Next week we'll move towards the intersection of paleoclimate and archeology, and discuss a set of papers that argue past climate change had profound impacts on ancient societies.
October 2010 Archives
I just found the animated version of the deglaciation maps I showed in class.
Tomorrow, we'll talk about proxy insights into the dynamics of large lakes in North America and Africa but first, I'll review how the concept of the 'Medieval Warm Period' has developed since its introduction in the mid 1960s.
Slides are here.
Also, the broader impact of the Shanahan article was discussed in an article on the prominent New York Times Dot Earth blog.
Next week, our discussion will turn back to drought but this time, the papers we'll read are entirely tree-ring free.
Students will submit citations for 10 articles that are related to the topics for their semester projects. These lists can be preliminary, so if you decide you want to add or subtract citations later, that will be fine.
Students will present brief (15-minute) summaries of their project topic. Key questions to keep in mind while preparing your presentation include: (1) What are the most important features of the modern climate in your region? (2) What proxies are available in your region, over the time interval specified? How are they related to climate? and (3) How different were past climates from modern conditions? Why is that important?
Student papers are due (in class). Papers should not to exceed 10 pages of text plus figures and references, and review the climate history of a single region over a specific time period during the Holocene (e.g., the mid-Holocene, the last 2ka or the last 500 yr).
This schedule is based on my memory of our discussion last Wednesday, but if I've inadvertently moved things around, please let me know ASAP.
In record time (!), here is the summary of our discussion this morning, courtesy of Keith H.
The Pacific Decadal Oscillation is a multi-decadal oscillation in North Pacific sea surface temperatures (SSTs) that affects precipitation and temperature patterns during the winter season in North America. When the PDO is in its warm phase, cool SSTs and anomalously low sea level pressure exist in the Northern Pacific Ocean. The PDO Index quantifies the strength of the Pacific Decadal Oscillation by taking the difference between Pacific SST anomalies north of 20N and global SSTs. Typical phases in the PDO last approximately 20-30 years, with observed abrupt changes in sign of the PDO Index in 1946 and 1977. Since the instrumental record for SSTs only goes back until approximately 1900, only three clear PDO phases have been observed. Therefore, tree ring records in regions that are significantly affected by changes in the PDO are used as a proxy for the PDO Index for the years before instrumental records are available. These records are chosen by finding sites where winter precipitation in the 20th century is highly correlated to the PDO Index and can therefore be used as a proxy for the PDO Index. If winter precipitation is highly correlated with the PDO Index, then it is assumed that trees in these areas are highly sensitive to the amount of winter precipitation that falls, and the winter precipitation mostly varies based on the Pacific Decadal Oscillation. A physical mechanism for the Pacific Decadal Oscillation has not been identified.
Biondi et al. (2001) gathered six tree ring chronologies from Southern California and Baja California to represent the PDO Index back to ~1650. Although the winter precipitation in this region does not appear to be correlated with the PDO Index (Figure 1, Mantua and Hare 2001), the Tree Ring Index matches the PDO Index remarkably well from about 1920-1995. Therefore, the Tree Ring Index from these trees is a suitable proxy for the PDO Index before 1900. The decadal-scale variability of the PDO that is observed in the 20th century is also observed in these tree ring chronologies. The reconstructed PDO Index from Biondi et al. (2001) has a correlation of 0.57 with the reconstructed PDO Index from MacDonald and Case (2005).
Mantua and Hare (2001) assembled various studies that used different proxies to represent the PDO Index back to around 1600. They observe that while the dendrochronologies are in sync during some periods, many periods exist where they show "little, if any correspondence with each other." This indicates the likelihood that other influences besides the PDO were affecting precipitation and/or the growth of trees in areas where tree rings were sampled.
MacDonald and Case (2005) used two tree ring chronologies (one from southern California and one from western Canada) as a proxy for the PDO Index back to approximately AD 900. These locations were chosen because they represent a dipole of precipitation from the PDO, where increases in precipitation one location due to the PDO is coincident with decreases in precipitation at the other location. The tree ring chronologies were significantly correlated to the PDO Index during the period of observed SSTs, but displayed rather low correlations with other PDO proxies (r = 0.19 with Gedalof and Smith (2001)). The low sample size of tree ring chronologies may not be truly representative of regional climatic variations that are caused by the PDO, but coincident local climatic variations instead. Perhaps the addition of more tree ring chronologies would provide more confidence about the structure of the PDO before the presence of instrumental records.
Last week we talked about tree-ring evidence for 'megadroughts' in the southwestern United States. Here's a summary of our discussion courtesy of Salli D.
Climate proxies such as lake sediments and tree rings are hydrologically influenced. Investigating hydrologic paleorecords gives scientists an insight into the past hydrologic regimes of a region and allows for better prediction tools for future hydrologic patterns. Discussion focused around 3 scientific articles that studied the hydrologic paleorecord of the Southwestern United States, a region that is heavily influenced by changing hydrologic patterns.
