Table of contents key findings


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7.1 Science-policy debate about the risks of danger
There now appears to be little doubt amongst researchers active in climate system and impacts studies that climate change is happening, that much of it can be attributed to human activities, and that there are serious risks of danger associated with such change if it continues unabated. However, the same level of confidence appears not as yet to have penetrated public opinion, particularly in North America. Recent American surveys, for example, suggest that, while there is an increasing public acceptance of the need to reduce greenhouse gas emissions, there is a continued lack of confidence in the science supporting such action. Polls conducted in 2005 and 2007, for example, found that about 39 and 56%, respectively, of Americans, still believe there is large disagreement amongst scientists as to whether or not the Earth is even warming. Most also do not accept human activities as the dominant cause of recent changes. Curiously, better information does not always create greater motivation to reduce personal emissions. Farmers in Ontario and forest managers in northwestern Canada indicate similar lack of awareness of the risks of climate change to their economic sectors. They perceive that economic and other environmental concerns will outweigh any risks due to climate change, even in the long term. There is evidence that one reason for such misinformation is a consistent media bias towards including the views of the relatively small number of skeptics who argue that recent climate change may be entirely natural and that the risks of future climate change have been exaggerated - regardless of whether or not those views are credible. Furthermore, there is a reluctance and/or lack of training of scientists in effectively communicating their research results to lay audiences. Communication experts argue that scientists need to be more pro-active in using analogies that relate their results to concrete experiences that lay audiences can process readily, and that the information provided must be relevant to the public, must be credible and objective and must clearly articulate what the personal risks to the individual are. This requires cross disciplinary, participatory and user oriented studies that remove institutional barriers and empower the public to help develop relevant mitigative and adaptive strategies. There are also indications that presenting the results as plausible future scenarios may be preferable to using probability language, since the latter does not adequately portray the risk of high-impact surprise scenarios that can arise out of a complex non-linear climate system64,910-924.
Policy makers also need to be reminded of the near irreversibility of global CO2 emissions. For example, if all of the carbon in known reserves of fossil fuels (about 5000 GtC) is combusted, most of the accumulated CO2 emissions into the atmosphere would still be there after two millennia. After 6800 years, atmospheric concentrations would still be about three times that of pre-industrial levels, and global temperatures would still be 6-8°C above current values. Furthermore, if total accumulated emissions exceed 3000 GtC, irreversible melting of the Greenland ice cap is likely unavoidable115,839,925.
Although scientists increasingly agree that the risks of future climate change are real and significant, there continues to be considerable debate about what level of human interference with the climate system might be tolerable. Some suggest that, if we are to significantly reduce the risk of strong positive feedbacks that have high risks associated with them (particularly those related to the Arctic, tropical storms, ice sheet melt and hydrate stability), future warming may need to be restricted to less than 1°C above current levels, or about 2°C above pre-industrial levels. Warming above this threshold could also cause the ocean circulation system to become unstable, triggering abrupt dangerous millennial scale climate anomalies like those of the past glacial period. While some studies have indicated that exceeding such thresholds could be avoided if future CO2 concentrations are limited to about 450 ppm and non-CO2 forcings are also decreased, others argue that much more stringent concentration thresholds would be required if climate sensitivities are at the high end of the estimated range, or if the risks of exceeding such temperature thresholds are to be kept low. They note that, under worst case scenarios, current concentrations may already be too high to do so320,926-932.
To keep CO2 concentrations below 450 ppm, global CO2 emissions would need to peak by 2015 to 2020. Emissions would need to be reduced to 25% below 1990 levels by about 2050, and eventually to near zero levels. Costs to achieve this early peaking would initially be large – about five times that required to limit concentrations to 650 ppm - but would decrease in subsequent decades. Studies note, however, that the costs of mitigation rise relatively linearly with the probability of meeting targets, which in turn relates to reduced costs of adaptation. They also suggest that an overshoot to a peak concentration of 510 ppm before returning to 450 ppm is a more cost effective scenario than those that focus rigidly on stabilization without overshoot. According to some studies, such an overshoot scenario would provide a 14 to 67% probability of achieving the 2°C target. Including potentially large positive global carbon cycle feedbacks into these calculations would likely enhance the required reduction in emissions by another 20% or so, making a 450 ppm target even more difficult to achieve. On the other hand, higher CO2 stabilization targets of 550 ppmv would only achieve the 2°C goal if climate sensitivities are low. Researchers also note that delaying the peaking of CO2 concentrations by 10 years or more will require a corresponding increase in the rate of decline in subsequent decades, which would likely be very costly. Hence, related legislation on implementing mitigative programs such as emissions trading need to include incentives for early action933-941.

