There's long been a general picture of the climate of the Holocene, the period of Earth history since the last ice age ended around 12,000 years ago. It goes like this: After a sharp stuttery warm-up following that big chill — to temperatures warmer than today — the climate cools, with the decline reaching bottom around 200 years ago in the period widely called the "little ice age." (A graph produced by Robert Rohde for his Global Warming Art Web site years ago nicely captures the general picture.)
In a new study, researchers from Oregon State University and Harvard have analyzed 11,300 years of data from 73 sites around the world and added more detail to this picture. The work, posted online today, is being published Friday in the journal Science. [Justin Gillis's news story is posted]
While the researchers, led by Shaun Marcott of Oregon State, conclude that the globe's current average temperature has not exceeded the warmth that persisted for thousands of years after the last ice age ended, they say it will do so in this century under almost every postulated scenario for greenhouse gas emissions.
In a news release, Candace Major, program director for ocean sciences at the National Science Foundation, which paid for the research, said:
The last century stands out as the anomaly in this record of global temperature since the end of the last ice age…. This research shows that we've experienced almost the same range of temperature change since the beginning of the industrial revolution as over the previous 11,000 years of Earth history – but this change happened a lot more quickly.
In sum, the work reveals a fresh, and very long, climate "hockey stick."
The hockey stick climate analogy arose from a variety of studies of the last millennium or two of temperatures in the Northern Hemisphere, Arctic and planet. There's a general pattern of a sharp warming from the 20th century onward. The shaft of the "stick" has a lot of wiggles and warps and still comes with substantial uncertainty, but the general pattern is well established. The Wikipedia entry is a reasonable starting point for reviewing varied views of this body of science.
This work is complicated, involving lots of statistical methods in extrapolating from scattered sites to a global picture, which means that there's abundant uncertainty — and that there will be abundant interpretations. Here you can listen to one of the lead authors, Jeremy Shakun, a visiting postdoctoral fellow at Harvard, discuss reactions from other scientists and answer some questions I raised:
Below, you can also review an e-mail exchange among some climate scientists familiar with the paper. Finally, there's a second part to my Skype conversation with Shakun in which he turns the tables on his interlocutor, asking me some questions about my view of that state of climate science and the complicated role of scientists thrust into public discussions not only of their work, but the options facing society.
Please read on:
Besides asking some scientists about the quality of the work, I sought their thoughts on its broader meanings, one of which, I proposed, was this:
While folks have long talked of "abrupt climate change" (as in NRC reports) as a plausible prospect, this paper builds on the idea that we've been in the midst of abrupt climate change since the early 20th century.
Here are a few initial replies (some e-mail shorthand is fixed), starting with a back-and-forth with Michael Mann of Pennsylvania State University. (This section is for deep divers; the replies are technical in spots and unavoidably contain references to graphs and other cotent in the paper that is behind the journal's subscription wall.)
Michael Mann:
This is an important paper. The key take-home conclusion is that the rate and magnitude of recent global warmth appears unprecedented for at least the past 4,000 years and the rate at least the past 11,000. We know that there were periods in the past that were warmer than today, for example the early Cretaceous period 100 million years ago. The real issue, from a climate change impacts point of view, is the rate of change –because that's what challenges our adaptive capacity. And this paper suggests that the current rate has no precedent as far back as we can go with any confidence — 11,000 years arguably, based on this study.
My only real concern is that their data and approach (e.g. the use of pollen records in the higher northern latitudes) seems to emphasize the higher latitudes of the Northern Hemisphere, during the summer season. This is an issue because we know there is a substantial long-term natural cooling trend for high-latitude summers because of Earth orbital effects, but the trend is nearly zero in the global annual average. One gets the sense from looking at their reconstruction that there is a very strong imprint of this orbital cooling trend — stronger than what one would expect for the global annual average.
The interesting thing about that, is that it suggests that the true conclusions might even be stronger than their already quite strong conclusions, regarding the unprecedented nature of recent warming. That is, it may be that you have to go even further back in time to find warmth comparable — at the global scale — to what we are seeing today. If you look at their tropical stack for example (Figure 2J) [a particular set of data], the modern warmth is unprecedented for the entire time period (i.e, the past 11,000 years). That's why I said that there results suggests recent warmth unprecedented for at leaat the past 4,000. It's possible, given the potential seasonality/latitudinal bias, that there is in fact no precedent over the past 11,000 years (and likely longer, since the preceding glacial period was almost certainly globally cooler than the Holocene) for the warmth we are seeing today. In that case, we likely have to go back to the last interglacial, i.e. the Eemian period (125,000 years ago) for warmth potentially rivaling that of today.
