How the Earth’s atmosphere shows its face

This is an article I wrote for the European Geosciences Union’s newsletter, GeoQ, issue 9. The issue’s theme is “The Face of the Earth”, and so my article is based on “The Atmosphere of the Earth”. It’s aimed at members of EGU (i.e. a variety of different geoscientists), and I’ve tried to make it understandable to an interested non-scientist audience. I’d love to know whether any non-scientists out there think I’ve pitched it right or not!

Looking from outer space, the Earth’s atmosphere appears as an encapsulating fluid that flows in patterns caused by the rotation of the planet and the heating from the Sun. Up close, however, the atmosphere shows its face in much more detail, helping researchers understand the complex interactions in the Earth system.

Temperature of the atmosphere

The temperature of the Earth is much like the temperature of a person: it is a symptom of everything else that is going on in that person’s body. It may seem like a basic property of the atmosphere, but it is a product of many other aspects of the Earth system, including land and oceans.

Recently, there has been much discussion of the so-called ‘temperature hiatus’, the weakening of the trend in global mean surface air temperature since the late 1990s. Observations, such as those from the HadCRUT4 dataset, appear to show temperatures in the past decade rising more slowly than in the preceding two decades (see figure).

The HadCRUT4 dataset is a combination of ground station and the sea surface temperature measurements, which represent about 85% of Earth’s surface. Recent analysis by Cowtan and Way has tested whether this data contains a bias due to incomplete coverage of the globe, and they conclude that it has led to an underestimate of recent global warming. The authors point out that satellite data, models and isolated weather station data show that regions not covered by the dataset, especially the Arctic, have warmed faster than other parts of the world. Accounting for this gives a trend two and a half times greater than that from HadCRUT4, for temperature since 1997.

So even establishing the magnitude of the temperature hiatus is an ongoing area of research. The range of different studies investigating the causes of it is indicative of just how many different factors affect the air temperature.

Work by Estrada, Perron and Martínez-López explores global temperature data sets and radiative forcing variables (greenhouse gases in the atmosphere, natural changes in composition and land use, and solar irradiance) using statistical techniques. Their method interrogates the data without the use of models, and the authors conclude that the temperature record and the radiative forcing (which describes whether the Earth system has a net warming or cooling) can be described by linear trends punctuated by breaks. In this picture, the hiatus is simply a period with a different trend following a break. But what caused this break to occur?

The results suggest that the predominant cause was an unintended consequence of the 1987 Montreal Protocol, the international treaty to stop the destruction of stratospheric ozone by chlorofluorocarbons (CFCs). CFCs are also greenhouse gases, so reducing them to protect the ozone layer also led to a relative cooling of the atmosphere. Pretis and Allen tested this finding in an energy balance model and found that global mean temperatures are 0.1 °C cooler because of the Montreal Protocol.

Estrada and colleagues also attributed a cooling from the reduction in the methane growth rate in recent years. Methane is a potent but short-lived (about a decade) greenhouse gas, with major natural and anthropogenic sources. The amount of methane in the atmosphere had been growing in the latter half of the 20th century, until it levelled off in the period around 2000 to 2006. The cause of this stagnation is in itself an active research area, with changes to agricultural practices, variability of wetlands, and changing fossil fuel emissions being likely factors.

Others have looked to the oceans to find a cause for the temperature hiatus. Modelling work by Kosaka and Xie shows that it can be explained by recent La Niña events. La Niña events are characterised by cooler tropical Pacific sea surface temperatures and cooler surface air temperatures. By putting observed tropical Pacific sea surface temperatures in to an atmospheric model (which also contained the observed greenhouse gas concentrations), the authors were able to reproduce the hiatus.

This is not necessarily in contradiction to the Estrada study, as Kosaka and Xie do not specify what is causing the sea surface temperatures to be La Niña-like, so the cause could be linked to greenhouse gas warming. A trend towards more La Niña-like conditions since 1950, coinciding with increases in global mean surface temperature, has been identified by L’Heureux et al..

