A significant revision to the climate impact of increasing concentrations of methane

Summary and Frequently Asked Questions relating to the paper:

Radiative forcing of carbon dioxide, methane and nitrous oxide: A significant revision of the methane radiative forcing by M. Etminan, G. Myhre, E.J. Highwood and K.P.Shine., Geophys. Res. Lett, 43, doi:10.1002/2016GL071930

Just when I thought it was safe to take a holiday, our paper presenting new detailed calculations of the radiative forcing for carbon dioxide, nitrous oxide and methane was published in Geophysical Research Letters on 27th December. For me, this paper was a blast from the past, as one of the first papers I wrote as a postdoc in 1998 was on a similar topic,  and shares 3 out of 4 authors, myself, Keith Shine and Gunnar Myhre.

The recent paper, co-authored with PhD student Maryam Etminan, describes new research on methane’s climate impact that has been performed at the Department of Meteorology at the University of Reading, UK and the Center for International Climate and Environmental Research – Oslo (CICERO) in Norway; it indicates that the climate effect of changes in methane concentrations due to human activity has been significantly underestimated. It also uses these detailed calculations to revise the simplified expressions for estimating radiative forcing adopted by the Intergovernmental Panel on Climate Change (IPCC). The new calculations indicate that the direct effect of increases in the concentration of methane on climate is 25% higher than represented by the expressions previously adopted by the  IPCC, making its present-day radiative forcing (relative to pre-industrial values) about one-third as powerful as carbon dioxide.

The paper has attracted some attention on social media as the calculations may have an impact on policy decisions in the future. Therefore I take the opportunity here, with much of the text below written by my co-author Keith Shine, to both summarise the study and answer some of the questions that have arisen so far.

Does this mean that our previous estimates of radiative forcing due to carbon dioxide have been over-estimated?

No. Carbon dioxide remains the most significant greenhouse gas driving human induced climate change.

In fact in this study we also looked at the estimates of forcing due to carbon dioxide using the same physical understanding as used for methane, and found forcing very similar to previous estimates, except for some underestimation at very high carbon dioxide concentrations.

So if the forcing due to methane has been underestimated in the past, why hasn’t the global mean temperature increased more?

The climate impact (e.g. temperature change) resulting from a radiative forcing change in the atmosphere depends on both the radiative forcing and how the climate system responds to that forcing. Although we have shown that the carbon dioxide forcings are little different to our earlier calculations, there are other changes that cause a radiative forcing that have documented very large uncertainties, for example aerosols (and in particular their impact on clouds) that could easily counteract the additional forcing from methane. Even if we knew the forcing accurately, the uncertainty in the climate response is also large enough that it isn’t a problem to reconcile the observed temperature changes. We are in fact refining the uncertainties through this type of study.

So why the focus on methane?

Human activity has led to more than a doubling of the atmospheric concentration of methane since the 18th century. Methane is a powerful greenhouse gas. It is the second most important greenhouse gas driving human-induced climate change, after carbon dioxide. Its warming effect had been calculated to be about one-quarter of that due to carbon dioxide. Methane emissions due to human activity come from agricultural sources, such as livestock, soil management and rice production, and from the production and use of coal, oil and natural gas.

What did you do that was different?

Previous calculations had focused attention on the role of methane in the “greenhouse” trapping of infrared energy emitted by the Earth and its atmosphere, primarily at wavelengths of around 7.5 microns. The vital element in the new research is that detailed account is taken of the way methane absorbs infrared energy emitted by the Sun, at wavelengths between 1 and 4 microns.

The effect of this additional absorption of Sun’s infrared radiation is complicated, as it depends on the altitudes at which the additional energy is absorbed. This determines whether the extra absorption enhances or opposes the greenhouse trapping. It has been known for many years that the absorption of the Sun’s energy by carbon dioxide reduces its climate effect by about 4%, because much of the additional absorption happens high in the atmosphere.

