Humanity Blamed for 9,000 Years of Global Warming

Not satisfied with blaming modern civilization for causing rampant climate change, now a small group of scientists are saying that everything went wrong once humans stopped being hunter-gatherers 9,000 years ago. The hypothesis, first advanced in 2003 by University of Virginia palaeoclimatologist William F. Ruddiman, remains controversial even among global warming true believers. Ruddiman now claims to have proven his critics wrong, much to the glee of the blame-humanity-first wing of the eco-activist community.

A paper in Quaternary Science Reviews by Ruddiman and Erle C. Ellis, entitled “Effect of per-capita land use changes on Holocene forest clearance and CO2emissions ,” claims Homo sapiens were bad news for the environment from humanity's first steps toward civilization. While most scientists discount the impact primitive humans had on the environment due to small populations and rudimentary tools, Ruddiman and Ellis think otherwise. They are proponents of the early anthropogenic hypothesis (EAH), which states that humans took control of greenhouse-gas trends thousands of years ago because of emissions from early agriculture. According to the paper:

Implicit in this view is the notion that per-capita land clearance has changed little for millennia, but numerous field studies have shown that early per-capita land use was large and then declined as increasing population density led to more intensive farming. Here we explore the potential impact of changing per-capita land use in recent millennia and conclude that greater clearance by early agriculturalists could have had a disproportionately large impact on CO2 emissions.

This hypothesis posits that thousands of years ago early agriculture caused large enough emissions of greenhouse gases to offset natural climatic cooling. In earlier work, Ruddiman and others claimed that the spread of rice production across large areas in China 4,000 to 6,000 years ago supported the hypothesis. Supposedly, early farming caused an anomalous reversal in methane levels (see Ruddiman et al.Early rice farming and anomalous methane trends”). Writing in the March 2005 edition of Scientific American Ruddiman pushed the date for man's earliest environmental transgressions back another 2,000 years, saying: “[E]vidence suggests that concentrations of CO2 started rising about 8,000 years ago, even though natural trends indicate they should have been dropping. Some 3,000 years later the same thing happened to methane, another heat-trapping gas. The consequences of these surprising rises have been profound.”

Now the threshold for the beginning of anthropogenic global warming dates to the Neolithic Revolution some 9,000 years ago. Based on a comparison of CO2 and methane (CH4) levels during the current Holocene interglacial versus data during similar portions of the three previous interglacials, human agricultural activity supposedly caused greenhouse gas levels to rise when they should have been shrinking. As a consequence of man's meddling Ruddiman claims that an incipient ice age, which would have begun several thousand years ago, was avoided. Without these early anthropogenic emissions, the theory goes, current temperatures in the northern parts of North America and Europe would be cooler by 3 to 4° C. Instead of a quick return to glacial conditions—something not expected if previous glacial terminations are any guide—Earth's climate has remained warm and stable for thousands of years. Are we supposed to view this as a bad thing?

The crux of the new paper's argument seems to be that early farmers used ten times as much land per person as modern farmers. Supposedly, ancient farmers practiced slash-and-burn agriculture, as some farmers in underdeveloped regions do to this day. Burning off large areas of forest or grassland temporarily enriches the soil with ash. After a few seasons crop yields begin to drop causing the farmers to move on to new land and repeat the cycle. However, the only way to reach the prehistoric land use levels required to make the theory viable is for neolithic farmers to have practice slash-and-burn on a tremendous scale. Naturally, Dr. Ruddiman is not without his critics.

A number of scientists have criticized the early anthropogenic hypothesis. One critic is NASA climate modeler Gavin Schmidt from the Goddard Institute for Space Studies (GISS). Schmidt states that crediting early human populations with the enormous environmental impacts that Ruddiman claims is extremely uncertain. He also says that recent studies of methane emissions have shown that methane increases over the last 5,000 years could be attributed to the development of the boreal wetlands and other areas uncovered by melting glacial ice. Indeed, a 2006 paper by Sergey A. Zimov et al. claims methane emissions from defrosting tundra are more significant than previously thought.

