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.

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.

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. 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! 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.

Peter Gleick