Category Archives: Inverse modeling

Methane growth and global OH concentration

There have been several recent papers exploring the growth in methane that has been ongoing since 2007. In a new paper in PNAS, we explore whether changes in the global OH concentration could be playing a role. By looking at trends in methyl chloroform, we infer an intriguing rise and fall in OH over the last 20 years, which could explain some of the methane change. There are significant uncertainties remaining, and much more work is needed before we can determine with confidence the role of changes in both methane sources and sinks. This is the focus of the new UK-wide NERC-funded MOYA project which will be on-going for the next three years.

A MOZART simulation of OH concentrations (Courtesy: Angharad Stell)

There’s a University of Bristol news item, which provides more details on this story. The paper can be found here: Rigby et al., Role of atmospheric oxidation in recent methane growth, PNAS, doi:10.1073/pnas.1616426114, 2017

 

Examination of HFC emissions in the UK leads to Government rethink on how levels are recorded

Dan Say’s work on our re-evaluation of the UK’s HFC emissions featured on the University of Bristol News. Based on an analysis of atmospheric data, we found that the UK’s estimates of the emissions of HFC-134a, used in car air conditioning, were likely too high. Dan took a closer look at the UK inventory and found that estimates of the frequency at which car air conditioning units were refilled, and the number of cars with air conditioning units in them were likely over-estimated in the inventory.

The government is now re-evaluating the assumptions that go in to their HFC-134a calculations.

 

University of Bristol impact case study

Our work was featured as an impact case study in the 2013 Research Excellence Framework. A summary is on the University of Bristol website:

SF6 around the UK.
SF6 in the atmosphere around the UK.

“The amount of greenhouse gases that the UK produces is calculated annually by the Department for Energy and Climate Change (DECC). Researchers at the University of Bristol independently verify these estimates using atmospheric measurements, making the UK one of only three countries in the world that does so.

“Our work is used by the DECC for monitoring compliance with international and domestic legislation; identifying priorities for improving inventory accuracy; assessing the UK’s progress towards targets set in the Montreal and Kyoto Protocols; evaluating the impact of policy; and informing international negotiations.” – Professor Simon O’Doherty

The UK is signed up to the Montreal Protocol (which aims to reduce the amount of ozone depleting compounds emitted) and the Kyoto Protocol (which aims to reduce the amount of anthropogenic greenhouse gases in the atmosphere). These require the UK to report its emissions on a regular basis, but it’s not as straightforward as simply measuring the gases in the atmosphere.

Reporting the amount of man-made greenhouse gases that the UK produces annually is a challenging task. Like the other 191 countries that have signed up to the Kyoto protocol, the UK uses an inventory approach to estimate the emissions, as directly measuring anthropogenic greenhouse gases is too complicated. This involves estimating emissions from a variety of activities such as burning fossil fuels, agriculture and energy production. But the UK is one of only three countries that go one-step further, by using atmospheric measurements to validate these inventory calculations.

This independent verification is performed by researchers from the University of Bristol’s Atmospheric Chemistry Research Group which is part of the Cabot Institute and the School of Chemistry. Using a combination of physical measurement and sophisticated modelling techniques, Professor Simon O’Doherty and Dr Matt Rigby work in collaboration with Dr Alistair Manning from the UK Met Office to monitor the greenhouse gases in the atmosphere above the UK.

In order to do this, the researchers developed the UK DECC Network – a unique national greenhouse gas monitoring system comprising six stations making high-frequency measurements of key atmospheric trace gases. Analysis and interpretation of these observations using state-of-the-art modelling techniques enables the independent assessment of the UK’s adherence to the Montreal and Kyoto Protocols.

“Our work started more than 20 years ago when we provided data to the Government on the accumulation of ozone-depleting gases in the atmosphere resulting in the signing of the Montreal Protocol,” says O’Doherty. “We have been able to show how the use of chlorofluorocarbons (CFCs) has risen and fallen over the years and that direct measurement of atmospheric gases can be used to monitor the impact of legislation such as the Montreal Protocol.”

Back then O’Doherty only had one monitoring station at his disposal, but today he can use data from a network of stations across the country as well as aircraft, satellites and even ferries that measure climatically important gases such as carbon dioxide, methane and nitrous oxide. When combined with models of atmospheric gas transport, these observations provide an independent means of assessing natural and man-made emissions. As well as monitoring the UK’s compliance with international treaties, these data have been central to recent World Meteorological Office (WMO) Scientific Assessments of Ozone Depletion produced between 2007 and 2010 and to the Nobel Prize-winning Inter Governmental Panel on Climate Change (IPCC) Assessment of Climate Change published in 2007.

The data will also form the basis for negotiations of future targets for UK emissions.

