GEOG8280: Frontiers in Paleoclimate

To anticipate the future, it’s crucial we understand how and why climate has changed in the past. But interpreting the history of Earth’s climate through the lens of natural archives has often sparked disagreement and debate.

The field of paleoclimatology, which uses information from natural and cultural archives such as tree rings, lake sediments, and historical documents, allows us to study the climate system outside of the constraints imposed by the comparatively short instrumental record. Paleoclimatology is also an intensely interdisciplinary subject that requires us to draw upon concepts, data, and techniques from the Earth sciences, climatology, archeology, and many other subjects. Piecing together climates of the past provides us with a ‘baseline’ to gauge current and future climate change, allows us to document potential ‘climate surprises’, and provides a broader perspective to evaluate the interconnections between climate, ecology, and society. But proxy data (and our interpretations of those data) can also be confounding, so much so that the field is often a crucible for debate among climate scientists and other parties.

In this seminar course, we will review current debates in paleoclimatology so that we may try to define the intellectual frontier(s) of this discipline. By the end of this course, students will be able to articulate the main arguments and points of controversy for several ongoing debates in paleoclimatology. They will have defined new areas for potential future research into ancient climates, and will have practiced writing concise, scientifically-accurate summaries for a broad scientific audience.

New frontiers in paleoclimate

posted May 5, 2016, 9:01 AM by Scott St. George   [ updated May 5, 2016, 12:29 PM ]

How might future research enhance our understanding of past climates, and answer (once and for all?) ongoing debates in paleoclimatology?


“Don’t tell me what you value. Show me your budget, and I’ll tell you what you value.”

Senator Joseph Biden
September 15 2008

Over the past semester, we have waded through several unresolved debates within the field of paleoclimatology, We’ve read some of the latest work on the risk of megadrought, the influence of early agriculture on the composition of our atmosphere, several attempts to link comet impacts to ancient climate change, and many other points of contention. And although much of the science we’ve reviewed has been quite recent, there is always something newer coming down the pipeline. For our final exercise, I’m going to ask you to take a peek into the future, and report on newly-funded projects that take aim at one or more of our central debates.

In the United States, the National Science Foundation is the main federal government agencies that supports paleoclimate research, as part of their funding of non-medical fields of science and engineering. Today in class, each of you will search the NSF archive to identify any and all active awards that connect to one of the topics we’ve discussed in class. Once your search is completed, you’ll give a short oral report explaining how these awards might address or even answer one or more of our key debates.

You’ll have about an hour to complete this task. Cooperation is perfectly OK, as would be addressing more than one debate.


Where to start
First, choose one of our debate topics and generate a short list of keywords to guide your search. Then go to the Awards Search function provided by NSF at https://www.nsf.gov/awardsearch/ and select the ‘Advanced Search’ option.  You might be to find several active proposals that connect to your topic simply by entering your keywords into the matching search field.

If you need to narrow your search, you can restrict your results to only those proposals funded by a specific NSF program. For example, click on the magnifying glass icon next to ‘Program’ and search for ‘paleoclimate*’ (don’t forget the asterisk). That option should restrict your search to those awards funded under by Paleo Perspectives on Climate Change (P2C2). Depending on your subject, you may need to revert back to searching all programs or enter another program instead.

I suggest you try to find several active proposals that connect to your topic. If you can’t find any appropriate awards, either switch to another paleo-debate or move your search elsewhere. 

Tip: Make sure you are searching active awards (so you find the latest funded work, not projects that have been expired/completed).


Where to go next
The Awards Search feature will provide a brief summary of the funded research and list recent products that have been generated by this study (if there are any yet). If need be, you can also look for other details about this project on external websites, such as the PI’s lab site. It’s not certain they’ll provide any more detail, but you might find something helpful.

Depending on your topic, you might consider expanding your search outside of the National Science Foundation. If you decide that you’ve exhausted your search at NSF, you are welcome to look at active proposals recently funded by other agencies (such as the National Oceanic and Atmospheric Administration, or the US Geological Survey), or other countries or funding programs. 


What to share
Once your search is complete, I’d like you to share a summary of your findings with the rest of our group. How you approach this challenge is up to you, but I would like you to (at minimum) (i) remind us of the fundamental questions associated with your particular debate, (ii) summarize the active grant (or grants) within this subject area, (iii) and explain how this work might address one or more aspects of the debate). 

