Common Climate Misconceptions

Recent Lower Global Temperatures Do Not Undercut CO2/Warming Relationship

Human emissions of carbon dioxide (CO2) are the primary factor contributing to the warming of the Earth’s surface over the past half-century.

However, for the past few years global temperatures have been stagnant or slightly decreasing even as atmospheric CO2 concentrations have been increasing faster than ever. This situation has led some voices in the media and blogging world to challenge the relationship between the CO2 concentrations and warming. These critiques are flawed, however, as short-term changes in global temperature are driven by numerous factors going beyond CO2, and the recent disconnect between the two is not particularly unusual in light of their past relationship.

There is little debate over the basic idea that, all things being equal, higher levels of atmospheric carbon dioxide would warm the planet’s surface. This is a basic result of the radiative properties of CO2 as a greenhouse gas: it absorbs a certain range of longwave radiation wavelengths and reradiates them down to Earth. Solar energy hits the Earth as shortwave radiation like light, and is absorbed and re-emitted as longwave radiation (e.g. heat). CO2 and other atmospheric greenhouse gases absorb this longwave radiation coming up from the Earth’s surface and re-emit a portion of it downwards, heating the Earth. This process, the Greenhouse Effect, is well understood and extensively backed up by empirical laboratory and satellite measurements.

The radiative forcing associated with greenhouse gases generally follows a logarithmic function: The amount of extra heating of the Earth’s surface resulting from an additional amount of CO2 in the atmosphere decreases as there is more CO2 in the atmosphere. This result occurs because parts of the CO2 absorption spectrum become saturated, leading to less additional forcing for additional CO2.

This saturation is never complete, however, as even at astronomically high CO2 concentrations like 20,000 parts per million (ppm) additional CO2 has a slight positive forcing. An easy way to grasp the logarithmic properties of CO2 is to think about it as a fixed temperature increase every time atmospheric concentrations double. So increasing atmospheric CO2 from 250 ppm to 500 ppm would result in a certain amount of warming, and increasing concentrations from 500 to 1000 would result in an additional warming of the same magnitude, all things being equal. In reality, global temperatures respond to various feedbacks and also to the direct forcing from CO2, and these feedbacks may not increase linearly with CO2 forcing.

By itself, doubling atmospheric CO2 would increase global temperatures by about 1.2 degrees C. Even most of the scientists skeptical of the severity of climate change agree on this basic point. The big question is the climate sensitivity of the Earth: how much that 1.2°C worth of forcing from CO2 increases the Earth’s temperature when various positive and negative feedbacks are factored in. These feedbacks include water vapor, clouds, the lapse rate, albedo (the reflectivity of the Earth’s surface), and other factors.

Each of these feedbacks has an estimated range of uncertainty (as shown in the figure below), but in combination they magnify the 1.2°C warming resulting from doubling CO2 into 2 to 4.5°C warming. This is a wide range of possible warming, as 2°C per doubling would not be that terrible, while 4.5°C per doubling would be potentially catastrophic. There is a healthy and impassioned debate in the scientific literature over what the actual climate sensitivity is likely to be.


View larger image
Range of estimated magnitudes of major climate feedbacks from the most recent IPCC report and Colman 2003. Figure taken from Soden and Held 2006 via Realclimate

In addition to feedbacks and greenhouse gas forcings, the Earth’s climate is affected by a number of natural factors such as solar cycles, El Niño Southern Oscillation (ENSO), and large volcanic eruptions that result in stratospheric aerosols. When combined together, these natural factors can have a strong effect on the climate for a few years.

For example, the current strong solar minimum combined with a modest La Niña (the cool phase of ENSO) likely contributed to colder temperatures in 2008. However, these natural factors are largely cyclical: solar cycles switch between minima and maxima, ENSO switches between hot El Niños and cold La Niñas.

Volcanoes are more sporadic and unpredictable, but have only a short-term cooling effect as a result of the limited lifespan of aerosol particles. While these natural factors can combine to create stagnant temperatures over a few years, over the long term their cyclical effects cancel out, leaving us with the underlying warming trend.

Over the past few years, global temperatures have slightly declined. This doesn’t mean that the last few years have been particularly cold; on the contrary, 11 of the past 13 years have been the warmest on record over the past 130 years. These declining temperatures have occurred during a time when atmospheric CO2 was increasing faster than ever, leading to those assertions challenging the connection between CO2 and temperature. The figure below shows atmospheric CO2 and global surface temperature over the past 28 years, with the divergence post-2004 clearly visible.