“Future climate treaties will take recent emissions estimates as a baseline from which to plan emissions reductions. Therefore, it’s really important that we are able to get these estimates right, both in the UK and around the world, so that the burden for emissions reductions is shared in a fair way,” says Dr Rigby.

One of the biggest challenges for the future is distinguishing between natural and man-made greenhouse gases. O’Doherty and Rigby are now investigating new techniques that could measure different isotopic compounds and thus distinguish between anthropogenic and naturally emitted greenhouse gases.

Further information

Key facts:
• The underpinning research was funded by the Department for Energy and Climate Change, the Natural Environment Research Council and NASA.
• Man-made greenhouse gas emissions cannot currently be measured directly but instead are calculated by estimating emission from a number of activities such as the burning of fossil fuels.
• Bristol researchers independently verify these calculations by taking atmospheric measurements and using atmospheric models to calculate the source of the emissions.
• The UK now has a network of stations, managed by Professor Simon O’Doherty, that continuously monitor important atmospheric gases.
• The data from the monitoring stations is used to verify if the UK is adhering to the Montreal and Kyoto protocols, as well as to inform international policy on climate change.
• The UK is one of only three countries in the world that collects data and verifies emissions in this way.”

Unexplained gap in global emissions of potent greenhouse gases resolved

In a new paper, published in the Proceedings of the National Academy of Sciences (PNAS), we examine the gap between the global emissions of HFCs that we infer from atmospheric observations and those reported by developed countries. The findings of the paper are summarised in a University of Bristol press release:

Until now, there has been little verification of the reported emissions of hydrofluorocarbons (HFCs), gases that are used in refrigerators and air conditioners, resulting in an unexplained gap between the amount reported, and the rise in concentrations seen in the atmosphere.  This new study shows that this gap can be almost entirely explained by emissions from developing countries. 

Currently only 42 countries are required to provide detailed annual reports of their emissions to the United Nations Framework Convention on Climate Change (UNFCCC). 

The study, led by Mark Lunt from Bristol’s School of Chemistry used HFC measurements from the international Advanced Global Atmospheric Gases Experiment (AGAGE), in combination with models of gas transport in the atmosphere, to evaluate the total emissions that are reported to the UNFCCC each year.

HFCs are potent greenhouse gases; per tonne of emissions, each gas measured in this work is hundreds or even thousands of times more effective than carbon dioxide at trapping the radiation that warms the Earth.

There is currently no global agreement to regulate the emissions of these compounds, although proposals have been made to begin phasing out their use. 

Mark Lunt said: “Any phase-out mechanism would likely be more stringent for the developed countries, but these results show that emissions from non-reporting countries are also highly significant.”

Meanwhile, the researchers note that although their estimates of total emissions from developed countries are broadly consistent with the reports that they compile, this does not necessarily mean that the emissions of each gas are being accurately reported. 

In fact, the results suggest that the most commonly used HFC is significantly over-reported whilst some other HFCs are under-reported. 

Dr Matt Rigby from the University of Bristol, who co-authored this work, said: “It appears as if the apparent accuracy of the aggregated HFC emissions from developed countries is largely due to a fortuitous cancellation of errors in the individual emissions reports.”

Professor Ron Prinn from the Massachusetts Institute of Technology (MIT), who leads the AGAGE network, added: “This study highlights the need to verify national reports of greenhouse gas emissions into the atmosphere.  Given the level of scrutiny these reports are under at the moment, it is vitally important that we improve our ability to use air measurements to check that countries are actually emitting what they claim.”

Anaesthetic gases have a (small) influence on climate

In a paper published in Geophysical Research Letters, we quantify the influence of anaesthetic gases on global atmospheric radiative forcing. It turns out that inhalation anaesthetics are potent greenhouse gases, with 1 kg of emissions of desflurane (a commonly used anaesthetic) having the influence of around 2,500 kg of CO2. However, emissions of these gases are very low compared to CO2, so the influence on climate is still relatively small.

This paper is nicely summarised in an American Geophysical Union press release, and was picked up by the Daily Mail.

A 6-year-old girl is prepared to go under anesthesia prior to undergoing a hydrocelectomy at Forward Operating Base (FOB) Farah, Afghanistan. A new study finds that anesthesia gases are accumulating in the Earth’s atmosphere, where they make a small contribution to climate change. Credit: ISAF Headquarters Public Affairs Office/Wikimedia Commons
A 6-year-old girl is prepared to go under anesthesia prior to undergoing a hydrocelectomy at Forward Operating Base (FOB) Farah, Afghanistan. A new study finds that anesthesia gases are accumulating in the Earth’s atmosphere, where they make a small contribution to climate change.
Credit: ISAF Headquarters Public Affairs Office/Wikimedia Commons

Little-studied man-made gases have big warming potential

We recently published a paper “Recent and Future Trends in Synthetic Greenhouse Gases” in Geophysical Research Letters describing recent trends in “synthetic” greenhouse gases (gases with no significant natural sources), and their possible impact on climate in the coming decades. The paper was summarised on the American Geophysical Union blog (thanks to Alexandra Branscombe!), and is described in the following University of Bristol press release:

Radiative forcing due to synthetic greenhouse gases. Historical values are based on AGAGE observations. Future values assume the HFC phase down schedule proposed by the US Environmental Protection Agency (2012), compared to previously published "non-policy" scenarios.
Radiative forcing due to synthetic greenhouse gases. Historical values are based on AGAGE observations. Future values assume the HFC phase down schedule proposed by the US Environmental Protection Agency (2012), compared to previously published “non-policy” scenarios.

