Geoengineering Is Unlikely To Succeed, Natural Defenses Are The Best
16 December, 2013
Reducing the amount of sunlight reaching the planet's surface by geoengineering may not undo climate change after all. 
Axel Kleidon and Maik Renner, two German scientists used a simple energy balance analysis to explain how Earth's water cycle responds differently to heating by sunlight than it does to warming due to a stronger atmospheric greenhouse effect.
They show that this difference implies that reflecting sunlight to reduce temperatures may have unwanted effects on Earth's rainfall patterns.
The results have been published in Earth System Dynamics , an open access journal of the European Geosciences Union (EGU).
Global warming alters Earth's water cycle since more water evaporates to the air as temperatures increase. Increased evaporation can dry out some regions while, at the same time, result in more rain falling in other areas due to the excess moisture in the atmosphere. The more water evaporates per degree of warming, the stronger the influence of increasing temperature on the water cycle. But the new study shows the water cycle does not react the same way to different types of warming.
Axel and Maik of the Max Planck Institute for Biogeochemistry in Jena , Germany , used a simple energy balance model to determine how sensitive the water cycle is to an increase in surface temperature due to a stronger greenhouse effect and to an increase in solar radiation.
The scientists predicted the response of the water cycle for the two cases and found that, in the former, evaporation increases by 2% per degree of warming while in the latter this number reaches 3%. This prediction confirmed results of much more complex climate models.
"These different responses to surface heating are easy to explain," says Kleidon, who uses a pot on the kitchen stove as an analogy. "The temperature in the pot is increased by putting on a lid or by turning up the heat -- but these two cases differ by how much energy flows through the pot," he says.
A stronger greenhouse effect puts a thicker 'lid' over Earth's surface but, if there is no additional sunlight (if we don't turn up the heat on the stove), extra evaporation takes place solely due to the increase in temperature. Turning up the heat by increasing solar radiation, on the other hand, enhances the energy flow through Earth's surface because of the need to balance the greater energy input with stronger cooling fluxes from the surface. As a result, there is more evaporation and a stronger effect on the water cycle.
In the new Earth System Dynamics study the scientists also show how these findings can have profound consequences for geoengineering.
Many geoengineering approaches aim to reduce global warming by reducing the amount of sunlight reaching Earth's surface (or, in the pot analogy, reduce the heat from the stove).
But when Kleidon and Renner applied their results to such a geoengineering scenario, they found out that simultaneous changes in the water cycle and the atmosphere cannot be compensated for at the same time. Therefore, reflecting sunlight by geoengineering is unlikely to restore the planet's original climate.
"It's like putting a lid on the pot and turning down the heat at the same time," explains Kleidon. "While in the kitchen you can reduce your energy bill by doing so, in the Earth system this slows down the water cycle with wide-ranging potential consequences," he says.
Kleidon and Renner's insight comes from looking at the processes that heat and cool Earth's surface and how they change when the surface warms. Evaporation from the surface plays a key role, but the researchers also took into account how the evaporated water is transported into the atmosphere.
They combined simple energy balance considerations with a physical assumption for the way water vapor is transported, and separated the contributions of surface heating from solar radiation and from increased greenhouse gases in the atmosphere to obtain the two sensitivities.
One of the referees for the paper commented: "it is a stunning result that such a simple analysis yields the same results as the climate models."
Humans threaten wetlands' ability to keep pace with sea-level rise
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 December 5 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 (CO 2 ) 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 CO 2 , 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.
‘Natural defenses' are best protection from rising sea levels
Citing a similar study finding Tim Radford reported :
Insidious things like sea level rise, coastal subsidence and the loss of wetlands could bring the sea water coursing through city streets in the decades to come.
Jonathan Woodruff of the University of Massachusetts Amherst in the US and colleagues report in Nature that shorelines are increasingly at risk, and humans must adapt and learn to live with increasing hazard.
Many of the world's great cities are on low-lying coastal plains, or on river estuaries, and are therefore anyway at risk as sea levels rise because of global warming.
But human action too – by damming rivers, by extracting ground water and by building massive structures on sedimentary soils – has accelerated coastal subsidence. Add to this the possibility of more intense tropical cyclones as sea surface temperatures rise, and coastal cities face a stormy future.
“A rise in sea level of one meter for the New York City region would result in the present-day 100-year flood events occurring every three to 20 years”, warn Woodruff and his fellow scientists in their Nature study.
“Most engineered coastlines are not designed for this increase in extreme flood frequency, and the dominance of sea level rise and landscape dynamics on impacts by landfalling tropical cyclones must be acknowledged for effective planning and management of our future coastlines.”
The scientists reviewed nearly 100 research studies of coastal change.
They also noted that, according to an international register of disasters, more than 60% of economic losses – around $400 bn – occurred in the North Atlantic , one of the areas least at risk from tropical hurricanes. The lesson is that governments and civic authorities will need to think more carefully about future threats.
“Sea-level rise, severe storms, changing climate, erosion and policy issues are just some of the factors to assess in order to understand risk”, says another of the authors, Jennifer Irish of Virginia Tech College of Engineering.
“We reviewed just three of the physical factors – tropical cyclone climatology, sea-level rise and shoreline change. If we look at them separately, we don't see how they are interconnected.
“But if we pull back to look at the whole picture, we stand a better chance of protecting our homes, roadways, energy and water networks, and the most critical and expensive infrastructure along the coastlines.”
 Story Source:
The story is based on materials provided by European Geosciences Union (EGU).
Journal Reference: A. Kleidon, M. Renner. A simple explanation for the sensitivity of the hydrologic cycle to global climate change. Earth System Dynamics Discussions, 2013; 4 (2): 853 DOI: 10.5194/esdd-4-853-2013
European Geosciences Union (EGU) (2013, December 5). Geoengineering approaches to reduce climate change unlikely to succeed. ScienceDaily. Retrieved December 6, 2013, from http://www.sciencedaily.com /releases/2013/12/131205092049.htm
 Story Source:
The story is based on materials provided by Virginia Institute of Marine Science. The original article was written by Kristen Minogue, SERC.
Journal Reference: Matthew L. Kirwan, J. Patrick Megonigal. Tidal wetland stability in the face of human impacts and sea-level rise. Nature, 2013; 504 (7478): 53 DOI: 10.1038/nature12856
Virginia Institute of Marine Science (2013, December 4). Humans threaten wetlands' ability to keep pace with sea-level rise. ScienceDaily. Retrieved December 5, 2013, from http://www.sciencedaily.com /releases/2013/12/131204132024.htm
 This article was produced by the Climate News Network.
RTCC.org, December 5, 2013, “‘Natural defences' offer best protection from rising sea levels”, http://www.rtcc.org/2013/12/05/natural-defences-offer-best-protection-from-rising-sea-levels/
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