Climate Science Glossary

Term Lookup

Enter a term in the search box to find its definition.


Use the controls in the far right panel to increase or decrease the number of terms automatically displayed (or to completely turn that feature off).

Term Lookup


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.

Home Arguments Software Resources Comments The Consensus Project Translations About Donate

Twitter Facebook YouTube Pinterest

RSS Posts RSS Comments Email Subscribe

Climate's changed before
It's the sun
It's not bad
There is no consensus
It's cooling
Models are unreliable
Temp record is unreliable
Animals and plants can adapt
It hasn't warmed since 1998
Antarctica is gaining ice
View All Arguments...

Keep me logged in
New? Register here
Forgot your password?

Latest Posts


CO2 was higher in the late Ordovician

What the science says...

During the Ordovician, solar output was much lower than current levels. Consequently, CO2 levels only needed to fall below 3000 parts per million for glaciation to be possible. The latest CO2 data calculated from sediment cores show that CO2 levels fell sharply during the late Ordovician due to high rock weathering removing CO2 from the air. Thus the CO2 record during the late Ordovician is entirely consistent with the notion that CO2 is a strong driver of climate.

Climate Myth...

CO2 was higher in the late Ordovician
"To the consternation of global warming proponents, the Late Ordovician Period was also an Ice Age while at the same time CO2 concentrations then were nearly 12 times higher than today - 4400 ppm. According to greenhouse theory, Earth should have been exceedingly hot. Instead, global temperatures were no warmer than today. Clearly, other factors besides atmospheric carbon influence earth temperatures and global warming." (Monte Hieb)

An argument used against the warming effect of carbon dioxide is that millions of years ago, CO2 levels were higher during periods where large glaciers formed over the Earth's poles. This argument fails to take into account that solar output was also lower during these periods. The combined effect of sun and CO2 show good correlation with climate (Royer 2006). The one period that until recently puzzled paleoclimatologists was the late Ordovician, around 444 million years ago. At this time, CO2 levels were very high, around 5600 parts per million (in contrast, current CO2 levels are 389 parts per million). However, glaciers were so far-reaching during the late Ordovician, it coincided with one of the largest marine mass extinction events in Earth history. How did glaciation occur with such high CO2 levels? Recent data has revealed CO2 levels at the time of the late Ordovician ice age were not that high after all.

Past studies on the Ordovician period calculated CO2 levels at 10 million year intervals. The problem with such coarse data sampling is the Ordovician ice age lasted only half a million years. To fill in the gaps, a 2009 study examined strontium isotopes in the sediment record (Young 2009). Strontium is produced by rock weathering, the process that removes CO2 from the air. Consequently, the ratio of strontium isotopes can be used to determine how quickly rock weathering removed CO2 from the atmosphere in the past. Using strontium levels, Young determined that during the late Ordovician, rock weathering was at high levels while volcanic activity, which adds CO2 to the atmosphere, dropped. This led to CO2 levels falling below 3000 parts per million which was low enough to initiate glaciation - the growing of ice sheets.

Last week, another study headed by Seth Young further examined this period by extracting sediment cores from Estonia and Anticosti Island, Canada (Young 2010). The cores were used to construct a sequence of carbon-13 levels from rocks formed during the Ordovician. This was used as a proxy for atmospheric CO2 levels, at a much higher resolution than previous data. What they found was consistent with the strontium results in Young 2009 - CO2 levels dropped at the same time that sea surface temperatures dropped and ice sheets expanded. As the ice sheets grew to cover the continent, rock weathering decreased. This led to an increase in atmospheric CO2 which caused global warming and a retreat of the glaciers.

Thus arguments that Ordovician glaciation disproves the warming effect of CO2 are groundless. On the contrary, the CO2 record over the late Ordovician is entirely consistent with the notion that CO2 is a strong driver of climate.

Intermediate rebuttal written by nancyk

Update July 2015:

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

Last updated on 8 July 2015 by pattimer. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page


Comments 1 to 10:

  1. A 2005 study shows that the late Ordovician glaciation actually occurred 10 million years before. This ice age did not occur when CO2 was at it's peak. Rather it began at a time when the concentration was between 180 and 200 ppm.
  2. John, what happened to "the sun being cooler" in this argument? You mention it in the summary box but nowhere at all in the body of the text; instead the argument drifts off into rising and falling CO2 levels, that frankly I'm having a hard time understanding. Where is the evidence for the sun being cooler?
  3. Tom_the_Bomb at 04:22 AM on 6 July, 2010

    The evidence partly comes from analysis of similar stars to our sun at different stages in their life cycle and well tested physics based models. Stars of a certain mass profile appear to display similar growth and fusion burn and brightness patterns, many have star spot cycle activity similar to our sun. I'll dig out some references.
    Response: How the sun was cooler and how this affects climate is discussed in more detail at CO2 was higher in the past.
  4. Tom_the_Bomb at 04:22 AM on 6 July, 2010

    Sorry "well tested" should be "well trusted"!