The total warming impact of 25 major synthetic greenhouse gases has been examined by an international team, led by researchers from the University of Bristol.

The study estimates that, without additional limits on synthetic greenhouse gas use, the resulting increase in warming could outweigh the climate benefits gained thus far from phasing down chlorofluorocarbons (CFCs).

CFCs—commonly used in refrigerators and air conditioners—garnered public attention for their role in creating a hole in the ozone layer over Antarctica.  As these chemicals were phased-down thanks to international agreements limiting their use, they were replaced by other synthesized gases that can still be harmful to the ozone layer and are greenhouse gases that contribute to climate change.  Despite this, synthetic greenhouse gases (SGHGs) beyond the CFCs have received relatively little attention from the research community—until now. 

The study, led by Dr Matthew Rigby in Bristol’s School of Chemistry, analysed observed atmospheric levels of SGHGs from 1978 to 2012, and then used these measurements to predict the impact these gases could have on global warming through 2050.

In response to the phase-down of CFCs through the 1987 Montreal Protocol, the researchers discovered that the use of other synthetic gases as refrigerants—such as hydrofluorocarbons (HFCs)—has risen.  HFCs had been limited in the now-defunct 1997 Kyoto Protocol, but there is currently no agreement restricting their use.  So, using HFCs as a test case, the researchers examined the effect of phasing down HFCs by amending the Montreal Protocol to include these gases.

Dr Rigby said: “We could avoid adding the equivalent of up to another three years of carbon dioxide emissions into the atmosphere if these gases were being phased down.”

HFCs are particularly strong greenhouse gases, so even relatively small levels in the atmosphere can contribute to warming.

“Per tonne of emissions, HFCs are much more potent greenhouse gases than carbon dioxide, and are very good at trapping the radiation that heats the Earth,” Dr Rigby said.

While HFCs are currently not a major driver of climate change compared to carbon dioxide or even other SGHGs, the researchers point out that if unabated they may contribute significantly to future warming.

The study used measurements of SGHG levels from the Advanced Global Atmospheric Gases Experiment (AGAGE), a global observing system developed by Professor Ronald Prinn of the Massachusetts Institute of Technology (MIT) and colleagues, and sponsored by NASA and other agencies.

Professor Prinn, co-director of the MIT Joint Program on the Science and Policy of Global Change and a co-author of the report said: “Addressing HFCs, and other SGHGs, now will ensure that they don’t contribute significantly to warming in the future.”

Meanwhile, the researchers note that due to extensive use, CFCs will continue to warm the planet for years to come.

“CFCs have contributed the most among the synthetic greenhouse gases to warming.  Their use peaked and levels are now declining, but these gases will remain in the atmosphere for many years.  This is likely the trend we will see with most SGHG gases, so it is important that we address these gases now before they do more severe damage,” said Professor Prinn.

(With particular thanks to Audrey Resutek who wrote the MIT press release that this is based on).

GAUGE kicks off!

On the 1st March, we officially began our new £3m NERC-funded consortium project Greenhouse gAs Uk and Global Emissions (GAUGE). The University of Bristol press release explains:

Scientists awarded grant to determine UK’s greenhouse gas emissions
Press release issued 1 March 2013

Researchers in the University of Bristol’s Atmospheric Chemistry Research Group (ACRG), in collaboration with scientists around the country, have been awarded funding from the Natural Environment Research Council (NERC) to provide an independent ‘top-down’ check on the UK’s greenhouse gas emissions estimates.

GAUGE schematic
GAUGE will involve measurements of greenhouse gases from the air, sea and space in an effort to better quantify UK emissions

The UK is required to estimate how much climate-warming carbon dioxide, methane and nitrous oxide it emits each year. However, at the moment, these estimates rely heavily on so-called ‘bottom-up’ accounting methods that may be subject to biases and inaccuracies.

The GAUGE project (Greenhouse gAs Uk and Global Emissions) is a three and a half year collaboration between several universities and research institutions across the UK.

ACRG’s Professor Simon O’Doherty, who also runs the UK greenhouse gas monitoring network funded by the Department for Energy and Climate Change, said: “It’s important that we expand our greenhouse gas observation capabilities in this country, if we’re really going to understand what we’re emitting. But it’s equally important that we begin exploring new types of measurement, which may help us understand emissions processes more fundamentally.”

