"...to stabilise concentrations of greenhouse gases in the atmosphere at a level which will prevent dangerous anthropogenic interference with the climate system, (continued)..." .
So which level should that be, and what is the best emissions pathway to reach it, considering the implications for both emissions and temperature?
This page discusses the stabilisation options available in JCM, which may be selected from the "mitigation" menu (top panel).
The model then calculates the CO2 emissions required to reach the target curve (an "inverse" calculation), as well as the consequences for temperature and sea-level.
See also:
In order to calculate the temperature, we also have to make an assumption about the contribution of the other greenhouse gases to radiative forcing. For this purpose IPCC-TAR SYR Q6 assumed that emissions of other gases are fixed according to SRES A1B scenario. To reproduce this, you should choose "SRES fixed" from the Other Gas menu (top panel), and "A1B" from the SRES menu.
The temperature rise slows quite soon after CO2 stabilisation, but continues to increase slowly due to the gradual transfer of heat to the deep ocean. The latter effect is much more apparent in the sea-level, which continues to rise for centuries after CO2 stabilisation. Note also that local, seasonal changes can be much greater than global average figures, as illustrated by the map.
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"Such a (stabilisation) level shall be achieved within a time-frame sufficient to allow ecosystems to adapt naturally to climate change, to ensure that food production is not threatened, and to enable economic development to proceed in a sustainable manner"
So we have to find a pathway that avoids abrupt changes, balancing climatic and economic considerations. If we reduce emissions more earlier, we don't have to reduce so dramatically later. On the other hand, reducing emissions later may be cheaper, if we use a delay to develop new energy-saving technology. This is essentially a question of "intergenerational equity".
IPCC considered two alternative sets of stabilisation pathways: the original formula from the IPCC 1994 technical paper, known as "S" or "WG1" scenarios, and a variant on this developed later by Wigley, Richels and Edmonds, known as "WRE" scenarios. The WRE scenarios followed the IS92A "business as usual" pathway for an initial period of 10-30 years (longer for higher stabilisation levels), before curving away to reach the stabilisation target.
A button to switch the "WRE delayed start" option on or off is provided on the carbon cycle plot (if you choose stabilise CO2 concentration, and switch off the Kyoto protocol). As you can see, the WRE option allows higher emissions initially, but later they must drop more steeply.
You can also experiment with other pathways, by dragging the endpoint of the curve (the black 4-pointed arrow) horizontally, changing the stabilisation year without changing the final level. The effect is similar: the earlier the target level is reached, the steeper the rise and fall in the emissions.
The choice of pathway makes little difference to the eventual equilibrium temperature, but it does influence the rate of temperature rise, which affects the ability of ecosystems and society to adapt (noting Article 2 above). You can experiment by watching how selecting the WRE option, or moving the black arrow horizontally, affects the temperature plot.
Note that, as WRE pointed out, the rate question is complicated by the short-term cooling effect of sulphate aerosols which are a by-product of burning coal. Considering this, the WRE pathway can actually lead to slightly cooler global average temperatures for the first few years! (note you will only see this subtle effect if other gas emissions are scaled to CO2)
WRE suggested that it might be economically more efficient to delay initial emissions reduction, although they did not apply any economic optimisation model in developing these scenarios. On the other hand, they also stressed that although emissions reductions might be delayed, the effort to develop new technology and infrastructure, anticipating reductions, should begin immediately.
Our current development path lies between the original WRE and WG1 pathways. Therefore, to get back to the original WG1 pathway from the present, would require rather abrupt reductions initially (this may partially explain why the economic mitigation costs shown in IPCC-TAR-SYR-Q7 seem rather high for the WG1 pathway. The choice of "discount rate" would also strongly influence any comparison of pathways, since discounting reduces later compared to earlier costs.)
To avoid this discontinuity, this model sets the starting point to 2000, or 2013 post-Kyoto, rather than 1990 as for original WG1.
For example, the European Union proposed that we should restrain the temperature increase to maximum 2C above the preindustrial level. Try setting this as a target, and then adjusting some scientific uncertainty parameters such as the climate sensitivity, or choosing different models from the GCM-fit menu. Now the final temperature should remain constant, so the emissions must change accordingly -what is a safe pathway given the range of uncertainty?
Even for a fixed set of model parameters, there are many possible routes to stabilise at a given temperature. Two different calculation methods are provided in JCM -either iterating to guess an appropriate CO2 stabilisation level (the default method), or adjusting emissions depending on deviation from a target curve (a "fuzzy control" method -expert level only).
The stabilise temperature option may be combined with options for mitigating other gases and distributing emissions between regions, as for stabilise concentration.
See also
You can explore this concept, by choosing "stabilise CO2 emissions" from the emissions menu in the top panel. Then you will see the CO2 emissions follow a simple curve, starting from the present trend, and eventually stabilising at a time and level determined by the 4-headed yellow arrow on a plot of regional CO2 emissions.
For example, it is often quoted, that we need to reduce global emissions by about 60% to stabilise the concentration of CO2 at current levels. This figure from the first IPCC report (1992) can be explained by considering that 3/5 of the fossil CO2 emissions stayed in the atmosphere whilst 2/5 was taken up by the ocean sink (these ratios are still approximately valid today).
You can test whether this works, by moving the moving the yellow arrow to stabilise CO2 emissions at 2.4 GtC/yr in 2010. This stops the atmospheric CO2 concentration (black curve on carbon cycle plot) at about 375ppm, but only instantaneously. If you look further into the future, the concentration starts to rise again, since the sinks reduce as the rate of atmospheric CO2 increase falls, the biosphere sink saturates, and the seawater becomes more acidic. If we really want to stabilise the atmospheric CO2 concentration, we have to keep reducing emissions for centuries, which is apparent when you choose instead the "stabilise CO2 concentration" option, and place the black arrow at 375ppm in 2010.
Note, if you want to stabilise emissions at current levels, simply select "constant emissions" from the mitigation menu (this may be useful for comparing different distribution options). Also at the "expert" complexity level, you can "fine-tune" an emissions stabilisation curve by adjusting the initial growth rate (%/year) or the integral of cumulative emissions (2000-2200).
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This may be confusing when considering the effect of scientific uncertainties. For example, if you adjust scientific uncertainty parameters affecting the ocean or biosphere carbon sinks, the CO2 concentration should not change significantly -instead the emissions are adjusted to remain on target. As there is also a biogeochemical feedback between the temperature and the carbon sinks, adjusting climate model parameters can also change the emissions, even when you have fixed the CO2 concentration.
So if you want to explore cause-effect relationships within the natural carbon-climate system, it is recommended to choose either "stabilise emissions" (for low emissions) or no-policy SRES scenarios (for high emissions).
It should also be emphasised that these are not predictions. They are useful for exploring mitigation policies, whereas the SRES scenarios may be more appropriate when exploring adapation policies. See also:
However, it may be considered unrealistic to reduce greenhouse gas emissions, without changing other underlying socioeconomic projections which are also part of the SRES scenarios.
Therefore IPCC has begun to review more complex mitigation scenarios, which still aim to reach specific CO2 stabilisation levels, but must also consider socioeconomic factors consistent with the original SRES storylines. The first stage of this "post-SRES" process is described in IPCCTAR WG3 Chap2.
These scenarios may help us to calculate the required mitigation effort, considering not only the difference in emissions, but also the interactive feedbacks with economic growth, population and technology development.