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Climate A Conceptual Framework to Assist Decision Making on International Climate Change Policy: Discussion Paper Online version
Decision 1: Global stabilisation goal
Atmospheric concentrations of GHGs have increased sharply over the last century and under a range of BAU scenarios are forecast to reach five times their historical level by the end of this century4. There is a close relationship between concentrations of GHGs in the atmosphere and temperatures on the earth. Table 1 presents the relationship between particular stabilisation concentrations, temperature increases, and the emission reductions required to achieve these concentrations.
Table 1: Concentrations, temperatures and emissions (source: IPCC)
| Category | Additional radiative forcing (W/m2) | CO2 concentration (ppm) | CO2-eq concentration (ppm) | Global mean temperature increase above pre-industrial at equilibrium, using "best etimate" climate sensitivitya), b) (°C) | Peaking year for CO2 emissionsc) | Change in global CO2 emissions in 2050 (% of 2000 emissions)c) | No. of assessed scenarios |
|---|---|---|---|---|---|---|---|
| I | 2.5-3.0 | 350-400 | 445-490 | 2.0-2.4 | 2000-2015 | -85 to -50 | 6 |
| II | 3.0-3.5 | 400-440 | 490-535 | 2.4-2.8 | 2000-2020 | -60 to -30 | 18 |
| III | 3.5-4.0 | 440-485 | 535-590 | 2.8-3.2 | 2010-2030 | -30 to +5 | 21 |
| IV | 4.0-5.0 | 485-570 | 590-710 | 3.2-4.0 | 2020-2060 | +10 to +60 | 118 |
| V | 5.0-6.0 | 570-660 | 710-855 | 4.0-4.9 | 2050-2080 | +25 to +85 | 9 |
| VI | 6.0-7.5 | 660-790 | 855-1130 | 4.9-6.1 | 2060-2090 | +90 to +140 | 5 |
| Total | 177 |
Notes:
a) Note that global mean temperature at equilibrium is different from expected global mean temperatures in 2100 due to the inertia of the climate system.
b) The simple relationship Teq = t2xCO2 x In([CO2]/278)/In(2) and ΔQ=5.35 x In([CO2]/278) are used. Non-linearities in the feedbacks (including e.g., ice cover and carbon cycle) may cause time dependence of the effective climate sensitivity, as well as leading to larger uncertainities for greater warming levels. The best-estimate climate sensitivity (3°C) refers to the most likely value, that is, the mode of the climate sensitivity PDF consistent with the WGI assessment of climate sensitivity and drawn from additional consideration of the Box 10.2, Figure 2, in the WGI AR4.
c) Ranges correspond to the 15th to 85th percentile of the Post-Third Assessment Report (TAR) scenario distribution. CO2 emissions are shown, so multi-gas scenarios can be compared with CO2-only scenarios.
Note that the classification needs to be used with care. Each category includes a range of studies going from the upper to the lower boundary. The classification of studies was done on the basis of the reported targets (thus including modelling uncertainities). In addition, the relationship that was used to relate different stablization metrics is also subect to uncertainity (See Figure 3.16).
There is general consensus that to avoid some of the most extreme impacts of climate change global mean temperatures need to increase by no more than 2-3 degrees Celsius above post-industrial levels5. Using the information presented in the table above, and taking a conservative estimate6, this would require GHG concentrations to stabilise between 445 and about 535 ppm CO2-equivalent.
Outcome A: Impacts of Climate Change
The level at which emissions are stabilised will have important implications for the magnitude and distribution of the physical impacts of climate change. These impacts could be quite large, even if a consensus is reached to stabilise emissions at the lower end of the concentration range. The impacts will also be unevenly distributed across the globe. New Zealand, for example, may experience some moderate increases in agricultural productivity and reduced health expenditure due to a warmer climate. It may however be subject to increase storm damage, and severe droughts. The impact of these direct effects on NNW will need to be considered7.
The other set of more indirect impacts which will affect NNW in New Zealand will be those that are experienced by other countries. For example, with rising sea levels it is likely that immigration to New Zealand from small island states in the Pacific will increase. This may have either positive or negative consequences on NNW. Furthermore, the effects of climate change will disrupt the supply and demand for commodities which New Zealand imports and exports. On the supply side, the loss of Amazonian forests through wild fires will increase the price of timber on international markets, while severe heat waves in Europe could reduce livestock numbers and increase prices of meat and wool. These impacts would positively affect NNW in New Zealand. On the demand side, if severe weather events disrupt economic activity in other countries, it may reduce demand for New Zealand exports, with negative affects on NNW.
The physical impacts of climate change on NNW are extremely difficult to accurately determine, however it will be important that when a decision is made on a stabilisation target the likely impacts on NNW of the associated warming are at least considered.
4 IPCC – 900ppm (relative to about 280ppm average in pre-industrial times)
5 Recent evidence predicts that with 2.7degrees warming the Greenland Ice Sheet (and potentially the West-Antarctic) will start an irreversible melting – adding 6-12m to sea level
6 This paper generally assumes the maximum desired increase is about 2.8 degrees – associated with the IPCC global stabilisation goal of 445-535ppm (Category I and II)
7 MED have recently undertaken some work to quantify the impacts of climate change on New Zealand – further work is however required
