4 essay question from lectures
Paleoclimates
Week 3 – July 6th
Announcements
Lab #2 is this week
Lecture Outline
Intro to Paleoclimates
Climate variability of the Holocene to now
Climate Proxies
A focus on dendrochronology
Readings With Lecture
Week 3 Readings Include:
Chp. 3 in textbook
Lost Cities and Climate Change Kate Marvel (2019) if you haven’t done so already
The Scientific Consensus on Climate Change Naomi Oreskes (2004) – PDF on Canvas
Environmental Justice, Fossil Fuels, and Telecoms Ayate Temsamani (2018) – link here: https://www.greenamerica.org/environmental-justice-fossil-fuels-telecoms
To Save Climate, Look to the Oceans Ayana Elizabeth Johnson (2020) – PDF on Canvas
And we’re off! Let’s start out with an introduction to what paleoclimates are and how we get the data!
Introduction to Paleoclimates
Research on paleoclimates, or climates of the distant past, answers questions including:
What was the climate of the past like?
When we say “the past,” when do we mean, and how has it changed into the future?
A word up front: Paleoclimates can be discussed at various scales and over various periods…for this class we’ll be focusing on the most recent 10K-20K years or so and onward, with specific focus on the Holocene…we may delve a little bit farther back if time permits…
Climate Variability
Climate is always changing, it’s never static
Using baseline climatology we can establish average climate variables (e.g. temp and precip) for an area
These averages are then used to evaluate and track climate anomalies. These are very important to climate research because anomalies show you deviation from the norm. Thus, we can say things like “the average global temperature in 2019 was warmer than normal.” We know it’s warmer because we have the baseline.
“Fun” fact – if you are younger than 32 years old, you have never experienced a cooler than normal month. Each month you have ever lived through has been as warm or warmer than the 1950-1980 baseline average.
Climate Variability of last 2000 years
Over the past 2K years Earth’s climate first transitioned from a moderate/mild regime during the Medival Warm Period (MWP) in the 800-1200s, to the cold period of the Little Ice Age (LIA) in the 1500-1800s.
However, spatial extent of these two climate regimes/events was not global
Current warming is both spatially extensive, but also occurring at a rate not seen in the proxy record we have
Notice this temperature reconstruction is based on various proxies, not just tree rings.
Side note – the PAGES 2k reconstruction referred to here and in your textbook is newly published. Check out the link to a commentary by Scott St. George, a dendrochronologist at the University of Minnesorta on the research publication at the end of the powerpoint if you’re interested in learning more!
Climate Variability of the Holocene
In general the Earth was warmer in the earlier parts of the Holocene (8000-6000 BC), but has since shown a cooling trend.
However, recent trends show rapid warming – yep, that’s right, the Earth has been in a cool phase compared to longer records.
You can really see the influence of humans on this graph
Need more evidence? Check out the CO2 graph on the next slide…
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Carbon Dioxide Variability of the Holocene
Here’s a graph from your textbook
You can also check out the Scripp’s data for the Keeling Curve to look at this as well (check back to Lab #1 if you need a refresher on The Keeling Curve!)
Milankovitch Cycles
Cycles in the precession, obliquity/tilt, and eccentricity of Earth’s orbit dictate the amount of insolation receipt at the surface
When these three cycles align such that northern hemisphere summers receive maximum insolation, glaciers retreat and ice ages phase out (and vice versa)
This occurs because snow melt from previous winter is lost during max insolation periods, thus no snow can accumulate and the ice sheets recede/retreat
Read more in your textbook on how these cycles impact ice ages for more information
Critical Learning and Comprehension Check!
Use the CO2 graph provided or one you acquire on your own from the Scripp’s Institute to explain the temperature graph. In other words, how does increased CO2 lead to increased temps?
What do you notice about the rate of temperature change? How does it compare to rates of temperature change in the past?
What additional picture does the Holocene temperature reconstruction provide that the one for the past 2000 years does not? In other words, how does the window with which you look at past climates matter in the current discussion of climate change?
