Managing Risks There is still considerable public confusion about the risks of climate change.
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).
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).
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).
A critical global temperature change threshold for avoiding dangerous changes in climate, of about 2°C above pre-industrial levels, is supported by a number of lines of evidence.
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).
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).
There are many options for reducing the risks of climate change, but their merits and liabilities need to be carefully assessed.
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).
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).
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).
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).
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).
Adaptation strategies need to be undertaken within the context of other global mitigation and development strategies.
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).
One challenge is the loss of traditional knowledge within the culture of northern people, which is slowly eroding their capacity to adapt (7.3).
1.0 INTRODUCTION This issue of the Atmospheric Science Assessment and Integration (ASAI) Report on New Climate Change Science provides a synthesis of some 1000 key scientific papers and reports relevant to climate change that have appeared within the international peer-reviewed literature during 2006 through 2008. (Previous such syntheses have been issued under the title of the CO2/Climate Report.) The preparation of this synthesis is part of an ongoing research literature review and assessment process undertaken by the Atmospheric Science Assessment and Integration (ASAI) group of Environment Canada’s Science and Technology Branch. Such assessments help keep Canadian scientists, industry stakeholders, policy makers and members of the general public apprised of new developments in the sciences related to atmospheric environment issues. As with past CO2/Climate Reports, this review is intended to be a concise summary of recent research highlights that builds upon the larger body of science extensively reported on in other comprehensive international assessment reports. For such comprehensive assessments, readers are referred to the three volumes of the Fourth Assessment Report of the Intergovernmental Panel on Climate Change1-3, released in 2007, as well as the 2008 Canadian assessment of impacts of climate change and adaptation options4.
In the interest of brevity and utility, this literature review for 2006-08 is based on a selection of key papers representative of the broad range of new contributions towards improved understanding of the science behind the climate change issue. The review is divided into six sections: past and projected changes in atmospheric composition; radiative forcing of the climate system caused by these changes; modeling the effect of changes in radiative forcing on the climate system; observations of past and current trends in various aspects of the climate system; assessing the potential impacts of future climate change on natural and socio-economic systems; and managing the risks of climate change. Since the report is intended to be a summary, it does not get into the details of the research involved. For such details, readers should consult the papers referenced in the relevant sections of the review. Undoubtedly, some important papers will have been missed. Any related annoyance to the authors of such papers or inconvenience to the reader is unintended.
2.1 Carbon Dioxide 2.1.1 Atmospheric Composition Various indicators of past climates, some extending back in time for many millions of years, consistently show a long history of strong correlation between past changes in global climate and the concentrations of greenhouse gases, particularly carbon dioxide (CO2), in the atmosphere. Data from Arctic Ocean sediments cores, for example, show that the intense warm period some 55 million years ago coincided with high atmospheric CO2 concentrations, while much cooler climates 10 million years later corresponded with low CO2 concentrations. However, the strongest paleo evidence for the important role of greenhouse gases as a major factor in past climate change has been obtained from ice cores extracted from the polar ice sheets. Such records from Antarctica now extend back 880,000 years. Oxygen and hydrogen isotope ratios obtained from the ice cores provide reliable proxies for regional air temperatures at the time the snow that eventually compressed into glacial ice actually fell. Meanwhile, air bubbles trapped within the ice provide an archive of concurrent atmospheric composition over time. The extended records show that eight interglacial-glacial cycles have occurred over the duration of the record, one about every 100,000 years. Superimposed upon these low frequency cycles are higher frequency fluctuations. Comparison of these temperature variations with the CO2 concentration record reveals a strong correlation between CO2 concentrations and climate. The lowest CO2 values of ~170 parts per million occur during the coldest parts of the glacial periods and the highest values of ~300 ppm occur during interglacial periods. These updated values suggest a slightly larger range in CO2 concentrations during glacial-interglacial cycles than estimated in previous studies5-8.
Figure 1. Annual increases in atmospheric CO2 concentrations (Data source: NOAA, ref. #15).
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 place9-13.