The new calculations of the effect of methane indicate that much of the extra absorption is in the lower part of the atmosphere, where it has a warming effect. The research shows that clouds play a particularly important role in causing this enhanced warming effect. Clouds scatter some of the sun’s rays back into space; it is the additional absorption of these scattered rays by methane that drives the warming effect, a factor that had not been included in earlier studies.

Are these results important for climate change negotiations?

The new calculations are important for not only quantifying methane’s contribution to human-induced climate change, but also for the operation of the Kyoto Protocol to the United Nations Framework Convention on Climate Change (UNFCCC). This takes into account emissions of many greenhouse gases in addition to carbon dioxide. The emissions of these other greenhouse gases are given a “carbon-dioxide equivalence” by multiplying them by a quantity called the “100-year Global Warming Potential” GWP(100); a similar approach is likely to be adopted by most countries for the operation of the UNFCCC’s more recent Paris Agreement.

The GWP(100) for methane includes not only its direct impact on the Earth’s energy budget, but methane’s indirect role, via chemical reactions, on the abundance of other atmospheric gases, such as ozone. Applying the results of the new calculations to the value of the GWP(100) presented in the Intergovernmental Panel on Climate Change’s (IPCC) most recent (2013) assessment, enhances it by about 15%. This means that a 1-tonne emission of methane would be valued the same as 32 tonnes of carbon dioxide emissions, up from the IPCC’s most recent value of 28. Hence for countries with large emissions of methane due to human activity, it would lead to a significant re-valuing of their climate effect, relative to emissions of carbon dioxide.

Can you be more specific about that?

In fact the GWP values have changed substantially due to new research since the Kyoto Protocol, and these changes are reported in the IPCC reports.

CO2 remains the dominant greenhouse gas emission from both developed (so-called Annex1) countries (77%) and non-Annex1 (65%) countries but using our revised value in place of the IPCC AR5 value, methane emissions now exceed 40% of CO2 emissions in developing countries, in CO2 equivalent terms, up from 36%. In developed countries, they are now almost one-quarter of the CO2 emissions (23% up from 20%).


So when might these new values influence policy?

The research team identified a number of uncertainties in the calculation of this enhanced absorption by methane, which will require further research to reduce. The new results are unlikely to be recommended for adoption in international treaties until they have been fully considered by the assessment process of the Intergovernmental Panel on Climate Change.

The new research was partly funded by the Research Council of Norway, and the UK’s Natural Environment Research Council.

The full paper is   Radiative forcing of carbon dioxide, methane, and nitrous oxide: A significant revision of the methane radiative forcing by M. Etminan, G. Myhre, E. J. Highwood, and K. P. Shine, Geophysical Research Letters, published online 27 December 2016, DOI: 10.1002/2016GL071930

It is open access and can be found at http://onlinelibrary.wiley.com/doi/10.1002/2016GL071930/full

 The IPCC working group 1 (2013) assessment report “Climate Change 2013, The Physical Science Basis can be found at https://ipcc.ch/report/ar5/wg1/


Climate poetry

In honour of National Poetry Day and inspired by the Royal Society meeting on “Next Steps in Climate” here is my rather un-sophisticated summary of the meeting in poem form.

The climate is a-changing

We observe that it is so

Surface temperatures are rising

AND there is less of ice and snow

We climate scientist are (95%) confident

That human emissions are involved

But there is still room for further study

Some puzzles remain unsolved

Aerosols and fluffy cloud

Deep ocean heat uptake too

Need more obs and better models

I accept the challenge. Do you?

Climate forcing: The impact of aerosols on mid-century temperature hiatus

This post is based on our recent paper: “The influence of anthropogenic aerosol on multi-decadel variations of historical global climate” by Laura Wilcox, Ellie Highwood and Nick Dunstone, which appeared in Environmental Research Letters. One of our main conclusions was:

  • Mid-century (1950s-1960s) temperature hiatus, and coincident decrease in precipitation, is likely to have been influenced strongly by anthropogenic aerosol forcing.

