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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

 

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.”

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).

35 years of AGAGE measurements

I’ve just returned from the 35th anniversary AGAGE meeting in Boston, MA. I’ve written a blog post for the Cabot Institute, summarising the changes AGAGE has seen during this time:

Concentrations of methyl chloroform, a substance banned under the Montreal Protocol, measured at four AGAGE stations.
Concentrations of methyl chloroform, a substance banned under the Montreal Protocol, measured at four AGAGE stations.

“I work on an experiment that began when the Bee Gees’ Stayin’ Alive was at the top of the charts. The project is called AGAGE, the Advanced Global Atmospheric Gases Experiment, and I’m here in Boston, Massachusetts celebrating its 35-year anniversary. AGAGE began life in 1978 as the Atmospheric Lifetimes Experiment, ALE, and has been making high-frequency, high-precision measurements of atmospheric trace gases ever since.

At the time of its inception, the world had suddenly become aware of the potential dangers associated with CFCs (chlorofluorocarbons). What were previously thought to be harmless refrigerants and aerosol propellants were found to have a damaging influence on stratospheric ozone, which protects us from harmful ultraviolet radiation. The discovery of this ozone-depletion process was made by Mario Molina and F. Sherwood Rowland, for which they, and Paul Crutzen, won the Nobel Prize in Chemistry in 1995. However, Molina and Rowland were not sure how long CFCs would persist in the atmosphere, and so ALE, under the leadership of Prof. Ron Prinn (MIT) and collaborators around the world, was devised to test whether we’d be burdened with CFCs in our atmosphere for years, decades or centuries.

ALE monitored the concentration of CFCs, and other ozone depleting substances, at five sites chosen for their relatively “unpolluted” air (including the west coast of Ireland station which is now run by Prof. Simon O’Doherty here at the University of Bristol). The idea was that if we could measure the increasing concentration of these gases in the air, then, when combined with estimates of the global emission rate, we would be able to determine how rapidly natural processes in the atmosphere were removing them.

Thanks in part to these measurements, we now know that CFCs will only be removed from the atmosphere over tens to hundreds of years, meaning that the recovery of stratospheric ozone and the famous ozone “hole” will take several generations. However, over the years, ALE, and now AGAGE, have identified a more positive story relating to atmospheric CFCs: the effectiveness of international agreements to limit gas emissions.

The Montreal Protocol on Substances that Deplete the Ozone Layer was agreed upon after the problems associated with CFCs were recognised. It was agreed that CFC use would be phased-out in developed countries first, and developing countries after a delay of a few years. The effects were seen very rapidly. For some of the shorter-lived compounds, such as methyl chloroform (shown in the figure), AGAGE measurements show that global concentrations began to drop within 5 years of the 1987 ratification of the Protocol.

Over time, the focus of AGAGE has shifted. As the most severe consequences of stratospheric ozone depletion look like they’ve been avoided, we’re now more acutely aware of the impact of “greenhouse” gases on the Earth’s climate. In response, AGAGE has developed new techniques that can measure over 40 compounds that are warming the surface of the planet. These measurements are showing some remarkable things, such as the rapid growth of HFCs, which are replacements for CFCs that have an unfortunate global-warming side effect, or the strange fluctuations in atmospheric methane concentrations, which looked like they’d plateaued in 1999, but are now growing rapidly again.

The meeting of AGAGE team members this year has been a reminder of how important this type of meticulous long-term monitoring is. It’s also a great example of international scientific collaboration, with representatives attending from the USA, UK, South Korea, Australia, Switzerland, Norway and Italy. Without the remarkable record that these scientists have compiled, we’d be much less well informed about the changing composition of the atmosphere, more unsure about the lifetimes of CFCs and other ozone depleting substances, and unclear as to the exact concentrations and emissions rates of some potent greenhouse gases. I’m looking forward to the insights we’ll gain from the next 35 years of AGAGE measurements!”