James Hansen's latest paper and implications for Ireland
Hansen's paper predicts AMOC shutdown in 2 to 3 decades if accelerated warming is not arrested
When reading about climate change I’m often reminded of the Albert Bartlett quote: “The greatest shortcoming of the human race is our inability to understand the exponential function”.
Exponential curves start off deceptively slowly before ramping up towards the end. If you were standing on the inflection point of one, and you looked behind, you wouldn't see much evidence that things were about to change, fast.
Imagine, for example, you were in the centre of the pitch in Croke Park, and you had a magic bottle of water. You pour out a teaspoon of water onto the pitch. Then a minute later, you pour 2 teaspoons of water. Then a minute later, you pour out 4 teaspoons of water. How long would it take to fill Croke Park - imagining it was enclosed all around - full to the brim with water?
The answer? 37 minutes. At 30 minutes, the water would only be up to your knees.
Exponential growth in climate instability comes to mind reading climate scientist James Hansen's latest paper “Global Warming Has Accelerated: Are the United Nations and the Public Well-Informed?”
Unusually for a paper on climate change, the paper shuns complex scientific terminology, and explains the situation in simple English, presumably with the intention of getting the message out far and wide. What follows is this layman’s interpretation of Hansen’s message.
The paper provides an explanation for the unprecedented 0.4 degree rise in global temperature in 2023 and 2024. Global temperatures had previously been rising at a rate of 0.18 degrees per decade from 1970 to 2010, and 0.36 degrees per decade from 2010 to 2023.
The paper argues the sudden 0.4 degree increase is due to a weak El Nino (El Nino is a natural warming of Pacific waters that happens every several years, increasing global temperatures briefly by 0.2 degrees - La Nina is the follow-on cooling of the Pacific waters), and an increase in the Earth's Energy Imbalance (EEI). The EEI is a measure of the amount of energy arriving on Earth from the sun, and the amount being reflected back out to space. Throughout the last 12,000 years, this has roughly been in balance.
This balance is now out of whack because of 2 factors: the retention of heat from the Sun due to its absorption by greenhouse gases such as CO2, and a decrease in the reflectivity of Earth’s surface (referred to as the “albedo”).
Two thirds of the decrease in albedo is due to the reduction in ice, snow and cloud cover (as the Earth warms, cloud cover lessens and ice melts). The remaining third is due to the reduction in aerosols from shipping. This turns out to be a crucial piece of evidence.
Aerosols are tiny pollutants floating in the atmosphere, with sulphates being a major component. They are mostly produced by the burning of fossil fuels. It has been known for decades that these pollutants slow down global warming by reflecting away sunlight from the Earth. These same aerosols, however, result in the death of about 8 million people globally per year. Those photos from a decade ago of Chinese cities enveloped in smog? Those are aerosols.
In 2020 regulations were introduced to drastically reduce sulphur content in shipping fuel thereby greatly reducing aerosol emissions from ships. This in turn reduced the reflectivity of Earth, with the question being how much this affects a key metric: climate sensitivity. Climate sensitivity defines how much Earth's surface temperature will eventually increase by should CO2 levels in the atmosphere double from their pre-industrial level of 280 parts per million.
Standard IPCC models peg this at 3 degrees. Hansen believes this is off by 50%, and the true increase is 4.5 degrees. The reason is this: the IPCC climate sensitivity calculations have aerosols contributing a constant level of reflectivity since the 1970’s, but Hansen’s team find aerosol contributions are not uniform and increase when aerosols are emitted over pristine skies (since releasing pollutants into clear skies will contribute more reflectivity than releasing pollutants into already polluted skies).
Much of the aerosols have been released over the clear skies of the North Atlantic and North Pacific, the 2 main shipping routes. Since the skies overhead are mostly free of pollutants, the aerosols have a larger reflectivity effect, thereby slowing global warming at a greater level than IPCC models have calculated.