Figure 17. CO2 emissions required for climate stabilization. (a) Simulated global mean surface air temperature relative to pre-industrial. (b) Simulated atmospheric CO2. (c) Simulated change in global mean ocean temperature relative to pre-industrial. (d) Cumulative carbon emissions relative to the year 2005 (where near-constant cumulative emissions reflect near-zero yearly emissions). Colors indicate climate stabilization at 1 (red lines), 2 (green lines), and 4 (blue lines) °C above pre-industrial temperatures. Line styles indicate rates of warming (between 2005 and the time of temperature-stabilization) of 0.1 (thick lines), 0.2 (medium lines), and 0.4 (thin lines) °C per decade. The solid grey line shows climate stabilization at year-2005 temperatures. (Mathews and Caldeira, 2008, ref. # 938).

However, there are arguments against focusing on developing an ‘optimum’ emission path to reduce the risks of climate change. For example, some note that large uncertainties related to the sensitivity of global temperatures to such emission paths make them unhelpful in determining near term action to achieve a 2°C threshold. Furthermore, investigations into alternative future control scenarios suggest that, because of the thermal lag in the climate system response to greenhouse gas forcing, the ability to meet long term CO2 concentration targets depends less on profile of the emission scenario over time than on the accumulated emissions. For example, alternative emission paths to achieve a CO2 stabilization target of 650 ppm all have cumulative emissions of about 2300 GtC, regardless of the path chosen. Strategies to address cumulative emissions would also need to address non-greenhouse gas and aerosol forcings, such as the effects of land use and land use change and urbanization. This would require unprecedented international cooperation, particularly since accumulated historical emissions of individual developing countries such as China may surpass that of the United States by 2050. China’s annual emissions now already exceed that of the United States942-946.

There are also cautionary voices that note that climate change is only one of a number of policy issues to be addressed at global to local scales. Effective action to mitigate risks of climate change will therefore need to integrate action on climate change with socio-economic development strategies that also address environmental and socio-economic issues. Some researchers suggest that ‘objective networks’ and ‘influence diagrams’ are useful tools for doing so. Experts argue that the largest co-benefits will arise from a development framework that focuses on reduction of poverty and vulnerability to disaster through improved rural economies, energy and transportation options. Best options will vary from region to region. For example, the greatest potential for simultaneous improvement in air quality and reduction of near term warming effects in developing Asia would be derived from reduced domestic fuel burning, while in North America it would be through reduction in surface transportation. On the other hand, reduction in emissions from the industrial and power generation sector would improve air quality but would also lower emissions of sulphate aerosols and their pre-cursors. This reduced offset to greenhouse gas forcing would initially cause a warming effect947-952.
Recent CO2 emissions trends also suggest that some of the SRES emission scenarios used as a basis for projections of future climate change and related mitigation discussions are already out of date. Relevant experts agree that they need to be revised, and that new scenarios that also integrate mitigative policies in an iterative manner should be added953-955.

