[Vision2020] The Lag Between Temperature And CO2

[Ted Moffett]

Your question comes from a short list of the most common skeptics arguments
trying to refute the conclusions of the IPCC regarding human sourced
emissions of CO2 inducing climate change.  This question has been answered
satisfactorily in detail by some of the world's most qualified climate
scientists.  I do not expect to change your mind on this issue. But in case
you, or anyone else who is concerned about the very serious problem of
anthropogenic climate change, wish to study the matter, the two entries
below from Realclimate.org from some very qualified climate scientists can
answer your question far better than any attempt I could make:

http://www.realclimate.org/index.php/archives/2004/12/co2-in-ice-cores/

 What does the lag of CO2 behind temperature in ice cores tell us about
global warming? Filed under:

   - FAQ<http://www.realclimate.org/index.php/archives/category/extras/faq/>
   - Greenhouse
gases<http://www.realclimate.org/index.php/archives/category/climate-science/greenhouse-gases/>
   - Paleoclimate<http://www.realclimate.org/index.php/archives/category/climate-science/paleoclimate/>

— group @ 9:42 AM - ([image:
Français]<http://www.realclimate.org/index.php/archives/2004/12/co2-in-ice-cores/langswitch_lang/fr>)


This is an issue that is often misunderstood in the public sphere and media,
so it is worth spending some time to explain it and clarify it. At least
three careful ice core studies have shown that CO2 starts to rise about 800
years (600-1000 years) after Antarctic temperature during glacial
terminations. These terminations are pronounced warming periods that mark
the ends of the ice ages that happen every 100,000 years or so.

Does this prove that CO2 doesn't cause global warming? The answer is no.

The reason has to do with the fact that the warmings take about 5000 years
to be complete. The lag is only 800 years. All that the lag shows is that CO
2 did not cause the first 800 years of warming, out of the 5000 year trend.
The other 4200 years of warming could in fact have been caused by CO2, as
far as we can tell from this ice core data.

The 4200 years of warming make up about 5/6 of the total warming. So
CO2could have caused the last 5/6 of the warming, but could not have
caused the
first 1/6 of the warming.

It comes as no surprise that other factors besides CO2 affect climate.
Changes in the amount of summer sunshine, due to changes in the Earth's
orbit around the sun that happen every 21,000 years, have long been known to
affect the comings and goings of ice ages. Atlantic ocean circulation
slowdowns are thought to warm Antarctica, also.

>From studying all the available data (not just ice cores), the probable
sequence of events at a termination goes something like this. Some
(currently unknown) process causes Antarctica and the surrounding ocean to
warm. This process also causes CO2 to start rising, about 800 years later.
Then CO2 further warms the whole planet, because of its heat-trapping
properties. This leads to even further CO2 release. So CO2 during ice ages
should be thought of as a "feedback", much like the feedback that results
from putting a microphone too near to a loudspeaker.

In other words, CO2 does not initiate the warmings, but acts as an amplifier
once they are underway. From model estimates, CO2 (along with other
greenhouse gases CH4 and N2O) causes about half of the full
glacial-to-interglacial warming.

So, in summary, the lag of CO2 behind temperature doesn't tell us much about
global warming. [But it may give us a very interesting clue about why
CO2rises at the ends of ice ages. The 800-year lag is about the amount
of time
required to flush out the deep ocean through natural ocean currents.
So CO2might be stored in the deep ocean during ice ages, and then get
released
when the climate warms.]

To read more about CO2 and ice cores, see Caillon et al., 2003, Science
magazine <http://icebubbles.ucsd.edu/Publications/CaillonTermIII.pdf>

Guest Contributor: Jeff Severinghaus <http://icebubbles.ucsd.edu/>
Professor of Geosciences
Scripps Institution of Oceanography
University of California, San Diego.
-------------------------
http://www.realclimate.org/index.php/archives/2007/04/the-lag-between-temp-and-co2/
The lag between temperature and CO2. (Gore’s got it right.) Filed under:

   - Arctic and
Antarctic<http://www.realclimate.org/index.php/archives/category/climate-science/arctic-and-antarctic/>
   - Greenhouse
gases<http://www.realclimate.org/index.php/archives/category/climate-science/greenhouse-gases/>
   - Paleoclimate<http://www.realclimate.org/index.php/archives/category/climate-science/paleoclimate/>
   - Climate Science<http://www.realclimate.org/index.php/archives/category/climate-science/>

— eric @ 2:45 PM

Eric Steig

When I give talks about climate change, the question that comes up most
frequently is this: “Doesn’t the relationship between CO2 and temperature in
the ice core record show that temperature drives CO2, not the other way
round?"

