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As a city grows, its impact on rivers doesn’t have to

Now, researchers have undertaken the first comprehensive study of how the infrastructure of U.S. cities alters rivers and their biodiversity. The effects are extensive, they reported yesterday in the Proceedings of the National Academy of Sciences, extending both upstream and downstream for tens or even thousands of kilometers like the branches of a nerve cell.

The researchers used data on river hydrology from the U.S. Geological Survey in a computer model to determine the effects of urban land transformation, electric power, and municipal water supplies on rivers nationwide. They also pulled information from a fine-grained database of species occurrence to assess the biodiversity of freshwater fish, mussels, and crayfish in urban-affected stream reaches.

The largest impacts are from urban land cover and electricity production, the researchers found. These aspects of infrastructure have altered at least 7% of the entire length of streams and rivers in the contiguous United States. “Although these estimates may not seem extensive, they result in very large biodiversity impacts,” the researchers write.

Urban infrastructure impacts 60% of all freshwater fish, mussel, and crayfish species in North America, over 1,200 species altogether. It has contributed to local extinctions of 260 species, sometimes in stream reaches distant from city boundaries themselves. It also influences 970 existing native species, 27% of which are threatened or endangered.

“Cities can impact far reaching areas due to the sheer intensity of resource demands,” the researchers write. For example, the energy and water infrastructure of Atlanta, Georgia stretches across four major river basins.

Moreover, the stresses from urban infrastructure are often compounded because multiple cities may be located on the course of the same river. Twenty-one cities lie along rivers downstream of Atlanta.

Surprisingly, however, the researchers found little connection between a city’s size and the severity of its effects on rivers. Atlanta’s infrastructure impacts 12,500 stream kilometers, and has contributed to 100 local extinctions of freshwater species. Las Vegas, Nevada has a similar population size, but impacts less than 1,000 stream kilometers and has contributed to only seven local extinctions.

Some of these differences have to do with differences in the layout of stream networks and the number of species found in the eastern compared to the western United States. But the findings also suggest that as a city grows, its area of hydrologic impact doesn’t necessarily have to expand along with it. For example, cities could check their impact on rivers by managing storm flows better, or choosing sources of electric power that minimize water use.

There are surely tradeoffs involved in such strategies (choices that take less of a toll on rivers may come with greater impacts on some other habitat). Still, the idea that city governments can not only cause ecological harm but also contribute to ecological integrity far beyond the city limits sounds like good news in an urbanizing world.

by  | Aug 22, 2017

Source: McManamay R. et al. “US cities can manage national hydrology and biodiversity using local infrastructure policy.” Proceedings of the National Academy of Sciences. 2017.


Cool the Planet with Saltwater

Scientists Make the Case for Spraying Saltwater Into Clouds to Help Cool the Planet

by Prachi Patel | Jul 27, 2017 Anthropocene Weekly Science Dispatch

Geoengineering is one of the most controversial proposals to combat climate change. The idea is to tinker with the Earth’s atmosphere on a large scale to counter rising temperatures.

In a new study, researchers at the University of Washington make the case for a geoengineering method known as marine cloud brightening. The technique calls for spraying saltwater into low-lying marine clouds, where they help create more clouds that reflect heat back into space.

The UW scientists say in the journal Earth’s Future that conducting small, controlled marine cloud brightening experiments would provide unprecedented data to understand the effects of aerosols on cloud formation and the resulting reflection of sunlight.

The effect of clouds on climate is one of the biggest uncertainties in today’s climate models. Climate scientists believe that increased pollution since the Industrial Revolution has created brighter, reflective clouds. But they don’t understand the scope of the effect.

There is a need for more controlled experiments to fill the gaps in our basic understanding of the physical processes that control clouds, and how sensitive clouds are to manmade emissions, the researchers argue. By running a series of small-scale experiments in which known quantities of particles are injected into the marine boundary layer, they should be able to observe the impact of the particles on cloud properties and compare them with the properties of naturally formed clouds.

The researchers are now developing a nozzle that can turn saltwater into tiny droplets and can spray trillions of these aerosol particles high into the atmosphere per second. This is the first step in their three-year plan.

Once the sprayer has been developed, they propose to test it in the lab; do preliminary coastal tests in Monterey Bay; and finally move to offshore tests. If these small-scale tests of the technology work, they hope that larger-scale versions could eventually be deployed over larger swaths of ocean.

This isn’t the only geoengineering test underway. Harvard physicist David Keith’s team has recently raised money to conduct a small geoengineering test to dim sunlight. They plan to release about 1 kilogram of calcium carbonate or other material from a high-altitude balloon to see how it affects the physics of the atmosphere. Keith has argued in a recent book that we need to understand geoengineering deeply before we dive into it.

Deliberately messing with the planet’s atmosphere brings up many technical and ethical questions. Reflecting sunlight could affect farming yields and solar panel outputs, for instance. It could trigger much more dramatic unforeseen side effects. Experts also worry that it would detract from efforts to reduce carbon emissions.

“There’s a science question about can we do it, but there’s also an ethical question about should we do it, and a policy question about how would we do it,” said Thomas Ackerman, a UW professor of atmospheric sciences and author of the new study in a press release. “I’m an agnostic on this. I want to test geoengineering and see if it works. But the whole time we’re working on this, I think we need to still be asking ourselves: ‘Should we do it?’”

Source: Robert Wood et al. Could geoengineering research help answer one of the biggest questions in climate science? Earth’s Future. 2017.

Photo: John MacNeill