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Why doesn’t the temperature rise at the same rate that CO2 increases?

Posted on 22 July 2013 by gpwayne

This post is a new 'basic' level rebuttal to the myth There's no correlation between CO2 and temperature

What The Science Says

Surface temperature measurements are affected by short-term climate variability, and recent warming of deep oceans.

Why doesn’t the temperature rise at the same rate that CO2 increases?

The amount of CO2 is increasing all the time - we just passed a landmark 400 parts per million concentration of atmospheric CO2, up from around 280ppm before the industrial revolution. That’s a 42.8% increase.

A tiny amount of CO2 and other greenhouse gases, like methane and water vapour, keep the Earth’s surface 33°Celsius (59.4°F) warmer than it would be without them. We have added 42% more CO2 but that doesn't mean the temperature will go up by 42% too.

There are several reasons why. Doubling the amount of CO2 does not double the greenhouse effect. The way the climate reacts is also complex, and it is difficult to separate the effects of natural changes from man-made ones over short periods of time.

As the amount of man-made CO2 goes up, temperatures do not rise at the same rate. In fact, although estimates vary - climate sensitivity is a hot topic in climate science, if you’ll forgive the pun - the last IPCC report (AR4) described the likely range as between 2 and 4.5 degrees C, for double the amount of CO2 compared to pre-industrial levels.

So far, the average global temperature has gone up by about 0.8 degrees C (1.4 F).

“According to an ongoing temperature analysis conducted by scientists at NASA’s Goddard Institute for Space Studies (GISS)…the average global temperature on Earth has increased by about 0.8°Celsius (1.4°Fahrenheit) since 1880. Two-thirds of the warming has occurred since 1975, at a rate of roughly 0.15-0.20°C per decade."

Source: NASA Earth Observatory

The speed of the increase is worth noting too. Unfortunately, as this quote from NASA demonstrates, anthropogenic climate change is happening very quickly compared to changes that occurred in the past (text emboldened for emphasis):

"As the Earth moved out of ice ages over the past million years, the global temperature rose a total of 4 to 7 degrees Celsius over about 5,000 years. In the past century alone, the temperature has climbed 0.7 degrees Celsius, roughly ten times faster than the average rate of ice-age-recovery warming."

Source: NASA Earth Observatory

Small increases in temperature can be hard to measure over short periods, because they can be masked by natural variation. For example, cycles of warming and cooling in the oceans cause temperature changes, but they are hard to separate from small changes in temperature caused by CO2 emissions which occur at the same time.

Tiny particle emissions from burning coal or wood are also being researched, because they may be having a cooling effect. Scientists like to measure changes over long periods so that the effects of short natural variations can be distinguished from the effects of man-made CO2.

The rate of surface warming has slowed in the past decade. Yet the physical properties of CO2 and other greenhouse gases cannot change. The same energy they were re-radiating back to Earth during previous decades must be evident now, subject only to changes in the amount of energy arriving from the sun - and we know that has changed very little. But if that’s true, where is this heat going?

The answer is into the deep oceans. Here is a graphic showing where the heat is currently going:


The oceans absorb most of the heat from global warming 

From Nuccitelli (2012)

The way heat moves in the deep oceans is not well understood. Improvements in measurement techniques have allowed scientists to more accurately gauge the amount of energy the oceans are absorbing.

The Earth’s climate is a complex system, acting in ways we can’t always predict. The energy that man-made CO2 is adding to the climate is not currently showing up as surface warming, because most of the heat is going into the oceans. Currently, the heat is moving downwards from the ocean surface to deeper waters. The surface gets cooler, humidity reduces (water vapour is a powerful greenhouse gas), and air temperatures go down.

The rate at which surface temperatures go up is not proportional to the rate of CO2 emissions, but to the total amount of atmospheric CO2 added since the start of the industrial revolution. Only by looking at long-term trends - 30 years is the standard period in climate science - can we measure surface temperature increases accurately, and distinguish them from short-term natural variation.

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Comments 1 to 23:

  1. Thanks; this will be useful, for those who actually seek to understand at least.

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  2. I have an analogy for this, and I would like everyone's opinion on whether or not it is valid.

    Today is the 23rd July, which means it is 32 days past the solstice, and the level of solar radiative forcing has been increasing (here in Australia) every day over the last month.  Yet interestingly, the temperature hasn't been increasing every day at the same rate.  The temperature has been going up and down, and it will continue to go up and down in response to other forcings.  Yet, over the next 6 months or so, the long term temperature trend will be upwards in response to the solar forcing.

