Climate change

Aerosols from space #1

Will Morgan August 23, 2013

A short post to illustrate the changing nature of aerosol in the atmosphere in terms of their spatial extent, source and properties. There are two images below showing the scene from the TERRA satellite as it passed over the Eastern Atlantic off the coast of Morocco. The first image shows the plume of smoke from wildfires from Madeira that swept through the island last weekend. The second image shows dust over the ocean that has likely originated from the Sahara Desert.

Image of the smoke plume from Madeira on 17th August 2013 from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on the TERRA satellite. Image courtesy of NASA. Click on the image for a larger view.

The smoke plume is quite well defined and doesn’t cover a large geographical extent. Typically, smoke plumes are dominated by small particles (less than 1µm) with very large concentrations in the plume. As the plume blows downwind it quickly mixes with the cleaner air in the region and is diluted by this mixing process in the same way that you mix cordial with water to make squash. At the closest point to the fire, the plume is like mixing the cordial with a glass of water, whereas downwind you are progressively mixing the same amount of cordial with more and more water (e.g. a bathtub, a swimming pool, a lake). Eventually you won’t be able to make out the smoke plume visibly although that doesn’t mean that it isn’t still around – you probably need some sophisticated instruments to pick it out of the atmosphere.

Image of the dust plume on 20th August 2013 from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on the TERRA satellite. Image courtesy of NASA. Click on the image for a larger view.

The dust plume on the other hand has a much greater geographical extent due to the much larger source region. The dust is emanating from a much larger area as high winds pick up dust from the desert and cast it into the atmosphere. In contrast to the smoke, dust plumes usually contain relatively more larger particles (for aerosols), with sizes often greater than 1µm extending up to around 100µm. In aerosol parlance, such particles are referred to as ‘coarse’. These particles generally can’t be transported as far as the smaller particles as they fall out of the atmosphere more easily. However, Saharan dust is famous for bucking this trend as it has been observed to traverse the Atlantic Ocean and reach the Amazon Rainforest. It can even make it to the UK!

These are just a couple of examples of how we can observe aerosols from space. The above are just visible images but satellite observations can provide us with more detail than this. I’m always quite amazed at what we can pick up so no doubt I will return to this in the future.

Sweeping soot out of the atmosphere

Will Morgan August 16, 2013

Efforts to slow the rate of global temperature rise in the 21st Century have for some time focussed on non-CO₂ species or so-called ‘short-lived forcers’. As far as aerosols are concerned, black carbon (often referred to as soot) has been the main avenue to explore due to its capacity to warm the atmosphere by absorbing sunlight. Black carbon contrasts with most other aerosol species which tend to cool the Earth, so reducing emissions of black carbon is attractive as it has the twin benefits of reducing future global temperatures and improving air quality. The hard part is that black carbon tends to get mixed up with other members of the atmospheric aerosol gang, with the outcome being highly uncertain. A recent review concluded that the overall impact of black carbon combined with its co-emitted partners was slightly negative but with huge uncertainties; the estimates ranged from cooling or warming that was comparable to the impact by carbon dioxide! Black carbon and its accomplices could either be offsetting the historical impact of carbon dioxide or adding extra warming on top.

Bearing the above in mind, the idea to slow global warming by reducing black carbon emissions has been around for more than a decade. Thirteen years ago, James Hansen and colleagues proposed reducing emissions of black carbon and non-CO₂ greenhouse gases in order to slow the rate of global temperature change due to increasing concentrations of CO₂by 2050. This was soon followed by a somewhat sceptical perspective in Science by Smith et al. that pointed out the difficulties in targeting individual pollutants with emission controls and the large uncertainties associated with aerosols. Mark Jacobson illustrated with a global aerosol simulation that eliminating black carbon and the associated organic aerosol from fossil fuel burning would reduce net warming by 20-45% within a 3-5 year period. For CO₂ emissions to have the same effect, they would need to be reduced by a third but the reduction in net warming would take 50-200 years. Tami Bond has contributed some typically thoughtful and thorough articles to the discussion, concluding that despite the large uncertainties, reducing all aerosol emissions from major sources of black carbon will reduce direct climate warming. Furthermore, she showed that the largest benefits can be gained by reducing emissions in developing countries, where a large proportion of the black carbon source occurs.

