|[ image credit: Bond et al., 2013 ]|
|[ image credit: Bond et al., 2013 ]|
Some aerosols, particularly sulfur dioxide, have a cooling effect, making that they partly mask the warming effect of other emissions by people. The IPCC AR4 image below shows that the direct and cloud albedo effect of aerosols equals a radiative forcing of as much as -2.7 W/m². In other words, if this masking effect were to fall away, warming would increase by as much as 2.7 W/m², according to IPCC AR4 figures.
A 2013 study by Levy et al. concludes that emission reductions by 35%–80% in anthropogenic aerosols and their precursors would result in some 1°C of additional warming. A 2016 paper by Zhang et al. calculates that from 1850 to 2010, anthropogenic aerosols brought about a decrease of about 2.53°C.
Anthropogenic aerosols are also suppressing the Pacific Decadal Oscillation, making that less heat gets transferred from oceans to the atmosphere. Recent research concludes that future reduction of anthropogenic aerosol emissions, particularly from China, would promote positive Pacific Decadal Oscillation, thus further speeding up warming over the coming years.
Dimethyl sulphide emissions from oceans constitute the largest natural source of atmospheric sulphur, and such emissions can decrease with ongoing ocean acidification and climate change. This could particularly impact specific regions such as Antarctica, speeding up warming and loss of sea ice there, as discussed at this paper.
|Global anthropogenic emissions weighted by GWP|
The lifetime of BC, SO₂ and CH₄ is much shorter than the lifetime of CO₂. Therefore, the changes by BC, CH₄ and SO₂ to temperature are felt much more strongly when using a shorter timespan.
Over a 10-year period, CH₄ produces more warming than CO₂, while BC is not far behind. As discussed in above box, SO₂ partly masks the warming by other emissions by people. For more on the warming impact of methane, also see this page.
The temperature impact for a one-year pulse of the various emissions by people is depicted on the image on the right.
The danger is that this masking effect will fall away, while warming will additionally increase, due to more emissions that have a strong immediate warming effect such as black carbon. If the masking effect of sulfur dioxide were to fall away, warming would increase by as much as 2.7 W/m², according to IPCC AR4 figures, and even more according to some other studies.
Imagine a scenario in which many people stopped burning fossil fuels for energy, including for heating, cooking and lighting. That would be great, but if many of them instead switched to burning biomass in woodburners (stoves, heaters and fireplaces) and in open fires, while burning more forests to create more pasture for grazing, while burning more waste in the absence of appropriate waste management, and while wildfires increased strongly, the net warming due to black carbon would increase dramatically.
Indeed, in the absence of comprehensive and effective government policies, further warming could result if people starting to burn more biomass in woodburners and open fires instead (as the sulfur masking effect falls away, i.e. in response to an absence of fossil fuel). Waste disposal is another source of aerosols. One study estimates that as much as 29% of global anthropogenic emissions of small particulate matter comes from trash fires. Furthermore, there could be more burning of forests to create more pasture for grazing and as a way to prevent wildfires (backburning and controlled fires), and a further rise in the frequency, intensity, duration and size of wildfires as the weather gets more extreme.
In the absence of appropriate policies, there will be much further warming, due to albedo changes and greenhouse gases such as water vapor, carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). In regard to aerosols, sulfur has a masking effect (or cooling effect). Reductions in sulfur will reduce the cooling effect that sulfur now has. The prospect is that this cooling effect will shrink rapidly, while at the same time other emissions will increase that have a huge immediate warming effect, such as black carbon (BC), carbon monoxide (CO) and nitrogen oxides (NOx), in particular nitric oxide (NO) and nitrogen dioxide (NO₂), resulting in a strong net radiative forcing.
In a recent paper by James Hansen and an earlier paper by Hansen & Sato (2011) a sensitivity of ¾°C per W/m² is used (see images on the right), while another recent paper suggest that the temperature rise per W/m² could be even stronger.
A high-end short-term increase in net radiative forcing combined with a strong temperature rise per W/m² could cause a temperature rise as a result of changes in aerosols of as much as 2.5°C in a matter of years, as indicated in earlier posts such as this one.
• The updated effective radiative forcing of major anthropogenic aerosols and their effects on global climate at present and in the future, by Zhang et al. (2016)
• The roles of aerosol direct and indirect effects in past and future climate change, Levy et al. (2013)
• Bounding the role of black carbon in the climate system: A scientific assessment, Tami Bond et al. (2013)
• Black carbon larger cause of climate change than previously assessed (news release of above study)
• Global and regional climate changes due to black carbon, Ramanathan et al. (2008)
• IPCC Climate Change 2007: Working Group I: The Physical Science Basis
• IPCC Climate Change 2007: Working Group I: The Physical Science Basis - Chapter 8
• For Air Pollution, Trash Is a Burning Problem
• Global Emissions of Trace Gases, Particulate Matter, and Hazardous Air Pollutants from Open Burning of Domestic Waste, by Wiedinmyer et a. (2014)
• Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation, Shakun et al. (2012)
• Paleoclimate Implications for Human-Made Climate Change, James E. Hansen and Makiko Sato (2011)
• Young People’s Burden: Requirement of Negative CO2 Emissions, James Hansen (2017)
• Climate Plan
• Extinction (this page)
• How much warming have humans caused?
• Temperature Rise
• Methane hydrates