Table of contents key findings

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  There is little doubt among researchers 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 unabated climate change. However, the same level of confidence appears not yet to have penetrated public opinion, particularly in North America (7.1).
  
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  Communication experts argue that scientists need to be more pro-active in communicating information to lay audiences in a manner that is clear, relevant, credible and objective. Information about personal risk is shown to be an effective communication approach (7.1).
  
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  For lay audiences, presenting climate change scenarios as plausible alternative futures may be preferable to using probability language when trying to convey the possibility of high impact surprise scenarios (7.1).
  

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  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 (7.1).
  
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  Alternative emission paths to achieve CO₂ stabilization targets appear to have similar cumulative emissions (i.e. total ‘emissions budgets’), regardless of the path chosen. However, delaying the time when global emissions peak and then begin to decline will increase the magnitude of emission reductions subsequently required (7.1).
  

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  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. However, these are unlikely to keep global temperature change below critical danger thresholds if global population and economies grow rapidly (7.2.1).
  
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  Bioenergy can have a major role to play in mitigative strategies to reduce CO₂ emissions from fossil fuel combustion; however, the total life-cycle emissions of all GHGs as well as other environmental and social impacts should be assessed when evaluating alternative fuel options (7.2.1).
  
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  Programs that convert agriculture or other non-forest lands to forests as a means of enhancing carbon sinks could have some negative biophysical consequences that should also be considered in evaluating net benefits (7.2.2).
  
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  CO₂ capture and storage is increasingly seen 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 (7.2.3).
  
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  Some prominent research scientists are now recommending that the technical feasibility and scientific implications of geoengineering solutions, such as injection of sulphates into the stratosphere and ocean iron fertilization, be properly investigated in case an emergency escape mechanism is needed in the future to stave off global disaster (7.2.4).
  

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  Numerous recent studies have drawn attention to the need to ensure that development of strategies to adapt to climate change should be integrated into, or at least undertaken as a complement to local, national and international agendas aimed at mitigating the risks of climate change. Furthermore, both should be an inherent component of broader sustainable development policies (7.3).
  
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  One challenge is the loss of traditional knowledge within the culture of northern people, which is slowly eroding their capacity to adapt (7.3).
  

2. 
   ATMOSPHERIC COMPOSITION
   

It is now generally accepted that the primary drivers of these interglacial-glacial cycles are the periodic changes in the Earth’s orbital patterns around the sun that can significantly alter seasonal solar radiation levels received at different latitudes of the Earth’s outer atmosphere. However, although with a lag period of multiple centuries to a few millennia, the close CO₂-climate correlation demonstrates that variations in CO₂ concentrations in response to the initial climate changes triggered by orbital factors provide a strong positive feedback that significantly amplifies the initial change. One plausible explanation for this feedback process is that, during the onset of glaciation, reduced summer solar insolation in the Northern Hemisphere causes a change in global ocean circulation that reduces vertical mixing in the Southern Ocean. This in turn reduces the outgassing of CO₂ from the Southern Ocean, causing atmospheric CO₂ concentrations to decrease. The build-up of ice sheets on land during the glaciation process also causes ocean levels to drop, exposing continental shelves. These exposed sea bottoms provide added nutrients to support ocean biological productivity and thus increase the rate of CO₂ uptake across all oceans. Increased winds during cold, dry periods of the climate cycles may enhance the transport of mineral dust from land surfaces and hence the iron deposition into the oceans as well, further increasing biological activity and CO₂ uptake. At end of the glacial period, the reverse process takes place^9-13.

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  Land use change. In addition to the direct effect of deforestation and forest harvesting activities on emissions of CO₂ into the atmosphere, afforestation and reforestation activities remove CO₂ from the atmosphere through decades of positive NEP while these forests are growing. The impact of land use change on current regional land sinks is particularly important in the eastern United States and western Europe. Eventually, as the forests mature, this factor will become less significant. However, old stands of forest continue to accumulate carbon in their soils, and can thus function as a weak net sink beyond maturation. Furthermore, recent studies also support arguments that, in mid to high latitudes, the positive radiative forcing effects of reduced surface albedo associated with reforestation may more than offset any climatic benefits of reduced atmospheric CO₂ concentrations resulting from these carbon sinks^49-52.
  
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  Effects of wildfire. Global CO₂ emissions due to wildfires have increased by more than 50% in recent decades. The initial effect of such fires is a large pulse of emitted CO₂ and, in low latitudes, reduced surface albedo that causes a strong local positive radiative forcing. However, the CO₂ uptake during subsequent regrowth (as well as rising albedo for low latitude regrowth) gradually changes this to a negative radiative forcing. Furthermore, some of the burned biomass is converted to black carbon, which decays much more slowly that other forms of soil carbon and thus serves as a long term sink. Hence long term effects of such fires, if followed by regrowth, may reduce climate warming^51,53-54.
  
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  Surface ozone. Increased surface ozone concentrations reduce biological growth⁵⁵.
  
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  CO₂ fertilization. While recent studies support the conclusion that much of the increased terrestrial sink in recent decades can be attributed to the direct effects of rising CO₂ concentrations, they also suggest that earlier estimates for the magnitude of this effect may be too high. Experimental studies, for example, show that NEP gains for trees grown under elevated CO₂ are short lived, and that net long term fertilization effects on trees may be small. In fact, if the fertilization affects species composition within forest ecosystems by favouring less productive species, the net impact on related NEP could be negative. While somewhat compensated for by higher C:N ratios in plant litter, limitations in nitrogen supply to plants also constrain the CO₂ fertilization effect. Furthermore, reduced land surface albedo associated with increased biomass generated through CO₂ fertilization has a positive radiative effect on climate that could offset as much as 30% of the negative climate feedback attributable to the CO₂ fertilization^56-60.
  
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  Nitrogen fertilization. Nitrogen deposition enhances biological growth (unless it is intense enough to cause major acidity problems). This appears to be an important factor in European forest sinks⁶¹.
  
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  Soil erosion. Some researchers argue that soil erosion reduces fertility and enhances oxidation processes within soils, causing increased CO₂ emissions. Others suggest that erosion often happens through heavy precipitation events that cause gullies and displace soils but do not deteriorate them. Hence, net related effects on the global carbon budget are as yet uncertain^62-64.