Quanto ci costa il riscaldamento globale se non facciamo niente?

The IPCC reports offer cost estimates for both adaptation and mitigation.  The 2014 Adaptation report reckons, assuming that the world takes no steps to deal with climate change, that "global annual economic losses for additional temperature increases of around 2°C are between 0.2 and 2.0 percent of income."  The report adds, "Losses are more likely than not to be greater, rather than smaller, than this range."

In a 2010 Proceedings of the National Academy of Sciences article, Yale economist William Nordhaus assumed that humanity does nothing to cut greenhouse gas emissions.  Nordhaus uses an integrated assessment model that combines the scientific and socioeconomic aspects of climate change to assess policy options for climate change control.  His RICE-2010 integrated assessment model found that "of the estimated damages in the uncontrolled (baseline) case, those damages in 2095 are $12 trillion, or 2.8% of global output, for a global temperature increase of 3.4
In a 2010 Proceedings of the National Academy of Sciences article, Yale economist William Nordhaus assumed that humanity does nothing to cut greenhouse gas emissions.  Nordhaus uses an integrated assessment model that combines the scientific and socioeconomic aspects of climate change to assess policy options for climate change control.  His RICE-2010 integrated assessment model found that "of the estimated damages in the uncontrolled (baseline) case, those damages in 2095 are $12 trillion, or 2.8% of global output, for a global temperature increase of 3.4° above 1900 levels." 
In his 2013 book The Climate Casino... Nordhaus notes that a survey of studies that try to estimate the aggregated damages that climate change might inflict at 2.5° comes in at an average of about 1.5 percent of global output.  The highest climate damage estimate Nordhaus cites is a 5 percent reduction in income.  The much criticized 2006 Stern Review: The Economics of Climate Changesuggested that the business-as-usual path of economic growth and greenhouse gas emissions could even reduce future incomes by as a much as 20 percent.

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The IPCC Mitigation report notes that the optimal scenario that it sketches out for keeping greenhouse gas concentrations below 450 ppm would cut future incomes by 2100 by between 3 and 11 percent... [P]rojected IPCC income losses that would result from doing nothing to adapt to climate change appear to be roughly comparable to the losses in income that would occur following efforts to slow climate change.  In other words, it appears that doing nothing about climate change now will cost future generations about the same as doing something now.
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http://econlog.econlib.org/archives/2016/04/global_warming_13.html