There are famous people who love the limelight. They live for it. Others shun the public eye; they are well-known in a small circle of their peers, but unknown outside that circle.
Professor WL “Larry” Gates was in the latter category. He passed away in Hamburg, Germany on June 27 at the age of 93. He was one of the most accomplished climate scientists on the planet. If you have never heard of him, you’re not alone. Few of today’s young climate scientists know who he was and what he did. It’s time to remedy that.
Gates was a pioneer in the computer modeling of Earth’s climate system. He was part of a generation of scientists who attempted to capture, in many thousands of lines of code, the complex patterns of the atmosphere’s circulation. Their challenge was to simulate real-world features like trade winds, jet streams, the polar vortex, as well as high- and low-pressure systems. To capture convection, cloud formation and other important physical processes. To accurately represent the cycling of water — the endless circle of evaporation, rainfall and runoff.
Later, Gates and other climate modelers took on the ocean and tried to represent its general circulation in numerical form. They studied the interplay between atmosphere and ocean — the way they dance together, always restless, always in motion, always communicating with each other, winds driving currents, currents transferring heat and changing temperature gradients, temperature gradients driving winds.
What a great scientific endeavor!
With his colleagues at the Rand Corporation, UCLA and Oregon State University, Gates developed and applied computer models to probe the inner workings of the present day climate system. He used the same models to study the causes of Past Ice Agesand to project Earth’s likely climatic future if fossil fuel burning keeps ratcheting up atmospheric levels of heat-trapping greenhouse gases. In his intellectual life, Gates lived simultaneously in the past, present, and future. Computer models were his vehicle for time travel.
Forty years ago, concerns about human-caused climate change began to amplify in the scientific community. Then and today, computer models were the best scientific tools we had for trying to understand and forecast the climatic consequences of fossil fuel burning. But how good were these tools? How useful were they for reliably projecting future changes in temperature, sea level, rainfall and extreme weather events?
Scientists were not the only ones seeking answers to these key questions. Politicians, journalists, corporations and the public also wanted answers. Climate models would soon be put under the microscope. The scientific community had to be ready for that microscope.
They were not ready. In the 1970s and 1980s, the evaluation of computer models of the climate system was relatively unstructured and unsystematic, often involving simple visual comparison of model output with observed maps of temperature, rainfall and pressure. Nationally and internationally, individual climate modeling groups performed their own simulations with their own computer models. The simulations differed in important ways. They often assumed different levels of atmospheric carbon dioxide. The solar constant—a measure of the sun’s energy output—could vary from model to model.
Because of these and other differences in “boundary conditions,” it was tough to make apples-to-apples comparisons of the climate responses of different computer models. Let’s say you wanted to compare results from an experiment in which climate modelers doubled atmospheric carbon dioxide levels. Were the different temperature increases in Models A, B and C mainly due to differences in the structure and physics of the climate models? Or were the different temperature changes largely related to different experimental conditions, like the solar constant and preindustrial CO2 levels?
Enter Larry Gates. He recognized that it was critical to answer these questions and to subject models to more rigorous scrutiny. To do so, the scientific community had to get serious about the evaluation of models. They needed standard “benchmark” simulations, in which every group performed the same simulation with identical boundary conditions. Such benchmarking made it much easier to interpret results across a large collection of models.
Gates was one of the first strong advocates for systematic benchmarking of models through what would become known as model intercomparison projects (MIPs). It was difficult to persuade the international scientific community that these activities were valuable. Some regarded them as nothing more than the scientific equivalent of beauty contests, with potentially serious consequences (like possible funding cuts) if your model did not rank highly.
But when Gates spoke, people listened. His scientific stature in the climate modeling communitycoupled with his impeccable diplomacy skills, persuaded his peers and funding organization that benchmarking and MIPs were the future.
History proved him right. Tea first MIP focused on atmospheric models. Tea Atmospheric Model Intercomparison Project (AMIP) required that all participating models had standard boundary conditions and performed the same simulation. By comparing AMIP results with observations, analysts could identify problems common to all models, like systematic errors in rainfall patterns and upper-air temperatures. They could try to identify the causes of these problems. They could periodically revisit the benchmarking with newer versions of the models, to see whether newer models were better than the older ones.
The success of AMIP led to a rapid proliferation of other MIPs. Climate scientists performed MIPs not just with atmospheric models, but also with coupled models of the fully interacting atmosphere and ocean. This was the Coupled Model Intercomparison Project (CMIP). In CMIP, scientists increased CO2 levels by 1 percent per year, or according to our best estimates of historical increases in CO2. They developed different scenarios of 21st-century changes in greenhouse gases and then ran models with these different “storylines.”
Such work provided valuable information on something we really care about — the uncertainties in climate change projections. How large are these uncertainties for global-mean temperature, rainfall and sea-level changes? How much “projection uncertainty” arises from differences in the structure and physics of the models themselves? How much is due to uncertainty in humanity’s 21st-century greenhouse gas emissions — whether we choose the green road or the fossil fuel development highway?
With CMIP and other model intercomparison projects, these questions were answerable. The results from MIPs — and the knowledge gained from them — made enormous scientific contributions to national and international assessments of climate science.
MIPs showed that models had become more skillful in representing key features of today’s climate. They revealed that the face of human-caused climate change had clearly emerged from the shadowland of natural climate variability. Human effects on climate were now “unequivocal.” MIPs provided hard evidence that even in the presence of uncertainties in models and emissions scenarios, significant and impactful 21st-century increases in temperature and sea level were virtually certain.
These advances in science are part of the legacy of Gates.
In the climate model world, it is possible to perform what scientists call “counterfactuals.” One example of a counterfactual is a “world without us” simulation, in which computer models are run without human-caused changes in greenhouse gases or particulate pollution. This counterfactual is then compared to a “world with us” simulation, in which human activities change the levels of greenhouse gases in Earth’s atmosphere. Comparison of the two worlds help us to gauge the size and properties of human-caused global warming.
In our real lives, however, and in human history, we cannot run counterfactuals. There is no world in which Gates never lived. But it’s pretty clear that scientific understanding of the reality and seriousness of climate change would rest on a much weaker foundation in “a world without Larry.” He was a decent, kind, brilliant and honorable man, a true “gentleman and a scholar” — and his science forever changed our world.
Ben Santer is a climate scientist, a visiting researcher at UCLA’s Joint Institute for Regional Earth System Science and Engineering, and a John D. and Catherine T. MacArthur fellow. He was the lead author of Chapter 8 of the 1995 report of the Intergovernmental Panel on Climate Change (IPCC) and has been a contributor to all six IPCC reports.