by Judith Curry

The IPCC Special Report on Global Warming of 1.5C is now published [link].

The good news is that it is better written and with better diagrams — no more turgid prose and obscure diagrams that we’ve come to expect from the IPCC.

The other ‘good’ news is facetiously articulated by the Australian:

The bad news is of course best articulated by Al Gore [link], including a dig at President Trump.

Oliver Geden tweets:

“One of the major governance ‘innovations’ of # COP21 – agreeing on a >3°C deal while putting a 1.5°C label on it.”

This tweet by Richard N. Haas, President of the Council on Foreign Relations, pretty much sums up the policy situation:

“Nothing suggests world will come close to meeting this goal on climate change. There is no US leadership, and the phrase “international community” is more rhetoric than reality. We’d better set aside $ for adaptation and accelerate R&D on geoengineering.”

I don’t see much in the way of new science or better ways of assessing uncertainty — same old, same old. Projections are based on the CMIP5 simulations. There are some more recent references cited in the ‘impacts’ chapter. Most of this addresses the difference between 1.5 and 2.0 C impacts, and also strategies for mitigation etc. The main issue is removing CO2 from the atmosphere, in the absence of being able to meet emissions targets.

The main climate science conclusion is that things would be a little better at 1.5C relative to 2C. For example:

“By 2100, global mean sea level rise is projected to be around 0.1 metre lower with global warming of 1.5°C compared to 2°C (medium confidence).” [0.1 m = 4 inches.]

They also assume that climate model simulations are useful at regional scales:

“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.” “Robust is here used to mean that at least two thirds of climate models show the same sign of changes at the grid point scale, and that differences in large regions are statistically significant.”

Without going into all the policy implications, I have a few comments about 1.5C Consider this figure from Berkeley Earth regarding land temperatures since 1750:

Over land, we have already blown through the 1.5C threshold if measured since 1890. Temperatures around 1820 were more than 2C cooler. There has been a great deal natural variability in temperatures prior to 1975 when human caused global warming kicked in any meaningful way.

And the IPCC climate model projections ignore the bottom third of the ‘likely’ climate sensitivity values – 1.5 to 4.5 C -, with only the outlier Russian model having a value of ECS as low as 2.1 C. Much of this problem goes away if ECS is actually 1.5 to 2 C.

And then there is the goldilocks issue. Who would prefer the climate of the 18th or 19th century relative to the climate of the early 21st century?

Apart from warmer temperatures, what evidence is there of potential catastrophes? An observed increase in extreme weather events is not well justified, if you correctly account for the influence of multi-decadal ocean oscillations. So, what is the possible worst cast impact for 1.5 or 2.0 C warming on the timescale of the 21st century?

collapse of the West Antarctic Ice Sheet, possibly resulting in up to 2.5 m sea level rise as per the NOAA (2017) report (actually, the IPCC does not even make this case, they are predicting SLR of 1-2 feet). This extreme scenario, which would maybe justify all this, is regarded as extremely unlikely, and we are not presently on such a trajectory. In any event, if the WAIS collapses it is more likely to be due to the geothermal heat flux and volcanoes beneath the ice sheet. Recent research shows portions of the WAIS rising at a rate of 41 mm/yr, acting to protect the WAIS from collapse. MASSIVELY uncertain.

species extinction. After alarming conclusions in AR4, the AR5 backtracked, and this new Report backtracks even further. What about the ocean – acidification and declining oxygen? Our understanding is in its infancy, but this needs to be looked at more.

IMO, even with erroneous attribution of extreme weather/climate events and projections using climate models that are running too hot and not fit for purpose of projecting 21st century climate change, the IPCC still has not made a strong case for this massive investment to prevent 1.5C warming.