Palaeotempestology: Tree rings

In my last blog, I explored how the layers of calcium carbonate, which build up as a coral skeleton grows, can be used as a climate proxy. We can find a similar process by looking at tree rings. One of the more established practices in palaeoclimatology is dendroclimatology (the use of tree rings to study the past climates). Like other palaeoclimatological proxies, it allows us to extend the range of our observational record beyond that of conventional weather recording instrumentation.

Just as corals live for hundreds of years (sometimes over a thousand years), trees can keep on recording the composition of the atmosphere in their layers of cellulose for many hundreds of years, and beyond when fossilised. Figure 1 below shows an example of Huon pine samples ready for analysis, each dark line denoting a season of growth.

Figure 1: Huon Pine ready for analysis. Source: Edward Cook, Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY

Isotopic differences

Ancient pines are often the favoured study subjects due to their longevity. They can give annual or seasonal information on atmospheric composition. To extend the record beyond a single sample, a variety of sources can be combined together using distinctive signatures as shown in Figure 2 below.

Figure 2: Sources of tree ring data showing how various samples can be linked together. Source: Laboratory of Tree-Ring Research, The University of Arizona

The main process that allow us to look at past storms is the fractionation of stable oxygen isotopes through condensation and evaporation. I touch upon this in my previous blog about corals, it is the difference atomic weight between the heavier oxygen-18 isotope and oxygen-16 isotope that allows us to glean clues about past climate events from tree cores.

The difference in atomic weight of oxygen isotopes is derived from the number of neutrons in the atomic structure. The most common natural isotope is oxygen-16 (over 99% of atmospheric oxygen) which has 8 protons and 8 neutrons (electrons are virtually weightless by comparison), but stable oxygen atoms can also have 9 or 10 neutrons to make up the different isotopes that we find useful for palaeoclimatology. As mentioned before, the water molecules with the lighter oxygen isotopes (oxygen-16) are preferentially evaporated in warm temperatures, while conversely the water molecules with heavier isotopic values (oxygen-18) tend to condense and form clouds or precipitation more easily. It is this property that allows us to identify different sources of precipitation in tree ring samples.

In extreme precipitation events associated with tropical cyclones, the level of oxygen-18 depletion in the rain water is high due to the highly efficient process of forming precipitation via condensation in the core of a tropical cyclone (Lawrence in 1998, Monksgaard et al. 2015). In Lawrence’s paper, five tropical cyclones that made landfall in Texas, U.S, were studied. They showed much lower oxygen-18 to oxygen-16 ratios (or δ¹⁸O) from tropical cyclones than normal summer convective storms.

This finding was further corroborated by a study of Hurricane Olivia by Lawrence et al. in 2002. Tropical cyclones are also large and long-lived and create vast areas of precipitation that can stay in the water system for weeks, giving different isotopic characteristics associated with the location of the heaviest rain bands and storm centre (Monksgarrd et al 2015). Deep soil water can remain unaffected by normal summer rainfall, and in the absence of further heavy rain events, it is allowed to be taken up by trees (Tang and Feng, 2001).

It seems clear that oxygen isotope analysis seems to be the favoured form of tree ring analysis for palaeotempestology.

Tapping the potential

Upon learning about these methods it also seems reasonable to assume that different intensities and characters of storms will result in different levels of oxygen-18 depletion. It seems likely that there would be much uncertainty in making assumptions of a storm’s intensity based on isotope fractionation (but I’ll keep looking for more research on this). At the moment, it seems that the uncertainty may preclude a reliable intensity measure of past storms using this approach.

The oxygen isotopes uptake into the tree’s structure will depend on many factors, including biological processes that are dependent on species, tree age, exposure to the storm, soil composition. Growth cycles are also taken into account. By doing so we can try to limit the degree to which uncertainty derived from the mismatch between growth season and storm season, can cloud useful information.

