• Students will develop fundamental geospatial skills and concepts needed to assess coastal processes that produce sea level change.
• Students will be able to explain what sea level is and differentiate the mechanisms that interact to produce changes in sea level over short-term and long-term time periods.
• Students will characterize trends in sea level over geologic time and conceptualize how changes in sea level can result in various spatial scales of coastline change.
• Students will use real tidal gauge data from diverse regions to identify recent trends in sea level change, and will use these trends and NOAA Sea Level Rise Viewer data to formulate projections for future sea level positions.

Learning Objectives

By the end of this module, students should be able to:

• describe the changes in sea level change in the Earth’s geological history, to gain a perspective of modern sea level rise;
• distinguish among methods used to determine sea level changes taking place in the past, present, and future;
• identify factors (intrinsic, extrinsic, and anthropogenic) that contribute to sea level change;
• discriminate among causes of local, regional, and global sea level change and associated impacts on coastal morphology and human communities;
• locate and use key datasets and data visualizing tools to analyze sea level dynamics at different temporal scales, and their impact on coastal landforms.

Module 4 Roadmap

Questions?

If you have any questions, please use the Canvas email tool to contact the instructor.

The following pages look at what sea level change is, and what mechanisms drive sea level change on a planetary scale.

Before we investigate these mechanisms further, let’s ask a couple of fundamental questions: What is sea level anyway? How is it measured...and why has it fluctuated during the course of geologic time? And why is it not even across the globe? As you watch the following quick video, make a list of forces mentioned that influence sea level. The video clip (3:25) was published on Nov. 25, 2013, by MinutePhysics.

Video: What is Sea Level? (3:25)

The Minute Physics video introduces a few key concepts that make measuring sea level pretty complex:

1. The Earth is not perfectly spherical, but an ellipsoid, due to its spin. This means that the Earth is “fatter” at the equator and slightly flattened at the poles, so that: “if you thought Earth was a sphere and defined sea level by standing on the sea ice at the North Pole, then the surface of the ocean at the equator would be 21km above sea level”.
2. Differential density of the interior of the Earth so that “gravity is slightly stronger or weaker at different points around the globe, and the oceans tend to "puddle" nearer to the dense spots”.
3. The mass of the continental plates creates a greater gravitational pull on ocean water than the ocean basin so that “mass gravitationally attracts oceans, while valleys in the ocean floor have less mass and the oceans flow away, shallower”.

These phenomena mean that there are peaks and valleys in the surface of the ocean – the ocean level is not uniform across the planet. These are important concepts to keep in mind as you read on.

We will also meet several other phenomena that drive sea level changes around the planet later in the module.

If you are interested in understanding climate change, and you pay attention to in-depth news stories on the topic, you have no doubt frequently heard or read references to sea level and temperature anomalies. Anomaly data are being shared with greater regularity in the media these days, so it is important to understand what we mean by terms like sea surface temperature anomalies and sea surface height anomalies. An anomaly is an inconsistency or deviation from the norm, so images are created to show where change is taking place in the ocean in either a positive or negative trend when comparing to previous data. This is sometimes referred to as Sea Surface Height Deviation data or SSHD. Sea surface height anomalies are calculated using data from satellite altimeters. Many years’ worth of thousands of measurements provide a historical mean sea surface height, and the difference between the historical mean and the sea surface measurement for a particular date is called the sea surface height deviation. This can be calculated for points over the ocean surface, providing the data for the incredible maps we are seeing that show color-coded variations in sea surface across the globe and the changes in these measurements over time and space.

In the figure below, the data show the sea surface height differences compared to the 1961 – 1990 average over the entire planet. By comparing sea surface height measurements for a particular time period with the average measured over a previous time period, the changes can be shown spatially. In the figure below, the warm colors are sea surface heights that are significantly greater than past measurements, while the cool colors are those areas showing significantly lower elevation in the sea surface. This is how sea level rise trends can be identified in different parts of the globe using satellite data.

Sea Level Anomalies: Comparing data. OSTM/Jason 2 map of sea-level anomalies from July 4 to July 14, 2008 (left) compared to the same 10-day period of data from Jason 1 (right).