Transmission electron microscopy (TEM) images of aerosol particles, including black carbon from Posfai et al. (1999). In panel A, the black carbon particles (denoted by the small arrows) are mixed with inorganic ammonium sulphate particles. In panel B, a typical chain-like black carbon aggregate is shown with the arrows pointing to a film of carbon that connects the individual spherules within the aggregate. In panel C, fly-ash spheres are shown, which are particles that are often associated with black carbon particles. The scales give an idea of the size of these particles – a human hair is around 100µm or 100,000nm.

Recently, reducing black carbon emissions has been highlighted by a UNEP report and an associated article in Science, which showed that targeting methane and black carbon emissions could knock off around 0.5°C (with a range of 0.2-0.7°C) by 2050 compared to the current projected temperature rise. This weeks’ Smith et al. paper in PNAS suggested a lower central result of 0.16°C (with a range of 0.04-0.35°C). The large ranges in temperature reduction in both studies were mainly due to the uncertainty in aerosol effects. Coverage of the most recent paper has focussed on the idea that such emission reductions will not “save us from climate change” (e.g. here, here, here and here). While this is very likely true, having read the work in this area, nobody has been suggesting this anyway! For example, Hansen et al. stated:

This interpretation does not alter the desirability of limiting CO2 emissions, because the future balance of forcings is likely to shift toward dominance of CO2 over aerosols.

While Shindell et al. stated:

…eventual peak warming depends primarily on CO2 emissions, assuming air quality–related pollutants are removed at some point before peak warming.

The argument for reducing short-term forcing species has been predicated on slowing global temperature rise over the first half of the 21st Century, not halting it. The potential benefits of this include more time to develop CO2 reducing policies/technologies, slower sea-level rise and reduced warming in susceptible environments, such as the Arctic. These are just some of the climate benefits, with advantages for human health and ecosystems also being highlighted.

The short atmospheric lifetime of black carbon also raises a question mark over whether a global temperature perspective is the most appropriate measure. Unlike the longer-lived greenhouse gases, the climate warming from black carbon is much more regional in nature and it is on these scales that the impacts are most keenly felt. For example, black carbon emissions for energy generation are concentrated in North America, Europe and Asia, so their impact is stronger in the Northern Hemisphere. Emissions from cooking stoves are more prevalent in developing countries in the tropics, while deforestation, which accounts for around a third of black carbon emissions, is also focussed in tropical areas. Consequently, any future policies or technological developments will need to take these into account if a reduction of black carbon emissions is to slow future temperature rises and the potential consequences of this. Whether the science is suitably mature to inform climate policy decisions in this area though remains to be seen; the studies highlighted all point to black carbon exerting a net warming up to 2050 but the magnitude of its effect and the feasibility of reducing emissions is highly uncertain.

Unfortunately, Sweep’s magic wand wasn’t enough to reduce Sooty’s contribution to climate change. Image from here.

If it wasn’t for those pesky aerosols…

Will Morgan August 8, 2013

Climate change is a subject that science knows a lot about; broadly, we can demonstrate that greenhouse gases have accumulated in the atmosphere over the past 200 years or so due to our burning of fossil fuels and that this has led to a rise in temperatures across the globe. However, our atmosphere is a complex beast and we have proven particularly adept at altering it. It turns out that as well as adding greenhouse gases to our atmosphere, we have also been adding a bunch of tiny particles into the mix. These particles are known as aerosols and they only stick around for short periods in the atmosphere, with a month-old aerosol particle being considered elderly. Many only last a few hours as they either crash into the ground or get washed away by rainfall. This contrasts with greenhouse gases, many of which stay around for decades to centuries.

While greenhouse gases have warmed our planet, aerosols act as a counter-balance to this warming – they cool the planet via a number of processes that reduce the amount of sunshine reaching the Earth’s surface. To complicate matters a little further, some of them, such as black carbon, warm the planet. Overall though, our best estimates of the impact of aerosols on our climate suggest that they have taken the edge off the warming expected from greenhouse gases. However, the major caveat here is that the level of cooling from aerosols is highly uncertain, both for the last 200 years and for the next century. This presents a problem for our ability to test our understanding of the climate system and project how it will change in the future.

