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2011 Literature Review Archives - Policy/Mitigation

Arora, V. K., and A. Montenegro. 2011. Small temperature benefits provided by realistic afforestation efforts. Nature Geoscience. Published online June 19, 2011; doi:10.1038/ngeo1182.

A study exploring the benefits of afforestation for reducing climate warming shows that planting trees is a viable climate change mitigation strategy. However, the temperature benefits of plausible afforestation schemes are limited when compared to the effects of ongoing human emissions of greenhouse gases and therefore cannot replace the need for very substantial emission reductions in order to limit the magnitude of global warming.

Afforestation involves the conversion of non-forested land to forest. The objective of afforestation is to enhance the uptake of CO2 from the atmosphere by trees and other vegetation through photosynthesis, and thereby contribute to lowering atmospheric CO2 levels and reducing anthropogenic warming.  A study by an Environment Canada scientist and a Canadian university colleague sheds some light on the potential size of that contribution.  At issue is not only the amount of CO2 that afforestation can remove from the atmosphere but also the extent to which this benefit may be offset by the impact that changing the land surface has on aspects of the climate system. For example, forests are less reflective than croplands, and even more so when covered in snow, and so afforestation, by increasing the absorption of sunlight, can contribute to climate warming.  The authors used a comprehensive Earth System Model to test various scenarios for afforestation within a standard increasing greenhouse gas emissions scenario (IPCC SRES A2).  Crop area was converted to forest cover over a 50-year period between now and 2060.  All scenarios considered resulted in lower atmospheric CO2 concentrations than in a scenario without afforestation; however, the affect this has on surface temperatures appears to be rather limited, with at most about half a degree of avoided warming by the end of the end of the century if all crop land globally could be forested (an admittedly unrealistic scenario but one to which more plausible scenarios can be compared). From scenarios that explored the effects of afforestation in different latitudinal bands, they find that tropical afforestation is about three times more effective than in northern temperate and boreal regions. This result arises because in tropical regions, both the biogeochemical processes (related to CO2 uptake) and biogeophysical processes (related to changing land surface) act to reduce warming while in more northerly regions, the biogeophysical processes act to offset some of the benefits of CO2 uptake. The results indicate that afforestation could be effective as part of a comprehensive climate change mitigation strategy where each individual measure makes a contribution to reducing global warming, but given the modest benefits simulated, it also cautions that planting trees will not reduce the need for major reductions in greenhouse gas emissions.

Peters, G.P, J.C. Minx, C.L. Weber, and O. Edenhofer. 2011. Growth in emission transfers via international trade from 1990-2008 PNAS, Vol 109, pp 8903-8908, DOI: 10.1073/pnas.1006388108.

A new accounting of international CO2 transfers indicates that developed nations have collectively increased their import of goods and services from developing nations; this indicates that CO2 emissions associated with production have been shifted elsewhere.  When CO2 emissions are tied to these imports, then the overall net transfer of emissions via international trade from developing to developed nations exceed territorial emissions reductions under the Kyoto Protocol commitments in developed nations.

Under the Kyoto Protocol to the United Nations Framework Convention on Climate Change (UNFCCC) mitigation policy applies only to greenhouse gas emissions and removals within the national territory or offshore in areas under the country’s jurisdiction.  This territorial-based approach does not consider transfers of emissions between nations (particularly developing and developed nations) and may lead to a misleading interpretation of factors driving emission trends and therefore mitigation policies.  Peters and colleagues developed a trade-linked global inventory (based on 113 regions and 57 economic sectors) of CO2 emissions through time (1990-2008).  They use this annual time series to (1) explore the role of international trade in national, regional and global emissions trends, and (2) determine the extent to which international trade may have stabilized emissions in developed countries.  The authors find that, globally, the proportion of emissions from the production of exported products increased from 20% to 26% (or 4.3 to 7.8 Gt CO2) and this was accompanied by large regional shifts in the location of emissions production and consumption.  Annual net emission transfers (tied to exported goods and services) from developing to developed countries have increased from 0.4 Gt CO2 in 1990 to 1.6 Gt CO2 in 2008.  Averaged over the 1990-2008 period, this emissions transfer exceeds the average emission reduction target (5% of 1990 levels or ~0.7 Gt CO2 per year) for developed nations under the Kyoto protocol by 18%.  If a consumption-based inventory is used (i.e. transfers via international trade are considered) 11% of the increase in global CO2 emissions over the period 1990-2008 can be attributed to consumption in developed nations. If only the territorial emissions are considered over this period (as in UNFCCC accounting), developed nations showed a 3% reduction.  The authors conclude that the growing spatial disconnect between where emissions are produced and consumed could have unintended consequences for climate policy and eventually make mitigation in developing nations more costly.

Rogelj, J., W. Hare, C. Chen and M. Meinshausen. 2011. Discrepancies in historical emissions point to a wider 2020 gap between 2°C benchmarks and aggregated national mitigation pledges. Environmental Research Letters 6, 024002 (9pp).

An adjustment to estimates of historical emission levels, from which committed pledges of emission reductions under the Copenhagen Accord are calculated, increases the size of the so-called ‘emissions gap’ in the year 2020. This gap represents the difference between projected emissions based on pledged reductions and the estimated emission levels compatible with limiting global warming to 2°C above pre-industrial.

There have been a number of studies assessing whether projected greenhouse gas (GHG) emission levels in 2020, taking into account commitments by Parties to the UNFCCC to reduce emissions, keep the world ‘on track’ to limit global warming to 2°C above pre-industrial. Rogelj and colleagues shows that at least some of those assessments (many of which used the IPCC Special Report on Emissions Scenarios (SRES) database to estimate historical emissions) may have been overly optimistic due to overestimates of historical emissions. Since committed emission reductions are generally expressed as a percentage of emissions in a reference year (i.e. x% of emissions in year Y), then if emissions in that reference year are overestimated, so will the emission reduction pledges be. The authors develop three composite historical global emissions paths based on data reported officially by countries to the UNFCCC along with various combinations of data from other inventory sources using the Potsdam Real-time Integrated Model for the Probabilistic Assessment of Emission Paths (PRIMAP). The IPCC SRES year 2000 emission values were compared to the three composite emission paths and significant discrepancies were found. For global total anthropogenic emissions, excluding land-use related emissions, this discrepancy is of the order of 5-6%, with SRES emissions higher than the composite estimate based on officially reported data. The authors present three different methods for resolving this discrepancy and evaluate how the ‘emissions gap’ is impacted with reference to a 44 GtCO2eq benchmark considered ‘on track’ with the 2°C target. Instead of an estimated 3.4 to 9.2 GtCO2eq shortfall in emission reductions by 2020, the actual gap may be as high as 5.4 to 12.5 GtCO2eq (a 22-88% increase in the size of the gap), with the range reflecting different assumptions about fulfillment of commitments.