GAUGE will bring together a more comprehensive suite of greenhouse gas observations around the UK than has ever been compiled. The project will determine emissions using information from satellites, aircraft, tall towers (including the BT tower in the middle of London), balloons and boats.

In addition to developing new measurements, GAUGE will use computer models to simulate how greenhouse gases travel through the air.

Dr Matt Rigby, a research fellow at ACRG, said: “By measuring the concentration of greenhouse gases in the atmosphere, and then using computer models to simulate where the air came from in the days before the measurements, we can determine emissions from the surrounding areas. This new funding will allow us to develop these methods with the help of the Met Office and GAUGE partners at other universities.”

Using the new measurements and modelling techniques, GAUGE researchers hope to make the UK’s emissions amongst the best-quantified in the world.

Using new measurement technology to estimate methane sources

Delta-13C-CH4
MOZART simulation of 13C-CH4 in the atmosphere. The blue areas show parts of the atmosphere with a smaller fraction of the 13C-CH4 isotope than the atmospheric average, due to near-by emissions from microbial sources. In contrast, the red areas show air that is enriched in 13C-CH4, probably due to biomass burning.

Although methane is the second most important greenhouse gas its sources are quite poorly understood. However, new methods of measuring atmospheric methane may be able to help.

Methane is a molecule containing one carbon and four hydrogen atoms. These atoms usually have an atomic mass unit of 12 (carbon) and 1 (hydrogen). However, they also occur in higher masses in nature, called isotopes. Carbon-13 and deuterium (hydrogen with a mass of 2) occur in one atom for every few thousand atoms of regular carbon or hydrogen. This becomes potentially useful for us, because different sources of methane emit molecules with slightly different ratios of carbon-12 to carbon-13 or different ratios of hydrogen to deuterium. For example, methane emitted from wetlands has less 13C than the average in the atmosphere, whereas wildfires emit methane with a relatively high deuterium content.

So, by measuring methane concentrations and isotope ratios in the atmosphere, we can hope to learn something more about where the methane came from.

In the last few years, advances in laser spectroscopy have meant that we can now measure the isotopic composition of methane by shining lasers through a sample and measuring the absorption of certain wavelengths. However, the variations in methane isotope ratio that we expect to see in the atmosphere are very small. Therefore, to be able to resolve small changes, some people are proposing to also “pre-concentrate” air samples, which means that we remove a lot of the nitrogen, oxygen and other major components of air, to leave a more concentrated sample of methane that can be analyzed. Similar systems exist for measuring concentrations of other gases, but not yet for methane isotopes.

In this paper, we asked the question: “If we had these instruments at each AGAGE station, how much better would we be able to constrain methane emissions from different sources than we can at present?”. The answer we found was a little mixed. We found that these new measurements would provide additional information about the methane emissions to the atmosphere. However, the amount by which the uncertainties in our current estimates of methane emissions would be reduced is a little smaller than we hoped for. For example, for wetlands (the single largest source) and other microbial sources, we found that global uncertainty reduction would be reduced by only around 3%. For smaller sources that had a more “distinct” source isotope ratio such as biomass burning, larger relative uncertainty reductions were possible (9%).

Despite the relatively modest uncertainty reductions, my feeling is that, given the importance of methane in the global climate system, these new instruments will have a role to play in a future methane observing system. Given the complexity of the system, no single measurement (or modeling) strategy will be able to fully determine the causes of the strange changes we see in methane. However, by combining many measurement types, we should be able to understand the system better than we currently do.

Combining two models for emissions estimation

In the AGAGE network, we have a small number of monitoring stations, which measure greenhouse gases at high frequency. I’m interested in using these high-frequency measurements to estimate emissions from the countries surrounding the sites.  To connect the measurements to sources, we require chemical transport models (see some animations here). However, when we use global models, they take a lot of computer time to run, particularly at high resolution, which is needed when we’re trying to estimate emissions on national scales.  Sometimes it makes sense to run a model at very high resolution close to the measurement sites (where we have the most information about emissions) and low resolution everywhere else.  This was the problem we tried to tackle in this paper, co-written by colleagues at the UK Met. Office.

The method we developed takes the output from two different types of model and couples them together so that we could estimate emissions at very high resolution close to the monitoring sites, and low resolution further away.

Average 'footprints' around four AGAGE monitoring sites. Generated by the UK Met. Office NAME model.

We’ve used this method, along with the the Met Office NAME model and NCAR’s MOZART model, to determine SF6 emissions around four AGAGE sites (see the figure), and will be extending it to all the other AGAGE gases in the near future.

The code for the project can be found at Google code: http://code.google.com/p/mr-cels/