This is the second of two articles focussing on stories highlighted by the Climate Contrarians over at the Register.
Over the last 0.8 million years, the mean temperature of the Earth’s Surface has been highly correlated with the atmospheric concentration of carbon dioxide. The figure below shows the data over the last 0.4 million years and there is an obvious correlation between global mean temperature (blue: inferred from the relative amounts of oxygen isotopes found in water in ice cores) and atmospheric CO2 concentration (green: deduced form the concentration of CO2 in bubbles in the ice). Very roughly 100 ppm CO2 corresponds to around 10 °C of warming – a truly terrifying statistic if it holds for warming as well as cooling. The role of carbon dioxide is perfectly clear.
10 million years ago there was less CO2 – but the Earth was WARMER.
The unspoken implication is that scientists don’t understand the Climate or the role of carbon dioxide, and so maybe carbon dioxide is not necessarily causing the current bout of global warming. This implication is mischievous. Just to put the time scale into context, there were of course, no human beings alive during this era. Our evolutionary path is unsure but modern humans probably evolved a little over 1 million years ago.
All those years ago, the continents were not in very different positions from the present day: for example the Atlantic was only around 100 km narrower. And so one might assume that oceanic and atmospheric circulation were probably similar. However, it is well known that this was not the case.
One of the key differences was that the Central American Seaway closed around 5 million years ago, creating a land bridge between North and South America. This closure process – partly volcanic and partly caused by plate movement – gradually split a single pan-global ocean into two, creating separate Pacific and Atlantic Oceans. Other sea passages have also changed significantly over this period, for example the Bering Straits, which affects communication with the Arctic Sea. But the closure of the Central American Seaway is probably the most significant.
This article by the Woods Hole Oceanographic Institute explains how this process dramatically affected oceanic and atmospheric circulation and led to the creation a few million years later of the Arctic ice sheets. Incidentally, the article points out that the Antarctic was already frozen at this point, an event triggered by the opening of the another ocean-linking region. The opening of the gap between the tip of South America, and the northern end of the Antarctic Archipelago around 34 million years ago allowed circumpolar weather systems to stabilise over the Antarctic, de-coupling it from the transport of heat from the Equator.
So in the era under discussion, ocean and climate circulation is known to have been significantly different from the present day. Given this background, the paper presents evidence from isotopic analysis of fossils for both sea-surface temperature and carbon dioxide concentration in the atmosphere – amazingly difficult data to get!
The key experimental result of the paper is shown in the figure below. The upper part shows estimates of sea surface temperatures rising as we go back in time by roughly 1 °C for each 1 million years. The lower part shows estimates of carbon dioxide concentration. I have added the current level of atmospheric CO2 concentration for reference. The gist of the paper is that back to around 5 million years ago, the elevated sea surface temperature is correlated (roughly) with CO2 concentration. But further back CO2 concentrations are lower, while sea surface temperature remains high.
The link between atmospheric carbon dioxide and global mean temperature is very clear from ice core data (purple) back to 800,000 years ago. Going back further to about 3 million years ago the data on carbon dioxide concentration are very widely spread and beyond 4.5 million years ago, there is very little data. To the extent that the older data is trustworthy, it indicates that 10 million years ago atmospheric CO2 was about the same level as it was in the late twentieth century (roughly 350 ppm) but the estimated global mean temperature was around 10 °C warmer.
The main author LaRiviere suggests some ways in which, with a radically different oceanographic and atmospheric circulation, this de-coupling of global temperature and carbon dioxide concentration might have occurred. In particular, he highlights fossil evidence for differences in the temperature profile of the surface layers of the oceans.
Despite reading the paper several times, I confess I could not follow all his arguments in detail. However I doubt The Register had studied the paper at all, because they make no mention of this radical change in oceanographic circulation. Instead they conclude with a quote from the author:
“It’s a surprising finding, given our understanding that climate and carbon dioxide are strongly coupled to each other,” LaRiviere says.
“In the late Miocene, there must have been some other way for the world to be warm.”
The Register‘s implication is that ‘this other way’ has not been understood by modern Climate Science. And indeed it hasn’t been fully understood: that’s why this is a paper in Nature! But LaRiviere is doing his best.
Does his paper yield any insight on our current situation? In my opinion, not directly. However:
- It does highlight how relatively small geographic changes can radically affect Earth’s Climate.
- It does highlight that ice free arctic regions were possible with a world around 10 °C warmer than the present day
- It does highlight that the estimated 0.75 °C warming in 100 years that humans have achieved is a little bit less than a MILLION times faster the rate of temperature change seen on the Figure.
However these are not insights to which the Climate Contraians over at the Register wish to draw to our attention. Instead they relish confusion. How I wish they would instead take the time to present scientific papers in context, and then instead of this turgid article, I could write short articles saying what great journalists they had, and we would all be the wiser. C’est la vie.