Credit: Sea Level Anomalies, NASA (Public Domain)

Revisit the NASA animation "22 Years of Sea Level Rise Measured from Space" from the Module 4 Overview that shows a 22-year period of sea level change using anomaly data. It is an excellent quick visualization of these phenomena.

Thinking in the Long Term: Sea Level Change in Geologic Time

The instrumental data we explored above gives a small window of time in Earth’s recent history. To put the recent changes into context, we need to also consider long term changes in sea level.

Humans typically have difficulty thinking about time beyond a human lifespan. Geologists may be the exception to this rule, but you may belong in the category of those who find it difficult to visualize the long distant past and the long distant future and to think in terms of millions or billions of years (or even thousands of years). But understanding the changes to atmospheric and ocean changes in the geologic history of the Earth is important if we are to understand what is going on with our climate and sea levels today.

Thinking REALLY long term: Below is a graph plotting sea level over the past 540 million years - since the Cambrian era. For reference, zero on the Y-axis is where the current sea level is. We don’t need to go into a lot of detail, but you can easily appreciate that sea levels have been much higher than today for much of this period of the Earth's history. Scientists have correlated these fluctuations with changes in atmospheric carbon dioxide and ocean and atmospheric temperatures, using methods described in the next few pages.

We also must acknowledge here that some people may argue that sea levels have always fluctuated, so why is sea level rise today a big deal? Hopefully, we can shed some light on this question by looking at the changes in sea level through the history of the Earth, while considering the causes for these changes. But, perhaps the simple fact that seas are rising faster than ever before in human history is enough to facilitate action and adaptation. You also may ask, “What can we do about it?” This question will be addressed in later modules.

For a rapid and fun overview of the history of the Earth’s climate changes, watch the following fascinating monolog video. It summarizes most of the concepts to be discussed in more detail in the materials that follow.

Video: A History of Earth's Climate (11:19)

There are several takeaways from studying the two graphs on the previous page. Look closely at the data presented and answer the questions below.

Learning Check Point

What were the causes of the changes in sea levels on the Earth over time? There are multiple causes that can be divided into two groups: intrinsic, or internal drivers, originating within the Earth’s system, and extrinsic drivers, which originate outside the Earth’s system. Some of these operate over the timeframe of the Earth’s history, and others operate over shorter timeframes. Some influences are global in scale, while others are more regional or localized. The following pages are part of a partial list of these influences. These drivers are also interconnected, with one influencing another in many cases.

In order for us to connect sea level with our discussion on intrinsic and extrinsic controls and feedback mechanisms, etc., let's see another dataset that links climate history and sea level. The image below shows warm and cool periods for the last 900,000 years and has an expanded inset for the last 140,000 years.

In the inset, you will notice the long-term decline in sea levels from the last interglacial warm period, which occurred about 130 ky before the present. It is labeled stage 5e on the graphic. Sea levels declined to the lowest levels during stage 2 that occurred between 13,000 and 20,000 years ago. During this time, despite some minor short-term rise/fall events, sea levels fell from near modern sea levels to some 120 meters below present. The long-term rate of sea level fall calculation shows that sea levels fell 120 meters in approximately 100,000 years, or 0.12 m / 100 years. That is, sea levels fell by some 12 cm per 100 years, or through a simple unit conversion, the rate can be stated as 1.2 mm/year.

In the same graph, sea level rise appears to have been occurring at least for the last 18,000 years. Modern sea levels (relatively similar to highs 120,000 years ago) were achieved quite rapidly, relative to the rate of fall. Based on these numbers, 120 meters of sea level rise appears to have occurred in 18,000 years. This represents a rate of almost 6 mm/year for a rise rate.

This asymmetrical pattern of sea level rise/fall rates is repeated again and again for many of the earlier glacial-interglacial episodes. Thus, there is evidence that shows sea level fall (and building of ice sheets) is a long, drawn-out process where cooling and various positive feedback loops help to reinforce the cooling. The end result is an extended period of sea level fall (i.e., iteratively more albedo, and less and less insolation delivery, more cooling, etc.).

Conversely, once the factors that initiate warming begin to turn on, warming and associated sea level rise apparently can proceed at a much faster rate until a maximum sea level is reached, and factors that influence cooling initiate and bring sea levels down again.

