Climate Change Impacts #1: Sea-level Rise

When I talk to people about the need to prepare for climate change in Carpinteria, the first thing they want to know is what sort of impacts I’m talking about. The ones I worry about the most are: 1) sea-level rise, 2) reduced availability of fresh water, and 3) price spikes in food and fuel. I want to talk here about the first one: sea-level rise.

I’m talking about it first not because I think it’s the most important climate-change issue we face; I think fresh water will probably be a bigger problem. But of all the impacts I’ve looked at, it’s the most straightforward. For one thing, it’s pretty much inevitable. Regardless of any progress we make in reducing greenhouse gas emissions, the inertia in the global climate system means that significant sea-level rise is essentially locked in. We will experience sea-level rise over the next several centuries. It’s just a question of how much and how soon.

Another thing that makes sea-level rise a relatively straightforward problem is that we know how it will play out. With detailed elevation measurements, we can make accurate predictions about what the effects will be of a given amount of sea-level rise. If we take the problem seriously and take action early, there are solutions that will be effective and comparatively cheap. If we ignore the problem, though, our options get worse.

Areas at Risk

A large part of Carpinteria’s downtown/beach neighborhood is only a few feet above the high tide line. It wasn’t that long ago that the Carpinteria salt marsh extended east to Carpinteria Creek, and the buildings in that area today are built on relatively shallow fill above what used to be a wetland that flooded at high tide.

The University of California Natural Reserve System has some neat images showing the extent of the Carpinteria salt marsh over the years. Here’s a map from 1869 (like most of the images in this page, you can click to enlarge it). You can see that at that time the salt marsh formed a large estuary, with (going west to east) Santa Monica Creek, Franklin Creek, and Carpinteria Creek flowing into it:

Here’s an aerial photo from 1929. This image does not extend as far east as the 1869 map. To help orient you, that little square of streets along the right edge of the photo, near the top, represents today’s Dorrance Way, Ash Avenue, and Third Street. The area south of there is still undeveloped salt marsh:

Next is an aerial photo from 1943. You can see that same square, although now another block has been added north of it bounded by Fourth Street, and Ash Avenue has been extended south across the marsh to the beach:

Finally, here’s a screenshot from Google Maps, showing what that neighborhood looks like today. That same square of streets doesn’t stand out as much in this shot, but you can see where it is if you know the neighborhood. The area south of it has been filled in by development, and the Silver Sands Mobile Home Park has been built between Ash Avenue and the Franklin Creek channel:

All that development south of the railroad tracks is quite vulnerable to sea-level rise.

How Bad Will It Be?

So, how much sea level rise can we expect?

The 2009 California Climate Adaptation Strategy (PDF) had this to say on that subject:

Sea-Level Rise Projections

Over the 20th century, sea level has risen by about seven inches along the California coast. Replacing previous projections of relatively modest increases of sea-level rise for the 21st century, the 2009 Scenarios Project built on scientific findings that became available in the last two years to produce estimates of up to 55 inches (1.4 meters) of sea-level rise under the A2 emissions scenario by the end of this century (Figure 7). This projection accounts for the global growth of dams and reservoirs and how they can affect surface runoff into the oceans, but it does not account for the possibility of substantial ice melting from Greenland or the West Antarctic Ice Sheet, which would drive sea levels along the California coast even higher. Projections of sea level rise under the B1 scenario are still several times the rate of historical sea-level rise, and would barely differ under a stringent “policy scenario” in which global emissions would be drastically reduced. This suggests that while mitigation will be important to minimize many climatic and ecological impacts, adaptation is the only way to deal with the impacts of sea-level rise that is anticipated under either emissions scenario during the 21st century.

In short, even on a lower emissions trajectory and without the addition of meltwater from the major continental ice sheets, sea levels in the 21st century can be expected to be much higher than sea levels in the 20th century

Here’s Figure 7 from that report:

A few things strike me about that graph. First, being realistic about what we have (and haven’t) accomplished in the area of reducing our greenhouse gas emissions, the red A2 path is probably pretty close to the one we’re actually on. That scenario predicts 4 feet or more of sea level rise by 2100, with that rise continuing and accelerating in the century after that. If the Greenland and Antarctic ice sheets melt quickly (and research published in the last few years suggests that may be happening), we could be looking at more than 6 feet of sea level rise by 2100, with (again) more to follow.

What would that mean in Carpinteria? A report prepared by the Pacific Institute for the California Energy Commission in 2009 gives an idea. See: The Impacts of Sea Level Rise on the California Coast. It includes this chart:

The light blue corresponds to “Current Coastal Base Flood (approximate 100-year flood extent)”. That is, it shows the areas that currently face a 1% risk of ocean flooding in any given year. The dark blue corresponds to “Sea Level Rise Scenario – Coastal Base Flood + 1.4 meters (55 inches)”. In other words, it shows the parts of Carpinteria that would be added to that 100-year floodplain under the A2 scenario (without including the effects of melting in Greenland and Antarctica) by the year 2100.

A few things to note about this map: For one thing, it isn’t very sophisticated. This report was about doing a quick assessment of the whole California coast, so it took a pretty basic approach. It just shows those areas that are at or below the predicted elevation of a 100-year flood. If there is no path for the water to follow (that is, if an area is surrounded by higher ground), that area would not necessarily experience flooding. Actual floodplain maps tend to be much more detailed, and include more analysis of local topography. Nor does this map attempt to show other effects that would accompany sea-level rise, like conversion of marshland to open water, and loss or migration of the beach.

