An Analysis of radiative forcing by anthropogenic economic sectors

When forming policies to mitigate climate change, it is imperative that the actions incorporate current levels of CO2 , ozone (O3), and aerosol particles. Of those three, O3 emissions are especially worrisome as the pollutant is mainly responsible for the warming that occurs in the atmosphere. Additionally, aerosols can also contribute to climate change by affecting the properties of clouds. These modifications include increased number of cloud droplets and smaller droplet sizes, when in turn can result in decreased precipitation and evaporation of clouds. While these chemical species are essential to understanding climate forcing, Unger, et al. (2010), argue that the emissions are not the cause, but the symptoms of climate change. The main focus when creating mitigation policy should therefore focus on the primary origin of climate change: anthropogenic factors. To better diminish changes from anthropogenic radiative forcing, Unger et al. suggest quantifying climate impact by economic sector. — Aly Stark 
Unger, N., Bond, T., Wang, J., Koch, D., Menon, S., Shindell, D., Bauer, S., 2010. Attribution of climate forcing to economic sectors. PNAS, 107: 3382 – 3387

 Unger et al. argue that the IPCC’s traditional method of examining radiative forcing is ineffective. This method examines individual pollutants and their changes over various times periods, but does not consider possible interactions between chemicals or the potential for effects caused by multiple pollutants. Therefore, to ensure smart climate policy, a complete evaluation that is sector-based is required. This study presented a comprehensive look at the total climate forcing, with emissions broken down into 13 components of the economic sector. Also included was an analysis short-lived species pollutants and aerosol radiative forcing.  Unger et al. found that aerosols from three different sectors (industry, power, and biomass burning) are contributing to larger quantities of cloud droplets, which in turn increases total cloud cover and cloud optical depth. These effects amplify reflectivity.  However, aerosol effects from other sectors were found to be insignificant. Additionally, positive radiative forcing was found in the power and industry sectors, suggesting that these sectors are major sources of SO2 emissions and contribute greatly to anthropogenic sulfate accumulation. Lastly, the climate impacts of emissions from 2000 to future time designations 2020 and 2100 are considered with regards to the current emission sectors. The biomass burning and industry sectors were found to have the greatest amount of negative radiative forcing, driven by considerable cooling effects. On the other end of the spectrum, the aviation sector was found to have the smallest amount of radiative forcing, likely due to less fuel use compared to other anthropogenic activities. Looking ahead at 2100, Unger et al. found that the radiative effects change and positive forcing is significantly perpetuated by the power industry.
To effectively combat climate change, emissions should be categorized by anthropogenic sectors. Although uncertainties in radiative forcing can occur from causes such as physical, chemical, and optical processes, this model is still stronger than past IPCC methods of examining climate forcing and the spatial effects of pollutant emissions. As a result of this radiative forcing assessment, Unger et al. found that both short-lived and long-lived species must be considered, as well as time period analysis of climate effects of cooling aerosols. Further, to best mitigate anthropogenic climate change, actions should be taken in on-road transportation, household biofuel, and animal husbandry sectors.

Contrasting climate effects of forests through radiative forcing

Forests play an important role in global temperature, as their processes involving CO2 and solar radiation have large impacts on the local and global climate. However, the interactions between forests and these climate changing agents produce contrasting effects on temperature. Rotenberg and Yakir (2010) observed these effects and explained the results by examining the differences in climate gradients and employing climate forcing analysis. As the forest absorbs carbon through carbon sequestration, a cooling effect occurs, driven by heavy vegetation during the peak photosynthesis months. However, the forests also absorb solar radiation by suppressing longwave radiation, which results in the opposing warming effect. These finding provide a basis for considering the consequences of surface energy balances when evaluating the success of land-based carbon sequestration. — Aly Stark 
Rotenberg, E. and Yakir, D., 2010. Contribution of Semi-Arid forests to the climate system. Science, 327: 451-454.