You should provide a brief oral summary - not too much longer than 5 minutes. If you like, you are welcome to illustrate your summary with a few visual aids, either something you’ve sketched yourself or found elsewhere. But let’s not break our streak of having a semester entirely free of PowerPoint.

The early Anthropocene hypothesis

posted Apr 21, 2016, 2:42 PM by Scott St. George

Did early civilizations save us from an impending ice age by slowly increasing atmospheric greenhouse gas concentrations via agriculture and land clearing? 


Yesterday (April 20) at a session at the European Geophysical Union meeting in Vienna, the majority of audience members agreed that our planet has entered the Anthropocene, a new epoch defined by the dominant influence of our species on its environment. In this admittedly small-but-timely sample of earth scientists, most thought the Anthropocene had begun relatively recently, coinciding with the industrial revolution or the mid-20th century. But if this new epoch began (and the Holocene ended) when human actions started to have a global impact on the atmosphere, biosphere, and lithosphere, what if humanity’s influence on our home planet started much earlier? Did the invention of agriculture cause an ‘early’ Anthropocene?


More than a decade ago, William Ruddiman, and earth and environmental scientist at the University of Virginia published his ‘Early Anthropocene’ hypothesis in the journal Climatic Change.  The central core of his argument was that, based on earlier interglacial periods, gradual changes in the Earth’s orbit should have caused atmospheric concentrations of carbon dioxide and methane to decrease through the Holocene. But instead, CO2 and CH4 trended upwards starting in the early or mid-Holocene, well in advance of our species’ widespread use of fossil fuels. Ruddiman put forward the idea that natural forcings could not have caused this anomalous trend in greenhouse gases, and instead claimed that agricultural expansion in Eurasia must have been the main cause. 


As you might expect, the idea that low-intensity agriculture could have slowly-but-surely revved up global biogeochemical cycles, tipped the balance towards more atmospheric carbon dioxide and methane, and staved off the return of the northern continental ice sheets was not immediately accepted by the scientific community. Early objections focused on the fact that state-of-the-art climate simulations did not produce the CO2 drawdown required by Ruddiman’s hypothesis. In parallel, at that time, our understanding of early human land-use in Europe and Asia was very limited and the large uncertainties about exactly how much land was dedicated to agriculture (and when it was cleared) made this hypothesis tough to test.


Your task

Next week we will first go back in time to 2003 and read Ruddiman’s initial paper on the early anthropocene, as well as one of the first comments on this provocative idea. Then we’ll fast-forward to the present and read two short papers (by Ruddiman) that provide an update on the status of this idea more than a decade later. Finally, I’ll ask each of you to choose one other article that (1) evaluates the early Anthropocene hypothesis and (2) was published in 2010 or later. Please upload electronic copies of your article to this post and provide a short description of its content prior to our next class. 


The ‘methane burp’ and Paleocene-Eocene Thermal Maximum

posted Apr 7, 2016, 11:57 AM by Adrian Wackett   [ updated Apr 7, 2016, 12:02 PM ]

Since the 25 year-old discovery of a major isotopic excursion in marine sediments suggesting 'hothouse' conditions at the end of the Paleocene, the Paleocene-Eocene Thermal Maximum (PETM) has commanded considerable attention from the scientific community because of its implications for understanding and predicting future anthropogenic climate change. However, pinpointing the specific event(s) that triggered Earth's last global warming episode has proven difficult. 


Roughly 56 Ma, thousands of gigatons of carbon were released into the atmosphere, pushing the Earth into its last major ‘hothouse’ phase.  This event, commonly termed the Paleocene-Eocene Thermal Maximum (PETM), was concomitant with dramatic increases in sea and land surface temperatures, rapid evolution and geographic redistribution of terrestrial and marine fauna, and extinction of benthic foraminifera at the hands of oceanic acidification.

Kennett and Stott (1991) were the first to identify a significant climatic warming event at the boundary between the Paleocene and Eocene epochs (~56) Ma after finding substantial shifts in the stable carbon and oxygen isotopic signatures of foraminifera in marine sediment cores off the coast of Antarctica.  This was the followed a year later by an equally groundbreaking study by Koch et al. (1992), who found a coeval isotopic excursion in mammalian tooth enamel and correlated the event to the biggest mammalian extinction of the Cenozoic.  These findings verified that the PETM was a global event affecting both marine and terrestrial environments, which set the precedent for a wealth of subsequent studies investigating the source of this negative carbon isotopic anomaly.

