1800 AD was in part caused by lower solar activity and more volcanic eruptions, although the magnitude of this change was much less than a true glaciation.

Over several years, events like El Nino or La Nina can move heat between the atmosphere and ocean, changing regional weather patterns and even impacting global temperatures for a couple years. However, these events don’t add energy to the whole planet over decades or centuries, and so cannot be responsible for larger trends in global temperature.

To Conclude…

In most cases, changes in the climate are forced by altering how much energy the planet absorbs from the sun (either by changing sunlight or how “reflective” the planet is), or perhaps even more importantly, changes in the composition of the atmosphere.  Changing the greenhouse content of the planet determines how efficient heat is lost by the planet to outer space.  In fact, the past climate record shows unequivocally a strong role of CO2 in acting like a “control knob” for Earth’s climate, so rather than past climate variations being evidence against human-caused global warming, they show that the same physics we think operates today was operating in the past.  This increases confidence in future projections and in our understanding of atmospheric physics.

Categories: General
Chris Colose

42 Responses

  1. prokaryotes says:

    Connecting the Dots: Climate Change drives Earthquake / Seismic activity


  2. prokaryotes says:


    I find this interesting, and is this a negative feedback? Or more like a two edge sword? Does the increase of weathering, builds up carbonates in soils – the surface layer and causes a range of other implications too, like acidifying the environment, increase of erosion and in general more stress and overall flux?

  3. Chris Colose says:

    There are several sub-cycles to the carbon cycle (such as the oxidation and burial of organic carbon), but the negative weathering feedback is ultimately thought to maintain Earth’s climate within a habitable range over timeframes of millions of years and longer. This was orignally proposed in this paper, but there has been a lot of work on it since then

    and described in some more detail in

  4. sam says:


    You say: “the past climate record shows unequivocally a strong role of CO2 in acting like a “control knob” for Earth’s climate”

    How do you explain then that CO2 very clearly lags temperature in ice core proxy reconstructions? I have heard the bizare claim that Co2 doesn’t start the warming or cooling but amplifies it… Yet there is literally no proof of that. In fact, if you look at the ice core data in detail you see that once co2 starts rising there is no acceleration in warming just the same slope. You have it exactly wrong when it comes to C02 and temperature — temperature drives C02. Case closed. You need to discard your C02 centric view of climate and come back to reality.

  5. Chris Colose says:

    Debt causes interest, so clearly interest can´t put you further in debt…

    I have issues with such logic.

  6. Patrick 027 says:

    Re sam – see physics. Increasing CO2 essentially can’t not cause warming.

    Re prokaryotes – I think chemical weathering is supposed to reduce acidification (of the ocean).

    There is also (related to celestial mechanics) the slow decrease of the coriolis effect over time. In the past, aside from other effects, I would guess that the pole to equator temperature gradient would have been larger (true? – if so, easier to have polar ice? harder to get snowballs?) and extratropical storm tracks would have reached into lower latitudes (Hadley cells more restricted), and maybe there could have been multiple defined tracks in each hemisphere. If circulation structures were more horizontally restricted, might the interiors of even smaller continents been drier, perhaps inhibiting ice sheet formation at times?

    There is the polar continent idea for ice ages; obviously it has to be cold enough globally for that to work, also, a continent that is too large could have summers that are too warm and/or be too dry.

    There is also the rate of sea floor spreading – I’m not sure if this is the best way to describe it, but if the average age of the sea floor is younger, there might be less carbonate sediment built up at any one time, implying more CO2 elsewhere. And (other) variatons in volcanic activity can cause changes in CO2 (Siberian Traps). Aside from variations on the million – 100 million year time scale, wouldn’t there have been a decrease in geologic emission over billions of years?

    I’ve sometimes wondered what the weather would have been like when the Earth’s atmosphere was dominated by H2O or had much more H2O in it (after the moon-forming impact if not at other times). How big would the rain drops be? Hail? Tornados?

    PS one interesting point is that the slowing rotation of the Earth and the outward shift of the Moon depend on the tides, which are shaped by the coriolis effect and the tidal forcing itself, and also by geography. Hence, tectonic processes could affect the rate of evolution of orbital cycles.