Global monthly surface temperature data is from HadCRUt and atmospheric CO2 data is from the Mauna Loa Observatory. Temperature data is smoothed using a 12-month running mean.

This figure shows that CO2 and temperature do not necessarily increase in lockstep. CO2 increases every year (with a strong annual cycle resulting largely from changes in hemispheric vegetative photosynthesis and respiration rates caused by seasonal effects), while temperature fluctuates as a result primarily of natural variability. If the divergence between temperature and CO2 over the past few years were unusual, one might maintain that the relationship between the two is not as strong as anticipated, or, more likely, that the recent years have seen natural forcings of a magnitude stronger than that generally modeled by climate scientists.

However, examining the present divergence in light of the past variability between the two factors suggests that the past few years’ divergence is not particularly unusual.

Simple linear regression of atmospheric CO2 concentrations on HadCRUt temperatures from 1980 to 2002 in red, as well as the line of best fit for the data projected out through 2009. 2003 to 2009 observations are shown in blue so they can be evaluated in light of the prior trend without influencing it.

The figure above shows a linear regression of temperature on CO2 concentration, and also the projection one would expect of 2003 to 2009 CO2 concentrations given the relationship between the two factors from 1980 to 2002. This projection allows testing of the post-2003 values consistent with the prior relationship between the two factors, without allowing the post-2003 observations to influence the trend. There is a noticeable dip at the end (and a spike a bit earlier representing the 1998 El Niño event), but it is difficult to see if it is significant, or if it is well within the bounds of the “noisy” relationship between CO2 and temperature.

To actually determine the significance of the recent divergence, one can remove the trend from the data and plot just the difference between the observed value and the expected value.

Residuals of the prior regression of atmospheric CO2 and temperature with the two standard deviation confidence intervals based on 1980 to 2002 observations shown as black lines. A 12-month running mean of the data is shown in red.

Based on the figure above, there are three months in the past five years (January 2008, in particular) that fall below the 95% confidence interval associated with the 1980 to 2002 trend in the relationship between CO2 and temperature. Given that there are 60 months in the past five years, one would expect that 5 percent of them, or exactly three months, would fall outside the 95 percent confidence interval.

So while the past five years have been decidedly below the long-term trend, one cannot say that it is particularly unusual given the past variability in the data. Further illustrating this point, the figure has a 12-month running average of the residuals shown in red (which we would expect to exceed the 95% confidence interval only once every 20 years or so), and this 12-month running mean is still within two standard deviations of the trend.

The declining temperatures and rising atmospheric CO2 concentrations over the past two years is best understood as the result of the natural variability of temperature, without casting any doubt on the underlying long-term relationship between CO2 and temperature.

Considerable uncertainties remain regarding the actual climate sensitivity (whether it is on the low or high side of the uncertainty range), and also the magnitude of short-term cyclical natural variability. Further studies will help diminish remaining uncertainties over the next few years, but for now the evidence of a fundamental long-term positive relationship between atmospheric greenhouse gases and the Earth’s temperature remains strong.

Zeke Hausfather

Zeke Hausfather, a data scientist with extensive experience with clean technology interests in Silicon Valley, is currently a Senior Researcher with Berkeley Earth. He is a regular contributor to Yale Climate Connections (E-mail: zeke@yaleclimateconnections.org, Twitter: @hausfath).
Bookmark the permalink.

11 Responses to Recent Lower Global Temperatures Do Not Undercut CO2/Warming Relationship

  1. Erl Happ says:

    Thanks for a closely reasoned analysis of recent temperature trends. It deserves a considered reply. In relation to your comment:
    “This process, the Greenhouse Effect, is well understood and extensively backed up by empirical laboratory and satellite measurements.” I must disagree.

    The atmosphere itself tests this theorem on an annual basis. It does so at the tropical tropopause, where outgoing radiation meets ozone, producing a strong temperature maximum in August at that level. That maximum is in turn due to the seasonal heating of the northern land masses and a loss of global cloud cover. Global temperature peaks in August, even though the sun is furthest from the Earth at that time. That should tell you something about the importance of clouds in determining surface temperature.

    At the surface near the equator the temperature peaks in May as it does in the atmosphere all the way to 200hPa. For those unfamiliar with this way of referring to altitude, the surface has an air pressure of about 1000hPa and the tropopause 100hPa. At 100hPa (about 10km) 75% of the atmosphere is beneath you. So this atmosphere is actually very thin.

