(Mis)Understanding Sea-Level Rise And Climate Impacts

Cross-posted from National Geographic

One of the most important and threatening risks of climate change is sea-level rise (SLR). The mechanisms are well understood, and the direction of changes in sea-level is highly certain – it is rising and the rate of rise will accelerate. There remain plenty of uncertainties (i.e., a range of possible outcomes) about the timing and rate of rise that have to do with how fast we continue to put greenhouse gases in the atmosphere, the responses of (especially) ice sheets in Greenland and Antarctica, and the sensitivity of the climate.

Even little changes can have big consequences. As we saw with Superstorm Sandy, where extremely severe weather was combined with a very high tide, on top of sea levels that have risen six to nine inches over the past century, even a little bit of sea-level rise around the world has the potential to cause hundreds of billions of dollars of damages and the displacement of millions of people.

The Pacific Institute, among many other organizations, has been working to understand and evaluate the nature of the threat of sea-level rise and the risks posed to coastal populations, property, and ecosystems. In 1990, a colleague and I published the first detailed mapping and economic assessment of the risks of sea-level rise to the San Francisco Bay Area, looking at populations at risk, the value of property in new flood zones, and the costs of building some kinds of coastal protection (“adaptation”) to protect higher valued assets. That early report can be found here.

Then, in 2009 and 2010, the Pacific Institute, with funding from the State of California, conducted a detailed, high-resolution mapping analysis of the entire coast from Oregon to Mexico. We analyzed a set of sea-level rise scenarios developed by the Scripps Institution of Oceanography and worked with the California Energy Commission, the Metropolitan Transportation Commission, the Ocean Protection Council, the National Oceanic and Atmospheric Administration, the US Geological Survey, FEMA, and others to evaluate the risks to people, property, transportation infrastructure, ecosystems, power plants, wastewater treatment plants, and more, should those scenarios of sea-level rise happen. The full peer-reviewed report, the high resolution maps, specialty maps, and all open source GIS data can be publicly downloaded here. (A peer-reviewed journal article was also published.) That analysis suggests coastal regions are highly vulnerable to even modest sea-level rises with hundreds of thousands of people and more than a hundred billion dollars of infrastructure already in zones at risk of future flooding.

I was reminded this week, however, of the difficulty some people have in understanding the nature of climate risks, when a climate skeptic who shall remain nameless started tweeting his misunderstandings to me without having read our studies (I know this because after I pointed out his errors, he asked me to send the studies to him). My internet-savvy sons have tried for years (only partly successfully) to teach me: DNFTT. But these tweets offer insights into what might be more general misconceptions, so let me address some of them for those who actually want to help the public understand the real risks of climate change.

Misunderstanding #1: Predication versus Scenario. There is a big difference between a prediction and a scenario. Scenarios are tools for examining how changes in some kind of conditions (such as greenhouse gas concentrations) might affect something else (such as climatic conditions or sea-level). They are stories of possible futures based on a range of assumptions. Almost all studies of climate impacts evaluate scenarios to examine possible future conditions, risks, and threats. Climatologist Gavin Schmidt sometimes uses the following:

  • Forecast: What you think will happen in the future (could be probabilistic), but with no conditionals. Used in weather forecasts, sales forecasts etc.
  • Prediction: A much broader category of scientific statement that implies a complete specification of the circumstances under which X would be expected.
  • Projection or Scenario: A conditional prediction about the future. i.e., if a certain set of circumstances come to pass, the climate will respond in the following way.

In the case of sea-level rise, climate modelers and oceanographers make projections of how sea-level would react to a range of assumptions about energy use and type, greenhouse gas emissions, and climate and ice sensitivities. These are not predictions. In the case of our reports, we evaluate the implications for coastal regions should these future sea-level rises occur. This is a risk and vulnerability assessment. In fact, for the estimates of sea-level rise in our study, we clearly note that changes could be both smaller or larger, and slower or faster than our evaluation. None of this is actually relevant to our estimate of the things currently at risk from a 1.4 meter rise.