7.2 Mitigating the risks
7.2.1 Reducing greenhouse gas emissions
The best no-regrets options for reducing the risks of dangerous interference with the climate system appear to include drastic energy efficiency improvements, carbon sequestration and the cautious expansion of renewable and other non-fossil fuel technologies. Furthermore, such measures need not be extremely costly. In Canada, for example, the costs of meeting its Kyoto targets are estimated to be as low as a 3.1% net reduction of the national GDP, providing strategies target the twelve largest emitter sectors rather than applying the same reduction targets across all sectors. However, no-regrets options alone are unlikely to reach threshold targets for global temperature change if global population and economies grow rapidly. Therefore, strategies for addressing climate change need to be both technological and socio-economic in nature. Measures to reduce CO2 emissions also affect aerosol emissions and have other environmental implications that depend on where measures are taken. For example, since sulphate aerosols cause an offset cooling effect, the coincidental reduction of their emissions while reducing CO2 emissions can reduce the effectiveness of the latter on net radiative forcing by between 14 and 47%, depending on where in the world they occur. This offset therefore also needs to be considered when selecting policy portfolios for individual countries956-958.
Greenhouse gas emissions from global food production (which is the primary anthropogenic source for both methane and nitrous oxide) are projected to increase by 50% over the next three decades. Mitigation options such as improved digestability of animal feed, better management of animal wastes and more efficient use of nitrogen on crops, can all help to significantly reduce these emissions. For example, improvements in the genetic quality of Canadian beef and dairy cattle and the energy content of feed have both helped to reduce the intensity of greenhouse gas emissions per kilogram of life weight or milk produced. Canadian studies also suggest that irrigation of croplands is also an effective strategy for reducing emissions. While such irrigation increases greenhouse gas emissions per unit of land, it enhances crop yields even more. Thus, net emissions per unit value of produced goods decreases959-963.
Another source of greenhouse gases often overlooked is hydro-electric reservoirs. Some of these can emit large amounts of methane and CO2 originating from the slow decay of flooded biomass. Recent studies indicate that much of the greenhouse gases generated in these reservoirs are emitted from rivers downstream. In the case of one Amazon system, these river emissions were larger than those from the reservoir itself. There are also modest N2O emissions from reservoirs, but only slightly above background levels. These emissions should be included in national emission inventories and in related emission reduction strategies964-966.
Reducing tropical deforestation by 50% is also a low cost option for cutting carbon emissions by as much as 1.5 to 2.7 GtCO2/year. Recent trends in the Amazon, where a rise in deforestation rates since 1998 was reversed in 2005 after tri-national agreement was implemented, demonstrate the potential of this option if the political will to do so is in place967-968.
There has been an increasing emphasis within the policy community on expanding the use of bioenergy as a replacement for fossil fuels in order to reduce related greenhouse gas emissions from fossil fuel combustion. Numerous studies, however, demonstrate that there are important caveats to the effectiveness of such strategies. If, for example, lands for growing bioenergy crops are obtained by clearing forests, draining peatlands or cultivating savanna lands, the carbon dioxide emissions caused by the land use change would exceed the reduced emissions from replacing fossil fuels with bioenergy by orders of magnitude. Soil emissions from such land use change would continue for many decades. Furthermore, if the biofuels are derived from cultivated crops such as corn, there would be a large increase in N2O emissions from agricultural soils that would at least partially offset gains for CO2 emissions. Thus, while bioenergy has a major role to play in mitigative strategies and needs to be deeply integrated into energy policies, the total life cycle emissions of all greenhouse gases as well as other environmental and social impacts must be considered when choosing biofuel options. Biofuels from wood fibre and perennial crops such as switch grass offer the greatest benefits, since such sources also store additional carbon into the soils during their production. In the United States, for example, these biofuels can offset carbon dioxide emissions from the fossil fuels they replace by 115% (assuming no incremental N2O emissions). That is, they provide a net sink. Another effective source is crop wastes, although their use can reduce potential carbon input into soils through litter. By comparison, the offset from corn crop sources is only about 40%969-980.
Other sources of alternative renewable fuels that can be used to replace fossil fuels and offset their greenhouse gas emissions include the conversion of manure to biogas and the generation of oil by photosynthetic microbes generated by photobiogenerators in open ponds. In the United States alone, adoption of a large scale manure biogas program could potentially reduce CO2 emissions from its electrical sector by 4%. Likewise, conversion of 7.3% of global arable land for the photobiogeneration of oil could, by 2050, reduce fossil fuel emissions of CO2 by 6.5 GtC/year. Proponents argue that the land requirement is much less than that needed for equivalent growth of biomass for bioenergy981-982.
7.2.2 Emission offsets through carbon sequestration
Carbon sequestration programs within managed forest and agricultural ecosystems have the potential to remove significant amounts of carbon from the atmosphere, storing it into surface biomass and soils, thus offsetting greenhouse gas emissions. Some estimates suggest the offset, if developed on a large global scale, could be between 0.5 and 0.7 GtC/year by 2050, with an accumulated sink by 2100 of 23-41 GtC. So, while not a long term solution for meeting global emission targets, this option provides an important, relatively low cost interim bridge that can complement other mitigative initiatives such as capture and storage, fuel switching and efficiency improvements until new technologies are developed to get to near zero carbon emissions983-984.
Agricultural systems which, in general, have a much more rapid response time to management initiatives than slower growing forest systems, have the greater potential for large increases in sinks in the near term (years to decades). Large scale global programs to enhance carbon sinks within temperate and tropical agricultural soils alone could remove enough carbon from the atmosphere to offset about 3-6% of global fossil fuel emissions. When grassland management is included, this combined sink could total about 0.36 GtC/year by 2050. Practices to achieve this include no till and liming of soils. Tillage, among other things, introduces microbes from fresh biomass carbon into the deep soils, where they begin to decompose old, stable carbon that has been present there for several millennia. Furthermore, there may be other biophysical co-benefits from agricultural initiatives that further reduce radiative forcing. For example, some crop management strategies for enhancing carbon sinks also increase surface albedo and thus reflect more energy back to space. Increased irrigation can also enhance reflective low cloud cover, with similar effects. It also increases uptake of energy as latent heat, thus cooling local temperatures. However, not all soils respond equally. Studies in eastern Canada show, for example, that no till on heavy clay soils can cause large increases in nitrous oxide emissions that more than offset any benefits of enhanced carbon sinks. Loam soils in the same region do not show a similar increase in N2O emissions. Furthermore, once carbon content in soils has increased significantly, the soil sink begin to saturate. Hence, while such programs buy valuable time to develop alternative technologies for reducing fossil fuel greenhouse gas emissions, they are of limited value in the long term182,983,985-990.
Over the next century, enhancing carbon sinks in forests provides another important option for long term carbon sequestration. Since trees grow relatively slowly, the effectiveness of initiatives such as afforestation projects will be weak during the first decade or so after implementation. However, by 2050, large scale forest management programs around the world could realize additional emission offsets through sinks equivalent to about 0.31 GtC/year. In the United States, lands afforested in recent decades are already providing a significant sink (above and below ground) of 2 to 3 tC/ha/yr. Adding calcium to soils could further enhance this rate of sequestration. Canadian model studies indicate that harvesting and replanting stands of jack pine would initially create a large CO2 source immediately following harvesting but would provide a net average sink over an entire harvest-regrowth cycle under current climate conditions of about 0.14 tC/ha/year. Despite risks of increased water stress, this could increase to 0.25 tC/ha/yr under warmer climates983,991-992.