A well-accepted explanation for the climatic excursion was proposed early on by Dickens et al. (1995), who argued that preferential heating of the deep ocean led to the dissociation methane hydrates (https://en.wikipedia.org/wiki/Methane_clathrate), which have a narrow stability range (with respect to temperature and pressure) and are thus typically found in the shallow lithosphere along continental shelves or in deep marine sediments.  While this ‘methane burp’ hypothesis did meet some opposition, it was generally accepted as the best available explanation for the rapid release of massive carbon stores.  Recent studies have favored a modeling approach in attempts to constrain how much (and how quickly) these carbon reservoirs had to mobilize to initiate the shift to a global ‘hothouse’, and many contend that there simply wasn’t enough carbon stored in these methane clathrate deposits to enact such a dramatic change in earth’s climate (e.g. Panchuk et al., 2008; DeConto et al., 2010).  Many of these studies point towards terrestrial carbon reservoirs, such as Arctic/Antarctic permafrost and/or widespread wildfires, as additional carbon sources that were likely mobilized at the onset of the PETM.

Which carbon source(s) were mobilized during the PETM and to what extent remains an ongoing debate today.  However, another debate surrounding the PETM remains arguably even less clear: the ‘trigger’ responsible for destabilizing earth’s methane hydrate and permafrost reservoirs.  This is what we will explore in our class discussion.

Your tasks

We will all read the review paper by McInerney and Wing (2011), which appeared in the Annual Review of Earth and Planetary Sciences.  Although it’s a little long, it is very easy to digest and does a good job of summarizing the available body of knowledge (as of 2011) regarding the PETM and its consequences.  Because the review focuses primarily on what is ‘known’, however, it only briefly alludes to these ongoing debates.  Then, we will split into two teams: the Cosmic Catastrophists and the Bountiful Basalts. Yes I realize I’m recycling one of Scott’s terms…

The Cosmic Catastrophists will read the 2003 paper by Kent et al., which argues that an extraterrestrial impact triggered greenhouse warming that led to the PETM hothouse.  I think it’s also worth reading the subsequent exchange between Kent and Gerald Dickens (author of the initial 1995 paleoceanography paper presenting the ‘methane burp’ hypothesis), which featured in later versions of EPSLAs you read, I’d like you to focus on the evidence presented by the authors in favor of a comet impact.  Can this only be explained by their impact hypothesis?  What evidence of an ET impact is missing?  If you’re dissatisfied with the evidence Kent et al. presented, what would you do (hypothetically) differently to test the validity of the P/E comet hypothesis?

The Bountiful Basalts will read two short papers by Svensen et al. (2004 – Nature) and Storey et al. (2007 – Science) that propose voluminous volcanism associated with rifting in the North Atlantic as a mechanism for mobilizing large stores of methane hydrate in organic-rich shelf sediments.  These papers use quite a bit of igneous geology terminology (which I will go over in class), but the focus should be on their analyses and what assumptions they made in order to arrive at their conclusions.  Could widespread volcanism in the Paleocene N. Atlantic really have ‘cooked’ a sufficient quantity of C-rich sediments and/or heated deep ocean water on a global scale enough to destabilize methane hydrates?  If so, why isn’t there evidence of this in other modern environments underlain by hot spots (for example Hawaii, Iceland, the Galapagos Islands, etc.)?  Is there anything else you would have liked to see the authors do to support their assertions?

I’ve also included a Scientific American article written by Lee Kump from Penn State, which draws comparisons between the PETM and modern climate change.  It’s an interesting article written for non-scientists that I think is worth looking at if you find yourself bored and/or fascinated by our last ‘global warmingepisode.  

Another link: Magnetic field and ancient civilizations

posted Apr 7, 2016, 11:26 AM by Michele Stillinger

Link between changing magnetic field and ancient civilization change in the Near East

The pulse of nations

posted Mar 31, 2016, 9:23 AM by Scott St. George   [ updated Mar 31, 2016, 9:25 AM ]

Paleoclimatologists argue that past climate events have been the underlying causes of major episodes in human history, but are the strategies used to integrate these two fields really falsifiable? 