These studies illustrate some of the complex interactions between atmospheric temperature, composition and climate. If temperature is the symptom, then we have seen that the make-up of the atmosphere is one of the many causes. To complicate things further, the symptom can also feed back into the cause. For example, wetland emissions of methane depend on temperature, so a warming Arctic may cause increased methane emissions and therefore even more warming.

The dome of the Jungfraujoch atmospheric observatory in Switzerland is seen  in the distance in this photo.

The dome of the Jungfraujoch atmospheric observatory in Switzerland is seen in the distance in this photo.

Composition of the atmosphere

We are finding ever more sophisticated ways of measuring the atmosphere’s composition: continuous ground-station measurements, sensors attached to weather balloons, aircraft- and ship-based instruments, drones, and satellites are all used to analyse the components of the atmosphere. This array of measurements at different scales is used in combination with models to paint the clearest picture of the atmosphere possible, within current understanding.

The MACC (Monitoring Atmospheric Composition and Climate) project has done just this, by assimilating satellite data into a global model of the atmosphere to produce an 8-year data set of atmospheric composition. The data for carbon monoxide, ozone, nitrogen dioxide and formaldehyde are evaluated against independent satellite, weather balloon, ground station and aircraft observations in Inness et al., which goes on to highlight where the discrepancies lie and also indicates the direction for future work. With so much varied data to consider, this kind of large modelling study is a good way of bringing together the current knowledge of atmospheric composition.

These are just a few facets of the atmosphere, with weather patterns, climate modes, aerosol, boundary layer flows, and interactions with the surface being some of the other parts of the atmospheric system that we take interest in studying in just as much depth.It is thanks to the multitude of ways of observing and describing this encapsulating fluid we have today, that we get the atmosphere to show its face.

You’ll find me over at the MAMM Arctic Methane blog…

I have been quiet here for a while, but I’ve been busy elsewhere! I am going to fly out to Kiruna, Sweden again next week, to do some field work to find out more about methane emissions in the Arctic.

Find out what I’m currently up to at You can see the welcome post by Prof John Pyle (my boss and head of the project), and my first post that talks about why it’s so interesting in the first place.

Photo diary from flights around the Arctic

This gallery contains 10 photos.

Here are a few snaps from the MAMM field campaign (July 2012). We went flying on the FAAM research aircraft, kitted out to measure many gases, aerosols, and other meteorological parameters. The main aim was to search out Arctic sources … Continue reading

Svalbard: no bubbles or bears detected

Svalbard airport. No bears (armoured or otherwise) in sight.

The only things that anyone* ever wants to know about Svalbard are: did you see armoured bears, and did you see any methane bubbling up form the clathrates? Well, I’m sad to report that on first look, we detected neither.

*OK, so maybe not anyone. Maybe just me. Most people probably have no idea what I’m on about. So for the 99%, I’ll explain. Armoured bears are in Philip Pullman’s His Dark Materials trilogy (highly recommended reading, IMHO). I won’t expand on that point. What I will expand upon is the bubbles, as we flew to Svalbard yesterday to see if we could find any evidence of them.

The bubbles of methane are released from structures on the bottom of the ocean, which are called methane clathrates, gas hydrates, or some variation thereof. I think these are very curious entities, probably because I don’t know enough about them. For now, I’ll just say that the gas hydrates are crystalline structures of water and methane ice, and methane is trapped within the structures. Sometimes, the methane can escape the structure, and bubble out into the ocean. There is a line of these gas hydrates just off the west coast of Svalbard, and methane has been observed bubbling up from the structures underwater. The methane dissolves in the water while it rises to the surface, but the question is whether all of it dissolves, or if some gas can escape to the air.

It was this source of methane that we went looking for off the coast of Svalbard. We didn’t observe higher concentrations of methane in the air while we were there, however it’s still possible we could detect some signature when we get the final analysis done in the lab (and by we, I mean colleagues at Royal Holloway, Manchester, FAAM, etc, and not me!).

