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The obvious big cost for Australia is that if the rise in global temperatures can be kept to below 1.5 degrees as much as a third of the Great Barrier Reef could still be alive in 2100 but if it rises by 2 degrees the reef will die.
So when Prime Minister Scott Morrison claims to be arguing from a hard-headed economics perspective in dismissing the IPCC, the Nobel committee respectfully disagrees.
Yale University Professor William Nordhaus.Credit:AP
Mr Morrison's response to the IPCC report confirms that the next election will pose one of the clearest choices on climate change policy in over a decade.
This will be the first election since 2004 where one of the major parties will campaign without even the skimpiest fig leaf of a policy on climate change.
Mr Morrison has responded by repeating his nonsense claim that Australia will meet targets for cutting emissions under the Paris treaty "at a canter" despite data from his own Ministry of Environment that emissions are rising and not falling as the government has promised.
While he is wary of the science, Mr Morrison appears to believe in magic. It is unclear why he thinks emissions would fall without any policy to do so. The government has ditched the National Energy Guarantee, its only emissions control policy.
The ALP's policy is for cuts in emissions which are at least in the ball park for the rapid and far-reaching target advocated by the IPCC. If they were serious, they would follow Professor Nordhaus' prescription of putting a price on carbon.
Government ministers responded to the IPCC by deploying the straw-man argument that Australia cannot survive without coal. True. It is is impossible to do it tomorrow. But that choice is false. Australia's old coal-fired electricity generators will inevitably give up the ghost over the next few decades. The issue is how fast they drop out of operation and what they are replaced with. With a time horizon of a couple of decades drastically reducing coal emissions is very possible.
Unfortunately, because Australia and the world have been slow to act, greenhouse gases have built up and it has become more expensive to stop global temperature rising at a dangerous pace. But the costs of doing nothing are ever more onerous. The smartest economist in the world says so.