Because it is first and foremost a physical science, the main goal of most paleoclimate studies is to uncover the evolution of past climates and understand the physical processes that influence the atmosphere, geosphere, and ecosphere over long timescales. Since human history has also marched along in parallel with changes in our planet’s environment, paleoclimatologists have, on occasion, branched out to consider whether past climate change may have influenced the arc of human history. But can paleoclimate archives give us insight into the impact of past climates on historical events, or do these comparisons tell us more about our own assumptions regarding the connection between nature and society?


Owing to our modern tools of geochronological dating and proxy archives, it is often possible compare directly paleoclimate evidence to history events. For this assignment, I’ll ask you to read closely two recent studies that argue past climate change acted as triggers for social collapse and expansion. In the first instance, Pederson et al. (2014) present tree-ring data to suggest the expansion of the Mongol Empire in the 13th century was enabled by a prolonged wet period (rather than as a response to drought, as had been argued earlier). In the second example, Malcom Weiner presents paleoclimate and archeological evidence to bolster the argument that a three-century long dry period prompted “migrations, the displacement of trading networks, warfare, the appearance of weapons made of bronze, and the first appearance of sailing vessels” in the eastern Mediterranean. 


Your tasks

I’d like everyone to read both papers (Pederson et al. 2014, and Weiner, 2014) carefully. We’ll start our discussion with a summary of the evidence presented by each study and a review of their major conclusions, so please come prepared to discuss the sources and analysis that form the core of each study. In addition to those items, I’d like everyone to consider the chain of causes linking the environmental event to a human social response. How do the authors imagine one affects the other? And what assumptions do they adopt (either explicitly or unspoken) to bridge the gap between paleoenvironmental data and human agency. 


In addition, we will read a short article that takes issue with the rising profile of what the author describes as ‘neo-environmental determinism’ in scientific research. Because this article assumes that readers are already familiar with the meaning of that term, I’d suggest you quickly skim the topic’s Wikipedia entry before reading the Sluyter article. Do the criticisms posed by Sluyter apply to the argument outlined by either Pederson et al. or Weiner? I also recommend reading this post by David Correia, which (somewhat casually) lobs the same critique at Pederson et al. 


Are paleoclimatologists really guilty of imagining the fingerprint of climate on human society wherever they look?


Sources

Pederson, N., Hessl, A., & Baatarbileg, N. (2014). Pluvials, droughts, the Mongol Empire, and modern Sluyter (2003) Neo-environmental determinism, intellectual damage control, and nature/society science. Antipode 35, 813-817. 

Pederson et al. (2014) Pluvials, droughts, the Mongol Empire, and modern Mongolia. Proceedings of the National Academy of Sciences 111, 4375-4379. 

Weiner (2014) The interaction of climate change and agency in the collapse of civilizations ca. 2300-2000 BC. Radiocarbon 56, S1-S16. 

Snowball Earth

posted Mar 24, 2016, 10:00 AM by Kerry Callaghan   [ updated Mar 24, 2016, 10:09 AM ]

The idea of the Snowball Earth is arguably the most contentious hypothesis in the Earth Sciences. The idea that the entire planet was once - or several times - entirely covered in snow and ice seems far-fetched, yet many view this as the best explanation for a combination of geological evidence. 

Researchers first aired the idea of a global glaciation over 30 years ago. These early observations did not take into account the now widely accepted idea of continental drift, and so the idea of the global glaciation was soon dismissed. Before long, however, magnetic evidence from new samples indicated that some were in fact located in the tropics during their glaciation. These data led to a brief article (Kirschvink, 1992) reiterating the idea of a global glaciation and coining the term 'Snowball Earth'.

Since then, various sources of evidence have been used to back the Snowball Earth hypothesis. These include magnetic data; the existence of carbonates (which usually form only in the tropics) directly overlying glacial deposits; anomalies of various trace elements which would have been deposited upon melting of the ice; and various types of glacial deposits which are said to have formed at low latitudes. 