Misunderstanding #2: Linear versus Exponential. There is sometimes confusion in some people’s minds about the difference between a linear trend and an exponential trend. In this case, data on actual changes in sea-level suggest that the recent rates of rise are between 3 and 3.5 millimeters per year. If sea-level changes are linear, then it is easy to project past trends forward: 100 years of rise would add between 0.3 and 0.35 meters. This is what my tweeter did, in an effort to say SLR is a smaller problem than the state-of-the-science 1.4-meter scenario we evaluated. Why the difference? Because climate change, and sea-level responses – are not linear; they are exponential. This means the sea level in the future will rise at an accelerating rate, leading to a much higher end point for any given year. Figure 1 shows this simple concept, but also shows that in the short term, it may be hard to distinguish between the two. A high-school student would get an F for assuming a linear rate for an exponential process. I know of no climate scientist who believes the climate will change in a linear fashion if there is continued exponential growth in greenhouse gas emissions.

Figure 1. Exponential versus linear growth. Note, for a while, it’s hard to tell the difference, but then the curves diverge dramatically.

Misunderstanding #3: Evaluating Average versus Extreme Risks. Climate scientists are a conservative lot (in the scientific sense, as shown in a recent journal article). As a result, assumptions and scenarios that are typically analyzed (including the ones we used, developed by the Scripps Oceanographic Institute) are in the middle of the range of what could plausibly occur. In particular, even the exponential rate that produces 1.4 meters of rise by around the end of the century includes no rapid acceleration of ice-sheet melt or ablation or other factors that could lead to even faster rates of increase or higher rises. There are some far more disturbing sea-level rise scenarios out there but we didn’t analyze them. Any criticism that the scenarios evaluated were too extreme could be equally balanced by criticism that they were not extreme enough. The most recent report on SLR scenarios for the U.S. offers a range from 0.2 meters to 2 meters by 2100 (see Figure 2).

Figure 2. USGCRP sea-level rise scenarios showing a range. Even more extreme increases are possible, just not considered likely. Also, note that SLR will not stop in 2100, just because the graph stops there!

Misunderstanding #4: Beware False Dichotomies and Ad Hominem Arguments. This skeptic opened his assault on the sea-level science discussion by arguing that I must not care about sea-level rise because my office was nearly at sea-level. First, a minute spent with Google Earth or a topo map would have shown that our offices are actually around +40 feet above mean sea-level – not in a vulnerable zone even with expected climate change over the next century (barring some more catastrophic scenario), and second, even if my office was in a vulnerable zone, it wouldn’t mean I didn’t care about the future risks of flooding. His ad hominem response was “OK I get it it [sic], the plan is to sit tight and laugh at others [sic] misfortunes.” I know, DNFTT.

Misunderstanding #5. Mitigation versus Adaptation versus Suffering: That same nasty tweet also reveals a deeper misunderstanding about the nature of responses to sea-level rise or any other climate impacts. We only have three options for sea-level rise: trying to reduce the rate of rise (mitigation), coastal defense or retreat (adaptation), and suffering the impacts. People and valuable property in zones threatened by sea-level rise will either suffer greater and greater damage, or will have to be protected with new costly infrastructure, moved away over time in advance of rising seas, or abandoned. These are issues discussed clearly in our studies. Moreover, our work at the Institute explicitly identifies vulnerable populations and strategies to protect them.

This particular climate skeptic lives nowhere near the coast. That could partly explain his lack of understanding or interest in the threats posed by sea-level rise to our extensive coastlines. But the risks facing his own community include growing heat stress and extreme temperatures, loss of inexpensive local hydropower generation, increased forest fire risks, greater air pollution, and, should sea-level rise get really bad, migration of lots of people to his community! More on these risks later.

Let’s put these errors and misunderstandings to rest and begin the necessary climate mitigation and adaptation responses, soon, or those exponential curves will begin to bite.

Dr. Peter Gleick is a scientist, innovator, and communicator on global water, environment, and climate issues. He co-founded and leads the Pacific Institute in Oakland.

15 Responses to (Mis)Understanding Sea-Level Rise And Climate Impacts

  1. Rabid Doomsayer says:

    One single glacial system could blow the 1 meter scenario out of the water. Go look at recent projections for the PIG/TW.

    As the ice shelves disappear, ice flow does speed up significantly. Ice shelves, even very thick ones can disappear with great speed. Larsen A, B, Wilkes all gone.

    Paleo climatic data suggests rises over 4 meters per century. We have changed climate forcing so much faster than anything in the paleo climatic record and in a way that exaggerates the warming at the poles. So why are we so certain that it will be less than that?

    Regardless of what happens this century the sea will keep rising. Current temperatures say 6 meters, current CO2 levels say tens of meters. Projected CO2 levels say no ice so 100 meters once other factors are added.

