|[ image credit: Bond et al., 2013 ]|
|[ image credit: Bond et al., 2013 ]|
Some aerosols, particularly sulfate, 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. Also see the study by Rosenfeld et al. and the news release.
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 resulting from sulfur dioxide emissions 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 when many of them instead switched to burning biomass in woodburners (stoves, heaters and fireplaces, for heating, preparing food and boiling water) and in open fires, while also burning more forests to create more pasture for grazing, and while burning more waste in the absence of appropriate waste management, the net warming (due to increased black carbon and brown carbon) could be a lot higher, especially when combined with a strong increase in forest fires.
Indeed, in the absence of comprehensive and effective government policies, further warming could result as the sulfate masking effect fell away (due to an absence of sulfates that are now co-emitted with fossil fuel burning) and as people started to burn more biomass in woodburners and open fires (instead of using 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, as said, 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.
|[ Rise of about 3.5°C from Last Glacial Maximum ]|
The danger is that, in the absence of appropriate policies, there will be much further warming, due to albedo changes and due to 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 fossil fuel will also reduce the cooling effect that sulfur now has, so this cooling effect will shrink rapidly.
The danger is that, at the same time, other emissions will increase that have a huge immediate warming effect, such as black carbon (BC), brown carbon (BrC), carbon monoxide (CO) and nitrogen oxides (NOx), in particular nitric oxide (NO) and nitrogen dioxide (NO₂), jointly resulting in a strong net additional radiative forcing.
In a recent paper by James Hansen and in an earlier paper by Hansen & Sato (2011), a sensitivity of ¾°C per W/m² is used (see images on the right).
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 therefore cause a temperature rise as a result of changes in aerosols of as much as 2.5°C in a matter of years, as discussed in posts such as this one and this one, which features an image showing sulfur dioxide levels as high as 3597.10 µg/m³ in East Asia on October 24, 2018.
As above image illustrates, there is a huge variation in sulfur dioxide levels regionally, implying that some areas could be hit much more strongly by rising temperatures than others.
The image on the right shows IPCC (2000) projections for sulfur dioxide emissions, from the same earlier post.
Sulfur dioxide (SO₂) gets oxidized in the atmosphere to form sulfate aerosols (SO₄), which have a strong cooling effect.
As a result of the COVID-19 lockdowns, traffic and large parts of industrial activity did grind to a halt worldwide. Yet, despite this, sulfate levels remained high, as illustrated by the image below which shows that τ, i.e. light at 550 nm as a measurement of aerosol optical thickness due to sulfate aerosols, was as high as 3.162 on March 30, 2020, at 11:00 UTC.
On December 17, 2020, at 10:00 UTC, sulfate aerosols (SO₄) were as high as 6.396 τ at the green circle. Wind on the image (below) is at 850 hPa.
|[ click on images to enlarge ]|
Additionally, sulfate aerosols strongly contribute to the impact of aerosol–cloud interactions, estimated in AR5 to provide as much as -1.2 W/m² cooling.
Taken together, the two add up to as much as -1.82 W/m² of cooling.
Besides sulfates, further aerosols contribute to aerosol–cloud interactions, such as dust aerosols.
According to AR5, dust has a direct impact of -0.1 W/m², while additionally contributing to aerosol–cloud interactions. When taking into account a 0.15 W/m² warming impact found to be caused by coarse dust, dust may well cause net warming, which is the more important since dust is likely to increase due to more fires, stronger winds and further desertification.
Above image shows that τ, i.e. light at 550 nm as a measurement of aerosol optical thickness due to dust aerosols, was as high as 8.9534 on March 28, 2020, at 07:00 UTC. The image on the right shows that τ was as high as 7.1567 on May 20, 2020, at 08:00 UTC, while a substantial amount of dust covers the Arctic.
The NASA visualization above can be viewed here. The visualization below covers a period from August 2006 to April 2007, showing dust (orange), sea salt (blue), smoke (green), and sulfates (white).
The image below shows smoke from fires on August 23, 2018, together with dust and sea salt aerosols.
• 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)
• We need to rethink everything we know about global warming
• Aerosol-driven droplet concentrations dominate coverage and water of oceanic low level clouds, by Daniel Rosenberg et al.
• 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)
• NASA On Air: NASA Launched CATS - Measuring Clouds and Aerosols (1/14/2015)
• NASA | GEOS-5 Aerosols (August 2006 - April 2007)
• NASA | Aerosols and Incoming Sunlight (Direct Effects)
• NASA | Just Another Day on Aerosol Earth (August 24, 2018)
• Climate models miss most of the coarse dust in the atmosphere - by Adeyemi Adebiyi et al.https://advances.sciencemag.org/content/6/15/eaaz9507
• Climate Plan
• How much warming have humans caused?
• Abrupt Warming, how much and how fast?
• Doomsday by 2021?
• Temperature Rise
• Methane hydrates