Climate models

How Climate Models Got So Accurate They Won A Nobel Prize


Climate modellers have a moment.

Last month, Time magazine listed two – Friederike Otto and Geert Jan van Oldenborg of the World Weather Attribution Project – among the 100 Most Influential People of 2021. Two weeks ago, Katharine Hayhoe of Texas Tech University was a guest on the popular talk show CBS Jimmy Kimmel Live! And on Tuesday, climate modeling pioneers Syukuro Manabe and Klaus Hasselman shared the Nobel Prize in physics with theoretical physicist Giorgio Parisi – recognition, said Thors Hans Hansson, chairman of the Nobel Committee in Physics, that “our knowledge of climate rests on a solid scientific basis, based on a rigorous analysis of the observations.

Climate modelers are experts in Earth or planetary sciences, often with background in applied physics, mathematics, or computer science, who use physics and chemistry to create equations, feed them into supercomputers, and apply them to simulate the climate of Earth or other planets. Models have long been viewed by climate change deniers as the soft underbelly of climate science. Being necessarily predictive, they were considered to be essentially unverifiable and the result of erroneous entries producing unreliable results.

A 1990 National Geographic The article put it this way: “Critics say modeling is in its infancy and cannot even reproduce the details of our current climate. The modelers agree and note that the predictions necessarily fluctuate with each refinement of the model.

However, more recent analyzes, dating back decades, have found that many of the early models were remarkably accurate in their predictions of global temperature increases. Now, as computing power increases and more and more refinements are added to modeling inputs, modelers are more confident in defending their work. As a result, says Dana Nuccitelli, author of Climatology versus pseudoscience: exposing the unsuccessful predictions of global warming skeptics, “There has certainly been a change from outright denial of climate science; because the predictions turned out to be so accurate, it becomes harder and harder to deny the science at this point. “

This 1990 article quoted Manabe – widely regarded as the father of modern climate modeling – as saying that, in some of the early models, “all kinds of crazy things happened … sea ice covered tropical oceans, for example.” But in a seminal 1970 article, the first to make a specific projection of future warming, Manabe argued that global temperatures would increase by 0.57 degrees Celsius (1.03 degrees Fahrenheit) between 1970 and 2000. The actual warming recorded was remarkably close to 0.54 ° C. (0.97 ° F).

A 2019 article by Zeke Hausfather of the University of California at Berkeley, Henri Drake and Tristan Abbott of the Massachusetts Institute of Technology, and Gavin Schmidt of the NASA Goddard Institute for Space Studies analyzed 17 models dating from the 1970s and found that 14 accurately predicted the relationship between global temperatures as greenhouse gases increased. (The estimates of two were too high and one was too low.) That’s because fundamental physics has always been strong, says Dana Nuccitelli, research coordinator at the Citizens’ Climate Lobby. and author of Climatology versus pseudoscience: exposing the unsuccessful predictions of global warming skeptics.

“We’ve understood for decades the basic science that if you put a certain amount of carbon dioxide into the atmosphere, we’ll get some warming,” he says. “These forecasts in the 1970s were remarkably accurate, but they also used fairly simplified climate models, in part because of our level of understanding of climate systems, but also because of computational limitations at the time. It is certainly true that climate models have come a long way.

The more things change …

In the realm of climate modeling, “What hasn’t changed over the years is the overall assessment of the extent of global warming as we increase CO2,” says Hayhoe, who is also Chief Scientist for Nature Conservancy and author of Saving Us: A Climate Scientist’s Case for Hope and Healing in a Divided World. “What has changed is our understanding at increasingly smaller spatial and temporal scales. Our understanding of feedbacks in the climate system, our understanding, for example, of the real sensitivity of the Arctic. “

As this understanding has grown, she says, it has enabled the development of what she calls “the cutting edge of climate science today” – the attribution of individual events, the specialty for which Otto and van Oldenberg were recognized in Time, which for the first time is able to establish strong links between climate change and specific weather events, such as heat waves in the western United States or the amount of rain deposited by Hurricane Harvey.

“We couldn’t do this without models,” says Hayhoe, “because we need the models to simulate a world without people. And we have to compare an Earth without people to the Earth we live on with humans and carbon emissions. And when we compare these two Earths, we can see how human-induced climate change has altered the duration, intensity, and even damage associated with a specific event.

In Hayhoe’s case, the actual act of modeling is “looking at thousands of lines of code, and it’s so intense that I often do it at night, when people aren’t emailing and that the lights are out and that I can focus on that bright screen. in a dark room. Then I blink and it’s suddenly half past four in the morning.

Much of the work, she says, involves trying to find things wrong with the models, to make sure they reflect reality. “If it doesn’t quite fit, we have to look more carefully because there is something that we didn’t quite understand. “

While such deviations can be flaws in the models, they can sometimes reflect errors in the observations. For example, a series of studies in 2005 found that satellite data which appeared to show no warming in the lower atmosphere, or the troposphere, and which was used to cast doubt on global warming patterns, was itself wrong. The models, backed by data from weather balloons, have always been right.

Irony, says Michael Mann, professor emeritus of atmospheric science at Penn State University and most recent author of The new climate war, is that “climatologists have been dismissed as alarmists for the predictions we made, but the predictions, if any, have been shown to be too conservative and we are seeing even greater impacts than we expected.” “

The apparent imminent collapse of the system that drives the currents of the Atlantic Ocean is, he says, an example. “It’s something we thought we could happen, but it’s not only happening, it’s happening sooner than expected,” he notes. Manabe, he points out, was one of those who first raised the possibility decades ago. “It just underscores that what’s going on in climate science is the worst thing that can really happen to a climate modeler: to see your worst predictions come true. “

Modelers recognize that science is not perfect; Even today, uncertainties remain, and not just of a single order.

“Do we have all the physical processes in the model? And if we have them in there, are they represented correctly or not? Hayhoe asks rhetorically. “Then there is a second source of uncertainty called parametric uncertainty. In addition, she says, some processes take place at such small scales, for example among cloud particles, that they cannot be measured directly but must be inferred. obviously this adds some uncertainty. However, the greatest uncertainty, she says, does not lie in physics, but in our own collective behavior, and how prepared we are to allow global levels of greenhouse gases to rise.

“If we didn’t know that carbon emissions produce all of these impacts on us, that it’s not just a curiosity about the increase in global temperature but also our food, our water, our health, our homes, then we would not act. “, says Hayhoe.

“That’s why I do what I do, and that’s why role models are so important, because they show what’s going on right now that we’re responsible for, and what’s going to happen in the future. I look forward to the day when we can just use climate models to just understand this amazing planet, but right now those models are telling us, “Now is the time to act! And if we don’t, the consequences will be serious and dangerous.


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