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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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Does CO2 always correlate with temperature (and if not, why not?)

What the science says...

Select a level... Basic Intermediate

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

Climate Myth...

There's no correlation between CO2 and temperature

"Twentieth century global warming did not start until 1910. By that time CO2 emissions had already risen from the expanded use of coal that had powered the industrial revolution, and emissions only increased slowly from 3.5gigatonnes in 1910 to under 4gigatonnes by the end of the Second World War. 

It was the post war industrialization that caused the rapid rise in global CO2 emissions, but by 1945 when this began, the Earth was already in a cooling phase that started around 1942 and continued until 1975. With 32 years of rapidly increasing global temperatures and only a minor increase in global CO2 emissions, followed by 33 years of slowly cooling global temperatures with rapid increases in global CO2 emissions, it was deceitful for the IPCC to make any claim that CO2 emissions were primarily responsible for observed 20th century global warming."
(Norm Kalmanovitch).

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 30°Celsius (54°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:

From Nuccitelli et.al (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.

Basic rebuttal written by GPWayne


Update July 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial

 

Last updated on 17 July 2015 by MichaelK. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

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Comments 26 to 50 out of 182:

  1. Sphaerica @ 21, you say "And for this reason, meeting your insistence on a direct correlation between CO2 and temperature is simply unrealistic.". That sounds "skeptic" to me. It's not that there is not a connection between temperature & CO2, (as noted by atmospheric absorption bands), but how much, and under what conditions. What I am pointing out, is the only direct data available are some long term temperature readings & relatively recent standardized GHG (CO2 since 1958) readings. So at this time, as I said before, I wouldn't bet the farm on proxy and model results. We might have to wait a few more years to see if there truly is a global temperature plateau, and note the corresponding GHG data to prove or disprove the model predictions.
  2. J Bob. I use MATLAB all the time. Just never heard of "System Identification" toolbox. Note I said " R instead of MATLAB for the tools if the statistics Toolbox wouldnt cover it" - I am well aware of the statistics toolbox in MATLAB. Just more likely to go to R for non-trivial stuff (or more honestly, hand them R experts here instead).
  3. From what I've seen, deniers focus on graphs of very recent events, but seem to forget the one time in Prehistory that is similar to what's happening today. The Permian mass extinction was caused by CO2 releaced from a massive flood-bassalt eruption in modern-day Siberia. This caused a chain reaction of events that involved anoxification of the oceans, release of frozen methane from the sea floor, and the desertification of almost all of Pangea. 95% of all species died. It was the biggest extinction in the history of the earth... so far.
  4. Looking at temp-CO2 correlations is a bad idea for the very fundamental reason that temperature indicates the accumulation of forcing, so there is an integration between flow (CO2 forcing) and stock (heat or temperature). In general, you can't expect to see a correlation between a flow and stock time series, even when causality is perfect. It happens that some relationship is evident for CO2-temp because the time constant of surface heat is fairly short, but this is not terribly informative. This topic gives the impression that it's OK to look for temp-CO2 or forcing-CO2 correlations, if you just look for them the right way. It would be better if the message were that looking at stock-flow correlations (CO2-temp, or worse, emissions-temp) is basically misleading, unless you explicitly consider the dynamics.
  5. Ive been debating a climate change denier for a while on a website, and he stated this and I was looking for a fair rebuttal:

    "

    I return to the fact that you should not make the assumption that I know nothing about climate warming - and you would learn more if you asked questions about what my beliefts are.

    Carbon is only one of MANY forcing elements in climate. And the IPCC has been spectacularly wrong on many of them - and even admitted the same. Such as, for example the importance of aerosol formation on clouds. The importance of ionizing radiation for aerosol formation.

    Finally, I return to a question I asked several weeks ago.

    When the temperature of the ocean increases, the solubility of gasses decrease - something known as Henry's law or Boyles law. This causes the oceans - when they get warmer - to release massive quantities of CO2 (and O2 etc).

    This is known fact.

    The opposite is in fact, not proven. In other words given increased CO2, AGW says that ocean temperatures increase.

    The observable fact is that increasing temperatures correlates with increased CO2 concentration. But it doesn't say a damn thing about causation.

    Can you restate my point here so that I can confirm you understand it?"

    Response:

    [TD] See the post "CO2 is Not the Only Driver of Climate," noticing the relative contributions of various forcings.  By "ionizing radiation" I'm guessing that he/she means cosmic ray influence on clouds; see "It's Cosmic Rays."  Notice there are multiple tabs--Basic, Intermediate, and Advanced.