 Rotenberg and Yakir conducted their study in a semi-arid, 2800 ha pine forest located in southern Israel. The characteristics of the forest include high productivity, a mean annual net ecosystem CO2 exchange (NEE) of 2.3 ton C ha -1, and a mean annual gross primary productivity (GPP) of 820. The ratio of NEE/GPP for this forest was 0.27, higher than the averages for both Europe and the global Fluxnet network, indicating high carbon use efficiency. As a result, there is found to be a range of eco-physiological adjustments in which the global climatic gradient is narrowed due to shifts of peak GPP (demonstrated by peak rates of photosynthesis) in different European regions. The trend is homeostatic and specific to a single vegetation type rather than changes in species composition. Therefore, the trend explains that plasticity of carbon sequestration with regards to a single species.
Vegetation cover is then considered in terms of its effects on surface radiation balance. The specific Israeli forest examined was unique in that its net radiation and sensible heat flux are both higher than those of any other eco-region (35% and 30%, respectively, greater than that of the Sahara). According to Rotenberg and Yakir, one significant implication of those characteristics is that the high global radiation, combined with a decrease in albedo associate with forestation, results in a drastic increase in shortwave surface radiation capacity. Also, as longwave radiation is important to semiarid forests, Rotenberg and Yakir examined its response to the shortwave albedo effect. As the surface cooling in areas of high vegetation suppresses longwave radiation flux, it is found that ecosystems with drier vegetation have increases in surface longwave radiation, which further doubles that shortwave effect.
These results were then syndicated with radiative forcing to gain a more complete knowledge of the consequences of surface radiation associated with carbon sequestration. This was illustrated by combining the observed albedo derived shortwave radiative forcing with the calculated radiative forcing associated with carbon intake to estimate the number of years until the two forcing values are balanced. Rotenberg and Yakir found that, in the “worst case scenario”, a net negative radiative forcing, one which produces cooling, will be found after approximately 80 years of forestation. These results hold important implications for addressing the anthropogenic effect on climate change and global temperature.

Estimating climate sensitivity of the global climate system using reconstructed temperature and CO2 records

As concerns about the global climate system increase, it is becoming increasingly important that projected warming be accurate. However, there are many uncertainties associated with such predictions, most of which can be attributed to a general lack of understanding of the behavioral trends of the carbon cycle. Frank et al. (2009), attempted to reduce these uncertainties by constraining the variable associated with global CO2/temperature feedback, γ. This particular attempt to constrain γ was unique because it considered numerous data sets and analysis periods, which resulted in 229,761 estimates and therefore facilitated distributions and confidence intervals that were previously unavailable. This also made it possible to assign likelihoods and create a bench mark for future simulations. The mean γ was found to be 7.7 p.p.m.v. CO2 per °C, and the range (80% of estimates), excluding upper tail sensitivities, was estimated to be between 1.7 and 21.4 p.p.m.v. CO2 per °C. Further, it was found that it was found that γ was likely to fall in the lowermost quartiles of estimates.  These predictions are key to understanding the global climate system and the impacts of amplifies anthropogenic warming. — Aly Stark 
Frank, D., Esper, J., Raible, C., Buntgen, U., Trouet, V., Stocker, B., Joos, F., 2010. Ensemble reconstruction constraints on the global carbon cycle sensitivity to climate. Nature, 463 : 527- 530.

 Frank et al. begin by examining the variability of reconstructed temperature and atmospheric CO2 from the past millennium. Long-term decreases in CO2 were attributed to the cooling that is associated with the shift from the Medieval Warm Period (MWP) to the Little Ice Age (LIA), characterized by changes in both atmospheric and oceanic carbon circulation. These two climate anomalies demonstrate that the amplitude of hemispheric to global-scale temperature change is significant. To further illustrate this notion, Frank et al. noted that peak LIA conditions (1601-1630) were 0.3-1.0 °C cooler than present day temperatures. Additionally, after deriving and analyzing ensembles of past temperature changes, it was determined that the most recent evaluated period (1971-2000) was, on average, 0.7 °C  warmer that the coldest LIA period, and 80% of the estimates yielding between 0.52 and 0.99 °C . These results may be explained by increased anthropogenic influences, which are predicted to have widened the temperature range by approximately 75% over the last millennium.
The data for reconstructed temperature and CO2 records were then regressed to derive γ estimates. Three different time periods were analyzed with respect to γ: 1050-1800, 1050-1549, and 1550-1800. The mean for all γ values was found to be 7.7 p.p.m.v. CO2 per °C.  The values for the earlier period were found to have a lower mean that the later period, with median γ values of 4.3 and 16.1 p.p.m.v. CO2 per °C, respectively. The higher γ values in the later period are recognized as a result of an influential CO2 drop during the LIA (1600). This drop was not solely a consequence of large-scale temperature, and it is possible that changes in atmospheric or oceanic CO2 states contributed to the carbon decrease.  When the γ estimates are subjected to a range of carbon-cycle climate models, the average γ increases and is estimated to be 8.5 p.p.m.v. CO2 per °C. Empirically, the estimates are twice as likely to be in the lower bound quartile (between 2.1 and 8.5 p.p.m.v. CO2 per °C). However, the empirical findings cannot be compared with the model-derived estimates due to the internal variability of the model. Nonetheless, the estimates of γ values are informative and assist in reducing uncertainties associated with projected warming and climate responses.