On the face of it, it sounds like a reasonable question. It is no surprise
that it comes up because it is one of the most popular claims made by the
global warming deniers. It got a particularly high profile airing a couple
of weeks ago, when congressman Joe Barton brought it up to try to discredit
Al Gore’s congressional testimony. Barton said:

   In your movie, you display a timeline of temperature and compared
to CO2levels over a 600,000-year period as reconstructed from ice core
samples.
   You indicate that this is conclusive proof of the link of increased
CO2emissions and global warming. A closer examination of these facts
reveals
   something entirely different. I have an article from *Science* magazine
   which I will put into the record at the appropriate time that explains that
   historically, a rise in CO2 concentrations did not precede a rise in
   temperatures, but actually lagged temperature by 200 to 1,000
years. CO2levels went up after the temperature rose. The temperature
appears to drive
   CO2, not vice versa. On this point, Mr. Vice President, you’re not just
   off a little. You’re totally wrong.

Of course, those who've been paying attention will recognize that Gore is
not wrong at all. This subject has been very well addressed in numerous
places. Indeed, guest contributor Jeff Severinghaus addressed this in one of
our very first RealClimate
posts<http://www.realclimate.org/index.php/archives/2004/12/co2-in-ice-cores>,
way back in 2004. Still, the question does keep coming up, and Jeff recently
received a letter asking about this. His exchange with the letter writer is
reproduced in full at the end of this post. Below is my own take on the
subject.

First of all, saying "historically" is misleading, because Barton is
actually talking about CO2 changes on very long (glacial-interglacial)
timescales. On historical timescales, CO2 has definitely led, not lagged,
temperature. But in any case, it doesn't really matter for the problem at
hand (global warming). We know why CO2 is increasing now, and the direct
radiative effects of CO2 on climate have been known for more than 100 years.
In the absence of human intervention CO2 does rise and fall over time, due
to exchanges of carbon among the biosphere, atmosphere, and ocean and, on
the very longest timescales, the lithosphere (i.e. rocks, oil reservoirs,
coal, carbonate rocks). The rates of those exchanges are now being
completely overwhelmed by the rate at which we are extracting carbon from
the latter set of reservoirs and converting it to atmospheric CO2. No
discovery made with ice cores is going to change those basic facts.

Second, the idea that there might be a lag of CO2 concentrations behind
temperature change (during glacial-interglacial climate changes) is hardly
new to the climate science community. Indeed, Claude Lorius, Jim Hansen and
others essentially predicted this finding fully 17 years ago, in a landmark
paper that addressed the cause of temperature change observed in Antarctic
ice core records, well before the data showed that CO2 might lag
temperature. In that paper (Lorius et al., 1990), they say that:

   changes in the CO2 and CH4 content have played a significant part in the
   glacial-interglacial climate changes by amplifying, together with the growth
   and decay of the Northern Hemisphere ice sheets, the relatively weak orbital
   forcing

What is being talked about here is influence of the seasonal radiative
forcing change from the earth's wobble around the sun (the well established
Milankovitch theory of ice ages), combined with the positive feedback of ice
sheet albedo (less ice = less reflection of sunlight = warmer temperatures)
and greenhouse gas concentrations (higher temperatures lead to more
CO2leads to warmer temperatures). Thus, both CO2 and ice volume should
lag
temperature somewhat, depending on the characteristic response times of
these different components of the climate system. Ice volume should lag
temperature by about 10,000 years, due to the relatively long time period
required to grow or shrink ice sheets. CO2 might well be expected to lag
temperature by about 1000 years, which is the timescale we expect from
changes in ocean circulation and the strength of the "carbon pump" (i.e.
marine biological photosynthesis) that transfers carbon from the atmosphere
to the deep ocean.

Several recent papers have indeed established that there is lag of
CO2behind temperature. We don't really know the magnitude of that lag
as well
as Barton implies we do, because it is very challenging to put CO2 records
from ice cores on the same timescale as temperature records from those same
ice cores, due to the time delay in trapping the atmosphere as the snow is
compressed into ice (the ice at any time will always be younger *older* than
the gas bubbles it encloses, and the age difference is inherently
uncertain). Still, the best published calculations do show values similar to
those quoted by Barton (presumably, taken from this
paper<http://www.sciencemag.org/cgi/content/abstract/291/5501/112>by
Monnin et al. (2001), or this
one <http://www.sciencemag.org/cgi/content/abstract/299/5613/1728> by
Caillon et al. (2003)). But the calculations can only be done well when the
temperature change is large, notably at glacial terminations (the gradual
change from cold glacial climate to warm interglacial climate). Importantly,
it takes more than 5000 years for this change to occur, of which the lag is
only a small fraction (indeed, one recently submitted
paper<http://www.cosis.net/members/journals/df/article.php?paper=cpd-3-435>I'm
aware of suggests that the lag is even less than 200 years). So it is
not as if the temperature increase has already ended when CO2 starts to
rise. Rather, they go very much hand in hand, with the temperature
continuing to rise as the the CO2 goes up. In other words, CO2 acts as an
amplifier, just as Lorius, Hansen and colleagues suggested.

