Based on the NC legislature’s decree about the science of sea level rise projections and some of the related propaganda we have seen from climate change deniers, I get the sense there is a lot of confusion about sea level rise. So here is a primer on what we know about sea level and climate change.
1) Sea level fluctuates naturally by 10s to 100s of meters but has been relatively stable for the last few thousand years. Changes in sea level are controlled in large part by atmospheric temperature, which in turn controls how much water is locked up on the continents as ice. Cool temperatures = lots of ice and low sea level. When the earth’s temperature increases (by just a few degrees C), water from the melting glaciers drains into the oceans and increases sea level – sometimes very rapidly and by tens of meters.
The graph below depicts global change in sea level since the end of the last “ice age” or “glacial period“. During this 15,000 year period, sea level has increased by a whopping ~125 meters or ~400 feet! You can also see some periods when the earth was transitioning to an interglacial period when sea level rose very rapidly, such as during “Meltwater Pulse 1A” when it appears to increase at least one to two feet per decade. These rapid increases in sea level are – as you might have guessed – caused by the rapid melting of glaciers. So the link is real and far from subtle; the earth warms slightly, the glaciers melt, and sea level rises.
Figure 1. This figure shows sea level rise since the end of the last glacial episode based on data from Fleming et al. 1998, Fleming 2000, & Milne et al. 2005. (Source and details)
During natural glacial cycles global temperature trends are influenced by a number of factors including slight changes in the earth’s obit around the sun and atmospheric composition. However, this time the earth’s warming isn’t due to an increase in heat coming in from the sun, but instead because greenhouse gases are letting less of that heat back out, i.e., the greenhouse effect.
2) Greenhouse gas emissions are causing sea level to rise (Fig. 2) via “thermal expansion” (warming a liquid increases it’s volume) and by melting mountain glaciers. Until human activities increased the concentration of greenhouse gases in the earth’s atmosphere, global sea level had been relatively stable for several thousand years (which can be seen in Fig. 1 above and with greater detail here).
Figure 2. Change in annually averaged sea level at 23 geologically stable tide gauge sites with long-term records as selected by Douglas (1997). The thick dark line is a three-year moving average of the instrumental records. This data indicates a sea level rise of ~18.5 cm from 1900-2000. (Source and details).
3) The rate of sea level rise appears to be accelerating, i.e., non-linear. The indented text in this section below is from Skeptical Science.
The blue line in Fig. 3 below clearly shows sea level as rising, while the upward curve suggests sea level is rising faster as time goes on. The upward curve agrees with global temperature trends and with the accelerating melting of ice in Greenland and other places.
Global mean sea level (e.g., the global average height of the ocean) has typically been calculated from tidal gauges. Tide gauges measure the height of the sea surface relative to coastal benchmarks. The problem with this is the height of the land is not always constant. Tectonic movements can alter it, as well as Glacial Isostatic Adjustment. This is where land which was formerly pressed down by massive ice sheets, rebounds now that the ice sheets are gone.
To construct a global historical record of sea levels, tide gauge records are taken from locations away from plate boundaries and subject to little isostatic rebound. This has been done in A 20th century acceleration in global sea-level rise (Church 2006) which reconstructs global sea level rise from tide gauges across the globe. An updated version of the sea level plot is displayed in Figure 3:
Figure 3: Global mean sea level from 1870 to 2006 with one standard deviation error estimates (Church 2008).
Tidal estimates from sediment cores go even further back to the 1300’s. They find sea level rise is close to zero in the early part of the sedimentary record. They then observe an acceleration in sea-level rise during the 19th and early 20th century. Over the period where the two datasets overlap, there is good agreement between sedimentary records and tidal gauge data (Donnelly 2004, Gehrels 2006).
What we’re most interested in is the long term trends. Figure 4 shows 20 year trends from the tidal data. From 1880 to the early 1900’s, sea level was rising at around 1mm per year. Throughout most of the 20th century, sea levels have been rising at around 2mm per year. In the latter 20th century, it’s reached 3mm per year. The five most recent 20-year trends also happen to be the highest values.
Figure 4: The linear trends in sea level over 20-year periods, with one sigma error on the trend estimates shown by the dotted lines. From 1963 to 1991, there were a series of volcanic eruptions which caused cooling and hence contraction of the upper ocean. This temporarily slowed the rate of sea level rise.
