The University of Texas at Austin
  • Humans now steer climate bus

    By Marc Airhart
    Marc Airhart
    Published: Nov. 17, 2010

    Climate scientists at the Jackson School of Geosciences address common myths about climate change in this eight part series.

    Myth No. 8: In the past, global temperatures rose first and then carbon dioxide levels rose later. Therefore, rising temperatures cause higher CO2 levels, not the other way around.

    Photo of Kerry Cook

    Kerry CookPhoto: Sasha Haagensen

    Ice cores from Dome C in Antarctica record surface temperatures and atmospheric concentrations of CO2 going back over 800,000 years. During that time, several ice ages came and went. After each ice age ended, temperatures rose first and then several centuries later, CO2 concentrations rose. This lag, some skeptics conclude, proves that CO2 increases are caused by global warming, not the other way around.

    According to Kerry Cook, professor of climate systems science, it isn’t an either/or proposition. Climate variations can have many different causes, which are known as “climate forcing factors.” The climate forcings for ice ages and warmer periods are well-known variations in Earth’s orbital parameters (the eccentricity of its orbit around the sun, the tilt of its axis of rotation, and the season during which Earth is closest to the sun).  These factors can be accurately calculated for any past or future time, and they vary on time scales ranging from 23,000 years to 400,000 years. The amount and distribution of solar radiation that reaches Earth changes with the orbital parameters, causing climate variations on these same time scales (tens of thousands of years).

    During glacial (cool) periods on Earth, atmospheric CO2 levels are lower, and during interglacial (warm) periods they are higher. That’s true in part because during warmer periods, the oceans store less dissolved CO2, just as your glass of soda releases its CO2 and goes flat when it warms to room temperature. Warming soils also release more CO2. So it makes perfect sense that in the past, CO2 variations followed temperature variations. The role of CO2 in natural glacial/interglacial climate change is to amplify the climate change, making warm periods somewhat warmer.  So changes in Earth’s orbital parameters likely kicked off the warmer periods and rising CO2 likely boosted the effects.

    That doesn’t prove that CO2 can’t cause global warming. In fact, since past natural warmings took about 5,000 years to complete, CO2 and temperature actually rose together for about 90 percent of the time. Climate scientists think much of the warming in ice core records during interglacials is due to CO2.

    Graph showing CO2 lag

    Data from Antarctic ice cores show that temperatures (brown) have changed before CO2 concentrations (blue) over a series of recent ice ages. That trend has been upset during the past 100 years, as a rapid increase in CO2 preceded the current warming. Credit: Michael Ernst/Woods Hole Research Center

    The game has changed dramatically in recent decades. There is no lag between rising temperatures and rising CO2. Both have spiked dramatically in the past 50 years. The rate of change and the lack of a lag are exactly what climate scientists theorize should happen if CO2 takes over as the main climate forcing.

    There are several reasons that it is useful to refer to the ice core records when we are thinking about CO2-induced climate change.  One is to simply understand that global climate can change, and that a change in the globally-averaged surface temperature of just a few degrees implies a huge change in climate. The difference between a glacial and an interglacial climate is only about 9 degrees Fahrenheit (5 degrees Celsius), which is comparable to predicted changes in globally-averaged temperature over the coming decades due to increasing greenhouse gas concentrations.

    We also learn from the ice core record that these natural, long-period changes take CO2 levels from about 200 parts per million by volume (ppmv) during a glacial (cool) period to about 280 ppmv during an interglacial (warm). This provides perspective on the current atmospheric CO2 concentration of about 390 ppmv, showing that it is far outside the range of natural variations even on time scales of tens of thousands of years.

    You are invited to post comments and follow-up questions on this site. You can also e-mail climate scientists questions. The scientists cannot respond to all questions individually but will address recurring themes with new entries.