But, again, the take-home conclusion: the rate of warming appears to be unprecedented as far back as the authors are able to go (to the boundary with the last ice age). And the rate of warming appears to have no analog in the past, as far back as the authors are able to go.
My followup question for Mann:
Separate from the potential northern bias, are you confident that jogs similar to the one recorded in the last century (a well-instrumented century) could not be hidden in the "smear" of millenniums of proxy [indirect] temperature data? (This is where my ignorance of the strengths/weaknesses of these statistical tools forces me to rely on expert judgment.)
Michael Mann:
Regarding the resolution issue, this was my main concern initially when I looked at the paper. But I'm less concerned now that I have read the paper over more carefully, because I think that Figure 1a and 1b give a pretty good sense of what features of higher resolution reconstructions (specifically, our '08 global reconstruction which is shown) are potentially captured. Based on that comparison, I'm relatively convinced that they have the resolution to capture a century-long warming trend in the past were there one comparable to the recent trend.
In the same e-mail thread, Robert Rohde, the chief data analyst behind the Berkeley Earth Surface Temperature project (and Global Warming Art, as I noted above), weighed in this way:
I found the Marcott et al. paper interesting and read it with some care. There are many details, such as proxy data locations and the exact method of averaging, that are not presented in the main text of the paper. In all likelihood many of these factors are discussed in the supplemental material, but since I don't have access to that I can only comment on the parts that I can immediately see.
That caveat aside, I have long believed that the last 10,000 years was a time period ripe for further study, and the authors seem to have approached the problem in much the same way I might have imagined doing it myself. Their results should help provide an important context for understanding the history of climate change.
The Marcott et al. results may refine our understanding the last 10,000 years; however, the broad picture of Holocene climate does not seem to have been significantly changed by their findings. Previous work had already pointed towards a period of early Holocene warmth somewhat higher than recent centuries. Similarly, earlier studies had also revealed some of the same details, such as warming peaking earlier in the South than the North, for reasons likely related to changes in the tilt of Earth's axis. The scientific community has also known for some time that the predicted future global warming in most climate models (more than 2 degrees C.) would probably be well above the long-term average temperature present at any time during the Holocene.
Hence, the Marcott et al. results are for the most part not surprising, and generally confirm the previous understanding of the Holocene. That said, the details are still important. Having better constraints on the timing and magnitude of past climate changes should ultimately lead to better constraints on climate sensitivity and the role of natural variability in the climate system. Like Crowley, I find the magnitude of estimated warming during the early Holocene to be somewhat larger than I would have expected based on prior work and known forcings, and if that magnitude holds up to further scrutiny then it may offer an important insight into past climate dynamics.
In discussing their result, there is one important limitation that I feel deserves more attention. They rely on proxy data that is widely spaced in time (median sampling interval 120 years) and in many cases may also be subject to significant dating uncertainty. These effects will both tend to blur and obscure high frequency variability. They estimate (page 1, column 3) that only 50% of the variance is preserved at 1,000-year periods. This amount of variance suppression is roughly what you would expect if the underlying annual temperature time series had been smoothed with a 400-year moving average. In essence, their reconstruction appears to tell us about past changes in climate with a resolution of about 400 years. That is more than adequate for gathering insights about millennial scale changes during the last 10,000 years, but it will completely obscure any rapid fluctuations having durations less than a few hundred years. The only time such obscuring might not occur is during the very recent period when dating uncertainty is likely to be low and sample spacing may be very tight.
Because the analysis method and sparse data used in this study will tend to blur out most century-scale changes, we can't use the analysis of Marcott et al. to draw any firm conclusions about how unique the rapid changes of the twentieth century are compared to the previous 10,000 years. The 20th century may have had uniquely rapid warming, but we would need higher resolution data to draw that conclusion with any certainty. Similarly, one should be careful in comparing recent decades to early parts of their reconstruction, as one can easily fall into the trap of comparing a single year or decade to what is essentially an average of centuries. To their credit Marcott et al. do recognize and address the issue of suppressed high frequency variability at a number of places in their paper.
Ultimately, the Marcott et al. paper is an interesting addition to the study of millennial scale climate variability during the Holocene. Their results are broadly consistent with previous findings, but the details are interesting and likely to be useful in future studies. However, since their methodology suppresses most of the high frequency variability, one needs to be cautious when making comparisons between their reconstruction and relatively rapid events like the global warming of the last century.
Jeremy Shakun responds to some of these points in our Skype chat above. Here's the second part of our conversation, in which he asks me some good questions about my climate views:
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