Nav: Home

Some of the latest climate models provide unrealistically high projections of future warming

April 30, 2020

A new study from University of Michigan climate researchers concludes that some of the latest-generation climate models may be overly sensitive to carbon dioxide increases and therefore project future warming that is unrealistically high.

In a letter scheduled for publication April 30 in the journal Nature Climate Change, the researchers say that projections from one of the leading models, known as CESM2, are not supported by geological evidence from a previous warming period roughly 50 million years ago.

The researchers used the CESM2 model to simulate temperatures during the Early Eocene, a time when rainforests thrived in the tropics of the New World, according to fossil evidence.

But the CESM2 model projected Early Eocene land temperatures exceeding 55 degrees Celsius (131 F) in the tropics, which is much higher than the temperature tolerance of plant photosynthesis--conflicting with the fossil evidence. On average across the globe, the model projected surface temperatures at least 6 C (11 F) warmer than estimates based on geological evidence.

"Some of the newest models used to make future predictions may be too sensitive to increases in atmospheric carbon dioxide and thus predict too much warming," said U-M's Chris Poulsen, a professor in the U-M Department of Earth and Environmental Sciences and one of the study's three authors.

The other authors are U-M postdoctoral researcher Jiang Zhu and Bette Otto-Bliesner of the National Center for Atmospheric Research. They say their study shows how geological evidence can be used to benchmark climate models and predictions of future warming.

The new study focuses on a key climate parameter called equilibrium climate sensitivity, or ECS. ECS refers to the long-term change in global temperature that would result from a sustained doubling--lasting hundreds to thousands of years--of heat-trapping carbon dioxide above the preindustrial baseline level of 285 parts per million.

The present-day CO2 level is about 410 ppm, and climate scientists say atmospheric concentrations could hit 1,000 ppm by the year 2100 if nothing is done to limit carbon emissions from the burning of fossil fuels.

For decades, most of the top climate models predicted an equilibrium climate sensitivity of around 3 degrees Celsius (5.4 F), with a range of 1.5 to 4.5 C (2.7 to 8.1 F).

But that changed recently with some of the newest climate models participating in CMIP6. The Coupled Model Intercomparison Project (CMIP) is an internationally coordinated effort between climate-science institutions, and it is now in its sixth phase. The next assessment report from the authoritative Intergovernmental Panel on Climate Change, which is due next year, will rely on CMIP6 models.

Ten of the 27 CMIP6 models have an equilibrium climate sensitivity higher than 4.5 C (8.1 F), meaning that they are more sensitive to CO2 increases than most previous-generation models. The CESM2 model (Community Earth System Model, version 2) tested by the U-M-led research team is one of those CMIP6 models and has an equilibrium climate sensitivity of 5.3 C (9.5 F).

The predecessor to CESM2, the CESM1.2 model, did a remarkably good job of simulating temperatures during the Early Eocene, according to the researchers. It has an equilibrium climate sensitivity of 4.2 C (7.6 F).

"Our study implies that CESM2's climate sensitivity of 5.3 C is likely too high. This means that its prediction of future warming under a high-CO2 scenario would be too high as well," said Zhu, first author of the Nature Climate Change letter.

"Figuring out whether the high climate sensitivity in CMIP6 models is realistic is of tremendous importance for us to anticipate future warming and to make adaptation plans," said NCAR's Otto-Bliesner.

The team's simulations of the Early Eocene incorporated the latest paleoclimate reconstructions and included data about paleogeography, vegetation cover and land surface properties. Reconstructions of atmospheric carbon dioxide levels from that time predate ice-core records and rely on geochemical and paleobotanical proxies.

The Intergovernmental Panel on Climate Change's Fifth Assessment Report, finalized in 2014, said the global surface temperature increase by the end of the 21st century is likely to exceed 1.5 C relative to the 1850 to 1900 period for most emissions scenarios, and is likely to exceed 2.0 C for some emissions scenarios.

The projections in that assessment were based on the previous generation of CMIP models, known as CMIP5 models. The newer CMIP6 models will likely lead to projections of even greater warming. The Paris climate accord's long-term temperature goal is to keep the increase in global average temperature to well below 2 C above preindustrial levels and to pursue efforts to limit the increase to 1.5 C.
-end-
The work was supported by grants from the National Science Foundation and the Heising-Simons Foundation to Poulsen, who is the associate dean for natural sciences at U-M's College of Literature, Science, and the Arts.