    I guess the radiation that is absorbed by ozone at the tropopause is coming from the near surface air as it is moved by the trades towards the equator. The surface could not be responsible for the radiation and temperature peak at the tropopause.

    The potential for downward transfer of energy from the tropopause where the maximum is in August to the 200hPa pressure level where the maximum is in May, is of course there, and if greenhouse theory were valid, we would see it. But it does not eventuate. I guess that testifies to the strength of the convectional force that cools the troposphere at all levels.

    Greenhouse theory is based on a misunderstanding of how the atmosphere works. The nature of the troposphere is apparent in the Greek derivation of the word ‘tropos’. Although I speak no Greek I believe it means ‘turning’.

    If one takes the trouble to actually look at the data from 1948 onwards the troposphere has not warmed at any level above the near surface layers that are in contact with a warmer ocean, layers that are warmed by surface contact, the release of latent heat of condensation and no doubt absorption of radiation by susceptible molecules.

    The warming in the tropics, where more energy is received than emitted is slight. Most of the extra energy received has gone into evaporation rather than increased sea surface temperature. So, the increase in temperature at the equator, at cloud level, where the latent heat is released is about three times that at the surface. The surface warming at high latitudes in winter is strong, amounting to about five degrees in both hemispheres. There has been no warming in summer. In fact Antarctica has cooled in summer. That winter warming at high latitudes, when radiation is at a minimum, should indicate the importance of energy transfer by the ocean.

    There is another explanation as to why the Earth warmed strongly between 1976 and 1983, more slowly until 2005 and has cooled since, and you will find it at http://climatechange1.wordpress.com
    The explanation you will find there actually fits the observed pattern of temperature change.

  2. Shoshin says:

    I see. So a 20 year trend of warming is evidence of AGW, but a 10 year trend of cooling is also evidence of warming.

    Sorry, but that doesn’t pass the stink test. You can’t have it both ways.

  3. Steve Case says:

    Zeke Hausfather wrote:

    “CO2 and other atmospheric greenhouse gases absorb this longwave radiation coming up from the Earth’s surface and re-emit a portion of it downwards, heating the Earth.”

    A semantic argument perhaps, but a cooler body, i.e., the greenhouse gasses will not heat the warmer body. Also there is the matter of energy, the mass of the earth’s surface compared to the mass of the greenhouse gasses would dictate that there is very little energy to transfer. Hence there would be very little warm-up. A more accurate scenario would be that the back radiation cancels out similar radiation from the surface, but it’s the sun that does the work and raises the temperature of the surface enough to achieve an equilibrium.

    And then there’s this:

    “By itself, doubling atmospheric CO2 would increase global temperatures by about 1.2 degrees C.”

    Last century temperature went up .6°C and if I follow the argument, a significant portion of that is feedback. Is 50 percent an acceptable figure? If it is, then a 30% increase in CO2 by itself resulted in an increase of 0.3 degrees C. Now I’m expected to believe that 100% rise will increase global temperatures by 1.2 degrees? Arithmetic says 3.3 times 0.3 degrees is less than 1 degree, not 1.2 degrees. Besides, we just got done talking about the logarithmic nature of the phenomenon. My editorial comment is that folks on Zeke’s side of the argument exaggerate the effect a little here and a little there and hope no one will notice.

    The chart of feedbacks

    http://www.yaleclimatemediaforum.org/pics/0409_ccm1.gif

    lists more positive than negative feedbacks, one would think that would have caused the temperature to runaway to Venus like conditions eons ago. It didn’t. There have to be as many negative as positive feedbacks.

    The CO2 vs. Temperature chart

    http://www.yaleclimatemediaforum.org/pics/0409_ccm2.jpg

    is a classic manipulation of data. Simply arrange the left and right “Y” axis on the graph so that the slopes are the same and the message becomes very dramatic.

  4. Chris Lane says:

    As I understand it, the theory of AGW can be broken into three parts:

    1) Global warming. Increasing levels of CO2 in the atmosphere from the burning of fosil fuels causes the globe to warm up significantly

    2) Consequences. The consequence of global warming is calamitous and includes rising sea levels, droughts, melting ice caps, farm land turning to desert, flood, famine, disease and death.

    3) Correction. Corrective action can be taken by the introduction of policies across the globe that will reduce the production of C02 from the burning of fossil fuels and thus reduce global warming and the resultant consequences.

