This ought to be good for some personal abuse from the usual suspects, along with the odd spot of scientific gobledygook from the resident expert.  

From Realclimate, Mozz's favourite, biased, climate change site.

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

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 CO2 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 CO2 could 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 CO2 rises 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 CO2 might be stored in the deep ocean during ice ages, and then get released when the climate warms.  

This old chestnut again? Once again, there are a few facts buried in there, but the conclusions are wrong.

Quote:


Well that's quite accurate for most of the pre-industrial past. For it to initiate the warmings, you'd need something to release CO2 - you know - a bit like burning fossil fuels?

Now as far as I'm aware there was no advanced civilisation which burnt CO2 prior to man, so no initiators of CO2 release. No CO2 release - therefore no initiation.

The fact that CO2 acts as an amplifier is exactly the kind of confirmation that we need to show that when there is a release of CO2 there will be an amplified temperature response and the record tells us the extent of that amplification, and confirms a climate sensitivity of around 3 degrees for a doubling of atmospheric CO2.

Once again. Nobody is saying that prior to the advent of mankind CO2 was any kind of initiator.

So the entire post misrepresents the position taken by climate scientists.

You know what that's called?  - A strawman.

muso wrote on Apr 13^th, 2010 at 9:07am:


... if there is already warming for other reasons. Isn't that what amplification means? It certainly does not mean 'initiate' or 'induce' warming/change/cooling. CO2 does not set anything in motion.


WHere does that the initial impetus come from then, for the current change? WHat is the cause of the change that CO2 amplifies?

Grendel wrote on Apr 13^th, 2010 at 1:00pm:


The problem I had was with the comments you added. As I said, what is written there makes sense, it's just that it doesn't click with you.

mozzaok wrote on Apr 13^th, 2010 at 2:35pm:



So what's the initiator of the current changes? It's not CO2.

Pst, Mozz - your amplifier does not make the music. Sorry, I know you are shocked. But it gets worse: it's not even your stereo that made the music.
Have a quick lie down, you must be shattered.  

To stay with your analogy, if that's easier to understand - We turned up the volume of WHAT?

What you are suggesting is that as far as you know there would be a steady state climate equilibrium if CO2 was kept steady and so whatever warming is occuring is because the extra CO2 is amplifying the otherwise unchanging climate.

Soren wrote on Apr 13^th, 2010 at 3:39pm:


Soren, the climate is never totally unchanging, but you're close.

What I'm saying is that  over a relatively short period of time (like the past 50 years - or 150 years - take your pick), any climate variation due to Solar variation has been relatively insignificant in the scheme of things. Any variation due to large stratospheric volcanoes has been of  short-term duration and provided a relatively minor perturbation.

The main effect has been due to the increasing CO2 emissions, and other greenhouse gases including methane and nitrous oxide (in order of importance - ie concentration and GHP). These GHG emissions come primarily from combustion processes involving fuels that contain carbon, but also from the clearing of Tropical rainforest and cement production. (Bake calcium carbonate - limestone - and it releases CO2.)  

Does that make sense so far?

I am particularly fond of this old denialist war horse. I like it because it is lifted from the earliest enhanced greehouse effect theories put together by climate researches back in the 1970s, perhaps earlier. Deniers simple quote the positive warming feedback part of the theory but leave out the first, rather crucial, bit about the current warming having been initiated by increased CO2 levels directly.

The problem for deniers is that unless they are very careful to also excise the feedback effects of carbon itself, they end up implicity accepting that CO2 causes warming, and this is what has happened once again today in this very thread.