The summary is still very much understating and underestimating the threats we might face if global warming does exceed 1.5°C and what we face already.
Professor Michael Mann, director of the Earth System Science Center at Penn State University and a former IPCC lead author, criticized people claiming the latest IPCC report would be too alarmist: “If anything, it is the opposite. Once again, with their latest report, they have been overly conservative (i.e. erring on the side of understating/underestimating the problem).”
He also warns that "We are closer to the 1.5°C & 2°C thresholds than they indicate & our available carbon budget for avoiding those critical thresholds is considerably smaller than they imply."
A number of scientists point out that the report fails to fully acknowledge the role of amplifying self-reinforcing feedbacks and tipping points, thresholds that, if passed, could cause the climate to destabilize even further and push the world on to an irreversible path of extreme warming.
One such paper, which was not considered in the IPCC report, describing how self-reinforcing feedbacks might cross tipping points leading to runaway effects, was published in August by the Proceedings of the National Academy of Science concluding that “even if the Paris Accord target of a 1.5°C to 2.0°C rise in temperature is met, we cannot exclude the risk that a cascade of feedbacks could push the Earth System irreversibly onto a ‘Hothouse Earth’ pathway.”
“Climate change is occurring earlier and more rapidly than expected. Even at the current level of 1C warming, it is painful,” Johan Rockström, a co-author of the recent Hothouse Earth.
“Even with its description of the increasing impacts that lie ahead, the IPCC understates a key risk: that self-reinforcing feedback loops could push the climate system into chaos before we have time to tame our energy system, and the other sources of climate pollution,” Mario Molina, who shared the Nobel prize in chemistry in 1995 for his work on depletion of the ozone layer said.
Climate change is not worsening in a simple, linear fashion, but rather by compounding and accelerating; “feedbacks could fall like dangerous dominos, fundamentally destabilizing the planet.”
The IPCC “fails to adequately warn leaders” about six climate tipping points that work in this way. One of the more well-known such tipping points is Arctic sea ice. The ice acts as a reflector of heat back into the atmosphere, so the more it melts, the more the Arctic waters absorb heat. This self-reinforcing feedback loop could lead to an ‘Arctic death spiral,’ where the loss of the sea ice accelerates the melting of permafrost, which some scientists believe could release large quantities of methane — a greenhouse gas 30 times more potent in driving warming than CO2 — into the atmosphere, which could again lead to much higher levels of global sea level rise.
Paris climate agreement pledges not enough
Due to the new report, global warming is likely to reach 1.5°C between 2030 and 2052 if it continues to increase at the current rate of 0.2°C (±0.1°C) per decade.
To limit global warming to 1.5°C, global carbon dioxide emissions would have to fall by more than 50% by 2030 and reach zero by 2050.
But countries’ pledges due to the Paris Agreement to reduce their emissions are currently not even in line with limiting global warming to 1.5°C. They would lead to a temperature rise of 3-4 degrees by 2100.
As the report says, warming will not be limited to 1.5°C or even 2°C unless fundamental transformations in all areas of society are undertaken. Emissions would need to decline rapidly across all of society’s main sectors, including buildings, industry, transport, energy, and agriculture, forestry and other land use. Actions to reduce emissions would have to include phasing out coal in the energy sector, increasing the amount of energy produced from renewable sources, electrifying transport, and reducing the ‘carbon footprint’ of the food we consume.