    There is a great deal of debate about all aspects of this very important theory and corrective action has been taken by a number of governments and organisations.

    The article, of course only dwells on the first part of the theory of AGW. It isn’t concerned with consequences or corrective action, only with the idea that as CO2 in the atmosphere increases with the burning of fossil fuels … so the temperature increases too.

    What is implied by the article, though not actually stated, is that this theory of AGW can be tested, by looking at temperature records and the levels of CO2 in the atmosphere. A correlation between the two, increasing temperature and CO2 levels is indicative that the first part of the theory is valid.

    As the article points out:

    “for the past few years global temperatures have been stagnant or slightly decreasing even as atmospheric CO2 concentrations have been increasing faster than ever”

    The general argument of the article is that this recent lack of correlation is …. well … a blip. I’m no Climate Scientist and the vagaries of the world’s climate are well beyond my understanding, but the article, for me raises a single fundamental question.

    What would we have to see in the temperature and CO2 records over the coming years for the AGW theory to be disproved?

    Paraphrasing the article “We look at the temperature record and we see the recent blip, this doesn’t disprove AGW”

    My question … What does?

    I’m looking for something explicit, something of the nature…

    If temperature doesn’t change over the next XXXX years then the theory of AGW is wrong.
    If the global temperature decreases by X over the next XXXX years then AGW isn’t happening.

  5. Stephen Thoms says:

    Why only take data from the last 130 years and compare with CO2 levels? Why not use data from the last 10,000 years which is readily available. I suggest you may come to a very different conclusion.

  6. Paul says:

    Thanks for the post. I am confused about the logarithmic relationship between GHGs concentration and global warming. Specifically I am confused about how this relationship would fit into a model trying to price the negative externality of GHG pollution. Do current pricing models, ie Stern Review take into account this logarithmic relationship? Do they need to? I am not one to give up in the middle of a fight but realistically I do not see us stabilizing below 450 ppm and I think we will overshoot this target. would this relationship imply that once we reach 450 ppm or 500 ppm the marginal damage of a extra emission would be significantly decreased.

    Sorry about the scientific ignorance but any enlightenment would be appreciated.

    Let’s hope the question is irrelevant and Copenhagen sees binding emission target with a long term stabilization target of 350 ppm.

  7. Thanks for the comments everyone.

    Erl,

    I’d suggest reading http://www.agu.org/pubs/crossref/2007/2006JD007956.shtml and perhaps directing your argument to Steve Sherwood here at Yale (Steven.Sherwood@yale.edu), as he is much better versed on radiative transfer modeling than I am.

    Shoshin,

    I wasn’t arguing that the last 10 years were themselves “evidence of warming” per se (though they were certainly warm relative to the past century, just not warming much). Rather, I was pointing out that the divergence between CO2 concentrations and temperatures over the past 10 years are not that unusual given the past variability between the two.

    Steve Case,

    The Earth has enough thermal inertia that the effects of increasing CO2 are not instantaneously realized as temperature increases. As you are probably well aware, there is still considerable debate in the literature over the extent of the Earth’s thermal inertia and what warming we may have in the proverbial pipeline, and knowing the thermal inertia better would probably allow us to better constrain climate sensitivity by inferring the magnitude of various feedbacks. Also bear in mind that feedbacks do not necessarily follow linearly with temperature. I’d be cautious of taking the projected feedbacks associate with doubling CO2 and applying them by simply linearly scaling the magnitudes to the past century, though I’m admittedly not an expert in this area.

    As far as a runaway greenhouse effect goes, the fact that outgoing longwave radiation is proportional to the fourth power of the temperature goes a long way in constraining run-away scenarios.

    I object to the accusation that the scales of my chart were deliberately chosen to “manipulate” the data. I simply used the default that Excel provided for the data. Showing CO2 concentrations with zero on the Y-axis would be about as useful as showing temperature with absolute zero on the axis; it would scrunch the data enough that its impossible to read. Additionally, the following chart that shows the regression of CO2 on temperature shows the relationship independent of any axis scale choice.

    Chris Lane,

    If the red line in the final chart dipped below two standard deviations more than once every twenty years or so, it would suggest that the relationship between CO2 and temperature in the current period was significantly different from the past 30 years, perhaps indicating that the magnitude of current natural forcings is greater than any natural forcings observed in the recent past. However, we are not there yet.

