Global wind and solar power are growing fast enough to keep the world roughly on track for 2 degrees Celsius of warming, but not fast enough to meet the more ambitious 1.5-degree target. That is the central finding of a new peer-reviewed forecasting model built by researchers at Chalmers University of Technology in Sweden, published in Nature Energy in early 2026.
The study arrives at a moment when governments are under pressure to show that pledges made at COP28 in Dubai, including a commitment to triple global renewable capacity by 2030, are backed by credible deployment trajectories. The Chalmers model suggests that even the strongest historical performers are not scaling renewables quickly enough to close the gap between a 2-degree world and a 1.5-degree one.
A forecast built on what countries have actually done
The model, called PROLONG, takes a different approach from most energy forecasts. Rather than optimizing for cost or engineering assumptions, it works backward from real deployment data. The researchers collected decades of national-level records showing how wind and solar electricity shares grew in dozens of countries, then used those trajectories to project what other nations are likely to achieve as they reach similar stages of adoption.
The method produces not a single forecast line but a probability distribution: a median projection flanked by upper and lower quartiles for each country and each year through 2050. In the paper, the authors describe the technique as a “time machine” approach, writing that it works by “matching a country’s current position on the adoption curve to the historical experience of nations that passed through that stage earlier.”
The technique builds on a 2021 Chalmers study, also published in Nature Energy, that measured the fastest rates at which individual countries have historically scaled onshore wind and solar as a share of total electricity. Those empirical speed limits feed directly into PROLONG, setting the growth-rate constraints the new model inherits.
Where the projections land
The researchers benchmarked PROLONG’s output against the IPCC’s Sixth Assessment Report scenario pathways, housed in the IIASA AR6 scenarios database. That database is the standard reference for defining what energy systems need to look like under 1.5-degree and 2-degree warming limits.
The result is a clean split. PROLONG’s median projections for combined wind and solar electricity shares fall within the envelope of pathways consistent with holding warming to 2 degrees Celsius. But they fall short of the steeper ramps required by 1.5-degree-consistent scenarios, which generally demand wind and solar to supply a much larger fraction of global electricity by mid-century. According to the paper, the model’s median projection places global combined wind and solar at roughly 50 percent of electricity generation by 2050, a level that sits comfortably within the 2-degree scenario range but below the approximately 70 percent or higher share that many 1.5-degree pathways in the IPCC database require by the same date.
For context, wind and solar together supplied roughly 16 to 17 percent of global electricity in 2024, according to Ember’s Global Electricity Review. The 1.5-degree pathways in the IPCC’s database typically require that share to climb steeply through the 2030s and 2040s, reaching levels that no large national grid has yet demonstrated at scale.
Why the gap matters for policy
The practical implication is pointed. If countries continue deploying wind and solar at rates consistent with the best historical performers, the world can plausibly stay on a 2-degree track. Reaching 1.5 degrees would demand acceleration beyond anything observed at the national level so far.
That distinction matters because many governments have formally endorsed the 1.5-degree goal, most recently through the COP28 Global Stocktake in December 2023, which called for “transitioning away from fossil fuels” and tripling renewable energy capacity by 2030. The Chalmers model does not say 1.5 degrees is impossible, but it does say that getting there would require breaking with historical precedent, not just matching it. As the authors note in the paper, “the historical record provides a credible basis for 2-degree-consistent deployment, but reaching 1.5-degree-consistent pathways would require growth rates that have not yet been sustained at the national level.”
The IEA’s own analysis points in a similar direction. Its recent renewables outlooks emphasize that even the tripling target faces structural obstacles: permitting backlogs, grid connection queues, and uneven investment flows to emerging economies. The agency treats alignment with 2-degree pathways as achievable under strengthened current policies, while framing 1.5 degrees as requiring much more aggressive action across multiple fronts.
Open data, open questions
One notable feature of the study is its transparency. The research team published a full reproducibility package on Zenodo containing model code, processed datasets, and scripts. Any researcher can regenerate the projections, check percentile distributions, and reproduce the comparison against IPCC pathway envelopes. That level of openness is uncommon in energy forecasting, where proprietary models and opaque assumptions often make external verification difficult.
Still, the model carries inherent limitations. Because it is grounded in historical patterns, it captures what has happened under past policy regimes, technology costs, and grid configurations. It does not automatically account for discontinuities. Rapid advances in solar cell efficiency, large-scale international technology transfer, or dramatic policy shifts could push countries onto faster adoption curves than any nation has previously achieved. Whether such shifts are plausible at the scale needed to close the 2-degree-to-1.5-degree gap is precisely the question the model leaves open.
There is also a question of representativeness. The historical record is dominated by wealthier economies with robust institutions and deep capital markets. Applying those trajectories to lower-income nations assumes similar growth dynamics can be reproduced under different financial and governance conditions. That assumption could prove conservative if technology costs keep falling and international climate finance scales up, or optimistic if investment risks and infrastructure gaps remain stubbornly high.
No public response from the IPCC or IEA specifically addressing the PROLONG model has appeared as of May 2026. The model stands as an independent, peer-reviewed contribution rather than an officially endorsed reference, though its use of the IPCC’s own scenario database as a benchmark gives it a direct point of comparison with established climate assessments.
What the baseline reveals about ambition
For policymakers and analysts, the most useful way to read the Chalmers work is as a stress test. If a national climate plan assumes wind and solar growth far above the upper quartile of historical experience, the burden of proof shifts: which concrete policies, investments, or innovations will deliver that break with precedent? Conversely, if a plan merely tracks the median PROLONG trajectory, it is likely aligned with a 2-degree world but inconsistent with the 1.5-degree ambition many governments have endorsed.
The picture that emerges is not one of inevitable failure or guaranteed success, but of a measurable gap between observed momentum and stated goals. Historical data, carefully modeled, show that current patterns of wind and solar deployment can plausibly deliver a 2-degree outcome while leaving a significant shortfall relative to 1.5 degrees. Closing that shortfall will depend on how quickly grids are modernized, how forcefully permitting is reformed, how reliably finance flows to emerging markets, and how willing societies are to accelerate a transition that is already underway but not yet fast enough for the targets they have set.
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*This article was researched with the help of AI, with human editors creating the final content.