Prior to the last few centuries, the system was controlled primarily by extrinsic and intrinsic variables. However, human impacts on landscapes, oceans, and climate (i.e., deforestation, greenhouse gas concentrations, nutrient runoff, aerosol pollution, and cloud development, etc.) have added variables that weren't present at any previous time (more on this later).

Now, let's look at a couple of more composite analyses that help us understand signals and signatures from the last few 1000 years. In the graphic below, a number of different datasets from Australia, South America, the Caribbean, the western Pacific, and other areas have been plotted to show sea level positions. Most of these datasets are derived from investigations of coral reefs that have drowned as sea level has risen.

From what is observed in the figure above, sea level rise rates appear to have been relatively low during the initiation of rise (i.e., from 20,000 to approximately 15,000 years ago), at which point a significant increase in sea level rise rates (Meltwater Pulse 1A), and several others ensued. Three rapid increases in rise rates ("pulses") are noted here so that the majority of the 100 meters of sea level rise occurred from 14,000 to approximately 8,000 years ago, or 90 meters in roughly 6,000 years. This yields a sea level rise rate of 0.015 meters per year or 1.5 centimeters per year or 15 mm per year.

This is an incredibly fast rate that is tied to the decay of large ice sheets, including both the Eurasian and Laurentide Ice Sheets. The Laurentide Ice Sheet on North America had mostly retreated from North America by 6,000 years ago, leaving behind only the alpine ice sheets. Check out the video below showing the retreat.

Video: Laurentide Ice Sheet (00:32) No audio.

As these large ice sheets and their albedo potential were removed, the rate of absorbance of incoming solar radiation was likely to have contributed to further warming and increased temperatures of seawater. Thus, geoscientists are increasingly confident in the two primary factors that have contributed to the sea level rise rate prior to human influences. The primary cause is thought to be tied to thermal expansion of seawater, as we discussed previously. The second is the role of melting glaciers and increased volumes of land-ice being moved to the oceans.

In the human timeline, the change from higher rates of sea level to lower rates of sea level rise coincided with the onset of the Neolithic interval ~4,000 years ago. Although human beings began to influence Earth in interesting ways within the last few millennia, (think Roman Empire, which began to expand about 2,000 years ago), anthropogenic impacts on sea levels likely occurred more recently. Feedback loops often have significant lag times.

As human populations grew and the demand for freshwater for agriculture and other industries increased, and as forests were deforested, significant volumes of water re-entered the ocean-climate system and contribute to sea level rise. Some calculations suggest that perhaps as much as 5 percent of the sea level rise observed in the last few decades may be from these sources.

In 2016, the Earth’s atmospheric carbon dioxide concentration reached the 400 parts per million threshold level. This was highly significant to climate scientists and all people concerned about the effects of climate change on the planet. The amount of CO2 in the atmosphere has increased from about 280 ppm since 1750 AD at the beginning of the Industrial Revolution to its current level of 420 ppm. This is significant because of the observed effect that these increased levels of, a powerful greenhouse gas, have on the warming of the atmosphere and in turn the ocean.

Climate scientists agree that this steady increase in carbon dioxide levels (as well as other greenhouse gases – water, methane, nitrous oxide) is the cause of the warming of the planet. The burning of fossil fuels by humans over the past few centuries has “unleashed vast reservoirs of fossil carbon stored in the Earth for hundreds of millions of years.” (Hearty). When the concentration of these compounds increases in the atmosphere, so to do global temperatures, as greenhouse gases limit the escape of long-wave radiation (thermal energy). These interactions result in a warmer climate, which means more glacial ice is being melted. When combined with more water released into the ocean, warmer ocean waters also expand in volume and result in higher sea levels.

If we look at the Earth’s history, it is extremely rare for Earth to have 400 ppm of carbon dioxide in its atmosphere. Paleoclimate research using ice core data shows that about 3 million years ago the atmosphere contained 400 ppm carbon dioxide. Then the sea levels were 10 – 40 m higher than today. In more recent Earth history, 800,000 years ago, during an interglacial period, there was 300 ppm CO2. As we discussed above, ice core data show that fluctuations in the Earth’s sea levels parallel atmospheric CO2 levels.