  7. Chris Colose says:

    Patrick, nice to see you, hope you drop by more often:

    Ray Pierrehumbert has a Neoproterozoic review paper just published this year, where the authors note that the length of day 700 my ago was ~22 hours (they give a citation but I haven´t read it), but apparently it is hard to change that in models, so they ran simulations with a modern rotation rate. They suggest that the impacts of the increased coriolis effect and the changed diurnal cycle are minimal, and certainly smaller than the effects arising from different cloud or albedo parameterizations (although some could argue this is just an assertion, and I don´t know how much detail it has actually been studied).

    There´s also some work out there on extrasolar planet dynamics concerning what happens for tide-locked bodies (very little rotation, higher rossby numbers). Typically Hadley Cells extend poleward, and horizontal temperature gradients collapse (as in the modern day tropics). The question for snowball initiation is interesting. I suspect going from a 24 to 22 hr day doesn´t have a huge impact on the CO2 threshold required to (de)glaciate, but it might be more important in the tidally-locked limit, though it would also depend on how good the atmosphere can transport heat to the night side (or whether it gets cold enough on the nightside for the whole atmosphere to condense out). I don´t think the science has converged to an answer on these topics…We have other contributors to this blog who have better dynamical insight and can probably think up of some more effects this might have.

    The Neoproterozoic paper is here

  8. Sam says:


    Warming from Co2 is far from being basic physics. If you thing about it, all that Co2 does in concentrate temperature into a lower portion of the atmosphere. It can not create energy, only change where it more likely to be. It changes the temperature gradient, not the total energy of the atmosphere. The real question is the amount that Co2 warms the atmosphere. Any honest person would have to admit is far from settled science.

    You and Chris had no real rebuttal to my previous statement, why is that? You really have nothing?

  9. Chris Colose says:


    I am going to let this comment through moderation, but if you want to discuss this topic intelligently here, you need to do so without the childish games of name-calling or saying ¨you have nothing.¨ There´s a lot of free-for-all blogs out there, and I don´t want this to devolve into one.

    Your argument is also a repeated point that has come up countless times over the last decade, and it has been so thoroughly addressed that anyone still using it is probably just asking to be ignored. I apologize if you are new to this topic, and are unaware of this, so I will briefly respond. But I urge you to consult the actual ice core studies that you seem to think support your case. If you are going to convince anyone here that we need to abandon a view held by virtually all of the scientific community, the existence of journal papers that support your case would probably be a good start -yet ice core experts do not accept your interpretation. Moreover, you need to frame your argument in a logical way. Saying A causes B therefore B can never cause A is not a good start.

    I see absolutely no reason why it should come as a surprise that temperature can lead CO2. Obviously if you change the ocean temperature, the wind patterns, the ice sheet extent, the size of the biosphere, etc, then you are going to change carbon inputs/outputs and alter atmospheric chemistry. I specifically noted in my article that the glacial-interglacial cycles were paced by orbital changed, and so CO2 did not come first. In fact, this was predicted before it was detected in ice core records See this paper. The rate of the CO2 response (the feedback) is small though, amounting to about 10 ppm/ deg C.

    What would actually be more surprising is if CO2 did lead temperature during the whole ice age cycles. If this were the case, we´d have to come up with a mechanism that would cause the atmospheric chemistry to change in a semi-cyclical way without being forced. If not for temperature changes, why would CO2 suddenly increase by over 50% on its own? That would be an incredibly surprising result.

    And I agree with Patrick, that CO2 causes warming IS basic physics. When we account for those physics, we explain the magnitude of temperature change between glacial and interglacial much better than with just orbital changes, which alone are fairly small. CO2 does not need to *create* energy, it just needs to slow the loss of energy out of the top of the atmosphere for a given surface temperature.

