A conference in Melbourne next  week featuring a who’s who of climate scientists will explore what warming of 4 degrees or more means, including for Australia. Apocalyptic is the only word for it,  and understanding the implications is equally important for policymakers, business and the  community. Keynote speaker Professor Hans Joachim Schellnhuber, director of the Potsdam Institute and climate adviser to the German Chancellor and to the EU, has  said that in a 4-degree warmer world, the population “carrying capacity estimates [are] below 1billion people”.

Professor Kevin Anderson, director of the Tyndall Centre for Climate Change in Britain, was quoted in The Scotsman  ahead of the 2009 Copenhagen conference saying the consequences were ‘‘terrifying’’.

‘‘For humanity it’s a matter of life or death ... we will not make all human beings extinct, as a few people with the right sort of resources may put themselves in the right parts of the world and survive. But I think it’s extremely unlikely that we wouldn’t have mass death at 4 degrees.

‘‘If you have got a population of 9 billion by 2050 and you hit 4 degrees, 5 degrees or 6 degrees, you might have half a billion people surviving.’’

Australian climate scientist Professor David Karoly, alongside Melbourne University and CSIRO colleagues, will give a paper next week on likely changes to our climate in a 4-degree scenario.  He  has warned that ‘‘we are unleashing hell on Australia’’, but says he can’t answer questions about Australia’s likely carrying capacity, which depends on variables such as  future willingness to pay, lifestyle and innovation. The hotter it gets,  ‘‘it just gets harder and harder to support more people’’.

Four degrees warming, says Karoly, is ‘‘a lot worse’’ than two. At  4 degrees, he says, there will be markedly reduced water availability in the Murray-Darling Basin, with rainfall decreasing by up to half. Coastal temperatures will increase by 3 to 5 degrees but inland temperatures will rise by 4 to 6 degrees. Drought frequency will more than double, as will the frequency of extreme bushfire conditions. Sea level rises will have a drastic effect on all coastal cities, with sea levels rising up to about 1.1 metres by 2100, increasing to more than 7metres over subsequent centuries even with no further global warming.

‘‘The Brisbane floods will bevery little compared to a 7-metre sea level rise,’’ Karoly says. Tourist attractions? The Great Barrier Reef will die. There will be no skiing industry in Australia —  he says  any snow will ‘‘fall and melt’’.

In a recent blog posting,  David Spratt, co-author of Climate Code Red, drew the threads together on the implications of 4-degree warming. The world would be warmer than during any part of the period in which modern humans evolved, he wrote, and the rate of climate change would be faster than any previously experienced by humans.

Three degrees may be the ‘‘tipping point’’, leaving us powerless to intervene as planetary temperatures soar and, quoting NASA climate scientist James Hansen, there will be ‘‘no return within the lifetime of any generation that can be imagined, and the trip will exterminate a large fraction of species on the planet’’. Half the world would be uninhabitable. Ocean ecosystems and food chains would collapse.

‘‘And we are talking about how we might adapt to a 4-degree warmer world,’’ Spratt wrote. ‘‘Have we gone mad?’’  paddy.manning@fairfaxmedia.com.auTwitter: @gpaddymanning