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Eocene landscape (56 to 34 million years ago)
Eocene landscape (56 to 34 million years ago). Illustration of a brown coal forest. Credit: Science History Images/Alamy Stock Photo.
CLIMATE SENSITIVITY
9 October 2017 7:00

Hyperthermals: What can they tell us about modern global warming?

Daisy Dunne

Daisy Dunne

10.09.17
Daisy Dunne

Daisy Dunne

09.10.2017 | 7:00am
Climate sensitivityHyperthermals: What can they tell us about modern global warming?

Around 50m years ago, Earth was struck by a series of short-term phases of rapid global warming, known collectively as “hyperthermals”.

Lasting for a few thousand years at a time, each hyperthermal saw the world’s temperatures rise by as much as 5C. This rise in global temperature is believed to have caused widespread changes to some of the world’s habitats and waves of species extinctions.

These hyperthermals were entirely natural climatic events, driven by large releases of gases, such as CO2 and methane, into the atmosphere. The likely causes of these events are still disputed by scientists, with some suggesting waves of volcanic eruptions, releases from natural methane stores and changes to the world’s soils could have been responsible.

The rate of CO2 release during the hyperthermals was much slower than the rate recorded by scientists today. However, climate scientists hope that getting a better understanding of these past events could tell us more about how human-driven emissions might affect the planet in the coming century.

At a conference held at the Royal Society in London, Carbon Brief spoke to some of the scientists that are studying what hyperthermals could tell us about future global warming.

Finding an analogue

One way that scientists can study the parallels between hyperthermals and current climate change is by finding a suitable analogue – that is, the hyperthermal that was most similar to the kind of global warming that we’re seeing today.

A number of scientists believe that the hyperthermal that is most comparable to current climate change is the Paleocene–Eocene Thermal Maximum (PETM), which was the first and largest of all the hyperthermals.

The chart below shows how the global ocean temperatures changed during the PETM (first spike on the graph), compared to temperature change during two later, smaller hyperthermals.

Change in ocean temperature (C) during the Paleocene–Eocene Thermal Maximum (PETM) and two later, smaller hyperthermal events. Source: Svensen (2012)

The PETM is comparable to current climate change because it experienced a relatively high rate of global warming, says Prof Appy Sluijs, a researcher of paleoceanography at Utrecht University in the Netherlands. He tells Carbon Brief:

“I think there are important things to learn from all kinds of time periods but certainly the PETM stands out in terms of rate of warming. Although, carbon input during the PETM was likely still 10 times as slow as in the modern [era], this is still the closest we can get in the geological record. Therefore, the PETM is often considered a nice analogue, although we all realise the analogy isn’t perfect.”

However, the PETM took place around 55m years ago, and trying to figure what happened to our planet’s atmosphere during this time is no mean feat.

One way that scientists can study changes in the Earth’s atmosphere during the PETM is by looking at deep ocean sediment cores. Analysing the ratios between different chemical isotopes can help scientists to work out what conditions were like in the oceans at that time – and, by extension, the atmosphere.

Research suggests that, during the PETM, huge stores of carbon were released into atmosphere. This caused average global temperatures to rise by around 5C, which in turn led to huge shifts in the world’s ecosystems.

However, what could have sparked this carbon release is still being debated, says Sluijs:

“What we know for sure is that large masses of carbon came from below the ground. A recent heavily debated paper suggested that it was caused by volcanism in the North Atlantic, which at the time was very active in terms of volcanic activity. Most colleagues think that carbon input came from sources such as methane hydrates below the seafloor or buried terrestrial organic matter, which is buried peat essentially.”

What can the PETM tell us?

Studying the PETM could give us clues about how the world’s animals and plants could react to future climate change.

Previous research suggests that, during the PETM, global temperature rise led to widespread ocean acidification. Ocean acidification occurs when CO2 in the atmosphere is absorbed by the sea. There it reacts with water to form carbonic acid, reducing the pH level and making the oceans less alkaline.

This rise in acidity is believed to have led to waves of extinctions of marine animals, with around 40% (pdf) of benthic foraminifera, a group of single-celled deep-sea organisms, going extinct.

But the rate of species loss from future global warming could be far higher.

This is because the rate of global warming today is much faster than during the PETM, says Prof Richard Zeebe, an ocean researcher from the University of Hawaii.