  10. Patrick 027 says:

    Re Chris Colose – thanks

  11. Patrick 027 says:

    Re Sam – the absorption/emission spectrum of CO2 has been studied; it’s known (along with other important atmospheric components). Satellites can measure the radiation emitted by the planet(s) and it’s what you’d expect based on the known physics. The changes in radiant fluxes caused by changes in composition can in general be calculated (and they have been measured by satellites). In some cases there are back-of-the-envelope ways to get a rough approximation; for more accuracy a lot of number crunching may be needed, but it’s based on very straightforward concepts (see ‘Schwarzchild’s Equation’). Radiation carries energy; radiation is generally (LTE) emitted as a function of temperature and of optical properties; it is generally (LTE) absorbed as a function of those same optical properties – the temperature distribution and optical properties thus determine radiative fluxes; where those fluxes converge there is a heating rate, where they diverge there is a cooling rate; these imbalances alter the temperature over time until equilibrium is achieved. When pure radiative equilibrium is unstable to convection, or otherwise locally disrupted by advection or cyclical forcing (day-night, etc.), the net radiative heating and cooling can remain, balanced by a rate of storage (related to temperature change or phase change) – which must average to zero over time in an equilibrium climate, or by horizontal advection – which must average to zero globally – or by vertical convection, which occurs mainly in the troposphere and from the surface to the troposphere (in isolation, that’s conduction and diffusion, but it’s a part of convection) (setting aside the oceanic circulation – which is also not generally unknown in it’s effects and causes).

    CO2 can change temperature gradients, but within the troposphere convection (vertical and otherwise) modifies temperature gradients. Changes in fluxes at the top of the troposphere add or remove heat to/from the troposphere+surface – however that is radiatively distributed, convection tends to redistribute it so that the resulting warming or cooling is more evenly distributed (within the surface-troposphere system), except as modified by the lapse rate feedback (physics of moist convection), and in some parts of the climate system where convection is more inhibited (and the distribution of feedbacks also has a particular pattern in space and time, altering the distribution of the temperature changes). The stratosphere tends to cool with increasing CO2, but that is dependent on some specific conditions (the greenhouse gases are not ‘grey’, there is direct solar heating of the upper atmosphere – even without that a transient cooling would tend to occur but the equilibrium would be different). The stratospheric cooling is not merely balanced by warming elsewhere; the stratosphere has a relatively small heat capacity and reaches radiative equilibrium much more quickly than the rest of the climate system – the troposphere is convectively tied to the land surface and very importantly, the ocean, and until they warm up to equilibrium, a radiative imbalance exists, with a net downward radiative flux (the difference between incoming solar radiation and net outgoing terrestrial radiation) from the top of the atmosphere to at least within the troposphere (this unbalanced flux must extend into space – it can’t stop within the stratosphere because the stratosphere isn’t contributing to it once it’s equilibrated). This imbalance causes an accumulation of heat.

    (PS even if CO2 didn’t change the total heat in the system, the redistribution would still be a climate change – it matters what happens at the surface, and in any given region and season, for that matter).

    There is a little uncertainty in the radiative forcing, but it’s not very big (at least for greenhouse gases – aerosols are another matter); the bigger uncertainty is in the (non-Planck) feedbacks (especially clouds) (The Planck response refers to the temperature change that restores equilibrium just from the temperature-dependence of the emission of radiation – this involves a negative feedback; in climatology one often refers to feedbacks as being the other feedbacks, which make the climate more or less sensitive to forcing than it would be relative to the Planck response). But we know there are feedbacks, and the evidence indicates they are (excluding Planck feedback) a net positive feedback. In addition to the physics and the model output based on that, there are observations of the recent climate, and paleoclimatic data (including from deeper geologic time as well as the recent ice ages), which are at least in broad agreement about climate sensitivity.

  12. Patrick 027 says:

    Re my (the greenhouse gases are not ‘grey’, there is direct solar heating of the upper atmosphere – even without that a transient cooling would tend to occur but the equilibrium would be different). – the transient cooling would extend from TOA down to some depth in the atmosphere; it wouldn’t necessarily be the entire stratosphere.

  13. sam says:

    Chris, You say that this argument about ice core lag has been brought up many times and it has been “addressed”… I disagree strongly with you on that. Just because this has been brought up many times does not make it invalid and just because you declare it addressed does not make it so. You say that a study predicted this before it was discovered in ice cores… I looked at the study you link to… it uses ice core data so how why would you say that it predicted this lag? This paper does not seem convincing to me at all btw. They claim that Co2 contributed ~50% of the warming seen in ice cores… because a climate model told them, so it must be true. This even though it flies in the face of basic reason from looking at the ACTUAL DATA. A simple timing analysis pretty much refutes the idea that Co2 is driving anything on that time scale. Have you climate people ever heard of Occam’s razor? You guys have destroyed that theory 10,000x over. The paper says that “climate and green-house gas concentrations were intensely interactive”. I only see interaction one way. You can’t make up information where none exists!