    We don't expect the temperature to increase at a linear rate every day from winter to summer, why would we expect temperature to do the same in response to CO2 forcing?

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  3. Why exactly is the ocean depth of 700 meters to 2000 meters used? Why is it 700 meters (rather than say 500 or 1000), and why not include the ocean below 2000 meters? Does the deeper ocean not interact as much/directly?

    @mandas: The forcings that dictate weather trends may be different, occur on a different scale, and occur in a shorter period from those that influence climate trends. It's an interesting point you make, and it's true that global termperature has not been increasing at a steady linear rate in short runs. But I think the analogy may be too weak to be substituted as a point on climate for longer runs.

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  4. DAK4Blizzard

    The difference between 700m and 2000m is historical. It is based on an earlier sensor technology and a later one. Prior to the 2000's, detailed measurement of heat content down to 700 meters was obtained using data from Expendable Bathythermographs. 700 meters was their maximum operating depth. Heat content below 700 was estimated from their data and other more sporadic deep sampling techniques.

    In the early 2000's, deployment of the ARGO array of smart robot diving buoys was commenced. These now drift around the oceans, diving to operating depth, sampling the water, surfacing and relaying their data back to satellites. And their maximum operating depth is 2000 meters.

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  5. DAK4Blizzard, Glenn directly covered most of your questions and the fact that ARGO buoys have a maximum depth of 2000 meters should explain why data below that point isn't included. There is no 'lack of interaction' in the deeper oceans, and indeed various studies of deep ocean temperatures have found evidence that significant additional warming is accumulating there. We just don't have widespread or continuous readings for those depths, and thus estimates of total additional heat accumulation in the deep ocean have a wide uncertainty range.

    Thus, the chart in the article above is 'conservative' in excluding the deep ocean heat content change... but only because the data on that isn't available at the same level of detail as the other items shown.

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  6. mandas @ 2: I like that analogy and I used it here:

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  7. For the sake of the example, suppose climate sensitivity is 30 for a doubling of CO2.  So from 200ppm to 400ppm we would see a temperature rise of 30.  We would then need to increase CO2 from 400ppm to 800ppm to see another 30.  In other words each additional increase in Carbon dioxide has less effect than a similar previous increase.  Of course this ignores an opposite effect, namely the inertia in the system.  The El'gygytgyn results hint that we should already be seeing more effects than we do.  It seems likely that we have set in motion a raft of interlocking feed back mechanisms that will have to work their way through before we are in equilibrium with 400ppm and hence be able to actually measure climate sensitivity. We won't be able to do this experiment since we are heading with gay abandon towards 500ppm and beyond. 

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  8. gpwayne, I think it would be a good idea to at least briefly mention the point that the warming due to carbon dioxide being put into the atmosphere at any given moment doesn't happen immediately, but involves a certain lag, that it is warming "in the pipeline" due to it resulting in an imbalance in the rate at which energy enters and leaves the system, and that we only gradually accumulate the energy that warms the Earth up enough that it radiates energy at the same rate that energy is entering the system.  I believe this is at least as relevant as the point that the rise of temperature due to a rise in carbon dioxide is nonlinear.  You wouldn't have to include much additional material, though.  A brief mention and a link to where it is covered in more detail elsewhere at this site, such as:

    What the science says... The argument that "Earth hasn't warmed as much as expected" generally relies on ignoring the factors which have a cooling effect on the Earth's temperatures, and the planet's thermal inertia, which delays the full amount of global warming.


    Has Earth warmed as much as expected?

    ... should be sufficient.

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  9. What is the current thinking as to why the oceans are now absorbing more heat than in previous decades? Is it some process which is occurring in response to the last centurys rise in air temperatures, or is it possibly related to other factors and cycles like ocean currents etc.?

    What are there promising research projects trying to sort this out, and will they likely result in improved climate models which better understanding of the role of the oceans?

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  10. David, you might want to check out:

    A Looming Climate Shift: Will Ocean Heat Come Back to Haunt us? by Rob Painting, 24 Jun 2013

    Basically, the Pacific Decadal Oscillation (or more recently "Interdecadal Pacific Oscillation) is in its negative phase, and during this time the ocean tends to store heat.  I may be wrong, but I believe this is primarily due to the fact that it la Ninas tend to dominate rather than el Ninos.

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  11. Correction:  last sentence should be "... I believe this is primarily due to the fact that la Ninas tend to dominate rather than el Ninos."  During the negative phase of the PDO, la Ninas are more common, el Ninos less so, and during la Ninas the ocean tends to store heat that is released during an el Nino.