Remember, if you ever want to discuss these with me, don’t hesitate to reach out! I will happily provide you with answers and further resources for these questions if you’re interested!
So we know the climate changes, and we know that it hasn’t changed the same in all places through time.
How do we track these changes? What are climate data and how do we get them?
Let’s go!
Where do we get climate data?
Historical weather records (NOAA for past ~120 years) are great sources of climatological and meteorological data.
Ship logs and the like can get you back a few hundred years, but nothing reliable too far after that
Check out this port log of Philadelphia in 1776. It has date, hour, and temp – neat right!?
Proxy Records of Past Climates
Tracking records of climate change over much longer time periods and farther back in time, we use proxy records
Proxies must reflect appropriate levels of temporal resolution AND depth
In other words, ideal proxies extend far back in time (depth), and have fine temporal resolution to give indications of change through time (e.g. trees form rings each year, thus tree rings provide fine temporal resolution as proxies)
What makes a good proxy?
Consistent relationships between physiological, ecological, geological, etc phenomena and proxy metric over space and time
Ability to track change – again referring to temporal resolution and depth
The more information extracted from the proxy the better…
Tree Rings
Tree rings can give you information on all of the following kinds of environmental signals you may be interested in, such as….
Rainfall
Temperature
ENSO, PDO, AMO
Wildfires
Volcanic Eruptions
Landslides
…
Here’s a picture of a slash pine cross section from a Big Pine Key, Florida in the Florida Keys. This sample was part of my PhD research focused on fire activity through time. The rings are almost hypnotic, no?
Tree-Rings
Tree rings can provide environmental data for the area they grew in 100s to 1000 years back into the past.
And because trees produce one ring per year, they have annual temporal resolution
In other words, anything that happens to the tree that year will be recorded in the ring. For example, let’s say a fire sweeps through an area in 1558, long before people were keeping records of fire for the area. The fire will produce a scar in the 1958 tree ring on the trees it harms. Then, over 450 years later, a dendrochronologists like me comes along and collets the data!
So how do trees grow and how are rings formed?
What do the characteristics of the tree rings tell us?
Trees grow like stacked cones, each new year brings a new “cone” or ring.
The oldest “cones” will be nearest the base, that’s where we sample!
Width variability = Climate!
1407
1396
Here’s a picture of tree rings up close…
Notice the 1407 ring is exceptionally narrow, and the 1396 ring is wide. This variability is a climate signal through time!
If you collect tree cores from across a region, you now have climate variability through time and space! Wicked cool, no!?
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Tree Rings Up Close!
What do you notice from this collection of six tree cores taken from a forest in Colorado? Look at the 1690s…each decade or so during the 1690s (the rings growing to the left of the 1700 marker on each tree core) is suppressed.
This suppression across the cores indicates a regional response to drought – these trees recorded drought in the 1690s, long before people were keeping records for the area!
Tree Rings
Patterns in ring width can be translated into records of wet and dry periods in past climate
Tree Ring Data
The only proxy with consistent annual resolution – trees grow one ring per year
Trees are found virtually everywhere
Here I am cheesing with a slash pine sample I just cut. It’s kind of hard to see, but my fingers are wrapped around fire scars, which track instances of fire occurrence through time. The tree forms a ring, the fire occurs and leaves a scar in that ring, and BOOM, I can come along and tell you when a past fire occurred down to the year! This is powerful stuff when you want to reconstruct past environments or climates for time periods before record keeping began.
Why might we care about how wildfires acted or “behaved” in the past? Why might it be helpful to know how often, and how severe wildfires were, compared to how they are now?
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Geoviz – Preliminary Research in Action!
If you’ll indulge my nerdiness here for a second, what you’re looking at is results fresh from my lab. Specifically, students and myself collected Ponderosa pine tree cores from the Cheney wetlands. We then used the wide and narrow variability in ring widths to reconstruct precipitation back 400 years! Any period above the baseline average is green and indicates a wet period, any period below the baseline average is in red and indicates dry periods. The vertical black line on the graph is the beginning of historical (weather) records for the area. Everything before that in time has been reconstructed from tree rings.
