At the presentation by the IPCC panel in Stockholm this morning, we heard that "Human influence on the climate system is clear: since the 1950s, many of the observed changes are unprecedented over previous decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased."
There has been much talk from sceptic circles that the recent hiatus in global warming means that the scientists have got it wrong.
But this hiatus was explained by the panel as being due to the oceans absorbing heat from the atmosphere. This is temporary. Thomas Stocker, one of the co-authors of the report, said that: "The ocean is taking most of the heat from the air (93%), but it is not saving us". He added that "warming will continue under every scenario available".
There are four scenarios considered in the report, depending on what action we take. They indicate a range of possible temperature increases over the rest of the century from 1.5°C to 6°C. But Stocker warned: "The low band only comes in if quick action is taken to stop carbon emissions. If no action is taken we will be in the high scenario band."
It does not mean that the danger has been averted. Quite the contrary. It is therefore vital to take action as soon as possible.
A climate sceptic in the audience who asked question, David Rose from the British Mail on Sunday newspaper, was given a lesson in basic science because the panel felt he clearly did not understand it.
He asked how long the warming hiatus would continue before the panel admitted that their models might be wrong. He was patiently told: "Your comment is based on a misunderstanding of how the models work. Just because there is temporary unpredictability it does not mean that longer trends are unpredictable. Climate models show absolute consistent agreement with all of the long-term observed trends."
2000 to 2010 saw highest temperatures of any decade in the records. According to the World Meteorological Organisation's Secretary-General Michel Jarraud, scientists are giving even more warning of extreme climate events.
The panel urged policymakers to listen because the review process for the report is as rigorous as it could possibly be. 110 governments contributed to it, with 831 authors and editors synthesising reviews of 9,200 scientific publications since 2007 and integrating 54,000 comments on these papers.
This means that the scientific consensus is overwhelming. Being certain of the global trends, scientists now need to do more research to improve predictions on local and regional levels.
So how should leaders respond?One of the particular issues highlighted was water use. Stocker said that: "Often people focus on temperature rises but, although important, local variations and underwater cycles are also vital, which impact on the water resources".
It was emphasised at the launch that action taken now will be cheaper than action taken later when conditions have worsened. This is exactly what was urged by Lord Nicholas Stern in his 2008 report.
The Summary for Policymakers says that: "The contrast in precipitation between wet and dry regions and between wet and dry seasons will increase, although there may be regional exceptions".
This means that to be resilient, cities and regions, and the land areas from which they source the water and their food, need to be far more water efficient and to implement strategies to manage excessive rainfall.
For those cities vulnerable to sea level rise the report says that "under all scenarios this rise will very likely exceed that observed during 1971-2010".
Over the next 80 years the rise could be between 40cm and 63cm (3ft6"-5ft). Together with predicted storm surges this guarantees increased flooding unless action is taken.
Oceans will acidify as they uptake carbon from the atmosphere, affecting marine sources of food.
"Most aspects of climate change will persist for many centuries even if emissions of carbon dioxide are stopped," the summary concludes.
Cities and regional and local authorities are in a better position than national governments to show leadership and respond more quickly to the challenge. Because they have control over planning and services locally they can choose to implement adaptation measures.
They also have a vital role in communicating the urgency of the situation to their citizens with local campaigns. Responding to the threat of climate change can be a source of pride for a city or region and its inhabitants and also improve quality of life.
Besides taking action to substantially and quickly reduce emissions of climate-warming gases, cities must also take steps to protect themselves: planting trees, using carbon-absorbing concrete instead of Portland cement, planning to reduce car journeys, making buildings as energy efficient as possible and generators of renewable energy, encouraging urban horticulture, using sustainable urban drainage surfaces.
This represents a business opportunity. There is room for cities to partner with industry to meet this challenge.
Tell me what you plan to do in responding to climate change.
How do you think we should respond?