Zimov et al. state that carbon contained in permafrost—permanently frozen ground—is an additional large carbon reservoir that is rarely incorporated into global carbon analysis. “The release of a large pool of radiocarbon-depleted carbon from permafrost could have contributed to declines in atmospheric radiocarbon during two strong warming events that occurred during the last deglaciation,” they claim. “These radiocarbon changes have previously been attributed to an assumed increase in deep- and mid-ocean venting, because no terrestrial pool that could readily release ancient carbon (such as permafrost carbon) was included in the analysis.” In other words, as the glaciers retreated and the ice melted significant amounts of carbon, in the form of CO2 and CH4, would be released from formerly frozen land. As tundra turned to grasslands and forests greenhouse gases would be released naturally.

In fact, a recent paper in Nature claims that tundra doesn't even need to fully defrost to emit significant volumes of greenhouse gas. The assumption has always been that a frozen, snowed-under environment did not release much in the way of GHG—turns out that assumption was wrong. The winter emissions, thought to be squeezed out by the growth of surface frost, match up with an atmospheric methane surge that had previously gone unexplained. “Mother Nature is showing us something that is really surprising," said lead author Torben Christensen, a biogeochemist at Lund University in Sweden. “Nobody would expect to have loads of gas seeping out from a frozen environment.” Since these sources of carbon have been overlooked scientists are free to cast about for other sources, like human activity.

Ruddiman's conclusions rest mainly on Vostok ice core data going back hundreds of thousands of years. The EAH claim is that this interglacial is different from previous ones and only humans could have caused that difference. However, calculations of the changes in the Earth's orbit that trigger ice ages, performed by Andre Berger of the University of Louvain-le-Neuve in Belgium, demonstrate that the current warm period is actually quite anomalous compared to the recent past. This means that comparing the Holocene with other recent interglacials is not meaningful. Indeed, recent results from the extremely long EPICA core show values in Marine Isotope Stage 11 are very similar to those seen in the pre-industrial Holocene. That interglacial, which occurred some 400,000 years ago is thought to be more like the Holocene than other, more recent interglacials. One really interesting thing about that interglacial is that it lasted significantly longer than other warmings—around 30,000 years verses 10-14,000 for more recent episodes. [For a good article on methane and its sources see “Methane: A Scientific Journey from Obscurity to Climate Super-Stardom” by Gavin Schmidt on the NASA GISS website].


The Stage 11 interglacial 400kya was similar to the Holocene.

If Dr. Ruddiman had looked a bit farther back in the paleoclimate record he would have found that the Holocene is not an anomaly, and if he had checked the recent research regarding GHG release during the early Holocene he would have had nothing to blame on our remote ancestors. There is nothing wrong with Dr. Ruddiman dusting off his theory ever couple of years and publishing a new paper. Theories like his force other scientists to critically review what they think they know about human beings and climate change, double check their data and re-validate the logic of their own theories. This is a good thing for science. Without improbable theories challenging existing dogma science becomes static and moribund. No, the problem with the airing of such theories comes when they get picked up by the news media and become fodder for the climate catastrophist crowd.

Even a staid, conservative newspaper like the Economist has picked up on Ruddiman's misanthropic message. In an article entitled “Nothing new under the sun,” it was reported that: “Long before the Industrial Revolution—indeed, long before a worldwide revolution in intensive farming, the results of which kept humanity alive—people caused unnatural exhalations of greenhouse gases that had an impact on the world’s climate.” The article closes with the observation “it looks as if humanity has been interfering with the climate since the dawn of civilisation.” What isn't mentioned, of course, is that all living things interact with the environment and to some extent affect climate—ants to elephants.

What gets lost in all of this shallow reporting is that the early anthropogenic hypothesis is not proven nor widely held by other paleoclimatologists. Oddball theories are useful in science but are easily blown out of proportion by elements on the lunatic fringe and if a marginal idea gains political momentum you end up with a mess like the current CO2 driven global warming debacle. The only cure for this sad state of affairs in a more informed and better educated public, and fewer air-head reporters who are more interested in a catchy headline than accurate reporting.


The tundra is melting! It must be those humans heating up the planet again.

