General Climate Discussion

[MORAY37]

27 April 2007
The lag between temperature and CO2. (Gore’s got it right.)

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 CO2 levels over a 600,000-year period as reconstructed from ice core samples. You indicate that this is conclusive proof of the link of increased CO2 emissions 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. CO2 levels 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, 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 CO2 leads 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 CO2 behind 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 by Monnin et al. (2001), or this one 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 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). 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 CO2 changes 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 CO2 is therefore by no means linear. That is, a given amount of CO2 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), 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).

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.

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.

[AKIron]

Many words there Moray but in essence it does not deny that co2 lags temperature increase. It claims (unproven) that the increased co2 resulting from an increasing temperature amplifies the effect. While that may be true it may also be true that the increasing level of co2 has no significant effect on the temperature. Fact is, Gore skipped over it because it was an "inconvenient truth".

[wrag]

Well.............

here it goes......................

http://www.desertdispatch.com/opinion/global_3006___article.html/warming_tax.html

[Donzo]

Well.............

here it goes......................

http://www.desertdispatch.com/opinion/global_3006___article.html/warming_tax.html

Yep....here we go.  Gotta get that tax in force so that it will look like it was man made global climate control that caused a reverse in warming.

Do think that this will be the end of it.

Cap and trade...someone's has to administer the exchange of carbon credits.  And guess what...I'm sure there will be a fee and someone will make money off it.

[texasmom]

http://www.desertdispatch.com/opinion/global_3006___article.html/warming_tax.html

[HIJACK] Hm. Desert Dispatch? I thought I was the only viewer of that small town newspaper. 
[/HIJACK]

[wrag]

[HIJACK] Hm. Desert Dispatch? I thought I was the only viewer of that small town newspaper. 
[/HIJACK]

NOPE!!!

NEENER NEENER 

[Angus]

Lagging or leading, it's basically known how it works, and without greenhouse effect we'd be frozen all over.

[WWhiskey]

Lagging or leading, it's basically known how it works, and without greenhouse effect we'd be frozen all over.

funny and yet plausable

[lazs2]

moot has it right..

It matters not.  What matters is who has the best show.  Who can give the best performance.

No one knows.. no one can even predict next year or five years..  in 1900 it could be said that the streets would be 30' deep in horse dung by now.

What will matter is this..  if the hand wringers try to bleed too much out of the people.. they will not buy the con.   They will say...  "not worth it" or.. they will run to the skeptics for comfort.   

If we have a few cold years.. the marks won't buy the con.

If we have a real hot summer and the handwringers get a big enough push right in the middle of a heat wave and exaggerate it enough.. they will be able to fleece a big chunk of change out of the sheeple.. even get some laws passed that generate billions or trillions of bucks a year for decades.

One thing is certain.. Mother nature will rule..  if it gets cold too soon..  the whole thing will fall apart.  they need to work fast but.. if they work too fast.. it will allow the sheeple to see what a hit to their lives they are taking.

Hence..  no real pushing during this winter.. They kept pretty darn quite you might have noticed all winter.  Hard to make people beg to pay more to end the warming when they are paying three bucks a gallon for heating oil to heat a frozen home

it will come this summer..  a few mild summers and cold winters like the last few and no one... NO ONE is gonna go for a couple thousand dollars a year hit on their income to maybe save (or not) a few polar bears.

In the end.. the worst thing for the hand wringers is that it has been very pleasant in the US.. mild summers and winters with no end in sight.. no green areas turning to desert.   

Myself... I have been enjoying this mild and pleasant warm period in our climate history... it will get cold soon enough..  I feel sorry for those of you who haven't got out and enjoyed it.

lazs

[Angus]

Hehe, Lazs, I'm going to save your day 

Al Gore was in Iceland, I think he left yesterday. In his wake came some cold air, and a blizzard, leaving some 6 inches of snow.

Not joking! It's a fact!

Maybe just me, and I still basically disagree with you on the major climate issues, but I still thought I'd please you with this one 

[AKIron]

Lagging or leading, it's basically known how it works, and without greenhouse effect we'd be frozen all over.

Certainly the greenhouse effect keeps us warm. Not so certain is what role co2 plays in this.

[Angus]

It is one of the gases included in the effect, and not the strongest at it.

[MORAY37]

Many words there Moray but in essence it does not deny that co2 lags temperature increase. It claims (unproven) that the increased co2 resulting from an increasing temperature amplifies the effect. While that may be true it may also be true that the increasing level of co2 has no significant effect on the temperature. Fact is, Gore skipped over it because it was an "inconvenient truth".

It says, just as the science does, that during interglacial warming CO2 can be predicted to lag until the retreat of the ice sheets worldwide. 

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 CO2 leads 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.

Why even attempt to show you where science is at if your preconceptions get in the way all the time? Marine biological photosynthesis is a major user of carbon.  While the levels go up, the microscopic plants in the ocean that do most of this, increase in population as well, due to higher levels of raw material for metabolic process, IE CO2.  At some point this population slows and or crashes, due to another mitigating population control. (surging water temperatures are most likely to be this) Thus CO2 will lag until this happens, which in some newer papers, is around 200 years, maybe less. 

Here is the problem with your types.  Arguing a point that is 17 to 20 years past argument.... It's already been agreed upon.  Yet denialists can't wrap their minds around it, as par for most issues that deal with anything scientific.

[AKIron]

It says, just as the science does, that during interglacial warming CO2 can be predicted to lag until the retreat of the ice sheets worldwide. 

Why even attempt to show you where science is at if your preconceptions get in the way all the time? Marine biological photosynthesis is a major user of carbon.  While the levels go up, the microscopic plants in the ocean that do most of this, increase in population as well, due to higher levels of raw material for metabolic process, IE CO2.  At some point this population slows and or crashes, due to another mitigating population control. (surging water temperatures are most likely to be this) Thus CO2 will lag until this happens, which in some newer papers, is around 200 years, maybe less. 

Here is the problem with your types.  Arguing a point that is 17 to 20 years past argument.... It's already been agreed upon.  Yet denialists can't wrap their minds around it, as par for most issues that deal with anything scientific.

Uh, did you just agree with me that co2 lags temperature rise? I never argued as to why this is. Arguing with you is like arguing with Arlo except he doesn't get quite so insulting.

[MORAY37]

Uh, did you just agree with me that co2 lags temperature rise? I never argued as to why this is. Arguing with you is like arguing with Arlo except he doesn't get quite so insulting.

Yes, just as is was said, "DURING INTERGLACIAL WARMING".  That caveat is very important.  In historical context, CO2 leads.  On interglacial contexts, it lags.  There is a reason for that and I just explained it to you..... Yet you still feel the point is important without obviously recognizing the difference.