Observational climate data only goes back roughly 400 years, and while that is enough data to discern a global climate trend for the Modern Era, it doesn’t give us a whole lot of data for comparison to Earth’s past climates. Paleoclimatology is the study of the Earth’s past climates. It is a growing scientific field which concentrates on understanding how climate has changed in the past and the forces that cause climate to change. Paleoclimatologist use a wide range of tools to understand the Earth’s climate, from studying tree rings, marine life and other biological climate markers, to ice cores and sediment cores. Constituents, such as oxygen isotopes or prehistoric marine organisms, which allow scientists to study paleo-climate are called proxies. Proxies are like a snapshot in time, which capture past chemical and temporal conditions of the Earth’s atmosphere and oceans.
Tree Rings
Have you ever counted the rings of a tree to try and determine its age? Not only do tree rings tell us a lot about the health, and age of a tree, but each ring is an indicator for the environment in which the tree grew. Unlike animals, trees live, grow, prosper, and whither away in the same location, sometimes for hundreds or even thousands of years (such as the bristle cone pine tree). Because trees are stationary, and can have pretty long lifespans, they are perfect for understanding regional climate. The scientists who spend their time hiking through the woods and boring into trees are called dendrochronologists.
So how do we extract vital climate data from tree rings? It all starts with groves of mature trees that have, for the most part, been untouched by humans. Ultimately, the perfect specimens will be old, healthy, and located in remote areas away from outside forces such as roads, streams or rivers which might mask certain measurements. Selected trees are then cored with a tool that looks like a giant corkscrew with a hollow bit (Figure 1). This allows a scientist to drill into the tree and remove a very tiny sample without harming the tree. Cores can also be sampled from dead trees. By sampling a dead tree scientists can cross date the sampled core with cores from live trees to find out when the dead tree began growing, it’s growth conditions, when it died, and the possible cause of death.
Typically when scientists set out into the field to collect core samples, they attempt to collect as many samples as possible. This helps increase resolution of the data collected, and allows them to create more accurate climate records. Once all of the cores have been collected they can be safely returned to the lab, where the real work begins. In the lab tree rings are examined under a microscope, where each ring’s width is measured and cell density is recorded. Each ring tells us whether or not that year was particularly hot or cold, or if there was an abundance or shortage of water supply. Also found within tree rings are scars which can indicate forrest fires or the occurrence of other natural disasters. After all of the data is collected it can be cross referenced with local meteorological sites, climate, and natural disaster records to determine if the record is an accurate representation of the regional climate from which the trees were sampled (Figure 2).
Ice Cores
Think of a desolate arctic environments, and a chill so cold that your bones ache. These harsh conditions are perfect for collecting ice cores. Permanent ice found in locations such as Greenland, Antarctica, the Himalayas, the Swiss Alps and other glacial paradises provides an excellent way to study Earth’s early atmospheric chemistry. The ice found in these locations is probably much older than you may think. For example the Vostok ice core from Greenland dates back nearly 750,000 years! This is because ice accumulation is very slow, and is directly affected by the Earth’s global climate. As snow precipitates on glacial locations during the winter months, it is compacted over time and creates layers of ice called firn. This process repeats itself year after year, causing glaciers to become hundreds of meters thick, and as each new layer of snow is turned into a solid ice, it traps surrounding atmospheric gases within it.
Just like dendrochronologists collect tree ring cores, climate scientists drill deep into glacial ice to learn more about the Earth’s past climate. When these cores are split open and analyzed, they reveal atmospheric levels of carbon dioxide, oxygen, methane, sulfur, hydrogen, and even dust. The relative abundance of each of these constituents enable us to compare Earth’s past climates to the Modern. Certain isotopes of hydrogen and oxygen are used to reconstruct prehistoric temperatures. Carbon dioxide and methane are also extracted to tell scientist a little bit about atmospheric chemistry. Other constituents like dust and sulfur allow us to understand past atmospheric circulation patterns or correlate the timing of geologic events, like volcanic eruptions or active plate tectonics, with climatic shifts.
