Six times since 1990, the Intergovernmental Panel on Climate Change (IPCC) has released major assessment reports on the climate. The most recent was last year. The results show, unequivocally, that human influence is leading to global warming and climate change, and it may soon be unstoppable.
But have we been looking at the wrong data?
A large amount of the work in the more than 14,000 scientific papers which went into the 2021 IPCC report is based on comparing climate data gathered since the industrial revolution with climate data from Earth’s ancient past. We can paint a picture of climatic changes and their impacts through geological history by analysing ice core samples, rocks, and fossil records to measure things from atmospheric methane and CO2, to sharp decreases in biodiversity.
But are we looking at the right periods of Earth’s history for our comparison?
An international team of researchers believes that we have not. The researchers’ paper, published in the Proceedings of the National Academy of Sciences (PNAS) journal, argues that we have wrongly been comparing today’s climate to historical episodes known as “greenhouse” phases. And the modelling would be more accurate if we compared modern climate data with data from previous “icehouse” periods in Earth’s history.
More on the climate: A barcode of Earth’s climatic past
The researchers’ results suggest that human-induced climate change will be worse and even more rapid than we thought.
The term icehouse refers to periods in Earth’s history in which the planet has large reserves of ice – like we currently do with the Arctic and Antarctic ice sheets.
“The research comes down to the different environments the Earth is operating under,” says University of Waikato, New Zealand, scientist and team member Terry Isson. “At the moment, we’re in an ‘icehouse’ phase, with glaciers and ice caps. The data we look at for forecasting the effects of global warming come from a period of ‘greenhouse’ when the Earth didn’t have massive stocks of ice.”
A major effect of global warming is the reduction of dissolved oxygen in the oceans – marine anoxia – leading to mass death of underwater life and marine dead zones. When ice caps and glaciers melt, they release fresh water on to the ocean surface, preventing circulation and cutting off oxygen supply to deeper water.
The authors note in beginning the paper: “Warming-mediated increases in marine anoxia may be more pronounced in a glaciated versus unglaciated climate state.”
“We know that global warming affects oxygen levels and how acidic our oceans are. This has a
flow-on effect to the kind of life that our oceans can sustain,” says Isson.
Instead of looking at greenhouse periods, the team looked at an event 304 million years ago – in an ice-capped world much like today – known as the Kasimovian–Gzhelian Boundary (KGB). Within about 300,000 years, atmospheric CO2 levels doubled, oceans became anoxic, and biodiversity dropped on the land and in the oceans.
Scientists theorise the massive carbon release may have been triggered by volcanic eruptions that tore through coal beds in the carboniferous period. The eruptions would have started fires, and warming may have melted permafrost, leading to the release of more organic carbon.
“It was one of the fastest warming events in Earth’s history,” says Isabel Montañez, professor in the University of California’s Davis Department of Earth and Planetary Sciences, and a member of the research team. This is the first identified rapid warming event in an icehouse Earth. The team’s results show that an icehouse climate may be more sensitive to warming than greenhouse phases when carbon dioxide levels are already higher.
Using carbon isotopes and trace elements from rocks and plant fossils, the researchers estimate that about 9000 gigatons of carbon was released into the atmosphere just before the KGB. “We don’t have a rate, but it was one of the fastest in Earth’s history,” Montañez says. The increase doubled atmospheric CO2 from about 350 parts per million, similar to levels just before industrialisation, to approximately 700 ppm.
The team also analysed rocks in present-day China to deduce oceanic oxygen levels 304 million years ago. A lack of oxygen can be seen in increased uranium isotopes in rocks on the ocean floor. The team estimates that about 23% of the world’s oceans became anoxic dead zones after the KGB, matching the fossil record showing mass extinctions on land and in the sea around the time.
“If you raised CO2 by the same amount in a greenhouse world, there isn’t much effect, but icehouses seem to be much more sensitive to change and marine anoxia,” Montañez says.
The new research doesn’t discount the decades of work that has already sounded the alarm about human-induced climate change, but rather it strengthens it.
Modelling is not a perfect science which spits out an exact, infallible answer. Understanding the infinitely complex system which is the Earth’s climate historically, and then making predictions about the future is gruelling. Models must be developed and are strengthened by the volume and relevance of the data we put in.
But the trends are clear. The climate is changing, and the planet is warming because of human activity. If anything, the new research is a warning that underlines this point. Comparing with other icehouse periods may provide a better picture of where our own ice-capped world is heading. “This research indicates that this process may be more rapid and more severe in our current
climate, and we may be seriously underestimating the rate and effect of global warming if we
continue to rely on greenhouse comparisons,” says Isson.
Evrim Yazgin has a Bachelor of Science majoring in mathematical physics and a Master of Science in physics, both from the University of Melbourne.
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