We know that there is a strong global trend towards urbanisation. Two hundred years ago the world's urban population was around 3 per cent. One hundred years ago, it was 14 per cent. According to a UN report (Global Environmental Outlook 2: Past, Present and Future Perspective, 2002), about half of the world's population now lives in urban areas; every week, over a million people leave their rural lives behind for the uncertain promise of the city.

Now more than half of the world's population live - or want to live  - in an urban environment. And here in South East Queensland the state and some local councils are encouraging people to come - for a perceived economic prosperity.

Climate change and urbanisation are two major drivers of change in the 21st century. A third is the rapidly growing population - wth China being the only country with the politcal will to atempt to manage that growth. The ways in which these trends interact and how we respond to them will be of great consequence to the wellbeing of human populations and all life.

We can now say with considerable certainty that as the 21st century unfolds, climate change will have an increasing impact on the environment and human society worldwide. As much as we might like to avoid thinking about it, most prudent people now understand that we must plan and prepare for a carbon-constrained future.

There are plenty of reasons for concern.

Urban populations consume more energy and other resources and export more carbon and waste per person, causing disproportionate harm to natural ecosystems. In many cases, cities are particularly vulnerable to the effects of climate change such as rising sea levels, increased storm surges, and temperature extremes. Large cities create their own 'urban heat island' - the heat given off from the city itself can make it warmer than the surrounding countryside. The health of many other vital environmental resources, including forests, oceans, rivers, and wetlands, that provide essential ecosystem services - such as stormwater management, air purification, and reduced heating and cooling - are also in jeopardy.

However, there are also solid reasons for hope.

Cities facilitate the peaceful exchange of ideas that drive social and economic innovation. Urban communities provide concentrations of human talent to envision and redesign sustainable and resilient cities, and the financial and technical resources to support these changes. When viewed as nested systems, central cities, the greater metropolitan communities that surround them, and the natural bioregions in which they are located, may prove to be the most effective forms and levels of organisation for creating, testing, refining and replicating innovative and ecologically appropriate solutions.

Innovations that emerge from one urban community can be adapted by different communities in ways that reflect the opportunities and needs of their particular bioregions. Through this process the original idea may becomes stronger and more robust, encouraging the next cycle of innovation. To a large extent, we can choose whether our cities become ecological sinks that suck up the resources of the countryside, or ecological arks where humanity gathers to sort out how best to respond to climate change and other environmental challenges.

A number of cities around the world are already demonstrating impressive leadership as they seek to become more sustainable. They include Amsterdam, the Netherlands; Bogotá, Columbia; Copenhagen; Denmark; Curitiba, Brazil, Reykjavik, Iceland; and Vancouver, Canada. Several cities in the United States have also accepted the challenge of becoming sustainable cities.

Portland with a population slightly over 568,000, while the greater Portland-Vancouver metro area approached 2.2 million people has performed well in a competition to determine the 'greenest' or 'most sustainable' city in USA.

Some of the key sustainability features of Portland include:
An extensive light rail network, bio-diesel powered buses and an aerial tram as part of a multi-modal transit system;Close to 450km of on-street bike lanes, bike boulevards and paved trails - bike commuting has experienced three straight years of double-digit growth;
Neighbourhoods consciously-designed to be pedestrian-friendly; A strong commitment by Portland State University to research and implement sustainability practices;City Repair, an organisation that helps neighbourhoods move their plans through city bureaucracy, coordinates meetings, provides experienced natural designers and builders, and helps find materials and funds; Portland's watershed management plan, including: a proliferation of 'eco-roofs' on houses and buildings, featuring a waterproof membrane, drainage material, a layer of soil and a cover of plants; permeable streets and parking lots that allow rain to soak into the ground; rain gardens and green streets with curbside bioswales to remove silt and pollution from run-off; and The conversion of a waterfront freeway into a three kilometre 'greenway' park