    • Quote 2
      Benjamin Smith said on May 10, 2012 at 1:15 p.m.
      Wow! I have a hard time wrapping my head around this argument for CO2 bieng the forcer in this graph. The graph clearly shows that temperatures rise AND THEN the CO2 rises. It has been going on with a pretty regular NATURAL cycle for many thousands of years. So that seems to indicate that the CO2 is a result of higher temps. not the inverse, and certainly can't be the driver, unless you prescribe to the "cart before the horse" theory. We have been on the upswing in temperatures for close to 20 thousand years now, and still we are not at a global temperature as high as the last 4 spikes. In fact it seems to have been leveling or stalling for quite some time taking in concideration the scale of time represented in the chart. Let's for a minuite assume the data on the chart is accurate. (Big leap given the ice core samples only go back a relatively short period of time.). Are we not looking at natural occurances through time, without the artificial insertion of CO2 from humans. Now all of a sudden the scientists say the cart is leading the horse and the sky is falling. What a leap it is to say the world climate decided to change it's mind and make the opposite true. Are these the same scientists that try and tell us that cosmic goo (That came from ?????) decided to get together and form themselves into the complex, unexplainable perfection we see in life itself. Sorry, I can't swallow this pill any more than I can swallow the one they are pushing now.
    • Quote 2
      Marc Airhart said on Oct. 12, 2011 at 2:18 p.m.
      Robert, thanks for the link to the Caillon et al. paper from Science 2003. They basically found that CO2 rose about 800 years (give or take 200 years) after global temperature increased (around 240,000 years ago). So they're acknowledging the challenges of pinning down the timing, but that the uncertainty is still small enough that you can comfortably say CO2 lagged behind temperature and it did so by quite a few centuries.
    • Quote 2
      Marc Airhart said on Oct. 12, 2011 at 2:11 p.m.
      Robert and Laurence, thanks for writing. You're right that the ice core data might not have the resolution to record the kind of rapid global warming event we're currently experiencing on a scale as short as a few decades or even a century or two. But the point with that graph was to show how CO2 and temperature rise and fall in relation to each other over long periods of time.
    • Quote 2
      Ken Calvert said on Oct. 12, 2011 at 3:28 a.m.
      The ice core data shows peaks of temperature and CO2 levels 800 odd years apart. When the CO2 peaks the temperature is already well on the way down again. I would expect that when the CO2 peaks that there would be at least a small bump or change of inclination of the temperature, and the curve would steepen as CO2 dropped again. Nothing like that happens! So, I am skeptical of CO2 warming, let alone forcing.
    • Quote 2
      Laurence Trafton said on May 20, 2011 at 6:27 p.m.
      It is illogical to compare on the same plot the CO2 ppt or the annual temperature rise of the last two hundred years with antarctic ice core data that has an effective plotted time resolution element of 2000 years or longer. This is because such spikes as seen in the last 200 years may have actually occurred frequently over the last half million years due to yet to be discovered (natural) processes, but they are not seen in these ice core data because these data are inherently smoothed over an extended time interval due to ice diffusion, core sampling, data processing, whatever. To be valid, you should first smooth the data of the last 200 years to match the effective time resolution of the core data before drawing any conclusion from their comparison.
    • Quote 2
      Robert Schleicher said on March 23, 2011 at 1:06 p.m.
      Yet another comment. This paper is interesting reading on trying to correlate the timing of CO2 fluctuations with temperature fluctuations, using ice core data:
    • Quote 2
      Robert Schleicher said on March 23, 2011 at 1:00 p.m.
      As an added comment, I did find a paper related to an analysis of the Vostok ice samples, that implied that the range of time encompassed within a given ice sample was around 20 years. But that the accuracy of the specific dates of that 20 year span was not as good. In any case, my question about the measurement process inevitably providing a smoothing or filtering of the data may or may not be relevant. It would take more digging into the actual core sampling process to figure out.
    • Quote 2
      Robert Schleicher said on March 23, 2011 at 12:40 p.m.
      In trying to find an answer to my question (regarding the time span that is encompassed in any given ice core sample from "ancient ice"), I ran across this paper summary, which questions the basic validity of CO2 measurements from deep, compressed ice cores. I mention that paper here, without personal opinion on it.
