Setting Better-Informed Climate Targets for New Zealand: The Influence of Value and Modeling Choices

Quantifying
greenhouse gas (GHG) emissions and setting GHG emissions
budgets for anthropogenic systems are influenced by several value
and modeling choices. This study, for the first time, quantified the
influence of choice of GHG accounting approach, GHG metric, time horizon,
climate threshold, global emissions budget calculation method, and
effort-sharing approach, taking New Zealand (NZ) as a case study.
First, NZ’s production- and consumption-based emissions were
quantified using multiregional input–output analysis and applying
different GHG metrics (global warming and temperature potentials)
and time horizons (20 and 100 years). Second, global emissions budgets
for 1.5 °C, 2 °C, and 1 W m–2 climate
thresholds were estimated. Budget shares were then assigned to NZ
using two effort-sharing approaches (grandfathering and economic value),
and emissions were benchmarked against the assigned shares. Finally,
the analysis was undertaken at the NZ sector level. The results showed
that, for each GHG accounting approach, NZ’s total emissions
exceeded their budget shares, irrespective of the choices; the largest
source of uncertainty was the choice of global emissions budget calculation
method, followed by GHG metric, climate threshold, effort-sharing
approach, and reference year for the grandfathering approach. The
sector-level analysis showed that, while most sectors exceeded their
budget shares, some performed within them. The ranking of uncertainty
sources was quite different at the sector level, with the choice of
effort-sharing approach providing the largest source of uncertainty.
Overall, the study indicates the importance of handling value and
modeling choices in a transparent way when quantifying GHG emissions
and setting emissions budgets for anthropogenic systems.