There’s a myopia in the climate discourse today.
“Everyone is focused on what happens by 2100. But that’s only 2 generations from today. It’s like: If the world ends in 2100 we’re probably OK!” says Professor Richard Zeebe of the University of Hawai’i. “But It’s very clear that over a longer timescale there will be much bigger changes.”
If the next century seems impossibly far off, bear in mind that if you have a young child now, we’re talking about the world her or his grandchildren will be trying to raise their kids in.
Scientists who take the long view on climate change see parallels between global warming today and mass extinctions in Earth’s past: “Apart from the stupid space rock hitting the Earth, most mass extinctions were CO2-driven global warming things,” says Professor Andy Ridgwell of Bristol University in the UK.
It has been a consistent pattern throughout geological time: “If you screw with the climate enough, you have huge extinctions,” says Ridgwell.
So much of what you read and hear about climate change is heavily based on instrument records that only go back 160 years or so. But Richard Zeebe and Andy Ridgwell are among a few scientists who look millions of years into Earth’s past to learn how the Earth responded to big additions of CO2 into the atmosphere before. I had the opportunity to chat with each of them about their work during the American Geophysical Union conference in San Francisco at the end of last year.
It’s true that scientists can learn about the ice age climate going back 800,000 years from bubbles of air trapped in ancient ice drilled from Greenland and Antarctica. They reveal many swings from warm to cold to warm again, in a low-CO2 world mostly cooler than today. “We went from very cold and low sea level to a mild climate with normal sea level,” says Ridgwell. But it was a very different scenario than today - those cycles were mainly driven by Earth’s wobbles as it circles the Sun, and those same orbital wobbles mean we should be cooling, not warming, today.
So both Ridgwell and Zeebe have been studying the best equivalent to modern climate change they have found so far, a relatively rapid global warming event that occurred 56 million years ago, called the “Paleocene-Eocene Thermal Maximum,” mercifully shortened to “PETM.” For Ridgwell, it’s a better analog for the future:
This is why I like the PETM, at least it’s a warming event. It had a peak global warming of about 5º or 6ºC, which is a little bit beyond the end-of-the-century worst case scenario.
For Zeebe it’s also about data quality:
We are focusing on the PETM is because we have relatively good sediment records. We are able to constrain timescales and ages relatively accurately. If you go further back in time, these constrains become very, very difficult.
Scientists use a wide variety of data from rocks exposed and drilled at various locations around the globe. They look at geology, fossils, and the chemical makeup of sediments, particularly chemical isotopes - different atoms of the same element that differ minutely in their mass. As Zeebe says,
We have two isotope systems that we can look at. One of those are oxygen isotopes and they are essentially a thermometer, they tell us about climate change. And the other isotope system we’re looking at is carbon isotopes and they tell us something about carbon release.
Zeebe and colleagues compared isotope samples with computer models of the climate system. They stretched the models’ carbon emission timescale until they got a match with the isotope measurements. This showed that the carbon emissions which caused the PETM took about 4,000 years - remarkably similar to an estimate of about 3,000 years just published by Ridgwell and his colleague Sandy Kirtland Turner. They used a completely different approach from Zeebe, focusing on the differences in the carbon signal between marine and land sediments.
Narrowing the PETM emissions timeframe to around 3,000 to 4,000 years shows that, like today, the global warming back then was caused by geologically-fast carbon emissions. But our emissions have taken just a couple of centuries so, as Zeebe points out:
What we’re doing with our emissions is unprecedented in the past 66 million years!
Even if the PETM isn’t a perfect equivalent of today’s climate (it was slower, and it happened in a world that was already warmer than today), it still tells us how the planet reacts to a sharp excess of carbon in the atmosphere. For example, the2013 Intergovernmental Panel on Climate Change (IPCC) report says we can expect a warming of between 1.5°C and 4.5°C if we double atmospheric CO2levels, but also acknowledges that the longer term warming (over centuries to millennia) “could be significantly higher” than that. The PETM tells us it is. Zeebe notes:
If we just try to explain the PETM with a climate sensitivity of 4.5°C, we only get maybe 60% of the warming. So my conclusion would be that long term sensitivity must be more than 4.5°C.
Ridgwell has also investigated global warming events going back deeper in geological time, including the big mass extinction events like the end-Permian and the end-Triassic, as well as so-called “Ocean Anoxic Events” in the Cretaceous. These involved profound and long-lived environmental disruption, with symptoms quite similar to today’s climate change, such as global warming, rainfall changes, and ocean acidification.
“Overall these are relatively slow events, on timescales of tens to hundreds of thousands of years,” says Ridgwell. But recent advances in rock dating, and in tracing Earth’s magnetic field frozen into ancient lava, suggests that within those slow events there may well have been intense, rapid episodes that came closer to today’s human emission rates.
You might have very short pulses of CO2 release within them. Some of these pulses of CO2 could look like what we’re doing now in terms of amount and rate. That’s an area of active research, because the estimates of individual pulses are getting better, but the estimates of how much CO2 would be released associated with an individual pulse are still uncertain.
Ridgwell is skeptical about ambitions to limit warming to 1.5°C:
Posted by howardlee on Friday, 29 January, 2016
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