Even if we don’t see any emissions from the gas hydrates, it doesn’t mean that it never reaches the atmosphere. The gas hydrates only trap the methane effectively at certain temperatures and pressures. If the water warms, the gas hydrates could potentially release considerable amounts of methane. If the sea is warming gradually, we may reach a point where lots of methane starts to be released. So it’s possible that under most conditions, no methane escapes. But then once the temperature crosses some threshold, it could then start to be released. What we want to know is whether any of this can get into the atmosphere, where it would cause more localised warming.

So that’s why we went off to Svalbard in summer. We also looked for, and found, regions of the atmosphere with more methane than the general background. This is one thing that I’m really interested in. I want to use a model to work out where these high concentrations of methane come from, and see if that’s consistent with the sources suggested by the isotopic analysis. Judging by the meteorology, I think the sources will be Russian (gas) or Scandinavian (wetlands). Watch this space (for a very long time) to find out if I’m right!

Arctic methane, here we come!

A quick snap of Stockholm, Sweden, today. Lucky it’s not possible to get to Kiruna from London in a single day, eh?

Today, I’m setting off for Sweden, to take part in field work for a project about Arctic methane (MAMM – methane in the Arctic, measurements and modelling). The research aircraft is going to be based in Kiruna, Sweden, and will be arriving there tomorrow. We’ve got a stop over in Stockholm to get there on a commercial flight, as we can’t do the journey in one day. It really is quite far north – the northernmost town in Sweden, where I don’t think it even gets dark at this time of year!

The aim of the project is to find out more about the methane (CH4) emissions in the Arctic, which are not very well known. Not only are the measurements in the Arctic quite sparse, as it’s rather remote, but the emissions are also very variable. One large source is wetlands, where bacteria produce methane (I’m no biologist, but wikipedia has an entry on wetland methane emissions). As the temperature increases, more methane is emitted. There are widespread wetlands in Scandinavia, and we have colleagues taking measurements there at the moment. Hopefully we will be able to fly over them and take more measurements, so we will be able to observe methane on the local scale, and the larger scale by the aircraft. Another source of methane is from “thermokarst lakes”. These are lakes that are frozen in winter and melt in the spring/summer, which releases methane.

One possible outcome of any Arctic warming is that there could be a positive feedback. This is because methane is a strong greenhouse gas (it is many times more potent than CO2 as a greenhouse gas, which I could go into in another post). Local emissions of methane will cause a local warming. As increased temperatures lead to more melting, and more release of methane, you can see how this could continue on and on! There is also a hypothesis that increases temperatures in the Arctic could be contributing to the bizarrely south-of-the-UK jet stream that we currently have, which has brought us immense amounts of rain. More on that on the Met Office blog.

So, what will I be doing for my field work, seeing as I usually live in the model world? Well, so far this week, I’ve been part of the group who are flight planning. We’ve been keeping a close eye on the forecasts for the European Arctic region, to try and plan the most suitable places to fly each day. We’re only flying Friday through to Monday, so we don’t have much room for manoeuvre, as it were. We want to link up some of the wetlands aircraft measurements with satellite measurements, but the satellite can only measure when there’s no cloud. Unfortunately, it’s looking like it might be cloudy at various times over the weekend.

We also want to go north to Svalbard. Hopefully we won’t see polar bears. What we do want to see is some methane coming out form the ocean. There is an undersea ridge, where methane trapped inside clathrates is released. There is definitely methane coming out form the vents (see a paper by Fisher et al, which I’ll have to find the link for later), but the question is whether it all dissolves in the water, or is some escapes to the atmosphere.  We shall hopefully find out more over the weekend!

So, that’s just a quick brain dump of what’s going on in my head just at the moment, while I’m on the train to London. Hopefully I’ll have time for more brain dumps over the weekend!