Improving Proxy Representations of Ocean Properties – Eos.org
A ship’s crew recovers the flotation sphere for an instrument mooring in the Irminger Sea. A new proxy approach could complement existing ocean observing efforts and identify beneficial locations for instruments. Credit: Isabela Le Bras
Source: Journal of Geophysical Research: Oceans
Ocean data are expensive and challenging to collect, especially in rough weather and ice-covered reaches. Yet ocean metrics are critical for understanding Earth’s climate: The ocean absorbs heat from the atmosphere and distributes it around the world.
To work around limitations of floating, moored, or ship-based instruments, oceanographers estimate quantities of interest by comparing them with proxy measurements. For example, sea level anomalies are used as proxies in predicting changes in the Atlantic Meridional Overturning Circulation, which warms Europe’s climate. Such proxies can reduce the need for expansive observation systems. However, correlation is not causation, and conventional approaches for assessing the value of a proxy in estimating another quantity do not describe how different variables are related.
Loose et al. report a new method developed to replace the conventional statistical approach. Called dynamical proxy potential, the technique relies on establishing physics-based relationships between variables. The method applies concepts from computational science like uncertainty quantification and optimal observing system design.
The authors demonstrated their approach in a case study by modeling heat transport across the Iceland-Scotland ridge, a North Atlantic gateway for waters traveling north toward the Arctic Ocean. The findings revealed that winds along the Atlantic Ocean’s eastern and northern boundaries drive subsurface temperature variability in the Irminger Sea and changes in heat transport along the Iceland-Scotland ridge. By isolating the effect of wind on both variables, the model quantified the relationship between subsurface temperature and heat transport. The results showed that a single temperature observation in the Irminger Sea could reduce uncertainty in the heat transport simulation by 19%.
By linking observations and quantities of interest, the new dynamical method complements existing ocean observing efforts. The authors emphasize that this quantitative approach could help in developing a cost-effective, long-term, and sustained ocean observing system, and this technique especially could help identify beneficial future locations for observational instruments. (Journal of Geophysical Research: Oceans,, 2020)
—Aaron Sidder, Science Writer
Citation: Sidder, A. (2020), Improving proxy representations of ocean properties, Eos, 101,. Published on 18 September 2020.
Text © 2020. AGU. CC BY-NC-ND 3. 0
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Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.
What Are “Proxy” Data?
In paleoclimatology, or the study of past climates, scientists use what is known as proxy data to reconstruct past climate conditions. These proxy data are preserved physical characteristics of the environment that can stand in for direct measurements. Paleoclimatologists gather proxy data from natural recorders of climate variability such as tree rings, ice cores, fossil pollen, ocean sediments, corals and historical data. By analyzing records taken from these and other proxy sources, scientists can extend our understanding of climate far beyond the instrumental record.
Historical Data
Historical documents, which are one type of proxy data, can contain a wealth of information about past climates. Observations of weather and climate conditions can be found in ship and farmers’ logs, travelers’ diaries, newspaper accounts, and other written records. When properly evaluated, historical documents can yield both qualitative and quantitative information about past climate. For example, scientists used historical grape harvest dates to reconstruct summer temperatures, between April and September, in Paris from 1370 to 1879.
Corals
Another type of proxy data, corals build their hard skeletons from calcium carbonate—a mineral extracted from seawater. The carbonate contains isotopes of oxygen as well as trace metals that can be used to determine the temperature of the water in which the coral grew. Scientists can then use these temperature recordings to reconstruct the climate when the coral lived. See Picture Climate: How We Can Learn from Corals to learn more about how scientists determine climate conditions from these beautiful ecosystems.
Pollen
All flowering plants produce pollen grains, which are another type of proxy data. Scientists can use the distinctive shapes of pollen grains to identify the type of plant from which they came. Since pollen grains are well preserved in the sediment layers in the bottom of a pond, lake, or ocean, an analysis of the pollen grains in each layer tells scientists what kinds of plants were growing at the time the sediment was deposited. Scientists can then make inferences about the climate of the area based on the types of plants found in each layer. See Picture Climate: How Pollen Tells Us About Climate to learn more about how scientists learn about climate from these tiny grains.
Ice Cores
Located high in the mountains and near the poles, ice—another type of proxy data—has accumulated from snowfall over many millennia. Scientists drill through the deep ice to collect ice cores, which often have distinct layers in them. These layers contain dust, air bubbles, or isotopes of oxygen, differing from year to year based on the surrounding environment, that can be used to interpret the past climate of an area. Ice cores can tell scientists about temperature, precipitation, atmospheric composition, volcanic activity, and even wind patterns. See Picture Climate: What Can We Learn from Ice? to learn more about how scientists study climate using ice cores.
Tree Rings
Trees and their unique rings also serve as proxy data. Because climate conditions influence tree growth, patterns in tree-ring widths, density, and isotopic composition reflect variations in climate. In temperate regions where there is a distinct growing season, trees generally produce one ring a year, recording the climate conditions each year. Trees can grow to be hundreds to thousands of years old and can contain annual records of climate for centuries to millennia. See Picture Climate: How Can We Learn from Tree Rings? to learn more about how scientists study climate using tree rings.
Ocean and Lake Sediments
Another type of proxy data can be found on the floors of the Earth’s oceans and lakes. Billions of tons of sediment accumulate in ocean and lake basins each year, providing a vast amount of information about the environment in them. Scientists drill cores of the sediments from the basin floors and examine their contents, which include tiny fossils and chemicals, to interpret past climates.
These are just a few examples of the environmental recorders scientists can use to learn about ancient climates. Learn more about the science behind the study of ancient climates at What is Paleoclimatology? and How Do Scientists Study Ancient Climates? Or visit NCDC’s Paleoclimatology Data page to access the Center’s proxy data holdings.
