Study Of Ancient Warming Suggests Future Warming Could Accelerate

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https://www.eurekalert.org/pub_releases/2019-09/uom-soa091219.php

NEWS RELEASE 18-SEP-2019

Study of ancient climate suggests future warming could accelerate

UNIVERSITY OF MICHIGAN

ANN ARBOR--The rate at which the planet warms in response to the ongoing buildup of heat-trapping carbon dioxide gas could increase in the future, according to new simulations of a comparable warm period more than 50 million years ago.

Researchers at the University of Michigan and the University of Arizona used a state-of-the-art climate model to successfully simulate--for the first time--the extreme warming of the Early Eocene Period, which is considered an analog for Earth's future climate.

They found that the rate of warming increased dramatically as carbon dioxide levels rose, a finding with far-reaching implications for Earth's future climate, the researchers report in a paper scheduled for publication Sept. 18 in the journal Science Advances.

Another way of stating this result is that the climate of the Early Eocene became increasingly sensitive to additional carbon dioxide as the planet warmed.

"We were surprised that the climate sensitivity increased as much as it did with increasing carbon dioxide levels," said first author Jiang Zhu, a postdoctoral researcher at the U-M Department of Earth and Environmental Sciences.

"It is a scary finding because it indicates that the temperature response to an increase in carbon dioxide in the future might be larger than the response to the same increase in CO2now. This is not good news for us."

The researchers determined that the large increase in climate sensitivity they observed--which had not been seen in previous attempts to simulate the Early Eocene using similar amounts of carbon dioxide--is likely due to an improved representation of cloud processes in the climate model they used, the Community Earth System Model version 1.2, or CESM1.2.

Global warming is expected to change the distribution and types of clouds in the Earth's atmosphere, and clouds can have both warming and cooling effects on the climate. In their simulations of the Early Eocene, Zhu and his colleagues found a reduction in cloud coverage and opacity that amplified CO2-induced warming.

The same cloud processes responsible for increased climate sensitivity in the Eocene simulations are active today, according to the researchers.

"Our findings highlight the role of small-scale cloud processes in determining large-scale climate changes and suggest a potential increase in climate sensitivity with future warming," said U-M paleoclimate researcher Christopher Poulsen, a co-author of the Science Advancespaper.

"The sensitivity we're inferring for the Eocene is indeed very high, though it's unlikely that climate sensitivity will reach Eocene levels in our lifetimes," said Jessica Tierney of the University of Arizona, the paper's third author.

The Early Eocene (roughly 48 million to 56 million years ago) was the warmest period of the past 66 million years. It began with the Paleocene-Eocene Thermal Maximum, which is known as the PETM, the most severe of several short, intensely warm events.

The Early Eocene was a time of elevated atmospheric carbon dioxide concentrations and surface temperatures at least 14 degrees Celsius (25 degrees Fahrenheit) warmer, on average, than today. Also, the difference between temperatures at the equator and the poles was much smaller.

Geological evidence suggests that atmospheric carbon dioxide levels reached 1,000 parts per million in the Early Eocene, more than twice the present-day level of 412 ppm. If nothing is done to limit carbon emissions from the burning of fossil fuels, CO2 levels could once again reach 1,000 ppm by the year 2100, according to climate scientists.

Until now, climate models have been unable to simulate the extreme surface warmth of the Early Eocene--including the sudden and dramatic temperature spikes of the PETM--by relying solely on atmospheric CO2 levels. Unsubstantiated changes to the models were required to make the numbers work, said Poulsen, a professor in the U-M Department of Earth and Environmental Sciences and associate dean for natural sciences.

"For decades, the models have underestimated these temperatures, and the community has long assumed that the problem was with the geological data, or that there was a warming mechanism that hadn't been recognized," he said.

But the CESM1.2 model was able to simulate both the warm conditions and the low equator-to-pole temperature gradient seen in the geological records.

"For the first time, a climate model matches the geological evidence out of the box--that is, without deliberate tweaks made to the model. It's a breakthrough for our understanding of past warm climates," Tierney said.

CESM1.2 was one of the climate models used in the authoritative Fifth Assessment Report from the Intergovernmental Panel on Climate Change, finalized in 2014. The model's ability to satisfactorily simulate Early Eocene warming provides strong support for CESM1.2's prediction of future warming, which is expressed through a key climate parameter called equilibrium climate sensitivity.

The term equilibrium climate sensitivity refers to the long-term change in global temperature that would result from a sustained doubling--lasting hundreds to thousands of years--of carbon dioxide levels above the pre-industrial baseline of 285 ppm. The consensus among climate scientists is that the ECS is likely to be between 1.5 C and 4.5 C (2.7 F-8.1 F).

The equilibrium climate sensitivity in CESM1.2 is near the upper end of that consensus range at 4.2 C (7.7 F). The U-M-led study's Early Eocene simulations exhibited increasing equilibrium climate sensitivity with warming, suggesting an Eocene sensitivity of more than 6.6 C (11.9 F), much greater than the present-day value.

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The research was supported by a Heising-Simons Foundation grant to Poulsen and Tierney.