Now, it there *is* a minor criticism one might level at Gore for his
treatment of this subject in the film (as we previously pointed out in our
review <http://www.realclimate.org/index.php?p=299>). As it turns out
though, correcting this would actually further strengthen Gore's case,
rather than weakening it. Here's why:

The record of temperature shown in the ice core is not a global record. It
is a record of local Antarctic temperature change. The rest of the globe
does indeed parallel the polar changes closely, but the global mean
temperature changes are smaller. While we don't know precisely why the
CO2changes occur on long timescales, (the mechanisms are well
understood; the
details are not), we do know that explaining the magnitude of global
temperature change requires including CO2. This is a critical point. *We
cannot explain the temperature observations without CO2.* But CO2 does not
explain all of the change, and the relationship between temperature
and CO2is therefore by no means linear. That is, a given amount of CO
2 increase as measured in the ice cores need not necessarily correspond with
a certain amount of temperature increase. Gore shows the strong parallel
relationship between the temperature and CO2 data from the ice cores, and
then illustrates where the CO2 is now (384
ppm<http://www.esrl.noaa.gov/gmd/ccgg/trends/co2_trend_mlo.png>),
leaving the viewer's eye to extrapolate the temperature curve upwards in
parallel with the rising CO2. Gore doesn't actually make the mistake of
drawing the temperature curve, but the implication is obvious: temperatures
are going to go up a lot. But as illustrated in the figure below, simply
extrapolating this correlation forward in time puts the Antarctic
temperature in the near future somewhere upwards of 10 degrees Celsius
warmer than present — rather at the extreme end of the vast majority of
projections (as we have discussed
here<http://www.realclimate.org/index.php/archives/2006/03/climate-sensitivity-plus-a-change>
).

Global average temperature is lower during glacial periods for two primary
reasons:
1) there was only about 190 ppm CO2 in the atmosphere, and other major
greenhouse gases (CH4 and N2O) were also lower
2) the earth surface was more reflective, due to the presence of lots of ice
and snow on land, and lots more sea ice than today (that is, the albedo was
higher).
As very nicely discussed by Jim Hansen in his recent *Scientific
American*article, the second of these two influences is the larger,
accounting for
about 2/3 of the total radiative forcing. CO2 and other greenhouse gases
account for the other 1/3. Again, this was all pretty well known in 1990, at
the time of the Lorius et al. paper cited above.

What Gore should have done is extrapolated the temperature curve according
this the appropriate scaling — with CO2 accounting for about 1/3 of the
total change — instead of letting the audience do it by eye. Had he done so,
he would have drawn a line that went up only 1/3 of the distance implied by
the simple correlation with CO2 shown by the ice core record. This would
have left the impression that equilibrium warming of Antarctica due to
doubled CO2 concentrations should be about 3 °C, in very good agreement with
what is predicted by the state-of-the-art climate
models<http://www.agu.org/pubs/crossref/2004.../2004GL020724.shtml>.
(It is to be noted that the same models predict a significant delay until
equilibrium is reached, due to the large heat capacity of the Southern
ocean. This is in very good agreement with the data, which show very modest
warming over Antarctica in the last 100
years<http://www.agu.org/journals/scripts/highlight.php?pid=2006GL027057>).
Then, if you scale the Antarctic temperature change to a global temperature
change, then the global climate sensitivity to a doubling of CO2 becomes 2-3
degrees C, perfectly in line with the climate sensitivity given by IPCC (and
known from Arrhenius's calculations more than 100 years ago).

In summary, the ice core data in no way contradict our understanding of the
relationship between CO2 and temperature, and there is nothing fundamentally
wrong with what Gore says in the film. Indeed, Gore could have used the ice
core data to make an additional and stronger point, which is that these data
provide a nice independent test of climate sensitivity, which gives a result
in excellent agreement with results from models.