    Regarding the relationship between el Ninos and the PDO, please see Comparing ENSO and PDO  and The Pacific Decadal Oscillation (Climate Impacts Group, University of Washington)

    However, there has also been talk of the character of ENSO changing as the result of global warming, where depending up the climate model, more "el Nino"-like or more "la Nina"-like conditions tend to dominate.

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  12. David Sanger

    I am no expert on this but the general sense I have is something like this:

    A major part of the ocean circulation patterns in the upper half of the ocean is currents generated by the winds, particularly the major permanent wind patterns, the Easterly Trade Winds near the tropics, and the Westerly Roaring Forties. These winds generate large circular currents, the Gyres, one per ocean basin with smaller child gyres as well. These rotate clockwise in the Northern Hemisphere and counter-clockwise in the south.

    This circular movement pushes water towards the center of the gyres which then creates a downwards water flow in the central region as this is the only place the inflow of water can get out. This is described in more detail in the theory of Ekman Pumping.

    Additional factors come into play in the Southern Ocean with the interaction between the gyres and the circumpolar current that circles the Antarctic. Also the fact that the cold surface temperatures in the Southern Ocean mean that the temperature gradient between the surface and the deeper waters is less so Ekman pumping can more easily move water down since the buoyancy gradient is lower.

    In recent years the speed of the Westerlies has increased. The speed of the Trade Winds may also have increased. And the latitiudes where the westerlies are blowing are shifting polewards as the Hadley Cells expand.

    In principle, increases in the wind speeds should 'spin up' the gyres, increasing the Ekman transport, thus more water being moved from the surface to the mid depths - 1000 to 2000 meters - and thus able to carry more heat with it. But the details are apparently rather complex due to the interactions in the Southern Ocean. This is why the Southern Ocean is a region of intense research right now.

    Things like the PDO and perhaps even the ENSO cycle may well be smaller scale consequences of a broader mechanism involving fluctuations in the winds and Ekman transport. With more research this area has the potential to really nail a lot of our uncertainties wrt natural climate variability.

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  13. @DAK4Blizzard #3: Extra info. Dr. Kevin Trenberth says in SFU lecture video posted as "The Role of the Oceans in Climate" oceans mix well to 20m depth in summer, 100m in winter - can take 90m as average and will delay response by 6 years. KT says 3.5m depth = all atmosphere heat. Dr. Randall says top 6.5m land is what's considered (insulates well) and equivalent to 3.25m ocean depth but I recall a lady climate scientist saying only ~2m ocean depth = all land heat (my memory vague on that latter).

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  14. To the extent that basic CO2 forcing formula is accurate and the reanalysis OHC is accurate and Dr. Randall was accurate saying 3.75 wm**-2 is balanced by +1.2C then it seems to me that 42% +ve feedback is what's been added average (presumably increasing). +0.8C thus far balances 3.75 wm**-2 * 0.5 = 1.87 wm**-2 (0.5 per t**4 at ~287-289 degrees K). OHC gradient looks like 0.85 wm**-2 (Dr. T says it's 0.9 wm**-2). CO2 forcing 5.35*ln(10/7) = 1.91 wm**-2. So, (1.87 + 0.85) / 1.91 = 1.42. What am I misunderstanding ?

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  15. @Me#16 Correction because delta-radiation ~proportional to delta-t for these <.01 dt ratios even though proportional to t**4. Seems to me that 75% +ve feedback is what's been added average (presumably increasing). +0.8C thus far balances 3.75 wm**-2 * 0.8 / 1.2  = 2.50 wm**-2. OHC gradient looks like 0.85 wm**-2. CO2 forcing 5.35*ln(10/7) = 1.91 wm**-2. So, (2.50 + 0.85) / 1.91 = 1.75 (75% +ve feedback). Right ?

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  16. grindupBaker @17.

    This post calls itself a 'basic' level rebuttal so we may be tending off topic with these calculations. Your "OHC gradient" number looks a little high given the manner you are using it. I wonder if the 0.9 W/sqM figure is for the area of the ocean and not for global area. Ice is not an insignificant addition to ocean heating, increasingly so in recent years (the same years that saw less increase/less significance in surface temperature). But the biggest omission is the other anthropogenic forcings. (See for instance Skeie et al 2011.). The other positive ones are reasonably easy to quantify and sum to a similar size as CO2. The negative ones are less easy to nail down. And additionally there are the natural forcings. So there's lots requiring adding into the calculation, which if you think about it isn't a surprise. This calculation is effectively calculating climate sensitivity and everyone would love to nail that baby.

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  17. Perhaps someone here can give me a simple explanation of how the heat gets from the atmosphere to the deep ocean (700m) without warming the upper 700m first. Also what changed to make the man made CO2 warm the ocean rather than the air, and how long will it be till it changes back?



