Dangerous Climate Change
Based on the best available scientific evidence, it is clear that increases in CO2, and other greenhouse gases in CO2e (carbon dioxide equivalent), which constitute F in the Kaya Identity (see earlier), result in temperature elevation. The relationship between the two is known as climate sensitivity. It is accepted by most scientists that a CO2 doubling (from pre-industrial 280ppm to 550ppm) will probably lead to a roughly 3C temperature rise. The IPCC calculated a range of 1.5C and 4.5C depending on the emission scenario. Unfortunately, this is far from exact since fossil records estimate that climate sensitivity could be as high as 6C (Hansen).
Are temperature increases dangerous? The overriding consensus is a resounding yes. There is no question that biomes, including pathogenic species, would survive and even proliferate. The concern is that widespread ecosystem collapse would occur specifically concerning biomes on which humans depend. For the human civilization's existence over the last 6,000 years, there has been near consistency in global climate. The next 100 years will almost certainly be very different. The question is from the point of view of our civilization: fatally different, or just seriously different?
Lynas, in his book Six Degrees (2007), collated hundreds of scientific reports to explore the probable effects of different temperature increases over the coming century. Whilst to some degree conjecture, they are nonetheless a good departure point for discussion.
Impacts at 2C
Lynas predicted: thirsty cities; acidic oceans; annual summer heat waves like 2003; and a hot Mediterranean climate like the Sahara. He also listed: dying corals; irreversible Greenland melt; polar bear extinction threat; conflict over Arctic sea routes; Indian heat waves; waterless Peru; and Californian dessication. Food shortages would be the norm and biodiversity would be lost with the possibility of the onset of the 6th Mass Extinction. For this temperature scenario. other sources have listed: disrupted ecosystems; concerns over water and food security; threats to health; rising sea levels; and cities at risk. And, of course, there is also the terrible IPCC report of 2018.
Impacts at 4C
Here, the effects become even more alarming with ocean encroachment an impossible challenge. Hundreds of millions of refugees would be stateless and worldwide agricultural drought would result. Mediterranean regions would experience extreme heat, 45C temperatures would be recorded in the UK while mainland Europe would be as hot as The Middle East is today. Lynas predicted that a State of Emergency would ensue as a result of these conditions. Glaciers would vanish globally.
Globally, the potential dangerous development for 4C is that the Arctic Ocean ice disappears and the tundra melts to release subterranean CO2 and methane. An additional factor to consider here is vegetation decomposition since just 1% of this potential carbon reservoir equates to a doubling of current emissions.
Impacts at 6C
Essentially, a near end of the world scenario would result. This is not to say that some humans would be eking out an existence somewhere in the world but conditions would become so difficult as to be almost unbearable.
History
The concept of dangerous climate change is not new. As early as the IPCC's Second Assessment in 1995 the concept was raised, ain in 2002 the United Nations Framework Convention on Climate Change (UNFCCC) incorporated the statement in its Article 2 that the world needed to stabilize atmospheric greenhouse gas concentrations to prevent dangerous anthropogenic climate interference.
In 2005, a conference was held in the UK to discuss 'dangerous' climate change avoidance. Based on the precautionary principle, it was agreed that a 2C limit should be the goal since higher temperature increases affect potentially large-scale adverse impacts as discussed above. In terms of atmospheric gas concentration, 450ppm is consistent with a 50:50 chance of not exceeding 2C. We are now at 426ppm (June 2024), with no sign of this abating anytime soon.
Given that 2024 concentrations exceeded 426ppm, a target of 450ppm may now be considered as ambitious although the 2C limit was adopted formally by both the EU in 1996 and in the 2009 Copenhagen Accord. In Paris in 2015 a new aim of 1.5C was set but as discussed we have now breached this.
Recent Evidence
Evidence provided at the December 2015 Paris Climate Change Summit suggested that more stringent standards should be enforced. Hansen, in the late 1980s, also strongly argued that the climate sensitivity is 6C, not 3C for a doubling of CO2 (from pre-industrial concentrations) and advocated that temperature increase should be restricted to no more than 1C. His CO2 atmospheric limit was 350ppm - which would necessitate an immediate emissions peak and subsequent reduction by carbon sequestration. As we mentioned earlier, the Paris Summit has since agreed to a target of 2C but aimed at 1.5C.
It is important to distinguish between emissions (a flow) and concentrations (stock). Emissions are annual but concentrations build up over time. Global warming is a stock problem. It is insufficient just to reduce or stop annual emissions - crucially we need to focus on the concentration in the atmosphere. Since it takes time to reach equilibrium, targets may be overshot with disastrous consequences - positive feedback such as the release of methane also a climate risk.
The Danger and Opportunity
In summary, since we have already passed the Paris aim of 1.5C and the clean energy transition has only just begun, we should aim for 2C at 450ppm. This gives us roughly a decade from now to put all effort into that clean energy transition barring events foreseen and unforeseen.
If we do not succeed we will have to adapt. The convincing arguments of Hansen over the years, and especially since his 1988 Senate testimony commend increased action to prevent irreversible climate change. When natural disasters, refugee crises, and system inertia are factored in it is not unreasonable to question whether adaption will be possible at all and, if so, how? This is an issue that is both global and intergenerational.
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