Emissions-Temperature-Uncertainty Framework (ETU)

The study by Jonas et al. (2014) “Uncertainty in an Emissions Constrained World” introduces the Emissions-Temperature-Uncertainty Framework (ETU) that allows any country to understand its near-term mitigation and adaptation efforts in a globally consistent and long-term context which includes all countries and stipulates global warming to range between 2 and 4 °C.

The revised and updated data set supporting the ETU framework is the basis for analyzing greenhouse gas emission targets scaled from global to regional (here EU 27) to national levels (here Austria).

Assumptions

• Emission targets derived for 2050 are exclusively available for technospheric emissions. The imperative for net emissions from LU activities is that these will be reduced linearly to zero by 2050. It is presupposed that deforestation and other LU mismanagement will cease and that net emissions balance.
• The hidden assumptions are that:
  1. The remainder of the biosphere (including oceans) stays in or returns to an emissions balance;
  2. This return, which refers to CO2-C, implies in turn that emissions and removals of CH4, N2O, etc. also return to an emissions balance;
  3. These returns happen without any unforeseen systemic surprises of the terrestrial biosphere.
• Additional assumptions exist when making the step from a 2 °C global warming target to global warming targets of 3 and 4 °C; namely that:
  1. The risk of overshooting is comparatively stable and independent of the particular warming situation, equilibrium or transient, when going from, e.g., 2 to 3 °C; and
  2. Deviations from this assumption are minor compared to the considerable change in risk when going from, e.g., 2 to 3 °C under either warming scenario, equilibrium or transient.

Background

Since their inception, climate treaty negotiations have set out to stabilize the Earth’s climate by implementing mechanisms that reduce global greenhouse gas [GHG] emissions and lead to sustainable management of the atmosphere at a ‘safe’ steady-state level (assumed to be characterized by an increase in global average temperature of not more than 2 °C above preindustrial levels).

In recent years, international climate policy has been increasingly focusing on limiting temperature rise (Rogelj et al. 2011).

The idea of limiting cumulative global GHG emissions by adhering to a long-term global warming target was first discussed publicly by policymakers at the 2009 United Nations climate change conference in Copenhagen. It appears to be a promising and robust methodology (Allen et al. 2009; Matthews et al. 2009; Meinshausen et al. 2009; WBGU 2009; Zickfeld et al. 2009; Raupach et al. 2011a; cf. also Box 1).

To comply with it, the emissions reductions required from the fossil-fuel and land use/land-use change [LUC] sector are daunting: 50%–85% below the 1990 global annual emissions, with even greater reductions for industrialized countries (Fisher et al. 2007; Jonas et al. 2010, 2014).

The underlying assumptions are equally daunting: terrestrial or oceanic sinks continuing to offset fossil-fuel and LUC emissions before achieving an emissions balance that goes beyond CO2-C (i.e., CO2-equivalents also including CH4, N2O, etc.), with no systemic surprises occurring during the transition process. In particular, net emissions from LUC activities will need to be reduced linearly to zero by 2050. That is, it is assumed that deforestation and other LU mismanagement will cease and that net emissions balance.