I’ve just seen a tweet of Bjorn Lomborg’s from the other day (yes, I’m a couple of days behind) on the cost of climate change versus the costs of mitigation. Here it is:
If you view his tweet on Twitter you will see that quite a few people have accused him of misrepresenting the report. For instance:
Is he making a false comparison? His 6% comes from this table in the Summary for policy makers:
Presumably he’s referring to the 2.1-6.2% reduction in consumption relative to a baseline in the row for 450ppm CO2eq. If so, then he’s chosen the upper value of the range which he tweets as being “Cost of policy higher than 6%”. This figure is also relative to a baseline, not annually. According to this IPCC presentation “this is equivalent to a reduction in consumption growth over the 21st century by about 0.06 (0.04-0.14) percentage points a year (relative to annualized consumption growth that is between 1.6% and 3% per year).”
Meanwhile, the IPCC WGII Summary for Policymakers puts the cost of climate change as:
With these recognized limitations, the incomplete estimates of global annual economic losses for additional temperature increases of ~2°C are between 0.2 and 2.0% of income (±1 standard deviation around the mean) (medium evidence, medium agreement). Losses are more likely than not to be greater, rather than smaller, than this range (limited evidence, high agreement).
On his Facebook page, Bjorn Lomborg says the 0.2-2% comes from a temperature rise of 2°C which we’ll reach sometime between 2055-2080. This seems to be consistent with the paragraph from the Summary for Policymakers above.
He’s comparing global economic losses annually with a reduction in consumption relative to a baseline. Is this a fair comparison?