How IPCC went from ‘not proven’ that we cause climate change in 1990 to ‘we are guilty’ in 2021

The Intergovernmental Panel on Climate Change (IPCC) advises the world’s governments on climate change science, impacts and response strategies, and recently released their latest report on climate change science. It was the sixth in a series of scientific assessments that began in 1990.

Much has been written about the Sixth Assessment Report (AR6). It attracted world-wide attention for its finding of “unequivocal” human influence on the climate system. The scientific jury comprised hundreds of experts in climate science. They reached a clear verdict: Humanity bears primary responsibility for the global warming of roughly 1.1 degrees Celsius since 1850.

Most commentators on AR6 view the report as a somber warning. A warning that planetary scale warming is already leading to more intense heat waves, droughts and floods. A warning that global warming is already driving sea-level rise, an existential threat to small island nations and millions living in low-lying coastal areas. A warning that all countries need to act swiftly and decisively to reduce emissions of greenhouse gases from fossil fuel burning. A warning that if we lack the political will to act, the forecast is poor for the well-being of present and future citizens of this planet.

Back in 1990, the first IPCC report concluded that the scientific jury was still out on the identification of human-caused global warming. Their finding? “The unequivocal detection of the enhanced greenhouse effect from observations is not likely for a decade or more.” The 1990 verdict of “not proven” has now been replaced by a very different verdict in 2021: unequivocal human influence on global climate.

Why did the verdict change?

I study the causes of climate change, and participated in each of the IPCC’s six climate science reports. This gives me an informed perspective on how “not proven” in 1990 became “humans are guilty” in 2021.

Many factors enabled this scientific progress. One factor was better, longer and more complete observations of climate change. We now study Earth’s climate from space, the atmosphere, the oceans and land surface. A wide variety of measurement platforms probe the climate system in amazing detail. Global changes in temperature, moisture, sea level and ice cover are routinely monitored. Less-familiar properties are also measured, like the liquid water content of clouds, the mass balance of major ice sheets and variations in ocean color driven by phytoplankton blooms.

Collectively, this rich set of observations provides crucial information for understanding causation. It’s tough to understand why climate is changing if you don’t know how it’s changing.

Another key factor shaping the trajectory from IPCC’s first 1990 report (AR1) to the sixth released this week was the development of better computer models of the climate system.

Einstein was fond of “Gedankenexperimente” — thought experiments you could perform in your head to shed light on important physical principles. He would have loved climate models. They are the ultimate thought experiment. A modeler can change different influences on climate, either individually or in concert. This makes it easier to identify the unique fingerprints that are caused by different human and natural influences on climate.

At the time of IPCC’s 1990 report, a half-dozen climate modeling groups existed. In 2021, the number of climate models assessed had swelled by an order of magnitude. The complexity of models also increased along the scientific road to the latest report.  The ocean’s role in the climate system was represented more realistically. Aspects of the climate system left out of the 1990 models were gradually added: the uptake and release of carbon from the oceans, soil and vegetation. Atmospheric chemistry, marine biogeochemistry and ice sheets.

Each addition of a new component of the climate system allowed scientists to study key “interaction terms.” Many of these interactions are critically important. We need to know the impact of global warming on the ocean’s ability to absorb the CO2 emitted from fossil fuel burning. We need to understand how ocean warming and sea-level rise affect the stability of massive ice shelves off the coast of Antarctica. We need to know how the melting of Arctic sea ice and the Greenland ice sheet might influence ocean circulation in the North Atlantic.

The availability of more and improved climate models had many added benefits. Each model has a different representation of the physics, chemistry and biology operating in the real-world climate system. By analyzing a large collection of models, scientists could quantify uncertainties in projections of 21st century climate change. They could assess whether detection of human fingerprints in observations was robust to model uncertainties. In the robust results, they could try to understand the physical processes driving common changes in climate. 