    Regarding oceans:  Of course climatologists know that the CO2 absorbing capacity of the oceans decreases as the oceans warm.  However, oceans continue absorbing until they reach a fairly high temperature.  And a counterforce is that the oceans also absorb more CO2 as the amount of CO2 in the atmosphere increases.  Climatologists figure all that into their calculations.  We know that currently the oceans continue to absorb, not emit, CO2, because (a) the oceans continue to acidify (i.e., become less alkaline) as a consequence of their CO2 absorption, and (b) the oceans absorb the bulk of CO2 from human activities, along with all the natural emissions of CO2:  See here and here.

    Causality of CO2 versus temperature was established when the mechanisms were understood (via experimentation) back in the 19th through mid 20th centuries.  Observations of global temperatures confirming those theoretical projections were made decades later, when global observations were sufficiently capable.  The exact mechanism by which CO2 warms the oceans was determined later, but again with experimentation.

    The fact that temperature increase causes CO2 increase in no way undermines the fact that CO2 increase causes temperature increase; see "CO2 Lags Temperature" and also "Warming Causes CO2 Rise."

  6. Also have another one I'd like answered:

    "If you take an atom of CO2 in the lower atmosphere and it absorbs energy and reradiates it - molecules can be modelled as radiating energy in all 360 degrees. Part of that energy gets radiated back to the earth (around 36% in the lower troposphere) and part gets radiated to space. (I'm simplifying, ignoring the convective, conductive etc elements of energy transfer between atoms, reradiation etc).

    As you ascend the atmospheric column ipast the troposphere, several things happen.

    a). The arc of reradiation to the earths atmosphere decreases (to a very small number - less than 1%). And the amount reradiated to space increases. (Virtually all).

    b). The contribution of convection, conduction decrease as well to almost zero, and the contribution by radiation dominates (due to concentration of molecules, primarily).

    So, by the time you get to the outer atmospheres, CO2 is no longer acting to warm the planet. In fact in the outer atmosphere, the amount of energy absorbed from IR emitted from the eart is for all intents and purposes - zero. Instead, in the outer atmospher, CO2 is energized (and in fact is almost always a plasma in some strata) by much more energetic radiation. And CO2 serves to reradiate this energy back into outer space.

    This is proven born out by the fact that the energy density of solar radiation at the outer edge of the atmosphere is roughtly 1347 per meter square. And the aereal density at the earth's surface is much less than a quarter - the rest of the energy (simplification) is reradiated to space.

    So in fact, by the outer atmosphere, CO2 is acting to COOL the planet.

    In the previous thread, I provided links to the study by Nasa that confirmed this finding, and confirmed that they would have to reconsider the NET effect of CO2 on global warming (warming in the low atmosphere, cooling in the high atmosphere)."

    Response:

    [TD] First, he needs to revisit his geometry; CO2 molecules would have be to pretty darn high for "nearly all" of their emitted radiation to go to space.  Here is one explanation of how CO2 cools the stratosphere while warming the troposphere:  "Stratospheric Cooling and Tropospheric Warming."

  7. tkman0...  It sounds like your denier-guy has a bunch of accurate facts but is interpreting entirely the wrong way. 

    For one, the IPCC isn't getting things "spectaculary wrong." They're merely putting out a report on the current science. If he wants to claim the IPCC is misinterpreting the research, I would have to say, I don't hear a lot of climate researchers jumping up and down about these supposed errors. 

    Your denier-guy correctly states that there are many forcings that act on the climate system. I'd have him watch Richard Alley's lecture "The Biggest Control Knob" which explains the science of why CO2 is the biggest (but not only) factor controling global temperature.

    Another weird one here: "This causes the oceans - when they get warmer - to release massive quantities of CO2 (and O2 etc)." Etc, etc.

    He's getting his knickers in a knot merely because he doesn't understand that CO2 can act as a feedback (as with ocean warming/glacial-interglacial cycles) but can also act as a forcing (as with burning fossil fuels).

  8. tkman0,

    Your description seems about right to me.  At the surface (where we live) CO2 causes warming.  In addition to the angle of emission, energy emitted upward is usually reabsorbed by another CO2 molecule at a higher altitude.  This enhances the warming.  Since the troposphere is cooler the higher you go, the higher molecule emitts less energy.  As CO2 concentration in the atmosphere increases, the hight in the troposphere where the energy can escape to space becomes cooler.  This is the basis of the greenhouse effect.  This post on hte greenhouse effect might help a little.