Ice core records also suggest that variations in atmospheric CO2 concentrations during the past millennium have been quite small– until the last two centuries. On the other hand, analyses of past variations in plant leaf stomata, which offer an alternate method of estimating past concentrations of CO2, suggest that values may have varied by as much as 34 ppm over the1000 – 1500 A.D. time period14.
In contrast to the above concentrations noted in paleo records, the atmospheric concentration of CO2 in 2007, averaged over a network of monitoring stations around the world, was about 384 ppm. This is approximately 0.5% above that for the preceding year, and 37% higher than pre-industrial levels. By the end of 2008, it had increased to 386 ppmv. Trends since 2000 indicate a current average annual increase of about 1.93 ppmv, which is about 30% more rapid than that observed in the 1990s. The Antarctic ice core data indicate that such CO2 concentrations have no precedence in at least the past 880,000 years15-16.
The primary reason for the acceleration in the rate of increase of atmospheric CO2 concentrations during the past decade or so is the rapid increase in CO2 emissions from fossil fuel combustion for energy and other industrial processes. Global emissions from this source rose from 6.75 billion metric tonnes of carbon (GtC) in 2000 to 7.98 GtC in 2005 - an annual rate of increase of 3.3% per year. This compares to average increases of 1.1% per year during the 1990s. While all geopolitical regions appear to be contributing to this increase, the largest contributors are now developing countries such as China that are experiencing major economic growth. During the 2000-2005 period, all countries who currently do not have emission control commitments under the Kyoto Protocol accounted for about 73% of the emission growth. Although emissions from land use change have remained relatively constant during this period, total anthropogenic emissions are now at the high end of the range of business-as-usual scenarios presented in the IPCC Special Report on Emission Scenarios (SRES) released in 1995. Furthermore, atmospheric concentrations have also been affected by a recent decline in land and ocean sinks. If these trends continue, CO2 concentrations may rise more rapidly than previously anticipated17-20.
2.1.2 Global carbon fluxes The natural global carbon cycle involves annual fluxes of carbon between land ecosystems, the oceans and the atmosphere that are more than an order of magnitude larger than that from human emissions of CO2. Fluctuations in this cycle significantly alter the global atmospheric concentrations of CO2 on seasonal, year to year and longer time scales. Hence, understanding the processes that govern this natural flow of carbon between the atmosphere and terrestrial and ocean reservoirs is necessary to both interpret current variations and trends in atmosphere concentrations of CO2 and to project future response of the global carbon cycle-climate system to human interference through greenhouse gas emissions and other activities. There are various tools available to researchers to help improve this understanding. Measurements from tall towers, for example, provide details on carbon fluxes between a local ecosystem and the atmosphere, while remote sensing by aircraft and satellite can help scale such information upwards to much larger regional and continental landscapes. Dynamic vegetation models that can simulate these fluxes by replicating the biogeochemical processes involved have also become more detailed and realistic. While these models do not do well in capturing year-to-year variability and episodic anomalies such as wild fire and insect outbreaks, they can be very useful, in conjunction with observed data, in investigating the likely causes of past changes in fluxes and projecting how these may change in the future. When combined, the range of monitoring and modeling tools now available can provide information on carbon fluxes on a region by region and continent by continent scale with increasing confidence21-24.
Global scale terrestrial fluxes of carbon. While they remain significant, the differences between estimates for fluxes derived from direct measurements and from model calculations are decreasing as both the quality of the measurements and the sophistication of the models improve. One recent analysis, based on both tall tower data and inverse atmospheric transport model calculations, estimated a net average global carbon sink over land areas for the 14 years between 1982 and 1995 of about 1.2 GtC/yr, with one third of this stored in Amazon ecosystems. Much of this sink is attributed to the effects of CO2 fertilization, with nitrogen fertilization and climate factors having secondary roles. Another inverse modeling study indicated a strong average Northern Hemisphere terrestrial sink of about 1.5 GtC/yr for the two decades between 1983 and 2003. However, for this longer study period, the tropics were a weak source that partially offset the large North American sink, resulting in a net average global terrestrial sink of only about 0.74 GtC/yr. Since land use changes such as deforestation cause large human-induced CO2 emissions from the tropics, a weak net source there still implies a large net natural sink that almost offsets the land use change emissions. There is also evidence that old forests continue to be significant sinks until disturbed, despite their mature state. A third analysis undertaken for the more recent period of 2000 to 2007, as part of the Global Carbon Project, also implied that average global natural carbon sinks of 2.6 GtC/year for this period were offset by deforestation and land degradation emissions of 1.5 GtC/year, for a net annual terrestrial carbon sink of 1.1 GtC. Almost all of the deforestation emissions come from South and Central America (41%), south and southeast Asia (43%) and Africa (17%). Experts note that these updated numbers for land sinks are sufficiently different from past estimates to suggest a need to re-examine some of the basic concepts of the global carbon cycle25-31.
