Mega-cities and human colony collapse disorder.

If, like me, you'd never flown into Sao Paulo at night on a clear evening, you'd probably not understand my reaction. The city - monstrous, sprawling to all horizons, twinkling and colorful in the dark, with apartments lit up like encrusted gems - has an immediate shock value. I had fallen asleep on the flights from Ushuaia and Buenos Aires. I could almost taste that shock as I woke.

Having just come from a month, almost, of not even seeing darkness - or almost any people outside our Antarctic leadership group - I was completely unprepared for the scale of its sprawl. Sao Paulo's airport was decked out in December darkness, like a vast Christmas playground of blue, red and green runway navigation lights.

What makes the mega-cities of the world places in which people are prepared to live? Economic opportunity, of course, but also personal history, inertia, economic imprisonment - sometimes enthusiasm and idealism. Mega-city living comes at a price. The sweet, friendly, thirty-something Paulito who issued my onward boarding pass at the airport gate gave me three adjectives to describe the city. "Dirty. Polluted. Noisy." He said he felt trapped there by his job. I asked him if he knew Spanish, or at least the word "limpia," which means clean. It's also the name of a Costa Rican advocacy group working on clean development and democracy, founded and run by my decarbonization strategist friend Monica Araya. "Life's too short," I smiled at him as I boarded the plane for South Africa. "Find yourself a nice clean country like Costa Rica - get out of the city, and go enjoy your life there."

I find cities a fascinating but generally uncomfortable mix of traits - unhealthy, noisy, crowded, polluted, dispiriting - but also full of delicious foods, great coffee, yoga centers, organic vegan food and great music from different countries. Like many, my husband and I live and work on the edge of a city. In our case, it's because our work is essentially international and big-picture. We have, I confess, become pretty privileged semi-urbanites who have been known to quaff lattes and croissants. We're lucky to have been living south of Cape Town, one of the world's most beautiful and liveable cities - for the middle and upper classes, anyway, less so for everyone else. And I've been working in an office above Cape Town - at the base of Table Mountain's towering cliffs and astonishing biodiversity. With regret, and only really because of the rising crime and corruption, and the falling currency and incomes, we took the big decision to move continents, and to set up somewhere near another beautiful city -- albeit one with its own risks -- Seattle.

Today, over 54% of humans live in urban areas. Since I entered high school in 1975, the population of Tokyo, the world's most sprawling megalopolis, has grown from 26.6 million to nearly 36 million. Sao Paulo has grown from 9.6 million to nearly 20 million. By 2025, there will be at least 10 cities with nearly 20 million people each. The strain on infrastructure, health care delivery, governance, air and water quality, and thus basic quality of life, is immense. Air pollution, noise pollution, traffic congestion, violent crime, dysfunctional government all erode the freedoms, sanity, physical health and well-being of their citizens.

Rampant urbanization has enormous apparent efficiencies. It's easy to provide services, energy and products to dwellers. But this has at least two pretty appalling consequences. One is the crippling dependency and complacency of urban dwellers who forget where water, food and fuel come from -- as they are delivered as if by magic.

I once had an epiphany of sorts, about 15 years ago. I was traveling in a hotel shuttle through the congested streets of Bangkok for a global conference, mesmerized by the towering apartment blocks above me. I realized, with some small horror, that if suddenly the distribution lines for water, food and fuel (and cellphone repairs) went dead, 95% of the occupants of those apartments would have no clue whatsoever what to do. Except perhaps to panic, and then within a few days, to start to riot and loot and pillage.

It was not a pretty thought. And I haven't really been able to shake it since. Coming back home, I became involved in one of the Transition Town structures (https://transitionnetwork.org/about-the-movement/), to help increase community resilience to such 'shocks' to the system and to its economies and distribution networks. It was a movement ahead of its time in South Africa, and we could not sustain the momentum. But current world events are showing the urgency of communities around the world preparing for global shocks to our seemingly safe ways of life.

A second tragic consequence of large-scale urban concentration - and there are many more - is declining health. People living near roads with busy traffic are apparently significantly more likely to experience dementia, brain damage, and other respiratory, neurological and cardiac problems. City life is stressful, polluted, and often divorced from the restorative presence of nature. Lead and other heavy metal concentrations in urban dwellers' bloodstreams are significantly higher than in rural dwellers'. Road rage, domestic abuse, crime and other social ills are arguably at least as high as among those who live within mega-cities as among those who live outside but commute to them. Crime is much easier to carry off when one can disappear in a tangle of shacks or alleyways.

We ecologists call this effect "density dependent behaviors." It's the lab rat syndrome. We humans are a resilient species, and we cope remarkably well with cities, on the surface. But I think it's fair to observe that we also have aggressive and sociopath behaviors arising with greater frequency in large cities, as well as stress, complex health problems that draw from the related problems of industrialized agriculture and a strong reliance on industrial chemicals in every day life since World War II. People around the world seem to be increasingly suffering from autoimmune, post-viral inflammatory ailments such as asthma, leaky gut syndrome, myalgic encephalomyelitis, and fibromyalgia. With some dismay, not least because these illnesses have appeared in my own family, I have come to think of this as a form of human colony-collapse disorder - like the complex immune disorder affecting honeybees. The colony-collapse metaphor is stronger than you may think. And it shouldn't surprise us that humans are now showing significant signs of it across the entire globe.

For years, my former boss, the director of environmental affairs in Namibia, argued to me that it was better from a sustainability and biodiversity perspective to take people off the land and condense them in cities. He was a visionary, but with the jovial overconfidence of someone whose main pastime was reading Churchill. I enjoyed arguing with him, but it wasn't just of academic consequence. I couldn't then shake the feeling that mega-cities, even small cities with weak supplies of local food or water, were a recipe for disaster. And I still can't.

I'll take up my idea of some of the solutions we have open to us in another essay. But meanwhile, I'd welcome your own thoughts on what they may be.