Here I describe how we came to this conclusion, and use an analogy with Newton’s laws of motion to describe the competing influences on climate.

Background: An increase in aerosol concentration (the particulates) in the atmosphere generally acts to cool the global climate, as sunlight is reflected back to space – we refer to this as the aerosol direct effect. Aerosols can also change the properties of clouds, making them more reflective, or last longer – this is the “indirect effect”.  There have been lots of recent studies linking aerosol changes to changes in important parts of our climate system: e.g.  Atlantic temperatures, Sahel rainfall and hurricanes.

But aerosol has played an important role in the historical changes of global temperature too. Unlike recent greenhouse gas changes, the sign of the influence of aerosols on global mean temperature has varied over time. Whilst greenhouse gas concentrations in the atmosphere have been growing consistently over the past 250 years, aerosol emissions and concentrations have a much more complex history. As aerosols can be washed out by rain, they last for only a few days to weeks if they remain in the lowest part of the troposphere. This means that the pattern globally is very non-uniform (see figure of Aerosol Optical Depth (AOD) below from NASA)  and that high concentrations tend to be close to emission regions. Thus if the emissions change over time (e.g. clean air act  in Europe reducing aerosol emissions on the grounds of improving air quality since 1960s), this would be seen in the time series of aerosol in the atmosphere too.


The timeseries of aerosol from climate models (we have to do it from models because although we have some idea of the changes in emissions, we have only been able to look at what aerosol is actually in the atmosphere globally for around 20 years) looks like the figure below which is taken from Wilcox, Highwood and Dunstone (2013). Thus there have been times of increase, and times of much smaller change.


What does this mean for the influences on global temperature? Let’s picture the global mean temperature as a floating ball. According to Newton’s laws of motion, this ball will remain stationary unless a net force acts upon it. Now imagine all the possible things that could be pushing the temperature higher (increases in greenhouse gases, decrease in aerosol) as a force on one side of the ball, and all the things that could be pushing the temperature lower (increase in aerosol) on the other side. If “cooling” influences win at any time, then the ball will move towards cooler temperatures. If stronger cooling influences exist, then it will move faster. If cooling and warming influences are more equal, then the ball will stay still, and we would see little trend in global mean temperature. See what we mean in an animation

What did we do?
We used a technique especially developed to deal with noisy timeseries, called Ensemble Empirical Mode Decomposition . This works by decomposing a noisy timeseries into a set of oscillating functions, as shown in figure 2.


We applied this technique to the timeseries of global mean temperature, rainfall, and the hemispheric gradient of temperature. We used simulations that included all things likely to affect climate change (greenhouse gases, anthropogenic aerosols, solar changes, volcanoes), as well as simulations that only included some of these things. We also separated models by the types of aerosol effect that are included.

What did we find out?

Using the different simulations, and imagining that the line in this animation  represents the position of the “global temperature ball”, we can show how the influences from aerosol, greenhouse gases and natural forcings changes over time, the arrows in this animation are the forces pushing on the ball.
Because adding up the simulations with single forcings produce the same total temperature time series (by coincidence?), we can put a percentage on the contribution due to each forcing. Aerosol forcing accounts for over 50% of the variability before 1970, and in excess of 70% in the 1940s-1960s.


What’s next?
One of the big uncertainties in the future is how aerosol emissions will change. The climate model simulations we have access to have assumed that aerosol emissions will decrease dramatically by the year 2100. Thus in the global mean aerosol will move from being a cooling push to being a warming push. Thus we will enter a period unlike all others in the recent past, where both greenhouse gases and aerosols are pushing in the same direction. There are enough non-linearities in the climate system to make us think that we might see some new types of changes – particularly in rainfall and circulation patterns. We will look at these competing influences for specific regions, and in the future, using EEMD and other techniques. Should be interesting!

Walk to School week… and travelling for work

This week it is “Walk-to-school” week in the UK. This morning on the way to school my son and I discussed driving to school vs walking to school – which was interesting, although his take on global warming “but then the planet might explode – it’s a good job we’re going on holiday” leaves some room for improvement.