    For another approach on when the past warming trend would be “disproven” by recent years, see Tamino: http://tamino.wordpress.com/2008/01/31/you-bet/

    Stephen Thoms,

    Accurate temperature data is not available on a global scale prior to 130 years ago (and, frankly, is probably even somewhat sketchy until the late 1940s). We have some idea through proxies what global temperatures have been in the last 600 years. See http://books.nap.edu/openbook.php?record_id=11676&page=R1 for more discussion on this.

    Paul,

    Most economic models worth their salt take into account the logarithmic relationship between CO2 and climate forcing when estimating the marginal cost of carbon emissions. And the diminishing returns of additional CO2 on temperature is counteracted by the fact that damages associated with warming increase geometrically as the temperature increases (e.g. 3 degrees warming is much more than 33% worse than 2 degrees of warming).

  8. Chris Lane says:

    Zeke Hausfather

    Thanks very much for your candid response. Not what I expected at all, I expected not to get a response at all, or some statement that AGW is a fact, regardless of what happens to temperature.

    The Tamino article was very interesting. Looks like the next five years could end a lot of arguments.

    Again thanks for your response.

  9. Chris,

    The complexity of the earth system and the sheer amount of interacting factors involved in modeling the climate should make us a bit humble. Nothing is ever certain in science; after all, even Newton’s laws got modified quite a bit when Einstein published his theories of general and special relativity.

    The real challenge in the climate debate is realizing where there is uncertainty, and where there is not. For example, it is certainly possible for climate sensitivity to be on the low end (or the high end) of projections. Similarly, its also possible that models under-account for various natural forcings. It is unlikely, however, that we will discover that the basic radiative properties of CO2 and other greenhouse gases are different than what we have observed through empirical experimentation. The uncertainties lie in the effects of aerosols, clouds, ocean heat transfer rates, the magnitude of solar forcings, and modeling various other feedbacks.

    Its my personal opinion that we are unlikely to see no warming over the next century due to the physics of radiative absorption by greenhouse gases. However, whether the warming due to the doubling of CO2 will result in 1.5 or 3.5 degrees warming is still an open question. Here is an interesting figure from Hare and Meinshausen showing various ranges of climate sensitivity estimates: http://i81.photobucket.com/albums/j237/hausfath/Picture23-1.png

  10. Erl Happ says:

    Zeke,
    Thanks for the recommendation re the article. It certainly looks interesting as a discussion of the factors determining temperature above the tropopause. Unfortunately I am not a subscriber.

    Radiative transfer theory is fine as far as I am concerned. What is in doubt is that the temperature of the air is materially altered by down-welling radiation given the countervailing force of convection. Observation reveals that radiative theory predicts a result that is not borne out by observation in this instance. As you rightly say: nothing is ever certain in science. Theory is just a best guess. We must always go and look.

    There has been strong visitor traffic from this site to my blog after my comment so I have formalized the critical elements of the argument and it appears here: http://climatechange1.wordpress.com/

    I argue that there is overwhelming evidence of a very strong increase in the energy gain from sunlight in tropical waters since the 1978 climate shift. The temperature increase at 850hPa is double that at the surface. That represents a lot more evaporation from tropical waters. That takes energy. The energy is available with a small increase in the proportion of sunlight that runs the gauntlet of the atmosphere.

    Similarly, the advance of temperature at 100hPa reflects a strong increase in outgoing long wave radiation, perhaps due to the extra release of latent heat. The subsequent gradual collapse of 100hPa temperature from 1984 and return to pre 1978 levels in 2005 testifies to a surge in energy gain in tropical waters due to repeated strong El Nino events in Solar cycles 21 and 22. The period of solar cycle 23 has restored the balance somewhat and it appears that a weak cycle 24 will tip us into La Nina dominance.

    Radiative transfer theory should tell us that under a regime of increased energy storage in the Earth system outgoing long wave radiation should decrease, not increase.

    I invite you to aggregate the monthly values of the Southern Oscillation Index for each solar cycle and observe the 20 to 30 year oscillation.

    The challenge in climate science is to explain that oscillation, quantify its effect and see what, if any residual change remains to be accounted for. That, as they say, is the elephant in the room.

  11. William Lee says:

    Simple questions:
    What percentage has atmospheric carbon dioxide increased in the last 10 years?
    What percentage has temperature increased in the last decade?
    You seem to be a bit hasty in declaring:
    ” Human emissions of carbon dioxide (CO2) are the primary factor contributing to the warming of the Earth’s surface over the past half-century.”

    I don’t posess a PhD, but I don’t think correlation equates to Causation.