A letter to my grandchildren: on the UNEP report "Limiting Overshoot" (2026) — and on what we still have to do

What the report says

We must be clear about what is stated in the United Nations Environment Programme report. Let me summarise it honestly, including the parts that are less comfortable.

Above 1.5 degrees of warming there is no good scenario left. The report is unequivocal about this: there is no version of "somewhat warmer" that is safe or acceptable. Sea levels rise faster. Coral reefs are on the verge of collapse. Glaciers may lose more than a quarter of their mass by 2100. Global food production could be up to 14 percent lower by 2050 if we do not adapt. Heatwaves grow longer, cities and infrastructure come understrain, and the risk of irreversible tipping points — the ice sheets, the warmGulf Stream, the Amazon rainforest — increases. Small island states and low-lying coastal cities may end up partly or entirely under water.

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The 1.5-degree goal still stands — but we now have to come back to it from above. That is the heart of it. Overshoot means that we are going to pass it. The task becomes keeping the peak as low as possible, keeping the period above 1.5 degrees as short as possible, and bringing the temperature back down before the end of the century. The report does not call this a desirable path. It calls it the best path we have left. Which means that every other path is far worse.

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Every tenth of a degree and every year counts. Even if all current national climate plans were carried out in full, the peak would come in at around 1.8 degrees. And above roughly 1.8 degrees, returning to 1.5 degrees within this century becomes steadily harder. The margin is, quite literally, a few tenths of a degree wide.

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Cutting emissions and adapting are one task,not two. They reinforce each other. Insufficient emission reduction simply increases the damage, and that damage then swallows the money we needed for the transition. Conversely, if we do not adapt, everything we build in order to become clean is itself put at risk.

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Removing CO₂ from the air is badly needed, but it is a supplement, not a cure. To make the temperature genuinely fall, we will in time have to removemore CO₂ than we emit. But the report does the sums soberly: planting and managing forests at today’s scale would need well over a hundred years to bring warming down by 0.1 degrees — and that assumes no residual emissions at all. Even optimistic scenarios deliver only a few tenths of a degree this century. Everything we do not emit now is something we will not have to take back later. Other technical solutions may yet prove possible, but they fall outside this analysis.

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The damage is permanent. What is lost will not come back. Even if the thermometer falls again, sea levels will keep rising for centuries, species will remain extinct, and large-scale migrations will already have taken place. Returning to 1.5 degrees is not returning to the world we have now, andcertainly not to the world of my youth. That is the observation that saddens me most. Even if we were to act effectively now, we would still leave you an earth that is far poorer and less inviting.

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The actions the report calls for

The report sets out a pathway in four stages —exceedance, peak warming, decline and stabilisation — and three response phases that have to run alongside one another: an immediate response, coping and containment, and long-term resilience. In concrete terms:

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1.      Cut greenhouse gases deeply, and now — CO₂, but with equal urgency methane and other pollutants of that kind. Methane accounts for roughly half a degree of the warming we already have. That is precisely where cheap and immediately available measures lie: abating leaks in the oil and gas sector,separating waste and capturing methane from landfills, and reducing food loss and waste.

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2.      No new fossil lock-in. Existing and planned fossil fuel infrastructure alone could exceed, by 120 percent, the CO₂ emissions consistent with 1.5 degrees in 2030, and would consume a substantial part of the remaining budget for 2 degrees. So nothing new should be added, and what exists should be retired or retrofitted faster.

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3.      Accelerate the shift to renewables — towards 60 to 70 percent of global electricity by 2030, with the grids, storage and flexible demand to match.

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4.      Bring energy demand down through insulation and efficiency, the electrification of heating and cooling, better mobility and different food choices. Cooling is a creeping risk: on a high-emissions pathway, global electricity use for cooling could rise from close to 5,000 terawatt-hours in 2022 to as much as 18,000 by 2050.

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5.      Adaptation that goes beyond patching things up. Flexible and reversible, with the most vulnerable people and ecosystems first.

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6.     Build up CO₂ removal under strict rules, as a supplement to reduction and never as a replacement for it. Protecting natural carbon stocks is part of that.

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7.      Money that gets ahead of the damage rather than compensating for it afterwards. Invest in measures that can be adjusted as welearn. Do not wait until every theoretical and practical uncertainty has been resolved.

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A call to business

Last Tuesday we attended a gathering of businessesin the Amsterdam port area. People whose firms have been involved forgenerations in the economic and social development of this region, and who have often taken matters into their own hands when that was needed. I hope that entrepreneurs of this kind will read this letter too.

To the owners, directors and supervisory boardmembers of my generation: we control the companies that can help make the difference now. Not only through our own emissions and impact, but through ourpurchasing conditions, our supply chains, our investments and our voice in The Hague and in Brussels.

Let us not treat this as a competitive battle. A single company that cleans up its supply chain is punished for it; an entire sector that does so jointly changes a market. To foster change we have to work together. Please draw up joint purchasing requirements for green steel, concrete, transport and food. Take part in that work. Share your measurement data instead of treating it as confidential. Sign long-term contracts so that suppliers have the certainty they need to invest in improvement. Ask government for stricter and more stable rules, precisely because that levels the playingfield. And help the small supplier further down the chain to adapt as well. They have the fewest resources, and that is where it breaks first.

And be honest and open in what you communicate. Simply show what you emit and what you intend to do about it. Be honest about carbon offsetting as well. A tree or a certificate is no substitute for not emitting in the first place. The report states plainly that carbon removal is needed in addition to sustained emission cuts, not instead of them, and it does the maths: even with fossil emissions at zero and deforestation halted, a hundred years of afforestation and forest management at today’s scale would bring warming down by only 0.1 degrees. Do both at once, and change the way your business is run. It is a heavy task, but there is no longer any alternative.

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My dear grandchildren, I am sorry that I cannot promise you that it will turn out all right. Within your lifetimes it will not.The damage we have done is already irreversible. The consequences of our complacency will echo for centuries. What I can promise is that we will not sit and wait any longer for you to solve it. The UN report says it plainly: overshoot is not a reason to lower our ambition. On the contrary, it is a reason to begin cutting and adapting sooner, to learn faster, and to worktogether more closely and more effectively.

I wish you well.

Your loving grandfather

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