How does the present Co2 concentration compare with those throughout earth history.?

Question

I've read, that for most of earth's history the co2 level has been much higher and very rarely lower.

Further, it's been claimed that the earth's temperatures have been much cooler even in times of high concentration.

If this info is correct doesn't it cast serious doubt on the current AGW theory?

I've also heard about a greenhouse signature that has yet to be found. How important is this signature to the theory?

Answer 1

True, but half the story. Throughout most of Earth's history, the Sun has been a lot cooler than it is now. Only the presence of high levels of CO2 in the atmosphere made early Earth habitable at all, under such cool-sun conditions.

http://www.sciencemag.org/cgi/content/ab...

So no, it doesn't cast doubt on AGW theory at all; in fact, it strongly SUPPORTS the theory.

The "signature" that had been alleged not present, is a bogus story.

When CO2 in the air absorbs outgoing infrared from the earth, it gets warmer. This heat causes the surface of the earth to get warmer too. Since the CO2 is in the air, the mid and lower troposphere should be warming up just a bit faster than the surface ... just a very, very small bit, mind you, but it should be there. And until recently, we couldn't find it.

Here's the problem: it turns out to be pretty darned difficult to measure air temperatures from space, especially air temperatures above the ground. Early satellite measurements had not been calibrated quite right because of decays in the satellite oribt. (Satellite orbits decay at irregular rates, and this is hard to predict.) Once the problem was found and the data re-calibrated, the small excess warming above the surface appeared, just like it was supposed to.

Oddly enough, the biggest, easiest "signature" for increased greenhouse effect isn't that tiny little difference between the surface and the troposphere; it's the big, big difference between the surface and the stratosphere. As more heat is trapped at the surface by more greenhouse effect, less heat escapes into the stratosphere, which means that the stratosphere should be cooling. And in fact, it has been cooling for as far back as we have records. Here's the data:

http://hadobs.metoffice.com/hadat/images...

Answer 2

CO2 has not been higher than current levels in millions of years.  When it was higher, global temperatures were usually hotter (despite the Sun being cooler than today).

http://ff.org/centers/csspp/library/co2w...

It's possible that there were some exceptions to this.  Of course, this doesn't disprove AGW, because AGW recognizes that there are many factors which impact global temperature.  For example, you could have a series of massive volcanic eruptions and/or a large meteor striking the Earth and kicking large amounts of dust into the atmosphere, blocking sunlight and thus causing cool temperatures despite high levels of atmospheric CO2.

As for the 'greenhouse signature', I assume you're referring to the tropical troposphere 'hot spot'.

For one thing, this 'hot spot' should exist regardless of the cause of warming.  For example, solar warming would also create a tropical troposphere hot spot.  

http://www.realclimate.org/index.php/arc...

Secondly, we don't know if such a hot spot exists because of problems with the radiosonde data.  Recent research shows that there may not be a model-data discrepancy after all.

"To conclude, the structural uncertainty in the radiosonde data is large, and while these attempts to improve the homogenisation are a step in the right direction, the degree of adjustment is a concern. The bottom line is that the observations may well be closer to the model data than preliminary analyses suggested but that the structural uncertainty remains high. Coming to dramatic conclusions based on any of this remains unwise."

http://www.realclimate.org/index.php/arc...

Of course there is an even better 'greenhouse signature' - that being the cooling of the upper atmosphere, which is consistent with AGW and inconsistent with solar warming.  This signature is quite evident.

http://www.realclimate.org/index.php/arc...

http://www.ssmi.com/msu/msu_data_descrip...

Answer 3

If you look at the graph that Bubba supplied, you will notice that CO2 levels have obviously risen naturally for the last few thousand years as we have warmed.  Long term trends indicate we live in fortunate times, at near temperature maximum of the interglacial period of our current ice age.  Even though CO2 levels have risen in the past, the period of glaciation still occurred indicating that CO2 was not the driver of that climate change.  The periodicity corresponds to the Milankovitch Cycle which is determined by variability in our orbit and axis of rotation.  I am skeptical of the accuracy of the graph but in any event, we are naturally at a point that would be near record for high CO2 concentration for 600,000 years simply because of our fortunate circumstance to be born at this time.  The exact amount of extra CO2 that humans may have added is debatable and the effect is also debatable.  IMO, the political left has used that extra CO2 to exaggerate the potential consequences to push their anti-capitalist agenda.  They exaggerate the consequences, exaggerate the level of their knowledge, underestimate potential benefits, underestimate the costs of the proposed solutions, and always tell us we only have 10 years to act or we may all die.

Answer 4

According to all the hard science people I have read it matters a lot!

Answer 5

Unfortunately, this is the highest the CO2 concentration has been in the last 400,000 years looking at ice core samples and direct measurements.  Other evidence of warming is also presented.

http://climate.jpl.nasa.gov/evidence/

Much of this increase is due to human activity. I think this calls into question you whole argument denying the problem more than anything else.  Face the facts.

Answer 6

CO2 is not a easy gas to teas for. They find gas in ice and it is CO2 what caused it. Was it methane that was trapped with oxygen and over time the oxidation occurred. CO2 is a very heavy gas it is used to smother a fire . If U measure at your wast it is much higher at ground level.