In summary, the historical record suggests that the rate of sea level rise appears to have increased since the late 19th century, however, we don’t know whether this acceleration will continue, i.e., whether this non-linearity is a short or long-term trend. My money is on the latter given the current work in Greenland and Antarctica indicating a frightening degree of melting.
4) There is a lot of variation in the rate of sea level rise. This is a point frequently missed or ignored by all kinds of people (including some scientists) talking about sea level rise. Take a look at the figure below (from NASA). Changes in sea level clearly vary from place to place. In some parts of the western Pacific ocean, sea level rise has been nearly 1cm per year. In others, sea level is falling. This variability is caused by several factors and nature cycles. This variability is not surprising and does not in any way challenge the fact that globally, average sea level is rising. It just means that the impacts of sea level rise, like very other aspect of global climate change, are variable. For some people they are trivial, to others they are catastrophic.
5) How much sea level rise should we expect this century? (some of the text in this section is from Skeptical Science)
We have a pretty good sense of how much sea level rise to expect from the two mechanisms mentioned above (thermal expansion and the melting of mountain glaciers) but a third important mechanism is a lot harder to deal with: the melting and calving of ice from the massive glaciers covering Greenland and Antarctica. Recent work suggesting that both processes may be accelerating.
Calving is accelerated by warming (in some really interesting ways) but the processes are not well understood. Therefore, scientists cannot predict what will happen to the massive amounts of water locked up in the glaciers on Greenland and Antarctica (although we suspect that calving will accelerate). Therefore, IPCC did not include the effects of what it calls “dynamical processes” like this, arguing they couldn’t be modeled: “Dynamical processes related to ice flow not included in current models but suggested by recent observations could increase the vulnerability of the ice sheets to warming, increasing future sea level rise. Understanding of these processes is limited and there is no consensus on their magnitude.”
In 2001, the IPCC Third Assessment Report (TAR) projected a sea level rise of 20 to 70 cm by 2100. In 2007, the IPCC Fourth Assessment Report (AR4) gave similar results, projecting sea level rise of 18 to 59 cm by 2100. How do the IPCC predictions compare to observations made since these two reports?
Figure 6: Sea level change. Tide gauge data are indicated in red and satellite data in blue. The grey band shows the projections of the IPCC Third Assessment report (Allison et al 2009).
Observed sea level rise is tracking at the upper range of model predictions (just as many scientists argued they would).
Why do climate models underestimate sea level rise? The main reason for the discrepancy is, no surprise, the rapid (likely accelerating) loss of ice from Greenland and Antarctica, i.e., the “dynamical processes” that are not considered in the models. Even East Antarctica, previously considered stable and too cold to melt, is now losing mass.
An alternative way to predict future sea level rise is from the known relationship between sea level and global temperature from the recent and distant past (Vermeer 2009). For example, Fig. 7 shows the strong correlation between observed sea level (red line) and reconstructed sea level (dark blue line with light blue uncertainty range) from 1880 to 2000.
Figure 7: Observed rate of sea-level rise (red) compared with reconstructed sea level calculated from global temperature (dark blue with light blue uncertainty range). Grey line is reconstructed sea level from an earlier, simpler relationship between sea level and temperature (Vermeer 2009).
Using this relationship, the amount of global sea level rise can be predicted based on different IPCC “emissions scenarios”, which make different assumptions about the future global economy and how much carbon dioxide it will emit (and thus how much the earth will warm).
The range of projected sea level rise by 2100 is 75 to 190 cm. As you get closer to 2100, the contribution from ice melt grows relative to thermal expansion. This is the main difference to the IPCC predictions which assume the portion of ice melt would diminish while thermal expansion contributes most of the sea level rise over the 21st Century.
Figure 8: Projection of sea-level rise from 1990 to 2100, based on IPCC temperature projections for three different emission scenarios. The sea-level range projected in the IPCC AR4 for these scenarios are shown for comparison in the bars on the bottom right. Also shown in red is observed sea-level (Vermeer and Rahmstorf 2009).
Fig. 8 shows projected sea level rise for three different emission scenarios. These predictions for sea level rise are roughly 3 times greater than the IPCC predictions. Note the IPCC predictions are shown as vertical bars in the bottom right. For the lowest emission rate (B1), sea levels are expected to rise around 1 metre by 2100. For the higher emission scenario, which is where we’re currently tracking, sea level rise by 2100 is around 1.4 m. Grim.
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