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2009 Literature Review Archives - Radiative Forcing

Benestad, R.E. and G.A. Schmidt, 2009, Solar trends and global warming. Journal of Geophysical Research, Vol, 114, D14101, doi: 10.1029/2008JD011639.
A new methodological paper is critical of previous estimates of a large contribution of solar forcing to recent global warming based on linear analytical methods. This work concludes that solar forcing most likely accounts for 7 ± 1% of 20th century warming.
Benestad and Schmidt (2000) explore methodologies used to detect and attribute the contribution of solar forcing to global warming over the 20th century. To accomplish this, they test the ability of different statistical techniques to correctly quantify solar forcing in GCM (GISS ModelE GCM) simulations of 20th century climate that have been generated with known forcing contributions. Similar analyses are applied to the observed temperature record. Their results demonstrate that, in the presence of internal climate variability and multiple colinear forcings, linear analytical methods (such as regression) yield non-robust results. The authors are also critical of previous studies by scientists Scafetta and West that have yielded high estimates of the solar contribution to global warming (50-69% since 1900) based on such methods. As such, the authors repeat their analyses and evaluate their robustness using modeled and observed data. The authors conclude that the linear approach of Scafetta and West is subject to the problems noted above and suggest that the solar contribution to global warming is unlikely to have been larger than 7 ± 1% (0.1-0.2°C) over the 20th century and is negligible for warming since 1980.

Erlykin, A.D., Sloan, T., and Wolfendale, A.W. 2009. Solar activity and the mean global temperature. Environ. Research Letters 4 (2009) 014006.
New study provides support for a minor role for solar forcing in global temperature changes of the past half-century, but rules out cosmic radiation as a factor.
Several past studies have suggested that changes in cosmic radiation caused by variations in solar behavior may have affected ion densities in the Earth's atmosphere, which in turn influenced cloud properties and hence the Earth's climate. However, others have provided contradicting evidence that indicates changes in cosmic radiation have not been a significant factor in cloud behaviour and climate during at least the past half-century. A team of British scientists have now added to that evidence by carefully analyzing the variations in cosmic radiation and atmospheric ionization rates since 1956 and comparing these to climate behavior. They find a cycle in the cosmic radiation-ionization rates of about 22 years, but also note that this cycle lags by several years similar cycles for changes in both direct solar radation intensity and global temperatures. Thus, while the temperature does appear to respond to variations in direct solar radiation, it is unlikely to be linked to cosmic radiation effects. They further conclude from their analysis that about 14% of the observed warming of the past half-century can be attributed to solar radiation effects. Given that volcanic aerosol forcings during this same period have caused a cooling effect, it remains clear that the dominate cause of recent climate warming has been human in origin.

Myhre, G., 2009. Consistency Between Satellite-Derived and Modeled Estimates of the Direct Aerosol Effect, Science 325, 187; DOI: 10.1126/science.1174461.
A new study finds that direct aerosol effect is slightly weaker than that estimated in the IPCC-AR4, because increases in black carbon have partially offset the cooling effect of the overall increase of anthropogenic aerosols.
The direct radiative forcing effect of aerosols includes both the scattering (cooling) and absorption (warming) of short and long wave radiation. In the IPCC AR4, the direct aerosol effect between pre-industrial and present time was estimated at -0.5 W m-2, (i.e. a cooling effect) with an uncertainty range from -0.9 to -0.1 W m-2. In this new study, the author used updated satellite data of aerosol optical depth, and a global aerosol model that includes all the main atmospheric aerosol components, to quantify the direct aerosol effect and compare model and observation-based approaches. Both approaches for determining the direct radiative forcing indicate very close agreement and produce a value that is weaker than in earlier studies. The author attributes this difference primarily to absorbing black carbon aerosols (which have a climate warming effect) and notes that emissions of these aerosols have increased by more than a factor of six between pre-industrial and present time, whereas emissions of all anthropogenic aerosols have increased by only a factor of three to four. Consequently, the warming effect of the large increase in black carbon aerosols has offset some of the cooling effect of other anthropogenic aerosols. The best estimate from this study (-0.3 ± 0.2W m-2) suggests that the direct aerosol effect offsets only a modest 10% of the radiative forcing that is due to increases in well-mixed greenhouse-gas concentrations at their current concentrations.

