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What did the IPCC say about aerosols?

Will Morgan February 11, 2014

Aerosols dominate the uncertainty in the total anthropogenic radiative forcing. A complete understanding of past and future climate change requires a thorough assessment of aerosol-cloud-radiation interactions.

This is one of the conclusions about aerosols and their impact on our climate from the the final report from the Intergovernmental Panel on Climate Change (IPCC) on the physical science basis that was released recently. What brought them to this conclusion and what does it mean? In this piece, I’ll go through the headline statements and findings.

The IPCC are concerned with assessing how various factors affect the Earth’s climate and it dedicates an entire chapter to aerosols and clouds, which is accessible here.

How do aerosols impact our climate?

Aerosols, can broadly influence the climate in two ways; aerosol-radiation interactions and aerosol-cloud interactions. The IPCC included some nice diagrams of how these processes occur, which I’ve included below.

Illustration of aerosol-radiation interactions produced by the IPCC. This figure and more are available here.

Illustration of aerosol-cloud interactions produced by the IPCC.

Both of these interactions are well established but the problem is pinning down the magnitude of these effects at larger scales and over time.

Headline numbers

The IPCC often refer to a term known as radiative forcing when assessing climate impacts. This illustrates the change in the amount of incoming versus outgoing radiation to our atmosphere. This regulates global temperatures and can affect things like circulation patterns and precipitation. Positive radiative forcing values increase temperature, while negative values reduce them. This value is usually referenced back to before the Industrial Revolution (typically 1750), when human influences on the composition of the atmosphere were more limited.

To put this in some context, the radiative forcing for carbon dioxide is 1.82 W/m², with a reported range from 1.63 to 2.01 W/m². What this means is that since 1750, adding carbon dioxide to our air has altered the radiation balance of the atmosphere and that this will cause a warming in the absence of any cooling factors.

In the table below, I’ve collated the various estimates for the aerosol influence from the report. As well as the central estimate, it is important to examine the range as this is one of the crucial aspects that drives the statements about our uncertain understanding of aerosols. The central estimates and the forcing range all suggest that aerosols exert a cooling influence on our climate; aerosols offset some of the warming influence from carbon dioxide and other greenhouse gases. If we first focus on the IPCC 2007 and 2013 values, we see that the central estimate has for the latest report (AR5) indicates that aerosols are exerting less of a cooling effect than estimated previously. The range of the forcing estimate has also reduced.

The IPCC is effectively saying that the cooling influence from aerosols is slightly weaker than previously estimated and that our understanding has improved.

Aerosol radiative forcing estimates.

The other values refer to different ways of calculating the impact and it is these numbers that inform the overall value in the report. The satellite based value refers to studies where satellite measurements of aerosol properties are used in conjunction with climate models; they are not wholly measurement based. In terms of studies using climate models on their own, the IPCC used a subset of climate models for their radiative forcing assessment, choosing those that had a “more complete and consistent treatment of aerosol-cloud interactions”.

The satellite based central value of -0.85 W/m² is less negative than the central value from the climate models, which means the models indicate more cooling than the satellite based estimate. Compared to the subset of climate models that the IPCC used for their radiative forcing judgement, there is little overlap between their ranges also.

Why so uncertain?

This lack of agreement is a big driver for the large uncertainty range. It’s important to stress that there isn’t a strong reason to “trust” one set of results over another here as both satellite observations of aerosol properties and their representation in climate models are prone to many biases and it isn’t currently clear how these will impact the results.

Both methods are very sensitive to the assumed pre-industrial conditions assumed by the studies. Both methods have difficulties dealing with clouds but for different reasons.

Satellites can’t typically see through clouds, so they can miss aerosol trapped below them. Another major challenge is when layers of aerosol exist above clouds, which can affect the satellite measurements. Satellites are best suited to measuring the total amount of aerosol in the atmosphere. They have difficulty identifying different aerosol types, which means they can lump together natural and anthropogenic aerosols. They also have difficulty determining the relative importance of scattering or absorbing aerosols, which will determine the climate impact. Typically they can’t determine where the aerosol is in the vertical dimension of the atmosphere, which again will influence the climate impact. There are certainly advances in these areas but as ever, more work is required.