The easily dismissed EAH example aside, there exists a significant thread that runs through much of the bad climate science done over the past several decades. The mainstream climate science community seems to have blinkered itself with respect to historical precedents in the climate record. The IPCC seems only interested in the past few hundred years, probably because the well documented climate variations which occurred over the course of the Holocene don't fit their models or their world view. Others, like Ruddiman and his colleagues, only go back far enough to convince themselves they have stumbled on something “unprecedented,” which naturally means they can blame it on H. sapiens. Moreover, none of the CO2 driven global warming proponents wish to acknowledge that there have been ice ages when the levels of carbon dioxide in Earth's atmosphere have been many times higher than today's. Climate science's myopic view of the past allows it to continue attributing climate change to CO2 and the blame to humanity with a clear conscience.

Be safe, enjoy the interglacial and stay skeptical.

Humans and interglacial periods.

Let's have a look at some of the interglacial periods of the Quarternary (the ice age we are currently in): Of the last four interglacials the two youngest, called “normal” interglacials, from 12,000 years and 125,000 years ago; and two older phases, called “super” interglacials, from 400,000 and 1.1 million years ago.

According to climate reconstructions based on pollen found in sediment cores, summer temperatures and annual precipitation during the super interglacials were about 4 to 5 degrees C warmer, and about 12 inches more saturated, than during normal interglacials.

The Holocene interglacial (that we are currently in) is considered a "normal" interglacial and as such is 4 to 5 degrees C cooler than the super interglacials, and is actually cooler than the last interglacial (125,000 years ago) as well.

Problem? Homo sapiens have only been around for the last 200,000 years, who are they going to blame for the super interglacial temperatures?

Homo heidelbergensis?

UPDATE: Humans off hook for Holocene warming

According to a new article in Nature, humans are off the hook for the early Holocene rise in methane ~5000 years ago. That rise in methane, which is often cited as the cause of the Holocene warming, has been found to have natural origins. In “Late Holocene methane rise caused by orbitally controlled increase in tropical sources,” by Joy S. Singarayer et al., it was those old orbital cycles that caused the methane release. Here is the abstract:

Considerable debate surrounds the source of the apparently ‘anomalous’ increase of atmospheric methane concentrations since the mid-Holocene (5,000 years ago) compared to previous interglacial periods as recorded in polar ice core records. Proposed mechanisms for the rise in methane concentrations relate either to methane emissions from anthropogenic early rice cultivation or an increase in natural wetland emissions from tropical or boreal sources. Here we show that our climate and wetland simulations of the global methane cycle over the last glacial cycle (the past 130,000 years) recreate the ice core record and capture the late Holocene increase in methane concentrations. Our analyses indicate that the late Holocene increase results from natural changes in the Earth's orbital configuration, with enhanced emissions in the Southern Hemisphere tropics linked to precession-induced modification of seasonal precipitation. Critically, our simulations capture the declining trend in methane concentrations at the end of the last interglacial period (115,000–130,000 years ago) that was used to diagnose the Holocene methane rise as unique. The difference between the two time periods results from differences in the size and rate of regional insolation changes and the lack of glacial inception in the Holocene. Our findings also suggest that no early agricultural sources are required to account for the increase in methane concentrations in the 5,000 years before the industrial era.

Looks like another blame-humanity-first theory bites the dust. Remember, it pays to stay skeptical.

real global warming views

Someone who respects Nature and regards mankind as a part of a larger whole would never dream about using up non-renewable resources in a short time nor would this person contaminate the environment with gigantic amounts of pollution. By contrary, someone who respects Nature and regards mankind as a part of a larger whole would in all decisions carefully evaluate any effects on Nature. The preservation of Nature would be given a very high priority. On this base, it wouldn't have been possible to deny and ignore global warming for more than 20 years!

historical CO2 levels

A paper in today's news (10/9/09) [to appear in Science] seems to contradict the statement about CO2 being many times higher during ice ages. It would be good to document that statement. What ice ages? How long ago? What was the condition of the planet?

Which Science article did you mean?

There were two related articles in the October 9 issue of Science: “Monsoons and Meltdowns” by Jeffrey P. Severinghaus and the more comprehensive “Ice Age Terminations ” by Hai Cheng et al. on which the first article is a perspective. What both articles report is that “the last four meltdowns began when northern sunshine was intensifying, in accordance with the classical Milankovitch or astronomical theory of the ice ages.” When an interglacial starts CO2 levels do increase from the lower levels of the preceding glacial period—this is part of the feedback loop that amplifies the termination warming trend. None of this is real news, the real purpose of the Cheng paper is to report a link between melting northern ice sheets and weakened monsoons.

