1°C not 2°C is the ‘danger threshold’ – Jim Hansen

James Hansen‘s latest paper, Assessing “Dangerous Climate Change”: Required Reduction of Carbon Emissions to Protect Young People, Future Generations and Nature, argues that the generally agreed aspirational target to limit global warming to 2°C is too high and that it would lead to ‘disastrous consequences.’ Rather he proposes that a 1°C target is the safe limit.


Given that we are already at 0.8°C of warming; this is concerning. Incidentally, it is worth noting that Hansen’s analysis matches up with Bill McGibbins proposed 350ppm target. It’s just another way of expressing it.

Regarding this paper as important INTELLIGENCE, a critical  SITREP or source of EARLY WARNING, let me offer a brief summary…

How are we tracking so far? GHG emissions are accelerating. Compare these fossil fuel emission growth rates:

  •  1980 – 2000: 1.5% p.a.
  • 2000 – 2012: 3% p.a.

How did he approach this study?

Hansen and his team assessed the Earth’s ‘energy balance’ – the amount of heat radiation coming into and out of the atmosphere – and how this would respond to different GHG emission scenarios. Presently, the energy is ‘unbalanced’ in that more heat radiation is coming ‘in’ than is going ‘out’ to space.

(1) ARGO FLOATS. Data obtained from the use of 3000 Argo Floats, which tested ocean temperatures  down the depths of 2km over the period 2005 to 2010, helped confirm that the energy imbalance is currently +0.58 watts per square metre, (W/m2). (With an error margin of plus or minus 0.15 W/m2).


(2) Consider slow feedbacks.

Hansen explains that while it may be appropriate with the present warming, (and if considering a 1°C temperature rise), to exclude slow feedback mechanisms; however, that this is not appropriate when considering a 2°C temperature rise.

  • Deep Ocean. At 2°C there is high risk of heat penetrating the deep ocean, which because of the ocean’s inertia, is not something that can be easily or quickly undone. Oceans drive climate, so this affects everything.
  • Forests and Soils. In many ‘mitigation’ plans, it is being assumed that improved forest and soil management will be able to soak up 100 Gigatonnes of Carbon (GtC). However Hansen warns that this may be naive optimism. He explains that, at 2°C of warming, it is “less likely that a substantial increase in forest and soil carbon could be achieved”.
  • History (palaeoclimatology) tells us so. Given the long lifetime of fossil fuel CO2 in the climate system, (100,000 years), Hansen writes that “it must be assumed that these slow feedbacks will occur if temperature rises well above the Holocene range.” (Holocene is ~ up to 1°C scenario.) Further, paleoclimate research has shown that it is the slow feedbacks that most impact ‘climate sensitivity’- that is – ‘how much’ the temperature changes in response to ‘radiative forcing’ (the overall energy imbalance spoken of earlier). Knowing the ‘climate sensitivity’ is sort of like knowing exactly how many straws it would take to break the camel’s back… [Such an unfortunate phrase!]
  • Current observations. The Earth is already seeing significant impacts now, (at 0.8°C). This must be taken into account when considering a ‘dangerous’ threshold. For example, Arctic sea ice thickness was found to be “declining a factor of four faster than simulated in IPCC climate models.” Hansen writes,

The paleoclimate record and changes underway in the Arctic and on the Greenland and Antarctic ice sheets with only today’s warming imply that sea level rise of several metres could be expected. Increased climate extremes, already apparent at 0.8°C warming, would be more severe. Coral reefs and associated species, already stressed with current conditions, would be decimated by increased acidification, temperature and sea level rise.” 

What is required  – according to the Science? – “We need to win on all three counts”

To achieve a safe climate, Hansen argues that three tasks must occur and that these are all “interactive and reinforcing”:

  1. Limit fossil fuel emissions;
  2. Limit and reverse land-use emissions; (agriculture, deforestation/reforestation; soil management) and
  3. Limit and reverse non-CO2 forcings, (principally methane).

Allowing a 2°C rise, with a promise of sharp reductions later (around 2030), is problematic as it inhibits the ability of Tasks 2 and 3 to be achieved.  (i.e. As the temperature rises, it is more likely that forests, agriculture and soils become carbon ‘sources’ rather than carbon ‘sinks’, which jeopardises Task 2; as permafrost melts, methane is released, jeopardising Task 3.)

Hanson proposes various GHG reduction strategies (below graphs). The longer the delay, the harder it all becomes.


What is required on the policy side? – A Legal or Human rights approach


Hansen concludes that urgent action is required, discussing two possible policy approaches that have been proposed by others: a legal or a human rights approach:

  • Legal. The atmosphere becomes a ‘public trust’. If this were so, Governments could be sued for failing to protect it.
  • Human rights. The United Nation’s Universal Declaration on Human Rights states that “all are equal before the law and are entitled without discrimination to equal protection of the law.” Likewise, the United States Constitution states that, “No person can be deprived of ‘life, liberty or property without due process of the law.” From a right’s perspective, Hansen concludes that the “failure of governments to effectively address climate change infringes on fundamental rights of young people.”



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