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2006 - 2008 Literature Review Archives - Radiative Forcing
Foukal, P., Frohlich, C., Spruit, H. and Wigley, T.M.L. 2006. Variations in solar luminosity and their effect on the Earth's climate. Nature 443:161-166; Scafetta, N. and West, B.J. 2006. Phenomenological solar contribution to the 1900-2000 global surface warming. GRL 33, L05708, doi:10.1029/2005GL025539; Clark, S. 2006. Saved by the sun. New Scientist, 16 Sept:32-36.
Several recent articles in science journals are a reminder that the debate about the role of variability in incoming solar radiation on the global climate system is far from over.
In one of these articles (Foukal et al.), several experts in solar physics speculate on what kind of solar dynamical processes might affect total solar irradiance (TSI) of the Earth, and hence Earth's climate. They conclude that more than 80% of TSI variability can be explained by changes in the area of the sun's surface covered by sunspots (reduced radiation output) and bright faculae (increased radiation output). In agreement with most other related studies, they suggest (using satellite measurements of TSI) that solar variability has not been a significant factor in climate change during the past three decades. However, in contrast to other past studies, they also use a limited TSI driven dynamic climate model to suggest that solar forcing may have had little net effect on climate for the past three centuries. They acknowledge, however, that there may be other solar-climate forcing, such as changes in UV, and of magnetized plasmas, that are not addressed in their model.
Authors of another article (Scafetta and West) report on results of a study using an empirical approach to analyzing TSI-climate relationships for the pre-industrial period between 1600 and 1900 AD. They assume that all trends in climate during that period were caused by direct and indirect influences of changes in TSI. Using this relationship as a predictor for the 20th century, they suggest that as much as 50% of the warming observed since 1900 can be explained by solar variability. This conclusion contrasts significantly with that by Foukal et al. Scafetta and West also agree, however, that most of the warming during the past 30 years must be anthropogenic in origin.
Finally, an news article in New Scientist reports on recent projections by several European solar experts that the unusually high level of sunspot activity observed in recent decades may begin to decrease fairly quickly again in the near future. Similar reduction events have happened in the past at 200 year intervals, with varying intensities and durations. The magnitude of projected cooling during such an event is modest (a total of about 0.2 C) relative to the warming effects of rising greenhouse gas concentrations (about 0.2 to 0.3 C each decade). The author cautions that a temporary weakening or cessation of the current warming trend caused by such a solar cooling event could lull us into a false sense of well-being and weaken global resolve to reduce greenhouse gas emissions. However, he also notes that the science behind such projections for cooling is as yet very speculative.
Joos, F. and R. Spahni. 2008. Rates of change in natural and anthropogenic radiative forcing over the past 20,000 years. PNAS Vol. 105 No. 5 pp 1425-1430.
Antarctic ice core experts have now reconstructed rates of change in GHG concentrations and in radiative forcing for the past 20,000 years. The results indicate that the 20th century rise in the combined radiative forcing from the three main greenhouse gases (CO2, CH4 and N2O) is exceptionally high within this record. Over the past millennium, anthropogenic radiative forcing is unusually high compared to natural forcing.
An important aspect of anthropogenic climate change is its rate of change, since it helps determine how well systems (both natural and human) can adapt. This study by Joos and Spahni compares rates of increase in anthropogenic forcing with rates of change in natural greenhouse gas forcing since the Last Glacial Maximum, as well as of solar and volcanic forcing of the last millennium. For century-scale rates of change, the authors conclude that the average rate of increase in the combined radiative forcing from CO2, CH4 and N2O during the Industrial Era, is larger than at any time during at least the past 16,000 years. In fact, radiative forcing due to the 20th century increase in CO2 concentrations alone rose more than an order of magnitude faster than any multi-century scale change during the past 22,000 years. (For comparison, the IPCC AR4 concluded that the rate of increase in the combined radiative forcing from CO2, CH4 and N2O was very likely unprecedented in more than 10,000 years). Decadal-scale rates were computed for the past 2,000 years from Antarctic Law Dome ice cores and from direct atmospheric measurements. The recent average rate of increase in the combined radiative forcing by all three greenhouse gases was at least six times larger than at any time during the period 1-1800 AD. Rates of anthropogenic forcing were then compared to other reconstructions of natural forcings over the past millennium, and were also found to be much higher.