These data are fresh, they are still being processed for errors, so this isn’t final by any means. BUT, given these preliminary results, what are some patterns you notice?
Interested? Who wouldn’t be?!
Geography minor = 15 credits
You’re 1/3 done after this class already, and the remaining 2 classes are offered online in the Fall – I’m teaching one of the sections too, so let me know if you’re interested or have questions!
Graduates with Geographic knowledge of human-environment interaction, especially those with skills GIS, are incredibly desirable in today’s job market. Just saying…
Lower Temporal Resolution Proxies
Extensive archives of tree-ring data allow for cross dating and the storage of annually resolved data – check out the end of the lecture slides for links to proxy datasets to explore!
Radiometric Dating must be used for proxies without the advantage of annual temporal resolution or for instances of deeper age
Radiocarbon for younger materials (~45-50K)
Potassium-Argon for mid-age to old (~100K or more)
Uranium-Lead for deep time and very old samples(~1 million to a couple billion)
So what are some climate proxies that are not annually resolved, like tree rings, and don’t form or produce a feature each year?
Specifically, what about climate proxies that are built or formed via rates (e.g. sedimentation rates)?
Let’s go!
Lake Sediments
From 500 to about 40,000 years
Lake sediments can tell us about past vegetation through pollen records
Past wildfires from charcoal
Past hurricane activity
and more…
Lake Sediments
Sedimentation rate is extremely important when establishing activity of the past
Ice Cores
1000s to 100,000s of years
VOSTOK project in Antarctica is at 1M now!
All the air bubbles are ancient atmospheres!
Ice Cores
Greenland (general, but not good for CO2 reconstructions due to impurities in the ice) and Antarctica (best for researching past-atmospheres due to purity of ice and minimal contamination from things like dust)
Ocean Sediments
From about 10,000 to 1 million years into the past
Ocean Sediments
Just like lake sediment cores, but from much deeper depths
Sedimentation rates can be MUCH slower, meaning it takes much longer for these sediments to build up. This is good because it means you can investigate processes from deep time.
Speleothems
Pollen gets trapped in the water that drips down to form the cave decorations.
The stalagtites and stalagmites form rings that can be isotopically dated
Varved Sediments
Annual deposition of layers of sediment
Similar to rings on a tree
Depends on complete lack of bioturbation or disturbance
Packrat Middens
Joshua Tree Natl Park, back 7000 years
Pollen analysis as proxy for temperature
Foraminifera (Forams)
Different species survive in different ocean water temps!
Concentrations of species at depth gives a temp proxy
Critical Thinking and Comprehension Check!
Which climate proxy provides the most temporally-resolved data? The proxy with the longest back in time?
You find a piece of carbon from a tree root buried deep in your lake sediment core. You suspect the sediment core in totality to be at most 10,000 years old. Which radiometric dating method do you use? Why?
Explain what this graph is showing. Be sure you include in your answer reference to temperature anomaly vs temperature observation.
Critical Thinking and Comprehension Check!
What are Milankovitch cycles? How do they impact climate? How might they be used to distort a climate change discussion?
As always, if you’d like to review these questions I’ve scattered throughout the lecture slides with me, please don’t hesitate to reach out!
Critical Thinking and Comprehension Check!
Links to Check Out!
Water in the West via Stanford University https://waterinthewest.stanford.edu/
Primer on paleoclimates, a newly updated paleo-temperature multi-proxy database was just published, this is cutting edge stuff!! https://www.nature.com/articles/s41597-020-0531-6
Scott St. George’s commentary on the global asynchrony discovered in the MWP and LIA via the PAGES 2k global temperature reconstruction
https://www.nature.com/articles/d41586-019-02179-2
Looking Ahead…
Week 4 is Discussion #2 and Exam #1 – you will have all week to complete the exam just like labs
We’ll be covering topics related to carbom and the carbon cycle
The only readings for Week 4 are Chp. 5 in your textbook