For each line of evidence used to support Snowball Earth, skeptics supply either reasons for invalidity of the data, or alternative processes which may have caused the observed features. Additionally, skeptics say that the Snowball Earth hypothesis is not yet fully explained. In recent years, the main aspect which is seen as problematic is the lack of a mechanism by which the Earth could escape a global glaciation. A frozen planet would have a higher albedo, reflecting more heat, and would only become colder. One possible mechanism to escape this state touted by Snowball Earth proponents is the build-up of carbon dioxide and other greenhouse gases in the atmosphere through volcanic eruptions over time. With enough greenhouse gases in the atmosphere, the Earth would start to warm up and create a positive feedback with rapid melting. Whether it is possible for this level of greenhouse gases to have built up remains controversial. 

Your task:

(1) read the two-page article by Kirschvink (1992). This older article is interesting because it makes use of the term Snowball Earth for the first time; it is also a good introduction to the topic. Take note of the tests suggested by Kirschvink to determine if Snowball Earth is valid.

(2) the review paper by Hoffman & Schrag (2002) revisits these tests proposed by Kirschvink. Although this paper generally argues in favour of the Snowball Earth, it provides a good overview of the various lines of evidence as well as alternative hypotheses for each. Consider the forms of evidence available in support of the Snowball Earth theory, how reliable each of these are, and the possible alternative hypotheses.

(3) the first group will read Sansjofre et al (2011), who reject the Snowball Earth hypothesis on the basis that there was not enough carbon dioxide present at the time to allow the earth to escape a global glaciation.

(4) the second group will read Abbot & Halevy (2010), who provide an explanation for lower carbon dioxide through the addition of dust aerosols as an important component to aid melting. 

Solar cycles

posted Mar 10, 2016, 11:59 AM by Scott St. George   [ updated Mar 10, 2016, 12:00 PM ]

The strength of association between variations in solar activity and the Earth’s climate is (to put it mildly) contentious. Do proxies provide robust evidence that long-term changes in solar irradiance have a discernible effect on climate? 


The Sun is the engine that drives the Earth’s climate system, so it stands to reason that fluctuations in solar radiation should serve to either warm or cool the planet, depending on the direction of change. Despite its name, it is known that the ‘solar constant’ (mean solar electromagnetic radiation at a distance of one astronomical unit from the Sun) is not actually constant, and instead varies by roughly 0.1 to 0.2 percent.The largest portion of this change is due to the solar cycle (more properly, the solar magnetic activity cycle or Schwabe cycle), which has an average duration of nearly 11 years and is accompanied by shifts in solar irradiance, solar flare activity, and the number of sunspots. Is it possible the Sun’s 11-year cycle has a discernible (and perhaps predictable) influence on the Earth’s climate?


Proxies play an outsized role in the ongoing debate about the relative importance of solar variability on global climate. For this discussion, we will work through a review of solar cycle evidence, examine the breadth of proxies used to investigate the solar fingerprint, and critically evaluate one study linking climate, tree growth, and solar activity. On top of all that, I’ll introduce you to an Anglo-Norweigen paleoecologist who has taken a hard look at the literature connecting the ups and downs of the Sun to life here on Earth.


First up, I’ll ask you to read this review of ‘long-period cycles’ in the Sun’s activity. Although this article was published in the journal Solar Physics, it’s mainly devoted to (i) introducing the various sources that describe past solar variability and (ii) presenting a spectral meta-analysis intended to highlight the most important ‘beats’ within the solar variability record. When reading this article, try to answer these questions: What are the Gleissburg and Suess cycles, and how do they differ from the Schwabe cycle? What are the types of observational evidence used to track solar variability? What evidence do changes in solar insolation leave behind in geological or biological systems? In what way are climate proxies used to identify either past solar variability or its impact on past climates?


Second, Engels and van Geel (2012) set out a long list of paleoclimate studies that reported a connection to solar variability. After skimming the article, take a closer look at Section 4 (4. Historical and paleoclimatological evidence for the influence of TSI on Earth’s climate). In general, what is the chain of causes proposed to link solar variability with these proxies? Are those mechanisms consistent within or between regions or proxies? 


Third, please read Duan and Zhang (2014), which is a recent paper that argued temperatures on the Tibetan Plateau are related to solar activity. When reading this article, pay close attention to each step within their analysis. Do they make any choices that might introduce artifacts to their data? If you were providing a formal review for this article, what questions would you put to the authors?