Climate Proxy Datasets: How Do We Know How Sea Levels …
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Can Rock and Ice Cores be Used as Proxies to Quantify Past Climate Change?
As mentioned on the last page, geologists are exploring the connections between sedimentary rocks and ice cores to help evaluate past climate change and associated sea levels both locally and globally. In order to evaluate this a little further, we will explore briefly some of the connections they are making.
The Vostok ice core from Antartica has been analyzed for its carbon dioxide and methane concentrations. Temperature is calculated from measuring ratios of oxygen isotopes (oxygen 18 and oxygen 16) that were trapped in air bubbles within the layers of ice. These isotopic ratios change according to the global temperatures in place during the interval when each layer of snow was deposited. More info about the ice core record can be found on the CDIAC website if you are interested.
A great site for visualizing the data collected from the Vostok Ice Core and other related climate proxies has been beautifully organized into an informative, interactive website which you are encouraged to explore on your own if you are interested. The data is presented on the Temecula Valley Bright Stars website. When you look at the data you will see that it is organized in such a way that you can toggle individual variables to compare each in turn, or all at the same time. A couple of pointers if you spend some time looking at the site. Remember, temperature is a temperature anomaly relative to modern average temperatures and modern oxygen isotopic compositions.
Figure 4. 29: Screen shot for the last 20, 000 years of paleoclimate data from ice cores and other proxies. This output shows more recent data on the right and prehistoric data on the left. Specific climate factors are shown and labeled on the right along the y‐axis. These include Milankovitch’s orbital parameters, variations in incoming solar radiation (insolation), methane (CH4) concentrations, carbon dioxide (CO2), and temperature anomaly data. Note that any of these parameters can be toggled on/off by clicking on the buttons on the left hand side of the screen.
Credit: Screenshot is from the Temecula Valley Bright Stars website.
What you are seeing in Figure 4. 29 is that, at 20, 000 years (left side of the graph) average temperatures would have been some 5 to 6 degrees Celsius lower than modern levels. Temperatures then began to rise (with some up-down wiggles) so that by around 11, 000 years ago temperatures may have reached modern average temperatures. This interval of time roughly correlates with the end of the Pleistocene and the beginning of the Holocene Epoch.
Since the beginning of the Holocene, temperatures have varied a little bit (ranging from approximately -1. 5 degrees Celsius to about +1. 0 degree Celsius), but, in general, much of the Holocene has shown average temperatures below the modern average. When looking at the methane and carbon dioxide records, the number of data points are too few to evaluate high-resolution changes, so we can only look at the longer term trends. Other websites report some of these higher resolution data and explore some of the controversies surrounding the linkages between temperature and greenhouse gases, for instance see this Joanne Nova webpage and the scientific references therein for more info.
We would venture that temperature variation appears to track changes in carbon dioxide and to a lesser degree changes in methane concentrations. Although cause and effect cannot be established from these data, temperature co-varies with changes in greenhouse gas concentrations such that warming occurred as greenhouse gas concentrations went up.
Given that our last glacial maximum occurred between 20, 000 and 13, 000 years ago, and that global temperatures began to rise to near modern values after this time, we still need to think about how this warming may have occurred. One variable that we haven’t looked at in detail is the insolation factor. In our graph above, you were asked to turn on the insolation estimates for 65 degrees N. So, what does this variable tell us, if anything? I
Insolation and Recent Climate Change?
Today, as in the Pleistocene, the biggest proportion of land is located between 30 to 60 degrees. By implication, as white reflective glaciers melt in response to warming and expose dark soils and rock, it is possible that changes in the amount of incoming solar radiation (and albedo) could be a driving factor in melting glacial ice and causing sea level rise. Check out the USGS Repeat Photography Project that compares photos of different glaciers today relative to historic photos. This will help you understand visually what changes in albedo can look like. There are many dramatic photo comparisons, but see the Chaney Glacier in Glacier National Park as a representative example. Located in the northern Rocky Mountains in Glacier National Park, Chaney Glacier had already begun to retreat by the early 1900s. The striking piece is that by 2005 the glacier is almost non-existent, so much so that many scientists worry that in another decade or two, Glacier National park may no longer have any glaciers.
Exploring our dataset above shows us that Earth saw larger incoming solar radiation values (at all northern latitudes), and likely reduced albedo (reflection of insolation). In our dataset, summertime (June) insolation (measured in watts per square meter) began to rise during the period that temperatures began to rise and preceded the rise in greenhouse gases.
Interestingly, northern latitude insolation began to fall at around 11, 000 years ago, about the time we reached near-modern temperatures. At the same time, greenhouse gas concentrations became elevated and remained we see insolation values at all northern latitudes (where most land is located) are near low points, but are beginning to rise once again, suggesting that more energy will be delivered to these latitudes in the future. These observations should raise some questions on your part! Keep writing them down in your notebook, and stay engaged with your instructor and the rest of the class!
One thing that is clear here is that there are lag and offsets in data set responses that need to be explored further. Are these lags real, or are they related to errors in alignment/calibrations between different datasets? We can’t answer this question here, but Joanne Nova does explore this question if you are interested in her website.
Frequently Asked Questions about sea proxy
Is sea level a proxy record?
Sea-level “proxies” are natural archives that record rates of sea-level rise prior to the mid-19th century, when tide gage measurements became relatively common. Proxy indicators are generally calibrated against data from modern instruments and then used to reconstruct past sea levels.
What are three types of proxy indicators?
List three types of proxy indicators. Ice cores, ancient sediments, tree rings.
What is a proxy in geology?
Paleoclimate proxies are physical, chemical and biological materials preserved within the geologic record (in paleoclimate archives) that can be analyzed and correlated with climate or environmental parameters in the modern world.