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Yellow scorpions have a poisonous, though not often deadly, sting. José Roberto Peruca / Flickr, CC BY 2.0

By Hamilton Coimbra Carvalho

I live in São Paulo, the biggest city in Brazil, home to some 12 million people—20 million if you count the outskirts, which have been sprawling for three decades.

That makes it a good place to observe the phenomenon I research: complex social problems. In academia, this concept refers to problems like corruption, crime and traffic—problems that, in practice, cannot be solved. They must simply be mitigated or managed.

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This narrow street, lined with parked cars but devoid of people, is both unwelcoming and unsafe for cyclists. Anne Lusk, CC BY-ND

By Anne Lusk

Designing for bikes has become a hallmark of forward-looking modern cities worldwide. Bike-friendly city ratings abound, and advocates promote cycling as a way to reduce problems ranging from air pollution to traffic deaths.

But urban cycling investments tend to focus on the needs of wealthy riders and neglect lower-income residents and people of color. This happens even though the majority of Americans who bike to work live in households that earn less than $10,000 yearly, and studies in lower-income neighborhoods in Brooklyn and Boston have found that the majority of bicyclists were non-white.

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Marshes at Blackwater National Wildlife Refuge on Maryland's eastern shore. Ataraxy22 / Wikimedia, CC BY-SA

By Jennifer Weeks

World Wetlands Day on Feb. 2 marks the date when 18 nations signed the Convention on Wetlands in 1971, in the Iranian city of Ramsar on the shores of the Caspian Sea. Since that time, scientists have shown that wetlands provide many valuable services, from buffering coasts against floods to filtering water and storing carbon. These five articles from our archive highlight wetlands' diversity and the potential payoffs from conserving and restoring them.

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Illegal deforestation in Pirititi indigenous land, Roraima, Brazil on May 8, 2018. Felipe Werneck / Ibama / CC BY-SA 2.0

By Robert T. Walker

Over the past 25 years that I have been conducting environmental research in the Amazon, I have witnessed the the ongoing destruction of the world's biggest rainforest. Twenty percent of it has been deforested by now—an area larger than Texas.

I therefore grew hopeful when environmental policies began to take effect at the turn of the millennium, and the rate of deforestation dropped from nearly 11,000 square miles per year to less than 2,000 over the decade following 2004.

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Before and after renderings of Tillary Street near the Brooklyn Bridge. New York City DOT

By Sonja Dümpelmann

Many cities, in recent years, have initiated tree planting campaigns to offset carbon dioxide emissions and improve urban microclimates.

In 2007, New York City launched MillionTrees NYC, a program designed to plant 1 million new trees along streets, in parks and on private and public properties by 2017. They hit their goal two years ahead of time.

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Applying nitrogen fertilizer to corn at the W.K. Kellogg Biological Station, a research site in Michigan. National Science Foundation

By David Kanter

Nitrogen pollution is produced by a number of interlinked compounds, from ammonia to nitrous oxide. While they have both natural and human sources, the latter increased dramatically over the past century as farmers scaled up food production in response to population growth. Once these chemicals are released into the air and water, they contribute to problems that include climate change and "dead zones" in rivers, lakes and coastal areas.

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