Finally (and really, this is a bonus), I recommend you introduce yourself to Dr. Richard Telford at the University of Bergen (in Norway). Dr. Telford is a quantitative paleoecologist whose blog is titled (appropriately enough) ‘Musings on Quantitative Palaeoecology’. Follow this link to see all of his blog entries that relate to solar variability. You are absolutely not required to read all of these entries! But I do suggest you read through one or two to get a sense of what Richard thinks is important to consider, as it relates to the question of the Sun, cycles, and the Earth’s climate. 


Your Science Highlight (Due March 24)

Finally, a reminder that each of you are required to submit a title and one-sentence summary outlining the current debate in paleoclimatology that will be the focus of their ‘Science Highlight’. I recommend that you submit those details as electronic comments to this blog entry (so, place it directly below).


Sources

Ogurtsov, M. G., Nagovitsyn, Y. A., Kocharov, G. E., & Jungner, H. (2002). Long-Period Cycles of the Sun's Activity Recorded in Direct Solar Data and Proxies. Solar Physics, 211(1-2), 371–394. http://doi.org/10.1023/A:1022411209257 

Engels, S., & van Geel, B. (2012). The effects of changing solar activity on climate: contributions from palaeoclimatological studies. Journal of Space Weather and Space Climate, 2, A09–9. http://doi.org/10.1051/swsc/2012009 

Duan, J., & Zhang, Q.-B. (2014). A 449 year warm season temperature reconstruction in the southeastern Tibetan Plateau and its relation to solar activity. Journal of Geophysical Research: Atmospheres, 119(20), 11,578–11,592. http://doi.org/10.1002/2014JD022422

ENSO and the mean climate

posted Mar 3, 2016, 12:54 PM by Scott St. George   [ updated Mar 3, 2016, 4:04 PM ]

Climate models show no consensus regarding the future behavior of the El Niño-Southern Oscillation. Can paleo evidence help us predict how the tropical Pacific might act as part of a warmer world?


Outside of the annual cycle, the El Niño-Southern Oscillation (ENSO) is by far the strongest source of year-to-year variability in the Earth’s climate. Its two twin aspects of El Nino and La Niña dominate the regional climate of the tropical Pacific Ocean, but also regularly disrupt weather, ecology, and society at scattered locations around the globe. Because the effects of ENSO are so strong, so widespread and so regular, any change in the behavior of this system would have “serious climatic and ecological consequences” (Latif and Keenlyside, 2009). Given that global temperatures are expected to continue their rise through the next century, is it possible that a warmer world will also lead to different marching orders for ENSO?


This week’s exercise does not focus on a debate in paleoclimatology, but instead connects to our limited understanding of the connection between global mean temperatures and ENSO. Instead of choosing a side, I’m asking everyone to read two papers that try to address this issue from two different angles.


The first paper by Latif and Keenlyside reviews the state of knowledge connected to the future behavior of ENSO. While reading this article, consider how you’d answer the following questions (but keep in mind, some questions are not answered in this review, and will require some additional searching): What is the ‘mean state’ of the equatorial Pacific? What does that term mean, and how might chances in the mean state affect ENSO? What is a ‘thermocline’ and why is it an important component to the ENSO system? What is the ‘Bjerknes’ feedback? How do climate models simulate the ENSO phenomenon? And most importantly, what aspects of ENSO are accurately reproduced by state-of-the-art climate models, and which aspects of its behavior are hard to simulate?


The second article by Ford et al. (2015) makes inferences about ENSO during the Last Glacial Maximum (LGM). Considering that during the LGM, much of northern North America, Europe, and Asia were under two miles of glacial ice, what use is understanding ENSO’s strength or orientation under such different boundary conditions? How can proxy estimates of the LGM mean state contribute to our efforts to forecast the future of ENSO?

Are trees becoming less sensitive to the recent warming?

posted Feb 25, 2016, 12:50 PM by Udya Kuwar Thapa   [ updated Feb 25, 2016, 2:12 PM ]

Summary

Tree ring series from environments where temperature limits growth have been useful in creating annually resolved temperature reconstructions going back thousands of years. However, some researchers have observed a weakened response of tree rings to temperature over the last 50 years of the instrumental record, a phenomenon called the “divergence problem.” This week, we will dig in to some potential causes and consequences of tree ring divergence. 