    The major issue I took with your post was your statement that “past climate record shows unequivocally a strong role of CO2 in acting like a “control knob” for Earth’s climate”. That statement isn’t even supported by or seem to fit with the rest of you post which overall seemed fairly informative and non-controvertial.

  14. Chris:
    Congratulations on the new home.

    Re: Control Knob — see Richard Alley’s excellent presentation which I link to from

    Re: CO2 being driven by temperature, see

    In both cases, the presentations are data driven.

  15. Chris Colose says:


    Unfortunately, I don’t know where you are getting information or what independent analysis you have done, and without analysis, I cannot reproduce the calculations you have done to lead you to your answer. Part of the purpose of this blog is educational, so any other reader trying to get information out of these comments is simply in a position to trust one of us.

    But there has also been an abundant literature on this, and the one I linked to was only among the first papers relating the role of CO2 to the glacial-interglacial changes. There are many others, including Nicolas Caillon and other’s 2003 paper in Science here (for termination 3, 240,000 years ago), Monnin et al 2001, Shakun and Carlson 2010 (from the last glacial maximum to the present day), and some others mentioned below. Note also that one of the ice core experts who published a paper that was routinely misrepresented about this point wrote about the subject here

    There does not need to be a single cause for the ~5000 year long warming trends, and the ~70,000 year cooling trends, and it is completely illogical to argue that because something doesn’t initiate it, the CO2 has little role over the whole cycle. As I and Patrick 027 have noted, the reason CO2 warms the climate is well rooted in basic physics. And yes, when a physical model is run with CO2 we get a much closer answer to the *actual data* than one where CO2 is not allowed to vary, and it also serves as a mechanism to communicate climate change globally (see Alley, 2003 Phil. Trans. R. Soc. Lond.), and this is widely used as a sensitivity test to see how CO2 can impact global temperature. Note also that the GHG feedback is nearly
    linear in global temperature (see Hansen et al 2007, Phil. Trans. Royal. Soc. A; Hansen et al 2008 Open Atmos. Sci. J., Fig. 2)

    You are welcome to come up with a model that comes up with a better answer, and submit it for publication, but simply declaring something useless because it is a “model” is not productive; virtually all of science is based on a “model” in some form or another. Dropping a ball off the building and writing physical equations to describe its velocity is a “model” and while larger scale GCM’s used to simulate the climate may not be perfect, this does not allow you to arbitrarily ignore the laws of physics built into them or interpret information in any conceivable way.

    My later statement about CO2 being the “control knob” was not only in relation to its feedback role over the glacial-interglacial cycles, upon which this whole “CO2 lags temperature” stuff is based on, but over a much deeper part of geologic time as well, including its role in the weathering thermostat that maintains Earth’s climate, the ability to help deglaciate a snowball, and not mentioned was its role in forcing the PETM warm event that is labeled in the first panel on the graph.

    I doubt this response will satisfy you but at this point the scientific community has been satisfied by this interpretation, it yields coherent explanatory and predictive power, obeys the laws of physics, and is a robust conclusion in a large number of studies that have done ice core analysis. So unless you publish work that seriously challenges this, I’m not sure I have any more to say than many of the above links and countless prior explanations on the web can provide. Apparently going back and forth with words and no physics or analysis is not going to lead us anywhere.

  16. Chris Colose says:

    Dr. Grumbine, thanks for coming by! Hope to see you again

  17. M says:

    Sam: The ice core data in 1990 did not have the temporal detail to determine how the timing worked, but the point is that the paper anticipated that there might be a lag anyway.

    Your “simple timing analysis” is anything but, and you should watch out before nicking yourself with Occam’s razor. Occam’s razor, to my mind, states that “it is simpler to believe that CO2 acts as 100-year old physics dictates it should, than to declare that it magically has no effect, and then some other magical force stepped in to fill in the remaining half of the temperature variation that our knowledge of physics (as embodied in climate models) suggests that orbital variations and ice-sheet retreat are insufficient to explain”.

