What is the ideal percentage of CO2 in the atmosphere?

Photosynthesis stops when you go below 180 ppm. That would kill ALL plant life and by extension ALL higher order life forms on the planet. So it is over 180 ppm.

Much of our current flora and fauna are stressed at CO2 levels below 300 ppm. I can not remember the study but when the reults of proper CO2 levels for a large portion of current flora and fuana it was suprising to many scientists.

My personnal beleif is while we know some absolute lower limits that we should not drop below, and some upper limts which would put those who breath O2 under stress and in danger, we do NOT know what the "ideal" CO2 concentration is. I do not beleive a true scientific study to define this has been performed.

James, your question is over-broad, but answerable. Where you get your bad information from, you will have to answer for yourself, however. Just fyi: I don't "believe" in global warming, I *trust* in science, and science tells me the world is warming and primarily from human actions. I certainly do not, however, believe "any increase in CO2 would be catastrophic."

Let's be totally honest here, the animals we favor are human, and so the answer to your question is what the ideal percentage for humans is. Now, I presuppose a working and thriving and varied ecosystem, as that is required for human long-term needs as well as that of all the other life on Earth.

How do we tell what the ideal number for humans is? Well, let's look at the situation from 2 points of view, what are humans doing, and what is the climate doing? Humans are growing crops in areas where the climate has been fairly stable for some time. But "fairly stable" in human terms leaves a good bit of leeway, and the long term climate trend was a slowly dropping temperature as the planet headed into the next Ice Age. So a bit more CO2 in atmosphere to warm things up a little would be fine, but only a bit. We have clearly gone past "a bit". The storms we're seeing, and the droughts, heat waves, floods, even some of the terrible winter storms we see in the middle and higher northern latitudes, these are amped up by the extra energy in the climate system. We're also pushing 400 ppm. So we can probably safely say that 400 ppm is a bit high, but 280 is a bit low. Splitting the difference gives 340, which falls very much in line with recommendations from scientists that it would be very good if we held atmospheric levels of CO2 to 350 ppm or less. We do not need brutal heat waves, devastating rains and floods, fires, droughts or the occasional ridiculous cold spells, because these things just totally screw up our crops. As we are seeing.

Humans also ship food from ports that have had the benefit of a highly stable sea level for thousands of years - we know, because Roman ports on the Mediterranean are still about at sea level now, 2000 years later. Yet the ice is melting around the world, and rising sea levels, coupled with worse storms, will hammer ports around the world ever harder, causing ever more damage to infrastructure vital to world trade. So we want the ice to stop melting. Again, this means we need to drop the amount of CO2 in atmosphere by about 50 - 60 ppm or more. We don't want to go 300 or below, because that would likely eventually put us back on that slow slide to ice advances. A lot of people depend on meltwater for crops. And peak flow is coming earlier and dropping lower for the ice-fed rivers. This tends to cause late-season water shortages, which hurts crops. You want most of that ice back. That implies you'd probably want 310 - 325 ppm, as a guess.

What you really want to do is warm things up a little bit, but don't let much ice disappear, so you get a better overall growing season without too much weather disruption. Based on current evidence, we've already gone past that sweet spot. All in all, I'd suggest 325 /-15 ppm CO2. ;)

In a reasonable sense, the truest answer is "not too different from what it was 100 years ago".

The problem is not climate so much as change in climate. If our civilization had arisen during one of the periods when the Earth was ice-free, then the sudden (as in, within a century) onset of an ice age would have been catastrophic. However, going from full-on glaciers-in-Chicago ice age to our current climate in 100 years would have been similarly catastrophic.

Every living thing alive today (including us) is used to the climate being more or less as it has been. Natural climate change is slow--a 1C change in the long term average temperature over 500 years is (as far as we can tell) fairly fast. And a slow change is easy to adapt to. If temperatures were going up less than 1C/century (and likely to *remain* at that speed, I know we only had about .8C in increase last century), then this would be a tolerable rate of change.

Most living things could adapt (mostly by moving to the area that now has the appropriate climate conditions for them). And human infrastructure could adapt easily. Sea level changes would be slow enough that by the time we needed to move a port, we probably needed to replace the buildings anyways. Our crops could adapt to changes in climate, or farmers could adapt to changes by planting new crops, without anyone suddenly having to switch entirely from one type of crop to a completely different type. And so on.

But, the thing is, we're not going to get that slow change with 400+ ppm of CO2. If we'd kept to, say, a 15-20% change in CO2 concentration per century (so, if I have the math right, ending up around 325 this century), then we'd be golden. We could go up to about 390 by the end of next century and still be on track. And so on.

But, a 40% or so change in much less than a century? Serious bad news for anyone who cares about living things, including most humans.

They seem to believe that 200 ppmV is ideal as they are sure that is what it would be without those evil fossil fuels. It probably isn't desirable that we are pumping huge quantities of CO2 into the air but I am still unconvinced it isn't actually generally better overall for most of the environment even assuming it is causing warming.

The question is without answer. Ideal to whom or what? Plants love CO2, so the higher the better. As I understand it for us, we breath automatically because of CO2 levels so I don't know what level is dangerous.

Well, we were at 280 ppm prior to the industrial revolution. My best estimate would be from 280 to 340 ppm.http://www.esrl.noaa.gov/gmd/ccgg/trends/history.h...

>>believers in AGW believe any increase in CO2 would be catastrophic. <<

Individually, stupidity and lying are often excusable; however, the only excuse for telling such stupid lies is either terminal stupidity or pathological liar.

800 to 1200 ppm that is what greenhouse growers strive to maintain for ideal plant growth.

1500 ppm

The same concentration that it would be without human interferenece. 270ppm.