Kaufman, Yoram J. and Ian Koren, July 13, 2006. Smoke and Pollution Aerosol Effect on Cloud Cover. Science express DOI: 10.1126/science.1126232
In addition to their radiative properties, aerosol particles from urban-industrial pollution and smoke from fires have been shown to affect cloud microphysics, cloud reflection of sunlight and the onset of precipitation. This study examines the relationship between aerosols and cloud cover by using aerosol absorption of sunlight as a signature for aerosol effects on cloud cover. This data came from an international network of sunphotometers (AERONET), rather than from satellite analysis, which can be affected by cloud artifacts. AERONET measures the aerosol attenuation of sunlight in cloud free conditions, and thus repeated measurements can be used, along with the aerosol optical thickness, to estimate cloud cover. The study was restricted to areas of minimal dust interference to focus on pollution and smoke effects. The authors used their data to derive a mathematical relationship between aerosol optical depth and cloud cover, which they then tested to see if it was a true relationship, or else just a correlation caused by some variation in the meteorological field. The same relationship was found in different locations and at different seasons, and for both smoke and pollution aerosols. A significant impact of total precipitable water vapour on this relationship was found, explaining about 25% of the effect. A "back of the envelope" calculation was also done and showed that this aerosol effect could have a profound effect on the hydrological cycle and climate.
Lubin, D. and Vogelmann, A.M. 2006. A climatologically significant aerosol longwave indirect effect in the Arctic. Nature 439:453-456.
For many years, aerosol experts have argued that rising concentrations of aerosols in the atmosphere induce an indirect radiative forcing through altered cloud properties that is strongly negative. However, in the above paper, American scientists Lubin and Vogelmann note that the reverse is true in Arctic regions. Using a new data set of radiation measurements along the North Slope of Alaska, they show that changes in properties of frequently occurring cloud types in the region caused by enhanced aerosol concentrations can increase surface longwave radiation fluxes by an average +3.4 watts per square meter. This strong positive effect is comparable to that from enhanced greenhouse gas concentrations, and may therefore be a significant factor in observed climate changes in the region.
Mazzarella, A. 2007. The 60-year solar modulation of global air temperature: the Earth's rotation and atmosphere circulation connection. Theoretical and Applied Climatology 88:193-199.
In a recent paper published in an international climate journal, an Italian researcher (Mazzarella) proposes a 60-year low frequency variability in global surface temperatures that may be linked to the Earth's geomagnetic activity and rate of rotation, and therefore predictable. He projects a slow global cooling between today and 2030 due to this oscillation, everything else being equal.
It should be noted that such variability is just one aspect of background noise within the climate system, and is small relative to the predicted effect of greenhouse gas emissions over future decades. He bases his arguments for the 60 year cycle on his analysis of long term variability of various parameters of Sun-Earth behaviour, including solar winds, earth's rotation rate, geomagnetic activity, zonal activity and global air temperatures. When the related data are filtered to remove variability of less than 23 year periodicity, a sixty year cycle becomes apparent in each of the data sets. He then uses a Sun-atmosphere-earth system model to simulate relationships between these variables, then uses this model as a predictor of long term surface temperature variability.
McConnell, J.R., R. Edwards, G.L. Kok, et al. 2007. 20th-Century industrial black carbon emissions altered Arctic climate forcing. Science Express, 9 August 2007, 7 pages.
Measurements from ice cores in Greenland show that black carbon (soot) from forest fires and from industrial activities in North America and Asia has had a strong influence on Arctic climate since 1850.