Here is an article I wrote at the same time last year for theWeather magazine of the Royal Meteorological Society.

“This week it is “walk to school week” and “eco-week” at my children’s nursery. So, as well as wearing green and making models out of recycled materials,  we’ve been taking the bus in the mornings and walking home in the evenings (well alright, I’ve been walking, the little ones have been riding in the luxury of their double buggy). They love counting the passengers at each bus stop in the morning, and the opportunity to run across a daisy-filled park on the way home. I am discovering muscles I forgot I had! However, it does at least allow me to temporarily ease my guilt at the carbon footprint I have just by being a climate scientist .

Our success at tackling global scale problems such as climate change relies on meaningful and productive collaborations between institutions across the globe. This leaves many climate scientists with a quandary- how to maximise the usefulness of their research whilst minimising their carbon footprint?  The development of telephone conferencing via the internet which allows presentations to be shared across many desktops is one possible option. However there is no doubt that a face-to-face meeting at a conference or workshop is often the only way to make progress fast and unambiguous. For example, the Intergovernmental Panel on Climate Change holds its work meetings around the globe to allow the inclusion of as many scientists and policy makers from different regions as possible to attend at some point.

However, the transport sectors are responsible for around 20% of the global CO2 emissions, as well as producing various gaseous and particulate emissions which also contribute to air pollution and climate change. Over the past few years the impact of aviation on climate has received much attention – carbon dioxide emissions, contrails and cirrus cloud that develops from contrails contribute to global warming. This has led many organisations with a climate conscience or a public commitment to reduce carbon emissions to discourage the use of air travel. What received less attention in the media at least is the fact that globally, the greenhouse gas emissions from road traffic produces several times the change to the energy balance of the earth (leading to climate change) as global air travel. The energy change due to particles is also substantially larger for road traffic as these aerosols absorb solar radiation and generally warm up the atmosphere. The effects of maritime shipping receives some press in the guise of the “food miles” debate, but its overall effect on global temperature is probably to cool it since high sulphur content fuel results in more sulphate aerosols which scatter solar radiation away from the surface. Reducing the sulphur content in shipping fuel (desirable from an air pollution standpoint) may actually exacerbate global warming slightly.

Some scientists subscribe to carbon offset schemes either privately or more publicly, but the choice of off-set mechanism requires careful thought – not all schemes are equal in effectiveness or scientific value. Additionally, some of my group’s research relies on flying on research aircraft – potentially contributing to the particles that they are measuring (obviously we don’t fly around measuring our own emissions directly!). This has always seemed like something of a contradiction to me (and  many others, see  for example Kevin Andersons blog and this one.. ) . Striking a balance between travel that makes our research more effective, efficient and scientifically sound and minimising our impact on the environment is a considerable challenge, but should be applied to everyday car driving as well as flying). At least I’ll have something to ponder as I push the buggy up the hill towards home.”

Note that references to scientific journals were not included in the original article due to it’s intended audience, but details can be found in the following papers.

Balkanski, Y., Gunnar Myhre, Michael Gauss, G. Rädel, E Highwood and Keith P. Shine, 2010. Direct radiative effect of aerosols emitted by transport: From road, shipping, and aviation. Atmos. Chem. Phys., 10: pp. 4477-4489.

Skeie, Ragnhild Bieltvedt, Jan S. Fuglestvedt, Terje Berntsen, Marianne Tronstad Lund, Gunnar Myhre and Kristin Rypdal, 2009. Global temperature change from the transport sectors: Historical development and future scenarios. Atmospheric Environment, 43 (39): pp. 6260-6270.

Fuglestvedt, Jan S., Terje Berntsen, Gunnar Myhre, Kristin Rypdal and Ragnhild Bieltvedt Skeie, 2008. Climate forcing from the Transport Sectors. Proceedings of the National Academy of Sciences (PNAS), vol 105 (no. 2): pp. 454-458.