    The stratosphere is also cooled as you describe.   This is one of the key predictions of greenhouse theory and demonstrates that the warming is caused by increased greenhouse gases and not the sun.  There is no significant transfer of  energy from the stratosphere to the troposphere so the cooling effect in the stratosphere has no significant temperature effect at the surface of the Earth.  This effect was predicted decades ago.

    Most of the energy that reaches the outer edge of the atmosphere reaches the troposphere.  Half of the difference between the 1347 and the about 270 w/m2 at the surface is caused by the fact that half the Earth is in darkness all the time (the night side) so you have to devide 1347 by 2.  Much of the rest of the difference is due to the fact that the remaining 675 W/m2 is a square meter perpendicular to the sun and the surface of the Earth is round so most does not receive the light directly.  About 1/3 of the light is reflected by clouds.  Scientists studying climate (the IPCC) have measured all this.  While some energy is emitted from the upper atmosphere by CO2, the net effect on the surface of the Earth is warming.  Much of the reflected energy and the energy emitted from the stratosphere has always been emitted.  It is the changes in energy flux casued by changes in CO2 that we are interested in.  Those energy fluxes cause the surface warming.

  9. tkman0 @30:

    "...you should not make the assumption that I know nothing about climate warming..."

    From what follows, the assumption seems well warranted.

    "... Such as, for example the importance of aerosol formation on clouds. The importance of ionizing radiation for aerosol formation."

    Your interlocuter here refers to Svensmark's hypothesis that cosmic rays cause clouds.  He is definitely wrong about the IPCC being mistaken about this hypothesis.  In AR4 all they had to say was:

    "Whether ... solar-induced heliospheric modulation of galactic cosmic rays (Marsh and Svensmark, 2000b) also contribute indirect forcings remains ambiguous."

    What the IPCC AR5 actually says on the effects of cosmic rays on climate is:

    "Changes in solar activity affect the cosmic ray flux impinging upon the Earth’s atmosphere, which has been hypothesized to affect climate through changes in cloudiness. Cosmic rays enhance aerosol nucleation and thus may affect cloud condensation nuclei production in the free troposphere, but the effect is too weak to have any climatic influence during a solar cycle or over the last century (medium evidence, high agreement) .  No robust association between changes in cosmic rays and cloudiness has been identified. In the event that such an association exists, it is very unlikely to be due to cosmic ray-induced nucleation of new aerosol particles."

    (My emphasis)

    Now, either your interlocuter thinks Svensmark's hypothesis is falsified, and was falsified as of AR4 (in which case they have indeed corrected an "error"), or he is completely misrepresenting the IPCC, presenting a stronger rejection of the hypothesis as being a correction of prior rejection of the hypothesis (which does not exist).

    "... This causes the oceans - when they get warmer - to release massive quantities of CO2 (and O2 etc)."

    Absolutely it is a known fact.  What is uncertain is the exact ratio of increase in atmospheric CO2 per degree C increase in Global Mean Surface Temperature (GMST), as other factors are involved.  We can quantify it approximately, however, by the 100 ppmv increase in CO2 for an approximately 5 degree C increase in GMST between the last glacial and the Holocene.  That gives an expected increase of 20 ppmv in atmospheric CO2 per degree C.  Alternatively we could quantify it from the 10 ppmv increase in atmospheric CO2 during the MWP, although a massive human cause deforestation may interfere with that calculation.  In either case, it is clear that this effect is not adequate to explain even half of the increase in CO2.  Deniers hide that fact by referring to Boyle's law, but never quantifying the details.  It is as though they have something to hide, and know it.

    Even worse for your interlocuters argument is the fact that CO2 concentration in the ocean has been increasing.  His theory predicts that it would be decreasing.  Further, the decrease in proportion of C14 in the atmosphere indicates the source of the CO2 is C14 free, ie, like fossil fuels, and unlike oceanic CO2.  Indeed, six out of 10 lines of empirical evidence preclude oceanic outgassing as being the source of the excess atmospheric CO2, with another two lines of evidence providing contrary evidence, but not decisively so.