    • Quote 2
      Robert Schleicher said on March 23, 2011 at 12:28 p.m.
      My question on ice core data is this: What time window is encompassed in any given ice core sample, especially for ones that are representative of more than 100,000 years ago? My guess, based on the practical constraints of collecting ice core samples, is that the ice within any given sample probably represents a time period of at least 100 years, and quite possibly much more. This inevitably provides a filtering or smoothing effect on the data. Short term spikes in CO2 concentrations that are within the time window of the ice core sample will be essentially averaged into the longer time period. Given that the "spike" in CO2 concentration in the last century or so is of less than 100 years duration, it is not obvious to me that the ice core data precludes the existence of similar "short term" (i.e. 100 years or less) changes in CO2 levels. Can anyone elaborate on the time span of a typical ice core sample, that is looking at 100,000 year old ice?
    • Quote 2
      Jim said on Feb. 22, 2011 at 2:25 p.m.
      Yes, but these predictions have yet to actually prove true. Remember the 1980's, there were many grim climate prophecies that are not going according to predictions. And don't say it's because of all we've done for the climate because it's already pretty much accepted that anything we do today won't actually have any noticeable improvements on our climate until several decades later. Actually I'm being generous because I'm pretty sure it was much longer that that. Also, unless I'm mistaken,the global temperature has been steadily rising, not exponentially rising. Also, what you're saying doesn't really prove anything. All it does is say that the temperature rises and CO2 rises as well. That's been accepted by both sides of the argument. However, it sounds like you're saying that they both, warming and CO2 levels, effect each other simultaneously. That makes sense, but it sounds like a lot of sketchy evidence at best. Imagine that we reduce our CO2 and other greenhouse gas outputs, thus we severely reduce our industrial sector and reliable efficient power and in it's place we go without, within tolerable levels, or we switch to a power-type that is expensive, somewhat unreliable, depending on the type, and it turns out that our globe is naturally warming, and that all these predictions come true anyway . . . Will we be prepared to stand the shock as a nation when much of our cheaper, more reliable, more simplistic power and industrial production is no longer a major part of this nation? Imagine it. Clouds are blocking out the solar power plants over regions of the nation because the globe is still warming, and the back-up generators of hospitals are continuously starving for fuel because our fuel production and supply has been drastically reduced. It would be nice to have a system of biodiesel supply and production in place, but there are too many that argue there isn't enough food to do that. Imagine people are unable to get to where they need to go because their battery/solar powered cars can't get even get past 15 mph, thus production suffers in entire regions around the nation. Imagine the exponential effects of a global disruption that happens upon a nation that utilizes as a majority of it's energy and production needs, expensive and complicated to supply and produce, green power. It'll be the newly industrialized third world nations that can't afford green power that survive that environmental onslaught. We have to be ready to survive first. Survival is utmost. Everything else takes a back seat. No one's going to protect Alaska's wilderness from oil drills if our nation falls or has to let go of Alaska due to too much disaster withing it's core, the contental U.S. We can't allow our needs to suffer for the sake of something that may actually be nature.
    • Quote 2
      Marc Airhart said on Nov. 22, 2010 at 1:59 p.m.
      Mark, thanks for the correction. You're right, we should have written "Ice cores from Dome C in Antarctica ..." I'll ask our web editors to correct that in the above text.
    • Quote 2
      Mark said on Nov. 19, 2010 at 7:01 p.m.
      The article is good explanation about the relationship between CO2 and temperature. My problem is the opening sentence: "Ice cores from Lake Vostok in Antarctica record surface temperatures and atmospheric concentrations of CO2 going back over 800,000 years." The core is not fro Lake Vostok but Vostok Station and the record goes back 420,000 years. It is the Dome C record that goes back 800,000 years and Lake Vostok is below the ice core that was collected. Thanks for the explanation but please check your facts. Thanks
    • Quote 2
      Bug Guy said on Nov. 18, 2010 at 2:34 p.m.
      Might the current CO2 rise and correlated warming trend forestall the next ice age?
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