    So building new infrastructure high above the sea level would be a good idea. Knowing that many of the the world’s airports will need to be replaced, suggests starting to plan now would be reasonable.

    Our unwillingness to consider all but the mildest of possibilities, could leave us with one failed adaption attempt after another. Adaptions for a 1 meter rise will be washed away well before centuries end.

  2. Merrelyn Emery says:

    Good detail but it’s not just the ice shelves and glaciers: the ice sheets themselves appear to be melting from the bottom which crews down there are trying to get a handle on at the moment, ME

  3. D. R. Tucker says:

    What can we expect from President Obama’s choice to head the EPA? Gina McCarthy is being
    mostly cheered by environmentalists, including Frank O’Donnell, President of Clean Air Watch,
    who says we should all be breathing easier soon. Is the political logjam on climate change a serious
    environmental threat? We’ll talk to a young columnist who says citizens need to wake up and smell
    the carbon and not vote for any candidates who are naive, neglectful, incompetent or ignorant on science!
    Last, Alexis Madrigal, author of “Powering the Dream” on the history and future of green technology.

    Read more:
    Under Creative Commons License: Attribution

  4. Mark E says:

    Just wanted to add an under-appreciated effect of rising seas…..


    Storm surges are salty. Between bigger surges and underground “saltwater intrusion” pushing further inland, critical agriculture places like the Nile and Mekong Delta face ever increasing challenges growing crops that won’t tolerate the salt.

    It’s one thing if industry or wastewater plants close. It’s something else if we can’t feed people.

    , and “salt

  5. Mark E says:

    filter check please; might be clogged with jellyfish

  6. prokaryotes says:


    A very good video (the first 1 in this post) with Richard Alley, which outlines some prospects of potential abrupt threats in regards to SLR.

  7. Dan Miller says:

    As Jim Hansen points out on his web site, the Greenland ice sheet may be losing ice at an exponential rate with a doubling time of 5 to 10 years. There is not enough satellite (GRACE) data yet to be sure, but in a few more years there should be. If it is exponential, then we will have 3 feet of sea level rise by 2045~2067 depending on the doubling time! Of course, that means much more than 6 feet at the end of the century if Greenland is exponential. And then what about Antarctica and the PIG and Thwaites glaciers?

  8. Robert in New Orleans says:

    An excellent article about sea level rise in SE Louisiana:

    I am posting it again for the benefit for those who may have missed it the first time.

  9. Jim Baird says:

    Six measures to counteract the most important and threatening risks of climate change.
    1. Convert ocean heat to mechanical energy to offset thermal expansion.
    2. Convert liquid volume to gas by electrolysis to enable the Hydrogen Economy.
    3. Desalinate ocean water.
    4. Capture runoff before it enters the ocean for use in irrigation and the replenishment of depleting aquifers.
    5. Move surface heat to a depth of 1000 meters where the coefficient of expansion is half that of the surface – see 1, ocean thermal energy conversion moves 20 times as much heat as is it converts to work.
    6· Throttle the movement of tropical heat to the poles by storms by converting the heat that drives the storms to mechanical energy and the moving more to the depths – see 1 and 5.

    Or just pray the problem goes away, which of course it will not.

  10. My sympathies for dealing with trolls… Internet form of the school bully.

  11. Solar panels and wind farms cost much less than all of the above. Although some mitigations may well be needed once ff use is reduced to zero.

  12. Carol says:

    and this:
    Irreversible mass loss of Canadian Arctic Archipelago glaciers

  13. prokaryotes says:

    When are we starting with public broadcast of climate warnings? Scientist explain to the public officially what is going in our Atmosphere and what does it mean – future prospects.

  14. Mulga Mumblebrain says:

    Internet form of the school dunce, more like it. Their primary weapon of intimidation is their ability to bore their victims to death, or by their arrogant imbecility and ignorance, drive the rational human beings to despair for their species.

  15. Jim Baird says:

    The greenhouse that is causing the accumulation of ocean heat is already in place. So called clean energy like wind and solar won’t stop the accumulation if this heat and the damage it is producing. They would only prevent more rapid accumulation. The current effects will be with us for a thousand years.

    See: “Irreversible climate change due to carbon dioxide emissions” published in the Proceedings of the National Academy of Sciences of the United States of America, December 16, 2008.