A final point. In Barton's criticism of Gore he also points out that CO2 has
sometimes been much higher than it is at present. That is true. CO2 may have
reached levels of 1000 parts per million (ppm) — perhaps much higher — at
times in the distant geological past (e.g. the Eocene, about 55 million
years ago). What Barton doesn't bother to mention is that the earth was much
*much* warmer at such times. In any case, more relevant is that CO2 has not
gone above about 290 ppm any time in the last 650,000 years (at least),
until the most recent increase, which is unequivocally due to human
activities <http://www.realclimate.org/index.php?p=87>.
------------------------------
Below is the letter written to Jeff Severinghaus, and his response:


Dear Jeff,

I read your article "What does the lag of CO2 behind temperature in ice
cores tell us about global warming?" You mention that CO2 does not initiate
warmings, but may amplify warmings that are already underway. The obvious
question comes up as to whether or not CO2 levels also lag periods when
cooling begins after a warming cycle…even one of 5,000 years?

If CO2 levels on planet Earth also lag the cooling periods, then how can it
be that CO2 levels are causally related to terrestrial heating periods at
all? I am not sure what the ice core records are related the time response
of CO2 to the cooling trends. If there is also a lag in CO2 levels behind a
cooling period, then it appears that CO2 levels not only do not initiate
warming periods but are also unrelated to the onset of cooling periods. It
would appear that the actual CO2 levels are rather impotent as an amplifier
either way…warming or cooling. We are talking about planet Earth after all
and not Venus whose atmospheric pressure is many times larger than Earth's.

If there is also a time lag upon the onset of cooling, then it appears that
some other mechanism actually drives the temperature changes. So what is the
time difference between CO2 levels during the onset of a cooling period at
the end of a warming period and the time history of the temperature changes
in the ice cores?

Dear John,

The coolings appear to be caused primarily and initially by increase in the
Earth-Sun distance during northern hemisphere summer, due to changes in the
Earth's orbit. As the orbit is not round, but elliptical, sunshine is weaker
during some parts of the year than others. This is the so-called
Milankovitch hypothesis [this really should say "theory" — eric], which you
may have heard about. Just as in the warmings, CO2 lags the coolings by a
thousand years or so, in some cases as much as three thousand years.

But do not make the mistake of assuming that these warmings and coolings
must have a single cause. It is well known that multiple factors are
involved, including the change in planetary albedo, change in nitrous oxide
concentration, change in methane concentration, and change in CO2
concentration. I know it is intellectually satisfying to identify a single
cause for some observed phenomenon, but that unfortunately is not the way
Nature works much of the time.

Nor is there any requirement that a single cause operate throughout the
entire 5000 - year long warming trends, and the 70,000 year cooling trends.

Thus it is not logical to argue that, because CO2 does not cause the first
thousand years or so of warming, nor the first thousand years of cooling, it
cannot have caused part of the many thousands of years of warming in
between.

Think of heart disease - one might be tempted to argue that a given heart
patient's condition was caused solely by the fact that he ate french fries
for lunch every day for 30 years. But in fact his 10-year period of no
exercise because of a desk job, in the middle of this interval, may have
been a decisive influence. Just because a sedentary lifestyle did not cause
the beginning of the plaque buildup, nor the end of the buildup, would you
rule out a contributing causal role for sedentary lifestyle?

There is a rich literature on this topic. If you are truly interested, I
urge you to read up.

The contribution of CO2 to the glacial-interglacial coolings and warmings
amounts to about one-third of the full amplitude, about one-half if you
include methane and nitrous oxide.

So one should not claim that greenhouse gases are the major cause of the ice
ages. No credible scientist has argued that position (even though Al Gore
implied as much in his movie). The fundamental driver has long been thought,
and continues to be thought, to be the distribution of sunshine over the
Earth's surface as it is modified by orbital variations. This hypothesis was
proposed by James Croll in the 19th century, mathematically refined by
Milankovitch in the 1940s, and continues to pass numerous critical tests
even today.

The greenhouse gases are best regarded as a biogeochemical feedback,
initiated by the orbital variations, but then feeding back to amplify the
warming once it is already underway. By the way, the lag of CO2 of about
1000 years corresponds rather closely to the expected time it takes to flush
excess respiration-derived CO2 out of the deep ocean via natural ocean
currents. So the lag is quite close to what would be expected, if CO2 were
acting as a feedback.

The response time of methane and nitrous oxide to climate variations is
measured in decades. So these feedbacks operate much faster.

The quantitative contribution of CO2 to the ice age cooling and warming is
fully consistent with current understanding of CO2's warming properties, as
manifested in the IPCC's projections of future warming of 3±1.5 C for a
doubling of CO2 concentration. So there is no inconsistency between
Milankovitch and current global warming.

Hope this is illuminating.

Jeff

---------------------------------------------

Vision2020 Post: Ted Moffett
On 2/19/09, lfalen <lfalen at turbonet.com> wrote:
>
> Ted
> If CO2 is the main offender, why do charts show that a raise in temperature
> precedes a rise in CO2,  not the other way around?? Are they just being
> faked?
> Roger
>
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