All this is evidence for the argument that as we continue to pour more CO2 into the atmosphere with continued fossil fuel energy reliance, we can expect sea temperatures and sea levels to continue to rise in tandem with the CO2 concentration. There are many other implications that go along with this scenario. We must remember the positive feedback mechanisms, such as that in which the melting of Arctic ice reduces the albedo effect – or the reflective nature of the white snow and ice. These types of mechanisms have the effect of accelerating warming and therefore increasing the melting of ice and, in turn, increasing sea levels.

Let’s look at the data from tide gauge records, graphed over the entire 20th century and into the 21st Century. The graph below represents a composite of 23 tide gauge records from around the globe for the last century (1880 to 2000), and altimetry data superimposed since 2000 (red line).

Recent sea level curve for the last 120 years or so (1880 to 2000) based on instrumental records (not proxy records) taken by both tide gauges and by satellites. Each tide gauge station is represented by a different color and the average of all sites is shown by the bold black line. Altimetry data, a relatively recent contribution, is shown in red for the last couple of decades.

Credit: Recent Sea Level Rise by Robert A. Rohde, Gloabal Warming Art, CC BY-SA 3.0, via Wikimedia Commons

Each individual record shows the volatility (ups/downs) in water levels attributed to seasonal sea level changes that we have also previously discussed. To reduce this volatility, scientists employ a statistical averaging technique that calculates the average sea level of each successive three-year interval for all sites and then plots the average point on the graph. In this case, the moving average line is represented by the thick black line. Although it is still "wiggly," it is much smoother than the highly wiggly lines from which it is derived. This technique helps to "see the forest through the trees," so to speak.

Although a clear trend is visible in the 12 decades of data, the smoothed record helps to identify longer-term trends that are obscured by the highly variable local datasets. In this case, you will notice that there are essentially two modes in the longer-term trend. The first mode lasts roughly from 1880 through 1920, with a fairly stable sea level. In fact, the average sea level for this interval is used as the datum for this graph.

By 1920, the rate of sea level rise accelerates, and water levels begin to rise at a relatively constant rate through 2000 when satellite altimetry methods and data (red line) become available and help to substantiate tidal gauge methods.

These data show that sea level has risen an average of 20 cm (200 mm) over the last 80 years. This equates to a sea level rise rate of about 2.5 mm/year for the 80-year interval. If we take another look, including data collected since 2000, NOAA scientists show that the rate is accelerating again and a rate of 3.2 mm/yr. Has been established as the most current estimate.

The International Panel on Climate Change (IPCC) has set an estimated projected sea level rise at between 0.5 and 1.5 m by 2100. Many scientists argue that this is a too conservative estimate.

These estimates are based largely on the thermal expansion of seawater as the ocean surface heats up. However, the possibility of a significant contribution from the melting of ice sheets in Greenland and East and West Antarctic is not currently an important factor in IPCC predictions. Stay current as this research develops and estimates are adjusted accordingly.

One of the biggest uncertainties that has caused scientists to tend towards conservative estimates of sea level changes in recent years (and result in the frequent announcements of increased projections in the past few years) is the rate at which the Earth’s polar ice sheets are melting. In the 2007 IPCC report, the sea level rise projection agreed upon was a conservative 60 cm (~ 2 ft.). This number did not account for the possibility of rapid ice flow from Greenland or the Antarctic into the sea. These two ice sheets alone hold enough water to raise sea levels by 65 meters compared to 0.4 meters from all the world’s mountain glaciers. But, at that time, researchers felt that there was insufficient understanding of the ice sheets to be certain, so the IPCC resisted putting a number on it.

The most recent IPCC report (2013) increased this estimate to 98 cm or almost 1 meter.

This number has been bumped up further since 2014 with the most recent projections ranging from 0.2 to 2.0 meters (See NOAA Sea Level Rise viewer information in Module 4 Lab for more on these ranges).

Eliminating the uncertainty around quantifying the contribution of ice sheet melt-water to sea level increases is attributed to observations from NASA/German Aerospace Center’s twin Gravity Recovery and Climate Experiment (GRACE) satellites. These data indicate that between 2002 and 2016, Antarctica shed approximately 125 gigatons of ice per year, causing global sea level to rise by 0.35 millimeters per year.

