The Budget Approach
Although the targets-and-timetables approach (eg an agreed percentage reduction in greenhouse gas emissions by 2020) remains the most common approach to defining trajectories for climate mitigation, the budget, or cumulative emissions, approach is rapidly becoming the favoured approach in analyses in the scientific community.
It offers a much simpler, easier-to-understand, transparent and powerful framework to estimate what level of emission reductions is required to meet the 2°C guardrail. …
- The budget approach directly links the projected rise in temperature to the aggregated global emissions in Gt CO2 or Gt C for a specified period, usually 2000 to 2050 or 2100. …
- Given an overall carbon budget between 2000 and 2050, the approach does not stipulate any particular trajectory, so long as the overall budget is respected.
Implications for Emission Reduction Trajectories
Although the budget approach allows more flexibility in the economic and technical pathways to emissions reductions than does a targets-and timetables approach, the fact that we have already consumed over 30% of our post-2000 budget means that much of that flexibility has been squandered if we wish to avoid the escalating risks associated with temperature rises beyond 2°C.
Thus, there is no room for any further delay in embarking on the transition to a low- or no-carbon economy. …
- Reducing emissions of CO2 does not reduce or stabilise its concentrations in the atmosphere; it slows the rate of increase of CO2 concentration.
To stabilise the concentration of CO2 requires emissions to be reduced to very near zero. - The peaking year for emissions is very important for the rate of reduction thereafter.
The decade between now and 2020 is critical. - Targets and timetables are, in principle, less important in the budget approach, but the urgency of bending emission trajectories downwards this decade implies that more ambitious targets for 2020 are critical in preventing delays in the transition to a low- or no-carbon economy.
Relationship Between Fossil and Biological Carbon Emissions and Uptake
Carbon “offsets”, in which emitters of CO2 from fossil fuel combustion can meet their emission reduction obligations by buying an equivalent amount of carbon uptake by ecological systems, are often proposed as a way of achieving rapid emission reductions at least cost.
However, although the immediate net effect on the atmospheric concentration of CO2 is the same for both actions, the nature of the carbon cycle means that the uptake of CO2 from the atmosphere by an ecosystem cannot substitute in the long term for the reduction of an equivalent amount of CO2 emissions from the combustion of fossil fuels.
In fact, the offset approach, if poorly implemented, has the potential to lock in more severe climate change for the future.
(p 56, emphasis added)
Although it is very important to sequester atmospheric CO2 into land ecosystems [it] is not a good idea to consider such biological sequestration as an offset for fossil fuel emissions. …
- Avoiding emissions by protecting ecosystem carbon stocks is a necessary part of a comprehensive approach to mitigation.
Sequestering CO2 into degraded ecosystems is also an important mitigation activity because it reverses an earlier emission.
However … the sequestered carbon is vulnerable to human land use and management … - The only way that CO2 sequestered into land ecosystems can permanently “offset” fossil fuel combustion is if the sequestered carbon is subsequently removed from the land ecosystem and stored in an inert state or in a stable geological formation, thus locked away from the active atmosphere-land-ocean cycle.
Contents
The Budget Approach
Implications for Emission Reduction Trajectories
Relationship Between Fossil and Biological Carbon Emissions and Uptake
Climate Council
- The Critical Decade: Climate science, risks and responses, Climate Commission Secretariat, Department of Climate Change and Energy Efficiency, Commonwealth of Australia, June 2011.
Will Steffen.
3.1 The Budget Approach
Conceptual framework
[If] we wish to have a 75% chance of observing the 2°C guardrail, we can emit no more than 1000 Gt (one trillion tonnes) of CO2 in the period from 2000 to 2050.
If want to achieve a 50:50 chance of observing the guardrail, then we can emit 1440 Gt in the period.
In the first nine years of the period (2000 through 2008), humanity emitted 305 Gt of CO2, over 30% of the total budget in less than 20% of the time period.
Strategic implications
This approach allows [a more] flexible approach that delivers least cost to the economy …
As it is the cumulative emissions over time that must be limited, rather than a series of interim emission reduction targets that must be met, many emission reduction trajectories are possible.
However, the later emission reduction trajectories are initiated, the more difficult and costly they become.
(p 53)
3.2 Implications for Emission Reduction Trajectories
Emissions trajectories
[Delaying] the peaking year by only nine years, from 2011 to 2020, changes the maximum rate of emission reduction from 3.7% … to 9.0% per annum [to achieve a 67% probability of meeting the 2°C guardrail]
Targets and timetables
[A] peaking year of 2015, has global emissions level for 2020 of about 32 Gt CO2, which is about the same as for 2010, and much higher than for 1990, the Kyoto baseline.
[However,] industrialised countries would be expected to have much larger emission reductions than the global average. …
(p 55)
[Once] a desired trajectory is established based on a nation’s overall carbon budget [this] in effect, sets a series of targets within a specific timetable …
The budget approach … has a subtle but important psychological advantage over the targets-and-timetable approach in that it focuses attention on the end game – essentially decarbonising the economy.
[Investment] decisions can be taken from a long-term perspective, knowing that a limited budget is most efficiently allocated to invest in new infrastructure that eventually delivers very low or no emissions by mid-century, rather than to invest in shorter-term measures aimed at meeting an interim target that are perhaps less effective in delivering longer-term emission reductions.
[The] biggest challenge to implementing the budget approach is allocating the global budget to individual countries, where equity issues become important.