However, programs that convert agriculture or other non-forest lands to forests could have negative biophysical consequences that must also be considered in evaluating net benefits. In temperate and boreal latitudes, for example, reduced albedo caused by such land use change can cause a positive radiative forcing that is greater than the forcing reduction attributable to the carbon sink accumulating in the forest. A key factor in the reduced albedo is the replacement of snow covered fields in winter with lower albedo forest cover. While this is not the case in the tropics, similar land use change there would cause changes in atmospheric circulation that could reduce rainfall in north Africa and central Asia. Studies in California also suggest that fire suppression, which some argue is an effective means of enhancing forest carbon stocks, may in the long term reduce net stocks because of increased build-up of litter and dead biomass that result in very large wild fires that are much more difficult to control. It may also result in forest stands with smaller trees that store less net above-ground carbon then large trees. When these factors are considered, avoiding deforestation and promoting reforestation in order to reduce net radiative forcing makes more sense in the tropics than at mid to high latitudes179,180,993-997.

One of the Kyoto Protocol criteria for effectively including human induced carbon sinks in forests as emission offsets in national emission inventories is the ability to separate those sinks originating from human activities undertaken since 1990 from those accumulated due to earlier management activities and from sources and sinks occurring due to natural processes. An improved carbon budget model for Canadian forests is now available to do that, provided adequate supporting data are available. However, for the first reporting period, countries that seek to report human induced carbon sinks must still report total Net Biome Productivity, regardless of whether it is natural or human in origin. Model simulations suggest that, when all fluxes are considered, the Canadian forests were a net sink until 2001. However, multiple simulations for the first Kyoto Protocol reporting period of 2008-2012 indicate that risks of fire and insect disturbances may cause Canadian forests to revert to a source of carbon of between 30 and 245 Mt of CO2. Since this discourages Canada from including forest sinks in its emissions inventory, there is currently little incentive to pursue management strategies for enhancing related sinks998-999.
Finally, adding limestone in upwelling regions of oceans may also be an effective method to enhance the ocean carbon sink by up to 0.6 GtC/yr within 50 years. This would also help mitigate ocean acidification1000.
7.2.3 Carbon Capture and Storage
Another strategy for reducing carbon dioxide emissions is carbon capture and storage. The most common approach is to extract CO2 from industrial smokestack effluents, liquefy it and transport it to locations where it can be deposited into geological formations for indefinite storage. Most environmental groups now accept such capture and storage as a viable and necessary option for reducing risks of climate change, although it should not replace the primary objective of working towards a near zero-carbon emission future. Recent studies have also suggested other alternative methods of capture and storage. Some argue, for example, that there is a vast potential for injecting liquid CO2 into deep sea basalts in a manner that is safe, non-toxic to the environment and has a low risk of leakage. Alternatively, artificial enhancement of weathering of exposed surface peridotite in regions like Oman could help store carbon in land surfaces. Such weathering absorbs atmospheric CO2 as it transforms the peridotite into solid calcium carbonate, and could thus provide long term removal of CO2 from the atmosphere without the cost of extracting, liquefying and transporting CO2 from fossil fuel combustion. Creating biochar by heating biomass in the absence of oxygen and adding this to agricultural soils could also lock up carbon for long term storage, meanwhile improving soil structure and fertility. Such alternatives need to be further investigated for practical feasibility and net benefit to the environment and society1001-1006.

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