In 2002, NASA launched the GRACE satellites, which track both ocean and ice mass by measuring changes in the Earth's gravitational field. The paired satellites orbit the Earth together and are spaced roughly 200 kilometers apart. Ice and water moving around the Earth exert different gravitational forces on the GRACE satellites. The satellites can sense the minuscule changes in the distance between one another caused by the change in gravitation force, which they measure and used to track water and ice mass change. It's thanks to GRACE that we know where the water flowing into the ocean came from. According to GRACE, melting of ice in Greenland increased sea level by 0.74 mm/year and melting in Antarctica by 0.25 mm/year since 2002. (Source: Smithsonian)

How do the IPCC and other agencies arrive at these numbers: Current rate of sea level rise: 3.2 mm/ year (Source: NASA: Global Climate Change); Sea Level rise predictions: 0.2 – 2.0 meters rise by 2100 (NOAA)? The numbers, which are frequently adjusted and may vary according to the source, are achieved using modeling methods of different kinds.

Earth System Models integrate the interactions of atmosphere, ocean, land, ice, and biosphere to estimate the state of regional and global climate under a wide variety of conditions. They numerically model the atmosphere, oceans, land, and sea ice, and have biogeochemical components (for example, dynamic global vegetation models) to study the carbon cycle.

Earth System Models of Intermediate Complexity (EMICs)

Models of intermediate complexity bridge the gap between conceptual models with many simplifying assumptions, and extremely complex, three-dimensional models that attempt to include as many known processes as possible. (Source: NASA: Sea Level Change)

The data gathering by the many satellite missions described above is producing datasets that are used to populate the models. Modeling ocean and sea surface height is responsible for the creation of the incredible simulations seen in this module.

Although models can help tremendously in understanding complex interactions among components of Earth systems and their outcomes, they are never perfect, and all are given ranges of confidence due to uncertainty. They must be compared to observations to iron out their inherent errors. The challenges involved in modeling the contributions of the ice sheets to sea level rise have reduced the ability to provide a full picture of what is happening in the oceans. This area is a growing field, and we are likely to hear about breakthroughs in modeling these aspects of climate change in the near future. The figure below shows sea level curves based on the IPCC AR4 report, which estimates a sea level rise of 0.8 meters by 2100.

Projected sea level rise for the 21st century. Purple bar represents uncertainty from thermal expansion of seawater; red bar represents uncertainly from ice sheet melting.

Credit: This image is from Australia's Marine and Atmospheric Research, Commonwealth Scientific and Industrial Research Organisation (CMAR/CSIRO) Climate Science Center website. View more excellent images showing sea level change data.

The figure above illustrates the levels of uncertainty that exist in the current data when using models to project future sea level rise. The AR4 IPCC data here show projected sea level curves through the year 2100. At 2095 the magenta bar shows a range of model projections (90% confidence limits), and the red bar shows the upper range extended to allow for the potential but poorly quantified additional contribution from a dynamic response of the Greenland and Antarctic ice sheets. Because there is a great deal of uncertainty surrounding the rates at which these ice sheets will melt in the future, a wide range of potential sea level rise scenarios are plotted. As the understanding of the mechanisms at work causing the melting of the ice sheets, the uncertainties shown here will most likely become more resolved in the future.

The overall takeaways from the detailed discussions of sea level in this module are:

• sea levels are not uniform across the planet, so when we talk about mean sea level rise, these averaged numbers used tend to gloss over the fact that some places are experiencing greater or lesser effects, depending on many variables;
• the drivers of sea level changes over the history of the Earth are complex (including intrinsic and extrinsic factors) and feedback loops work to create cycles of sea level rise and fall, while maintaining an overall equilibrium;
• mean global (eustatic) sea level has been rising steadily during the 20th and early 21st Centuries;
• relative sea level rise causes large local differences in rates of sea level rise. Subsidence of the land can exaggerate sea level rise, while geologic uplift can cancel out some of the effects;
• the current rate of global sea level rise is quantified at 3.2 mm/ yr., a number which has increased in recent years, based on strong evidence that the rate of sea level rise is increasing;
• the chief driver for the acceleration in sea level rise in the past one hundred to two hundred years appears to be anthropomorphic in origin and caused by rapidly increasing atmospheric carbon dioxide levels due to fossil fuel combustion;
• paleoclimate research has established that sea levels have been much higher than today for much of the Earth’s history, with many factors at play, but atmospheric carbon dioxide levels appearing to be a chief driver in climate changes;
• glacial and interglacial periods during the past 500,000 years have been responsible for periods of rapid sea level rise, followed by periods of slower sea level fall as climate cools, glaciers form, and water is once again locked up in ice;
• the exact rates of increasing sea level depend on factors, such as rates of ice melt from polar ice caps, that are currently not understood well enough to give accurate projections;
• the Earth system science models are being refined continually, using more exact data from satellites, and will improve the ability of scientists to predict future sea levels;
• current projections for the end of the 21st Century are between 1 and 2 meters of sea level rise;
• a worst-case scenario suggests that if all the ice in Antarctica and the Arctic were to melt, there would result in a rise in sea level of 65 meters!

