The UN climate chief on Sunday called on governments to take bold steps to tame carbon emissions after a landmark report said the worldwide aim to limit global warming was still attainable.

"The world can still combat climate change but only if nations raise their collective ambition to achieve a carbon-neutral world in the second half of the century," said Christiana Figueres, executive secretary of the UN Framework Convention on Climate Change (UNFCCC).

"We cannot play a waiting game where we bet on future technological miracles to emerge and save the day."

Figueres oversees UN negotiations towards a global pact on curbing climate-altering greenhouse-gas emissions that must be agreed in Paris late next year.

Its goal is to limit warming to two degrees Celsius (3.6 degrees Fahrenheit) over pre-industrial levels, but progress has been hampered by bickering over how to divide up the bill.

The report, issued in Berlin by the United Nations' Intergovernmental Panel on Climate Change (IPCC), said the challenge to curb warming to levels deemed relatively safe by scientists can still be met through a "large-scale" shift to greener energy.

There is a "likely" chance of meeting the target if the world cuts annual greenhouse gas emissions 40-70 percent by 2050 from 2010, it said.

It would also entail a "tripling to nearly a quadrupling" in the share of energy from renewable and nuclear sources and from traditional fossil or new biofuel sources whose emissions are captured and locked away.

But the document, a summary for policymakers that government representatives scrutinised line-by-line in Berlin over six days from Monday, also warned that on present trends the planet would be 3.7-4.8 C warmer by 2100 -- a level scientists say could be catastrophic.

"This new report challenges decision makers by presenting to them alternative futures and spelling out the pathway to each," said Figueres, in a statement issued by the UNFCCC secretariat in Bonn.

"We already have the finance and proven technologies needed to do what is required now. The challenge and the opportunity is to increase the speed and scale of action."

She called for "bold policy incentives" to reduce emissions, and constructive negotiations towards the 2015 climate pact.

And she urged governments "to step forward this year with generous capital contributions" to help developing nations switch to cleaner energy to fuel their rise out of poverty.

Points from UN report on carbon emissions
Berlin (AFP) April 13, 2014 - The main points from the IPCC report issued Sunday on options for mitigating carbon emissions:

Overall

- It is not too late to meet the UN's target of limiting average global warming to 2.0 degrees Celsius (3.6 degrees Fahrenheit) over pre-industrial levels.

- For a "likely" chance (at least 66-percent or more) of achieving this goal, the level of greenhouse gases in the atmosphere should not exceed about 450 ppm CO2eq by 2100.

- Delay beyond 2030 will reduce the chances of reaching this goal without harsher and probably more expensive mitigation measures in the latter half of the century.

- On current emission trends, the mean global surface temperature would be 3.7-4.8 degrees Celsius (6.7-8.4 degrees Fahrenheit) higher by 2100 than pre-Industrial Revolution levels.

Costs

- Achieving 450 ppm CO2eq by 2100 would put a brake of about 0.06 percentage points annually on growth in consumption, a measure of spending activity, which was otherwise projected to increase 1.6-3.0 percent per year over the century.

Energy revolution

Safely reaching the warming limit will require a massive switch in energy sources and use, which will in turn have an impact on business strategy.

For example, reaching about 450 ppm CO2eq by 2100 will likely require a cut in greenhouse gases of 40-70 percent from 2010-50.

Achieving this will entail a "tripling to nearly a quadrupling" over the same period in the share of energy from renewable and nuclear sources as well as traditional fossil and new biofuel sources with a mechanism for carbon capture and storage.

Several scenarios for 450 ppm CO2eq foresee emissions from the energy supply sector declining by 90 percent or more from 2010 levels between 2040 and 2070, and below zero thereafter.

For levels between 450 and 530 ppm CO2 by 2100, investment in alternative electricity sources must increase by $147 billion (106 billion euros) per year from 2010-2029, coupled to an annual investment decline of $30 billion in conventional fossil-fuel technology.

Energy efficiency investments in transport, buildings and industry must increase by about $336 billion per year.

In the short term, readily available natural gas is a cleaner option than coal, and renewable sources are the choices for the longer term.

The report also highlights carbon capture, which means sequestering CO2 emissions from fossil fuels at source, such as power plants, rather than spewing them into the atmosphere to add to the greenhouse effect. The technology for this exists but has only been used on small pilot scales.

Options for major sectors

Transport:

- Boost energy efficiency and vehicle performance

- Invest in low-carbon, mass-transport infrastructure

- Encourage pedestrian access and travel by bicycle

Construction

- Retrofit old buildings with low-energy technology

- Apply low-energy building codes to new projects

Industry

- Use energy more efficiently

- Reduce leaks of hydrofluorocarbons used in refrigeration and air conditioning, and recycle refrigerants

- Recycle and reuse waste

Agriculture and forestry

- Reduce the thinning of carbon-capturing forests and plant new ones

- Cut losses in the food supply chain

The report warns that biofuels may also compromise livelihoods and ecosystems, and may not contribute substantially to curbing greenhouse gas emissions.

Urban planning

- The global urban population is expected to almost double by 2050, and the expansion offers a "window of opportunity" for mitigating emissions. In addition to energy-efficient transport and buildings, planners should overlap work and living areas to avoid wasteful commuting.

Consumption

- Encourage a change in consumer behaviour and lifestyles to limit energy demand and use

- Switch to longer-lasting products and diets based on less energy-intensive food sources

- Reduce food and energy waste

SOURCE: Intergovernmental Panel on Climate Change (IPCC) Working Group III, Summary for Policy Makers.