Sediment Cores
To collect even longer climate records, geologists like to get a little dirty. Sediment cores are just like ice cores, but instead of drilling into ice, a hollow tube is sent into the ground. If you have ever been to the beach and shoveled a few feet beneath the sand (to build the perfect sand castle) you might have noticed that something was very different about the top layer of sand than the bottom. In many cases the lower layer of sand is either coarser/finer and maybe a darker color too. This is because sediment is typically deposited horizontally (known as the law of superposition), creating many layers which are an indication of the environment in which it was deposited. This is useful to scientists because approximately 75% of the world’s land mass is made up of sedimentary rocks! So what better way to understand the climate than to study the stuff which has existed in a range of climates for millions, sometimes billions of years (diamictite anyone)?
There are many types of sediment cores that can be collected, but if you’re trying to understand the Earth’s climate, then you want sediment that has remained undisturbed for a long period of time. Many of the sediment cores collected for climate research come from the bottom of the ocean, where sediment has settled and is far away from any outside disturbances (Figure 3). Other locations include permanently frozen lake beds, marshes, and even beaches. Moreover, scientists want to be able to collect the core without displacing or mixing any sediment in the process. This requires specialized equipment that either vibrates a hollow tube into the ground, uses powerful hydraulics to shove the core deep beneath the surface, or simply uses weight and velocity to drive the core tube through the surface. Similar to a tree ring and ice cores, once revealed, sediment cores display many layers which can be measured in all sorts of ways to deduce information about our natural world. Data collected includes sedimentation rates, relative age, biologic species abundance/extinction/evolution, geologic events, atmospheric chemistry, and regional temperature proxies.
How far back does sediment coring data go? This really depends on the location in which the core was collected, and the depth. In many cases, the deeper you can drill the more data you can collect. It is not uncommon for sediment records to go back anywhere from a few hundred to thousands or even millions of years. Recent studies have collected hundreds of sediment cores from the sea floor which date back roughly 65 millions years (about the time of the extinction of dinosaurs). These longer records are created by cross dating or “stacking” multiple cores from many different locations. Using modern chemistry and geochronology techniques, the data from each core is pieced together to create one giant climatic timeline.
Figure 3: an illustration of data from over 40 deep sea sediment cores. Covering approximately 65 Million years, this data correlates biotic, tectonic, and climatic events and reconstructs average global temperatures. (Zachos et al., 2001)
Accuracy
These techniques all sound really cool, but just how accurate are they? When reconstructing climate, or anything in the past, there will always be room for error. All of the scientists who study climate understand and account for a margin of error, because no matter how perfectly you collect a sample mistakes can be made, and machine calibrations can be incorrect. However, you must understand that climate proxies aren’t just “educated guesses,” they’re used because they have been proven, experimentally, to be reliable. For example, the stable isotopes oxygen 16 and 18 which are extracted from ice cores, sediment cores, the atmosphere, and the oceans can be verified by comparing modern samples to observational climate data.
Trusting in the scientific method of hypothesizing, experimenting, and repeating is crucial to scientific progress. Scientists set standards that must be met in order to become empirically verified. Research is rigorously criticized and tested before it is accepted, and the current scientific method has remained tried and true for centuries. While it is important to question research and to pry further, we must remember that scientists spend their careers understanding the field in which they work. We dedicate countless hours to understanding data and questioning our own work to ensure that what we produce is reliable informative, and progressive, but more importantly correct.
Sources:
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CDIAC, 2012, 740,000-year deuterium record in an ice core from Dome-C Antarctica: http://cdiac.esd.ornl.gov/trends/temp/domec/domec.html (accessed March 2017)
Grissino-Mayer, H., D., Principles of Dendrochronology: http://web.utk.edu/~grissino/principles.htm (accessed March 2017)
NOAA, 2017, Picture Climate: How can we learn form tree rings?: https://www.ncdc.noaa.gov/news/picture-climate-how-can-we-learn-tree-rings (accessed March 2017)
NOAA, 2017, Ice Core: https://www.ncdc.noaa.gov/data-access/paleoclimatology-data/datasets/ice-core (accessed March 2017)
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Zachos et al., 2001, Trends, Rhythms, and Abberations in Global Climate 65 Ma to the Present: Science, v. 292 p. 686 – 693