Assessing the role of natural and anthropogenic climate forcing in estimating temperature reconstructions

Despite recent advances in climate proxy data and models, many attempts to estimate spatially resolved temperature patterns have been limited to periods only extending back a few centuries. Mann et al. (2009), however, employ a climate field reconstruction method to attempt to estimate global and hemispheric trends from the past 1500 years onward. The proxy based temperature reconstruction was derived by using both global proxy data and surface temperature information that is created by the annual mean surface temperature field over a recent overlap of proxy and instrumental data. Mann et al. highlighted the Little Ice Age (1400 to 1700 C.E.) and the Medieval Climate Anomaly (950 to 1250 C.E.) as two periods which have some climatologically interest.  The one period is found to have a La Nina-like nature, while the other follows an opposite trend.  The difference between these two periods has a consistent pattern of surface temperature and atmospheric circulation for the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO). Mann et al. conclude that these findings provide evidence for a thermostat-like response of anthropogenic climate forcing.— Aly Stark 
Mann, M., Zhang, Z., Rutherford, S., Bradley, R., Hughes, M., Shindell, D., Ammann, C., Faluvegi, G., Ni, F., 2009. Global Signatures and Dynamical Origins of the Little Ice Age and Medieval Climate Anomaly. Science 326: 1256-1260.

 The reconstruction of Mann et al. not only greatly extended further into the past, but also produces multiple degrees of freedom, allowing for a more telling estimate. By observing patterns of variation, it was determined that the climate 1000 years ago (the time of the Medieval Climate Anomaly) displays La Nina conditions. The Medieval Climate Anomaly (MCA) was then characterized with warmer hemispheric conditions, while the Little Ice Age (LIA) corresponds to colder conditions. In nature of La Nina, the MCA had a warmer northern hemisphere and a colder southern hemisphere, while the LIA had a cold north and a warmer south.
To make meaningful comparisons with current climate stimulations, Mann et al. examined their findings against two coupled models. The comparison of global mean temperature difference between MCA and LIA was similar to the estimates that were made by Mann et al. However, while the estimates of the global mean temperature difference for these periods is consistent with other climate models, the spatial patterns of response for the two comparison models varies distinctly. Mann et al. attribute these differences to the tropical Pacific “thermostat”, which is not present in either comparison model. The thermostat mechanism is also linked to the surface temperature changes in the MCA positive phase of the NAO-AO pattern.  Therefore, it is possible that the tropical Pacific thermostat reaction may be indicative of forced climate changes.

Impacts of regional and global radiative forcing on regional climate change

Although regional climate change is attributable to many effects, the relevance of those effects is vague because the response to regional forcings is not adequately understood for the last century. Shindell and Faluvegi (2009) thus examined the susceptibility of various regions to changes in forcing. By determining the relationship between forcing and location reaction, and integrating observations of climate change from the twentieth century, Shindell and Faluvegi derived the importance of aerosols and resulting radiative forcing throughout time and location. This information explained that radiative forcing location influences climate response. Aerosols were proven to have great importance for both global and regional climate change. Further, the results demonstrate that, over the past three decades, the Arctic warming trend is influenced by the black carbon and aerosol emissions of the Northern Hemisphere.— Aly Stark
Shindell, D. and Faluvegi, G., 2009. Climate response to regional radiative forcing during the twentieth century. Nature Geoscience, 2, 294–300.

 Shindell and Faluvegi began by creating latitude bands and finding the response of the surface temperature to various levels of well-mixed greenhouse gases (WMGHG), ozone, and aerosols. Through this modeling, they found that when the forcing occurs within a specific latitude band, the mean temperatures follow the forcing per local unit area. Therefore, when the global mean radiative forcing is considered, the mid-latitude forcing and polar forcing must be greater than the tropical forcing (~ 2.4 and ~7 times, respectively). Higher latitude forcings are thus more sensitive to global forcing when compared to other regions.
Shindell and Faluvegi then compared the results from the modeling to the past patterns of surface temperature over various regions and time periods. These comparisons examined global and gradient responses and found significant implications for Arctic trends. The surface temperatures of the Arctic are warm until 1930, cooler from 1930–1975, and rapidly warmer onwards. Although there were differences between the observed and the recreated global and gradient responses, Shindell and Faluvegi accredit the discrepancies to internal variability and aerosol forcing. The models demonstrate a necessity for aerosol provided cooling.

