Nav: Home

Climate models may underestimate future warming on tropical mountains

January 27, 2017

PROVIDENCE, R.I. [Brown University] -- In few places are the effects of climate change more pronounced than on tropical peaks like Mount Kilimanjaro and Mount Kenya, where centuries-old glaciers have all but melted completely away. Now, new research suggests that future warming on these peaks could be even greater than climate models currently predict.

Researchers led by a Brown University geologist reconstructed temperatures over the past 25,000 years on Mount Kenya, Africa's second-highest peak after Kilimanjaro. The work shows that as the world began rapidly warming from the last ice age around 18,000 years ago, mean annual temperatures high on the mountain increased much more quickly than in surrounding areas closer to sea level. At an elevation of 10,000 feet, mean annual temperature rose 5.5 degrees Celsius from the ice age to the pre-industrial period, the study found, compared to warming of only about 2 degrees at sea level during the same period.

"When we run state-of-the-art climate models backward in time to this period, they underestimate the temperature changes at high elevations," said James Russell, an associate professor in the Department of Earth, Environmental and Planetary Sciences and a fellow at the Institute at Brown for Environment and Society. "That implies that the models may similarly underestimate high-elevation warming in the future."

The study, which Russell led with Shannon Loomis, his former graduate student, is published in the journal Science Advances.

Temperature differences

Questions among scientists about how global warming affects tropical high elevations date back about 30 years. In 1985, influential research by Brown geologist Warren Prell showed that from the last ice age to the pre-industrial period, sea surface temperatures in the tropics rose only a degree or two. Meanwhile, temperature records estimated from high-altitude tropical glaciers suggested much more dramatic warming at high elevation.

"The climate modeling community thought there must be something wrong with one of these temperature records," Russell said, "because the models simply can't reproduce such a big difference in warming between high and low elevations."

Subsequent work has largely confirmed the sea surface temperature estimates, but questions about the high-elevation data remained. This new study aimed to generate new, more robust high-elevation records.

Over the past decade, Russell's co-author Jaap Damsté of the University of Utrecht and colleagues have developed a new method of tracking temperature through time by studying the remains of ancient microbes. Specifically, they look at organic compounds called GDGTs that are produced in microbial cell walls. The chemical makeup of GDGTs is sensitive to temperature. In order to keep GDGTs and cell walls in a stable and permeable state, microbes alter the chemical makeup of GDGTs in response to temperature changes. Russell and his team have been able to precisely calibrate GDGT composition found in lake sediments with air temperatures through time.

"We thought we could use this new temperature proxy to create a record of high-elevation temperature since the last ice age that either confirms or refutes the glacier-derived record," Russell said.

For the study, Russell and his colleagues looked at sediment cores taken from the bottom of Lake Rutundu, a volcanic lake on Mount Kenya at an elevation of around 10,000 feet. The cores preserve the signature of GDGT chemistry dating back more than 25,000 to the ice age. The data suggested that mean annual temperatures at Lake Rutundu increased about 5.5 degrees Celsius since the last ice age -- a figure consistent with the previous high elevation temperature proxies. Meanwhile, temperature data from two lakes closer to sea level -- Lake Tanganyika and Lake Malawi -- suggest much more modest temperature changes of about 3.3 degrees and 2 degrees respectively.

Climate models are able to reproduce the temperature changes at low elevations, but they underestimate the high-elevation change by 40 percent, Russell says. That suggests there's something amiss in the way the models simulate changes in the atmospheric lapse rate -- the rate at which air temperature varies with altitude.

"All climate models calculate a lapse rate -- it's integral to the output of the model," Russell said. "What this work shows is that there's a problem in the way the models make that calculation."

Implications for future climate change

It's difficult to diagnose exactly what that problem is, Russell says, but it likely has something to do with the way models treat atmospheric water vapor content. Water vapor content is the strongest controlling factor in governing the lapse rate (moist air cools more slowly with altitude).

"We would argue that there's probably a problem in the water vapor concentrations and therefore the feedback," Russell said.

Whatever the source of the problem, the ramifications for tropical mountains may be significant. The models miss almost half the temperature change at high elevations in the past, and they may be underestimating future change as well.

"These are very fragile ecosystems that house extraordinary biodiversity and unique environments such as tropical glaciers," Russell said. "Our results suggest future warming in these environments could be more extreme than we predict."
The work was supported by the National Science Foundation (EAR-1226566). Russell's coauthors were Shannon Loomis (Brown), Dirk Verschuren (Ghent University) Carrie Morrill (University of Colorado, Boulder), Gijs De Cort (Ghent), Jaap S. Sinninghe Damsté (University of Utrecht), Daniel Olago (University of Nairobi), Hilde Eggermont (Ghent) F. Alayne Street-Perrott (Swansea University) and Meredith A. Kelly (Dartmouth).

Note to Editors:

Editors: Brown University has a fiber link television studio available for domestic and international live and taped interviews, and maintains an ISDN line for radio interviews. For more information, call (401) 863-2476.

Brown University

Related Climate Change Articles:

The black forest and climate change
Silver and Douglas firs could replace Norway spruce in the long run due to their greater resistance to droughts.
For some US counties, climate change will be particularly costly
A highly granular assessment of the impacts of climate change on the US economy suggests that each 1°Celsius increase in temperature will cost 1.2 percent of the country's gross domestic product, on average.
Climate change label leads to climate science acceptance
A new Cornell University study finds that labels matter when it comes to acceptance of climate science.
Was that climate change?
A new four-step 'framework' aims to test the contribution of climate change to record-setting extreme weather events.
It's more than just climate change
Accurately modeling climate change and interactive human factors -- including inequality, consumption, and population -- is essential for the effective science-based policies and measures needed to benefit and sustain current and future generations.
Climate change scientists should think more about sex
Climate change can have a different impact on male and female fish, shellfish and other marine animals, with widespread implications for the future of marine life and the production of seafood.
Climate change prompts Alaska fish to change breeding behavior
A new University of Washington study finds that one of Alaska's most abundant freshwater fish species is altering its breeding patterns in response to climate change, which could impact the ecology of northern lakes that already acutely feel the effects of a changing climate.
Uncertainties related to climate engineering limit its use in curbing climate change
Climate engineering refers to the systematic, large-scale modification of the environment using various climate intervention techniques.
Public holds polarized views about climate change and trust in climate scientists
There are gaping divisions in Americans' views across every dimension of the climate debate, including causes and cures for climate change and trust in climate scientists and their research, according to a new Pew Research Center survey.
The psychology behind climate change denial
In a new thesis in psychology, Kirsti Jylhä at Uppsala University has studied the psychology behind climate change denial.

Related Climate Change Reading:

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Jumpstarting Creativity
Our greatest breakthroughs and triumphs have one thing in common: creativity. But how do you ignite it? And how do you rekindle it? This hour, TED speakers explore ideas on jumpstarting creativity. Guests include economist Tim Harford, producer Helen Marriage, artificial intelligence researcher Steve Engels, and behavioral scientist Marily Oppezzo.
Now Playing: Science for the People

#524 The Human Network
What does a network of humans look like and how does it work? How does information spread? How do decisions and opinions spread? What gets distorted as it moves through the network and why? This week we dig into the ins and outs of human networks with Matthew Jackson, Professor of Economics at Stanford University and author of the book "The Human Network: How Your Social Position Determines Your Power, Beliefs, and Behaviours".