UN member states sign the IPCC reports, accepting its findings. But despite the warning sounded by AR5 - like the ones preceding it - the gap between what science tells should be done to prevent uncontrollable climate change and the actions taken by governments across the world is huge.

It brings me back to the issue of risk management. In spite of the IPCC and other organizations - which include even the World Bank - saying that the risks posed by a warming world are enormous, why is our response so feeble?

We know what the solutions are. Since the energy sector is the largest source of manmade GHG emissions, we have to urgently de-carbonize it. The world's dependence on fossil fuels carries huge environmental, social and economic risks. So we need to reduce the use of fossil fuels and switch to clean and renewable energy.

We have to phase out investment in fossil fuels, especially coal, and use that money to develop and generate renewable energy. Recognizing this, the World Wide Fund for Nature's (WWF's) Seize Your Power global campaign is calling on governments and financial institutions worldwide to increase investment in renewable energy by at least $40 billion over the next year.

That may be just a start, but switching to renewables in the long run is the only option. The Energy Report, the results of a WWF study, shows that a world run entirely on renewable energy is possible. Switching to renewable energy isn't just the best choice, it's the only choice we have, because the way the world produces and uses energy today is not sustainable.

Don't make the mistake of thinking that there are shortcuts. It would be nice to think that, as some argue, we can simply switch to "clean" fossil fuels like shale gas or "clean coal". The fact is, those terms are misnomers.

In the end, I come back to the simple message that science is sending us. The climate is changing. Mankind is largely to blame for that. We aren't doing enough to prevent the change. The window of opportunity is rapidly closing. We do still have time to prevent the worst effects of climate change, but the world must do much, much more to deal with this existential risk.

The author is director General of WWF International.