Image caption In industrialised nations, urban living demands more water, natural resources and energy

Urban areas are set to become the battleground in the global effort to curb climate change, the UN has warned.

The assessment by UN-Habitat said that the world's cities were responsible for about 70% of emissions, yet only occupied 2% of the planet's land cover.

While cities were energy intensive, the study also said that effective urban planning could deliver huge savings.

The authors warned of a "deadly collision between climate change and urbanisation" if no action was taken.

The Global Report on Human Settlements 2011, Cities and Climate Change: Policy Directions, said its goal was to improve knowledge of how cities contribute to climate change, and what adaptation measures are available.

Worrying trend

Joan Clos, executive director of UN-Habitat, said the global urbanisation trend was worrying as far as looking to curb emissions were concerned.

"We are seeing how urbanisation is growing - we have passed the threshold of 50% (of the world's population living in urban areas)," he told BBC News.

"There are no signs that we are going to diminish this path of growth, and we know that with urbanisation, energy consumption is higher.

According to UN data, an estimated 59% of the world's population will be living in urban areas by 2030.

Every year, the number of people who live in cities and town grows by 67 million each year - 91% of this figure is being added to urban populations in developing countries.

The main reasons why urban areas were energy intensive, the UN report observed, was a result of increased transport use, heating and cooling homes and offices, as well as economic activity to generate income.

The report added that as well as cities' contribution to climate change, towns and cities around the globe were also vulnerable to the potential consequences, such as:

Increase in the frequency of warm spells/heat waves over most land areas

Greater number of heavy downpours

Growing number of areas affected by drought

Increase in the incidence of extremely high sea levels in some parts of the world

Image caption Southern Africa is considered to be one of the areas at most risk from the impacts of climate change

The authors also said that as well as the physical risks posed by future climate change, some urban areas would face difficulties providing basic services.

"These changes will affect water supply, physical infrastructure, transport, ecosystem goods and services, energy provision and industrial production," they wrote.

"Local economies will be disrupted and populations will be stripped of their assets and livelihoods."

A recent assessment highlighted a number of regions where urban areas were at risk from climate-related hazards, such as droughts, landslides, cyclones and flooding.

These included sub-Saharan Africa, South and South East Asia, southern Europe, the east coast of South America and the west coast of the US.

Time to act

Dr Clos told BBC News that while climate change was a problem that affected the entire world, individual towns and cities could play a vital role in the global effort to curb emissions.

"The atmosphere is a common good, which we all depend upon - every emission is an addition to the problem," he explained.

But, he added: "Consumption is carried out at an individual level; energy consumption is also an individual choice.

"This is why local governments and communities can a big role, even when their national governments do not accept or acknowledge the challenges."

The report called on local urban planners to develop a vision for future development that considered climate change's impact on the local area.

It said that it was necessary to include mitigation measures (reducing energy demand and emissions) as well as adaptation plans, such as improving flood defences.

In order to achieve the most effective strategy, it was necessary for urban planners to seek the views of the local community, including businesses and residents.

However, the UN-Habitat authors said international and national policies also had a role to play in supporting urban areas.

These included financial support, reducing bureaucracy and improving awareness and knowledge of climate change and its possible impacts.

Dr Clos was launching the report on Monday evening at an event in central London, hosted by the London School of Economics.