Models on the other hand, have difficulty representing aerosol-cloud interactions due to cloud systems typically being smaller than the resolution of a climate model, the complex number of processes at play and the lack of real-world measurements to test them. There is some evidence to suggest that climate models at the global scale tend to overestimate the size of the aerosol effect on cloud properties.

If we examine more details from the climate models, we see that there are large differences between models in terms of what types of aerosol it considers to be important. For example some models say that dust is a major contributor to the global aerosol burden, while others disagree. These are important details as climate models can sometimes broadly agree in terms of the radiative forcing estimate they provide but for very different reasons. Black carbon is another species that can contribute to varying degrees in different models, which is important as it warms the atmosphere; how a model represents black carbon is going to have a strong influence on the reported cooling. Nitrate is a potentially important species that often isn’t even included in climate models.

To summarise: it’s complicated.

What does this mean for climate change?

The short answer is: probably not a lot.

The change from the previous report is overall quite small (0.3 W/m²) and this is predominantly a result of improvements in our ability to represent aerosol processes (particularly clouds) since the last report. To put the change in context, between 2005 and 2011, the radiative forcing by greenhouse gases increased by 0.2 W/m² due to increased concentrations in our atmosphere alone. At present, the uncertainties in aerosol radiative forcing mean that making definitive statements are likely unwise and putting faith in any one particular result will be fraught with difficulties. Focussing on the global scale also ignores the much larger regional impacts that aerosols can have, which is of more relevance to the wider issue.

The current state of understanding of aerosols suggests that they’ve exerted a cooling influence on our climate, which has offset some of the warming expected from the increase in greenhouse gases in our atmosphere. Improving this understanding will be crucial for assessing both past and future climate change.

AGU 2013 roundup

Will Morgan December 15, 2013

Now that the 2013 AGU Fall Meeting has ended, I thought I would roundup what I’ve been involved with over the week for both this blog and the Barometer Podcast, which I was recording each day with Sam Illingworth. Links to each piece are available below. Many thanks to all who have read and shared these over the past week.

Recording the podcast at conferences is becoming a trend as we’ve covered AGU now in 2012 and 2013 plus the EGU in 2013. Recording these is a lot of fun and particular thanks should go to Dave Topping, Bethan Davies and Mark Brandon for giving up their time to chat to us this week. Lastly, many thanks to Sam for his infectious enthusiasm and for being the only person I’ve ever met with a louder laugh than me.

The conference itself was excellent throughout, even if the amount of science on offer was overwhelming at times. The sessions on science communication I attended were also fantastic, thought-provoking and often inspiring. I’m planning to write a separate post on this aspect over the coming days.

So long San Francisco! Image: Will Morgan

So long AGU 2013 and thanks for all the science! Image: Will Morgan

Blog posts

AGU 2013 day 1: Short-lived climate forcers

AGU 2013 day 2: aerosol emissions, climate & the IPCC

AGU 2013 Day 3: secondary organic aerosol – animal or vegetable?

AGU 2013 Days 4 & 5: measurements & models

Podcasts

http://thebarometer.podbean.com/2013/12/10/fires-beer-and-satellites-day-1-at-agu/

http://thebarometer.podbean.com/2013/12/10/disappointment-aerosols-and-methane-burps-day-2-at-agu/

http://thebarometer.podbean.com/2013/12/12/hansen-nuclear-power-and-geologists-day-3-at-agu/

http://thebarometer.podbean.com/2013/12/13/science-communication-viscosity-and-londons-greenhouse-gases-day-4-at-agu/

http://thebarometer.podbean.com/2013/12/13/communicating-big-data-and-a-love-of-models-day-5-at-agu/

AGU 2013 Days 4 & 5: measurements & models

Will Morgan December 14, 2013

My fourth and fifth days at the AGU Fall Meeting involved dashing between multiple sessions to take in a number of talks on (surprise, surprise) aerosols! The main strand running through them from my point of view was how there are major efforts to construct large datasets of aerosol properties that can be used to test our understanding via numerical models.

Aerosols are complex and tend to stick around in the atmosphere for hours-to-weeks, rather than years-to-centuries. This means that they are spatially very diverse and also vary with time e.g. with the passage of seasons. This presents a challenge for our understanding but by constructing large datasets, it can also be used to constrain our numerical models.

For example, by looking at years of winter data for European pollution we might find that our models represent this quite well but it might be that there are significant errors when we look at the summer. This information is useful and might point us towards places to study further and improve our understanding. Just looking at a global annual average, this information is lost.

