[steve williams]

steve williams asked the Naked Scientists:

No one can deny that the Earth is warming and global weather is changing. - But I do wonder what evidence we have that suggests CO2 is the key contributor to that warming. Is CO2 simply being given bad press backed by a political agenda and powered by public hysteria or is there hard statistical evidence linking CO2 and Global Warming?

Many thanks

What do you think?

[LeeE]

The evidence that CO2 is linked to climate change comes from a number of different sources but one of the most easily understood sources is ice cores.  The bubbles of air frozen in the ice cores show different concentrations of CO2 and when these variations are plotted over time they correlate with climate changes in the past.  I believe another similar source are the fossilised shells of marine crustaceans and molluscs, which show the same trends.

Overall, it's because there seems to be a large degree of correlation between all the different sources that the issue is being taken so seriously.

[frethack]

he bubbles of air frozen in the ice cores show different concentrations of CO2 and when these variations are plotted over time they correlate with climate changes in the past.

This is a bit of a misnomer.  When the CO2 correlation was first discovered in the GRIP cores, scientists had a very low temporal resolution, and they first suspected that CO2 was a main driver of past climate.  Now that we can interpret the ice cores on centennial and decadal scales, we have found that rises in CO2 concentrations generally lag temperature increases by about 800 years, which quickly overturned the previous school of thought.  This is not to say that CO2 isnt providing some radiative forcing in the modern warming, but only that it was a temperature dependent feedback mechanism in the recent and distant past.

I believe another similar source are the fossilised shells of marine crustaceans and molluscs, which show the same trends.

You are correct.  Oxygen 18 uptake in marine carbonate shells correlates very well with the improved temporal resolution from the GRIP cores.

Again...I am not asserting that CO2 has no effect on our current climate.  To the contrary, I believe that we have some effect on our climate through industrialization...especially through deforestation and land use changes. 

Recently, it has been argued that temperature correlates better with Be 10, O 18, and C 14 proxies, and this data shows very strong trend lines that follow interpreted temperature very closely.  Be 10 and C 14 are solar induced (well...actually by cosmic radiation, but that is moderated by solar magnetic activity), which would indicate that temperature is closely tied to solar activity.  Some believe by luminescence and radiance, and some believe by feedback mechanisms such as cloud nucleation which would increase the earths total albedo.  There is a lot of current research in these areas, especially with overly anomalous weather during our current solar minimum and the lack of appreciable warming since the very moderate solar cycle 23 (the current one) began in 1996 (actually the warming leveled off in 1998 after we neared solar maximum).  Many studies are also being conducted over solar cycle length and temperature, which seems to also correlate very well (our current cycle has lasted a very long 12 years...they averaged between 9 and 11 in the 20th century).

[LeeE]

I don't want to seem to be 'nit-picky' but while the questioner pondered the issue of whether CO2 causes climate change, the actual question asked came down to: Is there a hard (indisputable) link between CO2 and climate change?

Which there is.

I believe that the temporal resolution of the early ice core data was always considered to be a qualifying factor.  Everyone knew that the initial ice core studies couldn't prove cause & effect, because of the resolution, but the degree of correlation could only be regarded as very siginifcant, especially in view of the time scales being considered.  As you say yourself, it was suspected, but only the most excitable claimed it as proof.

Thanks for mentioning that the resolution now seems to have be cleared up - I hadn't seen that.

I agree that it certainly isn't a straight-forward, clear-cut issue.  As you point out, there also seems to be correlation between Solar activity, and it's the Sun that ultimately drives everything that happens on Earth.

However, even while elevated CO2 levels couldn't be regarded as proof of cause, it had to be regarded as alarming.  If elevated CO2 levels are an effect and not a cause, it's difficult to see how a scenario where elevated CO2 levels predate climate change is going to end well.

As CO2 is heavier than 'air' I guess it could be claimed that the sky is falling []

[frethack]

I don't want to seem to be 'nit-picky'...

Be as nit-picky as you like!  []  Thats why I love this site...you can have a civil and open discussion about detail without fear of flaming and knee jerk reactions.  There are other sites that I wont mention that do not have this luxury.

However, even while elevated CO2 levels couldn't be regarded as proof of cause, it had to be regarded as alarming.  If elevated CO2 levels are an effect and not a cause, it's difficult to see how a scenario where elevated CO2 levels predate climate change is going to end well.

I would absolutely agree that there is cause for concern, and much further research is needed not only into detrimental effects of a warming climate, but also into beneficial effects as well.  The climate will continue to warm, even if we halted all man-made greenhouse emissions today, and it is essential that we learn to live with it.  Im not sure that we should be entirely 'doom and gloom' about a changing climate, but adding additional anthropogenic CO2 into the climate system without knowing its saturation point (CO2 acts logarithmically...the higher the concentration the less the added effect) is probably not the greatest of ideas.