Shindell, D.T. et al., 2009, Improved Attribution of Climate Forcing to Emissions, Science, 326: 716-718.
Taking gas-aerosol interactions into account amplifies the warming effect of shorter-lived greenhouse gases such as methane.
Understanding the role of short- and medium-lived climate forcers in global warming is an active area of research. What makes this difficult is the complexity of the atmospheric processes and chemistry involved such that changes in the emissions of one substance can indirectly effect the atmospheric concentration of a number of other substances. In this paper, Shindell and colleagues look in particular at how gas-aerosol interactions can affect the lifetime and warming impact of various atmospheric species. They used the GISS coupled composition-climate model to calculate atmospheric composition changes, historical radiative forcing, and forcing per unit of emission due to aerosol and tropospheric ozone precursor emissions. Their results show that emissions of NOx, CO and methane have substantial impacts on aerosol concentrations by altering the abundance of oxidants, especially hydroxyl, which convert SO2 into sulfate. Since both methane and CO reacts with hydroxyl (OH) and reduce the availability of this oxidant, increases in their emissions slow the formation of cooling sulphate aerosols. Including this indirect methane effect leads to an increase in methane's 100 year GWP of about 10% (and about 20 to 40% when you consider the aerosols indirect effects on clouds) compared to previous estimates. Similarly, the GWP for CO is also higher when gas-aerosol interactions are taken into account, than when these interactions are neglected. In the case of NOx, interactions with aerosols lead to an even greater cooling effect. As GWPs are used as the basis for evaluating multi-gas mitigation strategies, even small changes in GWPs could have large implications for optimal emission reduction strategies.

Tong, J.A., You, Y., Müller, R.D. and Seton, M. 2009. Climate model sensitivity to atmospheric co2 concentrations for the middle Micoene. Global and Planetary Change 67:129-140.; Mills, T.C. 2009. How robust is the long-run relationship between temperature and radiative forcing? Climatic Change 94:351-361.
Two new studies, one based on past climate behavior millions of years ago and the other comparing climate and radiative forcing trends during the past 150 years, suggest a global warming in response to a doubling of co2 concentrations on the order of 2°C. This is within, but at the lower end of the large range of estimates provided by past studies.
There has been long standing debate about how sensitive the global climate system is to rising greenhouse gas concentrations. Most estimates indicate at least a 1.5°C equilibrium warming for a doubling of carbon dioxide concentrations, while some suggest it could be 7°C or higher. Two recent studies have added to the debate. In one of these, a team of Australian modelers investigate the response of the earth's climate to changes in carbon dioxide some 15 million years ago. Their results suggest a climate sensitivity during that period of time of about 2.2°C per doubling of co2. In the second analysis, a British economist uses a statistical comparison of changes in temperatures over the past 150 years with concurrent estimated changes in radiative forcing. Assuming that net radiative forcing from multiple causes has the same effect on global climate as that due to co2 alone, he concludes that global climate sensitivity is likely in the range of 1 to 3°C per co2 doubling. Both of these estimates are within the range of past estimates, but at the lower end of the range.

Trenberth, K.E. and Fasullo, J.T. 2009. Global warming due to increasing absorbed solar radiation. GRL 36, L07706, doi:10.1029/GL037527, 2009.
Reduced global cloud cover is projected with climate warming. This will both reduce the amount of sunlight reflected back to space, and allow more heat energy to escape to space. This study suggests that the impact of these feedbacks will enhance the role of net absorption of solar radiation within the climate system relative to that of reduced loss of long wave heat radiation to space.
It is often assumed that, during the next century, the changes in climate will be primarily due to reduced heat loss to space from the top of the Earth's atmosphere due to rising greenhouse gas concentrations. A new study indicates, however, that the net changes in radiative fluxes at the top of the atmosphere will be more complex than that. An evaluation of simulations by climate models used for the IPCC Fourth Assessment report shows that, as the climate warms due to enhanced greenhouse gas concentrations, global cloud cover will likely decrease. Since clouds also absorb outgoing heat radiation, this allows more heat radiation to escape than if cloud cover remains constant. However, reduced cloud cover also reduces the amount of solar radiation reflected back to space, allowing more sunlight to be absorbed within the climate system. Thus a significant and perhaps dominant part of the net warming arises from the feedback caused by reduced cloud cover and the related increase in net short wave solar radiation forcing at the top of the atmosphere. This underscores the importance of improved modelling of cloud behaviour.

G.J.M. Velders, D.W. Fahey, J.S. Daniel, M. McFarland, and S.O. Andersen. 2009. The large contribution of projected HFC emissions to future climate forcing. PNAS, doi: 10.1073.
Study estimates that global HFC emissions will significantly exceed previous estimates. This will contribute a radiative forcing equivalent to 6 - 13 years of CO2 emissions by 2050.
Hydrofluorocarbons (HFCs) are a replacement gas for CFCs and HCFCs that are being phased out under the Montreal Protocol to the Vienna Convention for the Protection of the Ozone Layer. This study projects that the consumption and emissions of HFCs will increase significantly, especially in the developing world, as a result of compliance with the control measures of the Montreal Protocol and the increased demand for refrigeration, air-conditioning, and insulating foam products. The projections are based on new baseline scenarios out to 2050. The new global HFC emission projections are significantly higher than previous estimates especially after 2025 in developing countries. By 2050, the emissions are equivalent to 9-19% (CO2 eq. basis) of the projected global CO2 emissions in a business-as-usual (BAU scenario and contribute a radiative forcing in the range of 0.25-0.40W/M2, which is about a factor of 3 larger than previous estimates. By 2050, the HFC radiative forcing fraction is projected to be 7-12% of that for CO2. Several options for mitigation of HFC emissions are discussed, from reducing HFC emissions under a climate treaty to addressing concerns under the Montreal Protocol.

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