Black carbon aerosols (or soot) are known to have a warming effect on the atmosphere and, upon deposition, to decrease the albedo of snowy/icy surfaces, allowing these to absorb more energy from the Sun, accelerating melting. However, little is known about black carbon emission and deposition histories. A new study reports on how measurements in two central Greenland ice cores have allowed scientists to assess both the origin and the climate forcing of black carbon (BC) in the Arctic over the period 1788 to 2002. The authors found that, up to 1850, winter concentrations remained quite low and stable, while summer concentrations were highly variable and correlated with biomass burning from forest fires. After 1850, coinciding with the beginning of widespread burning of coal, soot levels started to rise, especially during winter. Concentrations peaked around 1908, varied but stayed quite high until 1951, then declined gradually the last 50 years although concentrations remained higher than before 1850. For the same period from 1850 to 2002, forest fires remained a significant source of BC during summer, but the main contribution all year-round was from industrial emissions. Using results from air mass back-trajectory modeling, the authors were able to establish that from 1850-1951 most of the industrial BC deposited in central Greenland precipitation likely came from North America while, since 1951, Asia may be the primary source. The authors also calculated the effect of measured soot concentrations at the two Greenland sites on absorbed solar radiation (i.e. the radiative forcing) for early summer - the period of highest sensitivity to black carbon deposition. They then estimate the impact of the measured changes in BC throughout the Arctic region, extrapolating the results from the ice core site, using global model simulations of radiative forcing from BC. Their results show that the average Arctic warming effect from soot has been 1.13 W/m2 from 1850-1951, peaking at 3.2 W/m2 in the 1906-1910 period - eight times the typical forcing prior to industrialization. For the period post-1951, during which BC concentrations began to decline, the warming effect was 0.59 W/m2. These results provide evidence of an anthropogenic component to the strong Arctic warming that occurred in the early 20th century.
Myhre, G., Nilsen, J.S., Gulstad, L. et al. (2006) Radiative forcing due to stratospheric water vapour from CH4. Geophysical Research Letters, vol 34, L01807.
New estimates of stratospheric water vapour contribution to the methane Global Warming Potential.
The authors of this study used the vertical profiles of methane, obtained from HALOE satellite data, to calculate the rate of oxidation of methane to water in the stratosphere. Their calculation of the radiative forcing for this indirect greenhouse effect of methane was estimated to be slightly weaker than 0.1 w/m2. However, this is 15-20% of the direct radiative forcing due to methane. The authors indicate that this water vapour contribution to the methane global warming potential (GWP) is larger by 10-15% than the 5% used in the 2001 IPCC Assessment. The study also found that when methane is oxidized to water in the stratosphere it results in a different temperature profile in the stratosphere compared to that expected for a uniform vertical distribution of stratospheric water vapour (SWV). This difference is important for the attribution of stratospheric water vapour and temperature trends.
Perovich, D.K., J.A. Richter-Menge, K.F. Jones, and B. Light. 2008. Sunlight, water, and ice: Extreme Arctic sea ice melt during the summer of 2007. Geophys. Res. Lett., 35, L11501, doi:10.1029/2008GL034007. Schweiger, A. J., J. Zhang, R. W. Lindsay, and M. Steele. 2008. Did unusually sunny skies help drive the record sea ice minimum of 2007?Geophys. Res. Lett., 35, L10503, doi:10.1029/2008GL033463.
Reductions in aerosol concentrations over mainland Europe since 1980 have contributed significantly to recent warming in that region. However, on a global scale, rising aerosol emissions in developing regions have more than offset reductions in North America and Europe, resulting in a small net aerosol cooling effect. This aerosol cooling effect is likely to further increase during the next few decades.
Two recently published scientific reports provide new results on the extent to which past and future changes in aerosol concentrations may contribute to regional and global climate forcing. In one of these, published in the journal Geophysical Research Letters, Ruckstuhl and a team of European colleagues analyze past trends in aerosol optical depth measurements taken at a large number of sites in Northern Germany and Switzerland. They conclude that aerosol concentrations in the region have declined by up to 60% since 1986, resulting in an increased total regional solar irradiance by about 1 W/m2 per decade. Most of this increase is due to the direct aerosol effect on incoming sunlight (rather than indirect effects due to changed cloud properties). They suggest that this solar 'brightening' has been a significant factor in the large (~1°C) warming seen over mainland Europe since 1980. In the second study, published in Environmental Research Letters, a group of American scientists led by S. Menon use the GISS climate model coupled to an aerosol source and transport model to assess the total global scale radiative effects (direct and indirect) of past and future changes in estimated concentrations and distributions of anthropogenic and natural aerosols. Indirect effects include those due to chemical processes affecting other greenhouse gases as well as those involving clouds. Between 1980 and 1995, radiative cooling effects of increasing concentrations of aerosols over Asia more than offset the warming effect caused by decreasing concentrations over North America and Europe. The net global cooling effect was about -0.1 W/m2. By 2030, this cooling effect is projected to increase by another -0.94 W/m2. They note that the related radiative/climatic impacts are particularly significant and anomalous for Arctic and Asian regions.