Image of the global aerosol distribution produced by NASA. The image was produced using high-resolution modelling by William Putman from NASA/Goddard. The colours show the swirls of aerosol particles formed from the numerous sources across the globe. The colours show aerosol particles as dust (gold/brown), sea-spray (blue), biomass burning/wildfires (green) and industrial/urban (white).

Aerosols have saved us from global warming then? Not really – aerosols are harmful to our health, specifically contributing to respiratory and heart disease when we breathe them in. We also expect the amount of aerosol in the atmosphere to decline in the future so they won’t provide as strong a cooling effect in the long term.

Embracing uncertainty

So, why are we so uncertain about aerosols? Here are a few of the issues.

1. They are hard to measure.

The first port of call when we try to understand aerosols is actually observing them in either the laboratory or in their natural habitat in the atmosphere. In an ideal world, we want to know the size, shape and chemical makeup of each particle while also understanding how it interacts with both light and water in the atmosphere. Now, consider that these particles range from approximately 10-100,000 times smaller than a human hair and that the number of particles in a piece of air around the size of a sugar cube can range from a few to hundreds of thousands. We want to know how they change over the course of a few minutes, a day and seasonally, as well as having an idea of their historical evolution. We also want to know how they vary across the globe in different environments and how their properties change in different vertical slices of the atmosphere. The really tough part is that because of their short lifetime, they don’t get mixed evenly throughout the atmosphere so there are large regional and vertical gradients in their properties. Keeping an eye on all of that is difficult!

2. We’re not sure where they come from.

On the face of it, the answer is quite simple; we know that aerosols come from both natural and man-made sources. However, we don’t know what proportion is from each of these sources. There is the added complexity of them interacting with each other to form new aerosols. For instance, naturally emitted compounds from trees can interact with emissions from car exhausts to form aerosol particles; is that natural or man-made? If we want to control them in order to alleviate their harmful impacts, we need to know what to control. Our knowledge of aerosol history is patchy, so even if we knew everything about present-day aerosols (which we certainly don’t) we would still struggle to work out the impact of them today compared to the past.

3. Modelling aerosol is hard.

There are a vast array of existing aerosol models which aim to aid our understanding of their properties and impacts. A recent comparison between 10 current climate models that include aerosols found that they are similar in terms of the global amount of aerosol and that they compare reasonably well with satellite observations. The trouble is they don’t always paint a consistent picture in terms of which aerosol components drive this. Some models say dust is the most important species, others say sea-salt, while others say sulphate. This seems like a classic case of being right for the wrong reasons. Furthermore, the models strongly underestimate the absorption of sunlight by aerosols (which can lead to heating rather than cooling) in many regions, which suggests that we don’t yet really understand this important aspect. These are just a couple of examples of current issues with modelling aerosols.

4. We don’t have a crystal ball.

Bearing all of the above in mind, if we are to make projections of future climate impacts due to human activities we need to know how aerosols will change over the coming decades. Historically, changes in man-made aerosol emissions and their properties have been driven by concerns surrounding their impacts on our health and environmental effects, such as acid rain. This has led to large reductions in chemical species such as sulphate and black carbon in North America and Europe, where Clean Air Acts were introduced decades ago. The aerosol landscape in rapidly developing countries in Asia, South America and Africa is difficult to predict. Overall, we expect aerosol amounts to decrease in the future but we might see other chemical species playing a more dominant role. For instance, as the importance of sulphate aerosol decreases, we might see our attention turning towards nitrate aerosol. How aerosol properties evolve in the future is highly uncertain, which presents a challenge when we try to project how the climate will change during the 21^st century.

So there we have it, just a few of the reasons why aerosols are tricky to understand in terms of climate change. This is without considering how they impact our health or their effect on ecosystems. My aim with this blog is to explore this vast area of research that I unwittingly stumbled into around eight years ago.