In the North Atlantic basin for example, hurricane season runs from early June to late November and as such overlaps mainly with latewood (as opposed to earlywood) growing phase. Therefore it is these sections of the layers of tree rings which are focussed upon for palaeotempestological studies.

Miller et al. 2006 presented the emerging case for using oxygen isotopes more widely after the devastation left behind by the busy 2004 and 2005 hurricane seasons, by building a 220-year record to identify past storms from unusually low oxygen-18 isotopes in pine forests. This is potentially very useful for engineering and loss modelling concerns.

“Can’t see the wood for the trees”

There are many uncertainties in the application of tree ring data to palaeoclimatology, let alone palaeotempestology, as summarized in the review paper by Sternberg et al. in 2009, including complex cellulose uptake biology, changes in isotopic composition of soil water, assumptions based on the relationship between leaf temperature and ambient temperature.

However, every study adds to the wealth of information and since each site represents a single location slice through time, it seems as though the science of dendroclimatology will only continually benefit from new data. And there still seems to be push to collect and analyse more data. The National Climatic Data Center, hosted by NOAA, is a font of old and recent tree ring datasets.

A recent review of the data by Schubert and Jahren published in October this year (2015) takes a wide view. It aims to unify tree ring data sets, to bring together a global picture of past extreme precipitation events based on low oxygen-18 isotope records. They conducted 5 new surveys and used 28 sites from the literature to create a relationship using seasonal temperature and precipitation, which can explain most of the isotopic oxygen ratio in tree cellulose. This seems to be a step up in resolution, as looking at seasonal variations rather than annual cycles may provide a step closer to identifying individual storms or storm clusters using tree ring data. It is interesting to see a comment in the conclusion of this paper about the fact that much of the uncertainty that still remains in this link, is derived from disturbances, such as storms.

Figure 3: Comparison between measured δ¹⁸O in the cellulose of studies trees and the calculated δ¹⁸O using the model developed by Schubert and Jahren which uses known climate characteristics. It shows a good correlation on relating seasonal temperature and precipitation to oxygen-18 isotope ratios. Source: Schubert and Jahren, 2015

It seems clear that it would be much more difficult to develop a simple equation to explain the extremes of the isotopic ratio chronologies to identify extreme storms. However, Schubert and Jahren seem to have taken a step forward while remaining focussed on average seasonal conditions. Nevertheless, I can’t help but wonder if there is a way for extreme events to be linked in to somehow.

Alternatives to isotopes

When looking specifically at past storms in trees rings, I did find a couple of other approaches to using tree ring data that may also be worth a mention.  

Firstly, an interesting couple of papers by Akachuka in 1991  and another in 1993, used a method where trees that have been forced to lean after a hurricane. This phenomenon is examined for any extra clues that it may provide by assessing how these trees recover from such disturbances. Although the papers do not look specifically at characterising the storms themselves (i.e. there is no wind speed to bole displacement relationship), I couldn’t help but wonder if there is some extra information to gather from these trees and whether we could build a relationship to specific storms or storm seasons.

Another paper by Sheppard et al. in 2005 looks at the effect of a tornado in 1992 on a specific dendrochronology and re-evaluates the pre-historical records from wood samples retrieved from an 11^th century ruin in Arizona. He looks for similar patterns in wood growth (see Figure 2 for conceptualisation). Unfortunately, the patterns found in the tree rings which were caused by the tornado in 1992 were not replicated in the ring patterns of the 11^th century sample. This is certainly interesting work, but I imagine that finding enough data for trees that are damaged but still survive tornadoes is not easy, especially when comparing to single older samples.

Conclusions

Although individual studies using tree lean or damage from specific events like tornados, are interesting and worthwhile academic endeavours to help us understand the ways in which storms of various scales impact certain tree growth, they do seem somewhat less applicable to thinking about climate change and how frequency and severity of storms are changing over a wide area.

With so many subtleties based on factors such as tree species or topography of a study site, I feel that the broader synthesis approaches (as per Schubert and Jahren above) using stable oxygen isotopes offer greater immediate potential for aiding our understanding of past changes in storm activity with possibility for application to risk assessments and projecting impacts of future climate change. 