Better observations and models are only part of the story of scientific progress from AR1 to AR6. Progress has many parents. There are now more rigorous protocols for evaluating and testing climate models. There is greater computational power — a necessary condition for running increasingly more complex models with finer spatial granularity. There are better platforms for sharing and analyzing enormous volumes of observational data and climate model output. From ice cores, tree rings, corals, sediment cores and many other paleoclimate records, there is improved understanding of climate changes over deep time, showing that global warming since the Industrial Revolution is unusually large and rapid.

And there is now a larger community of climate scientists — a community driven by a common desire to understand how and why our atmosphere and oceans are changing, and what those changes portend for the future.

Progress has also been made by responding to those who have kicked the tires of climate science. Let me give an example. Back in 1995, I was the convening lead author of chapter 8 of the IPCC AR2 report. Our chapter concluded that “the balance of evidence suggests a discernible human influence on global climate”. Many folks kicked that tire. They claimed satellite data showed “no warming whatsoever” of the lowermost layer of Earth’s atmosphere, thus invalidating the detection of a global warming signal.

The analysis of the satellite data underpinning such “no warming” claims was simply wrong. A lot of hard work had to be done to understand flaws in the satellite data. That hard work advanced the science, ultimately leading to better satellite observations — observations which now show robust warming of the lower atmosphere.

Another popular criticism of the IPCC’s 1995 “discernible human influence” conclusion was that it rested primarily on studies of surface temperature changes. Critics argued that if there really was a human-caused climate signal lurking in observations, it should be manifest in many different aspects of climate change — not just in surface thermometer measurements.

This was valid criticism. Climate scientists did not ignore it. You don’t get far in any scientific endeavor if you ignore valid criticism.

The response of the scientific community was to examine dozens of different independently monitored aspects of climate change, and to eventually show that human-caused climate fingerprints were all over the climate system — not just in surface temperature. As AR6 reported:

“It is unequivocal that human influence has warmed the global climate system since pre-industrial times. Combining the evidence from across the climate system increases the level of confidence in the attribution of observed climate change to human influence and reduces the uncertainties associated with assessments based on single variables. Large-scale indicators of climate change in the atmosphere, ocean, cryosphere and at the land surface show clear responses to human influence consistent with those expected based on model simulations and physical understanding.”

That’s how we got from AR1 to AR6. With better data, better models and better understanding of key physical processes. With improved paleoclimate reconstructions. With detailed scientific responses to valid criticism. With individuals willing to devote years of their lives to IPCC assessment reports — a scientific “coalition of the willing” that brought together experts from physics, chemistry, biology, oceanography, meteorology, glaciology, computer science, statistics, social sciences, risk management and many other disciplines. 

There will be critics of AR6, just as there were critics of all previous IPCC assessments. The critics will likely opine on Fox News and in the Wall Street Journal about a lack of “maturity” of climate science. The uncertainties are too large, they will argue — the costs of action are too great.

Those who deny the reality and seriousness of climate change are wrong. Dangerously wrong. Their incorrect views on the science make it more difficult to achieve real action on climate change. Inaction harms all of us. 

The AR6 report diminished the space in which climate change denialism can thrive. Denialism is a tough sell when cautious scientists use words like “unequivocal” in international reports. It’s a tough sell in 2021, when climate change manifests in your own backyard, and your backyard is in Siberia, the Pacific Northwest, British Columbia, Greece, Turkey, Algeria, Germany and China. It’s tough to sell “Don’t worry about climate change” when smoke from uncontrolled forest fires reaches the North Pole.   

If you do one thing this weekend, read the Summary for Policymakers of the AR6 report. Understand how our planet’s climate is changing. Then act and vote with purpose. Hold political leaders accountable if they fail to address climate change. As the AR6 report clearly states, Earth’s climate future is in our hands.    

Ben Santer, Ph.D., is a climate scientist and John D. and Catherine T. MacArthur Fellow. He was also the lead author of Chapter 8 of the 1995 IPCC report and has been a contributor to all six IPCC reports.