This is a political rather than a scientific question …
3.3 Relationship Between Fossil and Biological Carbon Emissions and Uptake
Carbon from land ecosystems
[About] 15%-20% of CO2 emissions globally originate from land ecosystems, primarily from deforestation. …
[Deforestation] is a human-driven redistribution of carbon among the three active stocks – from land to the atmosphere, and then, in part, to the ocean. …
Natural processes such as climate variability also redistribute carbon among these three stocks.
Fossil fuel combustion
The combustion of fossil fuels represents the injection of additional carbon from an inert, underground stock into the active atmosphere-land-ocean system. …
A little less than half of the additional, inert carbon activated by the combustion of fossil fuels remains in the atmosphere;
the rest is redistributed about equally to the land and ocean.
So the combustion of fossil fuel is fundamentally different from deforestation because fossil fuel combustion introduces additional carbon to the active cycle, rather than redistributing the existing amount of carbon in the active cycle among the three major stocks.
(p 57)
[However,] atmospheric carbon cannot be sequestered into land ecosystems indefinitely.
[Sequestration is] a rapid way to begin reducing the anthropogenic burden of CO2 in the atmosphere.
[It] yields some quick gains while the slower process of transforming energy and transport systems unfolds [and] can lead to many other co-benefits, such as enhanced soil condition, more productive agricultural systems, and better biodiversity outcomes.
[General principles] for designing and implementing an appropriate land carbon mitigation scheme:- The size of the stock is the important factor in the carbon cycle, not the rate of flux from one compartment (e.g. atmosphere) to another (e.g. a land ecosystem). …
Although a fast-growing, mono-culture plantation forest may have a rapid rate of carbon uptake for the years of vigorous growth, it will store less carbon in the long term that an old growth forest or a secondary regrowth forest on the same site. - Natural ecosystems tend to maximise carbon storage …
In general, forests with high carbon storage capacities are those in relatively cool, moist climates that have fast growth coupled with low decomposition rates, and older, complex, multi-aged and layered forests with minimal human disturbance.
This … underscores the importance of eliminating harvesting of old-growth forests as perhaps the most important policy measure that can be taken to reduce emissions from land ecosystems. - If designed carefully, a bio-sequestration approach can yield significant co-benefits.
[In] deforested, degraded and intensively cropped lands … bio-sequestration schemes … can improve the productivity of cropping systems through the replacement of soil carbon that was lost in tillage, can deliver additional ecosystem services such as improved water quality on landscapes, and can maintain or enhance biodiversity. …
[More] diverse forests have higher productivity, store more carbon, and are more resilient towards disturbance than those with impoverished biodiversity.
[The] land sink is highly variable on time scales of a few years, varying by as much as 2-3 Pg C in those timeframes.
The strong fluctuations are driven largely by modes of climate variability such as ENSO and by volcanic activity, which induce rapid changes in soil respiration and plant growth through changes in solar radiation, rainfall/drought and temperature.
In the longer term, climate change can significantly weaken or even reverse the land sink through droughts, increased soil respiration and disturbances such as fire and insect outbreaks.
Simulations by dynamic global vegetation models using the IPCC IS92a emissions scenario show a levelling off of the land sink in the second half of the century with two models showing a significant weakening.
When coupled to a climate model in interactive mode, all vegetation models show a weakening of the land sink by 2100 with a net release of carbon back to the atmosphere corresponding to an additional rise in concentration from 20 to 200 ppm CO2.
The 2003 drought and heatwave in central Europe triggered a 30% reduction in gross primary productivity over the region, which resulted in a strong net source of 0.5 Pg C yr^-1 to the atmosphere, undoing four years of a net carbon sink for the region.
A multi-decadal study of the carbon balance of Canadian forests has demonstrated that since 1970 they have become a weaker carbon sink despite a longer growing season, owing to a sharp increase in disturbances such as fire and insect outbreaks triggered by a warming climate.
As cited earlier, the Amazon rainforest, an important stock of carbon on a global level, has suffered severe droughts and fires in 2005 and 2010, leading to estimated losses in carbon storage of 2.2 and 1.6 Pg C for the two drought events, respectively.
[These droughts] offset a decade of carbon sink activity, estimated to be about 0.4 Pg C uptake per annum;
[At] temperatures above the 2°C guardrail, the Amazon rainforest is at risk of extensive dieback and conversion to a savanna …
If this occurs, then Amazonian ecosystems will continue to hold significant carbon stocks but at lower than current levels.
(pp 58-9)
Geosequestration as an offset
It is theoretically conceivable that biosequestered carbon could be removed and stored in a stable geological formation, locked away from the active atmosphere-land-ocean system.
Another approach, equivalent to geosequestration, is to replace fossil fuel combustion with biofuel combustion to produce energy. …
[However care must be taken] to limit the emissions associated with the production process to low levels relative to the amount of energy produced, and avoid undesired side effects such as competition with food production, loss of natural ecosystems and thus generation of large carbon emissions (see point 2 on page 58) and losses of biodiversity.
In general, biofuels made from ‘waste’ biomass from plantation forests or from perennial vegetation grown on abandoned agricultural land offer the most advantages and avoid the undesirable side effects.
Focussing on the end game
Responsibly implemented bio-sequestration schemes … can remove carbon quickly from the atmosphere …
The challenge is to ensure that linking bio-sequestration to the fossil fuel emissions sectors does not lead to any delays in the investment or deployment of low- or no-carbon technologies in those sectors.
(p 59) - The size of the stock is the important factor in the carbon cycle, not the rate of flux from one compartment (e.g. atmosphere) to another (e.g. a land ecosystem). …