Stein (1994) focused on tree-ring records and carbon dating of preserved woody matter to prove that lake levels in the San Francisco region had dropped significantly during two periods: one centering around 1100 CE and the other at 1350 CE. The large variability in the estimated droughts (especially drought 1) brought up the uncertainty in that can be present some carbon dating analyses. Despite the variability, the establishment of trees on the lake bottoms presents an acceptable argument that drought did occur over the time period, even if the exact date cannot be pinpointed. To prove that these droughts were a global phenomenon, Stein also analyzed proxies from Patagonia. Located in the same Pacific rain-shadow, this region in Argentina can be thought of a Southern-Hemisphere equivalent to the study sites in California. Results were consistent with those found in California, leading Stein to conclude that the "Medieval Warm Period" should instead be referred to by a different name (such as the "Medieval Climate Anomaly") to denote that changes were global and anomalies not only occurred in the temperature regimes but other climatic conditions as well.
Cook et al. (2004) used the Palmer Drought Severity Index (PDSI) and corresponding tree-ring records to create a grid map reconstructing annual drought over North America. Using Drought Area Index (DAI) as a metric for whether or not a drought occurred in a given year, the authors were able to create drought trends for the Western U.S. dating back to 800 CE. Their results showed that conditions were drier during the "Medieval Warm Period" and that the climate has tended toward wetter conditions in recent years. What is notable about the paleo-droughts is both the duration and intensity. No modern drought has been recorded on such a scale. Cook et al. suggest that the droughts were caused by La Nina conditions off of the Pacific Coast, which is known to create drought conditions in the Western US. The results of this paper proved consistent with Stein (1994), further suggesting evidence of Stein's proposed "Medieval Climate Anomaly."
Meko et al. (2007) looked at the relationship between tree-ring chronologies and streamflow in the Colorado River. Using various sampling sites throughout the watershed, the authors created a plot of the 25-year running mean of discharge in the river (Figure 2). The results suggest that flow is extremely variable and reached its highest known discharge in the past 100 years. A large and lengthy drought was detected during 1130 and 1154 CE, which did not precisely overlap with the results from Stein (1994). If the variability in Stein's data was expanded to include 2 standard deviations, the data would likely overlap, suggesting an evidence for widespread drought around 1140 CE. It is also likely that spatial heterogeneity of droughts and precipitation patterns are harder to detect at large spatial scales. Also, it was noted that the Colorado River drought did correspond to droughts in other locations, such as the Great Basin.
Next week, we'll read three paper on the Pacific Decadal Oscillation, or PDO. The PDO a is a persistent pattern in the climate of the North Pacific Ocean that is similar to El Niño in many respects but changes much more slowly.
The first paper by Nick Mantua and Steven Hare describes how the PDO operates in time and space and how it affects Pacific fisheries and the climate of western North America.
The other two papers use proxy records (tree rings, again) to estimate how the PDO behaved prior to the 20th century.
We have a lot to talk about tomorrow: the summary of our global temperature discussion, topics for climate history projects and MEGADROUGHTS! Until then, here are my graphics for tomorrow:
I've decided to re-arrange our class schedule, so next week we'll discuss proxy records of severe drought during the past millennium. We'll talk about a pair of recent tree-ring papers that have had a big impact on discussions of water resources in the western US and interconnections between this region and the broader climate system.
We'll also read a classic paper that used a very simple approach - dating dead, submerged tree stumps - to make very powerful inferences about drought in California during the medieval period.
Also a reminder: I'll make slides for the major figures from the papers but please bring your own copies of the articles.
Courtesy of Nick Y., here is his summary of our discussion of proxy estimates of global temperature changes during the last two thousand years:
The Hansen paper focused on temperatures over the 20th century and how its trend in the latter portion fit his expected scenarios. We agreed with Hansen that the late 20th century warming fit Scenario B, a scenario that expected moderate warming because of moderate CO2 emissions and volcanic eruptions. We decided the importance of studying global temperature was because it isn't as local as precipitation, its impacts are more wide ranging, and it is directly correlated with the effects of increased CO2. Lastly we discussed Hansen's beliefs about increased frequency of super El Niño events and that it may be hard to show a correlation with just two data points.
The other two papers (by Osborn and Moberg) dealt with using proxies to look at the statistical importance of 20th century warming. Both tended to focus on proxies located in the Northern Hemisphere, but Moberg included some in the lower latitudes. Moberg also seemed to have a wider range of proxy types whereas Osborn's study relied heavily on tree-rings. We discussed the reasoning behind the two studies. By looking at periods with increased deviation, Osborn simplified any comparison by counting the number of proxies below or above normal to normalize them. He also looked to see at how many standard deviations away these proxies were from the 20th century mean. Moberg used climate models to help get around the problems with comparing low-resolution proxies, such as lake sediment records, and the instrumental record. Both studies showed strong signals of warming in the twentieth century. Osborn highlighted the exceptional nature of this warming by showing a large discrepancy between the great number of positive deviations and the small number of negative deviations.
We ended the class by discussing the reason for conducting these studies. These proxies allow for a greater period of time to study and then compare to the most recent interval. It also compares the 20th century to warm periods without anthropogenic CO2 forcing and therefore improves our estimates of the sensitivity of the global climate system.