Whatever the reasons, levels of CO2 are now higher than at any time in the current ice age (assuming the ice core records are being correctly interpreted), and that is 750,000 years.
I dont believe that this has any implications for worrying short term future temperature but there has to be a reason.
I do not believe that it has been caused by human activity for the following reasons:

Murry Salby shows a graph where human emissions of CO2 are plotted along with global levels of CO2 (on a year to year basis). This same graph is shown for totally different reasons by Gavin C Crawley (of dreaded UEA origin). This graph shows that human emissions of CO2 have been rising steadily year on year by a fairly consistent small amount. The total CO2 graph is all over the shop, up 2ppm one year and down 2ppm over the next two years. There is no correlation on a year to year basis.
You can see the graph here:
http://scienceblogs.com/deltoid/2011/08/04/murray-salby-and-conservation/
Critics of Salby claim that he is wrong to ignore the trend in total CO2 which no doubt would be a similar graph to human emissions.
These criticisms invite two questions.
First what really is the mechanism by which CO2 is supposed to warm the atmosphere and why is temperature not "all over the shop" just like CO2?
Second just what exactly is causing this dramatic year on year fluctuation in global levels of CO2?
My own conclusion is that the fluctuations of global CO2 show that human emissions are totally disconnected from total global levels of CO2.

My own "knowledge" of past temperatures over the life of the planet is from http://www.scotese.com/climate.htm
perhaps someone can point me at a better one? In this record most of the earth's history has been much warmer than today and could be said to be the norm. This actually fits quite nicely with the work of Svensmark.
You will know that Svensmark links the levels of cosmic rays reaching the surface of the earth to cloud formation and thus to cooling/warming. Experiments at CERN are confirming this.
The basic theory is that cosmic rays or sub atomic particles entering our atmosphere collide with molecules in the atmosphere and create new larger molecules. The large molecules that reach the surface are large enough to seed low clouds and so cool the earth.
The numbers of cosmic rays striking the surface of the earth are affected by a number of factors based on the sun.
The strength of the sun's magnetic field (Solar Wind) varies in cycles.
The earth's orbit around the sun is not circular and there are times when we are further away and times when we are closer.
The other sun factor is that the sun is moving around our galaxy, sometimes it is passing through one of the spiral arms and at other times it is far from other suns. This is important because when passing through a spiral arm we are closer to the super novas that are the source of all cosmic rays.
According to Svensmarks latest paper we are just leaving one of the spiral arms and about to return to deep space (this takes us away from any danger posed by the inhabitants of LV41 ^.^). The further from the spiral arm we get then the less cosmic rays will strike earth and the warmer it will become.
All of our records of sun spots (strength of the suns magnetic field) relate to a time when we are within a spiral arm and therefore receiving a lot of cosmic rays. We know that normally a solar minimum will mean cold but when we are out of the spiral arm I doubt it will make a difference.
Is the rise in CO2 some kind of effect of this transition that we are making from ice age to warm period?

My apologies Martin and as ever you are correct :)

Salby did not ignore the trend. He showed that the high correlation of CO2 to temperature, which explains observed changes between years, also explains much of the observed trend in CO2:

http://wattsupwiththat.com/2012/04/19/what-you-mean-we-arent-controlling-the-climate/

It seems to even explain the change in the trend.

MARTIN

Our "knowledge" of the Earth's past on CO2 informs us that in the earliest days of its history, the atmosphere was 80% CO2 or 800,000 ppm, now we understand that it is 400 ppm.
If the record at http://www.scotese.com/climate.htm is correct then we assume that during all of that time the average global temperature varied between 10 deg C and 25 deg C.
According to The University Corporation for Atmospheric Research the average global temperature at the moment is about 14.5 deg C.
Life on Earth has survived and developed through this range of temperatures although obviously it has had to adapt.
What worries me is not the fact that temperatures might rise by 10 deg C but that in relative terms CO2 has almost disappeared from the atmosphere.
At around 180 ppm all currently existing vegetation will die and all life dependent on that vegetation will die. This I find somewhat worrying.

Dung

At around 180 ppm all currently existing vegetation will die and all life dependent on that vegetation will die. This I find somewhat worrying.

This is puzzling. CO2 levels fell below 180ppmv for about 3000 years during the glacial maximum 660 - 670ka (Luthi et al. 2008) and there wasn't a planetary extinction of all vegatation and all life dependent upon it. Unless vegetation and all life dependent upon it re-evolved from scratch over the last 660ka.

Carbon dioxide levels are below 180 parts per million by volume (p.p.m.v.) for a period of 3,000 yr during Marine Isotope Stage 16, possibly reflecting more pronounced oceanic carbon storage. We report the lowest carbon dioxide concentration measured in an ice core, which extends the pre-industrial range of carbon dioxide concentrations during the late Quaternary by about 10 p.p.m.v. to 172–300 p.p.m.v.

