This is one of the planet's most important figures: 1,000 gigatons, or around 2.2 quadrillion pounds. Scientists said this is the amount of carbon dioxide the Earth can still take in order to keep global warming levels below two degrees Celsius, the globally-agreed "safe" threshold.

People often throw the figures to the big players, the industrial companies who continue to emit their carbon dioxide by the bulk. Countries around the world have already submitted their climate change control plans in preparation for the 2015 United Nations Climate Change Conference in Paris. These countries focus largely on their industrial operations. It seems, however, carbon dioxide emissions in city planning remain under the radar when it comes to climate change checks.

A recent report urged a greener approach to city-building. The study was conducted by Peter Erickson and Kevin Tempest of the Stockholm Environment Institute (SEI), an international research institute that has been independently analyzing development and environment issues for over 25 years. The research was funded by a group of global megacities who are driven towards fighting climate change through sustainable growth. The consortium is called the C40 Cities Climate Leadership Group.

The paper focused on two urban development possibilities in the next 15 years. The first, which the researchers refer to as the "reference scenario," suggests that energy-inefficient developments of urban areas are capable of "locking in" future emissions of carbon dioxide. Findings showed that approximately 30 percent of potential carbon dioxide emissions carried out on a yearly basis are due to transportation systems and urban expansion.

This dire scenario can be solved by the second development possibility, which the researchers call an "aggressive 'urban action' scenario."  Here, committing future emissions can be avoided if policymakers can employ stricter policies among urban developers, pushing them to use sustainable and greener city designs and energy-efficient technology.

"The report calculates that moving from a business-as-usual scenario to low carbon urban development across all the world's cities would save 45 gigatons of CO2 by 2030 - equivalent to eight times the current emissions of the United States," said C40 Chairman Eduardo Paes in a press release published Oct. 8, 2015. Paes is also Rio de Janeiro's mayor.

The findings are crucial in future urban development projects and a lesson to existing cities experiencing costly retrofits due to carbon dioxide emissions. Smart choices towards greener and more energy-efficient cities will add to the plans to lower global warming levels worldwide.

Keeping within the carbon budget is just a matter of making those smart choices now to avoid having to do expensive modifications later on, say the researchers. 

The researchers published their findings on the SEI website.