After one degree of temperature rise we have huge deluges, more storms and temperatures of 47C in Europe.
Another degree, which could be achieved by 2040, would make things that much worse again.
"If we don't take enough action then we can get changes in the climate that are just unmanageable. It will damage the global economy and disrupt the lives of hundreds of millions of people. The stakes are high," he said.
It's hard for people to believe anything bad is happening.
"We are used to the idea that things don't change much, and that's just not true any more."
Disaster can be averted though.
"It's not about doom and gloom and let's give up and have a beer," he said.
Individuals can be more energy efficient, take public transport and drive electric cars. But more than that they need to talk about the situation, especially to business leaders and politicians.
It's possible to change the economy "almost overnight", as the United Kingdom and United States did before World War II. That's what he thinks should be done.
"We know we can do that. It's a question of understanding that this is an emergency situation."
Renwick is one of about 10 authors of IPCC reports in New Zealand. He's been part of the panel since 2003.
The work is irregular, and for authors it's completely unpaid. They meet about every nine months, work crazy hours before publication, and communicate online. Renwick has met a lot of people and learned about everything from clouds to ice sheets.
"I can't think of another enterprise like it. It's quite a community-based thing."
He is a Professor of Physical Geography at Victoria University, and a climate scientist. His talk was arranged by the Whanganui Science Forum.
++ It starts at 7.30pm in Whanganui Regional Museum's Davis Lecture Theatre on August 28. Cost is $4 for forum members, and $5 for others.
