"We have the means to limit climate change": Intergovernmental Panel on Climate Change Chairman Rajendra Pachauri. Credit:AP Launching the Intergovernmental Panel on Climate Change report, Mr Ban said that science had spoken unambiguously and now it was time for the world's leaders to act on climate change. "The good news is that, if we act now immediately and decisively, we have the means to build a better and more sustainable world," Mr Ban said. "Many tools and technologies are available. Renewables are increasingly competitive. "There is a myth … that climate action will cost heavily. But I am telling you that inaction will cost much more. "With this report, science has spoken yet again - with more clarity and greater certainty.

"There is no ambiguity in their message. Leaders must act, time is not on our side. Let's work together to make this world, our only planet Earth, sustainable economically, socially and environmentally for our succeeding generations." Mr Ban also urged companies such as pension and insurance firms to reduce their investment in fossil fuel-based industries. He said at the previous attempt at global consensus on climate change action, in Copenhagen in 2009, the world's leaders "might not have been fully ready to engage" because they were more focused on national priorities. This time he was "confident that we will do it, we can make it happen".

After a rocky, sometimes controversial six years, climate experts settled on their message to the world's governments: climate change can and must be stopped but the window of opportunity is closing fast. In Copenhagen on Sunday, the IPCC produced its final report before next year's summit in Paris where a new global agreement on climate is due to be hammered out. The IPCC's fifth "synthesis report" pulls together the latest climate science and economics to paint a picture of how the planet's climate is changing, and the options we have to step on the brakes. The 45-page "summary for policymakers" found that: Evidence of global warming is "unequivocal, and many of the observed changes are unprecedented [in recent history]".

Human influence on the climate is clear, and "extremely likely" to be the cause of warming since the mid 20th century.

The Earth's weather is already changing because of human influence, and the 21st century will "very likely" see more and longer heatwaves, more and fiercer storms and more acidic and higher seas.

The report found with "very high confidence" that, in urban areas, climate change would increase the risk of "heat stress, storms and extreme precipitation, inland and coastal flooding, landslides, air pollution, drought, water scarcity, sea-level rise and storm surges". Some of the worst effects would be felt by disadvantaged people and communities, the report said, driving poverty and economic shock that would increase the risk of violent conflicts. If climate change is not tackled, the globe is more likely than not to be 4 degrees warmer by 2100, accompanied by "substantial species extinction, [and] global and regional food insecurity". The IPCC's previous round of reports, finished in in 2007, was mired in controversy, including predictions on melting Himalayan glaciers that were not founded in good science. This round has also faced challenges such as the recent decade-long "pause" in global warming, which initially confounded predictions.

The synthesis report acknowledges "the rate of warming over the past 15 years ... is smaller than the rate calculated since 1951". However, it puts this down to "natural variability" and the problem with cherry-picking the start and end dates of short-term trends. Recent research suggests that much of the extra heat generated over that time has been absorbed by the world's oceans. The report also held good news: there were several ways the world could limit warming by 2100 to below 2 degrees relative to pre-industrial levels. These scenarios require the world to move swiftly to low carbon and renewable energy, and to use only a fraction of existing fossil fuel reserves.

The slower the switch, the more it will have to rely on (so far unproven) technology to capture and store carbon emissions. The "synthesis report" distils three separate IPCC reports published over the past 13 months. Each was worked on by hundreds of experts in climate science, economics and even philosophy. "We have the means to limit climate change," IPCC chairman Rajendra Pachauri said. "The solutions are many and allow for continued economic and human development. All we need is the will to change, which we trust will be motivated by knowledge and an understanding of the science of climate change." Harjeet Singh, a spokesman for climate action group ActionAid, said the report "puts the writing on the wall about the grim future we face if we don't act now". "It paints a devastating picture of food shortages, extinction of species and increasing natural disasters with the world's poorest continuing to suffer most. Governments cannot continue to ignore the warnings and dodge their responsibilities."

But Greenpeace climate policy adviser Kaisa Kosonen said the IPCC report was already out of date, in a positive way. The latest data showed that renewable energy – especially solar energy – was even more affordable and effective than the IPCC had calculated. "The science is in and it's game over for fossil fuels," Ms Kosonen said. "The IPCC spells out the benefits of scaling up the transition to renewable energy, such as affordability, better public health and more jobs." Several Australian experts were involved in producing the synthesis report.

CSIRO researcher Dr John Church from the University of Tasmania, an expert on rising sea levels, said the report's central message – based on stronger and more authoritative science than ever – was that global warming was "unequivocal", and it was very clear that humans had contributed. He said the impact was being seen in increased heatwaves, bushfires and floods, ocean acidity and coral bleaching. It was this visible impact, even more than the research, which would convince governments to act, he said. "What's going to win this argument is the observable impacts [of climate change]," he said. "The same will occur around the world over time and has already occurred to some extent. New record level temperatures, changing the trends of the past 15 to 20 years, that will have the biggest impact."

Australia had made significant progress in renewable energy and now needed to raise its renewable target. Australia has "lots of unexploited solar and wind energy", he said. Dr Scott Power, a researcher at the Australian Bureau of Meteorology, said the world would use up its "carbon budget" in the next two decades, and, if it overspent that budget, then the Earth would warm more than 2 degrees, leading to more severe impacts from climate change on people and ecosystems. "Limiting climate change requires substantial and sustained reductions in greenhouse gas emissions," he said. Emissions, now at an all-time high, would have to drop by 20 per cent to 40 per cent in the next 30 years, he said, replaced by wind and solar energy.

At an IPCC meeting in Berlin this year, there was heated argument over the economics of climate change, and the relative obligations of countries in the developed and developing world. Loading The focus now switches to next month's climate change conference in Lima, which should come up with a draft of a low-carbon agreement to be ratified a year later in Paris. Early next year, countries such as the United States and China are expected to announce their carbon emission targets for that agreement.