Description

The Arctic has seen the greatest increase in temperature over the last several decades of the instrumental period. Reliable paleoreconstruction of temperature in the Arctic would help us understand whether the recent warming is unprecedented. Tree ring chronologies from temperature-limit environments such as the Arctic have long served as useful proxies for past warming. However, Jacoby and D’Arrigo (1995) found that their tree ring data from Alaska and Canada significantly underestimated observed summer (May-August) warming over the past 50 years of the instrumental record. This disagreement between tree ring indices and summer warming in the recent decades in the Arctic is termed the “divergence problem” in dendrochronology.

The anomalous reduction in sensitivity of tree-ring indices with summer temperature has been tested by several studies. Briffa et al. (1998) tested this problem using a larger network of trees in the northern hemisphere and reported reduced agreement between tree indices and summer temperature. Bungten et al. (2008) on the other hand did not find the divergence problem in the European Alps.

Scientists have proposed several hypotheses to explain the divergence problem in tree rings, including drought stress, solar dimming, or simply data processing artefacts. During this discussion, we hope to sort out whether divergence is a “real” or perceived problem, and what could be causing it. We will also connect the divergence debate to our previous class topics and discuss the reliability of a range of climate proxies.

Your task

All of us will read Briffa et al. (1998). Then we will split into two groups to assess the tree-ring divergence problem. The first group will take the position of Bungten et al. (2008) to assess if divergence is a region specific problem. The other group will describe solar dimming as a potential mechanism for divergence. This second group will read Stine and Huybers (2014).

In addition to the assigned readings, each of you should bring an additional paper that adds a different perspective on divergence or explains a different hypothesis for the divergence problem.

While reading, focus on the same standard questions as in previous weeks: (i) What is the main argument authors are trying to make? (ii) What evidence do they have to support their argument? (iii) What are their limitations? (iv) Is there any such problem in the proxies you are familiar with? (v) What are the implications of divergence in tree rings?

 You can skim Jacoby and D’Arrigo (1995) and the D’Arrigo et al. (2008) review for background if you choose. 

How to prepare your PAGES-style 'Science Highlight'

posted Feb 25, 2016, 12:28 PM by Scott St. George   [ updated Feb 25, 2016, 12:31 PM ]

As part of this class, each of you will be asked to compose a short article that summarizes a current debate in paleoclimatology (potentially connecting to other related fields). In this post, I'll give you more specific instructions on how to prepare your Highlight, and remind you of a few deadlines related to this task. 

Twice a year, the international scientific group PAGES (PAst Global changES) publishes a very snappy-looking newsletter that features recent developments in paleoclimatology.  We'll use their guidelines to authors to prepare our own Highlight articles. I've attached a complete copy of one of their issues to this post, and suggest you skim through them to get aquatinted with the format, tone, and depth of their writing. Unfortunately, the PAGES website has been offline since last week, so at the moment, we can't refer to the published guidelines for their newsletter. Until that resource is back up-and-running, I recommend we agree to use the following instructions:

Guidelines
The body of the article, including figure captions, will not exceed 1,300 words.
The contribution should be divided logically using short, explanatory subheadings and should contain two figures.
A short (max. 40 words) and catchy abstract should be provided, highlighting the key finding and/or importance of the contribution.
Articles may include a maximum of 15 references.

Other helpful items
In addition to the copy of the full PAGES newsletter, I've also attached the final draft and published version of an article I wrote for the last issue of Past Global Changes magazine. If the first half of this article sounds familiar, it's because I used it as the introduction to our 'missing ring' debate in Week 2. I've included the final draft (as a Word file) so that you can refer to this example to see how to handle formatting, references, and other details. You may notice that I broke one of the PAGES guidelines for my own contribution, but please do not follow my own bad example. 

Due dates
On March 24each student must submit a title and one-sentence summary outlining the current debate in paleoclimatology that will be the focus of their ‘Science Highlight’.
By April 21students must have prepared a complete draft of their science highlight to share with their peers.
And on May 5, final drafts of the ‘Science Highlight’ are due.

If the PAGES website lurches back to life, I will update these instructions with links to their site, so you are able to review other examples and read the full guidelines for yourself. 

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