He underscored that limiting climate change will require large and sustained reductions of greenhouse gas emissions. "We need to act now, otherwise we will jeopardize the future of our children, grandchildren and many future generations," said Mr. Jarraud. "Time is not on our side," he added.

The Greenhouse Gas Bulletin reports on atmospheric concentrations – and not emissions – of greenhouse gases. Emissions represent what goes into the atmosphere, the agency pointed out. Concentrations represent what remains in the atmosphere after the complex system of interactions between the atmosphere, biosphere and the oceans.

At the same time, the Emissions Gap Report 2013, involving 44 scientific groups coordinated by the UN Environment Programme (UNEP), urges wide-ranging global action to close the emissions gap.

If the international community fails to take action, the report warned, the chances of remaining on the least-cost path to keeping global temperature rise below 2 degrees Celsius this century will quickly diminish and open the door to a range of challenges.

Under the UN Framework Convention on Climate Change (UNFCCC), governments have agreed to limit the global temperature rise to 2 degrees Celsius to avoid the worst impacts of climate change.

The report, which was released yesterday as leaders prepare to meet for the upcoming UN Climate Change Conference in Warsaw, finds that although pathways exist that could reach the 2-degree Celsius target with higher emissions, not narrowing the gap will exacerbate mitigation challenges after 2020.

This will mean much higher rates of global emission reductions in the medium term; greater lock-in of carbon-intensive infrastructure; greater dependence on often unproven technologies in the medium term; greater costs of mitigation in the medium and long term; and greater risks of failing to meet the 2-degree Celsius target.

"As the report highlights, delayed actions mean a higher rate of climate change in the near term and likely more near-term climate impacts, as well as the continued use of carbon-intensive and energy-intensive infrastructure," said UNEP Executive Director Achim Steiner.

"This 'lock-in' would slow down the introduction of climate-friendly technologies and narrow the developmental choices that would place the global community on the path to a sustainable, green future.

"However," he added, "the stepping stone of the 2020 target can still be achieved by strengthening current pledges and by further action, including scaling up international cooperation initiatives in areas such as energy efficiency, fossil fuel subsidy reform and renewable energy."