1.5 C Larry Kummer has a post today Did the IPCC predict a climate apocalypse? No. Excerpts from the IPCC Special Report on 1.5C, Summary for Policy makers. B1. Climate models project robust differences in regional climate characteristics between present-day and global warming of 1.5°C, and between 1.5°C and 2°C. … B1.1. Evidence from attributed changes in some climate and weather extremes for a global warming of about 0.5°C supports the assessment that an additional 0.5°C of warming compared to present is associated with further detectable changes in these extremes (medium confidence). … B1.3. Risks from droughts and precipitation deficits are projected to be higher at 2°C compared to 1.5°C global warming in some regions (medium confidence). … B2. By 2100, global mean sea level rise is projected to be around 0.1 metre lower {4″} with global warming of 1.5°C compared to 2°C (medium confidence). … B2.1. Model-based projections of global mean sea level rise (relative to 1986-2005) suggest an indicative range of 0.26 to 0.77 m by 2100 for 1.5°C global warming, 0.1 m (0.04-0.16 m) {4″} less than for a global warming of 2°C (medium confidence). … B3. On land, impacts on biodiversity and ecosystems, including species loss and extinction, are projected to be lower at 1.5°C of global warming compared to 2°C. … B3.1. Of 105,000 species studied,9 6% of insects, 8% of plants and 4% of vertebrates are projected to lose over half of their climatically determined geographic range for global warming of 1.5°C, compared with 18% of insects, 16% of plants and 8% of vertebrates for global warming of 2°C (medium confidence). … B3.2. Approximately 4% (interquartile range 2–7%) of the global terrestrial land area is projected to undergo a transformation of ecosystems from one type to another at 1ºC of global warming, compared with 13% (interquartile range 8–20%) at 2°C (medium confidence). … B4. Limiting global warming to 1.5°C compared to 2ºC is projected to reduce increases in ocean temperature as well as associated increases in ocean acidity and decreases in ocean oxygen levels (high confidence). … B4.1. There is high confidence that the probability of a sea-ice-free Arctic Ocean during summer is substantially lower at global warming of 1.5°C when compared to 2°C. With 1.5°C of global warming, one sea ice-free Arctic summer is projected per century. This likelihood is increased to at least one per decade with 2°C global warming. Effects of a temperature overshoot are reversible for Arctic sea ice cover on decadal time scales (high confidence). … B4.4. Impacts of climate change in the ocean are increasing risks to fisheries and aquaculture via impacts on the physiology, survivorship, habitat, reproduction, disease incidence, and risk of invasive species (medium confidence) but are projected to be less at 1.5ºC of global warming than at 2ºC.

Larry Kummer’s comments:

“Most of the findings in the SPM of this Special Report are of two kinds. First, stating that the effects of 1.5°C warming are less than those of 2.0°C warming. Pretty obvious, but it means little unless we know the effects of 2°C warming. It seldom quantifies the difference in effects from that extra 0.5°C warming, which is the key information necessary to know when assessing the cost-benefit of limiting the coming warming.

Second, there are more specific findings – bad but not disastrous – given at a “medium” level of confidence. The IPCC uses five levels of confidence: very low, low, medium, high, and very high. “Medium” is a weak basis for extreme measures to restructure society and the global economy. Especially since it is human nature to overestimate confidence more often than to underestimate it.”

JC note: with regards to IPCC confidence definitions, seem my previous post A crisis of overconfidence

“There is nothing in this Special Report justifying belief that the world will end, that the world will burn, or that humanity will go extinct. It has been misrepresented just as past reports have been (e.g., the 4th US National Climate Assessment). The disasters described the Climate Emergency and Extinction Rebellion activists are those of RCP8.5, the worst-case scenario in the IPCC’s Fifth Assessment report – or even beyond it. RCP8.5 is, as a worst-case scenario should be, a horrific but not apocalyptic future that is improbable or impossible.”

JC note: with regards to RCP8.5, see my previous post What’s the worst case? Emissions/concentrations scenarios

JC conclusion

Bottom line is that these timelines are meaningless. While we have confidence in the sign of the temperature change, we have no idea what its magnitude will turn out to be. Apart from uncertainties in emissions and the Earth’s carbon cycle, we are still facing a factor of 3 or more uncertainty in the sensitivity of the Earth’s climate to CO2, and we have no idea how natural climate variability (solar, volcanoes, ocean oscillations) will play out in the 21st century. And even if we did have significant confidence in the amount of global warming, we still don’t have much of a handle on how this will change extreme weather events. With regards to species and ecosystems, land use and exploitation is a far bigger issue.