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While climate models of atmospheric carbon dioxide concentrations and global temperatures have closely fit real world observations, the models have done less well in predicting sea levels. The following graph illustrates the disparity. The blue shaded region contains the upper and lower bounds of the IPCC climate model predictions. Sea level changes from satellite measurements (red line) and tidal gauges (orange line) since 1990 are consistently higher than predicted from those models.

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Since sea levels are a big deal for coastal populations and infrastructure, we cannot afford to guess wrong. So just how much water might we have to contend with in the 21st century and beyond?

A number of studies suggest that sea levels could rise between 1 to 2 meters by the end of the century. Taking thermal expansion and ice sheet melt into consideration, we would need to keep CO2 concentrations below 450 ppm to keep sea levels from rising more than a meter by the year 2100. Since a sea level rise of 3 feet or so would create quite an expensive mess, it would be wise to change our carbon polluting ways.

Far less comforting is data from studies looking at the geological record. It is all too clear that sea levels have been substantially higher than present day levels, perhaps as much as two hundred feet higher. That would be something of a game-changer. Sea levels have also been substantially lower than current levels during the peak of glacial periods.

A new study published in the Proceedings of the National Academy of Sciences (subscription required) examined the relationship between CO2 and sea levels in the geological record over a 40 million year time scale. Here is the abstract from the article.

On 103 to 106-year timescales, global sea level is determined largely by the volume of ice stored on land, which in turn largely reflects the thermal state of the Earth system. Here we use observations from five well-studied time slices covering the last 40 My to identify a well-defined and clearly sigmoidal relationship between atmospheric CO2 and sea level on geological (near-equilibrium) timescales. This strongly supports the dominant role of CO2 in determining Earth’s climate on these timescales and suggests that other variables that influence long-term global climate (e.g., topography, ocean circulation) play a secondary role. The relationship between CO2 and sea level we describe portrays the “likely” (68% probability) long-term sea-level response after Earth system adjustment over many centuries. Because it appears largely independent of other boundary condition changes, it also may provide useful long-range predictions of future sea level. For instance, with CO2 stabilized at 400–450 ppm (as required for the frequently quoted “acceptable warming” of 2 °C), or even at AD 2011 levels of 392 ppm, we infer a likely (68% confidence) long-term sea-level rise of more than 9 m above the present. Therefore, our results imply that to avoid significantly elevated sea level in the long term, atmospheric CO2 should be reduced to levels similar to those of preindustrial times.
Researchers Gavin L. Foster and Eelco J. Rohling looked at 2000 paired observations of CO2 concentrations and sea levels across time. CO2 concentrations were estimated using ice cores and chemical profiles from deep ocean sediments. Sea levels were based on chemical profiling of foraminifera, single-cell organisms, stratified in deep ocean sediments.

The reconstruction of the 550,000 years of the Pleistocene epoch prior to 1800 provides a fascinating look at the variability of CO2 concentrations and sea levels before the industrial age. CO2 levels ranged from 180 to 280 ppm, while sea levels ranged from 10 to 110 meters below current levels. CO2 concentrations and sea levels were highly correlated in this time series (r2=0.68).

What these data show is that CO2 concentrations in the past 50 years (currently around 394 ppm) are simply unprecedented as part of the normal fluctuations between glacial and interglacial periods during the past 500,000 years or so. We are tinkering with what has been a stable and predictable climate system. The longer we maintain CO2 levels above 280 ppm, the more sea levels are also likely to rise well above current levels. How fast and how much sea levels rise will depend on the dynamics of ice sheet melting in Greenland and Antarctica.

Combining all the proxies for CO2 and sea levels allows a reconstruction of the geologic record from the Pliocene epoch into the Eocene. Sea levels stabilize around 14 meters above current levels as CO2 concentrations fluctuate between 450 and 650 ppm. This plateau in sea levels may be the result of ice gain in East Antarctica compensating in part for the collapse of ice sheets in Greenland and West Antarctica. When CO2 concentrations increased from 650 to 1000 ppm, sea levels again rose rapidly to a maximum of 65 meters above current levels. The collapse of the East Antarctic ice sheet resulted in an ice-free planet with very high sea levels and very limited dry land.

If there is a silver lining in this story, it is that continental ice sheet losses that produce sea level rise above thermal expansion tend to take centuries. Even though other climate catastrophes (e.g., droughts, floods, wildfires, and deadly cyclones) are likely in the near future, we may have centuries to curb carbon pollution before sea levels rise to unmanageable levels. However, the data indicate that stabilizing CO2 concentrations at 450 ppm will eventually result in sea levels 9 to 14 meters above current levels. The authors summarize the situation as follows:

Clearly our relationship has limited relevance to short-term sea-level projections for the next century. However, accurately determining the long-term response of sea level to CO2 forcing has significant implications for the long-term stabilization of greenhouse gas emissions (by natural processes or human activity) and for decisions about the “acceptable” long-term level of CO2/warming. For instance, our results imply that acceptance of a long-term 2 °C warming [CO2 between 400 and 450 ppm (46)] would mean acceptance of likely (68% confidence) long-term sea-level rise by more than 9 m above the present. Future studies may improve this estimate, notably by better populating the interval between CO2 concentrations of 500–280 ppm (i.e., the Pliocene/middle Miocene). Regardless, the current relationship is sufficiently refined to imply that CO2 would need to be reduced significantly toward 280 ppm before any lost ice volume might be regrown (similarly over many centuries).
We are engaged in a very stupid experiment as we allow fossil fuels industries to play with the CO2 climate control knob to maximize paper profits. And we are kidding ourselves if we think the consequences will be trivial. Given the lack of political will and public pressure, the chances of CO2 stabilizing under 450 ppm are remote and under 300 ppm virtually impossible. Hell and high water are coming.

Originally posted to DWG on Sun Jan 27, 2013 at 05:58 AM PST.

Also republished by Climate Hawks, DK GreenRoots, and Community Spotlight.

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