Over North America, detailed results from model studies and data analysis of regional carbon fluxes indicate a large sinks in the regenerating hardwoods of the eastern United States, as well as a significant sink over coniferous forests (primarily Canadian boreal forests) and a lesser sink over Prairie grasslands and crop agricultural lands. These fluxes vary substantially from year to year. However, on average, Canadian forests and wetland soils have been a large carbon sink for most of the past century, with forest age, climate and CO2 fertilization and nitrogen deposition all having been significant factors. However, during the past two decades, losses from increased ecosystem respiration have largely offset gains from net primary productivity (NPP), and the role of Canadian forest ecosystems as sinks have virtually disappeared23,32-33.
Decaying permafrost is another active part of the global carbon budget that is expected to become of increasing significance in the future. In Siberia and the Canadian Arctic, these soils are estimated to contain 450 and 76 GtC, respectively. Much of this carbon could be released quickly once the soils thaw34-36.
Net Primary Productivity (NPP) is the metric generally used to describe the balance between vegetation uptake of carbon through photosynthesis and release through respiration. Satellite based indexes for greenness of vegetation, when compared with climatological and CO2 concentration data, suggest a strong positive feedback between terrestrial NPP and surface temperatures. They also imply a significant CO2 fertilization effect in mid to high latitudes. Both of these influences enhance biological productivity, particularly in the boreal forests at mid-latitudes of North America. In the tropics and subtropics, where precipitation has a dominant influence on NPP, there is much less evidence for a vegetation–temperature feedback. One reason for the increased NPP in response to rising temperatures in northern ecosystems is the earlier onset of photosynthetic activity in the spring and later cessation in the fall season. Since cool soil temperatures result in little increase in ecosystem respiration in spring, this results in enhanced CO2 uptake in the early part of the extended growing season. However, during the extended fall part of the season, the loss of carbon through increased respiration exceeds increased net primary productivity (NPP), resulting in a net loss of carbon that almost offsets the gains in the spring. Thus, the warmer climate of today appears to cause only a small increase in the net ecosystem productivity (NEP) across the entire season. If fall seasons warm more rapidly than spring seasons in future decades, a net decline in mid-latitude NEP could result because of the enhanced loss of carbon due to respiration, creating a positive feedback. Furthermore, reduced albedo associated with greater vegetation cover would also cause a regional positive feedback. Any concurrent increase of methane emissions from these ecosystems would further add to the positive feedback of northern ecosystems. Furthermore, indirect climate factors or other influences such as land use change, disturbances and natural succession processes would affect NEP and further weaken its relationship to climate variations and change. Not surprisingly, all coupled carbon cycle-climate model studies appear to provide supporting evidence for a strong positive carbon cycle feedback that results in reduced NEP under warmer climates over the next century. Uncertainty about the role of soils and other factors result in significant disagreement in model estimates of the magnitude of this feedback. However, ice core evidence of past climate variations suggest the carbon cycle feedback could result in an increase in atmospheric CO2 of as much as 20 to 60 ppm per degree of warming. This exceeds estimates from most models37--48.
Various research studies have explored other factors that may be important in the variations of terrestrial carbon fluxes over time and from one region to another. Leading factors include:
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 sinks49-52.
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 warming51,53-54.
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 fertilization56-60.
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.
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 uncertain62-64.