    There seems to be a misconception among a certain set that if temperatures rise by 1 degree in 1000 years, there’s no reason they can’t rise by another in the next 1000. (alternatively, see smaller changes, shorter timescales for post-little-ice-age). We’ve internalized a certain amount of Newton’s laws, and this one speaks to the “an object in motion stays in motion”. However, climate physics doesn’t work that way. Temperatures don’t change unless forced, and if the forcing is removed, they’ll want to return to their previous level. So, if you see temperatures increase over any given time period, that’s likely due to some influence, whether it be external (solar or orbital changes), internal-forcing (ice sheet retreat/growth, greenhouse gas changes, or a volcanic eruption), or internal-shifting-around of heat (El Nino). For each of these, the expected behavior is that when a forcing is applied, there will be an asymptotic change towards a new equilibrium. If the forcing is removed, the temperature will drop towards the old equilibrium. A slow feedback, like CO2 release from the oceans, can help a rise go far longer than it would otherwise.

    Also, I’d suggest watching Richard Alley’s “CO2 control knob” talk:


  18. prokaryotes says:

    Re Patrick 027 “I think chemical weathering is supposed to reduce acidification (of the ocean).”

    Thanks for the input Patrick. Though with uptake in weathering, the surface layer and small depth waterways acidify initially i believe. When you consider extreme climate changes which cause a wide range of environmental alteration of chemical compositions.

    For example suboxic zones/dead zones(hypoxia) which through denitrification can create No2, 10.000 times faster , another feedback.

    I working on a blog where i collect related science about weathering and it seems with higher temperature the it is much harder to reduce acidification (of the ocean).

    Pedology – Erosion & Weathering during the PETM

    Atmospheric carbon dioxide leads to Solution Weathering.

    One of the most well-known solution weathering processes is Carbonation the process of dissolving carbon dioxide in water. Carbonation can also describe a chemical reaction, one example of which is a key step in photosynthesis.


    The development of a thick suboxic zone with high iron bioavailability – a product of dramatic changes in weathering and sedimentation patterns driven by severe global warming – may have resulted in diversification of magnetite-forming organisms, likely including eukaryotes.

  19. prokaryotes says:

    Thanks for the links Chris Colose, im going to read the suggested papers.

  20. Chris:
    Bob, not Dr. :-) If we’re ever in the same area, it’ll be good to sit down and chat.

    Now that I’ve gotten back from travel, and you’ve mentioned your new location in a comment at my place so that I find it (laziness is rampant in my house), I’ll be in from time to time.

    I’ll encourage you to post links to reference relevant articles at my place, and perhaps encourage your readers to include citations in their comments. Sam’s bald assertions as to what is or is not the case regarding CO2 are cheap and easy to make. Doing the analysis that I did took a couple of hours, and I’m sure that Richard spent days of real working time preparing his presentation.

  21. Patrick 027 says:

    Re prokaryotes – CaCO3 solubility does decrease with increasing temperature; however, reaction rates are also important. (PS see the “OA is not Okay” series in the “Skeptical Science”.) When CaCO3 dissolves into Ca(2+) and CO3(2-) ions, CO3 ions can react with CO2 to form bicarbonate ions, which allows more CO2 to be taken up by the water (for a given partial pressure of CO2); as this removes CO3 ions, it should also allow more CaCO3 to dissolve, I think. I don’t know about the solubilities of other carbonate minerals.

    But ultimately hydrolysis of silicates (as opposed to dissolution of carbonates) is necessary to balance geologic emissions of CO2 and remove C not just from the atmosphere but from the ocean as well (excluding organic C and it’s burial). A representative net reaction is CaSiO3 + CO2 = CaCO3 + SiO2, but in reality there are a number of silicate minerals that are involved on both sides of the reaction; some minerals can be the product of one such step and the reactant of a subsequent step, and other carbonate minerals can be produced. Before these carbonate minerals precipitate, they can allow additional CO2 to be held in the water (because of reaction that forms bicarbonate ions) – when the carbonate minerals form, that extra CO2 (extra relative to conditions) will tend to be released, but because we started with silicate minerals and CO2, there is a net removal of C from the atmosphere+ocean+surface organic C reservoir.

    I don’t know about how the solubility of silicate minerals is affected by temperature, but I’ve read that hydrolysis is favored in warm wet conditions – I think because of the chemical reaction rate. However, physical erosion also enhances the chemical reaction rate. liquid water can be involved in that; so can ice. Freeze-thaw cycles can make cracks grow. Glaciers, of course. I’d guess that the flow of glaciers could enhance chemical weathering by providing sediments to locations where they can be picked up by rivers or wind and carried to warmer wet locations where the chemical reactions can procede; alternatively, if you have temporal cycles, debris left behind after each frozen night or winter or ice age might then be chemically eroded. So I wonder if a succession of ice ages can help contribute to long-term cooling whereas a permanent ice age wouldn’t so much (?). But these are things I only understand conceptually, not quantitatively.