But it’s an interesting starting point for thinking about the problem. Clearly, the entire area south of the railroad tracks, and some low-lying areas north of the tracks, are going to be at risk between now and 2100. That risk will worsen, and the affected area will expand further north, in the century after that.

What About the Beach?

The beach is going to be on the front lines of this battle. Here’s a cropped version of the Google maps image I showed earlier:

Carp has already experienced some beach loss, due to things like the reduction in sediment from channelization of local creeks and the creation of Santa Barbara Harbor. Those homes along Avenue Del Mar (on that narrow strip of land between the salt marsh and the ocean, west of Ash Avenue) are already protected by a riprap seawall. The beach in that section has become narrow enough that it disappears at high tide.

Moving eastward, the next section, between Ash Avenue and Linden along Sandyland Road, has somewhat more beach. There is no riprap here, but the city uses a bulldozer each winter to pile up the sand into a high berm directly in front of the houses, in effect creating a temporary seawall to protect the homes from winter waves, while sacrificing some of the recreational value of the beach while the berm is present.

The last section of beach, in the lower right of the image, is the Carpinteria State Beach. This section (which you can also see in this blog’s header graphic) includes the widest beaches in Carp, backed by a dune remnant, a parking lot, and a campground (which continues onto the higher ground east of Carp Creek).

If we continue to deal with sea-level rise the way we have in the past (that is, largely ignoring it until circumstances force us to take action), we can expect conditions on these three sections of beach to gradually worsen, with beach loss shifting from west to east in a stepwise fashion:

  • Avenue Del Mar will lose its beach entirely, ending up with open water south of the seawall.
  • Sandyland Road will become like Avenue Del Mar is today, with a permanent seawall of some sort and usable beach only at low tide.
  • The State Beach and/or the dunes behind it will be reduced, eventually requiring shore protection if the current edge of development (meaning the parking lot and campground) is going to be maintained.

The process won’t stop there, but will continue at an accelerating rate, with the beach loss working its way east along the coast: Wide beach will become narrow beach, then low-tide-only beach, then no beach at all. The water won’t stop, though. It will keep rising, locking defenders of coastal property into an escalating arms race of higher seawalls and greater flood risk, until eventually, when will or resources have been exhausted, they surrender and move somewhere else.

Besides the beach, Carp has a second shoreline where this issue will play out: The internal shoreline of the Carpinteria Salt Marsh. If we keep the marsh open to the ocean, the development that surrounds it will be threatened by sea-level rise. Maintaining the current boundary of the marsh will mean creating dikes, which will cause changes similar to those described above for the beach. Eventually the marsh will be gone, replaced by a permanently flooded basin.

What Can We Do About It?

We basically have two choices when it comes to sea-level rise: Defend the current shoreline, or retreat.

Defending means building a permanent seawall and dike, and raising it as the sea rises. It’s an expensive proposition, made more so by the fact that the sea’s rise over the next several centuries is going to be inexorable. Like the Terminator in the James Cameron movie, it Will. Not. Stop. So if we’re going to commit ourselves to defending the current edge of development, we need to be prepared for what that means.

I expect homeowners on the Carpinteria beachfront, as well as those on low-lying ground behind the beach, will be vocal advocates for defending the current shoreline. From the community’s standpoint, though, I’m not sure it makes sense. For one thing, it means sacrificing the beach. Beaches are the product of an open, unprotected shoreline. For any shoreline we choose to protect, that’s a shoreline that eventually will lose its beach. For a place that promotes itself as the one of the last great southern California beach towns (“home of the world’s safest beach”), that’s worth considering.

More importantly, climate change is going to create many issues we need to deal with, and every dollar spent defending the shoreline is a dollar that won’t be available for addressing other problems. As is sometimes the case in war, retreat — saving our resources and pulling back to a more easily defended position — may be the difficult but necessary choice, even if that is hard to appreciate from the perspective of someone outside the newly redrawn perimeter.

“Retreat” in this context means some sort of managed process of shifting the shoreline inland. Such a shift obviously would be much easier to accomplish in the case of the salt marsh and the state beach, which are relatively undeveloped. The fact that Carpinteria’s low-lying areas have not been as densely developed as other coastal communities is going to be a big help in this fight.

For the developed areas, retreat will be more painful, especially if we ignore it until it becomes a crisis. But we don’t have to ignore it. If we address the problem early enough, we can use zoning provisions to limit new development in areas that we know are going to be hard to defend. In that way, attrition will gradually reduce the developed footprint in those areas, eliminating the need for heroic defenses. Sea-level rise is happening quickly in geological terms, but that’s still fairly slow in human terms. We have time to deal with it effectively — if we start soon enough.

That’s the whole point of having a public planning process: It helps us make long-range decisions wisely, using the best available information. Carpinteria’s current planning laws are based on the assumption that sea level will remain constant. But that’s not reality. The best available scientific evidence tells us the sea will rise, and at an accelerating rate, for at least the next several centuries. The sooner we face that the better off we will be.

Update: This video segment from LA’s PBS affiliate, KCET, does a good job of explaining some of the issues behind adapting to sea-level rise in southern California:

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