Humans threaten wetlands' ability to keep pace with sea-level rise

December 04, 2013

Left to themselves, coastal wetlands can resist rapid levels of sea-level rise. But humans could be sabotaging some of their best defenses, according to a Nature review paper published Thursday from the Virginia Institute of Marine Science and the Smithsonian Environmental Research Center.

The threat of disappearing coastlines has alerted many to the dangers of climate change. Wetlands in particular--with their ability to buffer coastal cities from floods and storms, and filter out pollution--offer protections that could be lost in the future. But, say co-authors Matt Kirwan and Patrick Megonigal, higher waters aren't the key factor in wetland demise. Thanks to an intricate system of feedbacks, wetlands are remarkably good at building up their soils to outpace sea level rise. The real issue, they say, is that human structures such as dams and seawalls are disrupting the natural mechanisms that have allowed coastal marshes to survive rising seas since at least the end of the last Ice Age.

"Tidal marsh plants are amazing ecosystem engineers that can raise themselves upward if they remain healthy, and especially if there is sediment in the water," says co-author Patrick Megonigal of the Smithsonian Environmental Research Center. "We know there are limits to this, and worry those limits are changing as people change the environment."

"In a more natural world, we wouldn't be worried about marshes surviving the rates of sea level rise we're seeing today," says Kirwan, the study's lead author and a geologist at the Virginia Institute of Marine Science. "They would either build vertically at faster rates or else move inland to slightly higher elevations. But now we have to decide whether we'll let them."

Wetlands have developed several ways to build elevation to keep from drowning. Above ground, tidal flooding provides one of the biggest assists. When marshes flood during high tide, mineral sediment settles out of the water, adding new soil to the ground. It's one of the more convenient response systems to today's threat: When sea level rise accelerates and flooding occurs more often, marshes can react by building soil faster. Below ground, the growth and decay of plant roots adds organic matter--an effect rising carbon dioxide (CO2) seems to enhance. Even erosion can work in favor of wetlands, as sediment lost at one marsh can be deposited on another. While a particular wetland may lose ground, the total wetland area may remain unchanged.

But everything has a threshold. If a wetland becomes so flooded that vegetation dies off, the positive feedback loops are lost. Similarly, if sediment delivery to a wetland is cut off, that wetland can no longer build soil to outpace rising seas.

Direct human impacts, not rising seas or rising CO2, have the most power to alter those thresholds, the scientists report. Groundwater withdrawal and artificial drainage can cause the land to sink, as is happening right now in Chesapeake Bay. Because of this kind of subsidence, 8 of the world's 20 largest coastal cities are experiencing relative sea-level rise greater than climate change projections. Dams and reservoirs also prevent 20 percent of the global sediment load from reaching the coast. Marshes on the Yangtze River Delta survived relative sea-level rise of more than 50 mm per year since the 7th century C.E., until the building of more than 50,000 dams cut off their supply of sediment and sped up erosion.

In addition to building vertically, marshes can also respond to sea-level rise by migrating landward. But, the authors note, human activities have hindered this response as well. Conventional ways of protecting coastal property, such dykes and seawalls, keep wetlands from moving inland and create a "shoreline squeeze," Kirwan says. Because rates of marsh-edge erosion increase with rates of sea-level rise, the authors warn that the impacts of coastal barriers will accelerate with climate change.
-end-
A copy of the abstract will be available at doi:10.1038/nature12856. To receive a full copy of the paper or speak to Patrick Megonigal, contact Kristen Minogue at minoguek@si.edu or 443-482-2325. To speak with Matt Kirwan, contact David Malmquist at davem@vims.edu or 804-684-7011.

Virginia Institute of Marine Science

Related Climate Change Articles from Brightsurf:

Are climate scientists being too cautious when linking extreme weather to climate change?
Climate science has focused on avoiding false alarms when linking extreme events to climate change.

Mysterious climate change
New research findings underline the crucial role that sea ice throughout the Southern Ocean played for atmospheric CO2 in times of rapid climate change in the past.

Mapping the path of climate change
Predicting a major transition, such as climate change, is extremely difficult, but the probabilistic framework developed by the authors is the first step in identifying the path between a shift in two environmental states.

Small change for climate change: Time to increase research funding to save the world
A new study shows that there is a huge disproportion in the level of funding for social science research into the greatest challenge in combating global warming -- how to get individuals and societies to overcome ingrained human habits to make the changes necessary to mitigate climate change.

Sub-national 'climate clubs' could offer key to combating climate change
'Climate clubs' offering membership for sub-national states, in addition to just countries, could speed up progress towards a globally harmonized climate change policy, which in turn offers a way to achieve stronger climate policies in all countries.

Review of Chinese atmospheric science research over the past 70 years: Climate and climate change
Over the past 70 years since the foundation of the People's Republic of China, Chinese scientists have made great contributions to various fields in the research of atmospheric sciences, which attracted worldwide attention.

A CERN for climate change
In a Perspective article appearing in this week's Proceedings of the National Academy of Sciences, Tim Palmer (Oxford University), and Bjorn Stevens (Max Planck Society), critically reflect on the present state of Earth system modelling.

Fairy-wrens change breeding habits to cope with climate change
Warmer temperatures linked to climate change are having a big impact on the breeding habits of one of Australia's most recognisable bird species, according to researchers at The Australian National University (ANU).

Believing in climate change doesn't mean you are preparing for climate change, study finds
Notre Dame researchers found that although coastal homeowners may perceive a worsening of climate change-related hazards, these attitudes are largely unrelated to a homeowner's expectations of actual home damage.

Older forests resist change -- climate change, that is
Older forests in eastern North America are less vulnerable to climate change than younger forests, particularly for carbon storage, timber production, and biodiversity, new research finds.

Read More: Climate Change News and Climate Change Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.