(CNN) -- Humans can now officially be called an urban species. More than half of the global population now live in cities and the United Nations says that by 2030, 60 percent of us will live in them.

People in the Tsim Sha Tsui district of Hong Kong, regarded as one of the most crowded places on Earth.

Yet according to U.N. Habitat, the world's cities emit almost 80 percent of global carbon dioxide as well as "significant amounts of other greenhouse gases."

Put simply, if you want to tackle climate change, tackle the cities.

The UK's Tyndall Centre for Climate Change Research recently went as far as saying that "the fate of the Earth's climate is intrinsically linked to how our cities develop over the coming decades."

Two centuries ago there was only one city on the planet that could say it had a million inhabitants -- that was London.

Today more than 400 cities can boast that (408 to be precise, according to the Earth Policy Institute).

But today a population of 1 million people means nothing; we are moving into the era of megacities of 10 million-plus people.

Today, there are 20 so-called megacities, whose populations -- and therefore energy needs -- easily exceed that of countries, according to Earth Policy Institute (more people now live in Tokyo than Canada, for example).

Naturally, these cities are -- and will continue to be -- resource-hungry.

Despite only representing 2 percent of the world's surface area, they are responsible for 75 percent of the world's energy consumption. London, for example, requires a staggering 125 times its own area in resources to sustain itself, according to the New Scientist.

London's population is around 7.4 million, so it's nowhere near megacity status yet, but according to the Tyndall Centre, it already consumes more energy than Ireland (and the same amount as Greece or Portugal).

On first glance, however, some modern cities aren't as bad as many may think. Take New York, for example: Its 2005 carbon emissions were 1 percent of the country's total (79 percent coming from energy-hogging buildings), while on a per capita basis its emissions represented less than a third of the national average, according to Environmental News Service.

New York is sometimes viewed as being so much more energy-efficient than suburban or rural areas that some have started to call it The Big Green Apple. Back in 2004 even, the New Yorker wrote: "If it were granted statehood, New York City would rank 51st in per-capita energy use."

Why? People living in confined spaces essentially, and less people driving cars. According to a study of the Greater Washington area, for example, per capita transportation emissions in the city were less than the national average, reports Grist.

However, while the cities may be internally efficient, the problem doesn't just lie in the stuff that gets consumed within city limits. More often than not the bigger environmental issue lies in how that stuff gets to the city in the first place.

Beijing, for example, receives its water supply from the Yangtze River basin -- 1,500 kilometers (930 miles) away, Treehugger reports, also pointing out that "the oil that provides much of the energy to move resources into and out of cities itself often comes from distant oil fields."

As Treehugger quotes economist Tyler Cowen as saying about New York: "Manhattan sells services ... and in turn draws upon industrial outputs, which of course include steel and glass. It is also no accident that Gary, Indiana, is near Chicago and those rather aesthetically thrilling factories off the New Jersey Turnpike are right outside New York City ... Praising Manhattan is a bit like looking only at the roof of a car and concluding it doesn't burn much gas. Manhattan supports its density only by being surrounded by a broader load of crud."

Whether they are energy-efficient or energy-wasteful, cities such as New York and London are employing ambitious programs to improve efficiency.

Some programs are simpler than others. Miami, for example, has found that planting trees can help reduce air conditioning bills, for example, in summer months. One tree can release as much as 400 liters of water every day which can cool the air.

Miami recently found that its electricity bills were 10 percent lower in areas with more than 20 percent tree cover compared to those areas without any trees, the New Scientist reports.

This can make a difference in urban areas: Cities during the summer season tend to be about 1 degree Celsius warmer during the day and as much as 6 degrees Celsius warmer at night, according to the New Scientist.

Developing world poses new challenges

But some cities are much more ahead of the curve than others. In Sweden, the city of Växjö generates 51 percent of its energy, for example, from non-fossil fuel bases energy sources such as solar, geothermal and biomass, according to the Clinton Initiative's C40 program.

In slightly more than 10 years it has slashed its emissions by 24 percent on a per capita basis, C40 says, to 3.5 tons of carbon dioxide a year (the world average is 4 carbon dioxide tons annually). C40 believes that by 2015 it could be the world's first genuine fossil-free city.

But the problem, scientists say, is that future urban growth will mostly take part in the developing world. Manhattan or Växjö can be as green as they like, but in 20 years time the world will be more concerned with what is going on in the cities of Asia, Africa and Latin America, many analysts say.

Currently 75 percent of the world's billion poorest humans live in urban areas. Tyndall believes that by 2030 the developing world will have contributed another 2 billion people to its cities (from 2000) while the developed world is expected to only contribute an additional 100 million city-dwellers in the same time period.

The U.N.'s Intergovernmental Panel on Climate Change says that in the coming years, two-thirds of the growth in global electricity usage will be down to the needs of the developing world -- and mostly down to the needs of their cities.

It is expected that by as early as 2010, electricity use globally will have grown by 37 percent. By 2020 it will have shot up by 76 percent.

Without additional investment from companies -- and appropriate incentives from world governments -- this growth in demand will have to be served by fossil fuels. Coal use, therefore, is expected to grow by 50 percent by 2020; while the use of natural gas could double. E-mail to a friend

All About Urban Planning • Global Climate Change