Beyond the Point of No Return 

Climate scientist James Hansen on 2°C, sea rise, and the race against time 

Dr. James Hansen has spent decades warning policymakers that the laws of physics will not bend to politics. Former director of NASA’s Goddard Institute for Space Studies and now adjunct professor at Columbia University’s Earth Institute, Hansen’s 1988 testimony to the U.S. Congress marked a turning point in global climate awareness. 

Speaking with journalist Ahmetcan Uzlaşık during the ATLAS25 Climate Tipping Points Conference in Helsinki, Hansen, perhaps the world’s best-known climatologist, discussed the limits of 1.5°C and 2°C warming, the risk of irreversible sea-level rise, and why he believes it is time to face scientific reality. 

We have already reached 1.5°C. The planet is out of energy balance, more energy coming in than going out by over one watt per square metre. That may sound small, but it means several tenths of a degree more warming are inevitable. 

So yes, we will hit 2°C within the next couple of decades. Without purposeful action to alter the planet’s energy balance, that threshold will be exceeded. The IPCC’s RCP 2.6 scenario, meant to cap warming around 2°C, is already off track. 

It is frustrating to hear leaders insist that 1.5°C is still possible with “more ambitious promises.” That is simply not true. The physics says it is too late. We should have acted decisively 10 or 20 years ago. 

It is frustrating to hear leaders insist that 1.5°C is still possible with “more ambitious promises.” That is simply not true. The physics says it is too late. We should have acted decisively 10 or 20 years ago. 

Sea-level rise is complex, but history tells us it can happen fast. During the last interglacial period, the Eemian, about 120,000 years ago, sea level rose several meters in under a century, likely due to West Antarctic ice-sheet instability. 

That is the real danger now. We are eroding the ice shelves that stabilisze West Antarctica. Within 50 to 150 years, we could see multimeter sea-level rise, a largely irreversible disaster. 

However, it is not yet inevitable. If we could cool the Southern Ocean, we might halt or even reverse ice-shelf loss. But first, emissions must come under control. Without that, nothing else matters. 

At present, renewables alone cannot provide consistent baseload power because they are intermittent. We still need a complement, and right now that is either fossil fuels or nuclear. 

We should have invested long ago in next-generation, ultra-safe reactors that can burn existing nuclear waste, use thorium, and produce high-temperature heat for industry. These designs exist but remain underdeveloped. We cannot build them overnight. 

Partnerships, especially with countries like China that can deploy technology rapidly, could accelerate the transition. Cooperation, not competition, will help us phase down emissions fast. 

Because we have failed to implement effective emissions policies, SRM will probably be needed to avoid catastrophic sea-level rise. That is not ideal, but it is the situation we have created. 

We urgently need research – careful, transparent, and internationally – governed. But we must keep emphasizing that emission reduction remains the top priority. Without that foundation, any intervention becomes meaningless or dangerous. 

I do not believe we have crossed an irreversible threshold yet, but signs are troubling. The Atlantic overturning circulation (AMOC) does appear to be weakening, and a shutdown is possible. That would disrupt Europe’s climate, inconvenient, yes, but survivable. 

The true catastrophe would be several meters of sea-level rise. Over half of the world’s major cities lie on coasts. That is something humanity simply cannot afford to let happen.