9.1: Analog Forecasting

Can we use the past to predict the future?

In Investigation 7, we covered El Niño and La Niña and briefly explored some of the impacts of these variations in the Walker Circulation on the Equatorial Pacific. These phenomena have global consequences through a set of “teleconnections,” which is just a fancy way of saying that a variation in one part of the world influences another. Let's consider one example of how these teleconnections work: In 1982/83 and 1997/98, extraordinarily strong El Niño events occurred, and in both cases, many regions of California saw record-high rainfall. There have been other weak-to-moderate El Niño events that also enhanced rainfall in the region. As a result, whenever an El Niño event is occurring, Californians know to expect higher-than-normal chances for rainfall during the rainy season, and weather forecasters consider the presence of an El Niño event in predicting how much rain will occur during a rainy season. Basically, what everyone is doing is a type of Analog Forecasting: Using a past outcome to predict a future outcome. Since the El Niño events of 1982/83 and 1997/98 produced significant rainfall, a future El Niño should do so as well. If it happened before and the conditions are the same, it makes sense to predict it could happen again. We can do the same with Climate Change… we can look at prior abrupt climate shifts to help us predict what may happen in the future.

Reconstructing Past Climates

To find a potential analog for today’s rapid climate change, we need to use various tools to look back into the past. Since we only have direct temperature records dating back to approximately 1880, we need to find alternatives, known as Proxies, to look further into the past. Thankfully, the Earth System provides a number of such proxies for us. While there are many proxies we can look at, the two most fundamental for looking far into the past are Sediment Cores and Ice Cores. The collection methods of both are very similar, but the cores themselves are very different:

Sediment Cores: Gases, rainwater, fossils, and other elements of the current atmosphere/land makeup are constantly being mixed together through the process of weathering, which occurs when rainwater dissolves a small amount of rock from the Earth’s surface. This dissolved mixture becomes sediment, which then travels down to the ocean via rivers and streams, precipitates in the deep ocean, and finally deposits onto the ocean floor. Over millions of years, additional sediment precipitates and accumulates on top, creating a series of sediment layers that contain many elements of the Earth system at that time. Figure \(\PageIndex{1}\) is a set of sediment cores, which are drilled from the ocean floor and studied by Paleoclimatologists. These sediment cores can be chemically analyzed to measure Carbon Dioxide concentrations, detect fossils, and identify useful Oxygen isotopes that can tell us much about past climate. We’ll investigate the data from a sediment core in the next section on the Paleocene-Eocene Thermal Maximum.

Figure \(\PageIndex{1}\): A set of sediment cores representing climate from tens of thousands, to millions of years ago. (CC-BY-SA 2.5; Hannes Grobe AWI/CRP via Wikimedia Commons). Alternative Description of Image.

Ice Cores: The concept is very similar for ice cores, with the main difference being that they are retrieved from research bases on ice sheets and in permanently frozen regions (such as Antarctica). The most famous research station for collecting ice cores is the Vostok Research Station in Antarctica. Figure \(\PageIndex{2}\) is a map showing its location, and Figure \(\PageIndex{3}\) is an image of an ice core.

Figure \(\PageIndex{2}\): Location of Vostok Research Station in Antarctica (Circled in red). The station is located in East Antarctica, southeast of the center of the continent and inland from the coast. (Public Domain; NASA via Wikipedia)

Figure \(\PageIndex{3}\): Image of an ice core taken from the WAIS Divide Research Station, in Antarctica. The dark band is a layer of volcanic ash that settled from an eruption approximately 21,000 years ago. (Public Domain; Heidi Roop via National Snow and Ice Data Center). Alternative Description of Image.

We can reconstruct the past using various elements extracted from these cores, including bubbles of ancient air in ice cores, and the presence of oxygen-18 isotopes in both ice and sediment cores. Let’s now look at one particularly interesting period found in these data – The PETM.

The Paleocene-Eocene Thermal Maximum (PETM)

In order for us to find a “similar” period of sudden, intense warming caused by a rapid increase in Greenhouse Gasses, we need to look back millions of years (Carbon Dioxide Concentrations haven’t been as high as they currently are in hundreds of thousands of years, and the rate of increasing temperature hasn’t been as fast as it currently is in millions of years… in fact, we are warming at a rate over 10-times faster than we did when coming out of the last ice age). While there obviously isn’t an ancient period when humans induced rapid warming, the temperature record shows a similar spike at the end of the Paleocene, about 10 million years after the extinction of most Dinosaurs. Figure \(\PageIndex{4}\) is an infographic showing temperature reconstructions for the past 65 million years.

Figure \(\PageIndex{4}\): Reconstruction of Global Temperature estimates based on Polar Ocean Sediment Core data until approximately 35 million years ago and Ice Core Data from the Vostok Ice Core for the past 12 million years. All values are differences from the 20th-century average. (CC-BY-SA 3.0; Robert A. Rhode via Wikimedia Commons). Alternative Description of Image.

1. We’re going to focus on the Tan-highlighted rectangle in the upper-left-hand corner. The PETM is labeled next to a green spike in temperature (the green line represents the temperature difference from the modern average). From this, we can see that the PETM occurred approximately 56 million years ago, when global average temperatures were approximately _________ warmer than they are today.
   1. 2°C
   2. 10°C
   3. 6°C
   4. Temperatures were cooler
2. During this time, global average temperatures briefly spiked approximately _______ degrees from your answer in question #1 to 12°C
   1. 2°C
   2. 4°C
   3. 6°C
   4. 10°C

One way we can reconstruct past temperatures is by examining the presence of Oxygen-18 (¹⁸O), a heavy isotope of Oxygen. The green line in Figure \(\PageIndex{4}\), used to detect temperature in the above questions, is actually a graph of the Ratio of Benthic (bottom-dwelling) Oxygen-18 to normal Oxygen-16 (the ratio is indicated by the Greek letter “δ”) found in fossilized organisms in the sediment. A simple way to interpret this graph is that the larger the δ¹⁸O value, the more Oxygen-18 is present in the sample. Because temperature is also being plotted on this figure, you need to use the scale on the far right of the graph to identify the amount of Oxygen-18 present, and the amount increases as you travel down the axis (you’ll find out why in question 3).

3. Look at the small “blue” shaded box in the lower-right of the graph, which shows the difference in temperature from today vs. the time taken from the Vostok Ice Core data. The equivalent temperature difference is on the left side of the box, while the amount of Oxygen-18 is on the right. From this (just comparing the two scales), we can see that the more Oxygen-18 is present in the sample:
   1. The lower the temperature is believed to be
   2. The higher the temperature is believed to be

The sudden shock to the Earth system occurred over a period of approximately 1000-10,000 years – what may seem long to us, but in terms of geological time, is actually a very short period of time. The sudden increase was due to a sudden, rapid increase in greenhouse gases. There is still much debate about the exact cause of this spike in greenhouse gases, but this Carbon Brief article briefly discusses some possible causes, along with a more detailed description of PETM indicators. The PETM was the largest of a series of “Hyperthermals”, periods when a sudden “burp” of greenhouse gases caused a short-lived spike in temperatures. During this time, the rate of Carbon Release was still significantly slower than it is now, but we can still look to this period of rapid warming to see how human activity may affect our planet.