Prathur, M.J. and Hsu, J. 2008. NF3, the greenhouse gas missing from Kyoto. GRL 35, L12810, doi:10.1029/2008GL034542, 2008.
Researchers have identified nitrogen triflouride as a very potent greenhouse gas that has the potential of becoming a significant contributor to global warming. However, they note that as yet, it is not one of the gases controlled under the UNFCCC.
Nitrogen triflouride (NF3) is a synthetic gas that was initially used in small quantities for rocket fuel and lasers. However, the market for NF3 use is growing rapidly, particularly as a plasma etchant and equipment cleaning agent. In a recent article in the science journal Geophysical Research Letters, American scientists Michael Prathur and Juno Hsu note that the global warming effect over the next century of one tonne of NF3 is about 17,200 times that for a tonne of CO2. Of the greenhouse gases identified in the Kyoto Protocol, only SF6 is more potent. Thus, if all the NF3 produced globally in 2008 were released into the atmosphere, it would have the climatic effect of 67 million tones of CO2 (about 10% of Canada's annual greenhouse gas production). Furthermore, NF3 has a long atmospheric lifetime of about 740 years. The authors note that the high cost of NF3 will ensure users avoid its release into the atmosphere as much as possible. However, they suggest that the science community should at least begin to monitor its atmospheric concentration carefully.
Ramanathan, V., M.V. Ramana, G. Roberts, D. Kim, C. Corrigan, C. Chung and D. Winkler. 2007. Warming trends in Asia amplified by brown cloud solar absorption. Nature, Vol. 448, 2 August, pp. 575-578.
Atmospheric brown clouds - polluted clouds laden with aerosols - are shown to contribute as much as anthropogenic GHGs to warming of the lower atmosphere over the Indian Ocean and Asia. The combined warming effect is thought to have caused the widespread melting of major Himalayan glaciers.
Aerosols are a heterogeneous mix of particles, some of which cause atmospheric warming and some atmospheric cooling. The net effect of emissions of aerosols from human activity is estimated to be a cooling, which has offset some of the warming from GHGs. However, uncertainty about aerosol forcing remains large and one source of uncertainty is the heat-absorbing capacity of black carbon. A recent paper in Nature provides some new insights on this issue.. Atmospheric brown clouds (ABCs) are aerosol-laden clouds that are known to travel great distances and influence regional climate and air quality. In their study, the authors deployed, three small unmanned aerial vehicles (UAVs), closely stacked vertically (separated by only a few meters), to measure aerosol and soot concentrations, and solar fluxes, at altitudes of 0.5, 1.5 and 3 km. The differences in solar fluxes between two heights give the solar atmospheric heating rates. The experiment was conducted over the Northern Maldives in March 2006, during the dry season, when polluted air masses travel south from the Asian continent to the Indian Ocean. Their results show that during a polluted period, ABCs enhanced lower atmospheric heating by about 50 %, and soot particles were responsible for about 90% of that increase. To assess the warming impact that ABCs might have had over a longer time period, the authors used a climate model combined with earlier observations and estimates of ABCs to simulate the warming effect in the lower atmosphere of both greenhouse gases (GHGs) and ABCs from 1950 to present. Their results show that the warming due to ABCs is about the same magnitude, during that period, as that from GHGs. What's more, the combined simulated warming of GHGs and ABCs at atmospheric levels of 3 to 5 km is about twice the simulated surface warming trends, which is consistent with satellite temperature trends. These results have substantial implications for the elevated region of the Himalaya, where rapid reduction of glaciers has been observed during the past several decades.
Romanou, A., Liepert, B., Schmidt, G.A. et al. 2007. 20th century changes in surface solar irradiance in simulations and observations. GRL 34,L05713, doi:10.1029/2006GL028356, 2007.