Study: High climate sensitivity in CMIP6 model not supported by paleoclimate

University of Michigan

Related Climate Change Articles:

Mapping the path of climate change
Predicting a major transition, such as climate change, is extremely difficult, but the probabilistic framework developed by the authors is the first step in identifying the path between a shift in two environmental states.
Small change for climate change: Time to increase research funding to save the world
A new study shows that there is a huge disproportion in the level of funding for social science research into the greatest challenge in combating global warming -- how to get individuals and societies to overcome ingrained human habits to make the changes necessary to mitigate climate change.
Sub-national 'climate clubs' could offer key to combating climate change
'Climate clubs' offering membership for sub-national states, in addition to just countries, could speed up progress towards a globally harmonized climate change policy, which in turn offers a way to achieve stronger climate policies in all countries.
Review of Chinese atmospheric science research over the past 70 years: Climate and climate change
Over the past 70 years since the foundation of the People's Republic of China, Chinese scientists have made great contributions to various fields in the research of atmospheric sciences, which attracted worldwide attention.
A CERN for climate change
In a Perspective article appearing in this week's Proceedings of the National Academy of Sciences, Tim Palmer (Oxford University), and Bjorn Stevens (Max Planck Society), critically reflect on the present state of Earth system modelling.
Fairy-wrens change breeding habits to cope with climate change
Warmer temperatures linked to climate change are having a big impact on the breeding habits of one of Australia's most recognisable bird species, according to researchers at The Australian National University (ANU).
Believing in climate change doesn't mean you are preparing for climate change, study finds
Notre Dame researchers found that although coastal homeowners may perceive a worsening of climate change-related hazards, these attitudes are largely unrelated to a homeowner's expectations of actual home damage.
Older forests resist change -- climate change, that is
Older forests in eastern North America are less vulnerable to climate change than younger forests, particularly for carbon storage, timber production, and biodiversity, new research finds.
Could climate change cause infertility?
A number of plant and animal species could find it increasingly difficult to reproduce if climate change worsens and global temperatures become more extreme -- a stark warning highlighted by new scientific research.
Predicting climate change
Thomas Crowther, ETH Zurich identifies long-disappeared forests available for restoration across the world.
More Climate Change News and Climate Change Current Events

Trending Science News

Current Coronavirus (COVID-19) News

Top Science Podcasts

We have hand picked the top science podcasts of 2020.
Now Playing: TED Radio Hour

Our Relationship With Water
We need water to live. But with rising seas and so many lacking clean water – water is in crisis and so are we. This hour, TED speakers explore ideas around restoring our relationship with water. Guests on the show include legal scholar Kelsey Leonard, artist LaToya Ruby Frazier, and community organizer Colette Pichon Battle.
Now Playing: Science for the People

#568 Poker Face Psychology
Anyone who's seen pop culture depictions of poker might think statistics and math is the only way to get ahead. But no, there's psychology too. Author Maria Konnikova took her Ph.D. in psychology to the poker table, and turned out to be good. So good, she went pro in poker, and learned all about her own biases on the way. We're talking about her new book "The Biggest Bluff: How I Learned to Pay Attention, Master Myself, and Win".
Now Playing: Radiolab

Uncounted
First things first: our very own Latif Nasser has an exciting new show on Netflix. He talks to Jad about the hidden forces of the world that connect us all. Then, with an eye on the upcoming election, we take a look back: at two pieces from More Perfect Season 3 about Constitutional amendments that determine who gets to vote. Former Radiolab producer Julia Longoria takes us to Washington, D.C. The capital is at the heart of our democracy, but it's not a state, and it wasn't until the 23rd Amendment that its people got the right to vote for president. But that still left DC without full representation in Congress; D.C. sends a "non-voting delegate" to the House. Julia profiles that delegate, Congresswoman Eleanor Holmes Norton, and her unique approach to fighting for power in a virtually powerless role. Second, Radiolab producer Sarah Qari looks at a current fight to lower the US voting age to 16 that harkens back to the fight for the 26th Amendment in the 1960s. Eighteen-year-olds at the time argued that if they were old enough to be drafted to fight in the War, they were old enough to have a voice in our democracy. But what about today, when even younger Americans are finding themselves at the center of national political debates? Does it mean we should lower the voting age even further? This episode was reported and produced by Julia Longoria and Sarah Qari. Check out Latif Nasser's new Netflix show Connected here. Support Radiolab today at Radiolab.org/donate.