For the Instructor

These student materials complement the Coastal Processes, Hazards and Society Instructor Materials. If you would like your students to have access to the student materials, we suggest you either point them at the Student Version which omits the framing pages with information designed for faculty (and this box). Or you can download these pages in several formats that you can include in your course website or local Learning Managment System. Learn more about using, modifying, and sharing InTeGrate teaching materials.

Initial Publication Date: December 7, 2016

Instrumental Records: Direct Measurements of Environmental Change

What does the most recent record of climate and sea level change indicate about changes in the climate system coincident with human modification of global carbon cycles and land use activities?

Although we still utilize proxy records to investigate our most recent patterns of climate change and sea level rise, fortunately, we have direct observations and instrumental records for at least the last couple of centuries. Humans began keeping climate records in England some 300 years ago in the 1700s. Elsewhere in Europe records were also kept on tidal measurements. For some additional details on instrumental records check out NOAA's NCDC website.

As a result of these efforts, scientists have amassed large datasets and tidal records from around the globe. In the United States, NOAA and the USGS combine efforts to collect, archive, and analyze these data. Other organizations, including the Permanent Service for Mean Sea Level (PSMSL), provide access to global datasets on tidal records. The joint effort is sponsored and supported by numerous organizations. The PSMSL website represents one of the easiest user-interfaces for sharing and accessing these data records from around the globe.

For your summative assignment on this module, you will be working with data from this site. The image below (Figure 4.35) shows the PSMSL Data Explorer and the distribution of observation areas around the globe. In this visual, the color of the pin denotes the duration for which tidal records are available. The white pins show records with at least 100 years of data.

So what do these tidal records indicate? So, what observations and inferences can be made from this dataset? You will remember that we discussed earlier in the module how tidal records are collected, i.e., by tide gauge and by satellite altimetry. The graph below (Figure 4.36) represents a composite of 23 tide gauge records from around the globe for the last century (1880 to 2000).

Recent sea level curve for 1880 to 2000 based on instrumental records taken by both tide gauges and by satellites.

Figure 4.36: 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.

Each individual record shows the volatility (ups/downs) in water levels attributed to seasonal sea level changes that we have also previously discussed. In an attempt to remove the overprint of the annual 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. In this case, you should notice that there are essentially two modes in the longer-term trend. The first mode lasts roughly from 1880 through 1920, and shows a fairly stable sea level. In fact, the average sea level for this interval is used as the datum (0 on the y axis) for this graph. 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 might actually be accelerating once again.

The dataset below (Figure 4.37), shows data from 1993 to 2012. Trends in these data indicate that the 2.5 mm/year average through 2000, might actually be increasing to 2.8 mm/year or even more. Given more precise recent NASA measurements (from 2012, 2013, etc.) of glacial melting in Greenland and elsewhere, sea level rise rates might once again be responding to the sudden input of melt water produced from glaciers that were relatively stable at high latitudes and elevations.

Melt water might not be the only contributor to acceleration of sea level rise rates. Data produced by NOAA shows an increase in heat content of the surface ocean (0-700 m). Shown in Figure 4.38 below, ocean heat content in 2010 was more than double the heat content prior to 1990. Given what we have already learned about sea surface temperatures and thermal expansion, the heating of surface waters globally is likely to only enhance and amplify sea level change from melt water sources. And we might not be done there... Remembering way back to our discussion about sources of freshwater other than ice, you will recall that significant volumes of water were historically stored in rocks on land in the form of aquifers, in inland lakes, and in rainforests.

As human populations grow and the demand for freshwater for agriculture and other industries increase, and as forests are deforested, significant volumes of water are now re-entering the ocean-climate system and are contributing 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. This so called "de-watering" of the continents by some estimates could eventually be as much as 25% of sea level rise. See this National Geographic News page.

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