Palaeotempestology series: Introduction

In a previous blog, I talked about the various ways in which historical documents, records and anecdotal evidence are used in climatology. I mentioned briefly some of environmental proxies used to derive information about the climate throughout the whole of the Earth’s history using palaeoclimatological techniques. Studying past climates is an essential part of any debate on climate change and there has been a huge amount of science produced in this field both in terms of improved methods and developing datasets.

Depth of data

Ice cores, lake sediments, tree rings records, coral analyses and more, have been conducted around the world for the last few decades to build the picture of past climates that we have today. The National Oceanic and Atmosphere Administration (NOAA) in the U.S. has an online portal and interactive map (Figure 1) that shows the geographical spread of data. I knew there was a lot of data out there but this map really puts into perspective the amount of work that has been done to gather information around the world, but also shows that there are still many gaps and much more that could be done. Check out the Climate Data Online interactive map of palaeo records here.

Figure 1: Screen shot on NOAA's Paleoclimatology interactive map at Climate Data Online. Source: NOAA (https://gis.ncdc.noaa.gov/map/viewer/#app=cdo&cfg=paleo&theme=paleo)

Depth of study

As a snapshot to show the amount of research into palaeoclimatology, a useful list of just one year’s worth of research is compiled here by the team at the 'Skeptical Science' website.

Palaeoclimatological proxies are signatures left behind in the natural environment that can tell us something about the climate in the past. They require detective work and often sophisticated laboratory analysis, but can provide windows into the past to show us data that are otherwise not available.

They are often used to derive at temperature trends over thousands of years from which drought periods can be inferred, or to develop records of past atmospheric composition (useful for revealing changes in greenhouse gas concentrations) but certain proxies can also used to investigate past storm activity.

Pre-historical storm evidence

Since I am obsessed with storms, when thinking about pre-historical records, I couldn’t help but be drawn towards Palaeotempestology (a term coined by Professor Kerry Emanuel at MIT) which is the study of pre-historic storms. In this context 'pre-history' refers to the time before the beginning of observed instrumental record of weather and climate data, which is generally no more than 100-150 years long at best, shorter still if you consider that observations and full representation of all storms that occur has only really been possible since weather has been observed using satellites.

The first satellite used to observe weather conditions was TIROS I, launched on April 1^st 1960 and initially could only tell us some basics about locations of clouds, as analysed by hand. This image below (Figure 2) shows the very first image from this satellite.

Figure 2: The first image sent back from the first satellite used to observe the weather. SOURCE: NOAA/NESDIS

Satellite technology and application has come a long way since then (I’ll likely cover this in a future blog).

Palaeotempestology aims to look back hundreds or even thousands of years, so I’ll take a bit more time on this subject. In my next few blogs, I shall aim to investigate, and share, more on the various sources of data used to drill down in to using sediments (Figure 3), 

Figure 3: Heavy duty sediment core retrieval. Source: NOAA image by Ane Jennings. (ftp://ftp.ncdc.noaa.gov/pub/data/paleo/slidesets/heinrich/heinrich08.jpg)

swim through the information on coral cores (Figure 4),

Figure 4: SCUBA scientists extracting a core from coral. Source: NOAA image by Maris Kazmers. (ftp://ftp.ncdc.noaa.gov/pub/data/paleo/slidesets/coral/coral12.jpg)

and circle around the subject of tree rings (Figure 5).

Figure 5: Scientist preparing to take a sample from a Giant Sequoia tree. Source: NOAA image by Peter Brown. (ftp://ftp.ncdc.noaa.gov/pub/data/paleo/slidesets/treering/tree01.jpg)

Great Scott! 'Doc' Brown harnesses lightning using historical climatology!

Just over a couple of weeks ago, on October 21st, it was the 30th anniversary of the release of legendary sci-fi 80's classic ‘Back to the Future’.