My statement that there have been ice ages when the level of CO2 in the atmosphere was much higher than today was not a refernce to the conditions that have prevailed during the Pleistocene Ice Age, which has been going on for the past 3 million years or so. It was a reference to previous ice ages of which there have been many. For details see my article, “The Grand View: 4 Billion Years Of Climate Change.” There have indeed been ice ages when CO2 levels have been several times higher than the “unprecedented” levels so alarming to the climate catastrophists.

New Study Contradicts EAH Claims

A new study in Nature contradicts the claims made by Ruddiman and other EAH proponents. In a paper entitled “Stable isotope constraints on Holocene carbon cycle changes from an Antarctic ice core,” Joachim Elsig et al. used an isotopic analysis of ancient air trapped in Antarctic ice to show that humans caused little if any of the preindustrial buildup of CO2. The findings negate previous thinking about the role of early humans in the process and should help scientists develop better baseline models to forecast future climate. The preindustrial bump that EAH attributes to humans was natural accorting to the report, a combination of vegetation buildup after the ice age and, more prominently, the slow reaction to this change by ocean chemistry. Looks like H. sapiens is off the hook at least for this period of CO2 increase.

warming from 10,000 due to ..................

the human specie but I don't see how that can be. Even hunter gathers maintain a local area from which they draw their sustenance. Imagine that the ice retreated,and the animal life, of which we are only one, followed.
This anthropomorphic emphasis pales when one reads E.O. Wilson's research that indicates insects make up 60% of the mass of living tissue on Earth.

And, analysis of the mammoth tissue indicates it was flash frozen and in order for that to happen, the temp would have had to have been 450 degrees below zero to penetrate the flesh before digestion of the buttercups.

I am waiting for a resurgence of Dr Carey, Austrailian geographer's plain explanation of the mountain chains. Did you know that our planet's size before acquiring surface water was the same size as Mars? Why is water stored deep in the Earth? If all inside was expelled outside would we be a water world? Would we become Ice Ball Earth?

expanding earth

The expanding earth theory has been debunked as well as the young earth creationist arguments. I am constantly suprised when someone trots it out with all the evidence against it.

As to the Berezovka mammoth you are referencing, it showed evidence of having been buried in a landslide with the cold mud acting as preservative and the underlying permafrost completing the process by freezing the carcass.

Please avoid the psuedoscience that can easily be dismissed with a little reading.

More on an expanding Earth

I had not interpreted the post as suggesting such unscientific ID/creationist drivel as the "young earth" hypothesis—perhaps I should have read more carefully. I was agreeing that Earth probably did grow some from the early great bombardment period, but you are correct in saying that most scientists think that the accretion of the planets occurred fairly quickly. It almost grew considerably when the object that caused the formation of the moon struck. But Earth certainly did not swell up like a sponge because it absorbed a large volume of water.

As for the "flash-frozen" mammoth, there is no need for such an improbably explanation. The normal hazards of a glacial period are perfectly sufficient, as you pointed out. My remarks about E. O. Wilson stand, however.

E. O. Wilson

I fully believe that E. O. Wilson would like people much more if we all had six legs and had our skeletons on the outside. I believe that krill may give the bugs a run for the title. As for Earth starting out much smaller than it is today that is a reasonable hypotheses. The process of accretion continued for many hundreds of millions of years as the leftover "junk" that littered the early solar system was swept up by the major planets. This process actually continues to this day.

According to an article by Brooks Hanson in Earth Planet. Sci. Lett., there are 174 characterized impact structures on Earth, ranging from the 2-billion-year-old Vredefort "crater" that stretches 300 km across South Africa to 20 or so smaller structures less than 1 km in diameter. Most of the smaller craters are less than 1 million years old because Earth's tectonic and erosion processes continually recycle the planets crust. We really don't know how much extra material Earth has collected over the past 4.6 billion years but about 40,000 tons of meteoric dust rains down on the planet every year. As for your other questions and statements, unanswered riddles are why it's fun being a scientist.