In 1990, the newly formed Intergovernmental Panel on Climate Change released its First Assessment Report — the world’s first coordinated scientific attempt to project what a warming planet would actually look like decades into the future. It was a bold thing to do. Computing power was a fraction of what we have today, satellite records were thin, and the scientists involved knew they’d eventually be graded on their work by history.
Thirty-six years later, we can check their homework. The results are, in a word, sobering — not because the scientists got the science wrong, but because they got it largely right, and we didn’t act on it.
In 1990, atmospheric CO2 stood at about 354 parts per million (ppm). The IPCC’s “business as usual” scenario — the pathway assuming no serious climate policy — projected concentrations would reach roughly 410 ppm by the early 2020s.
Today, atmospheric CO2 is at approximately 429 ppm. We didn’t just meet the business-as-usual line; recent years have added carbon to the atmosphere faster than that scenario assumed, with annual increases now running as high as 3.75 ppm — the largest single-year jumps ever recorded. Pre-industrial CO2 was about 280 ppm, meaning we’ve now pushed concentrations up more than 50% above the atmosphere our civilization was built in.
This is the clearest evidence in the whole comparison that the 1990 report’s blind spot wasn’t the science — it was the assumption that the world would begin curbing emissions. It hasn’t, at least not at the pace or scale required.
The 1990 report projected roughly 0.3°C of warming per decade under business as usual. That estimate has held up remarkably well. Global average temperature is now running about 1.4–1.5°C above pre-industrial levels, with 2024 becoming the first calendar year to cross the 1.5°C threshold and 2025 ranking as the third-warmest year on record. The ten warmest years ever measured have all occurred since 2015.
Given the modeling tools available in 1990 — years before the modern satellite and reanalysis datasets we rely on now — this level of accuracy is genuinely remarkable. It’s a reminder that the physics of the greenhouse effect, understood since the 1800s, doesn’t bend to political debate.
The 1990 report predicted sea levels would rise at roughly 6 centimeters per decade under business as usual, essentially a steady, linear climb.
The actual numbers land close to that range — global sea level has risen about 10 centimeters since satellite measurements began in 1993. But the shape of the rise tells a different story than the 1990 report expected. The rate has nearly doubled, climbing from about 2.1 millimeters per year in the early 1990s to close to 4 millimeters per year today. Where the original projection assumed a straight line, reality has produced a curve bending upward — driven by accelerating ice sheet loss in Greenland and Antarctica that wasn’t well understood at the time.
Unlike the other categories, the 1990 report made no confident quantitative prediction about hurricane frequency or intensity. The climate models of the era simply weren’t sophisticated enough to resolve tropical cyclones with any reliability, and the IPCC said so plainly.
That gap has since been filled by modern attribution science. Researchers now have high confidence that warming is intensifying peak wind speeds and rainfall rates in the storms that do form, even though there isn’t yet a clear long-term trend in the total number of storms each year. It’s a useful example of science being appropriately cautious where uncertainty was genuinely high, and updating as better tools became available.
Line these four predictions up, and a pattern emerges. The physics-based projections — temperature and sea level — held up well, sometimes almost exactly. The one area where reality diverged sharply from the 1990 forecast was carbon dioxide concentration, and that divergence wasn’t a failure of climate modeling. It was a failure of political will. The “business as usual” scenario the IPCC warned us about wasn’t a worst case dreamed up to scare policymakers — it was, in large part, exactly the path we chose to take.
That distinction matters. It reframes more than three decades of climate history: this was never primarily a story about scientists being wrong, or about technology not existing. Solar power, wind power, energy efficiency and grid storage have all existed in workable form for decades. What’s been missing isn’t the knowledge or the tools — it’s the collective decision to use them at the scale the science has been telling us, consistently and accurately, that we need.
The barrier to a stable climate has never really been a lack of understanding. It’s been, and remains, a matter of political choice.
Wes Golomb taught energy and sustainability for 34 years, and developed the first Energy Services Degree in the Northeast. He chairs the Deerfield Energy Committee and was the NH Energy Conservation Coordinator.