    These are the two references that stood out in my memory. Minton 2007 is a good introduction, and explores the difficulties with the standard solar model and young sun model with regard to the very early climate, and Gudel 2007 gives a comprehensive exploration of models and other stars as well as the sun. Hope they are of interest.
  5. It is remarkable how rapidly the science has been moving in this area.  In 2005, Dana Royer wrote:

    "There is unequivocal evidence for a widespread but brief Gondwanan glaciation during the end-Ordovician (Hirnantian Stage; 445.6–443.7 Ma). Several reports argue for a longer interval of ice centered on the Ordovician–Silurian boundary (e.g., 58 my in Frakes et al., 1992), and alpine glaciers may have indeed persisted in Brazil and Bolivia into the early Silurian (Crowell, 1999), but most recent studies demonstrate that the dominant glacial phase was restricted to the Hirnantian (Brenchley et al., 1994, 2003; Paris et al., 1995; Crowell, 1999; Sutcliffe et al., 2000).  There is one CO2 data point available that is close in age to this glaciation, and it suggests very high CO2 levels (5600 ppm; see Fig. 3A; Yapp and Poths, 1992, 1996); moreover, GEOCARB III predicts high CO2 levels at this time (4200 ppm; see Fig. 1D). Apparently, this event presents a critical test for the CO2-temperature paradigm (e.g.,
    Van Houten, 1985; Crowley and Baum, 1991). However, it is unclear what CO2 levels were during this event. The single proxy record is Ashgillian in age, which spans the Hirnantian but also most of the preceding Stage (450–443.7 Ma); if the CO2 data point dates to the pre-Hirnantian Ashgillian, then this is consistent with a well-described mid-Ashgillian global warm event (Boucot et al., 2003; Fortey and Cocks, 2005)."

    That appears to be the point at which "skepictal" analysis of this issue stops.  Genuine scientists kept on analyzing the issue.  As noted above, Seth Young and coworkers made major contributions in 2009 and 2010, and now it is well accepted that there was a major carbon isotope excursion in the Hirnantian, coincident with the Hirnantian glaciation.

    The magnitude of the C13 excursion can be seen by comparing it with the similar, but opposite signed excursion in the Paleo-Eocene Thermal Maximum, which was associated with greatly increased CO2 concentration.

    First the PETM:

    Then the Hirnantian:

    (Please note the inverted scales for the C13 escursions.)

  6. I feel like the evolution of land plants at the same general time of the Silurian-Ordovician glaciation is not a coincidence.  It seems to me, this should have had a similar effect on climate as the initial evolution of plant life.


    If these plants were to have drawn down atmospheric carbon into swamps and bogs which remained there until being digested by newly evolved animals or burned because of retreating water, this wouldn't show in the strontium signal, right?

    How would it be possible to detect such an event?  Would the ¹³C delta be higher because marine organisms were carbon-deficient and picking it up more frequently?

  7. CBlargh @6, a large increase in the quantity of plants would show up in the fossil record as an increase in dC13 as the C12 isotope in CO2 was preferentially taken up in photosynthesis.  The release of O2 by photosynthesis would also show up as elevated oxygen levels.  However, this effect would have been very minor in the Ordivician when land plants were small, non-woody plants resembling liverworts, and mostly confined to shore lines, at least according to wikipedia.

    However, as plants evolved and colonized the land, that is exactly what happened.  At least, it was what happened until the evolution of land animals reduced the extent of forestation and reduced the time from photosynthesis to decay/metabolism, thereby restoring the balance:

  8. @Tom: Thanks for your reply.

    It looks from the chart that this occurred later.  Is increased weathering the accepted cause of the dC13 spike during the Hirnantian?

    Never underestimate the power of a liverwort!

  9. CBlargh @8, I haven't really looked into it.  Certainly it is plausible, and the Taconic orogeny occurs at about the right time to explain it.  I do not, however, know the precise timing, nor what other events might be simultaneious with the Hirnantian cC13 excursion.  Therefore it is a possible explanation only.

  10. @Tom:

    Wait!  They may not be unrelated!  I just read a paper suggesting roots increase weathering.

    <a href = "">Falkowski and Rosenthal</a>

    If the late Ordovician was the first time land rhizomes evolved, it would suggest to me it could have caused a pulse of calcium which pulled down atmospheric carbon in the sea, not necessarily in freshwater bogs through the land plants themselves.

Post a Comment

Political, off-topic or ad hominem comments will be deleted. Comments Policy...

You need to be logged in to post a comment. Login via the left margin or if you're new, register here.

Link to this page

The Consensus Project Website



(free to republish)



The Scientific Guide to
Global Warming Skepticism

Smartphone Apps


© Copyright 2015 John Cook
Home | Links | Translations | About Us | Contact Us