Art of the Anthropocene: The Scythe

By Tom Yulsman | March 13, 2013 12:13 am

This graph shows how temperatures have departed from the long-term mean over more than 11,000 years. The present time is at the right. The different colored lines are the result of a technique for dealing with uncertainty in data called Monte Carlo simulation. (Image: Science/Shaun Marcott, Oregon State University)

NOTE: Updates below include clarifications that Shaun Marcott, the lead author on this research, wants to make.

The graph above, based on new research showing how global temperatures have changed since the last Ice Age, has gotten some attention in recent days. — because it shows how the warming of the last hundred years has been unprecedented in its abruptness, and almost so in its degree.

Part of the reason is that when the research was first published, scientists were quoted as saying that the data showed how the global temperature increase of the last hundred years has been unprecedented in its abruptness  has happened a lot more rapidly than warming over the previous 11,000 years.  (For an example, see comments from Candace Major, program director in the NSF’s Division of Ocean Sciences, which funded the research.) It turns out that the issue is complex and requires a bit of explanation. But more about that below.

First, I decided to post this graph mostly because I think it is an exquisite example of an emerging genre that might be called the “art of the Anthropocene.” And that’s what I’d like to focus on here.

Much has been written about this “new hockey stick” graph, or “scythe,” resulting from research published last week in the journal Science. But not much that I’ve seen approaches the subject from the visual perspective.

The graph was buried deep in the supplementary materials accompanying the paper by Shaun Marcott of Oregon State and his colleagues. Just what exactly are you looking at? Marcott reports that some news reports have answered that question incorrectly. So if you’ve already seen this image, keep reading…

Using fossils of tiny organisms contained in cores of seafloor and lake sediments, he and his colleagues gauged past temperatures going back to the end of the last ice age — much further than other reconstructions using records like tree rings. These samples came from 73 sites around the world.

But there was a challenge they had to overcome: Each one of those 73 records had a degree of uncertainty associated with it — uncertainty in the temperature data and also when, exactly, the fossils were deposited. The graph was an attempt to deal with that.

To understand how, let’s start with the basics. The vertical axis of the graph shows how temperatures have departed from the long-term average, according to those 73 records. And the horizontal axis shows time, from more than 11,000 years ago on the left, to the present on the right.

What’s that colorful spaghetti about? As Marcott puts it:

Each color represents one realization (or simulation, or calculation) of the global temperature reconstruction from our study.  In this plot there are 1,000 of those realizations, which are based on all 73 of the datasets.

But if there were only 73 datasets, what are those 1,000 “realizations”?

To figure out the range of possibilities given the uncertainties in the data, Marcott and his colleagues used a method called Monte Carlo simulation.

Here’s how he explained it to me in an email message:

How do the Monte Carlo simulations come into all this? Each record has some uncertainty associated with the temperature and with the chronology (the time component, typically associated with radiocarbon dating).  For each realization, all 73 records are allowed to vary independently within their stated uncertainties. And after allowing them to vary within that uncertainty, we averaged them together to produce a global temperature reconstruction.  This was done one thousand times in this case to produce the plot you see.

I know. If you’re like me, you find it a little difficult to wrap your head around. But that’s the beauty of the visual. It gets across the basic idea. Given the known uncertainties in the data, the colored lines represent all the possibilities. And they sketch out the bounds within which temperature has varied over time in these records.

For me, at least, the result is indistinguishable from art.

The spike on the right represents the warming that has occurred since the Industrial Revolution. The suddenness of the rise compared to past changes in temperature is seems obvious — and unprecedented in this record. But since I wrote the first version of this post, Marcott has cautioned me that the graph does not really show that. Here’s a relevant section from an email he sent me to clarify the significance of the work:

We cannot say whether this change is unique across the entire Holocene because of the resolution (i.e., the sampling of temperature per unit time) of the entire dataset is about 120 years, and the nature of the Monte Carlo simulations smooth everything out to less than about 300 years.

[So] it would be incorrect to say the rate is greater than anything else seen in the past 11,000 years.  When looking at our reconstruction of temperature it appears this way. [But] it is, in fact, just a resolution issue related to this dataset.

Jeremy Shakun of Harvard, one of Marcott’s co-authors on the paper, made this comment recently on Andrew Revkin’s post at DotEarth about the research:

Just a quick point here. I state in the interview that we can’t be sure there aren’t any abrupt global warming blips during the Holocene similar to the current one due to chronological uncertainties and the relatively low time resolution of our global temperature reconstruction. It is worth considering though that we do have several high resolution proxy climate records from various regions around the world (think ice cores), and if abrupt global warming events happened in the past, then we might expect these local records to show them…..but my sense is they don’t. So, this isn’t hard and fast proof that there weren’t any abrupt global events like today during the rest of the Holocene….but if I had to lay down a bet, it might make me place my wager on that side of the argument.

Lastly, I want to point that while the graph shows that it has been as warm in the past as it is today, it seems obvious that we will probably be heading soon into unprecedented territory, with no caveats necessary. 

  • Buddy199

    That spike also directly corresponds with the spike in global prosperity over the past 150 years resulting from fossil fuel based industrialization: more food, improved medical care and public health, increased longevity and lower infant mortality, higher incomes and an unprecedented rise in human living conditions with billions lifted out of subsistance poverty. The fossil fuel boom is also the only real bright spot in the U.S. economy. Ironically, it is also likely to be what eventually leads the economy out of the morass; certainly not windmills or the millions of promised green jobs that never materialized. While improved efficiency and pollution control are worthwhile goals, the idea that continued global prosperity can be fueled by solar cells and windmills is an unreachable mirage. Unreachable not because of politics or public policy but because of physics and chemistry.

