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How do you make a dinosaur burp in a bag? Measuring prehistoric methane

Cows’ methane emissions can be measured with lasers, but it’s not that easy to measure emissions from an extinct sauropod. Mark Witton

Last week my colleagues and I published a paper showing how methane emitted by dinosaurs could have affected the world’s climate. The media response was huge, with 100+ interviews by email and phone, and live radio interviews on three continents. Google news says nearly 600 outlets republished the story.

With the benefit of hindsight, it’s an obvious speculation, and one that must have occurred to other people in the past. Indeed, since publishing our paper, we have heard from Simon Brassell (a geology professor at Indiana University, USA) that he made similar speculations in a conference talk over 20 years ago. But after receiving rather negative reactions to these ideas, he never pursued them to formal publication.

Several things have changed since then. There is now a much greater realisation of the importance of the interactions between biology and the chemistry and physics of the planet – an area that has become known as Earth Systems Science.

In addition much of the data we used in our own study was only published quite recently; it would have been much harder to make these ideas quantitative at the start of the 1990s. Putting numbers to such an idea is a key part in trying to work out if it’s likely to be correct.

It occurred to my colleague Graeme Ruxton and I that if modern cattle can potentially be a source of climate-altering amounts of methane then the extinct sauropods, so much larger than any living terrestrial animal, could have been an important source of greenhouse gasses in the past.

Neither Graeme or myself are specialist dinosaur researchers. Instead we are ecologists and are intrigued by sauropods because they are so different, in size and shape, from animals alive today. This makes thinking about them – and how their biology may have worked – particularly interesting.

The idea that methane from herbivorous dinosaurs may have been produced in quantities able to have a measurable effect on the global climate is an intriguing possibility, but how do you calculate the amount of methane produced by long extinct animals?

Leaf-eating lizards are a little bit like tiny dinosaurs, but comparing their methane emissions is tricky. DDanzig/Flickr

It turns out that there are equations which describe how methane production by microbes living in modern plant eaters is related to their body size. There are several such equations for different types of animals – such as various types of mammals with different digestive strategies and reptiles too.

How do you decide which to use for a dinosaur? The idea that dinosaurs are most closely related to birds is now well known, but birds don’t help much in this case. Their adaptations to flight make them a very specialised group and few of them eat large quantities of leaf material, as flight tends to require more energy rich foods.

What about reptiles? Until recently most people assumed these were the closest living relatives of dinosaurs so they may seem a good “second best” if bird data isn’t available.

However, it’s actually the microbes that produce the methane, not the animals. So – to us – the obvious question to ask was, were the conditions for these microbes in sauropods closer to modern mammals or reptiles?

Because these herbivorous dinosaurs were so large most biologists think that they would have had a high and constant body temperature (as large animals lose heat slowly). From this microbial perspective, the modern mammal data is probably a better match. It has the additional advantage of including data from elephants; currently the closest terrestrial animal in size to a large dinosaur.

Because of these arguments we decided to use relatively conservative mammal equations to make an informed guess about the likely methane output of a large sauropod (some mammals, such as ruminants like cows, produce even more methane than the mammals we based our sauropods on).

Elephants are the closest thing we have to sauropods - size wise - and their body temperature and resultant microbes may be similar to dinosaurs’. Shachi Sitaram

Other scientists had estimated the likely biomass – that is the weight of sauropods – that might have been found living on a given area of land, mainly based on theoretical arguments about energy use, along with a limited amount of fossil data. Taking these figures we estimated global methane production by sauropods and – with the help of Euan Nisbet, a geologist and methane expert from London University – worked out what this implied for methane levels in the Earth’s atmosphere.

Our result is obviously an educated guess – but to our surprise the answer was extraordinary! The methane output of these dinosaurs may have been equivalent to total amount of methane produced on Earth today, that is from both natural and man-made sources! Even if our results are twice as large as reality, these animals would still have been giving out climatically significant amounts of methane.

We know the climate of the time was very warm and that some sort of greenhouse effect is the likely explanation for this. Our results cannot prove that dinosaur methane played a role in these warm conditions but it does show that it’s an idea certainly that needs taking seriously.

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