If you like the film, you'll no doubt remember Emmett 'Doc' Brown - the mad scientist who builds a time machine (housed in a DeLorian sports car). Our hero, Marty McFly, gets stuck in the past and has to work out how to get ‘Back to the Future’. Upon reminiscing on this film, it occurred to me that when 'Doc' refers to himself as “a student of all sciences” he is including historical climatology. 

***SPOILER ALERT***

In the film, they need to find an energy source big enough to power the time-travelling DeLorian back in the 1950's. ‘Doc’ notices a “Save the Clock tower” flyer from the future, showing exactly when a lightning strike will hit the town's clock tower, which coincidentally, is set to happen in just a few days time. Madcap shenanigans ensue, but they end up performing the ultimate in verification of their historical climatological evidence when they manage to take advantage of this information to harness the power of the lightning strike, to send Marty back to the future!

Marty's flyer in "Back to the Future". Source: http://backtothefuture.wikia.com/wiki/Clock_Tower_flyer

This is actually a pretty good (fictional) example of how evidence derived from human society might relate to past weather records, especially for extreme or newsworthy events. Much of the uncertainty in the detail of how our climate has changed is down to a lack of data, and so using new sources of information is an important element of any discussion on climate change. 

With that thought in mind, for this blog, I have investigated historical climatology to study climate through human history (the Anthropocene), and how it may be used to support palaeoclimatological data which differs in that it studies climate throughout the history of the Earth.

It’s how you tell ‘em

In general, from checking a number of definitions, anecdotal evidence differs to scientific evidence in that it cannot be proven or disproven using scientific means. Historical climatology generally only stretches back a few thousand years too, since it focuses on the study of climate through representations in human historical evidence. The goal of a historical climatologist is to use what evidence can be gleaned from anecdotal and more official human records to infer observations about the climate. A good background of the different sources of historical climatological data was put together by Phil Jones in 2008. Data sources include:

• old news paper reports and photographs,
• mariner’s reports,
• town hall records,
• agricultural records,
• old maps,
• and one I am particularly intrigued by and probably more appealing than most other sources of ‘data’, the use of old paintings, and even cave paintings.

Here are some great old pictures (collated by ITV from PA) of the cold winter of 1963 in the UK, with parts of the River Thames apparently frozen. And further back still, here’s an old painting from 1683 of a Frost Fair,

Thames Frost Fair, 1683-84, by Thomas Wyke. Source: Wikipedia.com

and another from 1677 of the Thames freezing over with London Bridge in the background by Abraham Hondius.

The Frozen Thames, 1677, by Abraham Hondius. Source: Wikipedia.com

There are many depictions of cold weather through the 17th Century that coincide with the “Little Ice Age” and therefore it might seem to be a causal link. However, a lot has changed since the 17th Century. The river Thames for example was wider and shallower in places, and human structures such as the various bridges and flood defences, would have also affected river flow differently, perhaps allowing for freezes to occur in the past.

In short, many other factors determine whether a river freezes over, aside from temperature alone. I was asked about whether the Thames could freeze again while working as a weather forecaster, just a few years ago with the cold winter in 2010, and the question normally crops up during any cold winter season, as it certainly captures the imagination whenever freezing weather arrives in London. There is more on this subject at RealClimate.org here. This article goes on to describe how one of the most useful applications of anecdotal evidence is in the assessment of glacial retreat and ice melt.

Cool photographic evidence

A recent paper in Nature by Anders A. Bjørk et al. in 2012 uses 80-year old photographs of southeast Greenland to gain an insight into glacier and ice cover before satellite data became available. Photography offers greater ability to measure and analyse than an artistic impression, and so in this way, this paper was able to characterise some glacial responses over recent decades and link them to external forcings. It discusses how the warming in air and sea surface temperature led to rapid glacial retreats, but the study also notes that those glaciers that stop over the sea may respond faster than those terminating on land. A good article here covers the paper also.

Proxy validity?