    • Linda Neely

      You are correct and I applaud your true assessment. They utilize a chart that starts in an unknown with no links to proofs of claims. What was the methodology and the peer reviewed data? As ususal too much data might confuse the Nudge that is being applied to the feelings instead of the intellect.

      • Kieran Suckling

        Linda, the graph is from a peer reviewed paper in Science. There is a link to it at the beginning of the blog. The methodology is described above, but to see more details you should read the original paper.

        Many teams of scientists have reconstructed historic temperature trends using different datasets and different methodologies, but they all end us with essentially the same result. That what’s makes the conclusion so compelling.

    • David S. Leaton

      Buddy, the first part of your comment is absolutely correct: cheap energy has driven unprecedented economic expansion. Now, will you take responsibility for this expansion when it’s absolutely clear that it cannot continue? The cheap energy that drove the economic expansion is not unlimited. When it begins to fail, the economy that supports 7+ billions will also begin to fail. That constitutes a monumental failure of economic vision.

      Further, cheap energy has not lifted “billions out of subsistence poverty.” There were not billions to lift out when cheap energy became available. Capitalism found work for a few million in Europe during a specific historical moment, but it then proceeded to lock a certain percentage into poverty and unemployment. There are more humans living in poverty now than there were in the early 18th century. Poverty has little to do with cheap energy, except in the areas where cheap energy is produced (what was the poverty rate in Venezuela before Chavez came to power, and at that time where did Venezuela rank in oil exports?).

      Cheap energy will run out, unless a technological miracle occurs. Renewables will delay that moment. There’s every reason to develop these technologies. If the delay is long enough, then the global economy/population will adjust with fewer “transitional deaths.” If we end support of these technologies, then we have no delay, no safety net.

  • Jeremiah Brown

    Since you are addressing visuals, it would be more informative, for you and the original authors, to include an additional figure that zooms in on the last 100 years, including x-axis ticks and labels for every 10-years. Ticks at 1900, 1910, and so on up to 2010 would give a clearer picture of the runup the authors intended.

    I would like to make one additional point regarding your last paragraph and the spike. While it is helpful to look at trends and the mean, the point SHOULD BE to consider the information in its entirety when drawing conclusions. Without considering the range of uncertainty, which again would be aided by zooming in on the most recent 112 years, any final points are skewed. And skewing judgement is the antithesis of data analysis. We should seek instead to discuss complex issues in their full complexity–for instance, how does the temperature-anomaly uncertainty of -1 degree Celsius during some part of the 1900s (again, I can’t tell the years without the zoom) jive with the notion of global warming?

    • David S. Leaton

      Jeremiah, there are many studies on global and regional temp change over the last 150 years. The Marcott et al. (2013) research question focuses on getting a global look at period for which we have little global analysis (i.e. the first 10,500 years of the Holocene). In fact, in Marcott’s doctoral thesis, the basis for the current study, Marcott just runs the analysis up to the year 2000 without, as far as I know, taking advantage of studies that have a far greater temporal resolution. He just wasn’t interested in the last 150 years. We have instrumental data for much of that:

      The uncertainty bar is the uncertainty bar. It cuts both ways. The most likely value is the line. Furthermore, partially due to the fact that all surface warming for a given enhancement of the greenhouse effect will not occur instantaneously, and partially due to the fact that solar variation was overwhelming the signal, we don’t see the global warming signal in surface temp analysis until about 1960 (see Pasini et al. 2012 among many others).

      Also, the “notion” of global warming is not based on the surface temp trend. It is based on physics. We can be critical (in the best sense) of model projections when they fail to accurately (a loaded term in this case) project surface temp on the scale of climate (at least 30 years). We cannot be critical of the basic theory of anthropogenic global warming on the basis of the surface temp trend.

  • Brian W. Allan

    Not to belittle the effort that has gone into this analysis BUT I think determining temperature from fossil records is almost impossible in the first place…

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  • JonFrum

    I see some lines have been struck out. Don’t you think we should know why?

  • TallDave

    Core tops were redated in violation of all common sense and propriety (no this is not just the minor redating algorithm some have alluded to, the core tops are completely wrong), there is no hockey stick in this data. None. At. All.

    Up until now I was willing to give people the benefit of the doubt. After this, I’m forced to assume every pro-AGW paper is deliberately lying to me.

  • mikes

    When was this article updated?

  • SalientTruth

    . . . the 20th century portion of our paleotemperature stack is not statistically robust, cannot be considered representative of global temperature changes . . .

    This statement admits the data from the last 120 years is WORTHLESS – there is no blade to the hockey stick- the fact that taxpayers fund this garbage borders on insane.

    As dissected by McIntyre at CA:
    And further clarified by Pielke here:

    I’ll let readers decide whether this is fraud or incompetence.



ImaGeo is a visual blog focusing on the intersection of imagery, imagination and Earth. It focuses on spectacular visuals related to the science of our planet, with an emphasis (although not an exclusive one) on the unfolding Anthropocene Epoch.

About Tom Yulsman

Tom Yulsman is Director of the Center for Environmental Journalism and a Professor of Journalism at the University of Colorado, Boulder. He also continues to work as a science and environmental journalist with more than 30 years of experience producing content for major publications. His work has appeared in the New York Times, Washington Post, Audubon, Climate Central, Columbia Journalism Review, Discover, Nieman Reports, and many other publications. He has held a variety of editorial positions over the years, including a stint as editor-in-chief of Earth magazine. Yulsman has written one book: Origins: the Quest for Our Cosmic Roots, published by the Institute of Physics in 2003.


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