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).

There were several talks detailing efforts to build satellite data sets of aerosol properties over long periods and use these to assess numerical models. Phillip Stier from the University of Oxford sounded a cautionary note by illustrating how different satellites can sometimes deliver different results for certain properties. He showed three different satellite images of cloud effective radius, which is a property of clouds that is important for how they interact with sunlight, and they were all very different. One of them agreed with his model but which set of measurements do you believe? He ended with a call to expand initiatives that seek to collate data from surface and aircraft measurements to test satellite measurements and models.

Dan Murphy from the National Oceanic & Atmospheric Administration paper presented some work on trends in aerosol over the past decade. He showed that while at a global level, the amount of aerosol in the atmosphere hasn’t really changed, it has changed significantly at the regional level. Aerosol concentrations have decreased in the USA and Western Europe, while they have increased in East Asia and in the Middle East. Aerosol has moved around a lot but there has not been a discernible change in their radiative effect at the global level. However, he said we couldn’t rule out that aerosols haven’t caused other adjustments such as circulation changes and clouds. You can access his Nature Geoscience paper on his work here. His work is consistent with several other papers that have shown negligible trends in global aerosol after the past decade.

These were highly interesting talks and should serve us well as we attempt to improve our understanding of aerosols, although there is much work still to do. No doubt we can anticipate much more work on this in the future – one of the talks did proclaim that:

Aerosol reanalysis is trendy.

No doubt we’ll be seeing the influence of this in the fashion houses of London, Paris and New York soon.

AGU 2013 Day 3: secondary organic aerosol – animal or vegetable?

Will Morgan December 12, 2013

My third day at the AGU 2013 Fall Meeting involved lots of talks on one of the trickiest parts of aerosol science – secondary organic aerosol (SOA). We’ve known for several years now that SOA is ubiquitous across the globe and it is often the most dominant aerosol chemical species in many environments and this is particularly true in the industrialised regions of the Northern Hemisphere. The trouble, we’re not sure where it comes from and how it forms…

SOA represents the carbon-containing fraction of atmospheric aerosol that forms as a consequence of chemical processes in the atmosphere. Instead of being directly emitted in particle form e.g. from a car exhaust, they form from gases known as volatile organic compounds (VOCs) which can be emitted by both ourselves via burning fossil fuels or from natural sources such as trees. This latter glass of “biogenic” compounds is what gives pine trees and others their distinctive smell.

Morning fog in the Great Smoky Mountains. Image from EGU Imaggeo image repository and is provided by Oliver Pratt.

Morning fog in the Great Smoky Mountains. See also the haze in the background in the top left. Image from EGU Imaggeo image repository and is provided by Oliver Prat.

An area where there are large emissions of these biogenic compounds is the South-East of the USA and they are responsible for the ‘smokiness’ of the Great Smoky Mountains, which are pictured above. The SE USA is also interesting as parts of it have not warmed like other parts of the USA due to global warming – in fact, some areas have actually shown a cooling trend. One theory is that aerosols formed from the biogenic compounds are involved.

Many of the talks on SOA this week have been based on measurements from an array of projects that took place in the SE USA in 2013. Some of the main themes/conclusions include:

SOA from biogenic emissions of a VOC called isoprene are an important component. Isoprene is the most abundant biogenic VOC worldwide (aside from methane), so it potentially represents a large source of SOA.
Several of the measurements showed a strong link between isoprene SOA and sulphate aerosol, which is typically from human sources such as power plants. This is consistent with the work of Jason Surratt’s group and others. This is particularly important as it demonstrates how emissions from human activities can interact with biogenic emissions to form pollution, which was postulated to be important by Allen Goldstein in 2009. Yet another example of our impact on the atmosphere.
Condensed water is a significant part of aerosols in the SE USA, which is important for the impact of aerosol on climate. However, the evidence for it being a major driver of SOA formation, as suggested by Annmarie Carlton was limited. Further studies are required klaxon!
ISOPOOH is an important oxidation product of isoprene that forms when Tigger and Winnie have an argument and potentially leads to SOA formation.

New toys

Overall, the talks on SOA from these measurement studies were a fantastic demonstration of many of the new techniques to characterise SOA. These new techniques will certainly improve our understanding of SOA formation, particularly in areas where large volumes of our own emissions interact with the biogenic emissions.