 

Link to full paper:

https://advances.sciencemag.org/content/5/9/eaax1874

Simulation of Eocene extreme warmth and high climate sensitivity through cloud feedbacks

1. Jiang Zhu1,*, 
2. Christopher J. Poulsen1 and 
3. Jessica E. Tierney2

 See all authors and affiliations

Science Advances  18 Sep 2019:
Vol. 5, no. 9, eaax1874
DOI: 10.1126/sciadv.aax1874 

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Abstract

The Early Eocene, a period of elevated atmospheric CO2 (>1000 ppmv), is considered an analog for future climate. Previous modeling attempts have been unable to reproduce major features of Eocene climate indicated by proxy data without substantial modification to the model physics. Here, we present simulations using a state-of-the-art climate model forced by proxy-estimated CO2 levels that capture the extreme surface warmth and reduced latitudinal temperature gradient of the Early Eocene and the warming of the Paleocene-Eocene Thermal Maximum. Our simulations exhibit increasing equilibrium climate sensitivity with warming and suggest an Eocene sensitivity of more than 6.6°C, much greater than the present-day value (4.2°C). This higher climate sensitivity is mainly attributable to the shortwave cloud feedback, which is linked primarily to cloud microphysical processes. Our findings highlight the role of small-scale cloud processes in determining large-scale climate changes and suggest a potential increase in climate sensitivity with future warming.

 

 

 

 

[donsutherland1]

IMO, this study by Dr. Zhu, Dr. Tierney et al., is particularly important. The newer climate models have been showing greater warming than the prior ones for the expected RCPs. A better handle on cloud-related feedbacks, not modeling error, might be producing this outcome.

[bdgwx]

Another way of stating this result is that the climate of the Early Eocene became increasingly sensitive to additional carbon dioxide as the planet warmed.

That's seems to be inline with other studies I've read. The sensitivity may start out low...say...0.5C per W/m^2, but as the planet warms it gets more sensitive to the radiative forcing and may rise to 1.0C per W/m^2 and beyond.

The equilibrium climate sensitivity in CESM1.2 is near the upper end of that consensus range at 4.2 C (7.7 F).

Ya know...in his 1908 book World's in the Making Arrhenius refined his estimate of the 2xCO2 sensitivity at 4C. Wouldn't that be something if Arrhenius' prediction from 100 years ago ended up being nearly spot on? 

[CAD_Wedge_NC]

The burning question is.... how did the earth rid itself of that excess CO2? Was a dramatic increase in plant-life the culprit?

[donsutherland1]

1 hour ago, CAD_Wedge_NC said:

The burning question is.... how did the earth rid itself of that excess CO2? Was a dramatic increase in plant-life the culprit?

This overview of the carbon cycle explains the processes involved.

http://www.columbia.edu/~vjd1/carbon.htm

The net reduction to levels that predated those of the Eocene occurs over geological timeframes.

 

[Fantom X]

I read that Dr Valentina Zharkova is predicting a natural increase of 3C by 2600 - and that's without the affects of AWG

[Vice-Regent]

You reach a point where the slow feedbacks "wake up" and off you go down the cliff and into hothouse Earth. Tipping points are a thing but our entire climate mitigation policy is based on their non-existence. It's that same scientific reticence. We don't listen to "90% certainty" yet nothing is ever completely certain. You can play this game ad-infinitum.

There's no way you can walk away from this anymore in 2019. I hope you guys are looking forward to the genocides of the 2040s. Where is our World War 2-level mobilization?

[bdgwx]

25 minutes ago, Fantom X said:

I read that Dr Valentina Zharkova is predicting a natural increase of 3C by 2600 - and that's without the affects of AWG

It's this study.

https://www.nature.com/articles/s41598-019-45584-3#Sec6

The claim is that the recent warming and future warming of 3C by 2600 can be explained by the distance between the Earth and Sun decreasing due to solar inertial motion (SIM) around the barycenter.

Except...the distance between the Earth and Sun doesn't actually change as a result of the SIM as the paper had assumed.

https://www.newscientist.com/article/2209895-journal-criticised-for-study-claiming-sun-is-causing-global-warming/

[bluewave]

5 hours ago, CAD_Wedge_NC said:

The burning question is.... how did the earth rid itself of that excess CO2? Was a dramatic increase in plant-life the culprit?

As Don said, it would be a very slow process from such high CO2 levels. Hopefully, we make a great technological leap in how we power the world  before CO2 levels get that high.

https://www.sciencedaily.com/releases/2019/02/190220112221.htm

A new study finds humans are pumping carbon dioxide into the atmosphere at a rate nine to 10 times higher than the greenhouse gas was emitted during the Paleocene-Eocene Thermal Maximum (PETM), a global warming event that occurred roughly 56 million years ago.

The results suggest if carbon emissions continue to rise, the total amount of carbon dioxide injected into the atmosphere since humans started burning fossil fuels could equal the amount released during the PETM as soon as 2159.

"You and I won't be here in 2159, but that's only about four generations away," said Philip Gingerich, a paleoclimate researcher at the University of Michigan and author of the new study in the AGU journal Paleoceanography and Paleoclimatology. "When you start to think about your children and your grandchildren, and your great-grandchildren, you're about there."

Scientists often use the PETM as a benchmark against which to compare modern climate change. But the new study shows we're on track to meet this benchmark much sooner than previously thought, as the pace of today's warming far outstrips any climate event that has happened since the extinction of the dinosaurs.

"Given a business-as-usual assumption for the future, the rates of carbon release that are happening today are really unprecedented, even in the context of an event like the PETM," said Gabriel Bowen, a geophysicist at the University of Utah who was not connected to the new study. "We don't have much in the way of geologic examples to draw from in understanding how the world responds to that kind of perturbation."

The exact environmental consequences of PETM-like carbon levels are unclear, but the increased temperatures will likely drive many species to extinction with the lucky ones being able to adapt or migrate, according to Larisa DeSantis, a paleontologist at Vanderbilt University who was not connected to the new study. In addition, it will take thousands of years for the climate system cool down, she said.

"It's not just about 100 years from now; it's going to take significant periods of time for that carbon dioxide to make its way back into the Earth's crust," DeSantis said. "It's not a short-term event. We're really committing ourselves to many thousands of years of a warmer world if we don't take action quickly."