Earl J. Ritchie, Lecturer, Department of Construction Management
One frequently sees articles claiming a certain amount of global warming or sea level rise is inevitable based on the amount of CO2 already in the atmosphere. Locked-in warming is commonly estimated to be 1.5 degrees C (2.7° F) above preindustrial levels, about a one-half degree above the current temperature. This is the aspirational target of the 2015 Paris Agreement of the United Nations Framework Convention on Climate Change.
Although there may be some rhetorical benefit in this number, it understates the actual amount of committed warming and sea level rise predicted by mainstream climate change theory. The IPCC says, “Stopping emissions today is a scenario that is not plausible.” Therefore, we will inevitably have higher CO2 concentration than the present, greater warming and more sea level rise.
Under the lowest of the IPCC’s four scenarios, RCP2.6, peak temperature rise of 2 degrees C will be reached before 2100, and sea level rise will be less than about a half meter. However, due to lag effects in ocean warming and ice melt, sea level will continue to rise for centuries. Rise can theoretically be reduced by negative carbon emissions or geoengineering.
The best-case scenario
If one accepts “locked-in” as actually involving some amount of future emissions, the door is opened to the numerous speculations about what is achievable and what will actually happen. RCP2.6 requires very rapid CO2 reduction and, ultimately, negative emissions. There is considerable question, even as expressed by the original authors, whether these reductions can be realized. Assuming the IPCC’s models are correct and RCP2.6 is achievable, one might say locked-in warming is the 2 degree primary target of the Paris Agreement, approximately 1 degree higher than today.
What happens after 2100
The IPCC projects temperatures to decline slowly after 2100 under RCP2.6 and rise slowly under RCP4.5, the second most favorable scenario. However, due to lag, sea level continues to rise under all scenarios. The graph below shows sea level projections to 2500 for scenarios roughly equivalent to RCP2.6 (low CO2) and RCP4.5 (medium CO2).
Source: Modified from IPCC Climate Change 2014
The maximum rise of about 2 meters (7 feet) in these scenarios is quite moderate compared to what could happen. The IPCC’s range for “multi-millennial” commitment is 3 to 13 meters (10 to 36 feet) for warming of 2 degrees C.
There is ongoing debate in the scientific community about melting thresholds and rates. As discussed in a recent post, several recent articles have predicted faster rise in this century. Differences of opinion over short-term rise, to 2100, are primarily a question of how fast the Antarctic and Greenland ice sheets will melt. Longer-term rise is a question of how much of the sheets will melt.
Comparison to past warm periods
Although I am somewhat skeptical of the ability of current climate models to predict melt rates, an independent estimate of long-term sea level rise can be made by analogy to earlier warm periods. The argument is shown in the graph below, with the different colored lines representing sea level reconstruction by different researchers.
Source: Modified from Siddall, et al.
In this case, the comparison is to the last interglacial period, known as the Eemian or MIS 5e, about 125,000 years ago.
Temperature in the Eemian is estimated to have been about 1 degree C higher than today and maximum sea level about 5 meters higher. The analogy argument is that peak temperature expected in the near future will be similar to the Eemian maximum; therefore, we can expect ultimate sea level rise to be similar.
Unfortunately, there are two reasons why the value of this comparison is limited. First, there is considerable disagreement about the actual temperature and sea level during the Eemian. The temperature difference is commonly quoted as 1 degree to 2 degrees C; however, estimates range from negligible to “several degrees.” Similarly, sea level has been estimated to have been 3 to 10 meters higher, with estimates around 5 to 6 meters most common.
Second, conditions during the Eemian are not similar to the present. Solar heating was higher, and CO2 was lower.
The Eemian and other warm periods are not great analogs. But they do indicate a high probability of substantial sea level rise over the longer term for temperatures that are already locked in.
How high will the water get?
Kopp, et al. say, “future sea-level rise remains an arena of deep uncertainty.” The range of projected sea level is very wide, and it depends upon how far into the future you project.
The main cause of variation is the amount of melt of Greenland and Antarctic ice sheets. It’s estimated that complete melt of the ice sheets would raise sea level by about 66 meters (217 feet). Since only a small fraction of the sheets melted in the Eemian, projected rise at 2 degrees will be much less.
Levermann, et al. modeled sea level rise 2000 years into the future at 1 degree and 2 degrees above pre-industrial levels, bracketing the Paris Agreement goals. Their median estimates are 2.3 meters at 1 degree and 4.8 meters at 2 degrees. Not too much should be made of the specific numbers, because the model range is large (1 to 4.9 and 2.6 to 9.8, respectively), and other articles have different projections.
It is fair to say ultimate sea level rise could be in the range of 3 to 10 meters (10 to 33 feet). The difference in rise predicted between different models is small for the next two or three decades so there will be little evidence in the near term pointing toward a clearer estimate.
If you can reverse it, is it locked in?
Both warming and sea level rise can theoretically be halted or reversed by geoengineering methods: removing carbon dioxide to reduce the greenhouse effect (carbon dioxide removal, CDR) or reflecting sunlight (solar radiation management, SRM). There are dozens of proposed methods of each. Some are pretty innocuous, such as growing more forest to remove carbon dioxide and having more white roofs to reflect sunlight. Others, such as fertilizing the ocean to encourage algae or phytoplankton growth, have side effects and could get out of control.
At present, they are considered by most to be impractical, too expensive or too dangerous. Keller, et al. say, “At present, there is little consensus on the impacts and efficacy of the 60 different types of proposed CDR.” This is also true of solar radiation management, about which there is considerable concern about adverse effects. In any case, it is unlikely these methods will be implemented on a significant scale for at least two or three decades.
Why should we care?
It’s hard to get people concerned about possible events hundreds, or even thousands, of years in the future. James Hansen says, “nobody cares about matters 1,000 years in the future.” However, these are serious matters. Sea level rise of several meters has significant implications for displacement of populations, damage to infrastructure and loss of land.
Several visualizations available online show the effect of even modest sea level rise. The photo below is a NOAA simulation of 1-meter (4 foot) rise at Galveston, TX.
We are very likely facing amounts of sea level rise with serious consequences even at temperatures we have already reached.
What is to be done
Significant sea level rise is unavoidable. Adaptation will be necessary.
The uncertain rate of sea level rise makes planning difficult. A 2015 report by the New Zealand Parliamentary Commissioner for the Environment discusses the choice of time horizons. They describe that a 50-year planning horizon may be sufficient for projects with a short intended life and a 100-year planning horizon may not be enough for those with a long life. Their recommendation is to use a timeframe “appropriate for different types of development.”
What is necessary or feasible will vary by location. Levees, tidal barriers, seawalls and elevating infrastructure may be possible. It would make sense not to allow new development in low-lying areas. Adaptation methods are extensively discussed in the IPCC Fifth Assessment Working Group II Report.
Earl J. Ritchie is a retired energy executive and teaches a course on the oil and gas industry at the University of Houston. He has 35 years’ experience in the industry. He started as a geophysicist with Mobil Oil and subsequently worked in a variety of management and technical positions with several independent exploration and production companies. He retired as Vice President and General Manager of the offshore division of EOG Resources in 2007. Prior to his experience in the oil industry, he served at the US Air Force Special Weapons Center, providing geologic and geophysical support to nuclear research activities. Ritchie holds a Bachelor of Science in Geology–Geophysics from the University of New Orleans and a Master of Science in Petroleum Engineering and Construction Management from the University of Houston.