  10. tkman0 @31:

    "If you take an atom of CO2 in the lower atmosphere and it absorbs energy and reradiates it - molecules can be modelled as radiating energy in all 360 degrees. Part of that energy gets radiated back to the earth (around 36% in the lower troposphere) and part gets radiated to space. (I'm simplifying, ignoring the convective, conductive etc elements of energy transfer between atoms, reradiation etc)."

    Your interlocuter is ignoring absorption and reradiation of emitted energy.  Therefore his claim purports to be based purely on geometry.  For only 36% of the sphere to be occupied by the Earth's surface, the angle from the closest point of the surface to the limb of the planet as seen from the point of emission must be 64.8 degrees.  Given that the radius of the Earth is 6,371 km, it follows that the point of emission must be 670 km above the Earth's surface.  Given that the troposphere only extends from 10-20 km above the Earths surface, your interlocuter's definition of the "lower atmosphere" is bizarre to say the least.  Indeed, it only just avoids being in the exosphere, the layer of atmosphere from "700 km to the Moon's surface", a layer higher than most satellites.

    The later 1% claim is even more outrageous.  It requires an altitude of 196,458 km, approximately half way to the moon.

    In fact, as you rise in the atmosphere, more and more radiation does escape to space - but that is entirely due to the decreasing atmospheric concentration rather than to changes in the subtended arc of the Earth's surface.  Further, the proportions he quotes are pure fictions.

    "So, by the time you get to the outer atmospheres ..."

    Does your interlocuter think the Earth has more than one atmosphere?  Given the absurdity of some of their other errors, it is hard to be sure what is mistyped, and what is another fallacy.

    What your interlocuter says from there on is reasonably close to true.  It is only true of the Thermosphere, however, ie, the atmosphere from 50 to 800 km in altitude.  At that altitude temperatures vary rapidly as the result of energy being dumped into the atmosphere from solar flares.  That energy is then rapidly reradiated away, primarilly by CO2, with about 90% going to space and 10% to Earth.  The effect is to buffer the Earth, reducing the increase in temperature at the Earth's surface from increased solar activity.

    The effect has nothing to do with the greenhouse effect, which occurs in the first 30 km of the atmosphere.  Above that, atmospheric concentration is to low to have much of an effect.  It certainly does not falsify the greenhouse effect which has been experimentally observed and shown to exist in no uncertain terms. 

  11. Thank you very much for your assistance Tom, I'm an 2nd year Environmental Science major in university right now and while climate and resource management is my focus, it helps to have a seasoned researcher assist me in dealing with this denier. 

  12. tkman0 @36, de nada!  I do need to correct a misapprehension.  I am not a researcher, let alone a seasoned researcher.

  13. tkman0... Don't let Tom fool you with his humility. He is probably the best informed non-researcher I've ever had the pleasure to come across. While actual researchers are likely to have a deeper understanding of their own area of the science, Tom probably has the broadest knowledge of climate of anyone I've seen.

  14. When ever I look at the air temperature data, my eye always falls on the two upward trends, from 1910-1943 and 1970 to 2001. I keep finding myself thinking they have pretty much the same slope and the same duration. I cant help but thinking they may not have a common cause.

    When I look at the modeling results of natural forcing only, like in the intermediate rebuttal of the 'models are unreliable page', niether period is modelled well. The man made forcing only model only captures the second increase. CO2 levels and increases are quite different in these two periods.

    I realize that weather and climate data can be variable and can make one imagine things, but has there been any kind of study comparing these two periods?

  15. Razo @39, yes there has been such a study.  Many of them, which are summarized in the IPCC.

    In summary the results are:

    1)  The early twentieth century warming was of a shorter duration, and with a lower trend than the late twentieth century warming;

    2)  Durring the early twentieth century warming, volcanic forcing, solar forcing and anthropogenic forcings were all positive relative to the preceding decades, and had similar magnitudes;

    3)  Durring the late twentieth century warming, volcanic forcing and solar forcing were both negative relative to preceding decades, while anthropogenic forcings were strongly positive.

  16. Razo wrote "I cant help but thinking they may not have a common cause."

    that is pretty irrational, given that we know that both anthropogenic and natural forcings have changed over the last century, and that neither can explain both sets of warming.  So for that hypothesis to be correct, virtually everything we know ablut natural and anthropogenic forcings must be wrong.  Personally I'd say the hypothesis was wrong.

    "When I look at the modeling results of natural forcing only, like in the intermediate rebuttal of the 'models are unreliable page', niether period is modelled well."