            From modeling and comparisons, the results illustrate the prevalence of aerosol presence in both global and regional climate response. Further, forcing in the northern hemisphere has a particularly strong effect on the Arctic climate. As the forcing at the mid-latitude northern hemisphere oscillates between positive and negative, the temperatures of the Arctic transition from warmer to cooler. It is estimated that aerosols are one of the main contributors to the increased Arctic surface temperature; responsible for 1.09 ± 0.81° C of the 1.48 ± 0.28°C increased warming. As increased aerosol forcing continues, coupled black carbon and tropospheric ozone contributions, Arctic warming will also increase

The importance of gas-aerosol interactions in policy making

In current emission comparisons, the most cost effective method is one which employs multicomponent climate change mitigation strategies. These strategies involve analyzing both the direct and indirect effects of various linked emissions. However, the information of indirect effects between most gaseous pollutants and aerosols is absent. Shindell et al. (2009) took two approaches to calculate the impact of emissions on aerosols and the influence of these on radiative forcing. They found that the global warming potential (GWP) is substantially larger when the direct effects of aerosol interactions are considered and increases further when aerosol-cloud interactions are taken into account. Therefore, as atmospheric chemistry links methane, ozone, and aerosols, the policies regarding multigas mitigation should take gas-aerosol interactions into consideration.  Alyson Stark
Shindell, D., Faluvegi, G., Koch, D., Schmidt, G., Unger, N., Bauer, S., 2009. Improved Attribution of Climate Forcing to Emissions. Science 326: 716-718.

Shindell et al. relied heavily on averaged radiative forcing (RF) to compare various emissions and to estimate GWP. To find the response of atmospheric composition to both the collective and individual effects of emissions, the researchers calculated abundance based RF, which measured the effects of all emissions changing concurrently, and emissions based forcing, which instead attributed the response to a specific pollutant. Using these two techniques, Shindell et al. estimated several 100-year GWPs. These computations demonstrated that, with the absence of aerosol responses, the results were similar to previous studies. However, once the radiative effects are regarded, the GWP becomes larger in both methane and carbon monoxide (CO). Compared to the initial situation with no aerosol, the possible methane emission range increases, from approximately 25 to a range of 25-40 over a 100 year horizon. Similarly, the CO emissions initially begin at a 1-3 range and increases to a 3-8 range.  However,  the accuracy of these values is essential, and it there still remains much uncertainty in the predictions. For example, increases in these values also increases carbon dioxide, which perpetuates higher GWPs, and leads to more uncertain policymaking. 

Farm soil fertility is severely limited by sediment from tillage erosion

In the Sele River Basin, farm soil fertility is severely limited by sediment that spurs from tillage erosion. The erosion events can further perpetuate soil loss and essential ecological functions. Although it has been demonstrated that climate change influences the relationship between erosive rainfall and decreased soil quality, Didato et al. (2009) attempt to expand current models by quantifying the past interactions between climate change and erosion to produce a model which incorporates various erosion situations at different time periods. This process began by constructing a Net Erosion Model (NER), which examined monthly sediments and attempted to evolve erosion processes from monthly to annual time scales. The NER model was used in conjunction with other equations to derive monthly, seasonal, and annual erosion rates. An adapted conceptual erosion technique, Climate Forcing and Erosion Modelling (CliFEM) was also considered throughout the study. Didato et al. found that the potential for future average soil erosion was high, although the more hazardous concern was the estimate of monthly soil loss. The monthly soil erosion from August to November is the most devastating as these months follow the soil tillage process, which promote soil erodibilty and decreased soil coverage.  Alyson Stark
Didato, N., Fagnano, M., Alberico, I., 2009. ClimFEM- Climate forcing and erosion modeling in the Sele River Basin (Southern Italy). Natural Hazards Earth System Science, 9, 1693 – 1702.

Didato et al. conducted this study in the Sele River Basin, which serves many land uses including spaces for crops and mixed-deciduous woods.  However, much of the forests have been cleared for agricultural use, increasing the potential erosion and placing soil quality at risk. With a volatile average annual precipitation, ranging from 700 to 2000 mm, weather has considerable influence over erosion and agriculture. This study therefore considered historical water and land records to present a more comprehensive knowledge of the climate drivers behind erosion.

The main model employed was the NER, which calculated the amount of sediment in the basin and presented the information at different time scales. These data were then converted to gross soil loss by dividing NER by the fraction of eroded soil that is delivered outside of the basin, or sediment delivery ratio (SDR). This basic model was altered and combined with other data to derive a monthly NER model, a gross erosion evaluation, and an estimation of tolerable soil loss. The models demonstrated many essential details about erosion and erosion patterns in the Sele River Basin. First, from 1973-1007, only 10 years exceeded the long-term average of soil loss, and soil erosion compromised of 60% of total loss, indicating that the determination of long-term erosion can be heavily influenced by a small number of events. Also, it was illustrated that anthropogenic events, such as landscaping and irrigation increased the erosion of soil during the considered time period. Lastly, the most important conclusion was that the time that is most dangerous for soil is that which follows tillage. Erosion is rampant in the August to November months, indicating that measures should be taken to increase soil coverage, such as conservative soil tillage and increasing the use of perennial cover crops.