Ok. So how is it that those animals evolved...got to this point... to begin with? I don't think I need to provide a reference supporting the premise that marine animals with calcium carbonate shells have been around for hundreds of millions of years. We all see pictures of fossils of them. They were around long before vertebrates were. Yet the atmospheric carbon dioxide level of today, in geological terms, is believed to be close to the lowest it's ever been.
See the model output at the bottom of the page at the NOAA web page at ftp://ftp.ncdc.noaa.gov/pub/data/paleo/ ... ic_co2.txt" onclick="window.open(this.href);return false;. "RCO2" means the ratio of atmospheric CO2 to present (I looked it up to be sure). Don't know why "0" on the time scale shows 0.98 instead of 1.0 but that's what "RCO2" means.
But anyway, that 0.98 at the "0" mark is the lowest value in the output. And the next value up, 0.99 at 10 million years ago, is the next lowest. We see pictures of ammonite shell fossils all the time. The linked model output includes the estimate that, when the ammonites were swimming around the oceans around 240 million years ago (it's believed), the Earth's atmospheric carbon dioxide level was 7 times what it is now. The article at http://www.dmns.org/main/minisites/foss ... rtres.html" onclick="window.open(this.href);return false; says ammonite shells required less calcium carbonate due to their structures. But the article at http://animals.nationalgeographic.com/a ... ammonites/" onclick="window.open(this.href);return false; says they fed on crustaceans; which also need carbonate ions for their shells. As far as I can tell crustaceans appeared between 500 and 600 million years ago. The article at http://www.cell.com/current-biology/abs ... %2901127-X" onclick="window.open(this.href);return false; describes one believed to have been around at about 520 million years ago. The model output estimate for 520 million years ago is 26 times current atmospheric CO2 levels TWENTY SIX times.
Marine animals that need carbonate ions for their exoskeletons have been around a very long time. There are other estimates of historical CO2 levels. The figure below depicts some and also depicts the model output I've been discussing. But they all suggest that atmospheric CO2 levels are very low in historical relative terms now and were much higher throughout most of the history of existence by those marine animals that need carbonate ions for their exoskeletons.
I am guessing someone has an explanation for why there is still reason for concern because the information is too obvious for someone not to have thought of it. But it's still something to think about.