    It has already been explained to you that this is likely an artefact of the baselining.  The fact that both periods are reasonably well modelled by including both natural and anthropogenic forcings kind of suggests that the two periods do not have a common cause.

    "but has there been any kind of study comparing these two periods?"

    Try the IPCC report, the chapter where the figure was taken from is a good start.

  17. Linear 'fit' to the last 38 years of ocean data yields 0.3 watts/m^2. Essentially the same as the Livitus et al results. This is ~93% of global warming power with a decent average time. Note this is only about 1/10 of the global warming power claimed by IPCC to reach 3C in a century.

  18. Hi from Spain. I have a question:

    "How much of the thermal radiation energy from the Earth in the band centered on the 14.77micron wavelength that is resonant with the vibrational mode of CO2 has already been affected by the current atmospheric CO2 concentration, and how much energy remains to be affected?”

    A greeting and thanks in advance

  19. SOMMERSWERD - See CO2 is saturated for a discussion of this. The real effects of greenhouse gases take place in the upper atmosphere when the concentrations drop to the level that IR can escape to space, and increased levels of CO2 just raise that altitude. There is no 'saturation' issue.

    Response:

    [PS] Fixed link

  20. KR

    My question was simple but you have not responded. No discussion; just a question that you do not respond directly.

    Response:

    [PS] KR pointed you to a more appropriate thread for discussion of question. It is off-topic here. Please take the discussion there. Offtopic discussion will be deleted from this thread.

  21. SOMMERSWERD - Actually, I did answer your question. Atmospheric absorption is concentration and hence pressure/altitude dependent, with sea level pressure and concentrations of CO2 absorbing all CO2 frequencies in a matter of meters. 

    What matters for the greenhouse effect is at what temperature (and hence rate) IR is emitted to space - that occurs at altitude, when there is insufficient CO2 above the effective emission altitude to absorb the majority of the outgoing IR. 

    Increasing CO2 simply raises the effective emission altitude - and there is in fact solid data showing that over the last 50 years the tropopause where the effective emission occurs has risen a few hundred meters. Given the lapse rate relationship, that altitude change accounts for the observed rise in temps over that period. 

    So in detail the answer is "it depends on what you're asking". All CO2 frequencies get fully absorbed at the surface (with corresponding thermal emission), the strongest absorption lines continue to absorb a majority of the IR at those frequencies until pressure drops enough to allow >50% through to space, and changes in CO2 concentration simply change that altitude - where again >50% of the emitted IR at those wavelengths escapes the atmosphere. Your question "...how much energy remains to be affected?" is therefore somewhat meaningless without more context. 

  22. SOMMERSWERD - ...And your question is in fact a query about CO2 absorption saturation, which is more appropriately dealt with on the appropriate thread

    Response:

    [PS] Fixed link

  23. The global temperature did not increase from 1940 to 1970.
    How do you explain this?

    Response:

    [PS] Read the article. (Net forcings, see figures 5 and 6 on the intermediate version of the article). Warning: Asking questions without any interest in the answers is simply sloganeering and forbidden by the comments policy.

  24. I had read the article and still I did not understand why the global temperature did not increase
    from 1940 to 1970. Could you tell me what is the main cause of it?

    Response:

    [PS] From the article, "If CO2 causes warming, why isn't global temperature rising over this period? To answer this, one needs to recognise that CO2 is not the only driver of climate. There are a number of factors which affect the net energy flow into our climate. Stratospheric aerosols (eg - from volcanic eruptions) reflect sunlight back into space, causing cooling. When solar activity increases, the amount of energy flowing into our climate increases. Figure 5 shows a composite of the various radiative forcings that affect climate."

    Figure 5 shows all the factors other than CO2 affecting climate of the period and Fig 6 sums all them, positive and negative,  to show little net forcing till 1970. Please take some time to study actually study and understand the figures.

  25. Ken @49, your question is answered in the thread "CO2 is not the only driver of climate".

    Quickest to go to the comments column there, and see Bozzza's (@20) laconic quote: "aerosols" ~ which is the short story . . . that could be expanded slightly to: "increasing industrialization produced reflective aerosols which counterbalanced the rising CO2, until the ever-higher CO2 levels overpowered the aerosol effect by around 1970 [combined with some clean-up of aerosol emissions].

    You will note Tom Curtis's (@21) longer explanation of additional factors which contributed to a "flat" 1940-1970 record.

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