    There’s also the idea that reduced sea level during an ice age exposes more sediment to erosion (either hydrolysis or dissolution could help pull CO2 out of the atmosphere) – however, exposed organic C could be oxidized and add CO2 to the air. (And it occurs to me that CH4 emissions that would have been stored as clathrate/hydrate might then be released instead(?)). I also wonder how much organic C might be trapped under ice sheets or permafrost during an ice age that could be released upon deglaciation (in different locations if under ice sheets;
    … PS one thing about ice sheets is they are more resistant to warming if they are thicker because higher elevations are colder (although loss at lower elevations combined with flow can thin an ice sheet) – one idea for why the timing of ice ages changed a little after a million years ago is that prior glaciations had removed much loose material and thus it became harder for the ice to flow, so the ice sheets built up to greater thicknesses).

    … but anyway, the thermodynamic stability of carbonate minerals relative to their corresponding silicate minerals + CO2 actually decreases with increasing temperature (that’s why the C can be cycled back into the atmosphere/etc. by geologic processes), but it is the reaction rate that provides a negative feedback.

    PS to the extent that the rate of precipitation is a factor, I wonder if the negative chemical weathering feedback might be weakenned outside a certain temperature range. Greenhouse gases don’t just make the surface+troposphere generally warmer by altering the flow of radiant energy through the tropopause and above, they also alter the flow of radiation through the troposphere; in particular they can in general increase the backradiation at the surface (not necessarily in proportion to the warming effect – contrast a well mixed gas to cirrus clouds, to water vapor, etc.), reducing net LW cooling and thus, for the same solar heating, would increase convection. With warming of the surface, it’s not obvious that the net LW cooling of the surface would decrease with warming caused by an increased greenhouse effect because there’s also the increased surface emission – but then there’s also lapse rate feedback, etc., and the loss of snow and ice cover would add to solar heating; however, over a certain range of temperatures, the water vapor feedback is quite powerful at the surface and it’s possible to bring the net LW cooling down to near zero – and that should happen with solar-forced warming as well. So convection in terms of the heat flux should increase (in terms of air movement it may decrease because the water vapor concentration increases more – something like that, see Held and Soden, “Robust Responses of the Hydrological Cycle to Global Warming”). And the fraction of that which is evaporation at the surface would tend to increase with increasing temperature (setting aside regional variations in precipitation, of course). However, that fraction can at most be 1 (well, locally you can have evaporative cooling exceeding the net surface convective cooling, such as with warm dry air over a cool ocean, but it’s hard to imagine that occuring in the global time average). Water vapor also absorbs solar radiation, and farther warming may shift solar heating vertically up from the surface (but I don’t know how much), which would reduce convection from the surface. So, depending on what clouds do, I wonder if their is a temperature beyond which global precipiation would decrease (perhaps there’d be more virga).

  22. Patrick 027 says:

    PS … so I would have thought that hydrolysis of such silicate minerals would tend to mitigate acidification locally by creating dissolved carbonate minerals, but there are other dissolved components so I don’t know.

    Corrections/clarifications –
    So as the temperature rises from an increasing greenhouse effect, the sensible heat flux and convective mass flux would peak first, then the total convective heat flux, then the latent heat flux. But-

    Warming driven by solar TSI increases would tend to keep convection increasing or at least delay the point where any measure or type of convection peaks.

    I’d think the distribution of increased solar heating, such as from albedo feedback, could affect the tendency for higher temperature to increase the ratio of evaporative to sensible cooling of the surface. Increased convection in colder places could boost sensible heating more.

  23. prokaryotes says:

    Patrick what means LW and TSI?

    And with the exceptional acceleration of climate change and no action to stop that, we could very well enter a runaway climate change state, where all ocean evaporates.

  24. Chris Colose says:

    LW = longwave (far-infrared radiation)
    TSI = Total Solar Irradiance

    P.S. See my response to Kevin Tyle on the ´How to Build a Greenhouse thread´; we can´t actually trigger a runaway on Earth. We don´t get enough sunlight to sustain one.

  25. prokaryotes says:

    Chris, but the sunlight is not really the main agent in certain runaway scenarios..