Once these aerosols were reduced through regulation in 2020 - and therefore Earth’s reflectivity of sunlight was reduced - there was a snapback effect as temperatures suddenly shot upwards, giving a clue as to the true climate sensitivity. The real sensitivity, Hansen finds, is 4.5 degrees.
The IPCC and Hansen have disagreed over the years with Hansen maintaining that the IPCC has consistently under-estimated global warming. This has been borne out with some climate scientists “mystified” by the recent warming. The conservative estimates of the IPCC is probably an artefact of having a large number of scientists contributing to one text - the lowest common denominator upon which they can all agree on has to be found. This has also led to what Hansen calls “model fog”, with IPCC models outputting a very wide range for climate sensitivity of 2.5 to 4 degrees. In his latest newsletter, Hansen provides an acid test: if the 2025 global temperature remains at or above 1.5 degrees rather than drop back by 0.2 or 0.3 degrees due to the La Nina effect (as IPCC models project), then Hansen’s climate sensitivity of 4.5 degrees is likely correct.
The paper also provides an update on the gravest near-term threat of all - an AMOC shutdown. The AMOC is the dominant current in the Atlantic that transfers enormous amounts of heat from the southwestern Atlantic to the northeastern Atlantic. School-goers in Ireland learn Ireland’s relatively mild temperatures, despite our northerly latitude, is due to the North Atlantic Drift, a branch of the AMOC.
Hansen’s team found ice melt injection into the North Atlantic from the Greenland ice sheets exceeded prior estimates, and with the leap in temperatures in recent years, this flow will increase. This water is less dense than the warmer water in the upper layers, preventing the sinking of the upper layers of water - the process which kickstarts the AMOC. Hansen and his team state that a shutdown of the AMOC is “likely” within 20-30 years. Heat in the southwestern Atlantic will therefore remain there with two major consequences: a significant drop in temperature and precipitation for Western Europe, and increasing ice melt in the Antarctic, locking in “several” metres of sea level rise this century.
On a personal note, it is difficult to reconcile these cataclysmic warnings of near-term catastrophe with the triviality dominating newspaper columns, social media sites and politics. 20 years until a possible AMOC shutdown is just 4 government terms away. It is the same time it has taken to build the National Children's Hospital (which isn't even complete yet). My own son will be 22 years old.
There seems to be this idea in Ireland that climate change is some fairly innocuous thing, mostly afflicting far away places. We could do with a bit more warmth, after all. But an AMOC shutdown would thrust Ireland front and centre of the climate change catastrophe. Here is a projection of the long-term response of surface temperature to a doubling of CO2 and an AMOC shutdown:
Wind speeds and storms are driven by temperature differences - the greater the difference, the stronger the storms. Of this graph, leading AMOC expert Stefan Rahmstorf says "this looks like a crazy stormy climate to me, with unprecedented climate extremes across Europe".
Ireland, if the AMOC shuts down, would find itself on a kind of superstorm fault line.
This unimaginably tragic story of “intergenerational injustice”, as Hansen calls it, isn’t just being kept off the front pages by asinine stories of parish pump politics and the culture wars - it doesn't even make the news at all.
“Global Warming Has Accelerated: Are the United Nations and the Public Well-Informed?”
What do you think?
References
James Hansen’s latest paper “Global Warming Has Accelerated: Are the United Nations and the Public Well-Informed?“: https://www.tandfonline.com/doi/full/10.1080/00139157.2025.2434494
James Hansen’s newsletter summarising his last three papers, with response to criticisms: https://mailchi.mp/caa/global-warming-has-accelerated-why-what-are-the-consequences?e=e42cfb319f
James Hansen’s newsletter on the Acid Test: https://mailchi.mp/caa/the-acid-test-global-temperature-in-2025?e=e42cfb319f
Launch of the paper:
Hansen's estimate is likely behind the curve too. The cold blob south of Iceland indicates an exponential increase in the fresh water coming off of Greenland's glaciers is not being absorbed into the underwater conveyor belt of the AMOC. I look for significant effects in climate cooling in the north by 2030, as well as more drought in Brazil.