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  18. keitho - How? Circulation, to be short.

    Consider the vertical temperature profile of the oceans (warmest at the surface, coldest at the deeps), and note that if under increased circulation 100-700m water moves into the cold deeps, while warmer 0-100m waters move into the 100-700m range, the effect will be a cooling surface and warming deep water without changing mid-level temperatures.

    Given circulated energy E:

    • Surface energy - E -> colder sea surface temperatures, colder air
    • 100-700 +E - E -> potentially no change in temp, depends on amounts
    • gt. 700 + E -> warmer, as per observations

    Increased greenhouse gases warm both oceans and air - with circulation changes such as ENSO affecting the relative amount of energy going into those two compartments. And given the relative thermal mass of oceans and atmosphere, a small percentage change in ocean warming rate translates into a large change in atmospheric warming rates. But the average energy uptake hasn't stopped, warming is still occurring, and short term variations like ENSO will cancel out over time. 

    Don't get distracted by short term noise. 

    Long term trends and short term variations around them

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  19. Keith,

    The upper ocean has warmed (a lot) more than the deep ocean.  As KR points out, the heat can also move through the upper ocean to the deep ocean with little change in the middle.  

    The ocean and the atmosphere were in equilibrium before man started adding so much greenhouse gas to the atmosphere.  With increased greenhouse gas the atmosphere started to warm.  Once the atmosphere was warmer, the ocean started to absorb more energy so that the ocean/atmosphere system stays in equilibrium.  Currently, the atmosphere is relatively warmer than the ocean so the ocean is absorbing most of the energy.  The ocean will continue to absorb energy for a long time (centuries) until a new equilibrium is established.

    Why is this important?  The rate that energy is transferred to the deep ocean affects the rate at which the surface heats up.  If more energy is transferred to the deep ocean the surface warms a little slower.   If less energy is transferred the surface warms a little faster.  The more we understand the transfer of energy to the deep ocean the better we can project future temperatures.  We can also determine which climate models give the most accurate projections for heating of the deep ocean and relate that to the accuracy of their surface temperature projections.

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  20. @Michael Sweet . . if the ocean is now keeping the atmosphere from warming and will do for centuries to come, why are we so worried about CO2? Surely if the cold ocean is keeping things from overheating now, even if it didn't in the recent past, then that indicates a stabilising feedback mechanism which is a good thing isn't it.


    Also why does the heat prefer to be down deep? I understand that it can from what you say, but why would it do so.

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  21. Keitho,

    The ocean will not stop the atmosphere from warming.  If the deep ocean absorbs a lot of energy it might slow warming down a little.  If the recent slowing in the increase of surface temperatures is caused by ocean warming (as compared to just random variation), it might mean that we have an additional 10 years before 2C is exceeded.  I would point out that with current climate change we have already experienced record climate damages in the past three years worldwide.  It might be a good thing if the deep ocean gave us more time, but it is no panacea (especially if we use it as an excuse to waste another decade).

    Heat always goes anywhere that heat is absent from.  First the surface warmed, then the middle and now we are seeing the deep ocean start to warm.  The question is how fast it will go to each location.  The movements of heat are complex and that is why scientists are working to understand the heat fluxes better.  If the heat currently accumulating in the middle levels of the ocean is somehow funneled under the Antarctic ice shelves that would be a big problem, because of sea level rise.  As scientists learn more about the heat fluxes they can better estimate the chance of problems like that.  It strikes me (without data) that while heating the ocean might make the surface temperature a little lower for a while, it would be more likely to melt Antartica.  Since I live in Florida that would be bad.  Which do you prefer?

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  22. keitho - The oceans will not stop warming, will not magically maintain a cooler atmosphere in the presence of a GHG driven forcing imbalance. I hope you are not being disingenuous in that regard - you are, however, giving me that impression. 

    Ocean warming simply lags behind that forcing. Meaning that we will (with variations such as ENSO) continue to see atmospheric warming, and even if were to stop emissions instantly we would continue to see the oceans and atmosphere warm up until the Earth again radiates as much as it receives. It's just a long climate response time due to the high ocean thermal mass - not a 'get out of jail free' card. 

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  23. Timothy @12 and Glenn @14

    Thanks for the explanation and references which led me to the Rob Painting posts, extensive comments, and the two Meehl papers (2011 and 2013). Very helpful.

    Modeling of the deep ocean currents and vertical mixing does indeed seem to be extraordinarily complicated and and I'm glad to see it's an area of ongoing development

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