The implications of sea level rise – even the conservative projections – are huge for the millions of people around the world living in coastal communities. In Modules 5 and 6, we will consider how coastal catastrophes impact societies and how these societies are responding.

Reminder - Complete all of the Module 4 tasks!

You have reached the end of Module 4! Double-check the Module 4 Roadmap to make sure you have completed all of the activities listed there before you begin Module 5.

Introduction

In this Lab, you will:

• Use the PSMSL website and Google Earth to explore historical records from tide gauges around the U.S. that provide reliable long-term data sets, to gain an understanding of different rates of sea level change on the Pacific, Atlantic, and Gulf coasts.
• Use NOAA's Sea Level Rise Viewer to make observations and compare sea level change predictions in locations around the U.S.

Lab Overview

There are two parts to this Lab.

• Part I is Analyzing Sea Level Change Using Tide Gauge Data. Download the tide gauge Google Earth KMZ data file from the PSMSL site. Use the tide gauge data to answer questions 1-14.
• Part II is the NOAA Sea Level Rise Viewer. After exploring and figuring out how to manipulate the viewer to give you the information for projected sea levels at the location of interest for different years, use the viewer to answer questions 9-15.

Part I: Analyzing Sea Level Change Using Tide Gauge Data

In Part I of this Lab, you will use data obtained from the Permanent Service for Mean Sea Level (PSMSL) in conjunction with Google Earth. A KML file is provided by PSMSL to open in Google Earth, making data access seamless. The sea level data are monthly and annual means referenced to a common benchmark and are referred to as a Revised Local Reference (RLR). This data is used to create accurate time series to observe trends. We are choosing four tide gauges with long time series. The longer the time series (more data), the more reliable the data are for looking at trends. You will notice short-term variability in the data – more in some locations than others. As you work through the lab, consider the difference between short-term and long-term time-series data and the reasons for the short-term variability that makes the data “noisy."

PART II: NOAA Sea Level Rise Viewer

Sea-level rise is expected to accelerate in the immediate future. However, even using the annual rates you calculated from the tide gauge data from the 20^th Century and early 21^st Century, a steady rise would be expected.

In Part II of this Lab, you will use the NOAA Sea Level Rise Viewer to help you visualize what these levels would look like in particular locations around the U.S.

You will keep your calculated rates in mind for each place from Part I, as you work with the viewer and consider the factors that influence the future projections of increased sea level rise.

NOAA built the viewer based on data that calculates a projected Global Mean Sea Level (GMSL) for 2100 to be between 0.3 m and 2.5 m. The model uses five GMSL rise scenarios: Intermediate-Low, Intermediate, Intermediate-High, High, and Extreme, which correspond to GMSL rise by 2100 of 0.5 m, 1.0 m, 1.5 m, 2.0 m, and 2.5 m, respectively (NOAA). For more detail on the science behind the Sea Level Rise Viewer, please take the time to read at least the Executive Summary and Introduction of NOAA Technical Report NOS CO-OPS 083 “Global and Regional Sea Level Rise Scenarios for the United States” listed in Resources.

Downloads/Resources

• Module 4 Lab Worksheet
• Tide Gauge KML File (see downloading directions below)
• Global and Regional Sea Level Rise Scenarios for the United States (Executive Summary and Introduction)
• NOAA Sea Level Rise Viewer Note: The viewer functions better if you use Google Chrome.