    The present paper presents a mathematical analysis of a tipping point or runaway feedback that can arise when the heat from microbial respiration is generated more rapidly than it can escape from the soil to the atmosphere. This ‘compost-bomb instability’ is most likely to occur in drying organic soils with high porosity covered by an insulating lichen or moss layer. However, the instability is also found to be strongly dependent on the rate of global warming. This paper derives the conditions required to trigger the compost-bomb instability, and discusses the relevance of these to the concept of dangerous rates of climate change. On the basis of simple numerical experiments, rates of long-term warming equivalent to 10°C per century could be sufficient to trigger compost-bomb instability in drying organic soils.

  26. Gilles says:

    Chris , you say :


    and then :


    But the mechanism you first describe does not *always* change how much energy is absorbed for the sun, and isn’t caused by a change of the composition of the atmosphere either. Only during sharp glaciations/deglaciations transitions you can invoke change in the reflectivity, but there are variations with smaller amplitude during the holocene that don’t belong to this category. It is a change of how the energy is transported in latitude, which can profoundly affect the average temperature even with a constant *effective* temperature (= same energy absorbed and reemitted). For instance if you solve the basic problem of computing the average temperature of the Earth without GHG, and with a given albedo, the result depends vastly on which assumption you take for the redistribution of heat. A perfectly conducting earth will have a temperature of around – 20 °C, whereas a perfectly insulated (with only a “local” temperature given by a local balance between insolation and black body emissivity) will be .. around 100 °C colder on average (a whole hemisphere at night will be at 0 K). Although the global energy balance is the same. So the average surface temperature (which is * not* a physically meaningful quantity, not entering in any energy budget), is very dependent on how the heat is transferred from one point to the other. This explains the impact of oceanic oscillation like ENSO , AMO, or PDO on the temperature.

    Now I don’t think you and other climate scientists in general have offered a strong argument of why the XXth century variation could not have been due, at least partly, to this kind of variation, lowering the effect of CO2 accordingly.

  27. Chris Colose says:

    I’d bet a lot of the 20th century variability was because of those things you mentioned, and they haven’t been ignored. I would tend to agree also that multidecadal natural climate variability should be accounted for in future predictions, especially for regional and decadal scale prediction; though, people have shown fairly small direct contribution of the AMO (etc) variability to global temperature evolution. Note also that the abrupt climate changes during the last glaciation tended to have a see-saw temperature pattern to them between the North and the Southern Hemisphere.

    I’ll try to make my next post on the attribution issue.

  28. Patrick 027 says:

    Re Gilles – it’s possible to have zero global average forcing with a climate response that has some nonzero global average feedback, and internal variability could do something similar (the snowcover in a given winter will fluctuate with the weather, of course. How much, though? And note that it takes time for radiative disequilibria to result in accumulation of heat).

    But the average temperature is a physically meaningful quantity. It’s the average temperature, that’s what it means. It is related to enthalpy and to radiation, but with a distribution effect, so if you know the distribution you can figure it out.

    I attempted my own estimate – it was a rough estimate – but I came up with, for Earth’s surface, approximated as a blackbody everywhere, that the temperature variation made a difference of ~ 1 K radiatively – that is, with a global average surface temperature of 288 K, it would emit the same flux globally as would an isothermal globe at ~ 289 K. Temperature variations tend to be smaller goint up into the troposphere than at the surface. I’d guess the feedback from temperature rearrangements couldn’t be that big – consider how much global warming or cooling would be required for the temperature variation to be halved or doubled. On the other hand, changing correlations among clouds, humidity, and temperature could change the global OLR flux. But…

  29. Patrick 027 says:

    Re prokaryotes – if we do enough, we might enter a runaway feedback of a different sort, which would be limited by the amount CO2 or CH4 that would be released, or ice that would be melted, before sensitivity comes back below positive infinity (where the sensitivity would have to be something other than Charney sensitivity so that the CO2, etc, can be included as feedbacks). Concievably you could have a lurch from 290 K to 291 K and then stop. Or not…

    But I think the point that Chris Colose has made is that even if we take all the C we can find and put it in the air, as hot as it would get, we couldn’t get into a runaway water vapor feedback.

  30. prokaryotes says:

    I dont believe you can calculate with CH4 limitation, because CH4 gets produced by decomposing. Beside this i think Hansen has it right when he warns of a runaway climate change when we burn all the coal reserves.