Note to Yarns from the Farm readers: this is the fourth instalment of a series on climate change that I’m writing for our local monthly newsletter. The first article is here, the second here and the third here. The image above is a comparison of greenhouse gas nitrous oxide emissions over China, pre- and post-lockdown, courtesy of NASA. This is not the article I meant to write this month—that one is now ready for next month. I thought this might be more uplifting than the one I drafted first, which involved the flawed and anti-social Captain Nemo from Jules Verne’s 20,000 Leagues Under the Sea. Nan
What do the ozone hole, coronavirus and climate change have in common? The simple answer is the use of chemical tracers to track where heat is going in the ocean. The full answer will take me a bit longer. But since we’re all sitting at home now, we’ve got plenty of time to learn new things. And one of the things I’m really enjoying about writing these climate change articles is that I’m learning stuff that I assumed I knew, but didn’t.
Let’s start with the ozone hole. By the late 1970s, scientists realised that chemical reactions in the outer atmosphere were causing ozone to be destroyed, creating a ‘hole’ (thinning) in the ozone layer over Australia. Ozone absorbs ultraviolet radiation, preventing it from causing cancer. The chemicals responsible are chlorofluorocarbons, or CFCs, used at the time as refrigerants (‘freon’) and the propellants in spray cans.
In 1987 an international agreement, the Montreal Protocol, banned the use of CFCs. Gradually the ozone hole began to heal, although CFCs take decades to break down completely. The ozone layer is still thinner over Australia than it is over the rest of the globe, hence our emphasis on ‘slip, slop, slap’ with sunscreen, and our relatively high incidence of skin cancer.
Once CFCs were banned, though, they became a useful geochemical tracer: by measuring minute concentrations of CFCs that are absorbed at the ocean surface, mostly in the northern hemisphere, we can trace the pathways of those water parcels through the ocean. CFC distributions have given us great insight into the way heat from increased CO in the atmosphere has been carried into the deep oceans. The absorption of heat by the deep oceans has masked the true impact of global warming (more on this in next month’s episode).
North-south vertical section through the Atlantic, from Antarctica on the left to North Atlantic on the right. Note the incursion of CFC concentrations (pink) into the deep waters at both ends of the section. Reference.
What’s special about geochemical tracers is that, unlike temperature, they don’t change with time and position. So a parcel of water that left the surface of the ocean with a particular CFC value will carry that value with it wherever it goes. That means we can infer how much heat has been carried into the deep ocean by knowing the CFC concentration and the (approximate) temperature of the parcel when it left the surface.
When I was a young ocean-going researcher, I had the dubious pleasure of working with one of the great geochemists of the day, a relatively young, mildly arrogant and distinctly persnickety fellow named Ray Weiss. His CFC measurements were an adjunct to a major deep-ocean cruise for which I was chief scientist. Ray’s part-per-billion, hyper-sensitive measurements were incredibly difficult to do accurately at sea, and his general feeling was that whatever he and his post-doc, John Bullister, needed I should make sure they got. They did. We are still friends.
The coronavirus connection to this story is as yet untold. A couple of weeks ago, NASA published a side-by-side comparison of potent greenhouse gas nitrous oxide emissions from China just before and just after the coronavirus lockdown. This huge and rapid ‘step’ change in emissions is happening all over the world as economies dial back their use of fossil fuels.
We don’t need to pay this high a price to turn the climate change juggernaut around. We just need to summon our collective will to use the tools we have. The past success of the Montreal Protocol gives me hope.