Global dimming during the 20th century likely due to air pollution.
A team of American researchers use a combination of global climate models and satellite observational data to show that there has been a decrease in the amount of solar energy reaching the earth's surface (often referred to as 'solar dimming') over the 20th century, although not as significant as some past studies have suggested. Their assessments indicate that this long term trend has occurred primarily due to rising concentrations of atmospheric aerosols released due to human activities. On shorter time scales of decades, however, changing cloud cover can also have an important role in changing the intensity of incoming surface solar radiation. Hence reports of recent 'brightening' may well be due to reduced cloud cover associated with El Nino behaviour and could be temporary. They also note that the warming influence of rising greenhouse gas concentrations has significantly exceeded the cooling effects of surface dimming and resulted in a net warming.
van Hoof, T.B., Wagner-Cremer, F., Kurschner, W.M. and Visscher, H. 2008. A role for atmospheric CO2 in preindustrial climate forcing. PNAS 105:15815-15818.
New data from leaf stomata densities suggest variations in CO2 concentrations during the past millennium may have had a larger influence on pre-industrial global climates than previously thought.
While climate model simulations of pre-industrial climates during recent millennia have used solar and volcanic forcings as primary causes of global climate fluctuations, most modelers assume that atmospheric CO2 concentrations during that period were relatively constant and hence minor contributors. This assumption is based on Antarctic and Greenland ice core data, which show variations in atmospheric CO2 concentrations during that period of less than 12 ppm. However, a new study suggests that pre-industrial CO2 concentrations may vary by almost three times that magnitude. In their article, published in the Proceedings of the National Academy of Sciences, van Hoof and colleagues report that high resolution analyses of CO2 concentrations, from leaf stomata densities in bog-preserved pine needles in the US state of Washington and oak leaves in the Netherlands, show CO2 concentrations varied by as much as 34 ppm between 1000 and 1500 AD. They suggest changes in European land use and variations in ocean temperatures may both be factors. When included in climate model simulations, these fluctuations caused a range of global temperature variations of as much as 0.25°C over this time period. While the authors agree that direct correlation of temperature variations with CO2 fluctuations is difficult because of concurrent solar and volcanic forcings of similar magnitudes, they argue that including CO2 forcing in pre-industrial climate simulations would both improve their accuracies and provide an observational constraint on climate modelling sensitivity.
Winton, M. 2006. Amplified Arctic climate change: What does the surface albedo feedback have to do with it? GRL 33, L03701, doi:10.1029/2005GL025244, 2006.
A robust feature of simulations of future climate has been the amplification of warming in the Arctic relative to that for the rest of the world. Work to date has demonstrated the complexity of Arctic amplification and the multiplicity of processes that contribute to it, although the "snow/ice albedo feedback" (surface albedo feedback) is perhaps the factor most often linked to enhanced Arctic warming. This paper by Winton seeks to place the surface albedo feedback in the appropriate context relative to other forcings and feedbacks that affect Arctic amplification. To do so, he analyses simulations from 12 models from the archive of climate model results for the IPCC AR4. The simulations show Arctic warming that is, on average, almost twice that of the rest of the world at the time of CO2 doubling (in 1%/year CO2 increase experiments). A mathematical expression is derived that gives the Arctic amplification as a function of five factors. Three of these contribute to Arctic amplification: the Arctic-global differences in surface-albedo shortwave feedback (SAF), longwave radiation feedback, and the net top-of-the-atmosphere flux forcing. Doubled CO2 forcing and the non-SAF shortwave feedback (which includes feedbacks from clouds and water vapour) oppose Arctic amplification. To quantify the impact of individual factors on Arctic amplification, each term is 'neutralized' in turn to examine the effect on Arctic warming. The largest impact comes from neutralizing the non-SAF shortwave term which increases Arctic amplification by a factor of 6 (implying that the Arctic-global difference for this factor strongly opposes Arctic amplification). Neutralizing the SAF reduces Arctic amplification but does not eliminate it. These and other results support the interpretation that the SAF is a contributor but not the dominating factor in Arctic amplification.
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