It is important to validate any proxy information so, where data is sparse, alternatives should be considered. Historical climatology plays a part here, and can provide useful evidence if properly analysed. The main use would be to validate scientifically derived climate information from sources that can extend further back in through antiquity and beyond. By providing analogues (periods or events when both datasets agree), historical climatology might help to give validity and confidence to other datasets such as palaeoclimatoloical proxies. The main sources of palaeoclimatological records include:

• ice cores (essentially compacted snow over millennia is examined and useful information can be extruded using thickness of layers and the analysis of trapped air to find and measure oxygen isotopes, pollen or volcanic ash)
• tree rings (dendroclimatology can provide information on growth rates of different tree species which allows us to deduce information about the climate),
• corals (sclerochronology is essentially like using tree rings, but different variables of the atmosphere and oceans are represented),
• sediment layers in lakes.

Related to my main topic of storms, lake sediments can be used in to measure storm activity in the past. The method is quite opportunist and cannot be applied everywhere (perhaps a bit like anecdotal evidence in that respect) but basically involves using lakes that routinely hit by storms, or storm surge, and measuring indicators of erosion from flooding and/or incursions of sea water that build up, into the sediment layers, over the years.

There are numerous studies that infer storm and flood rates and intensities from lake sediments such as Gilli et al. in 2013, Vermaireet al. in 2012, and Page et al. in 1994. Through using lake sediments to look at salt water incursions, one recent paper by Donnelly et al. published in February this year, suggests that pre-historic storms (before the mid-1800s) on the NE coast of the US were more intense than anything we have seen in the historical records. It seems that this is a chief proxy for looking into palaeotempestology. This has peaked my interest, so I’ll be looking at lake sediments and storm surge again in more detail in future blogs.

Everyone loves a good story

I have learnt that evidence analysed by climate historians is difficult to link with the more scientifically collected evidence from palaeoclimatological proxies, but it can provide a degree of verification if used carefully as discussed above. Some cases, such as using old painting and photographs, certainly capture the imagination easily, and old mariner’s reports and documentary accounts can be equally compelling. Perhaps its power is also effectively focussed on public engagement, allowing a narrative and immediacy that many graphs and figures do not. I’m sure the idea of science communication and engagement will come up again in my blog as I do find it fascinating.

New sources of weather data

So aside from Doc Brown using historical evidence in the form of an old flyer, Anders A Bjørk using old photos to study glaciers, and palaeoclimatologists using a whole host of proxies for peering further back in time through the ages, I’ve also wondered what types of past evidence there will be in the future, aside from our ongoing weather observations network. In a world of Big Data, there is plenty of unverified but still potentially valuable data around. With the dawn of the internet, we have become able to share and consume data from virtually anywhere on the planet.

Social media and smart phone technology provides unprecedented accessibility to environmental data. “Citizen Science” crowdsourcing projects such as mPING from the National Severe Storms Laboratory, or the Met Office's Weather Observations Website (WOW) provide portals for the general public to upload weather reports that can be used to improve forecasting.

mPING report data. Source: NSSL mPING website.

These are not calibrated observations by professional staff or sensitive equipment, but they do potentially provide a coverage and density of data points that cannot be matched by conventional ground-based weather observing. Calibration can be performed, by using other sources of data such as radar, satellite imagery and more credible weather stations. It is the hoped that this kind of data can provide better information on extreme weather events in the future.

Back to the beginning

But thinking back to the storm that started this line of enquiry - the fictional thunderstorm that hit the small fictional town of Hill Valley - it’s easy to find ourselves a little envious of ‘Doc’ and Marty. I find myself musing that if we had the time-travelling DeLorian, we could go back to verify as much historical or palaeoclimatological data as we want. Better yet, we could even warn our past selves that climate change would become such a major global issue, and advise ourselves that we should at least start thinking seriously about a more sustainable future.

Although, as we know from most films involving time travel, things always go wrong when you start messing around with the past. Perhaps, it’s best that we must stick to learning what we can from what’s left behind for us to find.