Here's the close-up of the CO2 reconstruction from the bottom of the EPICA Dome C Antarctic ice core:

CO2 below 180ppmv (EPICA Dome C)

Now this could be wrong. It's from right at the bottom of a deep core. But *all* Antarctic cores provide evidence for CO2 at ~180ppmv during glacial maxima 400ka - present. If this was a threshold kill value for vegetation, you'd expect evidence of die-backs in temperate latitudes that could not be correlated with temperature. To the best of my knowledge, none has ever been found. If you have some references I'd be interested to see them, so please post up the links.

Dung

Going right back to your original questions about how we know it's us etc.

Look at the last ten thousand years (10ka) from ice cores. Everything bumbles along at the bog standard interglacial ~280ppmv CO2 for the entire period but then, suddenly, in the second half of the C20th, BANG!

Now, what's changed radically in terms of CO2 sources over the last 10ka? With a huge increase from about the mid-C20th sufficient to account for the observations? Yup, 'clever monkeys' is the correct answer.

The red bit on the inset graph of CO2 concentrations from 1750 (top panel) is the direct atmospheric measurement aka the 'Keeling curve' after the chap who got it going back in the 1950s.

Here's the classic plot, showing CO2 measured by the Mauna Loa station. Thanks to the good offices of WfT it's bang up to date: 1958 - May 2012. The annual cycle you see is the uptake of CO2 by vegetation in the NH summer and the release by decay during the NH winter. The steep upward trend is the result of anthropogenic CO2 emissions accumulating in the atmosphere faster than natural sinks can remove them.

Plants 'prefer' the light δ12C isotope of carbon, and CO2 formed by burning plants - and fossil fuels - has more of the light δ12C isotope. CO2 from other sources has more of the δ13C isotope. If the majority of the increase in atmospheric CO2 is from burning fossil fuels then the amount of δ13C *relative* to the amount of δ12C would fall. This is exactly what has happened.

See the detailed monitoring work presented in Keeling et al. (2010):

The 13C isotope is stable and heavier than the normal form of carbon (12C), and plants tend to selectively assimilate the lighter isotopes during the photosynthetic process. This results in the following features of the 13C/12C ratio in the atmosphere: (1) a seasonal cycle occurs with the heavier isotope at relatively high concentrations during the summer, as plants selectively remove the lighter isotope from the atmosphere, and (2) a general decrease with time, as more fossil carbon (which originally was plant material, and consequently biased toward the lighter isotope) is injected into the atmosphere from the combustion of fossil fuels.

δ13CO2 Mauna Loa 1977 - 2008.

Same thing, but at the South Pole.

Same thing but Alert station, NWT Canada, the northernmost monitoring station (the best approximation to the North Pole)

There are 11 monitoring stations in the study - you can view the rest
here. The fall in the relative amount of δ13C is clear in all data sets and is obviously global.

So, we've got a known, quantified source sufficient to explain an increase in CO2 unprecedented in the last 10ka (actually, vastly longer than that - CO2 only gets above ~285ppmv once (MIS9) in the last 800ka). On top of that, we've got unambiguous isotopic fingerprinting. A powerfully persuasive combination.

A final point. You referenced Deltoid in your original comment at the start of this thread. The article there is helpful and I recommend that you revisit it.

Bearing our previous discussion about the lower bound for CO2 during glacial maxima, you might want to pay extra attention to the concluding quote from John Nielsen-Gammon. JN-G is recalling his discussion with Selby after the latter's presentation (emphasis mine):

In discussing what his [Salby's] model would mean for past variations of temperature and CO2, it eventually became clear that he believed all paleoclimate data that supported his statistical analysis and disregarded all paleoclimate data that countered his statistical analysis, even though the latter collection was much larger than the former.

Eventually I realized that if 0.8 C of warming is sufficient to produce an increase of 120ppm CO2, as Salby asserted, then the converse would also have to be true. During the last glacial maximum, when global temperatures were indisputably several degrees cooler than today, the atmospheric CO2 concentration must have been negative.

That was enough for me.

The big picture suggests that the increase in atmospheric CO2 and sunspot numbers don't correlate.

And what is the source of the light carbon, if not fossil fuels? Let's not forget that we *know* that we are adding about 30GT CO2/annum to the atmosphere. So, if it's not us, then what on Earth is it and why does our contribution somehow disappear? ;-)

Dung

Okay, never mind me, but what's up with WUWT? :-)

Ferdinand Engelbeen has a four part series there of guest posts explaining why the measured increase in CO2 is anthropogenic.

Part 4 has links to parts 2 and 3.

If plants prefer C12 CO2 then there would very soon be no C12 CO2 left unless its was constantly being replenished; by what?

Burning coal made of fossil plants with a fossilised light isotopic signature. I mentioned this above @ Jul 1, 2012 at 7:20 PM: Keeling et al. (2010).