His research suggests that the build up period that led to the PETM, in which around 3tn tonnes of CO2 was released into to the atmosphere, may have taken thousands of years. In comparison, the onset of current climate change has taken less than two centuries, sparked by the advent of the industrial revolution.

The upper chart in the figure below compares the rate of carbon release during the PETM (orange) and emissions under a “business as usual” scenario of modern climate change (grey), which assumes that a total of 5tn tonnes of carbon are emitted into the atmosphere in the next 500 years. This gives a sense of how much more quickly the rate of human-caused emissions are compared to the natural emissions that triggered the PETM.

The lower chart shows how these emissions translate into ocean acidification. The line shows levels of calcite – a mineral used by marine organisms to construct their shells – in the surface ocean. The lower it drops the less alkaline the ocean is becoming.

Chart A shows the rate of carbon release to atmosphere over time during the PETM and current “business as usual” climate change (in billion tonnes of carbon per year), which assumes that carbon emissions will rise to 5tn tonnes in the next 500 years. Chart B shows the state of ocean surface carbon saturation during the PETM (orange) and under projected “business as usual” climate change Source: Zeebe & Zachos (pdf) (2013)

The research suggests that the rate of carbon release as a result of human-driven climate change, and its resultant effect on the world’s oceans, could be “completely unprecedented”, says Zeebe:

“You can say: ‘Well, it’s definitely going to be worse in the future, because we’re releasing carbon much more rapidly.’ Because of this, the consequences of ocean acidification in the future are almost certainly going to be worse than they were during the PETM.”

Climate sensitivity

Understanding the PETM could also help us to shed light on the climate sensitivity – a measure of how much the climate warms in response to greenhouse gases.

Climate sensitivity is the warming that can be expected when the concentration of CO2 in the atmosphere reaches double what it was in pre-industrial times.

(Pre-industrial CO2 concentration levels were about 280 parts per million (ppm) and levels are currently around 404ppm.)

The latest assessment report from the Intergovernmental Panel on Climate Change (IPCC) puts the likely value of climate sensitivity between 1.5 and 4.5C. This estimate refers to “equilibrium” climate sensitivity – that is, the amount of warming once the full impact of the extra greenhouse gases has played out. This includes any long-term feedbacks that can dampen or amplify the warming from the increased greenhouse effect, Sluijs explains:

“What’s important here is that the warming will always be slow relative to carbon input. So essentially it means that, if you double the CO2 concentrations, you wait for a few hundred years until the new climate has equilibrated and then you measure how much warming has taken place.”

Sluijs hopes that getting a measure of the climate sensitivity during the PETM could help us to be more precise about the possible climate sensitivity of the future. He says:

“As a climate science community, we want to decrease this uncertainty because it might be the difference between retaining an ice sheet on Greenland or losing it in the far future. Although the details are complicated, the relation between changes in CO2 and temperature in the past might tell us climate sensitivity as well.”

“I’d have to add, though, that this research field of palaeoclimate sensitivity is in its infancy and we have a lot of work to do to improve our estimates.”

Understanding feedbacks

Studying the PETM could also help us tease apart different climate feedbacks that could contribute to climate sensitivity.

Climate feedbacks are natural processes that take place on the Earth’s surface and in the atmosphere that are triggered by global warming. “Positive” feedbacks strengthen warming, while “negative” ones can weaken it. The recent IPCC report concluded that the combined effects of all feedbacks is likely to significantly strengthen global warming.

Uncertainty around the full effects of these feedbacks is the main reason scientists use a range rather than an exact figure when estimating future climate change.

Some climate feedbacks happen on a relatively short timescale and so it is possible to observe their effects on warming. Such loops include the albedo feedback mechanisms of snow, sea ice and cloud cover.

However, it possible that there may be more long-term climate feedbacks may take thousands of years to show their effects. This is where studying the PETM could come in handy, Zeebe says.

“One thing to keep in mind is the climate sensitivity that we may be dealing with in this century, only includes processes that are active on a timescale of maybe centuries or less. If you look at the PETM, there the timescale is probably order of millennia. Over thousands of years, you have different feedbacks.

“By looking at the PETM, we can look at a lot of feedbacks that were active at this timescale. These could be the feedbacks that we may need to worry about in the future.”

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